Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G1: Recent Advances in Density Functional Theory III
Sponsoring Units: DCP DCOMPChair: John P. Perdew, Temple University
Room: 103/105
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G1.00001: Insight into Structural Phase Transitions from Density Functional Theory Invited Speaker: Adrienn Ruzsinszky Structural phase transitions caused by high pressure or temperature are very relevant in materials science [1]. The high pressure transitions are essential to understand the interior of planets. Pressure or temperature induced phase transitions can be relevant to understand other phase transitions in strongly correlated systems or molecular crystals.\textbf{ }Phase transitions are important also from the aspect of method development [2,3,4]. Lower level density functionals, LSDA and GGAs all fail to predict the lattice parameters of different polymorphs and the phase transition parameters at the same time. At this time only nonlocal density functionals like HSE and RPA have been proved to resolve the geometry-energy dilemma to some extent in structural phase transitions [1]. In this talk I will report new results from the MGGA\textunderscore MS family of meta-GGAs and give an insight why this type of meta-GGAs can give a systematic improvement of the geometry and phase transition parameters together [3,4]. I will also present results from the RPA and show a possible way to improve beyond RPA.\\[4pt] [1] Xiao, B., Sun, J., Ruzsinszky, A., J. Feng, and Perdew, J.P.,\textit{ Phys. Rev. B} \textbf{2012,} 86, 094109.\\[0pt] [2] Ruzsinszky, A., Sun, J., Xiao, B., and Csonka G.I., \textit{J. Chem. Theory and Comp. }\textbf{2012}$, $8, 2078.\\[0pt] [3] Sun, J., Xiao, B., and Ruzsinszky, A., \textit{J. Chem. Phys.} \textbf{2012,} 137, 051101.[4] Sun, J., Xiao, B., Fang, Y., Hao, P., Ruzsinszky, A., et al., \textit{Phys. Rev. Lett.} \textbf{2013}, 111, 106401. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G1.00002: New ways of computing effective potentials for orbital-dependent functionals Viktor N. Staroverov Orbital-dependent density functionals offer many advantages over local-density and gradient-corrected approximations, but also pose distinctive challenges to developers and users alike. In particular, evaluation of functional derivatives of orbital-dependent functionals is far from straightforward because of numerical difficulties and basis-set artifacts involved. In response to this challenge, we have developed a whole class of iterative methods for accurate and efficient calculation of Kohn-Sham potentials for various orbital-dependent functionals including exact exchange, hybrids, and meta-generalized gradient approximations. The presentation will overview these methods and demonstrate their dramatic advantage over existing approximations (KLI, LHF, etc.) in practical finite-basis-set calculations. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G1.00003: Rationalization of Hubbard U in CeOx from first principles: Unveiling the role of local structure in screening Deyu Lu, Ping Liu DFT+U method has been widely employed in theoretical studies on various ceria systems to correct the delocalization bias in local and semi-local DFT functionals with moderate computational cost. To rationalize the Hubbard U of Ce 4f, we employed the first principles linear response method to compute Hubbard U for Ce in ceria clusters, bulks, and surfaces. We found that in contrast to the commonly used approach treating U as a constant, the Hubbard U varies in a wide range from 4.1 eV to 6.7 eV, and exhibits a strong correlation with the Ce coordination numbers and Ce-O bond lengths, rather than the Ce 4f valence state. The variation of the Hubbard U can be explained by the changes in the strength of local screening due to O $\rightarrow$ Ce intersite transition. Our study represents a systematic, quantitative investigation of the relationship between the Hubbard U and the local atomic arrangement, enabling a DFT+environment-dependent U scheme that can have potential impact on catalysis research of strongly correlated systems. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G1.00004: Adiabatic Connection and Virial Theorem for Ensemble Density Functional Theory Aurora Pribram-Jones, Zeng-hui Yang, Carsten Ullrich, Richard Needs, Kieron Burke Ensemble density functional theory (DFT) establishes a natural framework for thermal DFT and provides excited state information inaccessible through other DFT methods. However, development of better exchange-correlation approximations is needed for this theory to be of practical use [1]. In this talk, the adiabatic connection [2] and virial theorem for ensemble DFT will be presented. In particular, their relation to exact ensemble DFT calculations, dependence on ensemble weights, and a new method for extracting exact ensemble exchange-correlation potentials will be explored. [1] Gidopoulos, N. I. and Papaconstantinou, P. G. and Gross, E. K. U. {\it Phys. Rev. Lett.}, {\bf 88}, 033003 (2002). [2] Nagy, A. {\it Int. J. Quant. Chem.}, {\bf 56}, 225 -- 228 (1995). [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G1.00005: Hybrid Density Functionals Tuned towards Fulfillment of Fundamental DFT Conditions Invited Speaker: Matthias Scheffler Hybrid exchange-correlation functionals (XC), e.g. PBE0 and HSE, have significantly improved the theoretical description of molecules and solids. Their degree of exact-exchange admixture ($\alpha )$ is in principle a functional of the electron density, but the functional form is not known. In this talk, I will discuss \textit{fundamental conditions} of exact density-functional theory (DFT) that enable us to find the optimal choice of $\alpha $ for ground-state calculations. In particular, I will discuss the fact that the highest occupied Kohn-Sham level of an $N$-electron system ($\varepsilon _{\mathrm{HOMO}}(N))$ should be constant for fractional particle numbers between $N$ and \textit{N-1 }[1,2] and equals the ionization potential (IP) [3, 4], as given by the total-energy difference. In practice, we realize this in three different ways. XC($\alpha )$ will be optimized (opt-XC) until it $(i)$ fulfills the condition: $\varepsilon_{\mathrm{HOMO}}(N) = \varepsilon _{\mathrm{HOMO}}$(\textit{N-1/2}) or the Kohn-Sham HOMO agrees with the ionization potential computed in a more sophisticated approach $\varepsilon _{\mathrm{HOMO}}(N) =$ IP such as \textit{(ii)} the $G_{\mathrm{0}}W_{\mathrm{0}}$@opt-XC method [5,6] or \textit{(iii)} CCSD(T) or full CI [6]. Using such an opt-XC is essential for describing electron transfer between (organic) molecules, as exemplified by the TTF/TCNQ dimer [5]. It also yields vertical ionization energies of the G2 test set of quantum chemistry with a mean absolute percentage error of only $\approx $3{\%}. Furthermore, our approach removes the starting-point uncertainty of \textit{GW} calculations [5] and thus bears some resemblance to the consistent starting point scheme [7] and quasiparticle self-consistent \textit{GW} [8]. While our opt-XC approach yields large $\alpha $ values for small molecules in the gas phase [5], we find that $\alpha $ needs to be 0.25 or less for organic molecules adsorbed on metals [9]. \\[4pt] [1] J. P. Perdew et al., PRL 1982.\\[0pt] [2] P. Mori-Sanchez et al., JCP 2006.\\[0pt] [3] M. Levy et al., PRA 1984.\\[0pt] [4] T. Stein et al., PRL 2010.\\[0pt] [5] V. Atalla et al., PRB 2013.\\[0pt] [6] N. A. Richter, et al., PRL 2013.\\[0pt] [7] T. K\"{o}rzd\"{o}rfer, N. Marom, PRB 2012.\\[0pt] [8] M. van Schilfgaarde et al., PRL 2006.\\[0pt] [9] O. T. Hofmann et al., NJP 2013. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G1.00006: Performance of optimally-tuned range-separated hybrid functionals in predicting molecular valence-electron spectra David A. Egger, Shira Weissman, Sivan Refaely-Abramson, Sahar Sharifzadeh, Matthias Dauth, Roi Baer, Stephan Kuemmel, Jeffrey B. Neaton, Egbert Zojer, Leeor Kronik Density functional theory with optimally-tuned range-separated hybrid (OT-RSH) functionals has been recently suggested [Phys. Rev. Lett. 109, 226405 (2012)] as a non-empirical approach to accurately predict the outer-valence electronic structure of molecules. Here, we provide a quantitative evaluation of the OT-RSH approach by examining its performance in predicting the outer-valence electron spectra of prototypical gas-phase aromatic rings. For a range of up to several eV, we find that the obtained outer-valence electronic structure agrees very well (typically within 0.1-0.2 eV) with both experimental photoemission and theoretical GW data. The sole exception found is a high-symmetry orbital that is particular to aromatic rings and occurs relatively deep inside the valence state manifold. We conclude that OT-RSH functionals offer a balanced description of differently localized electronic states, a feature we find to prevail also for the more complex terpyrimidinethiol. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G1.00007: Electronic structure of CuPc from an optimally-tuned range-separated hybrid functional Shira Weissman, Sivan Refaely-Abramson, David A. Egger, Egbert Zojer, Leeor Kronik The optimally-tuned range separated hybrid (RSH) functional approach [1] was recently shown to allow for the calculation of the outer valence electronic spectrum of different molecules [2], resulting in good agreement with both experiment and many-body perturbation theory. The functional is based on a separation of short-range and long-range exchange components, where the range-separation parameters is tuned based on satisfaction of physical constraints, notably the ionization potential theorem. Here, we apply this approach to copper phthalocyanine (CuPc), which is of much recent interest owing to its ability to form a highly stable organic semiconductor. CuPc offers a difficult challenge for the method, because it is an open shell molecule whose electronic structure involves strongly localized $d$ orbitals. We find that the spectrum obtained for CuPc using the optimally-tuned RSH functional is in very good agreement with both experiment and MBPT calculations throughout most of the outer valence range. [1] L.Kronik, T.Stein, S.Refaely-Abramson, R.Baer,\textit{J. Chem. Theo. Comp.} (Perspectives Article) \textbf{8}, 1515 (2012). [2] S.Refaely-Abramson, S.Sharifzadeh, N.Govind, J.Autschbach, J.B.Neaton, R.Baer, L.Kronik, \textit{Phys. Rev. Lett.} \textbf{109}, 226405 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G1.00008: Kinetic and Hole Contributions to the Exact TDDFT Correlation Potential Kai Luo, Johanna Fuks, Ernesto Sandoval, Peter Elliott, Neepa Maitra The recent report[1-2] that dynamical steps generically develop in the exact correlation potential of time-dependent density functional theory (TDDFT) triggers the present work on the investigation on correlation potential and its adiabatic approximation. We hope this understanding will be of use in the construction of new non-adiabatic functionals capable of modeling non-linear electron dynamics using time-resolved TDDFT. We decompose the exact correlation potential into kinetic and hole contributions, analogously to what was done in the ground-state some years ago (e.g.[3]). In the ground-state, it was found that the dominant contribution was typically from the correlation hole potential, except in cases of strong correlation, when the system is far from single Slater determinant. However, this is not true in the time-dependent case: the dynamical step feature is independent of the deviation from a single-Slater determinant. Instead, the steps appear to be correlated with local oscillations of time-dependent natural orbital occupation numbers and further understanding of this connection is part of on-going investigations. [1] Elliott, Phys. Rev. Lett. 109,266404(2012) [2] Fuks, J. Phys. Chem. Lett. 4, 735(2013) [3] Gritsenko, J. Chem. Phys. 104, 8535-8545(1996) [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G1.00009: First-Principles Studies of the Excited States and Optical Properties of Xanthene Derivative Chromophores Samia Hamed, Sahar Sharifzadeh, Jeffrey Neaton Elucidation of the energy transfer mechanism in natural photosynthetic systems remains an exciting challenge. In particular, biomimetic protein-pigment complexes provide a unique study space in which individual parameters are adjusted and the impact of those changes captured. Here, we compute the excited state properties of a group of xanthene-derivative chromophores to be employed in the construction of new biomimetic light harvesting frameworks. Excitation energies, transition dipoles, and natural transition orbitals for the low-lying singlet and triplet states of these experimentally-relevant chromophores are obtained from first-principles density functional theory. The performance of several exchange-correlation functionals, including an optimally-tuned range-separated hybrid, are evaluated and compared with many body perturbation theory and experiment. Finally, we will discuss the implication of our results for the bottom-up design of new chromophores. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G1.00010: Exact Factorization of the Electron-Nuclear Wavefunction: Exact Electronic Potentials in Coupled Electron-Ion Dynamics Yasumitsu Suzuki, Ali Abedi, Neepa T. Maitra, Koichi Yamashita, E.K.U. Gross We develop a novel approach to the coupled motion of electrons and ions that focuses on the dynamics of the electronic subsystem. Usually the description of electron dynamics involves an electronic Schr\"{o}dinger equation where the nuclear degrees of freedom appear as parameters or as classical trajectories. Here we derive the exact Schr\"{o}dinger equation for the subsystem of electrons, staying within a full quantum treatment of the nuclei. This exact Schr\"{o}dinger equation features a time-dependent potential energy surface for electrons (e-TDPES). We demonstrate that this exact e-TDPES differs significantly from the electrostatic potential produced by classical or quantum nuclei. \\[4pt] [1] Y. Suzuki, A. Abedi, N. T. Maitra, K. Yamashita and E. K. U. Gross, e-print arXiv:1311.3218v1 [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G1.00011: ABSTRACT WITHDRAWN |
Session G2: Focus Session: Quantum Control of Molecular, Nano, and Plasmonic Materials IV
Sponsoring Units: DCPChair: Thomas Weinacht, Stony Brook University
Room: 102
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G2.00001: TDDFT and RPA for mesoscopic systems with thousands to millions of electrons: understating the red-shift in silver clusters absorption around 5nm Invited Speaker: Daniel Neuhauser Two quantum approaches for describing mesoscopic quantum systems with TDDFT will be described: The first, in collaboration with G. Lu and colleagues, is a single-orbital Madelung-like TDDFT propagation incorporating the correct homogenous electron gas dependence of the susceptibility on frequency, wavevector and density. We used this approach to understand the red-shift in the absorption of silver clusters around 5nm diameters. The second, in collaboration with R. Baer and E. Rabani, obtains the exact TDDFT and RPA results by stochastic averaging where the system's time-dependent density and potential is obtained by propagating small set of randomly chosen stochastic-orbitals, each of which is initially a random combination of the system's occupied orbitals. For large systems $\sim$ 10-40 orbitals are sufficient to get the correct dynamics regardless of the number of electrons. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G2.00002: Binary Platinum-Based Nanoclusters: A Density Functional Theory Investigation Juarez L.F. Da Silva, Ricardo K. Nomiyama, Maur\'Icio J. Piotrowski, Diego Guedes Sobrinho, Anderson S. Chaves Binary Ptatinum-based nanoclusters have attracted great attention in the last years due to the possibility to improve the chemical and physical properties of Pt nanoclusters. In this work, we will report a theoretical study of the structure and electronic properties of the Pt$_n$TM$_{55-n}$ (TM = Fe, Co, Ni, Cu, Zn, Rh, Au) nanoclusters using density functional theory as implemented in the Vienna Ab-Initio Simulation Package (VASP). We found negative values for the excess energy for all systems, except for TM = Au, which indicates a gain in stability of the nanoclusters in comparison to the parent systems, i.e., Pt$_{55}$ and TM$_{55}$. We observed that platinum has a strong preference to occupy the nanocluster surface, except for TM = Au, which can be explained by the large atomic radius of Pt atoms compared with with the Fe, Co, Ni, Cu, and Zn atoms. Our results indicate that the core-shell configuration, in which the core (13 TM atoms) and shell (42 Pt atoms) are from different chemical species, has greater stability compared with other compositions for all systems (except for TM = Au). Furthermore, we studied the average effective coordination, bond lengths, magnetic, and electronic properties of all those systems as a function of the composition. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G2.00003: First principles theory for surface plasmon generation and decay to hot carriers Ravishankar Sundararaman, Prineha Narang, Adam Jermyn, Harry A. Atwater, William A. Goddard III Plasmonic resonances provide a promising pathway for efficiently capturing infrared photons from solar radiation and boosting photo-catalytic activity via local temperature enhancements and hot carrier generation. Previous calculations of plasmon decay to excited carriers employing a fully quantized model Hamiltonian \footnote{A. Jermyn, P. Narang and H. A. Atwater, \emph{(under preparation)}} indicate strong plasmon polarization dependence and momentum anisotropy of the generated carriers, in contrast with classical theories. An accurate first principles calculation for this process must account for microscopic details at the atomic scale for the electronic states as well as the effect of the 10-100~nm length scale particle and antennae geometries on the plasmon resonances. Here, we present a first-principles multi-scale model of plasmonics combining electronic density-functional theory with electromagnetic models on longer length-scales, and investigate the role of electronic structure and geometry on plasmonic light absorption, decay and hot carrier generation. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G2.00004: High Field Optomagnetic (OM) Polarization-Phase Selective (PPS) Monitoring of Structures and Controlling Reaction Agents Mechanisms in Complex Molecular Systems Kresimir Rupnik Using OM techniques, including new high filed 25T Split-Florida magnet at NHMF Laboratory, we have recently observed unusual metal cluster structures and electron transfer patterns in complex molecular systems of biomedical and material science interest. We report here some of the new technological solutions and (many) challenges that face OM and (quantum) control research. Of particular interest is identification of fast (10-100s fs) highly correlated electrons spin and vibrational coupling interpreted using adaptive molecular-photonic interaction models. Our observations question interpretations of previously proposed electron spin structure models and mechanisms and$_{\mathrm{\thinspace }}$indicate possible new controlling mechanisms through highly selective coupled channels that combine different specific redox and photonic agents. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G2.00005: Strong Field Coherent Control at the Space-Time Limit Invited Speaker: Tamar Seideman Strong field coherent control has proven advantageous for control of molecular dynamics largely since it is able to benefit from the advanced Ti-Sapphire technology available at 800 nm wavelength. Although the most popular and versatile of the class of strong field, induced dipole coherent control methods is molecular alignment, related techniques that rely on similar concepts, including torsional control and molecular focusing, have been shown similarly successful. Here we suggest that the strong field approach to will prove yet more advantageous for coherent control of nanoscale material systems. One reason is the naturally available strong field and strong field orientational and translational gradients in nanoscale plasmonic environments, such as light-triggered molecular conduction junctions and tip--molecule--surface systems. Another is the enhancement of the polarizability of molecules adsorbed onto a metal construct as compared to isolated molecules. In the talk, we will combine plasmonics physics with concepts and tools borrowed from coherent control of molecular dynamics with two goals in mind. One is to introduce new function into nanoplasmonics, including ultrafast elements and broken symmetry elements. The second is to develop coherent nanoscale sources and apply them to strong field coherent control of both mechanical motions and electric transport in the nanoscale. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G2.00006: Charge transfer and quantum coherence in solar cells and artificial light harvesting system Christoph Lienau In artificial light harvesting systems the conversion of light into electrical or chemical energy happens on the femtosecond time scale, and is thought to involve the incoherent jump of an electron from the optical absorber to an electron acceptor. Here we investigate the primary dynamics of the photoinduced electronic charge transfer process in two prototypical structures: (i) a carotene-porphyrin-fullerene triad, a prototypical elementary component for an artificial light harvesting system and (ii) a polymer:fullerene blend as a model system for an organic solar cell. Our approach [1] combines coherent femtosecond spectroscopy and first-principles quantum dynamics simulations. Our experimental and theoretical results provide strong evidence that the driving mechanism of the primary step within the current generation cycle is a quantum-correlated wavelike motion of electrons and nuclei on a timescale of few tens of femtoseconds. We furthermore highlight the fundamental role played by the flexible interface between the light-absorbing chromophore and the charge acceptor in triggering the coherent wavelike electron-hole splitting. \\[4pt] [1] C. A. Rozzi et al., Nature Comm. \textbf{4}, 1602 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G2.00007: High photoreactivity in a non-fluorescent photocleavable ligands on gold Hans D. Robinson, Chalongrat Daengngam, Stefan V. Stoianov, Steven B. Thorpe, Xi Guo, Webster L. Santos, John R. Morris We report on the photo-patterning of a gold surface functionalized with a self-assembled monolayer of an {\em o}-nitrobenzyl-based photocleavable ligand bound to the gold surface with a thiol anchor. We find that the dose of UV light required to induce the photoreaction on gold is very similar to the dose in an alcohol solution, even though many optical phenomena are strongly suppressed on metal surfaces. We attribute this finding to a combination of the large skin depth in gold at UV wavelengths, the high speed of the photoreaction, and the spatially indirect nature of the lowest excited singlet. Any photoreactive compound where the quantum efficiency of fluorescence is sufficiently low, preferably no larger than about $10^{-5}$ in the case of gold surfaces, will show a similarly high photoreactivity in metal-surface monolayers. The implications of this result for optically driven self-assembly in plasmonic systems will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G2.00008: The effect of Coulomb interactions on thermoelectric properties of quantum dots Natalya Zimbovskaya, Valery Kuzmin Thermoelectric effects in a quantum dot coupled to the source and drain charge reservoirs are explored using a nonequilibrium Green's functions formalism beyond the Hartree-Fock approxomation. We concentrate on theoretical analysis of the influence of Coulomb interactions on thermopower and the figure of merit $ZT. $ Obtained results show that Coulomb interactions between charge carriers on the dot significantly contribute to its thermoelectric properties. In the present work, we trace the transition from the Coulomb blockade regime to Kondo regime in the thermoelectric properties of the quantum dot which occurs when we gradually strengthen the coupling of the dot to the charge reservoirs. We show that within the Coulomb blockade regime (when the coupling of the dot to the leads is weak compared to the characteristic strength of the charge carriers interactions) thermoelectric characteristics of the dot display distinct features caused by Coulomb interactions. These features indicate possibilities of enhancement of thermoelectric efficiency of the considered systems. Within the Kondo regime, when the couplings of the dot to the leads became stronger, the influence of Coulomb interactions declines bringing a decrease in the the thermoelectric efficiency. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G2.00009: Quantum Control of Electrons in Atoms, Molecules and Materials - from Femtosecond to Attosecond to Zeptosecond Timescales Invited Speaker: Margaret Murnane This talk will discuss strong field quantum control in atomic, molecular and materials systems with applications across a broad range of chemical, physical and materials sciences. Using mid-infrared femtosecond lasers to drive the high harmonic (HHG) frequency upconversion process, strong time-gated phase matching results in bright \textit{coherent} keV soft X-ray beams on a tabletop for the first time [1]. The new photon energy range accessed of 0.2--1.6 keV (corresponding to wavelengths of 1 -- 6 nm) is of particular interest for applications in chemical and materials spectroscopy and imaging. X-rays can penetrate thick (opaque) samples and achieve high spatial resolution (2--50nm) imaging, with the added advantage of elemental and chemical specificity by employing characteristic elemental X-ray absorption edges and chemically-induced fine structure at these edges. Moreover, when atoms are ionized by mid-infrared light, the electron liberated during the HHG process can be driven back to the parent ion multiple times, resulting in quantum interferences and zeptosecond x-ray waveforms [2]. We also recently demonstrated that we can precisely control molecular dynamics on both nuclear (i.e. femtosecond) and electronic (i.e. attosecond) timescales [3,4]. Using vacuum ultraviolet light pulses that are tunable in wavelength and time structure, it is possible to switch population between electronic excited states on attosecond timescales, and use this ability to select specific pathways for ionization or dissociation of a molecule. Ultrafast lasers can also be used to switch the dissociation pathways of molecules as they explode after irradiation by ionizing light. Finally, we used ultrafast x-rays to capture coherent processes in materials, such how fast a material can change its electronic or magnetic state,... or how fast spin currents can control and enhance magnetization in materials.\\[4pt] [1] Popmintchev et al., Science \textbf{336}, 1287 (2012).\\[0pt] [2] Hernandez-Garcia et al., PRL \textbf{111},~033002 (2013).\\[0pt] [3] Zhou et al., Nature Phys. \textbf{8}, 232 (2012).\\[0pt] [4] Ranitovic at al., submitted (2013).\\[0pt] [5] S. Mathias et al., PNAS \textbf{109}, 4792 (2012).\\[0pt] [6] Rudolf et al., Nat. Comm. \textbf{3}, 1037 (2012).\\[0pt] [7] Turgut et al., PRL \textbf{110}, 197201 (2013). [Preview Abstract] |
Session G3: Outreach and Diversity
Sponsoring Units: FEdChair: David Rench, Pennsylvania State University
Room: 107
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G3.00001: Physics, Dyslexia and Learning: A Collaboration for Disabled Students Barbara M. Moskal, Lyndsey Wright, P.C. Taylor Researchers have found that children with dyslexia reason differently with respect to language from those who do not have dyslexia. Dyslexic students' brains work differently than do students without dyslexia. Some researchers speculate that these differences provide dyslexic students with an advantage in science. The presentation will describe an outreach activity which developed and delivered instructional modules in physics to students in grades kindergarten through sixth. These modules were tested on thirty students who attended a summer camp designed for students who have been diagnosed with dyslexia. Eighty percent of students who have learning disabilities have dyslexia. Many of the students who attended this camp have experienced repeated failure in the traditional school system, which emphasizes literacy with little attention to science. A number of science and engineering professors collaborated with this camp to build instructional modules that were delivered one hour per day, during two weeks of this five week summer camp (ten hours of hands-on physics instruction). Both quantitative and qualitative data were collected with respect to the impact that this camp had on students' understanding and interests in science. The results of these efforts will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G3.00002: Adapting diagrams from physics textbooks: improving the autonomy of blind students Adriana Dickman, Alexandre Martins, Amauri Ferreira In this work we elaborate and test a glossary consisting of a set of objects and their symbols. The symbols are designed to represent objects frequently used in mechanics diagrams, such as vectors, ropes, pulleys, blocks and surfaces, and can be used to adapt drawings of physics situations in textbooks for blind high school students. The educational product was tested at a specialized school for the blind. The results indicate that adequate training can help blind students to become familiar with the symbols, and to identify them in a problem without the need of a description. This educational product can help blind students to achieve the same conditions of autonomy as sighted ones, when studying physics. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G3.00003: The Longitudinal STEM Identity Trajectories of Middle School Girls who Participated in a Single-Sex Informal STEM Education Program Roxanne Hughes This study examined the longitudinal effects of participation in an all-girls STEM summer camp on young women's interest in STEM fields and motivation to pursue these fields. The SciGirls camp has been in existence since 2006, with its goal of providing a safe space for young women to explore STEM careers and strengthen their interest in these careers. Over 166 middle school age girls have participated in the program since it began in 2006. Of those participants, 60 responded to at least one of the follow up surveys that are sent every three years -- 2009 and 2012. The surveys attempt to determine participants' level of interest in STEM. The survey was qualitative in nature and asked open ended questions. Results indicated that the camp had a positive effect on participants' perceptions of scientists and their work. This study adds to the literature that looks at the longitudinal impacts of informal STEM educational programs that expose young women to female scientist role models and mentors. This study supports the research that claims that exposing young women at an early age to science role models can positively alter their perception of science careers which can eventually increase the number of women who pursue these careers. This increase is important at a time when men still outnumber women in many science and engineering fields. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G3.00004: Phun Physics 4 Phemales: Physics Camp for High School Girls Chuhee Kwon, Jiyeong Gu, Laura Henriquez The department of Physics and Astronomy with the department of Science Education at California State University, Long Beach hosted summer program of ``Phun Physics 4 Phemales (PP4P)'' during summer 2012 and summer 2013 with the support from APS public outreach program. PP4P summer camp was hosted along with a two-week summer science camp, Young Scientists Camp, which has been institutionalized for the last 14 years since 1999. More than 2,500 3$^{rd}$-8$^{th}$ grade students and 250 teachers have participated in the program. PP4P program provided the tools and support that female high school students need to pursue careers in physics and/or science, technology, engineering and math (STEM) field. This girls-only camp created connections among the girls and built confidence. In addition PP4P program introduced students to key principles in physics by a hands-on lab environment and demonstrated the real-world social impact of physics. In summer 2012, high school girls worked on physics experimental project on electronics and in summer 2013 they worked on the mechanics. I would share our experience in this program and the impact on the female high school students. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G3.00005: The Art of Science Ashwin Vaidya, Mika Munakata The Art of Science project at Montclair State University strives to communicate the creativity inherent in the sciences to students and the general public alike. The project uses connections between the arts and sciences to show the underlying unity and interdependence of the two. The project is planned as one big `performance' bringing together the two disciplines around the theme of sustainability. In the first phase, physics students learned about and built human-powered generators including hand cranks and bicycle units. In the second phase, using the generators to power video cameras, art students worked with a visiting artist to make short films on the subject of sustainability, science, and art. The generators and films were showcased at an annual university Physics and Art exhibition which was open to the university and local community. In the final phase, to be conducted, K12 teachers will learn about the project through a professional development workshop and will be encouraged to adapt the experiment for their own classrooms. The last phase will also combine the university and K12 projects for an exhibition to be displayed on Earth Day, 2014. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G3.00006: Teaching minority middle-school students to solder James Reardon, Billy J. Gates, Jr. We aspired to teach minority middle school students to solder. We found that important variables affecting our ability to do so included: student-to-teacher ratio, venue of instruction, relationships with community partners, and understanding of the structure of the student's worldview. Once the effects of these variables had been understood, we found the students readily learned to solder. We now want to see whether the acquisition of the skill of soldering leads the students to be more interested in technical careers and in going to college. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G3.00007: Zoom In! A Nanoscience Claymation Video Project Designed for Students K-3 Nancy Sandler, Sergio Ulloa, Kate Raney Nanoscience concepts are somewhat new and strange to the general public, and although simple to explain, have not permeated through the various information channels available for public education. This is particularly true for children. Because young people in levels K-3 are exposed to digital media on a daily basis, we recognized the importance of reaching them using a familiar format. Hence, we developed a claymation Zoom In! movie that follows the ``adventures'' of Gwen Pym, a girl ``nanoscientist,'' in her quest for a dress that cannot be stained. The pilot video presented in this talk provides a novel and imaginative way to capture young children's attention while focusing on basic nanoscience concepts. By reducing Gwen to a sub-milimeter scale, concepts of scale, surface tension, hydrophobicity induced by roughness, are all exemplified in simple terms accessible to this age range. The movie is accompanied by supporting material aimed at schoolteachers, covering the physics concepts involved in the various aspects of the adventure, and including suggested on-class activities that expand on these points. The final product is contained in a DVD that was distributed to the local elementary schools in the South East Ohio area. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G3.00008: ``A Penny Plain and Twopence Coloured,''-- How the Penny Theater format was used to animate and present Mary Chapin Carpenter's book, ``Halley Came to Jackson'' to preschoolers and their families as a STEM outreach program Elizabeth Jan Jablonski, Daniel Jablonski, Matthew Jablonski, Peter Jablonski, Maureen Green, Charles Green, Megan Wyble, Margaret Ardillo The goal of this project is to develop a program for young children (3 to 8 years old) that provides basic information about concepts related to space, comets, time, and timelessness in a stimulating, memorable manner. The vehicle for achieving this goal is an adaptation of the children's picture book \textit{Halley Came to Jackson} to a modern update of the historical Penny Theater format. The resulting ``show'' is rich in the concepts listed above, and has been presented in a variety of venues and with a variety of supplementary activities to several hundred preschoolers and their families. Based on a combination of prior research on how young children learn and careful observation and follow-up to performances of ``Halley,'' numerous findings have been developed. The Penny Theater concept and the findings of this project are discussed herein. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G3.00009: The Global Sensor Web: A Platform for Citizen Science Ariel Simons The Global Sensor Web (GSW) is an effort to provide an infrastructure for the collection, sharing and visualizing sensor data from around the world. Over the past three years the GSW has been developed and tested as a standardized platform for citizen science. The most developed of the citizen science projects built onto the GSW has been Distributed Electronic Cosmic-ray Observatory (DECO), which is an Android application designed to harness a global network of mobile devices, to detect the origin and behavior of the cosmic radiation. Other projects which can be readily built on top of GSW as a platform are also discussed. http://www.globalsensorweb.org/wiki/index.php/Home [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G3.00010: On the Sea, From the Sea, Of the Sea: The Physics of maritime Governance Royce James, Eric Page The United States Coast Guard Academy Physics Section is proud to present our initial conceptions of ``\textit{On the Sea, From the Sea, Of the Sea: The Physics of Maritime Governance,}'' a program funded by an APS Outreach Grant in 2013. In our classes, the Physics Section has focused on active student engagement for the past ten years. Recently, we have refined our program to make heavy use of Interactive Lecture Demonstrations (ILDs) and our own highly interactive adaptation which we call Interactive Lecture Labs (ILLs). ``On the Sea'' is a unique opportunity to investigate their use in a different learning modality from our standard college level military academic use. Multigenerational science, technology, engineering, and mathematics (STEM) projects are a prolific source of academic discourse, while learning through play has been touted as an effective learning tool. We plan to investigate group and individual participation, intragroup communication, demographics, and prior skill (or education) in comparison to outcomes in learning objectives through projects designed to educate the Coast Guard Academy and surrounding community on the physics of the Coast Guard's missions. Progress on the lab and demonstration designs, community participation, and our emerging ILL and ILD pedagogical methods, will be reported. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G3.00011: Intergalactic Travel Bureau Olivia Koski, Mark Rosin The Intergalactic Travel Bureau is an interactive theater outreach experience that engages the public in the incredible possibilities of space tourism. The Bureau is staffed by professional actors, who play the role of space travel agents, and professional astrophysicists, who play the role of resident scientists. Members of the public of all ages were invited to visit with bureau staff to plan the vacation of their dreams--to space. We describe the project's successful nine day run in New York in August 2013. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G3.00012: DEEP: Discover, Explore, and Enjoy Physics \& Engineering via High Impact Educational Experiences at Texas A\&M and Beyond Tatiana Erukhimova, Edward Fry We will present the first results of an innovative program at Texas A\&M University that aims to enhance the learning and research experiences of undergraduate and graduate students through their participation in high-profile outreach activities: principally the Texas A\&M Physics and Engineering Festival and the Physics Shows. The goals are to enhance students' knowledge of fundamental physics concepts through collaborative hands-on research and educational activities, to teach them effective communication skills and responsibility, and to enhance their opportunities for interactions with their peers and professors outside the classroom. The program activities include (i) students working side-by-side with their peers and professors on research, concept, design, and fabrication of physics demonstration experiments, (ii) presentation of these exhibits during the Festival and Shows in teams of several students and faculty members, (iii) assessment of students teamwork, and (iv) incorporation of new demonstrations in core curriculum classes. Texas A\&M Physics and Engineering Festival is a major annual outreach event at TAMU attracting over 4000 visitors and featuring over 100 interactive exhibits, public lectures by prominent scientists, and various hands-on activities. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G3.00013: Physics Outreach Grant Experiences Heide Doss Descriptions of two different Physics Outreach grant projects will be presented. I will discuss my experiences trying to engage and teach the public in my locality some physics through birthday parties for the laser in 2010. I will also discuss my experiences trying to reach the general public through greeting cards and bookmarks with physics on the back in 2012-2013. These efforts spilled over to a larger audience, which led to a larger impact. I will describe what worked, what didn't, and the value of these efforts. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G3.00014: Weird-World, Weird-World Ivan Schuller, Rich Wargo We will present the first in a series of videos designed and produced specifically as a pilot for the YouTube audience to playfully explore interesting and unusual phenomena that physics reveals, and their uses in modern life. No talking heads, no pedants, no complicated theory -- but rather a visually captivating and often kooky comical look at exclusion principle, entanglement, tunneling and the retinue of exceedingly strange things that happen in classical and quantum physics and how we understand and actually use this weirdness each and everyday. Produced by the UC San Diego-based creative partnership between an active physicist and established university based science media producer responsible for the highly successful and comical nanoscience caper When Things Get Small, this will pilot an on-going series with the specific goal of entertaining and engaging audiences of all ages. The series has planned distribution and marketing on YouTube though the unique programming and distribution capacities of University of California Television to commence in 2013. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G3.00015: Outreach with Team eS Through Science Festivals and Interactive Art Installations Amanda Yoho, Glenn Starkman The Team eS project aims to acclimate (pre)teens to scientific concepts subtly, with fun, accessible, and engaging art and activities hosted at public community festivals, online at a dedicated website, and using social media. Our festivals will be centered around an interactive art installation inspired by a scientific concept. We hope to provide a positive experience inspired by science that these teens can reflect upon when encountering similar concepts in the future, especially in settings like a classroom where fear and anxiety can cloud interest or performance. We want to empower teens to not feel lost or out of the loop -- we want to remove the fear of facing science. [Preview Abstract] |
Session G4: Focus Session: New Frustrated Materials
Sponsoring Units: GMAGChair: Jaime Fernandez-Baca, Oak Ridge National Laboratories
Room: 112/110
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G4.00001: Gapless Spin Liquid Behaviour in the S $=$ 1/2 Vanadium Oxyfluoride Kagome Antiferromagnet [NH$_{4}$]$_{2}$[C$_{7}$H$_{14}$N][V$_{7}$O$_{6}$F$_{18}$] Invited Speaker: Lucy Clark The ionothermal synthesis of the magnetic bilayer compound diammonium quinuclidinium vanadium oxyfluoride, [NH$_{4}$]$_{2}$[C$_{7}$H$_{14}$N][V$_{7}$O6F$_{18}$], or DQVOF was recently reported [1]. Its structure contains two crystallographically distinct vanadium sites. On one site sit V$^{4+}$ d$^{1}$ cations, which form a geometrically frustrated kagome network of S $=$ 1/2 spins. At the second site, between these kagome layers, reside V$^{3+}$ d$^{2}$ S $=$ 1 cations to give the kagome bilayer-type units. Here, I will show that DQVOF can be considered as an experimental realization of an S $=$ 1/2 kagome antiferromagnet, with non-interacting S $=$ 1/2 kagome planes, as evidenced by the paramagnetic behaviour of the interlayer S $=$ 1 spins of the V$^{3+}$ cations in our low temperature magnetization and specific heat data. Furthermore, I will show that the combination of strong geometrical frustration and quantum effects within the kagome planes results in exotic magnetic behaviour, with significant experimental evidence in the form of specific heat and muon spin relaxation measurements pointing towards a gapless quantum spin liquid ground state in DQVOF [2]. \\[4pt] [1] F. H. Aidoudi et al. Nat. Chem. 3, 801 (2011).\\[0pt] [2] L. Clark et al. Phys. Rev. Lett. 110, 207208 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G4.00002: Spin Waves in the FCC Kagome Lattice Martin LeBlanc, Byron Southern, Martin Plumer, John Whitehead The impact of an effective local cubic anisotropy [1] on the spin wave excitations and inelastic neutron scattering intensity peaks of the Heisenberg model on the 3D fcc kagome lattice are examined through a linear spin wave theory. Previous Monte Carlo simulations revealed that the addition of anisotropy to the fcc kagome lattice changes the order of the phase transition from weakly first order to continuous and restricts the $T=0$ spin configuration to a number of discrete ground states, removing the continuous degeneracy [2]. It is shown that the addition of anisotropy removes the number of zero energy modes in the excitation spectrum associated with the removed degeneracies. These results are relevant to Ir-Mn alloys which have been widely used by the magnetic storage industry in thin-film form as the antiferromagnetic pinning layer in GMR and TMR spin valves [2]. \\ {[1]} L. Szunyogh, B. Lazarovits, L. Udvardi, J. Jackson, and U. Nowak, Phys. Rev. B \textbf{79}, 020403(R) (2009). \\ {[2]} M.D. LeBlanc, M.L. Plumer, J.P. Whitehead, and B.W. Southern, Phys. Rev. B \textbf{88}, 094406 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G4.00003: Spin frustration and magnetic ordering in the Mott insulating fcc-Cs$_3$C$_{60}$ Yuichi Kasahara, Yuki Takeuchi, Tatsuaki Itou, Yoshihiro Iwasa, Denis Arcon, Matthew Rosseinsky, Kosmas Prassides The low-temperature magnetic state at ambient pressure has been investigated by specific heat and nuclear magnetic resonance (NMR) measurements in face-centered-cubic (fcc-) Cs$_3$C$_{60}$, which is characterized by a Mott insulating state with $S=1/2$ spins in C$_{60}^{3-}$ anions and a geometrical spin frustration inherent in the fcc lattice. Specific heat exhibited no sharp anomaly down to 0.4~K, but both magnetic specific heat and NMR relaxation rate revealed a broad peak around 2.5~K, indicating that the reported antiferromagnetic ordering is accompanied by a gradual freezing of electronic spins with distributed transition temperatures. These results are unexpected in the conventional fcc antiferromagnets. Interplay of geometrical frustration, orientational disorder of C$_{60}$ molecules, and weak Mottness gives rise to the unique magnetic ground state in fcc-Cs$_3$C$_{60}$. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:51PM |
G4.00004: A$_{3}$TeX$_{3}$Z$_{2}$O$_{14}$ (A$=$Ba, Pb, K; X$=$Co, Mn, Fe; Z$=$V, P): Understanding symmetry loss in langasites Invited Speaker: Harlyn Silverstein Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ (BNFS) is a langasite that displays simultaneous antiferromagnetic ordering and ferroelectric polarization below T$_{\mathrm{N}} =$ 26 K. But many langasites, including BNFS, crystallize in the nonpyroelectric space group P321. It has been postulated that ferroelectric domains in BNFS and related systems may arise from either the Dzyaloshinskii-Moriya interaction or through symmetry loss to either the P3 or C2 (or lower) space groups from magnetoelastic distortions. Indirect experimental evidence for symmetry loss to C2 exists, but such a distortion is too small to detect with synchrotron X-ray diffraction and implies polarization along the wrong axis. Here, we present another route to understanding symmetry loss in langasites. Rather than focusing on BNFS, where the observed small structural distortions are clouded by experimental uncertainties, we instead turn our attention to alternative chemical systems that are more prone to structural distortions. Unlike BNFS, these distortions can be directly detected using X-ray diffraction. In particular, emphasis is placed on Pb-containing langasites that distort away from P321 symmetry and the impact of this symmetry loss on the magnetism observed in each system.\\[4pt] Work done in collaboration with Arzoo Sharma, Kanisha Cruz-Kan, Avichai Stoller, University of Winnipeg; Haidong Zhou, University of Tennessee-Knoxville; Ashfia Huq, Oak Ridge National Laboratory; Pascal Manuel, ISIS - Rutherford Appleton Laboratory; Roxana Flacau, National Research Council; and Christopher Wiebe, University of Winnipeg. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G4.00005: Magnetothermal observables of geometrically frustrated systems: The case of Fe$_{2}$P-like layered structures Juan Manuel Florez, Oscar Andres Negrete, Patricio Vargas, Caroline A. Ross We study a Fe$_{2}$P-like structured material composed by alternating layers of distorted-Kagome and segmented-triangular lattices. The system is modeled as a stacked Heisenberg structure of mixed AFM/FM couplings, and the magnetothermal properties are calculated by using a Monte Carlo simulations framework. We focus on the question of whether the system could present or not a double-transition-like behavior as a consequence of an intermediate ordered state, which gives rise to a thermal delaying of the spin disorder after the planar 120$^{\circ}$ ordering of the Kagome layers is already broken. This double transition could be observed, e.g., in Fe$_{2}$P-like Iron-pnictides if the Fe-triangles behave like an effective spin center. In a more general case however, FM and AFM intra-triangular interactions lead to different magnetic specific heat sceneries, where the observable peaks evidence an opposed behavior as the strength of the couplings increases: FM promotes the shrinking of the paramagnetic-like zone of the phase-diagram; AFM boosts the global spin disorder but also triggers a competition between the canted orderings of the triangular and the Kagome lattices, which is evidenced through a doubly-bifurcated phase-diagram. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G4.00006: $^{31}$P-NMR Study of the Effect of Pressure on the Magnetic Properties of the 2d Frustrated Square-Lattice Compound BaCdVO(PO$_{4})_{2}$ at Low Temperatures Beas Roy, Ramesh Nath, David C. Johnston, Yuji Furukawa BaCdVO(PO$_{4})_{2}$ is a spin $S =$ 1/2 frustrated square-lattice compound with a nearest-neighbor exchange coupling $J_{1} = -$3.62 K and a next-nearest-neighbor exchange coupling $J_{2} =$ 3.18 K yielding \textbar $J_{2}$/$J_{1}$\textbar $=$ 0.88. A transition to an antiferromagnetic (AFM) ground state occurs below a temperature $T_{\mathrm{N}} =$ 1.0 K under ambient pressure $p$. Based on the $J_{2}$/$J_{1}$ ratio, the system is located close to the disordered ground state (``nematic state'') regime of the phase diagram. We carried out $^{31}$P-NMR measurements under high $p$, ranging from 0.74 GPa to 1.88 GPa, and at low temperatures $T$ down to 100 mK, to investigate the effects of $p$ on the magnetic properties of the system. With increasing $p$, the $T_{\mathrm{N}}$ does not change much, but the magnetization saturation field $H_{\mathrm{S}}$ is significantly suppressed from $H_{\mathrm{S}} =$ 4.2 T at ambient $p$ to $H_{\mathrm{S}} =$ 0.55 T at $p =$ 1.88 GPa. Our $^{31}$P-NMR spectra and spin-lattice relaxation rate (1/$T_{1})$ data establish the first $H$--$p$--$T$ phase diagram for this system. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G4.00007: A Monte Carlo Study of Magnetically Frustrated Chromium Andrew Macdonald, Sarah Burke, Doug Bonn, Yan Pennec As the thickness of engineered films decreases exotic magnetic configurations can appear because of boundary conditions, alloying, or metastable crystallographic phases. Characterizing the ground state of such films is imperative to building new and better magnetic devices and gaining a fundamental understanding of magnetic materials. Spin-polarized scanning tunnelling microscopy experiments have recently revealed unexpected types of magnetic order in a thin film of chromium grown epitaxially on gold. In this talk, I will discuss the characterization of the magnetic ground states of this film via classical Monte Carlo simulations. By modelling the film as an Ising system with a variable degree of lattice distortion and diluting with non-magnetic sites the simulations replicate the complexity of the magnetic ordering observed in the experimental data. Comparing the results of simulation and experiment we conclude that the observed magnetic order is a result of substantial gold inter-alloying combined with geometric frustration, making the ordered state highly sensitive to the degree of lattice distortion. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G4.00008: Magnetic short and long range order in a disordered perovskite Shravani Chillal, Severian Gvasaliya, Andrey Zheludev, Fred J. Litterst, Dennis Schr\"oder, Mathias Kraken, Sergey Lushnikov, Tatiana Shaplygina PbFe$_{1/2}$Ta$_{1/2}$O$_{3}$ (PFT) belongs to the family of PbB$'_{x}$B$''_{1-x}$O$_{3}$ perovskites with inherent chemical disorder at the B-site. We have studied the magnetic phase diagram of PFT through macroscopic techniques, neutron scattering and M\"ossbauer spectroscopy. We show that PFT undergoes two phase transitions: paramagnetic to antiferromagnetic transition at T$_{N}\sim$153K and a spinglass transition at T$_{SG}\sim$10K. Below T$_{SG}$, antiferromagnetism coexists with spinglass on microscopic scale. We suggest that the arrangement of magnetic moments in this unusual ground state of PFT is a speromagnet-like, similar to the one proposed for PbFe$_{1/2}$Nb$_{1/2}$O$_{3}$.\footnote{S. Chillal, M. Thede, F. J. Litterst, S. N. Gvasaliya, T. A. Shaplygina, S. G. Lushnikov, and A. Zheludev, Phys. Rev. B 87, 220403 (2013).} [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G4.00009: Muon Spin Relaxation Studies of the Magnetically Frustrated Double Perovskite Ba2CaOsO6 J.P. Carlo, C. Thompson, T. Medina, T.J.S. Munsie, J. Munevar, Y.J. Uemura, J.E. Greedan The double perovskite structure A$_2$BB'O$_6$, in which antiferromagnetically-correlated magnetic B$'$ cations form an edge-sharing tetrahedral network, is an ideal laboratory for geometric magnetic frustration. The versatility of the perovskite structure enables systematic studies as a function of lattice distortion and moment size, and with 4d and 5d cations, spin-orbit coupling (SOC). Systems with large moments (d$^3$) tend toward antiferromagnetic order, albeit at T $<$ $|\Theta_{CW}|$. Systems with small moments (d$^1$) tend toward disorder, including glassy and singlet ground states. d$^2$ systems form a ``middle ground'' in which a variety of ground states are observed, and theory indicates a wealth of accessible behavior in systems with sizable SOC. Here we report on muon spin relaxation experiments of the 5d$^2$ system Ba$_2$CaOsO$_6$, which exhibits an undistorted cubic structure down to low temperatures, and in which long-lived muon spin precession is observed below 50K. These results are compared to related compounds, including the isostructural Ba$_2$YReO$_6$, an isoelectronic 5d$^2$ system exhibiting glassy behavior, and Ba$_2$YRuO$_6$, a 4d$^3$ system exhibiting commensurate antiferromagnetic order. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G4.00010: Spin Ordering Studies of Edge-sharing Iridates Tess Smidt, Itamar Kimchi, Min Gyu Kim, Zahir Islam, Robert J. Birgeneau, Ashvin Vishwanath, Jeffrey B. Neaton, James G. Analytis We have synthesized a material that is related to the layered honeycomb iridates. The magnetic order shows that this material has highly spin-anisotropic interactions, a key ingredient of the exotic possibilities associated with these compounds. We present X-ray studies of the spin ordering and lattice parameters, which aid in understanding the origin of the magnetic anisotropy and assess the possible proximity to a spin-liquid state. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G4.00011: Antiferromagnetic Exchange, Hunds Coupling and the Origin of the Charge Gap in LaMnPO Daniel McNally, J.W. Simonson, G.J. Smith, V. Leyva, C. Marques, M.C. Aronson, K.W. Post, D.N. Basov, Z.P. Yin, M. Pezzoli, G. Kotliar, Y. Zhao, J.W. Lynn, L. DeBeer-Schmidt, A.I. Kolesnikov LaMnPO is an antiferromagnetic insulator with an ordering temperature T$_{N}$ = 375 K, ordered moment of 3.2 $\mu_{B}$/Mn and a charge gap $\Delta$ = 1 eV. We present inelastic neutron scattering and magnetization data that are well described by a Heisenberg model of magnetic interactions with nearest neighbour exchange SJ$_{1}$$\sim$39 meV and next-nearest exchange SJ$_{2}$$\sim$12 meV. These measurements also show magnetic correlations persist up to T$_{max}$$\sim$700 K, significantly larger than T$_{N}$ due to the effectively decoupled MnP layers. High temperature optical transmission measurements show the charge gap has decreased by $\approx$ 10\% by T$_{max}$ suggesting the rather small exchange interactions J $\ll$ $\Delta$ have only a small effect on the gap. Density functional theory and dynamical mean field theory DFT+DMFT reproduce the observed gap in the paramagnetic state of LaMnPO only in the presence of strong Hunds coupling J$_{H}$, as well as onsite Coulomb interactions U. In light of these experimental and theoretical results, LaMnPO should be considered a Mott-Hunds insulator. [Preview Abstract] |
Session G6: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Strain Effects
Sponsoring Units: DMP GMAGChair: Christian Urban, University of California, San Diego
Room: 108
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G6.00001: Anion-based approaches to tunable functionality in oxide heterostructures Invited Speaker: Steven May The ability to control the position and composition of the anion site is emerging as a promising route to tune properties in epitaxial perovskites. This talk will focus on recent and ongoing efforts aimed at developing anion-based approaches to tailor electronic and magnetic properties in oxide films. First, I will discuss how the position of the oxygen anions can be tailored to stabilize non-bulk-like bond angles and lengths, thereby altering electronic bandwidth. Recent work on La$_{2/3}$Sr$_{1/3}$MnO$_3$ will be presented in which ultrathin films under the same strain state exhibit dramatically different electronic and magnetic properties when grown on substrates with different symmetries. In the second half of the talk, I will describe efforts focused on altering the composition of the anion site. In La$_{1/3}$Sr$_{2/3}$FeO$_{3-\delta}$ films, a reversible change in oxygen content leads to dramatic changes in electrical, optical, and structural properties. Finally, the synthesis of oxyfluoride ferrite and nickelate perovskite films via topotactic reactions carried out following thin film deposition will be described. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G6.00002: The effect of uniaxial strain versus chemical doping on V$_{2}$O$_{3}$ thin films Christian Urban, Ivan Schuller Vanadium(III) oxide (V$_{2}$O$_{3})$ exhibits as a function of temperature a metal-insulator transition associated with a structural and a magnetic transition which can be influenced either by strain or chemical doping. We investigate the effect of doping and external pressure on V2O3 thin films. Due to the thin film geometry, application of pressure results in a uniaxial strain perpendicular to the surface. In contrast, chemical doping causes isotropic strain. The interplay of the different strains is reflected in the electrical transport behavior throughout the phase transition. Doping with Ti and Cr is employed in highly oriented films to cover a large portion of the phase diagram. Application of external pressure on doped films tests the commonly unquestioned equivalence of doping and pressure. Additionally, we investigate the interplay of doping and pressure on the transition temperature and transport properties. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G6.00003: Magnetic Excitations in Thin Film Ba2IrO4 and Sr2IrO4 Probed by Resonant Inelastic X-ray Scattering J.P. Clancy, A. Lupascu, H. Gretarsson, M.H. Upton, J. Kim, Z. Islam, M. Uchida, D.G. Schlom, K.M. Shen, J. Nichols, J. Terzic, G. Cao, S.S.A. Seo, V.M. Katukuri, L. Hozoi, J. van den Brink, H. Stoll, Y.-J. Kim We have performed resonant inelastic x-ray scattering (RIXS) measurements on epitaxial thin film samples of the layered perovskite iridates Ba2IrO4 and Sr2IrO4. These materials display a novel Jeff = 1/2 Mott insulating ground state driven by strong 5d spin-orbit coupling effects. By studying 10 to 50 nm thin film samples grown on a variety of different substrates (GSO, STO, LSAT), we have investigated the impact of applied tensile/compressive strain on the characteristic magnetic and electronic excitations of these materials. Unlike other perturbations, such as doping or applied magnetic field, we find that applied strain does not alter the magnetic structure of Ba2IrO4 or Sr2IrO4. However, strain does affect the magnetic energy scales of these systems, providing a means of tuning both the ordering temperature (Tn) and the magnetic exchange interactions (J). In addition, we show that the dispersion of the low-lying magnon and spin-orbit exciton modes is renormalized by strain-induced structural changes. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G6.00004: Strain induced magnetization reversal in Ba$_2$IrO$_4$ via strong orbital-lattice coupling Choong H. Kim, Craig J. Fennie We have studied from first principles the structural, electronic, and magnetic properties of the layered-perovskite iridate Ba2IrO4 as a function of epitaxial strain. In contrary to what is usually assumed, we find within density functional theory that the ground state structure displays oxygen octahedra rotations ($\sim 6^\circ$ about the $c$-axis). This leads to a canting of the nominally antiferromagnetic moments. It turns out that the magnitude and direction of the orbital moment canting, which is in the opposite direction to the spin canting moment, can be controlled with strain. This leads to a situation in which the total magnetization can be tuned and in fact be reversed with strain. Our observations highlight a difficulty with describing magnetism in Ba$_2$IrO$_4$ within a simple effective $j_{\rm eff}$-spin Hamiltonian. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G6.00005: Influence of the SrTiO$_{3}$ Phase Transformation on Magnetotransport Phenomena in Ultrathin SrTiO$_{3}$(001) / La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta}$ Srinivas Polisetty, Shameek Bose, Shun Wang, Chris Leighton The existence of electronic and magnetic ``dead layers'' at interfaces in complex oxide heterostructures presents a significant challenge to the realization of functional oxide devices. Our recent work on La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta}$ (LSCO) [1,2] has shown that the formation of these ``dead layers'' in cobaltites is due to strain-induced oxygen vacancy formation and ordering. Here, we present some of the remarkable array of complex magnetotransport phenomena that accompany this interfacial magnetic inhomogeneity in SrTiO$_{3}$(001)/LSCO. Reduction of the film thickness from 70 to 30 {\AA} results in a percolative metal-insulator transition, the onset of large magnetoresistance due to inter-cluster transport, and clear signatures of the 108 K cubic-tetragonal phase transition in the SrTiO$_{3}$. The latter include resistivity anomalies at 108 K, strong temperature hysteresis, in-plane anisotropy, and stochastic discontinuities in resistivity [3]. We attribute these effects to strain-mediated propagation of the substrate phase transformation into the pseudomorphic LSCO and argue that the interplay between this effect and the thickness evolution of the magnetic inhomogeneity provides qualitative understanding of all observed phenomena. \\[4pt] [1] Torija \textit{et al.}, Adv. Mater. \textbf{23}, 2711 (2011).\\[0pt] [2] Gazquez \textit{et al.}, APL Mater. \textbf{1}, 012105 (2013).\\[0pt] [3] Polisetty \textit{et al.} (\textit{unpublished)}. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G6.00006: Making Nonmagnetic Palladium Ferromagnetic by Antiferromagnetic CoO Srijan Kumar Saha, Piotr Ku\'{s}wik, Pedro L. Gastelois, Marek Przybylski, Valeri Stepanyuk, J\"urgen Kirschner We present a novel finding of our combined experimental and theoretical studies which have revealed unexpected spin polarization of the Pd(001) substrate in contact with antiferromagnetic CoO overlayers. We give an evidence that the ferromagnetism of Pd is caused by the zigzag positions of Co atoms with respect to the Pd interface, resulted from the lattice-mismatch driven structural relaxation. Thanks to the itinerant nature of its 4d electrons, we see that the ferromagnetic properties of Pd are highly sensitive to the local environment and can be enhanced further by increasing the thickness of CoO overlayer film or/and by applying an additional uniaxial pressure along c-axis exerted externally on the bottom layers of the Pd substrate. Our finding provides new functionality for the interfacial moments of the CoO/Pd system, which can be accessed experimentally, e.g., by the magneto-optical Kerr effect as we demonstrate here. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G6.00007: Enhancement of Curie temperature in thin-films of SrRuO$_{3}$ Sean Thomas, Bouwe Kuiper, Jun Hu, Zhicheng Zong, Ruqian Wu, Guus Rindjers, Gertjan Koster, Jing Xia SrRuO$_{3}$ (SRO) is an itinerant ferromagnet that has generated a large amount of interest due to its potential use as an electrode layer in complex oxide heterostructures. We present the results of our ongoing study of the ferromagnetic properties of thin-films of SRO, which have been measured using a scanning Sagnac microscope. By varying the thickness of the a non-ferromagnetic capping-layer, we have observed enhancement of the Curie temperature of over 20 K as compared to uncapped films of the same thickness. The amount of enhancement can be tuned by varying the thickness of the capping-layer. Further, we have performed density functional theory calculations that suggest the enhancement may be due to rotations of the oxygen octahedrons in the SRO near the interface between the SRO and capping-layer. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G6.00008: Strain-dependent, Extraordinary Magnetocrystalline Anisotropy in Sr$_{2}$CrReO$_{6}$ Epitaxial Films Jeremy Lucy, Jennifer Soliz, Molly Ball, Oscar Restrepo, Wolfgang Windl, Patrick Woodward, Fengyuan Yang, Adam Hauser, John Freeland We have grown Sr$_{2}$CrReO$_{6}$ films that exhibit one of the largest anisotropy fields shown to date (18.1 T) and a large uniaxial magnetocrystalline anisotropy energy $K_{u} =$ 9.05 $\times$ 10$^{6}$ erg/cm$^{3}$. We investigate strain-controlled magnetocrystalline anisotropy for epitaxial Sr$_{2}$CrReO$_{6}$ films grown on (LaAlO$_{3})_{0.3}$(Sr$_{2}$AlTaO$_{6})_{0.7}$, SrTiO$_{3}$ and Sr$_{2}$CrNbO$_{6}$/LSAT substrates using high resolution X-ray diffraction, in-plane and out-of-plane superconducting quantum interference device magnetometry, and density functional theory calculations. The substrates impose tetragonal distortions of $c$/$a =$ 1.025, 1.007 and 0.991, respectively, which lead to dramatic changes in magnetocrystalline anisotropy of order tens of tesla and a switching of the magnetic easy axis from in-plane for compressive strain to out-of-plane for tensile strain, as observed via magnetometry measurements. Density functional theory calculations elucidate the dependence of oxygen octahedra tilting and rotation on tetragonal distortions, which affect both electronic and magnetic properties of the films. Finally, X-ray magnetic circular dichroism measurements reveal strong magnetic moment contributions at the oxygen sites, as evident in oxygen-specific X-ray absorption spectra. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G6.00009: Tunable strain-induced phase transitions in manganite thin films on BaTiO3 substrate Wengang Wei, Jinjie Chen, Kai Zhang, Zhang Du, Wenbin Wang, LiFeng Yin, Jian Shen The transport and magnetic properties of manganites depend sensitively on the lattice parameters, which can be conveniently tuned by the epitaxial strain in thin films. In an extreme case of manganites thin films grown on ferroelectric BaTiO3 (BTO), sudden jumps of both magnetization and resistivity have been observed upon cooling (or warming) in accordance with the temperature-dependent structural transitions of the BTO substrate. Surprisingly, both up and down jumps have been reported for both magnetization and resistivity of the LCMO films at the same temperature point where BTO undergoes a structural transition from orthorhombic to rhombohedra. Here we solve the puzzle by showing that the physical origins of the up and down jumps of both magnetization and resistivity are tied to the relative orientations of c-axis of the BTO substrate with respect to the LCMO film plane during the structural transition. Based on this understanding, we demonstrate the ability to control the up and down jumps by electric field poling of the BTO substrate upon cooling. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G6.00010: Strain induced ferromagnetism in LaCoO3 and interface coupling in magnetic multilayers Francisco Rivadulla, Beatriz Rivas-Murias, Irene Lucas, Pilar Cavero, Andrey Chuvilin, Luis Hueso, Luis Morell\'on Bulk LaCoO$_{3}$ (LCO) is rhombohedral with the Co$^{3+}$ atoms in a low spin (LS) diamagnetic configuration. Intraatomic exchange splitting is of similar energy to the crystal field of Co$^{3+}$ in an octahedral oxygen environment, and a transition from LS to high-spin (HS) can be induced by epitaxial tensile stress. We have grown ultrathin films of LCO ($\approx $2 nm) on top of SrTiO$_{3}$ (STO, $\approx $1.5 nm) and La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO, $\approx $22 nm). Magnetization and conductive AFM (C-AFM) experiments in the trilayer demonstrate that the ferromagnetic insulating behavior is kept in ultrathin LCO, with a T$_{\mathrm{C}}\approx $90 K, and M$\approx $0.8 $\mu_{\mathrm{B}}$/Co. The magnetization of LCO and LSMO is decoupled by the STO barrier, and so can be independently switched. C-AFM experiments show I-V curves characteristic of tunnel conduction between the ferromagnetic electrodes across the STO barrier. Moreover, the magnetization of both layers can be conveniently coupled/decoupled by changing the order of deposition of the films (either LCO/LSMO or LSMO/LCO). These results show that new approaches for the design of insulating ferromagnets are possible. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G6.00011: Structure, strain, and control of ground state property in LaTiO$_{3}$/LaAlO$_{3}$ superlattice Alex Taekyung Lee, Myung Joon Han We examined the ground state property of LaTiO$_{3}$/LaAlO$_{3}$ superlattice through density functional band calculations. Total energy calculations, including the structural distortions, $U$ dependence, and the exchange correlation functional dependence, clearly showed that the spin and orbital ground state can be controlled systematically by the epitaxial strain. In the wide range of strain, the ferromagnetic-spin and antiferro-orbital order are stabilized, which is notably different from the previously reported ground state in the titanate systems. By applying $+$2.8{\%} of tensile strains, we showed that the antiferromagnetic-spin and ferro-orbital ordered phase become stabilized. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G6.00012: Strain-Tunable Magnetocrystalline Anisotropy in Epitaxial Y3Fe5O12 Thin Films Hailong Wang, Chunhui Du, P. Chris Hammel, Fengyuan Yang Magnetocrystalline anisotropy plays an essential role in many applications and there is intense interest in understanding the role of magnetoelastic coupling in phonon-magnon interactions in thermal spintronics. It is important to understand magnetocrystalline anisotropy in the presence of lattice distortion induced by epitaxial strain and the underlying magnetization-lattice coupling. Y3Fe5O12 (YIG) has been widely used in microwave applications and spin pumping. Most YIG epitaxial film fabrication has employed Gd3Ga5O12 (GGG) substrates with nearly perfect lattice match. In order to probe the magnetocrystalline anisotropy and control magnetization by epitaxial strain in epitaxial YIG films, we grow YIG epitaxial thin films on (001)-oriented Y3Al5O12 (YAG) substrate with -3.0{\%} lattice mismatch. We demonstrate strain-tuning of magnetocrystalline anisotropy over a range of more than one thousand Gauss in epitaxial YIG films of excellent crystalline quality grown on YAG substrates. Ferromagnetic resonance (FMR) measurements reveal a linear dependence of both out-of-plane and in-plane uniaxial anisotropy on the strain-induced tetragonal distortion of Y3Fe5O12. Importantly, we find the spin mixing conductance determined from inverse spin Hall effect and FMR linewidth broadening remains large in Pt/YIG/YAG heterostructures, quite comparable to the value found in Pt/YIG grown on lattice-matched GGG substrates. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G6.00013: Strain's role at the 4-point phase boundary of La$_{0.4}$Sr$_{0.6}$MnO$_{3}$ T. Zac Ward, Anthony Wong, Hangwen Guo, Christianne Beekman, Zheng Gai Bulk La$_{0.4}$Sr$_{0.6}$MnO$_{3}$ shows a 4-point phase boundary where ferromagnetic metal, paramagnetic metal, canted antiferromagnetic metal, and A-type antiferromagnetic metal phases meet at 240K. The strong spin-charge-lattice coupling inherent in these materials make this a prime candidate for exploring the impact of interfacial strain effects on phase behavior. We use pulsed laser deposition to grow single crystal thin films of this composition on several different substrates to induce a range of epitaxial strains. We will present data showing a high level of tunability in resistive and magnetic properties and show the results as being directly tied to changes arising from shifts in the orbital occupancy. [Preview Abstract] |
Session G7: Focus Session: Magnetic Thin Films
Sponsoring Units: GMAG DMPChair: Barry Zink, University of Denver
Room: 106
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G7.00001: Engineered materials for all-optical helicity-dependent magnetic switching Invited Speaker: Eric Fullerton The possibilities of manipulating magnetization without applied magnetic fields have attracted growing attention over the last fifteen years. The low-power manipulation of magnetization, preferably at ultra-short time scales, has become a fundamental challenge with implications for future magnetic information memory and storage technologies. Here we explore the optical manipulation of the magnetization of engineered materials and devices using 100 fs optical pulses. We demonstrate that all optical -- helicity dependent switching (AO-HDS) can be observed not only in selected rare-earth transition-metal (RE-TM) alloy films but also in a much broader variety of materials, including alloys, multilayers, heterostructures and RE-free Co-Ir-based synthetic ferrimagnets. The discovery of AO-HDS in RE-free TM-based synthetic ferrimagnets can enable breakthroughs for numerous applications since it exploits materials that are currently used in magnetic data storage, memories and logic technologies. In addition, this materials study of AO-HDS offers valuable insight into the underlying mechanisms involved. Indeed the common denominator of the diverse structures showing AO-HDS in this study is that two ferromagnetic sub-lattices exhibit magnetization compensation (and therefore angular momentum compensation) at temperatures near or above room temperature. We are highlighting that compensation plays a major role and that this compensation can be established at the atomic level as in alloys but also over a larger nanometers scale as in the multilayers or in heterostructures. We will also discuss the potential to extend AO-HDS to new classes of magnetic materials. This work was done in collaboration with S. Mangin, M. Gottwald, C-H. Lambert, D. Steil, V. Uhl\'i\v{r}, L. Pang, M. Hehn, S. Alebrand, M. Cinchetti, G. Malinowski, Y. Fainman, and M. Aeschlimann. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G7.00002: Domain Structures and Anisotropy in Exchange-coupled [Co/Pd]-NiFe and [Co/Ni]-NiFe Multilayers Larysa Tryputen, Sunjae Chung, Majid Mohseni, T.N. Anh Nguyen, Johan {\AA}kerman, Feng Guo, Robert D. McMichael, Caroline A. Ross Exchange-coupled multilayers [Co/Pd]$_{5}$-/NiFe and [Co/Ni]$_{4}$-NiFe with strong perpendicular magnetic anisotropy have been proposed to use in spin-torque switching and oscillators devices with tilted fixed and free layer to improve their functional performance. We present an experimental study of the magnetization behavior of [Co/Pd]$_{5}$-/NiFe and [Co/Ni]$_{4}$-NiFe multilayers measured using magnetometry, magnetic force microscopy (MFM) and ferromagnetic resonance (FMR) as a function of the thickness of the top NiFe layer. We varied the thickness of the NiFe layer in [Co/Pd]$_{5}$-NiFe (t), t $=$ 0 - 80 nm and [Co/Ni]$_{4}$-NiFe (t), t $=$ 0.5 - 2.5 nm in order to study the interplay between perpendicular magnetization of the Co/Pd or Co/Ni multilayers and in-plane magnetization of the NiFe. Our magnetometry and FMR data suggest that the [Co/Ni]$_{4}$/NiFe multilayer behaves like a homogeneous ferromagnetic film with anisotropy that reorients towards in-plane as the NiFe thickness increases, whereas the [Co/Pd]$_{5}$/NiFe multilayer reveals more complex behavior in which the [Co/Pd] layer retains out-of-plane anisotropy while the magnetization of NiFe layer tilts in-plane with increasing thickness. MFM showed that domains with $\sim$0.1 $\pm$m size were visible in [Co/Pd]-/NiFe with NiFe thickness of 20-80 nm. Multilayers were patterned into sub-100 nm dots using ion beam etching and their magnetization behavior are compared with unpatterned films. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G7.00003: Magnetic anisotropies in epitaxial Py/FeMn/Ni/Cu[001] films Ali Tan, Jia Li, Elke Arenholz, Zi Qiang Qiu The interaction between ferromagnetic and antiferromagnetic layer in a FM/AFM bilayer depends on the details of the spin configurations at the interface. By inserting a Ni layer of different thicknesses below FeMn in Py/FeMn/Cu(001) FM/AFM bilayer, we compared the effect of in-plane and out-of-plane magnetization on the FeMn spin structure which will subsequently influence Py/FeMn interfacial interaction. The Py/FeMn interface interaction is characterized by measuring four-fold and two-fold anisotropies of Py using rotating magneto-optic Kerr effect as a function of Ni and FeMn thicknesses. We found that out-of-plane Ni magnetization has little effect on the Py magnetic anisotropy, but in-plane Ni magnetization enhances the Py magnetic anisotropy in the region just above antiferromagnetic transition thickness. The underlying mechanism could be attributed to the FeMn 3Q spin structure [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G7.00004: Field Dependent Phase Front in Ni[1-x]Cu[x] Graded Alloy Films B.J. Kirby, H.F. Belliveau, D.D. Belyea, C.W. Miller For heterostructures composed of distinct, homogeneous layers, the magnetic properties of the individual layers can be strongly affected by interlayer exchange coupling, leading novel and useful properties (exchange bias, GMR, etc.). Less well understood are structures exhibiting a gradient in magnetic properties, with no discrete interfaces. Particularly interesting is the case where a phase transition is expected across the length of the gradient - do individual regions behave as they would in isolation, or does exchange coupling cause a single phase? Ni[1-x]Cu[x] alloy is a useful model system for, as the Curie temperature (Tc) varies with x. We have studied a 100 nm Ni[1-x]Cu[x] film with x that varies smoothly from 0.39-0.30 across the thickness. Magnetometry measurements of homogenous x=0.39 (x=0.30) samples reveal Tc near 200 K (300 K). Polarized neutron reflectometry measurements of the graded sample magnetic depth profile reveal a distribution of Tc, demonstrating that the sample is not completely coupled. At 300 K we observe evidence of a non-magnetized / magnetized boundary that moves vertically with applied field. Implications of a spatially controllable paramagnetic-ferromagnetic phase boundary will be discussed. Work at USF was supported by NSF-CAREER. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G7.00005: Coercivity Enhancement in V$_{2}$O$_{3}$/Ni Bilayers Siming Wang, Jose de la Venta, Thomas Saerbeck, Juan Gabriel Ramirez, Ivan K. Schuller We studied the temperature dependence of the coercivity and magnetization of V$_{2}$O$_{3}$/Ni bilayers. When the V$_{2}$O$_{3}$ is in the middle of the metal to insulator transition, we observe a maximum enhancement of the coercivity and a decrease of the magnetization. The maximum value of the coercivity shows a 300{\%} increment compared to the room temperature value. The decrease of the magnetization indicates magnetic domain formation. We propose a model in which the inhomogeneous V$_{2}$O$_{3}$ phase transition induces nanoscale stress and disorder in the Ni film. The local stress anisotropy and disorder break the Ni film into magnetic domains and pin the domain walls in Ni. The model is supported by micromagnetic simulations and shows that magnetic properties of ferromagnetic films are strongly affected by the proximity to materials that undergo inhomogeneous phase transition at nanoscale. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G7.00006: PNR studies of spin-flop and spin-flip processes in magnetic multilayer, NiFe/Ir system Haile Ambaye, Gary Mankey, Valeria Lauter, Jimmy Hwang Early GMR devices relied on antiferromagnetic interlayer coupling to work and it was shown that the interlayer coupling is in fact oscillatory, with both ferromagnetic and antiferromagnetic interlayer exchange depending on the thickness of the nonmagnetic layer [1,2]. Different competing interactions such as magnetic anisotropy and interlayer afm coupling occur in multilayer systems. Distinguishing the individual contributions is one of the major challenges in the study of multilayered systems. We used polarized neutron reflectivity (PNR) with full polarization analysis to understand how the magnetization is distributed through the system and how deep the flipping process of the magnetization goes into the system. Depending on the range of the external field applied parallel to the easy axis we studied the occurrence of spin-flop and spin-flip events in the system. \\[4pt] [1] S. S. P. Parkin, Phys. Rev. Lett. \textbf{71}, 1641 (1993).\\[0pt] [2] D. Elefant, et al., Phys. Rev. B \textbf{77}, 014426 (2008). [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G7.00007: Growth of Co/Ni Multilayers with Perpendicular Magnetic Anisotropy N. Soriano, M.H. Kilinc, H.F. Belliveau, C. Redondo, D. Navas, C. Garcia, R. Morales, Casey W. Miller Thin films with perpendicular magnetic anisotropy (PMA) have attracted wide interest for perpendicular recording media applications as well as for devices based on the spin transfer torque effect. In particular, Co/Ni multilayers with strong PMA have been considered one of the most promising candidates for these applications [1]. Understanding the elements which determine the preferred orientation of magnetization in these multilayers involves the study of several factors such as Co/Ni thicknesses, number of bilayers, deposition conditions (base and deposition pressure) and the material and thickness of the underlayer [2]. In this work, we outline our fabrication methods for ultrathin Co/Ni multilayers with a thickness ratio of 1:2 and 1:3 by sputtering at room temperature and using Cu as underlayer. The magnetic behavior of the samples was characterized by polar and transverse magneto optical Kerr effect magnetometer and structural studies were made by X-ray diffractometry.\\[4pt] [1] Guangzhong Wang et al., J. Appl. Phys. 113, 17C111 (2013).\\[0pt] [2] F.J.A. den Broeder, E. Janssen, W. Hoving and W.B. Zeper, IEEE Trans. Magn., 28, 2760 (1992). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G7.00008: Exchange coupling in MnBi/Fe-Co thin film bilayers Lei Fang, Tieren Gao, Sean Fackler, Shingo Maruyama, Ichiro Takeuchi, Jun Cui, M.J. Krammer, Duane Johnson, Elke Arenholz, Julie Borchers, Brian Kirby, William Ratcliff, Ralph Skomski, Samuel Lofland To achieve enhanced energy products of MnBi for rare-earth free permanent magnet applications, we studied the exchange coupled soft/hard bilayers based on MnBi films. By using DC magnetron sputtering, we fabricated pure MnBi films with magnetization of 500 emu/cc and coercivity of 1.6 T. A (BH)$_{\mathrm{max}}$ of 6.2 MGOe is obtained for pure MnBi films. A large enhancement in (BH)$_{\mathrm{max}}$ due to exchange coupling in MnBi/Fe-Co bilayers is observed with Fe-Co thicknesses between 2 and 5 nm. The highest (BH)$_{\mathrm{max}}$ obtained is 14.0 MGOe at room temperature with a single phase magnetization curve for a MnBi (20 nm)/Co (2 nm) bilayer. TEM and XPS studies indicate there is no oxidation between soft/hard interface. The XMCD results show that the soft moments of Fe/Co at a thickness of 2 nm are perpendicular to the MnBi plane, indicating nearly perfect hard-soft coupling. Moreover, a micromagnetic calculation on perpendicularly-coupled MnBi/Fe-Co bilayers suggests a critical coupling thickness of 4 nm of the soft layer. We will also discuss results from polarized neutron reflectometry measurements performed on the bilayers. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G7.00009: Effects of strain and surfaces on the antiferromagnetic and ferromagnetic phases of thin film FeRh Frances Hellman, Catherine Bordel, Chloe Baldasseroni, Cory Antonakos, Oliver Schneider, Gunar Pal, Sergio Valencia, Akin Unal, Florian Kronast, Slavo Nemsak, Chuck Fadley, Julie Borchers, Brian Maranville FeRh undergoes an unusual antiferromagnetic (AFM) to ferromagnetic (FM) first order transition just above room temperature. This transition can be tuned by pressure, magnetic field, composition, and strain. The underlying source of the transition is still under much discussion, but it is clear from a variety of measurements that electronic structure, lattice, and magnetic excitations all play roles in contributing the underlying entropy difference and hence the competition between AFM and FM states. The surface and bottom interface of thin films are often found to be FM even while the bulk of the film is AFM. The source of this effect, along with the dependence of strain on both anisotropy and transition temperature will be presented and discussed. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G7.00010: Local Atomic Structure and Magnetism in Amorphous Fe$_{\mathrm{x}}$Si$_{\mathrm{1-x}}$ Thin Films Frances Hellman, Yanning Zhang, Catherine Bordel, Kevin Stone, Catherine Jenkins, David Smith, J. Hu, Ruqian Wu, Steve Heald, Jeff Kortright, Julie Karel Amorphous FexSi1-x thin films exhibit a large enhancement in M compared to crystalline films with the same composition (0.45\textless $x$\textless 0.75). XMCD shows enhancement in both spin and orbital moments. Density functional theory (DFT) calculations reproduce this enhanced magnetization. DFT and EXAFS show the amorphous materials have decreased number of nearest neighbors and reduced number density relative to crystalline samples of same x, which leads to the enhanced moment. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G7.00011: Pulsed Laser Deposition of Thin Films of Binary Compounds of Gd and Si using Femto-Second Laser Ravi Hadimani, Yaroslav Mudryk, Timothy Prost, Vitalij Pecharsky, Karl Gschneidner, David Jiles Growth of thin films of Gd$_{5}$(Si$_{\mathrm{x}}$Ge$_{\mathrm{1-x}})_{4}$ has not been reported widely because of difficulty in obtaining the monoclinic phase that is responsible for the giant magnetocaloric effect. Our previous attempt resulted in multiple phases of the material in the film including oxides of Gd [1]. In this work, we therefore report growth of thin films of binary compounds of Gd and Si with Pt protection on top to prevent the oxidation. We have used femto-second laser that results in finer particle size and a composition closer to the target. Microstructure analysis using SEM, EDS was carried out to determine the film thickness, morphology and composition. Magnetic moment vs. temperature measurements were carried out at an applied field of 1000 Oe. The sample showed a major transition below 150~K and a minor transition around 335~K similar to the bulk sample. Magnetization measurements showed that the magnetization in the film saturated close to a field of 5kOe (0.397MA/m). References: [1] R. L. Hadimani, I. C. Nlebedim, Y. Melikhov, D. C. Jiles, ``Growth and characterisation of Gd$_{5}$(Si$_{\mathrm{x}}$Ge$_{\mathrm{1-x}})_{4}$ thin film'' \textit{J.Appl. Phys.}, 113, 17A935, (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G7.00012: A study of magnetic proximity effect in two-dimensional heterostructure Shanshan Su, Gen Yin, Darshana Wickramaratne, Mahesh Neupane, Roger Lake Recent research found the spin Hall effect and the inverse spin Hall effect in heterostructures composed of a ferromagnetic insulator, Y$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$, and transition metals with large atomic numbers [1]. It is also reported that graphene has an exchange-splitting with an adjacent EuO layer in both experiments and simulations [2, 3]. Our systems of interest are two-dimensional (2D) heterostructures composed of ferromagnetic insulators, ferromagnetic alloys, and graphene. Along the heterointerface, overlap of the wavefunctions of the ferromagnetic material and graphene leads to a proximity effect. To understand this magnetic proximity effect, density functional theory (DFT) is used. Exchange parameters, magnetic moments, magnetocrystalline anisotropy and exchange-splitting are calculated for the 2D heterostructures. \\[4pt] [1] S. Y. Huang, et. al. Phys. Rev. Lett., \textbf{109}, 107204 (2012).\\[0pt] [2] H. X. Yang, et. al. Phys. Rev. Lett., \textbf{110}, 046603 (2013).\\[0pt] [3] A. G. Swartz, et. al. J. Vac. Sci. Technol. B, \textbf{31}, 04D105 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G7.00013: Electric-field-induced modification in magnetocrystalline anisotropy, exchange interaction, and Curie temperature of transition-metal thin films K. Nakamura, M. Oba, T. Akiyama, T. Ito, M. Weinert, A.J. Freeman Magnetism induced by an external electric field ($E$-field) has received much attention as a potential approach for controlling magnetism at the nano-scale with the promise of ultra-low energy power consumption. For magnetocrystalline anisotropy (MCA) in transition-metal thin films, it is agreed that a change in the screening charge density due to an $E$-field, which causes a small change in band structures around Fermi energy, gives rise to a modification of the MCA energy.\footnote{Nakamura et.al, PRL{\bf 102}, 187201(2009); PRB{\bf 81}, 220409(2010)} Here, we extend our first-principles investigation to Curie temperature of an Fe monolayer in an $E$-field. Calculations were carried out using film-FLAPW method that treats spin-spiral structures in an $E$-field. Results predict that when the $E$-field is introduced, calculated magnon (spin-spiral formation) energy is modified, by a few tens of meV, compared to that in zero field. The exchange parameters within the classical Heisenberg model, by making the back Fourier transformation of the magnon energy, suggest the $E$-field-induced modification of Curie temperature. Taking a large MCA energy of the monolayer into account, the modification of Curie temperature by the $E$-field was demonstrated by Monte Carlo simulations. [Preview Abstract] |
Session G8: Focus Session: Spin-Dependent Phenomena in Semiconductors: Magnetism in Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Connie Li, Naval Research Laboratory
Room: 104
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G8.00001: Insulating origin of the layered antiferromagnetic semiconductor (LaO)MnPn (Pn$=$P, As, Sb) Koichi Takase, Yasuhiro Morosawa, Tadataka Watanabe, Yoshiki Takano The layered oxypnictides (LaO)MnPn are antiferromagnetic semiconductors. Considering a high spin magnetic state (s$=$5/2) of the Mn3d electrons, this system seems to be a Mott insulator. In this study, we have investigated the origin of an insulating character of (LaO)MnPn by carrier doping. The carrier doping is attempted by the fluorine substitution and deficient oxygen. Huge reductions of the absolute value of the electrical resistivity are found in all samples. Especially, the Sb system shows metallic characters in a highly doped concentration. If this system is a Mott insulator, the magnetic ordered phase should disappear in the metallic sample. However, ferromagnetic impurities increase with increase concentration and it hinders evaluation of the magnetic properties. From these results, these systems are robust against carrier doping. Only in the Sb system with relatively large Mn-Mn distances, the antiferromagnetic interaction is slightly weaker than those of P and As cases, the carrier doping might change the character from insulating to metallic. If this scenario is true, the origin of insulator should not be an usual band insulator but a Mott insulator. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G8.00002: Magneto-Optical and Time Resolved Spectroscopy in Narrow Gap MOVPE Grown Ferromagnetic Semiconductors M. Meeker, B. Magill, M. Bhowmick, G.A. Khodaparast, S. McGill, C. Feeser, B.W. Wessels, D. Saha, G.D. Sanders, C.J. Stanton We report on magneto-optical at high magnetic fields and time resolved studies, that provide insight into the band structure, time scales, and the nature of the interactions in ferromagnetic InMnAs and InMnSb grown by MOVPE. By probing the dynamical behavior of the nonequilibrium carriers and spins, created by intense laser pulses, we gain valuable information about different scattering mechanisms and observe the sensitivity and tunability of the carrier and spin dynamics to the initial excitation energy. Theoretical calculations are performed using an 8 band $k\cdotp$ model including non-parabolicity, band-mixing, and the interaction of magnetic Mn impurities with itinerant electrons and holes. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G8.00003: What is the valence of Mn in GaMnN? Ryky Nelson, Tom Berlijn, Juana Moreno, Mark Jarrell, Wei Ku Motivated by the potential high Curie temperature of GaMnN [1], we investigate the controversial Mn-valence in this diluted magnetic semiconductor. From a first-principles Wannier functions analysis [2] of the high energy Hilbert space we find unambiguously the charge state of Mn to be close to $2+$ ($d^5$), but in a mixed spin configuration with average magnetic moments of 4 $\mu_B$. Using more extended Wannier orbitals to capture the lower-energy physics, we further demonstrate the feasibility of both the effective $d^4$ description (appropriate to deal with the local magnetic moment and Jahn-Teller distortion), and the effective $d^5$ description (relevant to study long-range magnetic order). Our derivation highlights the general richness of low-energy sectors in interacting many-body systems and the generic need for multiple effective descriptions, and advocates for a diminished relevance of atomic valence measured by various experimental probes.\\[4pt] [1] Dietl, T., H. Ohno, and F. Matsukura, Phys. Rev. B 63, 195205 (2001).\\[0pt] [2] W. Ku et al., Phys. Rev. Lett. 89, 167204 (2002). [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G8.00004: TBD Invited Speaker: Tomasz Wojtowicz |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G8.00005: Interfacial exchange coupling in Fe/(Ga,Mn)As bilayers Abdel Khalq Alsmadi, Y. Choi, D.J. Keavney, K.F. Eid, B.J. Kirby, X. Liu, J. Leiner, K. Tivakornsasithorn, M. Dobrowolska, J.K. Furdyna We have carried out a systematic studies of magnetic order and coupling in Fe/(Ga,Mn)As bilayers using superconducting quantum interference device magnetometry, polarized neutron reflectometry, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism. Our results clearly show that Fe/(Ga,Mn)As bilayers display exchange coupling at the interface. Contrary to recent reports [e.g., F. Maccherozzi et al., Phys. Rev. Lett. 101, 267201 (2008)], a ferromagnetic coupling between the magnetic moment of the Mn ions and the moment of the Fe overlayer is observed. Furthermore, our element-specific data indicate that an ultrathin Mn-rich interfacial (Ga,Mn)As layer directly in contact with the Fe film is strongly coupled to the Fe layer, showing nearly identical coercive fields as the Fe layer, while the coercive fields of the bulk (Ga,Mn)As further from the Fe layer are distinctly weaker. We argue that the exchange coupling strength between Fe and Mn at the interface and throughout the (Ga,Mn)As layer is a function of the Mn concentration in the system, possibly arising from the diffusion of Mn interstitials during the MBE growth. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G8.00006: Valence band ordering restored by the p-d exchange interaction in GaMnAs Iriya Muneta, Hiroshi Terada, Shinobu Ohya, Masaaki Tanaka In ferromagnetic GaMnAs, it is predicted that the Mn impurity level or impurity band (IB) is formed in the band gap by the strong $p$-$d$ exchange interaction with the hybridization [1], which is consistent with the recent result of x-ray photoemission unveiling the disordered IB [2]. Although the hybridization might be expected to result in the disordered or merged valence band (VB), it was found that VB is almost unchanged by Mn doping in GaAs [2-4]. In order to understand the bandstructure and ferromagnetism of GaMnAs induced by the strong exchange interaction, we measured the VB ordering by using the resonant tunneling spectroscopy in the GaMnAs quantum-well (QW) double-barrier hetrerostructures with the Mn content $x$ varied from 0.4\% to 2.3\%. In $x<1\%$ (paramagnetic), the $d^2I/dV^2$ oscillations weaken as $x$ increases, which shows that VB merges with the {\it paramagnetic} IB and becomes disordered. However, the oscillations are restored at the onset of the ferromagnetism ($x>1\%$) and become stronger as $x$ increases. Our results show that the strong exchange interaction does not fluctuate VB but forms the disordered IB. [1] Krstaji\'c et al., PRB (2004). [2] Kobayashi et al., arXiv:1302.0063 (2013). [3] Ohya et al., Nat. Phys. (2011). [4] Muneta et al., APL (2013). [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G8.00007: Itinerant Magnetism and the Ferromagnetic Quantum Critical Point in Fe(Ga,Ge)$_3$ David J. Singh FeGa$_3$ is a tetragonal semiconductor with a band gap of $\sim$0.5 eV and interesting thermoelectric properties. It shows diamagnetic behavior but when modestly electron doped by Ge, a ferromagnetic quantum critical point emerges and the ground state becomes a ferromagnetic metal. We present first-principles calculations showing that the magnetism can be readily explained in an itinerant picture without the need for preexisting moments in the semiconducting state and without the need for correlation terms. We also present Boltzmann transport calculations of the thermopower. Itinerant magnetism implies strong coupling between the electrons at the Fermi energy that control transport and the magnetism. Thus, FeGa$_3$ may be a particularly interesting material near a quantum critical point. We find that the ferromagnetic state is half-metallic over a substantial composition range. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G8.00008: Quantitative chemical and structural ordering of Heusler Co$_{\mathrm{x}}$Mn$_{\mathrm{y}}$Ge$_{\mathrm{z}}$ (111) epitaxial films Frank Tsui, Brian Collins, Liang He, Yong Chu Heusler alloys are attractive spintronic materials, owing to the predicted half-metallicity and their compatibility with epitaxial semiconductor heterostructures. Chemical defects have been suggested as the cause of low spin-polarizations measured in these materials. We report a systematic investigation into the structural and chemical ordering of Co$_{\mathrm{x}}$Mn$_{\mathrm{y}}$Ge$_{\mathrm{y}}$ films grown epitaxially on Ge (111) substrates, as a function of composition near the Heulser Co$_{2}$MnGe stoichiometry. X-ray diffraction experiments show that the structural ordering is extremely sensitive to the Co-Mn atomic ratio with the best ordering occurring at compositions rich in Ge, i.e. off the Heulser stoichiometry. A new multi-edge anomalous diffraction technique has been employed to measure the elemental occupancy of the lattice sites. The measurements and analysis reveal that the dominant chemical defect is Mn-Ge site swapping with no detectable Co-Mn swapping, at variance with the predictions based on density functional theory. The observed shift for the most ordered composition from that of the bulk has been attributed to epitaxial constraints. The finding provides impetus for exploring spin polarization at off-stoichiometric compositions. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G8.00009: Annealing-induced enhancement of ferromagnetism and nano-particle formation in ferromagnetic-semiconductor GeFe Yuki Wakabayashi, Yoshisuke Ban, Shinobu Ohya, Masaaki Tanaka Ge-based ferromagnetic semiconductor GeFe is a promising material for future Si-based spintronic devices because of the high-quality single crystallinity and good compatibility with Si. However, its Curie temperature ($T_{C})$ is currently at the highest 170 K. In this study, we investigate the annealing effect on GeFe in order to enhance the ferromagnetism. The Ge$_{0.895}$Fe$_{0.105}$ thin film was epitaxially grown on a Ge(001) substrate by low-temperature molecular beam epitaxy. Then, post-growth annealing was carried out. We have analyzed GeFe films both crystallographically and magnetically by using transmission electron microscopy, transmission electron diffraction, energy-dispersive X-ray spectroscopy, magnetic circular dichroism, and superconducting quantum interference device. We have successfully increased the $T_{C}$ of Ge$_{0.895}$Fe$_{0.105}$ up to $\sim$ 220 K while keeping a single ferromagnetic phase when the annealing temperature was lower than 500$^{\circ}$C. In contrast, when annealed at 600$^{\circ}$C, single-crystal GeFe nano-particles with stacking faults and twins, which have a high $T_{C}$ nearly up to room temperature, were formed in the film. Both types of films have a flat surface (roughness of 2-5 MLs), and thus they are promising for Si-based spin devices. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G8.00010: Investigation of Room temperature Ferromagnetism in Mn doped Ge Leyla Colakerol Arslan, Burcu Toydemir, Aykut Can Onel, Merve Ertas, Hatice Doganay We present a systematic investigation of structural, magnetic and electronic properties of MnxGe1$-$x single crystals. Mn$_{\mathrm{x}}$Ge$_{\mathrm{1-x}}$ films were grown by sequential deposition of Ge and Mn by molecular-beam epitaxy at low substrate temperatures in order to avoid precipitation of ferromagnetic Ge-Mn intermetallic compounds. Reflected high energy electron diffraction and x-ray diffraction observations revealed that films are epitaxially grown on Si (001) substrates from the initial stage without any other phase formation. Magnetic measurements carried out using a physical property measurement system showed that all samples exhibited ferromagnetism at room temperature. Electron spin resonance indicates the presence of magnetically ordered localized spins of divalent Mn ions. X-ray absorption measurements at the Mn L-edge confirm significant substitutional doping of Mn into Ge-sites. The ferromagnetism was mainly induced by Mn substitution for Ge site, and indirect exchange interaction of these magnetic ions with the intrinsic charge carriers is the origin of ferromagnetism. The magnetic interactions were better understood by codoping with nonmagnetic impurities. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G8.00011: Two-carrier Analysis of a- and m-ZnO Thin Films W.C. Hsieh, Q.Y. Chen, P.V. Waderkar, H.C. Huang, Y.F. Cheng, C.F. Chang, K.H. Liao, S.Y. Lai, H.H. Ko, Q.J. Lin, W.Y. Lin, H.W. Seo, C.H. Liao, H.H. Liao, L.W. Tu, N.J. Ho, D. Wijesundera, W.K. Chu Thin films of a- and m-plane oriented ZnO have been produced, respectively, on r- and m-sapphire substrates, all demonstrating reasonable crystalline qualities as judged by the X-ray diffractornetry. Contrary to most reported, the samples all demonstrated p-type charge carriers determined by Hall measurement using a Quantum Design PPMS system. The I-V curves of All-ZnO p-n junctions all demonstrate the characteristic nature of diodes. These two measurements provide unambiguous evidences of p-type behaviors. However, there are some irregularity in Hall measurement and magneto-resistivity. In order to understand the origin, we conducted a two-carrier analysis of the Hall data taken over a wide range of temperatures (T) and magnetic fields (B). The dependence of Hall resistivity and MR on B as T and the existence of hysteresis, we speculate, possibly reflect the complex atomic defects and their mobile nature in the otherwise largely perfect crystalline lattices. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G8.00012: (Zn,Co)O alloyed magnetic semiconductor: giant magnetization and experimental determination of band structure Guolei Liu Magnetic semiconductor exhibits both ferromagnetism and semiconductor properties. Since the magnetic dopants tend to aggregate in, the thermo-dynamical miscibility of transition metals in semiconductors is in fact extremely low (generally <15\%, in diluted region). The miscibility of transition metals exceeding diluted region is challenge for material science and future spintronic applications. In this paper, we have been grown (Zn,Co)O thin films by oxygen plasma-assisted molecular beam epitaxy. The Co context in these films can be up to 45\%, which we defined as alloyed magnetic semiconductor (AMS). The measurements of RHEED, XRD and in. situ. XPS indicated that (Zn, Co)O films are spinodal decomposition phase with ZnO wurtzite lattice. The saturated magnetization at room temperature was remarkably enhanced with increasing the Co content. It reach 530emu/cm${3}$ at x = 45\% which corresponds to the average magnetic moment 1.6$\mu_{B}$ per Co. The electronic band structure of (Zn,Co)O films were determined by angle resolved photoemission. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G8.00013: Magnetoresistance of ZnO and SnO$_{2}$ diluted magnetic oxide thin films Christer Akouala, Sandhyarani Punugupati, Raj Kumar, Yi-Fang Lee, Jagdish Narayan, Justin Schwartz, Frank Hunte Robust ferromagnetism (FM) at room temperature (RT) in semiconductor materials is critical to the development of spintronic devices making use of both charge and spin of electrons. In this work, magnetotransport techniques were used to study the electronic and magnetic properties of pulsed laser-deposited ZnO and SnO$_{2}$ diluted magnetic semiconductor films on sapphire substrates. Results from thin films with high structural quality showed both semiconductor and ferromagnetic characteristics. Magnetization measurements of Co-doped ZnO showed clear hysteretic behavior indicative of ferromagnetic films even at room temperature. Measurements of resistance versus temperature and magnetic field showed consistency with semiconductor characteristics. All samples were n-type with magnetoresistance (MR) behavior being strongly dependent on carrier concentration.. The presence of magnetic ions significantly affects the scattering of the conduction electrons in cobalt-doped samples resulting in different MR behavior from undoped samples. Correlation of transport characteristics with magnetization will be discussed in the context of carrier-mediated ferromagnetism in diluted magnetic semiconductors. [Preview Abstract] |
Session G10: Focus Session: Physics of Cancer
Sponsoring Units: DBIOChair: Robert Austin, Princeton University
Room: 201
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G10.00001: The cytoskeleton significantly impacts invasive behavior of biological cells Anatol Fritsch, Josef K\"as, Kristin Seltman, Thomas Magin Cell migration is a key determinant of cancer metastasis and nerve regeneration. The role of the cytoskeleton for the epithelial-meschenymal transition (EMT), i.e, for invasive behavior of cells, is only partially understood. Here, we address this issue in cells lacking all keratins upon genome engineering. In contrast to prediction, keratin-free cells show a 60{\%} higher deformability compared to less pronounced softening effects for actin depolymerization. To relate these findings with functional consequences, we use invasion and three-dimensional growth assays. These reveal higher invasiveness of keratin-free cells. This study supports the view that downregulation of keratins observed during EMT directly contributes to the migratory and invasive behavior of tumor cells. Cancer cells that effectively move through tissues are softer and more contractile than cells that stay local in tissues. Soft and contractile avoids jamming. Naturally, softness has to have its limits. So neuronal growth cones are too soft to carry large loads to move efficiently through scar tissue, which is required for nerve regeneration. In synopsis, the physical bounds that the functional modules of a moving cell experience in tissues may provide an overarching motif for novel approaches in diagnosis and therapy. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G10.00002: Role of mismatch in mechanical properties in cancer cell migration Julian Butcher, Moumita Das Recent experiments suggest that the mechanical stiffness of cells and their interaction with their surroundings undergo remarkable changes during tumor progression [1,2]. An intriguing experimental result in this area suggests that the mismatch in the elasticity and adhesive properties between cancer cells and cells that have not yet transformed may lead to enhanced cancer cell motility in a binary cell population [2]. Motivated by this, we study the mechanical response and dynamics of a binary system of active and deformable particles using Langevin Dynamics simulations. We characterize their motility by studying particle trajectories, mean square displacements and correlation functions. Our study may provide an understanding of the interplay of mechanical and statistical mechanical properties underlying the enhanced motility of cancer cells during metastasis [2]. \\[4pt] [1] S. Suresh, Biomechanics and biophysics of cancer cells, Acta Biomaterialia 3, 413 (2007).\\[0pt] [2] M. H. Lee, P. H. Wu, J. R. Staunton, R. Ros, G. D. Longmore, and D. Wirtz, Mismatch in Mechanical and Adhesive Properties Induces Pulsating Cancer Cell Migration in Epithelial Monolayer 102, 2731 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G10.00003: Quantifying Collective Cell Migration during Cancer Progression Rachel Lee, Christina Stuelten, Kerstin Nordstrom, Carole Parent, Wolfgang Losert As tumors become more malignant, cells invade the surrounding tissue and migrate throughout the body to form secondary, metastatic tumors. This metastatic process is initiated when cells leave the primary tumor, either individually or as groups of collectively migrating cells. The mechanisms regulating how groups of cells collectively migrate are not well characterized. Here we study the migration dynamics of epithelial sheets composed of many cells using quantitative image analysis techniques. By extracting motion information from time-lapse images of cell lines of varying malignancy, we are able to measure how migration dynamics change during cancer progression. We further investigate the role that cell-cell adhesion plays in these collective dynamics by analyzing the migration of cell lines with varying levels of E-cadherin (a cell-cell adhesion protein) expression. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G10.00004: Interplay of Genes and Mechanics in the Disorganization of Multicellular Structures Invited Speaker: Jan Liphardt As of last count, there are at least 10 risk factors for breast cancer. Some of these risk factors are genetic, such as mutations in the BRCA1 and 2 genes. Other risk factors are based on bulk tissue characteristics such as the degree to which the tissue attenuates x-rays (``mammographic density'') or its mechanical stiffness. Finally, risk and outcomes are also correlated with specific micro-anatomical features, such as collagen lines or tracts that extend radially outwards from the tumor-stromal interface. Despite significant progress in discovering risk factors, it is not understood if and how these risk factors interact. We have developed a simple model system for studying how genes, mechanics, and geometry interact to drive defined multi-cellular structures to an invasive phenotype. We have found that pairs or groups of Ras-transformed mammary acini with thinned basement membranes and weakened cell-cell junctions can generate collagen lines that then coordinate and accelerate transition to an invasive phenotype. When two or more acini mechanically interact by collagen lines, the pairs or groups of acini begin to disorganize rapidly and in a spatially coordinated manner, whereas acini that do not interact mechanically with other acini disorganize slowly and to a lesser extent. When acini were mechanically isolated from other acini and also from the bulk gel by directed laser cutting of the collagen matrix, transition to an invasive phenotype was blocked in 20 of 20 experiments. Thus, pairs or groups of mammary acini can interact mechanically over long distances through the collagen matrix and these directed mechanical interactions are necessary for rapid transition to an invasive phenotype. This new model system may help to understand the interplay of genes, mechanics, and geometry in transition to an invasive phenotype.\\[4pt] In collaboration with Quanming Shi, Rajarshi P. Ghosh, Hanna Engelke, Bay Area Physical Sciences Oncology Center and University of California, Berkeley; Chris H. Rycroft, Bay Area Physical Sciences Oncology Center, University of California, Berkeley, and Lawrence Berkeley National Laboratory; Luke Cassereau, Bay Area Physical Sciences Oncology Center and University of California, San Francisco; James A. Sethian, Bay Area Physical Sciences Oncology Center, University of California, Berkeley, and Lawrence Berkeley National Laboratory; and Valerie M. Weaver, Bay Area Physical Sciences Oncology Center and University of California, San Francisco. \\[4pt] Reference:\\[0pt] Rapid disorganization of mechanically interacting systems of mammary acini Q. Shi, RP. Ghosh, H. Engelke, CH. Rycroft, L. Cassereau, JA. Sethian, VM. Weaver, and J. Liphardt, PNAS 111(2), 658-663 (2014) [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G10.00005: Emergence of therapy resistance in multiple myeloma in heterogeneous microenvironment Amy Wu, Qiucen Zhang, Guillaume Lambert, Zayar Khin, Ariosto Silva, Robert Gatenby, Hyungsung Kim, Nader Pourmand, Robert Austin, James Sturm Cancer chemotherapy resistance is always a problem that is not clear considering spatial heterogeneity in the tumor microenvironment. We culture multiple myeloma in a gradient from 0 to 20 nM of doxorubicin (genotoxic drug) across 2 mm wide region in a microfluidic device which mimics the tumor microenvironment with a chemotherapy drug gradient and microhabitats. Resistance of the multiple myeloma cells to doxorubicin emerged within two weeks. For the resistant cells evolved from the devices, the doxorubicin concentration that inhibits 50\% of the controlled population increased by 16-fold than the parental cells. Whole transcriptome sequencing revealed that 39\% of newly acquired mutational hotspots (the genes with more than 3 non-synonymous point mutation) of the resistant cells are involved in apoptosis and DNA repair. On the other hand, 40\% of the non-mutated genes that are abnormally regulated in the resistant cells, are involved in metabolism, biosynthesis, and biomolecular transport. Among them, metabolic drug efflux pumps and oxidative stress scavengers are up-regulated to reduce the cytotoxicity of doxorubicin and further result in the resistance. The roles of the spatial drug gradients and microhabitats in rapid emergence of cancer resistance will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G10.00006: Network of mutually repressive metastasis regulators can promote cell heterogeneity and metastatic transitions Gabor Balazsi, Eun-Jin Kim, Marsha Rosner The sources and consequences of nongenetic variability in metastatic progression are largely unknown. To address these questions, we characterize the transcriptional regulatory network around the metastasis suppressor Raf Kinase Inhibitory Protein (RKIP). It was previously shown that RKIP negatively regulates the transcription factor BACH1, which promotes breast cancer metastasis. Here we demonstrate that BACH1 acts in a double negative (overall positive) feedback loop to inhibit RKIP transcription in breast cancer cells. BACH1 also negatively regulates its own transcription. Analysis of the RKIP-BACH1 network reveals the existence of an inverse relationship between BACH1 and RKIP involving both monostable and bistable transitions between ``low BACH1, high RKIP'' and ``high BACH1, low RKIP'' cellular states that can potentially give rise to nongenetic variability. Single cell analysis confirmed the antagonistic relationship between RKIP and BACH1, and showed cell line-dependent signatures consistent with bistable behavior. Together, our results suggest that the mutually repressive relationship between metastatic regulators such as RKIP and BACH1 can play a key role in determining metastatic progression in cancer. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G10.00007: Role of Fiber Length on Phagocytosis {\&} Inflammatory Response Leonid Turkevich, Carahline Stark, Julie Champion Asbestos fibers have long been associated with lung cancer death. The inability of immune cells (e.g. macrophages) to effectively remove asbestos leads to chronic inflammation and disease. This study examines the role of fiber length on toxicity at the cellular level using model glass fibers. A major challenge is obtaining single diameter fibers but differing in length. Samples of 1 micron diameter fibers with different length distributions were prepared: short fibers (less than 15 microns) by aggressive crushing, and long fibers (longer than 15 microns) by successive sedimentation. Time-lapse video microscopy monitored the interaction of MH-S murine alveolar macrophages with the fibers: short fibers were easily internalized by the macrophages, but long fibers resisted internalization over many hours. Production of TNF-$\alpha $ (tumor necrosis factor alpha), a general inflammatory secreted cytokine, and Cox-2 (cyclo-oxygenase-2), an enzyme that produces radicals, each exhibited a dose-dependence that was greater for long than for short fibers. These results corroborate the importance of fiber length in both physical and biochemical cell response and support epidemiological observations of higher toxicity for longer fibers. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G10.00008: The Genetic Origins of Cancer Invited Speaker: Jonathan Licht |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G10.00009: Optical Properties of Human Cancer and Normal Cells Christopher Sander, Nan Sun, Jeffrey Johnson, Sharon Stack, Carol Tanner, Steven Ruggiero We have investigated the optical properties of human oral and ovarian cancer and normal cells. Specifically, we have measured the absolute optical extinction for both whole cells and intra-cellular material in aqueous suspension. Measurements were conducted over a wavelength range of 250 to 1000nm with 1 nm resolution using Light Transmission Spectroscopy (LTS). This provides both the absolute extinction of materials under study and, with Mie inversion, the absolute number of particles of a given diameter as a function of diameter in the range of 1 to 3000 nm. Our preliminary studies show significant differences in both the extinction and particle size distributions associated with cancer versus normal cells, which appear to be correlated with differences in the particle size distribution in the range of $\sim$ 50 to 250 nm. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G10.00010: Reaction rate theory of radiation exposure:Effects of dose rate on mutation frequency Masako Bando, Issei Nakamura, Yuichiro Manabe We revisit the {\it linear no threshold} (LNT) hypothesis deduced from the prominent works done by H. J. Muller for the DNA mutation induced by the artificial radiation and by W. L. Russell and E. M. Kelly for that of mega-mouse experiments, developing a new kinetic reaction theory. While the existing theoretical models primarily rely on the dependence of the total dose $D$ on the mutation frequency, the key ingredient in our theory is the {\it dose rate} $d(t)$ that accounts for decrease in the mutation rate during the time course of the cellular reactions. The general form for the mutation frequency with the constant dose rate $d$ is simply expressed as, $\frac{d F_m (t)}{dt}= A - B F_m(t)$, with $A = a_0+a_1(d+d_{eff})$ and $B = b_0+b_1(d+d_{eff})$. We discuss the solution for a most likely case with $B > 0$; $F_m(t) = [\frac{A}{B}- F_m(0)] (1-e^{-Bt}) + F_m(0)$ with the control value $F_m(0)$. We show that all the data of mega-mouse experiments by Russel with different dose rates fall on the universal scaling function $\Phi(\tau) \equiv \frac{[F_m(\tau) - F_m(0)]}{[A/B - F_m(0)]} = 1 - \exp{(-\tau)}$ with scaled time $\tau = Bt$. The concept of such a scaling rule provides us with a strong tool to study different species in a unified manner. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G10.00011: Modelling Spread of Oncolytic Viruses in Heterogeneous Cell Populations Michael Ellis, Hana Dobrovolny One of the most promising areas in current cancer research and treatment is the use of viruses to attack cancer cells. A number of oncolytic viruses have been identified to date that possess the ability to destroy or neutralize cancer cells while inflicting minimal damage upon healthy cells. Formulation of predictive models that correctly describe the evolution of infected tumor systems is critical to the successful application of oncolytic virus therapy. A number of different models have been proposed for analysis of the oncolytic virus-infected tumor system, with approaches ranging from traditional coupled differential equations such as the Lotka-Volterra predator-prey models, to contemporary modeling frameworks based on neural networks and cellular automata. Existing models are focused on tumor cells and the effects of virus infection, and offer the potential for improvement by including effects upon normal cells. We have recently extended the traditional framework to a 2-cell model addressing the full cellular system including tumor cells, normal cells, and the impacts of viral infection upon both populations. Analysis of the new framework reveals complex interaction between the populations and potential inability to simultaneously eliminate the virus and tumor populations. [Preview Abstract] |
Session G11: Focus Session: Active Soft Matter II - Dynamical Response
Sponsoring Units: DPOLY GSNP DBIOChair: Zvonimir Dogic, Brandeis University
Room: 203
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G11.00001: Active nematics of flat and spherical surfaces Invited Speaker: Zvonimir Dogic The laws of equilibrium statistical mechanics impose severe constraints on the properties of conventional materials assembled from inanimate building blocks. Consequently, such materials cannot exhibit spontaneous motion or perform macroscopic work; i.e., a fluid in a beaker remains quiescent unless driven by external forces. Inspired by biological phenomena such as ciliary beating or \textit{Drosophila} cytoplasmic streaming our aim is to develop a new category of materials assembled from animate, energy-consuming building blocks. Starting from a few well-characterized biochemical components we assemble and study far-from-equilibrium analogs of conventional liquid crystals. Released from the constraints of equilibrium, this internally driven polymeric material exhibits a host of highly-sought after properties including appearance of steady-state streaming flows that are accompanied by the spontaneous unbinding and annihilations of motile defects as well as appearance and subsequent self-healing of fracture lines. Active liquid crystals can serve as a platform for developing novel material applications, testing fundamental theoretical models of far-from-equilibrium active matter and potentially shedding light on self-organization in living cells. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G11.00002: Visualization of the material response in an actomyosin network at the onset of internal motor activity Samantha Stam, Margaret Gardel The actomyosin cortex of living cells generates forces that drive structural rearrangements at the sub-cellular, cellular, and tissue length scales during cell migration, cell division, and tissue formation.~In the cortex, filaments of myosin II motors actively generate stresses on actin filament networks and bundles to form an active contractile material. However, how the spatial and temporal regulation of contractile deformation is affected both by local stresses and the material response is not understood.~For instance, the extent to which the stress-strain relationship within active networks may be understood with governing equations from continuum elasticity is unknown. Here, we directly measure strain fields within quasi-2D actin networks subjected to varying degrees of internal myosin activity. We observe evidence that both the motor-generated stress and its propagation are regulated by network properties such as cross-linking. Stresses propagate anisotropically and produce a non-trivial material response even at motor concentrations much lower than those required to observe robust contractile behavior. Our data yield insights into how cellular networks make use of varying microstructures to regulate motor-generated force and the resulting strain. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G11.00003: Quantifying actin wave modulation on periodic topography Can Guven, Meghan Driscoll, Xiaoyu Sun, Joshua Parker, John Fourkas, Anders Carlsson, Wolfgang Losert Actin is the essential builder of the cell cytoskeleton, whose dynamics are responsible for generating the necessary forces for the formation of protrusions. By exposing amoeboid cells to periodic topographical cues, we show that actin can be directionally guided via inducing preferential polymerization waves. To quantify the dynamics of these actin waves and their interaction with the substrate, we modify a technique from computer vision called ``optical flow." We obtain vectors that represent the apparent actin flow and cluster these vectors to obtain patches of newly polymerized actin, which represent actin waves. Using this technique, we compare experimental results, including speed distribution of waves and distance from the wave centroid to the closest ridge, with actin polymerization simulations. We hypothesize the modulation of the activity of nucleation promotion factors on ridges (elevated regions of the surface) as a potential mechanism for the wave-substrate coupling. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G11.00004: Characterizing tunable dynamics in an active gel Gil Henkin, Stephen DeCamp, Daniel Chen, Zvonimir Dogic We experimentally investigate dynamics of an active gel of bundled microtubules that is driven to far-from-equilibrium steady states by clusters of kinesin molecular motors. Upon the addition of ATP, the coordinated action of thousands of molecular motors drives this gel to an active, percolating state that persists for hours and is only limited by the stability of constituent proteins and the availability of the chemical fuel ATP. We extensively characterize how enhanced transport in emergent macroscopic flows depends on relevant molecular parameters, including ATP, motor, and depletant concentrations, microtubule concentration and length, as well as structure of the motor clusters. Our results show that the properties and dynamics of this active isotropic gel are highly tunable, suggesting that this is an ideal system for studying the behavior of active materials. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G11.00005: Tuning mechanical relaxation through the regulation of non-equilibrium actin assembly Patrick M. McCall, David R. Kovar, Margaret L. Gardel Two of the most fundamental differences between the biopolymer filamentous actin (F-actin) and more conventional synthetic polymers are its semiflexibility and intrinsically non-equilibrium (active) nature. While the consequences of semiflexiblity on the mechanics of F-actin-based materials have received much study, less is known about the role of non-equilibrium dynamics. A major roadblock to experimental progress in this regard is that the assembly dynamics of purified actin at steady-state are too slow for appreciable effects to be observed. Taking a cue from living cells, we address this problem by polymerizing actin in the presence of the actin regulatory proteins formin, profilin, and cofilin, which promote filament elongation, nucleotide exchange on monomers, and filament disassembly through severing, respectively, to increase the rates of these processes. The mechanics of the resulting entangled F-actin solution is then monitored at steady-state with passive particle tracking microrheology. At fixed formin and profilin concentrations, the self-diffusion time of 1-micron tracer particles drops by more than two orders of magnitude as the cofilin concentration is increased above a molar ratio threshold of 10{\%}. In addition, the elastic plateau gives way to anomalous scaling with a crossover time that shifts with cofilin concentration. Interestingly, these effects are not observed in the absence of formin, indicating filament treadmilling as the likely mechanism of the enhanced relaxation. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G11.00006: Non-equilibrium States of Active Filament Networks Robert A. Blackwell, Meredith D. Betterton, Oliver M. Sweezy, Matthew A. Glaser Active networks of filamentous proteins and crosslinking motor proteins play a critical role in many cellular processes. Among the most important active networks is the mitotic spindle, an assembly of microtubules and crosslinking motor proteins that forms during cell division and that ultimately separates chromosomes into two daughter cells. To evolve a better understanding of spindle formation, structure, and dynamics, we have developed course-grained models of active networks composed of filaments, modeled as hard spherocylinders, in diffusive equilibrium with a reservoir of crosslinking motors, modeled as Hookean springs that can adsorb to microtubules and translocate at finite velocity along the microtubule axis. We explore the phase diagram and other characteristics of this model in two and three dimensions as a function of filament packing fraction, and of crosslink concentration, velocity, and adsorption and desorption rates. We observe a variety of interesting emergent behaviors including sorting of filaments into polar domains, generation of extensile stress, and superdiffusive transport. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G11.00007: Dynamics and rheological applications of chiral nanostructures Arijit Ghosh, Ambarish Ghosh We have developed a system of magnetic chiral (helical) nanostructures that can be actuated in fluidic environments with rotating magnetic fields. These objects demonstrate interesting dynamical behavior, determined by the counteracting applied and viscous torques. Under certain experimental conditions, a bistable chaotic dynamics could be observed. This is surprising, since motion at low Reynolds' numbers is typically deterministic. We have developed an analytical theory to understand this dynamics, which has led to the development of a novel tool for micro-rheological measurements. We will demonstrate how the helical nanostructures can be used to map a complex rheological environment with micron scale spatial resolution, in a measurement time significantly shorter than all other currently used techniques. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G11.00008: Confocal Rheometry of Active Networks Daniel Chen, Stephen Decamp, Daniel Blair, Zvonimir Dogic While much is known about the rheological responses of passive biopolymer networks, we currently lack a conceptual framework to describe active networks under shear.~ To this end, we have engineered an active gel composed of microtubules, bidirectional kinesin motors, and molecular depletant that self-organizes into a highly dynamic network of bundles.~ The network continually remodels itself under ATP-driven cycles of extension, buckling, fracturing, and self-healing [1].~ In this talk I will present comprehensive confocal rheometry measurements elucidating the interplay between the network's dynamic morphology and its linear and non-linear rheological responses. [1] T. Sanchez, D.T.N. Chen, S.J. Decamp, M. Heymann, and Z. Dogic Nature 491 (7424), 431-434 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G11.00009: Chain-configuration dependent rheological properties in transient networks Michelle Sing, Zhen-Gang Wang, Gareth McKinley, Bradley Olsen Complex associative networks capable of shear thinning followed by recovery on the order of seconds are of interest as injectable biomaterials. However, there is a limited understanding of the molecular mechanisms that contribute to rheological properties such as the network's yield stress and rate of self-healing. Here we present a transient network theory for associative physical gels arising from the chemical kinetic form of the Smoluchowski Equation capable of modeling the full chain end-to-end distance distribution while tracking the fraction of looped, bridged, and free chain configurations in the gel. By varying the equilibrium association rate relative to the material relaxation time, we are able to track the evolution of loop and bridge chain fraction as the system undergoes stress instabilities. We have evidence that these instabilities result from non-monotonic trends in loop and bridge chain fraction when the end group association rate is high relative to the dissociation rate. This behavior provides insight into the complex kinetic interactions responsible for certain mechanical behaviors while serving as a valuable predictive tool for gel design. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G11.00010: Effects of filament rigidity in myosin II-induced actin network contractility and dynamics Kimberly Weirich, Margaret Gardel Cells change shape, deforming to move and divide. The dynamic protein scaffold that shapes the cell is the cortex, a disordered, thin network of actin filaments. Random, local stresses generated by myosin II in the network create cellular-scale deformations. Myosin induced buckling and severing of actin filaments has been shown to underlie the contractility of two-dimensional disordered actin networks. This non-linear elastic response of actin filaments is thought to be an essential symmetry breaking mechanism to produce robust contractility in disordered actomyosin networks. To test this idea, we explore the effects of an actin bundling protein fascin, a crosslinker which induces polarity specific bundling of actin filaments, to create a network of F-actin bundles. We investigate myosin-induced stresses in a network of randomly oriented actin filaments, confined to a thin sheet at a supported lipid bilayer surface through a crowding agent. We find fascin-bundled filaments are less prone to filament buckling and show increased filament sliding, causing the myosin activity to induce network reorganization rather than contraction. Thus, changes in the filament bending rigidity in motor-filament systems can drive the system between distinct states with unique dynamic and mechanical signatures. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G11.00011: Defect Dynamics in Active 2D Nematic Liquid Crystals Stephen DeCamp, Gabriel Redner, Michael Hagan, Zvonimir Dogic Active materials are assemblies of animate, energy-consuming objects that exhibit continuous dynamics. As such, they have properties that are dramatically different from those found in conventional materials made of inanimate objects. We present a 2D active nematic liquid crystal composed of bundled microtubules and kinesin motor proteins that exists in a dynamic steady-state far from equilibrium. The active nematic exhibits spontaneous binding and unbinding of charge $+$1/2 and -1/2 disclination defects as well as streaming of $+$1/2 defects. By tuning ATP concentration, we precisely control the amount of activity, a key parameter of the system. We characterize the dynamics of streaming defects on a large, flat, 2D interface using quantitative polarization light microscopy. We report fundamental characteristics of the active nematics such as defect velocities, defect creation and annihilation rates, and emergent length scales in the system. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G11.00012: Simulation Study of Defect Dynamics in an Extensile Active Nematic Gabriel Redner, Aparna Baskaran, Michael Hagan This talk will describe a novel particle-based simulation scheme for studying extensile active nematic liquid crystals. Motivated by recent experiments on bundled microtubles and molecular motors, we have implemented a minimal simulation model which simplifies the complex details of the experimental system while retaining the correct symmetries of alignment and activity. Our simulations exhibit the same basic phenomena as the experimental system, including the spontaneous generation, unbinding, and annihilation of defect pairs. Our flexible simulation approach allows us to investigate in detail the dynamics of defects interacting with other defects and with system boundaries, as well as to explore a broad region of parameter space. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G11.00013: Living liquid crystal: collective bacteria motion in anisotropic viscoelastic media Shuang Zhou, Andrey Sokolov, Oleg D. Lavrentovich, Igor S. Aranson By transducing energy stored in the environment to drive systematic movements, bio-mechanical hybrids can move and reconfigure their structure and properties in response to external stimuli. Here, we create a fundamentally new class of bio-mechanical hybrid -- living liquid crystals (LLCs), by combining two seemingly incompatible concepts, living swimming bacteria and inanimate but orientationally ordered lyotropic liquid crystal. The coupling between the activity-triggered flows and director reorientations results in a wealth of phenomena, including: (a) a characteristic length $\xi $ to describe the coupling between the orientation of LLC and the bacterial motion, (b) periodic stripe instabilities of the director in surface-anchored LLCs, (c) director pattern evolution into an array of disclinations with positive and negative topological charges as the surface anchoring is weakened or when the bacterial activity is enhanced. Our study provides an insight in understanding hierarchy of spatial scales in other active matter systems, as well as providing basis for devices with new functionalities, including specific responses to chemical agents, toxins, or photons. [Preview Abstract] |
Session G12: Invited Session: Energy Research and Applications
Sponsoring Units: GERAChair: Rueben Collins, Colorado School of Mines
Room: 205
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G12.00001: Light-induced defect creation and recombination in organic solar cells Invited Speaker: Robert Street Prolonged exposure to visible, UV and x-ray radiation induces defects in some organic bulk heterojunction solar cells, and the resulting excess recombination reduces the cell efficiency. Optical transitions characteristic of deep localized electronic states are observed by photocurrent spectroscopy and allows the kinetics of defect creation to be measured. The defect creation rate varies greatly with photon energy and saturates after a long exposure. The states are reversible by annealing to about 100C with thermal activation energy of 1.1-1.3 eV. The results suggest that the defects arise from the recombination-induced dissociation of C-H bonds and the migration of the hydrogen to other sites in the polymer. First-principles calculations show that the resulting under-and over-coordinated carbon atoms are deep traps with properties consistent with the measurements. In the absence of light-induced defects, the dominant recombination process is a transition through localized band tail states, which is reflected in the diode forward bias current-voltage characteristics. The diode ideality factor is temperature dependent and related to the band tail slope. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G12.00002: Prospects of Omnidirectional Substrates for Light Trapping Invited Speaker: Teri Odom Nanostructured substrates are promising for light trapping in photovoltaic devices because they have the potential to manage and direct light absorption and scattering. Whether these structures should be periodic or randomly arranged is under some debate, although most texturing in inorganic devices has no short or long-range order. This talk will describe how different types of nano-textured substrates can result in broadband absorption over visible and near-infrared ranges. We will discuss different strategies to generate subwavelength features with moir\'{e} patterns, high rotational symmetries, and wrinkled features. The potential of these omnidirectional substrates in organic photovoltaics will be described. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G12.00003: Understanding and Optimizing Bulk Thermoelectric Materials Invited Speaker: Andrew May Thermoelectric materials research continues to expand due to the need to increase energy conversion efficiencies, as well as the expanding role of thermoelectrics in niche applications. I will review several examples demonstrating the successes and limitations of semi-classical models of electron and phonon transport in the study of bulk thermoelectric materials. Single band models provide a simple starting point for analysis and optimization, and can facilitate the development of more physically-accurate models. The importance of identifying intrinsically favorable electronic structures will be demonstrated with La$_{3-x}$Te$_{4}$, where a heavy-band/light-band configuration leads to desirable properties. Using other chalcogenides and intermetallics as examples, I will also discuss the importance of phonon dispersions and scattering mechanisms, and how they can provide insight for materials exploration efforts. Research supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G12.00004: An Aqueous Based Route to Fabricating 3-D Solid-State Lithium-Ion Batteries Invited Speaker: Derek Johnson |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G12.00005: Quantum Confined Semiconductors for High Efficiency Photovoltaics Invited Speaker: Matthew Beard Semiconductor nanostructures, where at least one dimension is small enough to produce quantum confinement effects, provide new pathways for controlling energy flow and therefore have the potential to increase the efficiency of the primary photon-to-free energy conversion step. In this discussion, I will present the current status of research efforts towards utilizing the unique properties of colloidal quantum dots (NCs confined in three dimensions) in prototype solar cells and demonstrate that these unique systems have the potential to bypass the Shockley-Queisser single-junction limit for solar photon conversion. The solar cells are constructed using a low temperature solution based deposition of PbS or PbSe QDs as the absorber layer. Different chemical treatments of the QD layer are employed in order to obtain good electrical communication while maintaining the quantum-confined properties of the QDs. We have characterized the transport and carrier dynamics using a transient absorption, time-resolved THz, and temperature-dependent photoluminescence. I will discuss the interplay between carrier generation, recombination, and mobility within the QD layers. A unique aspect of our devices is that the QDs exhibit multiple exciton generation with an efficiency that is $\sim$ 2 to 3 times greater than the parental bulk semiconductor. [Preview Abstract] |
Session G13: Focus Session: Fe-based Superconductors-Pairing Symmetry and Damage
Sponsoring Units: DMPChair: Roman Movshovich, Los Alamos National Laboratory
Room: 207
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G13.00001: Symmetry imposed line nodes in the superconducting gap of KFe$_2$As$_2$ Hyunsoo Kim, Makariy A. Tanatar, Yong Liu, Thomas A. Lograsso, Ruslan Prozorov, Chenglin Zhang, Pengcheng Dai The variation with temperature of the London penetration depth, $\Delta\lambda(T)$, was measured in single crystals of K$_{1-x}$Na$_x$Fe$_2$As$_2$ ($x=0,~0.07$) by using a tunnel diode resonator down to 50 mK. Electrical resistivity measurements on the same samples show that isoelectronic Na-substitution significantly increases residual resistivity $\rho_0$ (from 0.2 to 1 $\mu\Omega$), without changing the shape of $\rho(T)$, and changes $T_c$ from 3.6 K to 2.9 K for $x=0$ and 0.07 samples. In both pure and doped compounds, the penetration depth follows the power-law function, $\Delta\lambda(T)=AT^n$ below $0.3\,T_c$ with $n\approx 1.5$ and 2.0 for $x=0$ and 0.07 samples, respectively. This behavior is consistent with presence of line nodes in the superconducting gap with moderate scattering for $x=0$ evolving into dirty limit for $x=0.07$. The normalized superfluid density, $\rho_s=\lambda^2(0)/\lambda^2(T)$ was calculated with $\lambda(0)=200$ nm and 500 nm for $x=0$ and $x=0.07$, respectively. Detailed investigation of the calculated $\rho_s$ strongly supports the existence of symmetry imposed line nodes in KFe$_2$As$_2$ superconductor and is consistent with thermal conductivity data that concluded $d-$wave pairing in this compound. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G13.00002: Local disorder and superconductivity in K$_{x}$Fe$_{2-y}$Se$_{2}$ Despina Louca, Keeseong Park, Bing Li, Y.-Q. Yan, Joerg Neuefeind In the K$_{x}$Fe$_{2-y}$Se$_{2}$ family of Fe-based superconductors, superconductivity is observed between a semi-metallic region below x $\sim$ 0.7 and an insulating and antiferromagnetic region above x $\sim$ 0.85. By probing the local structure we observe that superconductivity emerges in a locally distorted Fe sublattice that accommodates two kinds of bonding environments, forming a double-well distribution. Consisting of short bonds which are metallic in nature and of long ones which are insulating and antiferromagnetic, their distribution changes with x. Even though crystallographically the atomic structure changes slowly on average by adding K, a continuous transition from the short to the long Fe bonds is observed across this region. What is unique to this system's superconducting state is the presence of the double-well distribution in equal proportions, in contrast to other Fe-based materials where only one kind of Fe bond is present. This suggests that superconductivity is intertwined with magnetism, appearing at the crossover from metallic to insulating conditions and is not due to phase separation. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G13.00003: Systematics of the temperature dependent inter-plane resistivity in (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ Makariy Tanatar, Yong Liu, T.A. Lograsso, B. Jensen, K.W. Dennis, R.W. McCallum, Ruslan Prozorov The single crystals of hole-doped iron-based superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ were studied in great details in underdoped to optimally - doped compositions. In contrast, the overdoped side was practically inaccessible due to the difficulty in crystals growth. Only recently high quality single crystals have become available, and here we report the temperature dependent inter-plane resistivity for the whole doping range $x$=0 to $x$=1. In a parent compound, BaFe$_2$As$_2$, inter-plane resistivity shows a broad cross-over maximum at a characteristic temperature $T_{max} \sim$200~K. With K doping this maximum first shifts to higher temperatures, reaches a maximum 250~K close to the optimal doping, $x=0.4$ and then decreases towards the K-rich compositions. In pure KFe$_2$As$_2$ the maximum shows clear correlation with features in magnetization measurements. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G13.00004: Universal Heat Conduction in the Iron-Arsenide Superconductor KFe$_2$As$_2$: Evidence of a $d$-wave State Invited Speaker: Jean Philippe Reid The thermal conductivity $\kappa$ of the iron-arsenide superconductor KFe$_2$As$_2$ was measured down to 50 mK for a heat current parallel and perpendicular to the tetragonal $c$ axis. A residual linear term at $T \to 0$, $\kappa_0/T$, is observed for both current directions, confirming the presence of nodes in the superconducting gap. Our value of $\kappa_0/T$ in the plane is equal to that reported by Dong et al. [Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity $\rho_0$ was ten times larger. This independence of $\kappa_0/T$ on impurity scattering is the signature of universal heat transport, a property of superconducting states with symmetry-imposed line nodes. This argues against an $s$-wave state with accidental nodes. It favours instead a $d$-wave state, an assignment consistent with five additional properties: the magnitude of the critical scattering rate $\Gamma_c$ for suppressing $T_c$ to zero; the magnitude of $\kappa_0/T$, and its dependence on current direction and on magnetic field; the temperature dependence of $\kappa(T)$. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G13.00005: Resistive Losses in Single-Crystal Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ Brendan Benapfl, Chenglin Zhang, Pengcheng Dai, H.A. Blackstead Recently, we conducted surface resistance measurements using electron spin resonance techniques on single-crystal Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ samples (\textit{rf} frequency = 20.3 GHz), testing temperature and field dependence. In the superconducting state, the samples exhibit dissipative losses which increase monotonically as a function of applied field for fixed temperature. The level of field-dependent dissipation increases as $T$ approaches $T_C$ from below, and vanishes at the transition. The dissipation is also dependent on the angle between the \textit{rf} field and the static field, consistent with flux-flow models of other high-temperature superconductors, such as YBCO. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G13.00006: Multiband, Strong coupling and Strong Correlations in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ single crystals. Mind the (nodal) gap! Fr\'ed\'eric Hardy, Dai Aoki, Thomas Wolf, Carley Paulsen, Robert Eder, Anna B\"ohmer, Rolf Heid, Martin Jackson, Robert A. Fisher, Christoph Meingast We report an exhaustive thermodynamic study (heat capacity, magnetization, thermal expansion) of the normal- and superconducting-state properties of the entire Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ series. We show that strong correlations develop with increasing hole concentration indicating the possible proximity to an orbital-selective Mott transition predicted by theory. In the superconducting state, these single crystals exhibit strong multiband and paramagnetic effects and we give evidence of a strong-to-weak-coupling crossover that occur near the concentration where the electron sheets disappear. Our data show no evidence for a symmetry change, from {\it s} to {\it d}-wave, of the superconducting state. Dissimilarities with the Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ series are emphasized. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G13.00007: High-field critical current enhancement by irradiation induced correlated and random defects in (Ba0.6K0.4)Fe2As2 Karen Kihlstrom, Lei Fang, Ying Jia, Bing Shen, Alexei Koshelev, Ulrich Welp, George W. Crabtree, Wai Kwong Kwok, Asghar Kayani, Shaofei Zhu, Hai Hu Wen Mixed pinning landscapes in superconductors are emerging as an effective strategy to achieve high critical currents in high, applied magnetic fields. Here, we use heavy-ion and proton irradiation to create correlated and point defects to explore the vortex pinning behavior of each and combined constituent defects in the iron-based superconductor Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ and find that the pinning mechanisms are non-additive. The major effect of p-irradiation in mixed pinning landscapes is the generation of field-independent critical currents in very high fields. At 7T and 5K, the critical current density exceeds 5\textunderscore MA/cm2. This work supported by the Center for Emergent~Superconductivity, an Energy Frontier Research Center funded by the U.S. D.O.E., Office of Science, Office of Basic Energy Sciences and by the D.O.E, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The operation of the ATLAS facility was supported by the U.S. D.O.E., Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357. The work in China was supported by the NSF of China, the MOST of China (2011CBA00102 and 2012CB821403) and PAPD. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G13.00008: Effect of electron irradiation on magnetic and superconducting transitions in underdoped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ Ruslan Prozorov, M. A. Tanatar, M. Ko\'{n}czykowski, R. Fernandes, B. Shen, Hai-Hu Wen Single crystals of (Ba$_{0.8}$K$_{0.2}$)Fe$_2$As$_2$ ($T_{c0} =$ 17 K) were irradiated by $2.5$ MeV electrons in several steps up to a total fluence of $2\times10^{19}$ electrons per cm$^{2}$. The sample resistance was measured both in situ at $23$ K during the irradiation, and as a function of temperature in a separate set-up, between the irradiation runs. Annealing of the induced defects by warming the sample up to different temperatures showed that the defects are stable as long as sample temperature remains at or below the highest temperature the sample was subject to. We found that both superconducting and magnetic transition temperatures decrease linearly with the increase of the residual resistivity. Surprisingly, both transitions are supressed at the same rate of 0.1 K/$\mu \Omega$cm. For the highest dose, the residual resistivity changed by $\Delta \rho(0) = 85~ \mu \Omega$cm, whereas $T_c$ changed from 17 K to 8 K and $T_N$ changed from 102 to 93 K. Our results provide a strong evidence that both superconductivity and magnetism of iron - based superconductors are derived from the interband nesting - like interactions. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G13.00009: Study of magnetic fluctuations and nematic phase transition in BaFe$_{2}$As$_{2}$ Xu Luo, Valentin Stanev, Bing Shen, XinSheng Ling, Wai-Kwong Kwok, Ulrich Welp We used high resolution AC micro-calorimetry and SQUID magnetometry to study the phase transitions in BaFe$_{2}$As$_{2}$ single crystals. A pronounced peak observed in the specific heat at T$_{\mathrm{N}}$ $\sim$ 132K, together with a step in the magnetization at the same temperature signifies the AFM/Structural transition in BaFe$_{2}$As$_{2}$. Annealing treatment of the sample shifts the peak in specific heat to 137K and reduces its width to $\sim$ 0.8K (FWHM). A thorough investigation of the specific heat up to 220K revealed no discernible, additional phase transitions above a typical background noise level of 5 to 10 $\times$ 10$^{-4}$ for $\Delta $C/C. However, strong in-plane magnetic fluctuations were observed to persist to as high as 180K in magnetization, which provide a possible explanation for previous reports of a nematic phase transition. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G13.00010: Ising-nematic spin correlations in the tetragonal state of uniaxial strained BaFe$_{2-x}$Ni$_x$As$_2$ Xingye Lu, Jitae Park, Rui Zhang, Huiqian Luo, A.H. Nevidomskyy, Qimiao Si, Pengcheng Dai Superconductivity in iron pnictides can be derived from electron or hole-doping to their antiferromagnetic (AF) ordered parent compounds. In the normal state above the superconducting transition temperature (Tc), an in-plane resistivity anisotropy has been observed in uniaxial strained iron pnictides BaFe$_{2-x}$T$_x$As$_2$ (T $=$ Co, Ni) near the AF ordered state. This anisotropy has been suggested as a signature of the spin Ising-nematic phase that breaks the in-plane four-fold rotational symmetry of the underlying tetragonal lattice, but direct evidence for a spin nematic state is lacking. Here we use inelastic neutron scattering (INS) to show that low-energy spin excitations in uniaxial strained BaFe$_{2-x}$Ni$_x$As$_2$ (x $=$ 0, 0085) change from four-fold symmetry to two-fold symmetry in the tetragonal phase at temperatures corresponding to the onset of the in-plane resistivity anisotropy. Our results thus indicate that spin excitation anisotropy plays a crucial role in the electronic nematic behavior of iron pnictides. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G13.00011: Evolution of the in-plane resistivity anisotropy in isovalenly substituted Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ Erick C. Blomberg, M.A. Tanatar, S. Ran, S.L. Bud'ko, P.C. Canfield, R. Prozorov, A. Thaler Recent studies of electronic anisotropy in iron-based superconductors have revealed a dramatic asymmetry between electron and hole doped compounds [1]. A natural question is: What effect would isovalent substitution have? The BaFe$_2$As$_2$ system shows little change in its Fermi surface and carrier concentration upon Ru-doping, even at the levels far beyond the point of total suppression of the AFM state, making it a valuable system to compare against the hole and electron doped system. Here we study in-plane resistivity anisotropy in detwinned single crystals of Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$. Polarized optical imaging was used to confirm detwinning. A quantitative comparison of our new results with the isovalently substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$ [2] system, as well as the charge doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Ba$_{1-x}$K$_x$Fe$_2$As$_2$ systems will be discussed. This work was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358.\\[4pt] [1] E. C. Blomberg, \textit{et al},. Nat. Comm. \textbf{4}, 1914 (2013).\\[0pt] [2] H-.H. Kuo, \textit{et al}., Phys. Rev. B \textbf{86}, 134507 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G13.00012: Influence of proton irradiation on the magnetotransport properties of BaFe$_{2}$As$_{2}$ and BaFe$_{1.985}$Co$_{0.015}$As$_{2}$ Dominc Moseley, Karen Yates, Will Branford, Lesley Cohen, David Mandrus, Athena Sefat Since their discovery in 2008, the iron-based superconductors have provided a new and unexpected system to probe the superconducting quantum phenomena. They have also presented the opportunity to investigate the intriguing interplay between magnetism, structure and transport properties. An additional complexity is the existence of Dirac Cones (DCs) within the electronic structure of the underdoped ferropnictides. In an attempt to elucidate the effects of these competing influences in the ferropnictides, we have performed a series of magnetoresistance (MR) experiments on proton irradiated undoped BaFe$_{2}$As$_{2}$ and sub-optimally doped BaFe$_{1.985}$Co$_{0.015}$As$_{2}$. Our findings show a non-saturating (up to 7T) linear MR above a temperature dependent critical magnetic field; in agreement with previous studies. Invoking the quantum linear magnetoresistance (QLM) model, it has been suggested that this infers the carrier transport dominance of DCs within the ferropnictides. Controlled proton irradiation allows us to test this concept by introducing random point defects into these materials. The conclusions drawn from this study will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G13.00013: Magneto-transport and magnetization studies on thermally activated flux flow in iron-based superconductors Martin Nikolo, Xiaoyan Shi, Eun Sang Choi, Jianyi Jiang, Jeremy Weiss, Eric Hellstrom We study the magneto-transport properties of three iron-based high temperature superconductors, polycrystalline samples, Ba(Fe$_{0.95}$ Ni$_{0.05})_{2}$As$_{2}$ ($T_{c} =$ 20.4 K), Ba(Fe$_{0.94}$ Ni$_{0.06})_{2}$As$_{2}$ ($T_{c} =$ 18.5 K), and Ba(Fe$_{0.91}$Co$_{0.09})_{2}$As$_{2}$ ($T_{c} =$ 25.3 K) in magnetic fields of up to 18 T. The thermally activated magnetic flux behavior was retrieved by plotting ln$\rho$ vs. 1/$T$ ($\rho$ and $T$ are resistivity and temperature, respectively) and obtaining the activation energies $U_{0}$ for flux motion near $T_{c}$. We show a 3-D plot of the distribution of $U_{0}$ as a function of $T$ and magnetic field $H$. We apply the WHH model by measuring \textit{dH}$_{c2}$\textit{/dT} at $T_{c}$ to estimate the upper critical field $H_{c2}(T=$ 0); we estimate the coherence length $\xi (T=$ 0). We study the broadening of resistive transition as a function of the applied magnetic field and compare it to Tinkham's prediction for high-T$_{c}$ materials. [Preview Abstract] |
Session G14: Invited Session: Toys and Mechanisms
Sponsoring Units: GSNPChair: James Hanna, Virginia Polytechnic Institute and State University
Room: 301-303
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G14.00001: Rotation with zero angular momentum: Demonstrations of the falling cat phenomenon go sour Invited Speaker: Andy Ruina It is well known that a system with zero angular momentum can, by appropriate deformations, rotate while preserving the condition of zero angular momentum. This effect explains how a cat that is dropped while upside down can turn over and of how certain gymnastic maneuvers are performed. These rotations are taken as a demonstration of the ``non-integrability'' of a ``non-holonomic'' constraint. There is a simple demonstration of this rotation-with-zero-angular-momentum effect with a rotating platform. But the demonstration often doesn't work because most floors are not perfectly flat. I found a simple better demonstration experiment. Unfortunately, the experiment came out all wrong for different reasons. But I figured out why and did a second demonstration experiment. And that came out wrong exactly in the opposite way. The talk presents the four puzzles: a) how can you turn while having zero angular momentum? b) Why does a rotating platform demonstration often not work. c) Why does a simple demonstration not work? d) Why does almost exactly the same demonstration not work in the opposite way? The talk starts with various personal stories about non-holonomic constraints and their relation to locomotion, like bikes skates and walking, and then gets into the 4 rotation puzzles. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G14.00002: Physics of Toys: The Joy of Asking Questions Invited Speaker: Beverley Taylor Children are natural scientists. They ask questions, they observe, they try things to see what happens. Often school-based science does little to nurture the young scientist and, in fact, may do just the opposite with thick textbooks, fact heavy lessons, and too many equations. The exploration of common toys produces deep learning by emphasizing concepts and connections before formal definitions and mathematics. It also connects the classroom to the familiar world outside of school and gets students writing and talking about physics ideas. At the university level, investigating what toys do and how they do it can be a challenging application of undergraduate physics from the introductory course up through senior mechanics. Toys provide an ideal system for the kind of open-ended inquiry that introduces students to what scientists really do. They can pose their own questions, explore the behavior of the system sufficiently to create a hypothesis, use their theoretical knowledge to make a simplified model of the system and predict an outcome, design an experiment, discover that the real world is messy, think about what they haven't taken into account with their simple model and try to improve it. I have spent close to 30 years thinking about how to use toys to enhance physics education from 4th grade through college. In the process I have collected hundreds of toys the majority of which relate to mechanics, but also to sound, light, electricity and magnetism. I will discuss the pedagogical reasons for using toys in physics education and the many different ways to use them from demonstrations to laboratory experiments to discussion starters as well as how it is possible to use the same toy with many different age levels by approaching the analysis differently. I will share a number of my favorite toys, but focus particularly on those related to energy concepts. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G14.00003: Water Bouncing Balls: how material stiffness affects water entry Invited Speaker: Tadd Truscott It is well known that one can skip a stone across the water surface, but less well known that a ball can also be skipped on water. Even though 17th century ship gunners were aware that cannonballs could be skipped on the water surface, they did not know that using elastic spheres rather than rigid ones could greatly improve skipping performance (yet would have made for more peaceful volleys). The water bouncing ball (Waboba\textregistered) is an elastic ball used in a game of aquatic keep away in which players pass the ball by skipping it along the water surface. The ball skips easily along the surface creating a sense that breaking the world record for number of skips could easily be achieved (51 rock skips Russell Byers 2007). We investigate the physics of skipping elastic balls to elucidate the mechanisms by which they bounce off of the water. High-speed video reveals that, upon impact with the water, the balls create a cavity and deform significantly due to the extreme elasticity; the flattened spheres resemble skipping stones. With an increased wetted surface area, a large hydrodynamic lift force is generated causing the ball to launch back into the air. Unlike stone skipping, the elasticity of the ball plays an important roll in determining the success of the skip. Through experimentation, we demonstrate that the deformation timescale during impact must be longer than the collision time in order to achieve a successful skip. Further, several material deformation modes can be excited upon free surface impact. The effect of impact velocity and angle on the two governing timescales and material wave modes are also experimentally investigated. Scaling for the deformation and collision times are derived and used to establish criteria for skipping in terms of relevant physical parameters. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G14.00004: Snap, crack and pop: What elastic instabilities in toys can teach us Invited Speaker: Dominic Vella The mechanism of many modern toys rely on some form or other of elastic instability, from the locomotion of the ``Hexbug nano'' to the snapping of a ``Hopper popper.'' In this talk I will discuss some fundamental mechanical problems that are inspired by the mechanism of such toys. A particular focus will be on the ``snap'' and ``pop'' phases of the Hopper popper but I will also discuss the ``crack'' of a whip and other examples of dynamic elastic instabilities. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G14.00005: The mechanics of trick roping Invited Speaker: Pierre-Thomas Brun Trick roping evolved from humble origins as a cattle-catching tool into a sport that delights audiences the world over with its complex patterns or ``tricks,'' such as the Merry-Go-Round , the Wedding-Ring, the Spoke-Jumping, the Texas Skip... Its implement is the lasso, a length of rope with a small loop (``honda'') at one end through which the other end is passed to form a large loop. Here, we study the physics of the simplest rope trick, the Flat Loop, in which the motion of the lasso is forced by a uniform circular motion of the cowboy's/cowgirl's hand in a horizontal plane. To avoid accumulating twist in the rope, the cowboy/cowgirl rolls it between his/her thumb and forefinger while spinning it. The configuration of the rope is stationary in a reference frame that rotates with the hand. Exploiting this fact we derive a dynamical ``string'' model in which line tension is balanced by the centrifugal force and the rope's weight. Using a numerical continuation method, we calculate the steady shapes of a lasso with a fixed honda, examine their stability, and determine a bifurcation diagram exhibiting coat-hanger shapes and whirling modes in addition to flat loops. We then extend the model to a honda with finite sliding friction by using matched asymptotic expansions to determine the structure of the boundary layer where bending forces are significant, thereby obtaining a macroscopic criterion for frictional sliding of the honda. We compare our theoretical results with high-speed videos of a professional trick roper and experiments performed using a laboratory ``robo-cowboy.'' Finally, we conclude with a practical guidance on how to spin a lasso in the air based on the results of our analysis. [Preview Abstract] |
Session G15: Membranes: Biological and Synthetic
Sponsoring Units: DBIOChair: Mark Bowick, Syracuse University
Room: 304
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G15.00001: Dynamical Clustering and the Origin of Raft-like Structures in a Model Lipid Membrane Francis Starr We investigate the dynamical heterogeneity of a model single-component lipid membrane using simulations of a coarse-grained representation of lipid molecules. In the liquid-ordered (LO) phase, lipid diffusion is hindered by the transient trapping of molecules by their neighbors, giving rise to two distinct mobility groups: low-mobility lipids which are temporarily ``caged'', and lipids with displacements on the scale of the intermolecular spacing. The lipid molecules within these distinct mobility states cluster, giving rise to transient ``islands'' of enhanced mobility having the size and time scale expected for lipid ``rafts''. These clusters are strikingly similar to the dynamical clusters found in glass-forming fluids, and distinct from phase-separation clusters. Such dynamic heterogeneity is ubiquitous in disordered condensed-phase systems. Thus, we hypothesize that rafts may originate from this universal mechanism, explaining why raft-like regions should arise, regardless of lipid structural or compositional details. This perspective provides a new approach to understand membrane transport. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G15.00002: Undulatory motion of bilayer membrane structures and fluctuation amplitudes on SANS/SAXS profile for large membrane wavelength Takumi Hawa, Victor Lee In this study, we have extended our previous investigation of the effect of bilayer membrane structures and fluctuation amplitude on small angles scatter (SAS) profiles to the cases with undulatory bending motions using 2D harmonic motion model. We consider the case that the aspect ratio (AR) = membrane wavelength (Y) / membrane thickness is larger than 5. Thicknesses of the bilayer membrane are accurately estimated based on the formula we have derived in our previous study. We have identified the shifting direction of the peak location, q$_{\mathrm{peak}}$, which is distinctively different from the cases of AR $<$ 1. We also identified a form factor q$_Y$, from which the value of Y (and consequently AR also) can be estimated accurately. We also found the relationship among AR, q$_Y$, and the amplitude and developed a chart which can be used to estimate the value of the membrane amplitude. Even though the applicability of the new chart is limited and some assumptions may be required, the values obtained through the new chart showed good agreements to the experimental results obtained in the past experiments. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G15.00003: The Cooperative Behaviour of $\alpha $-Helical Antimicrobial Peptides in Different Environments Marco Pinna, Janping Wang, Manuela Mura, Yuhua Zhou, Andrei Zvelindovsky, Sarah Dennison, David Phoenix A systematic analysis of the antimicrobial peptides (AMPs) cooperative action is performed by means of a full atomistic molecular dynamics simulation. The following peptide analogues: Aurein 2.5-COOH, Aurein 2.6-COOH and Aurein 3.1--COOH are investigated in different environments including aqueous solution, trifluoroethanol (TFE), palmitoyloleoylphosphatidylethanolamine (POPE), and palmitoyloleoylphosphatidylglycerol (POPG) lipid bilayers. Simulations conducted for monomer and trimer peptide highlight the importance of the cooperative behaviour and reveal the different mechanisms of antimicrobial peptides action in different lipid bilayers. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G15.00004: Line Tension of Multi-Component Bilayer Membranes Ashkan Dehghan, Kyle Pastor, An-Chang Shi The line tension of self-assembled multi-component bilayers is investigated using self-consistent field theory. The bilayer membranes are self-assembled from amphiphilic AB/ED diblock copolymers in a solvent modelled as C-homopolymers. We examine the effects of copolymer composition, geometrical shape and interactions on the line tension of bilayer membranes. Specifically, we calculate the line tension for membranes composed of symmetric, cone and inverse-cone shape amphiphilic molecules with neutral and/or repulsive E/D interactions. We show that an increase in the concentration of the cone shaped species results in a decrease in the pore line tension. In contrast, we found that adding inverse-cone shaped copolymers results in an increase in the line tension of the bilayer membrane. By examining the density profile of the membrane we determined that the different amphiphilic species phase separate within the membrane according to their local curvature. Our theoretical predictions are shown to be consistent with available experiments and theories. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G15.00005: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G15.00006: Molecular Transport Studies Through Unsupported Lipid Membranes William Rock, Sapun Parekh, Mischa Bonn Dendrimers, spherical polymeric nanoparticles made from branched monomers around a central core, show great promise as drug delivery vehicles. Dendrimer size, core contents, and surface functionality can be synthetically tuned, providing unprecedented versatility. Polyamidoamine (PAMAM) dendrimers have been shown to enter cells; however, questions remain about their biophysical interactions with the cell membrane, specifically about the presence and size of transient pores. We monitor dendrimer-lipid bilayer interactions using unsupported black lipid membranes (BLMs) as model cell membranes. Custom bilayer slides contain two vertically stacked aqueous chambers separated by a 25 $\mu$m Teflon sheet with a 120 $\mu$m aperture where the bilayer is formed. We vary the composition of model membranes (cholesterol content and lipid phase) to create biomimetic systems and study the interaction of PAMAM G6 and G3 dendrimers with these bilayers. Dendrimers, dextran cargo, and bilayers are monitored and quantified using time-lapse fluorescence imaging. Electrical capacitance measurements are simultaneously recorded to determine if the membrane is porous, and the pore size is deduced by monitoring transport of fluorescent dextrans of increasing molecular weight. These experiments shed light on the importance of cholesterol content and lipid phase on the interaction of dendrimer nanoparticles with membranes. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G15.00007: Diffusion of water on supported bilayer lipid membranes Andrew Miskowiec, Zachary Buck, Helmut Kaiser, Haskell Taub, Flemming Hansen, Madhusudan Tyagi, Souleymane Diallo, Eugene Mamontov, Kenneth Herwig We compare the temperature dependence of quasielastic incoherent neutron scattering from water associated with fully hydrated single bilayers of the charge-neutral DMPC (dimyristoylphosphocholine) lipid supported on a SiO$_{\mathrm{2}}$-coated silicon substrate to that of water in proximity to a similarly supported \textit{anionic} DMPG (dimyristoylphosphoglycerol) bilayer. The diffusion constant of water near the DMPC membrane decreases on cooling in two step-like transitions: 1) at the freezing point of bulk-like water (267 K); and 2) at a second transition of unknown origin at 261 K [2]. In contrast, we observe on cooling only a continuous decrease in the diffusion constant of water in proximity to the DMPG membrane. Water remains mobile to lower temperature on the anionic membrane; however, its diffusion is systematically slower than on DMPC in the temperature range above 255 K where water is mobile in both systems. [2] M. Bai \textit{et al}., Europhys. Lett. \textbf{98}, 48006 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G15.00008: Wetting and freezing of water on supported bilayer lipid membranes Zachary Buck, Andrew Miskowiec, Mia Brown, Helmut Kaiser, Gavin King, Renee Jiji, Jason Cooley, Haskell Taub, Flemming Hansen, Madhusudan Tyagi, Souleymane Diallo, Eugene Mamontov, Kenneth Herwig Temperature-dependent elastic incoherent neutron scattering shows qualitatively different behavior for water associated with single bilayers of the charge-neutral DMPC (dimyristoylphosphocholine) lipid than for the anionic DMPG (dimyristoylphosphoglycerol) bilayer supported on an SiO$_{\mathrm{2}}$-coated silicon substrate. For the neutral DMPC membrane, the membrane-associated water shows a step-like freezing transition somewhat below the bulk freezing point followed by a continuous freezing behavior and, on heating, a step-like melting transition at the bulk melting point of 273 K [2]. In contrast, water near the anionic DMPG membrane shows only continuous freezing that extends to much lower temperatures than for DMPC and continuous melting that is complete well below the bulk melting point. We suggest that these results may be explained by a film-like water structure in the DMPG case owing to the hydrophilic nature of the membrane surface, while most of the water in the DMPC system is bulk-like and dewets from this more hydrophobic membrane surface. [2] M. Bai \textit{et al}., Europhys. Lett. \textbf{98}, 48006 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G15.00009: Euler buckling, membrane corrugation and pore formation induced by antimicrobial peptide Leonardo Golubovic, Lianghui Gao, Licui Chen, Nana Jia, Weihai Fang Antimicrobial peptides serve as defense weapons against bacteria. They are secreted by organisms of plants and animals and have a wide variety in composition and structure. In this study, we theoretically explore the effects of the antimicrobial peptides on the lipid bilayer membrane by using analytic arguments and the coarse grained dissipative particle dynamics simulations. We study peptide/lipid membrane complexes by considering peptides with various structure, hydrophobicity and peptide/lipid interaction strength. The role of lipid/water interaction is also discussed. We discuss a rich variety of membrane morphological changes induced by peptides, such as pore formation, membrane corrugation and Euler buckling. Such buckled membrane states have been indeed seen in a number of experiments with bacteria affected by peptide, yet this is the first theoretical study addressing these phenomena more deeply. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G15.00010: Logarithmic Domain Growth in Ternary Mixture Lipid Multilayer Systems with Cholesterol Yicong Ma, Sajal Ghosh, David DiLena, Laura Connelly, Nirav Patel, Fernando Teran Arce, Ratnesh Lal, Sunil Sinha Cholesterol, one essential constituent of the cell membrane, is shown to perform an important function in maintaining the membrane integrity and fluidity. However, the role of cholesterol in mixed membrane phase behavior is still not completely understood. In our study of model membrane multilayer systems consisting of 1:1 DPPC/DOPC with 0 to 30{\%} Cholesterol, we have studied the kinetics of domain growth as a function of time. X-ray diffraction measurements reveal two different time scales of domain growth: fast growth at small time scale of minutes, and slow growth at large time scale which exhibits a logarithmic growth law. This logarithmic growth law may indicate that the barriers to domain growth increase linearly with the domain size [1,2]. Our detailed analysis of domain lipid structure from electron density profiles with different cholesterol concentrations, and comparison of the growth law time constants should shed new light on the effects of cholesterol on membrane domains. [1] J.D Shore, M.Holzer, and J.P.Sethna, Phys. Rev. B 46, 11376 (1992). [2] Z. W. Lai, G. F. Mazenko, and O. T. Valls, Phys. Rev. B 37,9481 (1988). [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G15.00011: Measuring Lipid Membrane Viscosity Using Rotational and Translational Tracer Diffusion Tristan Hormel, Sarah Kurihara, Matthew Reyer, Raghuveer Parthasarathy The two-dimensional fluidity of lipid membranes enables the motion of membrane-bound macromolecules and is therefore crucial to biological function. However, current methods of measuring membrane viscosity rely on particular membrane lipid compositions or geometries, making the comparison of different measurements difficult. We address this with a new technique for measuring lipid membrane viscosity, in which determination of both the rotational and translational diffusion coefficients of tracer particles enables quantification of viscosity as well as the effective size of the tracers. This technique is general, and can be applied to different model membrane systems to determine the effects of membrane composition and protein modulation. We present measurements of lipid membrane viscosity for two different lipids with phosphatidylcholine headgroups, finding a surprisingly wide distribution of effective tracer sizes, due presumably to a large variety of couplings to the membrane. We also compare the effective viscosity of two different structures -- black lipid membranes and membrane multilayers -- as well as changes in viscosity induced by peripheral protein binding. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G15.00012: Aspirin Increases the Solubility of Cholesterol in Lipid Membranes Richard Alsop, Matthew Barrett, Sonbo Zheng, Hannah Dies, Maikel Rheinstadter Aspirin (ASA) is often prescribed for patients with high levels of cholesterol for the secondary prevention of myocardial events, a regimen known as the Low-Dose Aspirin Therapy. We have recently shown that Aspirin partitions in lipid bilayers [1]. However, a direct interplay between ASA and cholesterol has not been investigated. Cholesterol is known to insert itself into the membrane in a dispersed state at moderate concentrations (under $\sim$37.5\%) [2] and decrease fluidity of membranes. We prepared model lipid membranes containing varying amounts of both ASA and cholesterol molecules. The structure of the bilayers as a function of ASA and cholesterol concentration was determined using high-resolution X-ray diffraction. At cholesterol levels of more than 40mol\%, immiscible cholesterol plaques formed. Adding ASA to the membranes was found to dissolve the cholesterol plaques, leading to a fluid lipid bilayer structure [3]. We present first direct evidence for an interaction between ASA and cholesterol on the level of the cell membrane. [1] MA Barrett, S Zheng, G Roshankar, {\bf RJ Alsop} {\em et al.} PLoS ONE 7, e34357, 2012 [2] MA Barrett, S Zheng, LA Toppozini, {\bf RJ Alsop} {\em et al.}Soft Matter 9, 9342-9351, 2013 [3] {\bf RJ Alsop} {\em et al.}, submitted [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G15.00013: Formation of Vesicles in Lipid -- Liquid Crystal Colloidal Mixtures Jeffrey Peters, Germano Iannacchione The formation and phase ordering / evolution has been studied in lipid and liquid crystal (LC) colloidal mixtures as a function of LC concentration and thermal history. The lipid used was 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (POPC) while the liquid crystal was pentylcyanobiphenyl (5CB). POPC is a naturally occurring lipid in eukaryotic cell membranes and mimics many of the properties of human cell walls. 5CB is a polar liquid crystal that exhibits a thermodynamically stable orientationally ordered (nematic) state at room temperature. Colloidal dispersions were made at various 5CB and POPC concentrations in water and studied via optical microscopy (phase contrast, confocal, florescence, and cross-polarizing) to probe phase order and evolution as well as by calorimetry to study phase transformations. Very large vesicles were observed to form that appear to use 5CB droplets as scaffolds as well as a unique promotion of lipid crystallization within defect regions of nematic domains. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G15.00014: Topological defects of tetratic liquid-crystal order on a soft spherical surface Yao Li, Han Miao, Hongru Ma, Jeff Chen We model the orientational and positional order of tetratically shaped molecules, each having four-fold structural symmetry, confined on a spherical surface. Our Monte Carlo simulation shows that at a high molecular density, a tetratic orientational order develops in the system, accompanied by eight disclinations arranged in an anticube form on the hard spherical surface. We also consider an elastic-energy model, which consists of both Helfrich and Frank energies for a soft surface; the solution confirms the Monte Carlo study and further predicts the tetratic morphology that can be realized on the surface of a soft vesicle. Assuming that the induced interaction between these disclinations are repulsive, we demonstrate that the anticube structure has a lower free energy than, for example, the cubic structure. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G15.00015: Tetratic order on elastic membranes: defects and geometry Oksana Manyuhina, Mark Bowick One of the fascinating problems in soft condensed matter is finding equilibrium shapes of elastic membranes subjected to additional constraints. It is known that confining nematic liquid crystal (with point symmetry group $D_{\infty h}$) on a surface of a sphere results in topological defects, which influence the overall shape of the closed surface and favour the formation of faceted tetrahedron (topologically equivalent to sphere). Nevertheless, because of the interplay between the local nematic orientational order and the global geometry of the closed membrane, identifying the ground state is not straightforward and requires numerical computation. We propose a phenomenological model to describe the tetratic order formed by particles with point symmetry group $D_{4h}$ on surfaces with various Gaussian curvature. Self-organization of defect patterns as well as buckling instability of membranes are discussed and further connections with other soft matter systems are established. [Preview Abstract] |
Session G16: Focus Session: Soft Matter Perspectives on Protein Self-Assembly I
Sponsoring Units: GSNP DBIO DPOLYChair: Patrick Charbonneau, Duke University
Room: 401
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G16.00001: Self-assembly of Model Microtubules: Shape, Chirality and Twist Mark Stevens, Shengfeng Cheng The efficient and controlled assembly of complex structures from macromolecular building blocks is a critical open question. Microtubules are one example of a biopolymer that possesses characteristics quite distinct from standard synthetic polymers that are derived from its monomer being a protein. In order to understand microtubule assembly and how to design artificial polymers that possess features similar to those of microtubules, we have used molecular dynamics simulations to study the self-assembly of model monomers into a tubule geometry. The self-assembly of free monomers into tubules yields a tubule pitch that often does not match the chirality of the monomer (including achiral monomers). We show that this mismatch occurs because of a twist deformation that brings the lateral interaction sites into alignment when the tubule pitch differs from the monomer chirality. In order to control the tubule pitch by preventing the twist deformation, we employ a lock-and-key interaction and obtain good control of the self-assembled tubule pitch. These results explain some fundamental features of microtubules. We generally find that the control of the assembly is limited, which explains the range of pitch and protofilament number observed in microtubule assembly. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G16.00002: Fibril-based, geometrical microtubule - kinetochore attachments Zsolt Bertalan, Caterina La Porta, Helder Maiato, Stefano Zapperi Mechanical factors involved in regulating the stability of microtubule-kinetochore attachments during cell division are poorly understood. Various aspects of these attachments are essential for proper chromosome segregation. We introduce and simulate a mechanical model of microtubule-kinetochore interactions in which the stability of the attachment is due to the geometrical conformations of curling protofilaments entangled in kinethochore fibrils. The main load of the simulations are done in two dimensions due to the geometric shape of the protofilament curl. However, since the microtubule-kinetochore fibril entanglement is inherently a three dimensional problem, we also model and test the attachment in 3D. The model allows us to reproduce with good accuracy in vitro experimental measurements of the detachment times of yeast kinetochores from MTs under external pulling forces. Numerical simulations also suggest a purely geometrical mechanism that does not require changes in chemical affinities to control the switch between stable and unstable attachments. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G16.00003: Nano-structured metallic amyloid fibril networks Kiersten Batzli, Brian Love Amyloid proteins form high aspect ratio fibrillar structures with great chemical and physical stability under specific conditions. By examining the produced networks as novel materials we can envision uses for these high aspect ratio fibrillar structures. Produced fibril networks can be used as templates for the creation of high surface area metallic meshes that may be of use as catalysts or in electronic applications. We have formed fibrillar networks from porcine insulin and have characterized them by TEM, showing that by varying environmental conditions, such as strain rate, the resulting network morphologies may be influenced. We have used electroless deposition techniques to coat insulin fibrils with platinum to produce metallized networks thought to have high catalytic activity. We will present our experience using these coated fibrils to facilitate the reduction of nitrophenol to aminophenol using UV-visible spectroscopy as a gauge. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G16.00004: Electrostatic effects in collagen fibrillization Svetlana Morozova, Murugappan Muthukumar Using light scattering and AFM techniques, we have measured the kinetics of fibrillization of collagen (pertinent to the vitreous of human eye) as a function of pH and ionic strength. At higher and lower pH, collagen triple-peptides remain stable in solution without fibrillization. At neutral pH, the fibrillization occurs and its growth kinetics is slowed upon either an increase in ionic strength or a decrease in temperature. We present a model, based on polymer crystallization theory, to describe the observed electrostatic nature of collagen assembly. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G16.00005: Surfaces Self-Assembly and Rapid Growth of Amyloid Fibrils Yichih Lin, E. James Petersson, Zahra Fakhraai The mechanism of surface-mediated fibrillization has been considered as a key issue in understanding the origins of the neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases. \textit{In vitro}, amyloid proteins fold through nucleation-elongation process. There is a critical concentration for early nucleating stage. However, some studies indicate that surfaces can modulate the fibril's formation under physiological conditions, even when the concentration is much lower than the critical concentration. Here, we use a label-free procedure to monitor the growth of fibrils across many length scales. We show that near a surface, the fibrillization process appears to bypass the nucleation step and fibrils grow through a self-assembly mechanism instead. We control and measure the pre-fibrillar morphology at different stages of this process on various surfaces. The interplay between the surface concentration and diffusion constant can help identify the detailed mechanisms of surface-mediated fibril growth, which remains largely unexplored. Our works provide a new insight in designing new probes and therapies. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G16.00006: Surface Mediated Protein Disaggregation Mithun Radhakrishna, Sanat K. Kumar Preventing protein aggregation is of both biological and industrial importance. Biologically these aggregates are known to cause amyloid type diseases like Alzheimer's and Parkinson's disease. Protein aggregation leads to reduced activity of the enzymes in industrial applications. Inter-protein interactions between the hydrophobic residues of the protein are known to be the major driving force for protein aggregation. In the current paper we show how surface chemistry and curvature can be tuned to mitigate these inter-protein interactions. Our results calculated in the framework of the Hydrophobic--Polar (HP) lattice model show that, inter-protein interactions can be drastically reduced by increasing the surface hydrophobicity to a critical value corresponding to the adsorption transition of the protein. At this value of surface hydrophobicity, proteins lose inter-protein contacts to gain surface contacts and thus the surface helps in reducing the inter-protein interactions. Further, we show that the adsorption of the proteins inside hydrophobic pores of optimal sizes are most efficient both in reducing inter-protein contacts and simultaneously retaining most of the native-contacts due to strong protein-surface interactions coupled with stabilization due to the confinement. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G16.00007: Dynamic renormalisation group reveals sequential mechanism of the secondary nucleation of proteins Thomas Michaels, Paolo Arosio, Tuomas Knowles Secondary nucleation has emerged as a key process in the self-assembly of amyloid fibrils associated with a number of neurodegenerative disorders. Secondary nucleation conceptually involves both aggregates and monomers, but a variety of ways exist, in which this process may occur. Elucidation of this complex mechanism using experimental data represents a theoretical challenge. A systematic coarse-graining procedure inspired by the renormalisation group is used to bridge the length- and timescale gaps between detailed microscopic descriptions and the processes observed in experiments. Various mechanisms of secondary nucleation are discussed at different levels of coarse graining and compact terms in the master equation are generated, that provide a single-step description of this process. This treatment is general and allows to test assumptions regarding mechanisms at the microscopic level and to filter their effect on the kinetics at the macroscopic scale. By analysing data from the polymerisation of amylin, we conclude that pre-critical nuclei in islet amyloid polypeptides stay attached to the aggregates during the process of secondary nucleation. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G16.00008: The breakup mechanism of biomolecular and colloidal aggregates in a shear flow Breannd\'{a}n \'{O} Conch\'{u}ir, Alessio Zaccone The theory of self-assembly of colloidal particles in shear flow is incomplete. Previous analytical approaches have failed to capture the microscopic interplay between diffusion, shear and intermolecular interactions which controls the aggregates fate in shear. In this work we analytically solved the drift-diffusion equation for the breakup rate of a dimer in flow. Then applying rigidity percolation theory, we found that the lifetime of a generic cluster formed under shear is controlled by the typical lifetime of a single bond in its interior, which in turn depends on the efficiency of the stress transmitted from other bonds in the cluster. We showed that aggregate breakup is a thermally-activated process where the activation energy is controlled by the interplay between intermolecular forces and the shear drift, and where structural parameters determine whether cluster fragmentation or surface erosion prevails. In our latest work, we analyzed floppy modes and nonaffine deformations to derive a lower bound on the fractal dimension df below which aggregates are mechanically unstable, ie. for large aggregates df $\simeq$ 2.4. This theoretical framework is in quantitative agreement with experiments and can be used for population balance modeling of colloidal and protein aggregation. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G16.00009: Mechanical and Assembly Units of Viral Capsids Identified via Quasi-Rigid Domain Decomposition Guido Polles, Giuliana Indelicato, Raffaello Potestio, Paolo Cermelli, Reidun Twarock, Cristian Micheletti Key steps in a viral life-cycle, such as self-assembly of a protective protein container or in some cases also subsequent maturation events, are governed by the interplay of physico-chemical mechanisms involving various spatial and temporal scales. These salient aspects of a viral life cycle are hence well described and rationalised from a mesoscopic perspective. Accordingly, various experimental and computational efforts have been directed towards identifying the fundamental building blocks that are instrumental for the mechanical response, or constitute the assembly units, of a few specific viral shells. Motivated by these earlier studies we introduce and apply a general and efficient computational scheme for identifying the stable domains of a given viral capsid. The method is based on elastic network models and quasi-rigid domain decomposition. It is first applied to a heterogeneous set of well-characterized viruses (CCMV, MS2, STNV, STMV) for which the known mechanical or assembly domains are correctly identified. The validated method is next applied to other viral particles such as L-A, Pariacoto and polyoma viruses, whose fundamental functional domains are still unknown or debated and for which we formulate verifiable predictions. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G16.00010: Viral genome structures, charge, and sequences are optimal for capsid assembly Invited Speaker: Michael Hagan For many viruses, the spontaneous assembly of a capsid shell around the nu-cleic acid (NA) genome is an essential step in the viral life cycle. Capsid formation is a multicomponent, out-of-equilibrium assembly process for which kinetic effects and thermodynamic constraints compete to determine the outcome. Understand-ing how viral components drive highly efficient assembly under these constraints could promote biomedical efforts to block viral propagation, and would elucidate the factors controlling assembly in a wide range of systems containing proteins and polyelectrolytes. This talk will describe coarse-grained models of capsid proteins and NAs with which we investigate the dynamics and thermodynamics of virus assembly. In con-trast to recent theoretical models, we find that capsids spontaneously `overcharge'; that is, the NA length which is kinetically and thermodynamically optimal possess-es a negative charge greater than the positive charge of the capsid. When applied to specific virus capsids, the calculated optimal NA lengths closely correspond to the natural viral genome lengths. These results suggest that the features included in this model (i.e. electrostatics, excluded volume, and NA tertiary structure) play key roles in determining assembly thermodynamics and consequently exert selec-tive pressure on viral evolution. I will then discuss mechanisms by which se-quence-specific interactions between NAs and capsid proteins promote selective encapsidation of the viral genome. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G16.00011: Hierarchical, Self-Similar Structure in Native Squid Pen Fei-Chi Yang, Robert Peters, Hannah Dies, Maikel Rheinstadter Proteins, chitin and keratin form the elementary building blocks of many biomaterials. How these molecules assemble into larger, macroscopic structures with very different properties is the fundamental question we are trying to answer [1]. Squid pen is a transparent backbone inside the squid, which supports the mantle of the squid. The pens show a hierarchical, self-similar structure under the microscope and the AFM with fibers from 500$\mu $m to 0.2$\mu $m in diameter. The chitin molecules form nano-crystallites of monoclinic lattice symmetry surrounded by a protein layer, resulting in $\beta $-chitin nano-fibrils. Signals corresponding to the $\alpha $-coil protein phase and $\beta $-chitin were observed in X-ray experiments \textit{in-situ}. The molecular structure is highly anisotropic with 90{\%} of the $\alpha $--coils and $\beta $-chitin crystallites oriented along the fiber-axis indicating a strong correlation between the structures on millimeters down to the molecular scale [2]. \\[4pt] [1] ``Self-assembly enhances the strength of fibres made from vimentin intermediate filament proteins'', N. Pinto, \textbf{FC. Yang}, \textit{et al}., submitted to \textit{Biomacromolecules}.\\[0pt] [2] ``Hierarchical, Self-Similar Structure in Native Squid Pen'', \textbf{FC. Yang}, \textit{et al.}, submitted to \textit{Royal Society Interface}. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G16.00012: Theoretical model of a soft particle with charged core Dustin Tracy, Anh Phan The numerical and analytical solutions of the electrostatic potentials of soft particles with an ion-permeable charged outer layer and a non-permeable charged core with constant charge densities are found using the Poisson-Boltzmann equations. The charged core is found to significantly alter the local potential within the soft particle, yet it has little effect on the potential outside its particle's boundaries. Previous experimental research into the electrical properties of the MS2 virus agree with these findings. Our results also suggest that there is only a slight change in the potential as the temperature is increased from 290 K to 310 K. The potential profile is found to be significantly affected by the ionic strength in the 1-600 mM range. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G16.00013: Stochastic Interactions of Two Brownian Spheres in the Presence of Depletants Mehdi Karzar-Jeddi, Remco Tuinier, Takashi Taniguchi, Tai-Hsi Fan The pair interactions between hard spheres play an essential role in many processes such as macromolecular crowding, binding, self-assembly of particles, and many chemical and food processes. Here we focus on theoretical analysis of the long-time correlated stochastic motion of two hard spheres in a non-adsorbing polymer solution. The hard spheres are held by hypothetical optical traps. The pair mobility tenser is found using a two-layer approximation with pure solvent in the depletion zone surrounding the particle and uniform polymer solution elsewhere. The resulting mobility computed by the boundary integral analysis is used to define the level of thermal fluctuation. Results show how the mobility and the decay of displacement correlation functions modified by the polymer depletion effect. The attractive osmotic potential increases the auto-correlation of the pair particle motion, while reduces the cross-correlation of the particles. This work gives better understanding of the pair interactions in a suspension of non-adsorbing polymers as an essential step toward many-particle interactions. [Preview Abstract] |
Session G17: Focus Session: Strong Correlations in Systems Far from Equilibrium II
Sponsoring Units: GSNPChair: Uwe Tauber, Virginia Polytechnic Institute and State University
Room: 402
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G17.00001: Multistable phase patterns in finite oscillator networks Daniel Goldstein Recent experiments on spatially extend arrays of droplets containing Belousov-Zhabotinsky reactants have shown a rich variety of spatio-temporal patterns. Motivated by this experimental set up, we study a simple model of chemical oscillators in the highly nonlinear excitable regime in order to gain insight into the mechanism giving rise to the observed multistable attractors. By allowing intrinsic fluctuations to influence a simple activator inhibitor model, switching between stable attractors is observed. When coupled, these two attractors have different preferred phase synchronizations, leading to complex behavior. We study rings of coupled oscillators and observe a rich array of oscillating patterns. We characterize the different modes of oscillation in the mean-field model and compare those to the oscillations observed in stochastic simulations. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G17.00002: Synchronization for Systems with Spatially Correlated Coupling Huan-Yu Kuo, Kuo-An Wu Synchronization phenomenon in systems of large populations is of great interest in physical, biological, chemical, and social systems. The Kuramoto model describes elements as coupled oscillators whose natural frequencies are drawn from certain prescribed distribution. For globally coupled oscillators, as the coupling strength exceeds a certain threshold, part of the oscillators spontaneously synchronizes while others remain incoherent. However, the interaction between oscillators in many biological and physical systems depends on the distance between two oscillators. We analyze the Kuramoto model with spatial correlated coupling, and we find the existence of a universal critical coupling strength, beyond which a phase transition occurs for certain spatial correlated coupling. We will discuss the relation between the universality of critical coupling strength and the form of specific spatial couplings. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G17.00003: A Subpopulation Approach to the Finite Kuramoto Model David Mertens The Kuramoto model is the canonical model for studying spontaneous collective synchronization, notable because the functional form of the order parameter in its second order transition can be calculated analytically. Since its introduction nearly four decades ago, nearly all of the work on the underlying model has focused on the behavior of the order parameter for very large populations. Surprisingly little interest has been paid to small, discrete populations. In this talk I will introduce a new approach to analyzing the finite Kuramoto model based on a remarkably simple resumation of the interaction terms. This representation of the Kuramoto model is mathematically identical to the original Kuramoto model. However, rather than frame the model as an all-to-all interaction between oscillators or an interaction between oscillators and a mean field, this representation suggests a model of interacting subpopulations. This approach provides a much more intuitive starting point for making meaningful approximations and calculating important transitions for specific finite populations, as I will demonstrate. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G17.00004: Contact process on generalized Fibonacci chains: infinite-modulation criticality and double-log periodic oscillations Hatem Barghathi, David Nozadze, Thomas Vojta We study the nonequilibrium phase transition of the contact process with aperiodic transition rates using a real-space renormalization group as well as Monte-Carlo simulations. The transition rates are modulated according to the generalized Fibonacci sequences defined by the inflation rules A $\to$ AB$^k$ and B $\to$ A. For $k=1$ and 2, the aperiodic fluctuations are irrelevant, and the nonequilibrium transition is in the clean directed percolation universality class. For $k\ge 3$, the aperiodic fluctuations are relevant. We develop a complete theory of the resulting unconventional ``infinite-modulation'' critical point which is characterized by activated dynamical scaling. Moreover, observables such as the survival probability and the size of the active cloud display pronounced double-log periodic oscillations in time which reflect the discrete scale invariance of the aperiodic chains. We illustrate our theory by extensive numerical results, and we discuss relations to phase transitions in other quasiperiodic systems. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G17.00005: Dynamic Phase Transitions in Driven Cyclic Kinetic Networks Todd Gingrich, Suriyanarayanan Vaikuntanathan, Phillip Geissler Many physical processes can be modeled by Markovian rate processes. When detailed balance is broken, as is generically the case in biological processes, the dynamics exhibits nonvanishing fluxes around cycles and produces entropy. We demonstrate that a particular class of kinetic networks, those with a nearly periodic, pseudo-one-dimensional cyclical character, yield a nontrivial statistics of the large deviations in the observed fluxes. This behavior can be understood analytically in the limit of large networks, where we demonstrate the existence of a dynamic phase transition. The observation suggests that interesting, and potentially useful, large dynamical fluctuations are common even in rate processes with a single degree of freedom. As the analysis holds for networks driven out of equilibrium, potential application to biologically relevant networks is especially intriguing. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G17.00006: Non-equilibrium transitions and critical points in a two-temperature Ising model Nick Borchers, R.K.P. Zia, Michel Pleimling From complex biological systems to a simple simmering pot, thermodynamic systems held out of equilibrium are exceedingly common in nature. Despite this, a general theory to describe these types of phenomena remains elusive. In this talk, we further explore a simple two-temperature modification of the venerable Ising model in hopes of shedding some light on these issues. Of particular interest is the ``freezing by heating'' transition, and a range of larger system sizes are considered in the hopes of determining the transitions critical temperature and exponents. While this transition initially appeared as second-order, evidence suggesting a possible weak first-order nature obscured by finite size effects will also be explored. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G17.00007: Exploring universal scaling laws far from equilibrium with turbulent liquid crystal Invited Speaker: Kazumasa A. Takeuchi Recent theoretical progress has revealed a variety of universal scaling laws describing various scale-invariant phenomena out of equilibrium, but even the most basic and important of these developments had largely remained without complete experimental verification [1,2]. Here, I show that chaotic convection of electrically driven nematic liquid crystal is an ideal system to overcome past difficulties, which allows thorough experimental tests of theoretical predictions and beyond. First I present the route to turbulence in the electroconvection, focusing in particular on the transition between two regimes of spatiotemporal chaos, called the dynamic scattering modes (DSM) 1 and 2. This transition is characterized by spatiotemporal intermittency, where DSM2 patches randomly migrate, coalesce, and sometimes disappear. Measuring both static and dynamic critical behavior, we identified the directed percolation universality class [3], which is theoretically known as the most fundamental class for absorbing-state phase transitions [1]. We also studied the DSM2 regime under higher applied voltage, where DSM2 domains grow with fluctuating interfaces. Measuring how the interfaces roughen in the course of time, we found evidence for the scaling laws of the Kardar-Parisi-Zhang class [4], the prototypical class for stochastic growing interfaces [2]. Remarkably, fluctuations in the interface positions are found to exhibit the largest-eigenvalue distribution of Gaussian random matrices [4], indicating universality of recent rigorous results for solvable models [5]. The distribution is classified into a few universality subclasses according to the global shape of the interface, or to the initial condition. I also discuss some open problems raised by the experiment [4] on this universality beyond the scaling exponents.\\[4pt] [1] H. Hinrichsen, Adv. Phys. \textbf{49}, 815-958 (2000).\\[0pt] [2] A.-L. Barab\'asi and H. E. Stanley, \textit{Fractal Concepts in Surface Growth}, Cambridge Univ. Press (Cambridge, 1995).\\[0pt] [3] K. A. Takeuchi \textit{et al.}, Phys. Rev. Lett. \textbf{99}, 234503 (2007); Phys. Rev. E \textbf{80}, 051116 (2009).\\[0pt] [4] K. A. Takeuchi and M. Sano, Phys. Rev. Lett. \textbf{104}, 230601 (2010); K. A. Takeuchi \textit{et al.}, Sci. Rep. \textbf{1}, 34 (2011); K. A. Takeuchi and M. Sano, J. Stat. Phys. \textbf{147}, 853-890 (2012).\\[0pt] [5] For reviews, see, T. Kriecherbauer and J. Krug, J. Phys. A \textbf{43}, 403001 (2010); T. Sasamoto and H. Spohn, J. Stat. Mech. (\textbf{2010}), P11013; I. Corwin, Random Matrices: Theory and Applications \textbf{1}, 1130001 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G17.00008: A friction driven Brownian ratchet Alberto Petri, Andrea Gnoli, Fergal Dalton, Giacomo Gradenigo, Giorgio Pontuale, Alessandro Sarracino, Andrea Puglisi Exploiting thermal fluctuations to produce mechanical work requires statistical non-equilibrium conditions. We propose a new mechanism where an asymmetric wheel in a thermal bath exhibits a preferential direction of rotation because of the Coulomb friction at solid-on-solid contacts. The presence of a net drift induced by friction is demonstrated by numerical simulations and analytical calculations. If the thermal bath is replaced by a granular gas, the well-known granular ratchet effect also occurs, and becomes dominant at high collision rates. Depending on the wheel shape, the granular medium can act in opposite direction with respect to the friction-induced torque, resulting in the inversion of the ratchet motion as the collision rate increases. Both these ratchet effects and the predicted inversion are observed in the novel granular ratchet that we have realized experimentally (A.Gnoli et al., Phys.Rev. Lett. 110, 120601 (2013)). This also suggests the possibility of micro and sub-micrometer Brownian motors in equilibrium fluids, based purely upon nano-friction. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G17.00009: Magnetic friction between a Potts wedge and a Potts block in three dimensions Linjun Li, Michel Pleimling Magnetic systems, whose surfaces are coupled by boundary spins, experience magnetic friction if one system is moving with a relative velocity along the coupled surface. In our current research, we focus on systems consisting of one three-dimensional (3D) magnetic Potts wedge and one 3D magnetic Potts block. For cases where the total number of Potts states is equal to 2 or 9, we systematically study the effects of different interface coupling strengths, relative velocities, and wedge sizes, using numerical simulations. We find that the magnetic friction between the wedge tip and the block surface can change the local magnetic fluctuations on the surface of the block as well as at the tip of the wedge. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G17.00010: Equilibrium-like phase transition of a dynamic system Ming Han, Jing Yan, Steve Granick, Erik Luijten Dynamic systems are considered to be intrinsically different from systems in thermal equilibrium. Despite this fundamental dichotomy, here we demonstrate that a non-equilibrium, fully dynamical system can display behavior that constitutes a complete analogy to thermal equilibrium phase behavior. This dynamical system, consisting of Janus colloids strongly controlled by external fields and over-damped by a viscous solvent, phase separates like a binary fluid mixture, with a coexistence curve separating mixed and demixed regimes and a critical point that we demonstrate to belong to the 2D Ising universality class. Within the coexistence curve, we locate the spinodal curve that separates spinodal decomposition from nucleation and growth. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G17.00011: Functional Representation and Response Behavior of Aging Anomalous Diffusion Processes Stephan Eule The functional representation of stochastic processes provides a powerful method to calculate average values of path dependent observables. Here, the functional representation of Continuous Time Random Walks (CTRWs) and Fractional Fokker-Planck Equations is presented. This formulation, which is based on an alternative formulation of CTRWs, is then used to tackle the delicate and open problem of calculating the response of a CTRW to an external time-dependent perturbation. For the fractional Ornstein-Uhlenbeck process, the response function is calculated explicitly. It is proven that the fluctuation-dissipation theorem holds when the process is perturbed away from equilibrium. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G17.00012: Comparing Record Dynamics Predictions with Simulations and Experiments of Aging Colloids Stefan Boettcher, Nikolaj Becker, Paolo Sibani We describe the spontaneous off-equilibrium relaxation process known as aging in a simple, real-space model: Kinetic constraints bind on-lattice particles together in ``clusters,'' where a phenomenological function of size controls their lifetime. But once a cluster breaks down, its particles can move independently in space, a process akin to ``cage breaking,'' to join other clusters.\footnote{S. Boettcher and P. Sibani, JPCM, \textbf{23}, 065103 (2011);} Known properties of glassy systems \emph{emerge}, such as spatial heterogeneity and record dynamics. Here we compare our simple model with recent molecular dynamics studies of hard-sphere colloids.\footnote{D. El Masri, L. Berthier, and L. Cipelletti, PRE, \textbf{82}, 031503 (2010).} We find agreement with the scaling properties of the particles mean square displacement, and the aging properties of the interface energy, of the intermediate scattering function, and of the probability density function of the particle displacements occurring within different time windows. These properties are related to an underlying Poisson process which describes the salient events or quakes which correspond to the break up of clusters and give a coarse-grained description of the model dynamics, confirmed by re-analysis of experimental data. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G17.00013: Dynamics of 2D Ising Model in linearly varying magnetic field Na Xu, Cheng-Wei Liu, Anatoli Polkovnikov, Anders Sandvik We consider non-equilibrium dynamics of systems driven out of equilibrium at some finite rate near phase transitions. In previous work [1] on systems with varying temperature, scaling behaviors have been tested in great detail. Here with Monte Carlo simulations, we investigate the 2D Ising Model with linearly varying magnetic field and demonstrate the applicability of similar scaling functions when approaching the critical point. Moreover, we have found an interesting power-law scaling behavior in this system also below the critical temperature (even close to T$=$0). [1]Cheng-Wei Liu, Anatoli Polkovnikov, Anders W. Sandvik, arXiv:1310.6327 (2013) [Preview Abstract] |
Session G18: Focus Session: Liquid Crystals, Nano to Meso Scale Structure in Ordered Matter and Liquid Crystal II: Mostly Smectic and Chromonics
Sponsoring Units: DCMP GSNP DPOLYChair: Luz J. Martinez-Miranda, University of Maryland, College Park
Room: 403
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G18.00001: Diastereomeric domains formed by chiral liquid crystals confined in a network of helical nanofilaments Michael Tuchband, Dong Chen, Balazs Horanyi, Eva Korblova, David Walba, Joseph Maclennan, Matthew Glaser, Noel Clark Mixtures of the bent-core liquid crystal material NOBOW with guest mesogens are well dispersed in the isotropic phase. Upon cooling, the NOBOW forms B4 helical nanofilaments which often nucleate and phase-separate directly from the isotropic melt, forming locally homochiral dendritic networks which act as a porous medium of large internal area. The guest material is then confined to the nanoscale interstitial volumes between the twisted filaments. A typical cell contains a conglomerate of independently nucleated left- and right-handed B4 domains many tens of microns across. Polarized optical microscopy reveals that chiral guest liquid crystal materials with a specific twist of the director form optically distinguishable diastereomeric domains in the left- and right-handed chiral domains. Due to the different twist environments of the pores in the left- and right-handed helical nanofilament networks, the molecular arrangements of chiral guest material and the corresponding changes of birefringence in the left- and right-handed chiral domains differ as the chiral guest transitions from isotropic to cholesteric, and then to smectic. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G18.00002: Effective theory and simulations of Smectic-A liquid crystals Danilo Liarte, Matthew Bierbaum, Muxin Zhang, Brian Leahy, Itai Cohen, James Sethna We present simulation results of focal conic structures in smectics-A. Smectic-A is a liquid-crystalline state of matter, with long-range orientational order, and a layered structure with liquid-like order within each layer. In experiments it exhibits striking focal conic domains - defect structures forming beautiful ellipses and hyperbolas. We derive dynamical equations for a vector field that is parallel to the layer normal, and whose size is related to the compression of the layers. In our simulations, focal conic structures spontaneously emerge from random initial configurations, and are characterized by several visualization tools that we developed. We study both coarsening and the effects of shear stress in this system, which are compared with experiments performed in Cornell. We also discuss generalizations to include the dynamics of dislocations. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G18.00003: Field-Induced Alignment of Polar Bent-Ccore Smectic A Liquid Crystals Yongqiang Shen, Lisa Goodhew, Renfan Shao, Joseph Maclennan, Noel Clark, Per Rudquist The SmAP$_{\mathrm{F}}$ phase is a promising phase modulator mode. To use the SmAP$_{\mathrm{F}}$ materials for applications, we need to obtain uniform, large-area alignment of the samples. However, bent-core liquid crystals are notoriously difficult to align with conventional surface treatment methods because most of them have no nematic phase. We have developed a powerful, new method using in-plane applied electric fields that allows us to create a perfect bookshelf alignment of orthogonal bent-core smectics. By using an interdigitated, finger-like electrode arrangement on one of the cell surfaces, we can align the materials by applying in-plane electric fields. This stripe geometry, which produces curved field lines, allows for only one smectic layer orientation, normal both to the cell walls and to the finger electrodes. After alignment, the cell can be operated in the conventional way by connecting the finger electrodes together to make one effective electrode, opposing continuous, common electrode on the opposite side of the cell. This alignment method opens up the use of these materials in perfectly aligned cells for both amplitude and phase-only modulation applications. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G18.00004: Rich Variety of Smectic Phases in an Achiral Bent-Core Liquid Crystal Renfan Shao, Cheol Park, Joseph Maclennan, Carlson Tschierske, Noel Clark The mesomorphic properties of an achiral bent-core liquid crystal (PAL30) have been recharacterized using polarizing optical microscopy, electro-optic response measurements and freely-suspended films. We find the phase sequence on cooling to be: I - SmA - SmCAPA' - SmCAPA --SmAPA -- Crystal , and see no evidence of the orthogonal SmAPR and SmAP$\alpha $ phases reported previously [Y.P. Panarin et al., Phys. Rev. Lett. 107,247801,2011]. The tilted SmCAPA' and SmCAPA phases show novel `tiger' pattern and stripe textures in the presence of applied electric field. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G18.00005: Double twist helical nanofilaments in bent-core liquid crystals Cuiyu Zhang, Nicholas Diorio, Oleg D Lavrentovich, Antal Jakli Cryo-TEM observations on 40-150 nm films of four bent-core liquid crystal materials in their helical nanofilament (HNF) phase show that the filaments get deformed near the substrate, and the subsequent arrays of nanofilaments are not parallel, but twisted with respect to each other. The effect can explain the mysterious properties of the HNF materials, such as structural color and ambidextrous optical activity. The observed double twist structure was not expected in the previous models of this phase. Being principally different from the packing of molecules in the twist grain boundary (TGB) and blue (BP) phases, the double-twist structure of HNF expands the rich word of nanostructured organic materials. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G18.00006: Liquid Crystal Phase Transitions and Defects to Sort and Soft-Assemble Microstructures Andrea Rodarte, L.S. Hirst, S. Ghosh The isotropic phase transition of thermotropic nematic liquid crystal (LC) doped with low concentrations of quantum dots (QDs) has previously been used to create ordered clusters of QDs centered at the LC defect points [1]. This process can be exploited to create ordered columns of QD clusters using a micropillar template. In addition, the capping agents on the QDs can be modified to liquid crystal like mesogenic ligands, allowing for better dispersion in the LC host material. At high concentrations of these LC-QDs, we observe a fluid-fluid phase separation in which the functionalized QDs create nematic droplets when dispersed in 4?-Pentyl-4-biphenylcarbonitrile (5CB). Defects and phase separation may be used to sort nanoparticles and create dynamic microstructured assemblies, taking one step closer to soft assembled metamaterials.\\[4pt] [1] A. L. Rodarte, R. J. Pandolfi, S. Ghosh and L. S. Hirst, J. Mat. Chem. C, 1, 5527, 2013. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G18.00007: Light guiding and lasing in Smectic-A liquid crystal fibers Venkata Subba Rao Jampani, Karthik Reddy Peddireddy, Shashi Thutupalli, Christian Bahr, Igor Musevic, Stephan Herminghaus We demonstrate for the first time the optical applications of self-assembled smectic-A liquid crystal fibers in an aqueous micellar medium. These fibers consist of smectic-A layers arranged in a cylindrical fashion so that inevitably a topological line defect exists along the core of the fiber. Light guiding through the fibers and Whispering Gallery Mode (WGM) lasing in a plane perpendicular to the fiber are shown. The light guiding as well as the lasing threshold significantly dependent on the polarization of the excitation beam. The observed threshold for WGM lasing is very low ($\sim$ 75 $\mu$ J/cm$^{2})$ when the pump beam polarization is perpendicular to the direction of the laser dye alignment. Further, these fibers are smooth and flexible and can be manipulated with laser tweezers demonstrating a promising approach for realization of soft photonic circuits. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G18.00008: An upper-bound on the discontinuity at the smectic A-nematic phase transition in octylcyanobiphenyl (8CB): A high-resolution birefringence study Mehmet Can \c{C}etinkaya, Sevtap Yildiz, Haluk Ozbek, Patricia Losada-P\'erez, Jan Leys, Jan Thoen Although the nematic-smectic $A$ ($N$-Sm$A)$ transition in liquid crystals (LCs) has been extensively studied, it is still quite controversial. Most theoretical studies predict that the transition should be of 3-dimensional (3D) XY type. However, the experimental results to date have not established a clear case of 3D \textit{XY} universality. Halperin, Lubensky and Ma (HLM) predicted that the crossover should always make the $N$-Sm$A$ transition first order with very small latent heats. HLM-type first order N-Sm$A$ transitions has been evidenced by ASC data, but inconsistencies remain in reported discontinuity values. Of particular relevance is pure 8CB LC with an estimated HLM contribution substantially smaller than the upper limit for the latent heat from ASC. We carried out high-resolution birefringence measurements near the N-Sm$A $transition of 8CB LC. We find that the Sm$A$-$N $phase transition is continuous. For a possible discontinuity in the $N$ order parameter $S(T)$ at $T_{AN}$ we obtain an upper limit of 0.0002, which is consistent with the ASC latent upper limit and HLM theory. The temperature derivative of $S(T)$ exhibits a power law divergence with a critical exponent that is consistent with $\alpha $ $=$ 0.31 $\pm$ 0.03 obtained from ASC. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G18.00009: Myelin structures formed by thermotropic smectic liquid crystals Karthik Reddy Peddireddy, Pramoda Kumar, Shashi Thutupalli, Stephan Herminghaus, Christian Bahr We report on transient structures, formed by thermotropic smectic-A liquid crystals, resembling the myelin figures of lyotropic lamellar liquid crystals. The thermotropic myelin structures form during the solubilization of a smectic-A droplet in an aqueous phase containing a cationic surfactant at concentrations above the critical micelle concentration. Similar to the lyotropic myelin figures, the thermotropic myelins appear in an optical microscope as flexible tube-like structures growing at the smectic/aqueous interface. Polarizing microscopy and confocal fluorescence microscopy show that the smectic layers are parallel to the tube surface and form a cylindrically bent arrangement around a central line defect in the tube. We study the growth behavior of this new type of myelins and discuss similarities and differences to the classical lyotropic myelin figures. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G18.00010: Thermotropic Liquid Crystalline Side Chain Elastomers David Thomas, Matthew Cardarelli, Peggy Cebe, Badel Mbanga, Timothy Atherton, Antoni Sanchez-Ferrer Nematic Liquid Crystal Elastomers (NLCE) are lightly cross-linked polymeric materials that exhibit rubber elasticity and liquid-crystalline orientational order. We investigated the thermal response and microstructure of side-chain NLCEs using real-time synchrotron wide-angle X-ray scattering (WAXS) and optical ellipsometry. During thermal treatment, the material displayed a highly anisotropic fiber-like diffraction pattern comprising narrow equatorial reflections characteristic of the nematic state. A fully reversible nematic to isotropic transition was observed in WAXS upon heating and cooling. The transition temperature from WAXS was correlated with thermal properties, using differential scanning calorimetry, and with optical properties, using polarizing optical microscopy and transmission ellipsometry. By transmission ellipsometry, with the sample in the low temperature nematic phase, we were able to observe the effect of mechanical stimuli (tensile drawing) on the degree of orientational order. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G18.00011: Lyotropic Chromonic Liquid Crystal Droplets, Faceted and Squeezed Zoey Davidson, Joonwoo Jeong, Matthew Lohr, Peter Collings, Tom Lubensky, Arjun Yodh We report on the structures of lyotropic chromonic liquid crystal (LCLC) droplets of spherical, spherocylinder, and faceted shapes dispersed in a background oil phase. The LCLCs phase varies as a function of the mesogen concentration, which in turn changes the elastic properties and the resultant liquid crystal structure. The various director configurations are investigated by brightfield and polarized optical microscopy and are compared with Jones matrix calculations based on model director configurations. In the nematic phase of the LCLC, the spherical droplets exhibit a twisted bipolar configuration with large chiral symmetry breaking as a result of the small twist elastic modulus. As mesogen concentration increases, the LCLCs enter a columnar phase that energetically prefers a pure bend structure and develops facets at high concentrations. Further, we create spherocylinders to study the effects of droplet shape on defect patterns by squeezing droplets into oil-filled capillaries. The observed director configurations reveal a localization of energy of the director field near the topological defects, and they are understood theoretically to be the result of the LCLC's giant elastic anisotropy. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G18.00012: Phase behavior of chromonic liquid crystal mixtures of Sunset Yellow and Disodium Cromoglycate Akihiro Yamaguchi, Gregory Smith, Youngwoo Yi, Charles Xu, Silvia Biffi, Francesca Serra, Tommaso Bellini, Noel Clark Chromonic liquid crystals (CLCs) are formed when planar molecules dissolved in water stack into rod-like aggregates that can order as liquid crystals. Isotropic, nematic, and M-phases can be observed depending on the degree of molecular orientational and positional order by variation of the CLC concentration. We focused on mixtures of two well-known CLCs, Sunset Yellow, a food dye, and disodium cromoglycate (DSCG), an asthma medication. In order to study the phase behaviors of these mixtures, we observed their textures in glass cells and capillaries using polarized light microscopy. We report here a ternary phase diagram describing the complete phase behavior of the CLC mixtures. We observed a variety of phase behaviors depending on species ratio and concentration. In the isotropic phase, no clear phase separation of the two dyes was observed, while separation did occur in many nematic and M-phase combinations. We will also describe phase observations made using a light spectroscopy and bulk centrifugal partitioning. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G18.00013: Chirality Amplification in Tactoids of Lyotropic Chromonic Liquid Crystals Chenhui Peng, Oleg Lavrentovich We demonstrate an effective chirality amplification based on the long-range forces, extending over the scales of tens of micrometers, much larger than the single molecule (nanometer) scale. The mechanism is rooted in the long-range elastic nature of orientational order in lyotropic chromonic liquid crystals (LCLCs) that represent water solutions of achiral disc-like molecules. Minute quantities of chiral molecules such as amino acid L-alanine and limonene added to the droplets of LCLC lead to chiral amplification characterized by an increase of optical activity by a factor of 10$^{3}$ -- 10$^{4}$. This effect allows one to discriminate and detect the absolute configuration of chiral molecules in an aqueous system, thus opening new possibilities in biosensing and other biological applications. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G18.00014: Orientations of Chromonic Liquid Crystals by Imprinted or Rubbed Polymer Films Youngwoo Yi, Aya Mcguire, Noel Clark A variety of novel alignment effects of chromonic liquid crystal phases of sunset yellow (SSY)/water, disodium cromoglycate (DSCG)/water, and their mixtures by thiol-ene polymer films topographically imprinted with linear channels are observed using polarizing optical microscopy. Nematic DSCG and SSY at low concentration and their nematic mixtures orient with the long axes of stacked chromonic aggregates on average parallel to the channels, that is, with the molecular planes normal to the channel axis. On the contrary, nematic SSY in contact with the rubbed polyimide films orients with the long axes on average in-plane perpendicular to the rubbing direction, arguably, due to a tongue-groove interaction between SSY and the stretched PI chains. Furthermore, multi-stable alignments are observed in SSY solutions of sufficiently high concentration, including preferential in-plane orientation of the long axes of the aggregates parallel to, perpendicular to, and 45$^{\circ}$ rotated from the channels. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G18.00015: Escaped-radial configuration with a twist: lyotropic chromonic liquid crystals confined to cylindrical cavities Joonwoo Jeong, Louis Kang, Zoey S. Davidson, Matthew Lohr, Daniel A. Beller, Randall D. Kamien, Tom C. Lubensky, A.G. Yodh, Peter J. Collings We report new chiral-symmetry-broken configurations of nematic liquid crystals (LCs) confined to cylindrical cavities with a homeotropic boundary condition. In order to relieve high splay deformation in the center of the cylinder with the homeotropic boundary condition, many nematic LCs adopt an escaped-radial configuration where LC directors are radial near the cavity wall but parallel to the cylindrical axis near the center. Interestingly, we find that achiral lyotropic chromonic liquid crystals (LCLCs) having an unusually small twist modulus can have a configuration that is both escaped and twisted radially. Sunset Yellow FCF, a nematic LCLC, is introduced into capillaries coated with a homeotropic alignment layer, and its configurations are investigated by polarized optical microscopy and numerical calculations. Additionally, we discuss other newly observed structures: 1) domain-wall-like defects separating regions of opposite handedness in the twisted- and escaped-radial configuration and 2) another chiral configuration having a double helix of disclination lines along the cylindrical axis. [Preview Abstract] |
Session G19: Polymer Composites
Sponsoring Units: DPOLYChair: Mircea Chipara, University of Texas Pan American
Room: 404
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G19.00001: Critical Effect of Segmental Dynamics in Polybutadiene / Clay Nanocomposites Characterized by Solid State 1H NMR Spectroscopy Xiaoliang Wang, Rongchun Zhang, Pingchuan Sun, H. Henning Winter, Gi Xue The segmental dynamics of rigid, intermediate, and mobile molecular components in end-functionalized polybutadiene (PB) / organo-clay systems was characterized by fully refocused 1H NMR FID. In addition, 1H DQ NMR experiments allowed to semi- quantitatively monitor changes in segmental dynamics near the interface. Both methods suggested a critical concentration of end-functionalized polybutadiene, indicating a saturation effect for the surface-adsorbed polymer. The critical concentration depended on molecular weight of PB and PB-clay interaction. Based on the 1H NMR results, a tentative model was proposed to illustrate the evolution of the structure and segmental dynamics in PB/organo-clay nanocomposites. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G19.00002: Percolation Threshold in Polycarbonate Nanocomposites Suresh Ahuja Nanocomposites have unique mechanical, electrical, magnetic, optical and thermal properties. Many methods could be applied to prepare polymer-inorganic nanocomposites, such as sol-gel processing, in-situ polymerization, particle in-situ formation, blending, and radiation synthesis. The analytical composite models that have been put forth include Voigt and Reuss bounds, Polymer nanocomposites offer the possibility of substantial improvements in material properties such as shear and bulk modulus, yield strength, toughness, film scratch resistance, optical properties, electrical conductivity, gas and solvent transport, with only very small amounts of nanoparticles Experimental results are compared against composite models of Hashin and Shtrikman bounds, Halpin--Tsai model, Cox model, and various Mori and Tanaka models. Examples of numerical modeling are molecular dynamics modeling and finite element modeling of reduced modulus and hardness that takes into account the modulus of the components and the effect of the interface between the hard filler and relatively soft polymer, polycarbonate. Higher nanoparticle concentration results in poor dispersion and adhesion to polymer matrix which results in lower modulus and hardness and departure from the existing composite models. As the level of silica increases beyond a threshold level, aggregates form which results in weakening of the structure. Polymer silica interface is found to be weak as silica is non-interacting promoting interfacial slip at silica-matrix junctions. Our experimental results compare favorably with those of nanocomposites of polyesters where the effect of nanoclay on composite hardness and modulus depended on dispersion of nanoclay in polyester. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G19.00003: Path-Integration Computation of the Transport Properties of Nanoparticles Jack Douglas There is need for effective computational methods for calculating the transport properties of polymers and complex-shaped particle aggregates arising in materials science and biology as a foundation for rational material design and the design of well-defined measurements assessing the environmental impact of nanoparticles. We focus on the problem of calculating basic solution transport properties (translational diffusion coefficient, intrinsic viscosity) of isolated particles having essentially any geometry using a novel computational method involving path integration developed by Mansfield and Douglas. The basic concepts behind the method are described and the method is validated in cases where exact analytic, or at least highly accurate numerical estimates, are known for comparison. After defining and validating our method, some applications of the program are given to some non-trivial problems illustrating the use of the program for charactering such as nanoparticles with grafted DNA brush layers, DNA orgami, carbon nanotubes, etc. The path-integration method is evidently a powerful tool for computing basic transport properties of complex-shaped objects and should find wide application in polymer science, nanotechnological applications and biology. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G19.00004: N/A Invited Speaker: TBD TBD |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G19.00005: Field-theoretic simulations of polymer nanocomposites containing grafted nanoparticles Robert Riggleman, Huikuan Chao, Jason Koski As polymer nanocomposite materials have found their way into an ever-growing number of applications, it has become clear that the dispersion state of the nanoparticles can play a key role in the resulting material properties. In some instances, it is ideal to have well-dispersed nanoparticles in a polymer matrix to facilitate high loading, while in other cases, self-assembled structures are preferred. A common route for controlling the dispersion state of the nanoparticles is to graft the particles with polymer chains, and efficient computational methods capable of predicting the limits of dispersion, aggregation, and the structure of any self-assembled particles could go a long way towards enabling the design of future polymer nanocomposite devices. To that end, we have extended the field-theoretic simulations framework to include grafted nanoparticles, where polymer chains can be grafted to the surface of hard nanoparticles. Our method is compatible with both self-consistent field theory and fully-fluctuating field theoretic simulations. Calculations will be shown for the distribution of grafted nanoparticles in homopolymer thin films as well as binary homopolymer blends, where our results agree very well with recent experiments. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G19.00006: Effect of interfacial entanglement density on the melt and glassy properties of attractive polymer nanocomposites Erkan Senses, Pinar Akcora Individual dispersion of silica nanoparticles of 13 nm and 56 nm sizes in poly(methyl methacrylate) is achieved by the right choice of a solvent.By using this well-defined model attractive system, it was shown in our previous work that the conformation of PMMA on attractive silica surfaces can be dynamically altered by applying large amplitude oscillatory shear (LAOS) well above the Tg of the polymer[1].Correspondingly, the entanglement density of polymer is increased due to dynamic heterogeneities between the matrix and the adsorbed polymer. Here, we investigate, on the same system, the effect of different interfacial entanglement densities on the melt and glassy properties (Tg, fragility and physical aging).Instead of surface modification of particles, which leads to poor control over the dispersion, we tuned the interfaces by applying LAOS above Tg of the composites and by using binary blends of short (Mw< |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G19.00007: Dynamics of nanoparticles in an entangled polymer matrix Subas Dhakal, Radhakrishna Sureshkumar Quantitative description of the dynamics and rheology of the extracellular polymeric substance (EPS) of bacterial biofilms is still a major challenge due to their structural complexity. Recent experiments suggest that the viscoelasticity of EPS is not governed by entanglements in the polymer matrix. Here, we investigate the microstructure, dynamics and rheology of a Dextran EPS by probing the motion of nanoparticles embedded in the matrix using coarse-grained molecular dynamics simulations. Specifically, these simulations show that for particle diameter D \textgreater entanglement length $l_{e}$, the probe particles exhibit normal diffusion, while for D \textless $l_{e}$ sub-diffusive motion modulated by the polymer chain dynamics is observed. Results will be discussed in the context of micro-rheology experiments. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G19.00008: Field Theoretic Simulations of Polymer Nanocomposites Jason Koski, Huikuan Chao, Rob Riggleman Polymer nanocomposites (PNCs) are materials comprised of nanoparticles immersed in a polymer matrix. PNCs are used in a broad range of industries due to the enhanced properties achieved from the coexistence between the polymer and nanoparticle. A global understanding of the physical principles that dictate the equilibrium morphologies of these systems would greatly assist further development of PNCs. While polymer field theory has long been an efficient method to model equilibrium morphologies of inhomogeneous polymer systems, extensions to incorporate nanoparticles are often limited by their accuracy, computational expense, or their restriction to mean-field descriptions. In this talk, I will present a method in which nanoparticles are incorporated into a polymer matrix using a pure field theoretic approach. I provide results that indicate our approach captures correlations in the particle positions that agree identically with particle-based simulations of the same model. Additionally, our method can be applied in a fully-fluctuating field theoretic simulation where the field fluctuations are sampled using complex Langevin dynamics. Finally, I will show demonstrative calculations of the distribution of spherical and cylindrical particles embedded in a diblock copolymer melt. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G19.00009: Micro-Scale Polymer Matrix Elastic Properties in Composites using Inelastic Light Scattering Measurements and Molecular Dynamics Simulations Michael Aldridge, Katherine Sebeck, Anthony Waas, John Kieffer Polymer matrix composites with carbon fiber reinforcement are used in a wide range of aerospace and industrial applications. Composite behavior predictions based on continuum mechanics have been inaccurate, and required empirical corrections, due to the lack of polymer materials property information. The involved length scales make measurement of the elastic properties within fiber tows and near to individual fibers difficult. Micro-Brillouin and Raman light scattering provide sufficiently high spatial resolution to probe the mechanical properties and chemical composition of the matrix, without interfering with the thermo-mechanical equilibrium of the material. Elastic properties of an epoxy resin have been measured between and within the fiber tows of a composite with this technique, and compared to a bulk epoxy resin. These experimental results are complemented with molecular dynamics simulations of the interface, allowing extrapolation of findings to nanometer length scales. A diminished elastic modulus is observed in close proximity to fibers. We identify the extent to which residual stresses, chemical inhomogeneities, or structural rearrangements near the interface contribute to this effect in order to explain the underlying reason for this finding. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G19.00010: Reinforcement in Natural Rubber Elastomer Nanocomposites: Breakdown of Entropic Elasticity Paul Sotta, Roberto Perez-Aparicio, Arnaud Vieyres, Pierre-Antoine Albouy, Loic Vanel, Didier R. Long, Olivier Sanseau Understanding reinforcement mechanisms, which are responsible for the remarkable mechanical properties of elastomers filled with nanometric particles, implies combining complementary techniques. Here, we propose an approach based on the combination of different experiments in order to discriminate various reinforcement effects in elastomers filled with carbon black or silica: mechanical response, independent measurements of the crosslink density by multiple-quantum proton NMR and of chain segment orientation under stretching by X-ray scattering, in unfilled and filled vulcanized natural rubbers with various crosslink densities. In unfilled materials, all measurements are nicely correlated, in agreement with rubber elasticity theory. In filled materials, analyzing the deviations with respect to the behavior of the pure unfilled elastomer matrix allows discriminating various physical mechanisms. We demonstrate that the mechanical response at medium/large strains is essentially driven by strain amplification effects, while, in the linear regime, there is a strong additional reinforcement which is not related to the properties of the elastomer matrix. [R. Perez-Aparicio et al., Macromolecules 2013]. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G19.00011: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G19.00012: Thermal Investigations on PVDF-BaTiO$_{3}$ Nanocomposites Mircea Chipara, David Garza, Dorina M. Chipara, Armando Salinas, Steven C. Tidrow, Jerry Contreras Nanocomposites of polyvinylidene fluoride (PVDF) and barium titatanate (BaTiO3) were obtained by melt. BaTiO3 with cubic structure and average size of 100 nm was purchased from Nanostructured {\&} Amorphous Materials, Inc. The mixing consisted of 3 segments at 190 $^{\circ}$C and 60 rotations per minute (rpm) for 30 minutes, 210 $^{\circ}$C and 80 rpm for 15 minutes, and 180 $^{\circ}$C and 6o rpm for 30 minutes. Nanocomposites loaded with various amounts of BaTiO3 ranging from 0 to 15 {\%} wt. were synthesized. Differential Scanning Calorimetry measurements were performed by using a DSC Q-100 TA Instruments equipment. Isothermal crystallization rates were recorded for 100 minutes in the temperature range 130 $^{\circ}$C to 150 $^{\circ}$C. Prior crystallization, the samples were annealed at 175 $^{\circ}$C for 10 minutes. Experimental results were analyzed within the Avrami model including subsequent improvements. The effect of the nanofiller on melting temperature, crystallization temperature, and degree of crystallization was investigated. Wide Angle X-Ray scattering data are also reported. Electron microscopy confirmed the dispersion of BaTiO3 nanoparticles within PVDF. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G19.00013: Dielectric Performance of Polymer Nanocomposites: Matrix Free, Hairy Nanoparticle Assemblies and Amorphous Polymer-Nanoparticle Blends Christopher Grabowski, Elizabeth Opsitnick, Hilmar Koerner, Jeffrey Meth, Michael Bockstaller, Michael Durstock, Richard Vaia Over the past decade, polymer nanocomposites (PNCs) have been developed for electrical insulation and capacitor films to achieve extreme energy-power storage. The quality of nanoparticle dispersion has been shown to greatly affect dielectric performance. Nanoparticle aggregates function as defect sites and dramatically reduce dielectric strength. It is unclear, however, to what extent enhanced nanoparticle order (or perfect dispersion) can improve energy storage properties. Uniform dispersions of silica colloids (15 and 29 nm diam.) in polystyrene (PS) and polymethyl methacrylate (PMMA) have been achieved by two methods: (1) solvent-annealed, two-component, polymer-nanoparticle blends and (2) single-component matrix free, hairy nanoparticle assemblies. The dielectric strength, permittivity, and energy storage over a wide range of silica loadings (0-50{\%} v/v) will be discussed. Our findings indicate PS NCs have comparable breakdown strength for blend and hairy nanoparticle assemblies, while at intermediate silica loadings ($\sim$ 15{\%} v/v) PMMA grafted silica assemblies show enhanced breakdown strength compared to simple blends. Dielectric permittivity follows the Bruggeman effective medium model for all materials. [Preview Abstract] |
Session G20: Focus Session: Theory and Simulations of Macromolecules V
Sponsoring Units: DPOLYChair: Mesfin Tsige, University of Akron
Room: 405
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G20.00001: Using thermodynamic integration to simulate the free-energy of bicontinuous phases formed by block copolymer/homopolymer blends Poornima Padmanabhan, Francisco Martinez-Veracoechea, Fernando Escobedo AB diblock copolymers can co-assemble with A-type homopolymers to form different bicontinuous phases whose 3D connectivity of both A and B domains is of interest for potential applications in nanolithography, photovoltaic cells and drug delivery. In this work, we use particle-based simulations to study the vicinity of a triple point where three bicontinuous phases (gyroid, double diamond and plumber's nightmare) were predicted to coexist by Self Consistent Field Theory. A key roadblock is that bicontinuous morphologies are highly sensitive to the commensurability of the simulation box size and the a-priori unknown unit cell size. Accurate estimation of free energies is thus crucial to the determination of the stable morphology. In this work, we apply thermodynamic integration over a constructed reversible path to calculate the free energies of these bicontinuous phases relative to a disordered phase and compare the predicted phase stability to results from alternative methods. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G20.00002: Binding large globular particles to long polymer chains Anton Souslov, Jennifer E. Curtis, Paul M. Goldbart We present a minimal model that captures the change in conformational properties of long polymer chains as a result of the binding of large suspended globular particles. The large globular particles, which we model as spheres, have a single binding site and interact with each other via excluded volume repulsion, causing the attached chain to swell. This swollen chain in solution can be described as a free chain with an increased effective persistent length at large length scales and as stretched chain at short scales. Within the context of our model, we examine the statistics of these bindings and the structure of dilute and semidilute solutions of such polymer assemblies. We also consider such polymers grafted at an interface with a sufficient surface density to form a brush. We show how this model applies to the macromolecular assemblies found in the synovial fluid and in the pericellular coat of mammalian cells. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G20.00003: Depletion induced coil-globule transition of a generic macromolecule: simulations and theory Martin Bertrand, Tyler N. Shendruk, Hendrick de Haan, James L. Harden, Gary W. Slater Entropic depletion forces play a role in the compaction of chromosomal material in simple cells such as bacteria but it remains debatable whether they are sufficient to account for complete chromosome collapse. Using Coarse-Grained Molecular Dynamics simulations we show that depletion induced attractive interactions are sufficient to cause the coil-globule transition of a model chain of supercoiled DNA structural monomers suspended in a bath of smaller generic crowding agents such as proteins. We present a simple theoretical model and quantitatively cast the action of depletants on a generic macromolecular chain as an effective solvent quality: as molecular crowding increases, the radius of gyration goes from its good solvent to globular value via a theta-point and a poor solvent regime. The abrupt collapse of the chain at the predicted volume fraction of depletants is a second-order phase transition. Such coarse-grained simulations may be useful for modelling the effects of molecular crowding on chromosomal DNA in more complex geometries. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G20.00004: Coil-globule transition of macromolecules in mixed solvent: A semi-grand canonical molecular dynamics approach Debashish Mukherji, Kurt Kremer Conformational transition of macromolecules in mixed solvents are intimately linked to large local concentration fluctuations of solvent components. The numerical studies in the field are limited to the closed boundary schemes, which, however, suffer from severe system size effects. To overcome this discrepancy, we have developed a semi-grand canonical molecular dynamics scheme for complex fluids [1]. Our method makes use of the adaptive resolution scheme (AdResS) [2] with a metropolis particle exchange criterion. In AdResS, an all-atom region, containing macromolecule, is coupled to a coarse-grained (CG) reservoir. The semi-grand canonical particle exchange is performed in the CG region. As the applications of the method, we study the concentration driven reentrant collapse and swelling transition of poly(N-isopropylacrylamide) (PNIPAm) and poly(N,N-diethylacrylamide) (PDEAm) in aqueous methanol and demonstrate the role of the delicate interplay of the different intermolecular interactions. [1] D. Mukherji and K. Kremer, Macromolecules, DOI:10.1021/ma401877c (2013). [2] M. Praprotnik, L. Delle Site, and K. Kremer, J. Chem. Phys. 123, 224106, (2005). [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G20.00005: Unified View on the Mean-Field Order of Coil-Globule Transition Delian Yang, Qiang Wang It is well known that a polymer chain immersed in a small-molecule solvent undergoes the coil-globule transition (CGT) as the solvent quality changes. In the study of CGT, a mean-field theory, either of the Flory-type or the self-consistent field theory, has been commonly used; the transition order predicted by the mean-field theory, however, has been controversial. By examining the first- and second-order derivatives of the Helmholtz free energy with respect to the solvent equality, the continuity of which defines the transition order but has not been reported in the literature, we concluded that the mean-field CGT of a polymer chain of finite length $N$ exhibits the type-I behavior; that is, it is either a first-order phase transition, a critical point, or a crossover depending on the location of the critical point. It becomes a second-order phase transition with respect to the solvent equality characterized by the Flory-Huggins parameter $\chi$ (or equivalently the second virial coefficient $v$ or the temperature $T$) only in the limit of $N\to\infty$. Even in this limit, it still has the type-I behavior with respect to $v N^{1/2}$ (or equivalently $(1-2\chi)N^{1/2}$). [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G20.00006: Electrophoresis of composite objects: effect of shape, topology and polymer stiffness Mykyta V. Chubynsky, Gary W. Slater In several methods of electrophoretic separation, DNA fragments are conjugated or form complexes with objects of various kinds (linear and branched polymers, globular proteins, gold nanoparticles, micelles) having a different electrophoretic mobility. With these applications in mind, we study the free-solution electrophoresis of various composite objects (diblock copolymers with blocks of different stiffnesses, a polymer attached to a sphere, a branched polymer). We use the approach of Long \textit{et al.} [J. Chem. Phys. 108 (1998) 1234], calculating hydrodynamic interactions within the Kirkwood-Riseman approximation, and we extend the approach to the case where some parts are solid objects, rather than polymers. We find, in particular, that for diblock copolymers the results depend strongly on the relative stiffness of the blocks. If the mobility of the complex is represented as a weighted average of the mobilities of the individual parts, then when a polymer is attached to a sphere or forms a branch the weights are lower for the parts near the attachment point. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G20.00007: New perspectives for molecular field simulations of complex fluids Invited Speaker: Friederike Schmid Molecular field simulations have been introduced a while ago as a dynamic extension of the self-consistent field theory, one of the most successful theories for the description of inhomogeneous polymer systems. They build on a continuous free energy functional, which however incorporates details on the structure and architecture of the molecules. In that sense, they bridge between particle-based simulations of complex matter and continuuous simulations based on phase field theories such as the Cahn-Hilliard theory. In the talk I will first very briefly review the basic concept of molecular field simulations, and then present three recent extensions developed in our group. (i) A method to introduce hydrodynamic interactions by coupling molecular fields to a Lattice-Boltzmann fluid model; (ii) A method to deal with permanent crosslinks, i.e., to deal with polymer networks; and (iii) An approach to couple molecular field simulations with particle-based simulations in an adaptive multiscale scheme. \\[4pt] [1] L. Zhang, A. Sevink, F. Schmid, Macromolecules 44, 9434 (2011).\\[0pt] [2] F. Schmid, Phys. Rev. Lett. 111, 028303 (2013).\\[0pt] [3] S. Qi, H. Behringer, F. Schmid, New J. Physics, accepted (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G20.00008: Effects of dipolar interactions on thermodynamic stabilities of polymer blends and diblock copolymer melts Rajeev Kumar, M. Muthukumar, Bobby Sumpter We present a generalized theory for studying effects of dipolar interactions on the phase separation in polymer blends and diblock copolymer melts. A new formalism is developed to construct free energy of these polymeric media with inhomogeneous dielectric function, which bears resemblance to the static part of Lifshitz theory for dielectric slabs with sharp interfaces. Using the formalism, effects of continuous dielectric function can be studied. We have applied the formalism to a study of co-existence curves in polymer blends and interfacial tension for a planar interface between the coexisting phases. The same formalism is used to study microphase separation in lamellar forming diblock copolymer melts. Results on the effects of mismatch between the dipole moments on thermodynamics of polymer blends and diblock copolymer melts will be presented in this talk. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G20.00009: Confined ring polymers as a model nucleoid Bae-Yeun Ha, C. Jeon, J. Kim, H. Jeong, S. Jun, Y. Jung The bacterial chromosome is tightly packed in an intracellular space called the nucleoid with its loci linearly and precisely positioned. Here we propose a model nucleoid: a ring polymer confined in a cylindrical space. When the cylinder-ring parameters are chosen properly, our model describes the observed locus distributions of the {\it E. coli} chromosome surprisingly well. Our results illustrate how the geometry and function of the nucleoid are interrelated. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G20.00010: The influence of topology on the free energy and metric properties of ring polymer confined in a slit Zhao-Yan sun, Bing Li, Li-Jia An, Zhen-Gang Wang An off-lattice model with no excluded volume is used to study the effect of topological constraint on the free energy and metric properties of ring polymer confined in a slit with height $d$. The topological state is conserved by forbidding bond crossing. This model was applied to ring polymers with chain length up to $N=10^{3}$. Umbrella sampling and weighted histogram analysis method (WHAM) are used to calculate the free energy and the radius of gyration. In the strong confinement limit, free energy of linear chain in our model scales as $d^{-2}$ and the in-plane radius of gyration $R_{||}$ is independent of confinement, which agrees with the theoretical prediction very well. However, unlike the linear chain, the scaling behavior of ring polymer shows a different trend. This abnormal scaling behavior is thought to be caused by the topological constraint: the knotting probability of ring polymer increases with decreasing the slit height, and in this case, ring polymer is forced to expand itself to conserve its topological unknotted state. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G20.00011: Shear-induced desorption of isolated polymer molecules from a planar wall Sarit Dutta, Kevin Dorfman, Satish Kumar Shear-induced desorption of isolated polymer molecules is studied using Brownian dynamics simulations. The polymer molecules are modeled as freely jointed bead-spring chains interacting with a planar wall via a short-range potential. The simulations include both intrachain and chain-wall hydrodynamic interactions. Shear flow is found to cause chain flattening, resulting at low shear rates in an increased fraction of chain segments bound to the wall. However, above a critical shear rate the chains desorb completely. The desorption process is nucleated by random protrusions in the shear gradient direction which evolve under the combined effect of drag, hydrodynamic interaction, and vorticity-induced rotation, and subsequently lead to recapture. Above the critical shear rate, these protrusions grow in length until the entire chain is peeled off the wall. For free-draining chains, the protrusions are not sustained and no desorption is observed even at shear rates much higher than the critical value. These simulations can help in interpreting experiments on shear-induced desorption of polymer films and brushes. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G20.00012: Evidence of random copolymer adsorption at fluctuating selective interfaces from Monte-Carlo simulation studies Igor Gazuz, Jens-Uwe Sommer We performed Monte Carlo simulations of a binary, strongly separated mixture of A- and B-type homopolymers with some amount of random AB copolymers added. We show that the copolymers tend to localize at the interface between A and B species. We also simulated random copolymers in a one-component surrounding and compared the free energy to the case of copolymers at the interface. The result shows that interface adsorption is energetically clearly favored compared to bulk micellization, contrary to the conclusion made previously in the literature. We calculate the reduction of the interface tension due to copolymers and check the theoretical predictions for the adsorption mechanism and scaling laws made in the previous works, where ideal interfaces were considered. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G20.00013: Adsorption of Polymers on Rough Surfaces Abishek Venkatakrishnan, Vikram Kuppa Most of the surfaces encountered in nature display irregularity and self-similarity at certain length scales. Such real surfaces can be mimicked via fractal surfaces using an algorithm that produces random surfaces. The problem of polymer chains adsorbed on smooth surfaces has been well understood whereas adsorption on rough surfaces still remains unclear due to the complexity involved in equilibration and sampling of molecules in such systems. The enthalpic interactions between the monomers and the entropic penalty arising due to adsorption on rough surfaces are significantly different from smooth surfaces. In this study, we investigate the adsorption of freely rotating polymer chains on fractal surfaces by Monte-Carlo molecular simulations. Random fractal surfaces are generated using the diamond-square algorithm for different values of the Hurst parameter. Properties like monomer-surface interaction, density profiles, chain orientation profiles and distribution of adsorbed chain fractions are investigated. We also demonstrate the significant effect of fractal dimension on adsorption of polymers on rough surfaces. [Preview Abstract] |
Session G21: Physics of Copolymers: Ordering and Application of Block Copolymers
Sponsoring Units: DPOLYChair: Issei Nakamura, Changchun Institute of Applied Chemistry
Room: 406
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G21.00001: Complex Ordering of Soft Spheres in Block Polymer Melts Sangwoo Lee, Jingwen Zhang, Frank S. Bates Hard sphere systems, such as low-z metallic elements and colloids, generally form densely packed crystalline states with BCC, FCC and HCP symmetry. In contrast, a certain self-assembling soft materials including block polymers, surfactants, and dendrimers have the capacity to order into more open crystalline structures. Recently, we have identified five discrete sphere-packing symmetries in poly(styrene-$b$-isoprene-$b$-styrene-$b$-ethylene oxide) (SISO) tetrablock terpolymers: BCC, Frank-Kasper $\sigma $-phase, dodecagonal quasicrystal (DQC), Pm$\bar{3}$n(A15), and non-close-packed hexagonal (sHEX). Some of these packing symmetries have been documented in certain heavy metals and organic dendrimers, and interestingly Landau theory anticipated a similar set of non-close-packed symmetries more than three decades ago. This talk will draw analogies between the ordering of spheres in soft and hard materials. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G21.00002: Chirality Effect on Self-Assembly of Chiral Block Copolymers Hsiao-Fang Wang, Ming-Chia Li, Rong-Ming Ho Here, we report the mechanisms of chiral transfer at various length scales in the self-assembly of enantiomeric chiral block copolymers (BCPs*). We show the evolution of homochirality from molecular chirality into phase chirality in the self-assembly of the BCPs*. The chirality of the molecule in the BCP* is identified from circular dichroism spectra, while the handedness of the helical conformation in the BCP* is determined from a split-type Cotton effect in vibrational circular dichroism spectra. Microphase separation of the BCP* is exploited to form a helical (H*) phase, and the handedness of helical nanostructure in the BCP* is directly visualized from transmission electron microscopy tomography. Moreover, the phase transitions from the H* phase to both the hexagonal cylinder phase and gyroid phase are found after long-time thermal annealing. Those results suggest that the H* phase is a long-lived metastable phase. To demonstrate the universal behavior of the chirality effect on BCPs*, different block copolymers containing chiral segment are synthesized and examined, suggesting that the chirality effect indeed plays an important role in the formation of H* phase. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G21.00003: Control of Block Copolymer Morphology through End-functional Groups Gyuha Jo, Moon Jeong Park Recently, poly(ethylene oxide) (PEO)-containing polymer electrolytes have attracted significant attention to be applied for lithium batteries. As the realization of high mechanical strength from the polymer electrolyte becomes of critical importance in high-energy lithium batteries, much effort has been devoted to developing PEO-based block copolymers comprising mechanically robust polymer chains. Interest in this topic has been further stimulated by multiple observations of significant electrolytic conductivity enhancement imparted by microphase separation of block copolymers. In the present study, we report an intriguing methodology for modulating the morphology of poly(styrene-ethylene oxide) (PS-PEO) block copolymers with a single ionic group tethered at the chain end of PEO. Unique intra- and inter-chain interactions deduced from the end functional group afforded enriched nanostructures, i.e. disorder, lamellae, hexagonal cylinder, and gyroid, with significant differences in conductivities depending on lithium salt concentration. In particular, a gyorid morphology with a twofold-enhanced lithium ion transport efficiency was found for the end-functionalized PS-PEO block copolymer, attributed to the structural advantages of the gyroid having co-continuous ionic channels. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G21.00004: Increase in the Domain Spacing from ARB-Type Triblock Copolymer Sanghoon Woo, Hyunjung Jung, June Huh, Du Yeol Ryu, Soo-Hyung Choi, Joona Bang It has been reported that the self-assembly of block copolymers (BCP) with very high molecular weight (MW) can achieve the length scale above 100 nm, which can be utilized as photonic band-gap materials or photonic crystals. However, due to slow chain dynamics, it is hard to fabricate well-controlled nano-patterns from high MW BCPs via thermal annealing process. In this work, we designed a new type of BCP, namely ARB-type BCP, where the R represents the short middle block composed of A and B random copolymer. It was expected that the R block provide the effect of increased polydispersity via compositional distribution, leading to an increased domain size compared to the AB diBCP with same MW and polydispersity. We prepared various ARB-type BCPs and AB diBCPs having the similar polydispersity via living-radical polymerization, and their morphologies were characterized by TEM, SAXS, and GISAXS. Consequently, it was shown that the ARB-type triBCP exhibited $\sim$30\% increase in the domain spacing compared to the AB diBCPs with same MW and polydispersity. These results were also compared with theoretical viewpoint. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G21.00005: The Order-Disorder Transition in Short Diblock Copolymers: Relaxation Calorimetry Experiments Timothy Gillard, Daniel Phelan, Sangwoo Lee, Chris Leighton, Frank Bates Fluctuation-driven, weakly first-order phase transitions occur in a variety of physical systems, including the order-disorder transition (ODT) in block copolymers (BCPs), and certain phase transitions in liquid crystals, magnetic materials, and superconductors. BCPs provide an attractive model system for studying this fascinating class of transitions since BCPs exhibit universal phase behavior dependent on a small number of parameters that are easily tuned during synthesis. However, thermal measurements of the ODT in BCPs are rare since the magnitude of the latent enthalpy of the transition scales inversely with $N$, the degree of polymerization. Here we extend a thermal measurement technique common in the inorganic materials community, relaxation calorimety, to accurately measure the temperature dependence of the heat capacity near the ODT of poly(1,4-isoprene-$b$-DL-lactide) BCPs that form ordered structures at low $N$. The transition temperature (371 K), latent heat (0.3 J/g), and temperature hysteresis ($\sim$ 1 K) were found to agree with values obtained from differential scanning calorimetry, rheology, and scattering experiments, establishing relaxation calorimety as a valuable new tool for studying the ODT in BCPs. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G21.00006: Alignment pathways and the effect of a Nematic-Smectic A transition on the orientational order of an LC block copolymer under magnetic fields Manesh Gopinadhan, Youngwoo Choo, Pawel Majewski, Chinedum Osuji We explore the effect of magnetic fields on the thermodynamics and alignment kinetics of a liquid crystalline block copolymer (LCBCP) using in situ SAXS. We examine the effect of magnetic fields on the order-disorder transition temperature (T$_{ODT})$, alignment pathways and the effect of liquid crystal ordering on the orientational order of the system. The application of the field did not result in any discernable shift in the phase behavior of the system. This is consistent with our observation that alignment occurred by slow grain rotation rather than by selective melting of field-destabilized grains, which is further supported by time and temperature resolved measurements. Zero field cooling conducted after field-annealing in the nematic phase result in highly-aligned structures despite the initial weak alignment in the nematic window. The strong enhancement of the alignment is correlated with the emergence of a smectic mesophase and highlights the strong orientational coupling of the BCP interfaces to the orientation of the smectic layer normal, k, and not to the molecular director field of the mesogens, n. These results suggest that the presence of a smetic A mesophase and the nematic-smectic sequence can be leveraged in designing schemes for magnetic field directed self-assembly of block copolymers. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G21.00007: Self-Assembly of Pluronic Block Copolymers in Solutions: Simulation and Neutron Scattering Zhe Zhang, Kunlun Hong, Changwoo Do Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers in water solution display various phase behaviors such as micellar, lamellar, and hexagonal phases and have been of great interest to researchers for their wide range of applications including templates of various nanostructures in solar cell and transportation of nanoparticles in drug delivery. In this study, we combined density functional theory-based mesoscale simulation and small-angle neutron scattering (SANS) experiments to investigate equilibrium structures of L62/water systems at different concentrations. Various simulation parameters found in the literature have been revisited with the experimental findings. Scattering experiments were found to be an excellent. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G21.00008: Chain exchange in block copolymer micelles Jie Lu, Frank Bates, Timothy Lodge Block copolymer micelles are aggregates formed by self-assembly of amphiphilic copolymers dispersed in a selective solvent, driven by unfavorable interactions between the solvent and the core-forming block. Due to the relatively long chains being subject to additional thermodynamic and dynamic constraints (e.g., entanglements, crystallinity, vitrification), block copolymer micelles exhibit significantly slower equilibration kinetics than small molecule surfactants. As a result, details of the mechanism(s) of equilibration in block copolymer micelles remain unclear. This present works focuses on the chain exchange kinetics of poly(styrene-b-ethylenepropylene) block copolymers in squalane (C30H62) using time-resolved small angle neutron scattering (TR-SANS). A mixture of h-squalane and d-squalane is chosen so that it contrast matches a mixed 50/50 h/d polystyrene micelle core. When the temperature is appropriate and isotopically labeled chains undergo mixing, the mean core contrast with respect to the solvent decreases, and the scattering intensity is therefore reduced. This strategy allows direct probing of chain exchange rate from the time dependent scattering intensity I(q, t). [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G21.00009: A Theoretically Informed Model for the Rheology of Entangled Block Copolymer Nanocomposites Yongrui Su, Abelardo Ramirez-Hernandez, Brandon Peters, Juan J. de Pablo The addition of nanoparticles to block copolymer systems has been shown to have important effects on their equilibrium structure and properties. Less is known about the non-equilibrium behavior of block polymer nanocomposites. A new particle-based, theoretically informed coarse-grained model for multicomponent nanocomposites is proposed to examine the effects of nanoparticles on the rheology of entangled block copolymer melts. Entanglements are treated at the two-molecule level, through slip-springs that couple the dynamics of neighboring pairs of chains. The inclusion of slip-springs changes the polymer dynamics from unentangled to entangled. The nanoparticles are functionalized with short polymer chains that can entangle with the copolymers. We study the nonlinear rheology of the resulting nanocomposites under shear flow with a dissipative particle dynamics (DPD) thermostat. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G21.00010: Baroplastic Behavior in Block Copolymer Blends Yonghoon Lee, Hyungju Ahn, Hoyeon Lee, Eunhye Kim, Young Soo Han, Du Yeol Ryu The study of block copolymer (BCP) blends, composed of the weakly interacting polystyrene-$b$-poly(n-butyl methacrylate) (PS-$b$-PnBMA) and polystyrene-$b$-poly-(n-hexyl methacrylate) (PS-$b$-PnHMA) presented the various composition-dependent phase behaviors arising from a miscible phase between the PnBMA and PnHMA blocks in the BCP blends. As the blend composition varied from PS-$b$-PnBMA to PS-$b$-PnHMA, a lower disorder-to-order transition (LDOT) to a closed-loop phase transition and to an order-to-disorder transition (ODT) on heating were observed. The hydrostatic pressure effects on the various phase behaviors of the BCP blends were further investigated using small-angle neutron scattering (SANS), depolarized light scattering (DPLS) and transmission electron microscope (TEM). [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G21.00011: Diblock copolymer bridges: the break-up dynamics and enhanced stability of structured liquids Robert Peters, Kari Dalnoki-Veress Liquid bridges form when a liquid is stretched between two boundaries, creating a freestanding fiber. The break-up of simple Newtonian liquid bridges has been studied both theoretically and experimentally for a wide variety of different initial conditions since Plateau and Rayleigh considered the instability of a liquid jet. Though the break-up of liquid bridges composed of linear polymer melts and polymer solutions have been well studied, very little focus has been placed on the dynamics of diblock copolymer bridges. When annealed above the glass transition temperature and below the order-disorder transition temperature (ODT), diblock copolymers can organize into well-defined structures determined by the weight fraction of each polymer block. Conversely, as the temperature is increased above the ODT, the diblock will disorder and have properties more similar to a linear polymer melt. In this work, we monitor the evolution of diblock copolymer bridges to study the effect that diblock order has on dynamics and stability. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G21.00012: Phase Transitions of Polystyrene-b-Polydimethylsiloxane in Solvents of Varying Selectivity Ting-Ya Lo, Chia-Cheng Chao, Rong-Ming Ho, Prokopios Georgopanos, Apostolos Avgeropoulos, Edwin L. Thomas A simple method to create a variety of nanostructures via the self-assembly of a single composition silicon-containing block copolymer (BCP) is developed. By using selective solvents for the self-assembly of polystyrene-block-polydimethylsiloxane (PS-PDMS), the phase behavior of intrinsic BCP can be enriched due to the strong segregation of the PS-PDMS enabling the clear-cut phase transitions during solvent evaporation. The solution-state phase behaviors of the strong segregation BCP system are systematically studied using temperature-resolved SAXS. Meanwhile, owing to the high etching contrast of the silicon-containing block versus the PS block, various nanostructured SiOC can be fabricated by using one-step oxidation. Furthermore, the recovery of the intrinsic lamellar phase can be achieved by thermal annealing the metastable cylinder and gyroid phases through order$-$order transition (OOT). Time-resolved SAXS and electron tomography are carried out to reveal the variation of the structural evolution in reciprocal space and real space, respectively. This result offers new insights into the phase behaviors of the OOT of BCPs. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G21.00013: Controlled orientation and ordering of nanostructured thin films from degradable block copolymer. Rong-Ming Ho, Ming-Shiuan She, Ting-Ya Lo, Yi-Hsiu Wu The fabrication of nanostructured thin films from the self-assembly of degradable block copolymers (BCPs) has attracted extensive attention. To create useful BCP thin films for practical uses, controlling the orientation of self-assembled nanostructures is essential. Here, we present a new method for forming well-ordered and oriented nanostructured thin films on a functionalized SiO$_{\mathrm{2}}$ surface, using homopolymers with hydroxyl group at the chain end to functionalize SiO$_{\mathrm{2}}$ surface, to give neutral substrate for the BCPs. To demonstrate the feasibility of suggested approaches, a series of degradable BCPs, polystyrene-$b$-poly(L-lactide) (PS-PLLA) with hexagonally packed cylinder and double gyroid phases, are used as model systems for creating nanostructured thin films with controlled orientation and ordering of BCP nanostructures. Different methods such as thermal and solvent annealing are utilized to exploit the fabricated neutral substrate for creating expected nanostructured thin films. By taking advantage of degradable character of PLLA, nanoporous PS thin film can be fabricated by hydrolysis and used as a template for synthesis of various nanohybrids and nanoporous materials. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G21.00014: Investigation of the self-healing mechanism of poly (ethylene co-methacrylic acid) copolymers utilizing ultrasonic time dependent resonant spectroscopy Nicholas R. Bowers, Kenneth A. Pestka II, Stephen J. Kalista, Jr. The ultrasonic resonant spectra of four different poly (ethylene-co-methacrylic acid) copolymers (EMAA copolymers) have been obtained using the Time Dependent Resonant Spectroscopy (TDRS) method. The spectra of these EMAA copolymers, developed by DuPont (commercially known as: Nucrel 925, Nucrel 960, Surlyn 8920, and Surlyn 8940), were analyzed. Evolution of resonances and the associated energy dissipation within the samples were observed over a period of 10 $\sim$ 50 hours. Results quantifying the influence of chemical properties (including molecular weight and ionic content), age, and damage mechanism on the self-healing response will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G21.00015: The Effect of Long Range Order on Ionic Conductivity in a Solid Block Copolymer Electrolyte Mahati Chintapalli, Jacob Thelen, Alexander Teran, Nitash Balsara Poly(styrene)-block-poly(ethylene oxide) (SEO) mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt is a promising material for battery electrolytes due to its high ionic conductivity and ability to suppress lithium dendrite growth. Ion conduction has been found to depend on many aspects of the electrolyte microstructure, including the morphology and degree of ordering. The effect of long range order on ionic conductivity was investigated in a lamellar SEO/LiTFSI mixture by in situ small angle x-ray scattering and ac impedance spectroscopy during polymer annealing. The observation that increasing long range order decreases ionic conductivity indicates that disorder, due to small grain size or defects, enhances the ionic conductivity of the electrolyte. [Preview Abstract] |
Session G22: Focus Session: Padden Award Symposium
Sponsoring Units: DPOLYChair: Stephen Cheng, University of Akron
Room: 407
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G22.00001: Exploring conditions for craze initiation and for absence of crazing in polymer glasses Shiwang Cheng, Panpan Lin, Shi-Qing Wang Although crazing in polymer glasses has been extensively studied in the past, it is still difficult to predict and explain when and why crazing would take place. There is even a recent proposal to suggest [1] that craze initiation is ``a frustrated fracture process than rather a yield mechanism'' [2]. In this work, we report the variable parameters that influence crazing. Specifically, we show that a ``young'' glass (prepared by mechanical ``rejuvenation'') can resist crazing and aging promotes crazing. Thus, the degree of vitrification is one variable. Under creep, crazes form faster at a higher tensile stress, showing that crazing is an activation process and depends on the external condition. We also show how the large-scale structure such as the degree of chain networking affects crazing behavior. For example, melt stretching suppresses crazing. Finally, we demonstrate crazing in absence of any ongoing extension when a cold-drawn polymer glass is held fixed when annealing well below T$_{\mathrm{g}}$ during the elastic yielding [3]. These new observations have inspired a molecular picture for large deformation of polymer glasses [4]. \\[4pt] [1] \textit{Polymer }2007\textbf{,} \textit{48}, 1030;\\[0pt] [2] \textit{Polymer }2011\textbf{,} \textit{52}, 2319;\\[0pt] [3] \textit{Phys. Rev. Lett. }2013, \textit{110}, 065506;\\[0pt] [4] submitted to Polymeric Glasses session. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G22.00002: Traversing the crystalline phase space of contorted hexabenzocoronene to maximize charge transport Anna Hiszpanski, Arthur Woll, Nan Yao, Yueh-Lin Loo Alternative crystal structures of molecular semiconductors may exhibit increased intermolecular charge transport, but methods to controllably access non-thermodynamically-favored crystal structures are lacking. Starting with an amorphous film of contorted hexabenozocoronene (HBC) and applying thermal and solvent-vapor annealing to induce crystallization, we have accessed three distinct HBC polymorphs, two of which have previously not been observed. HBC films crystallize as polymorph I upon thermal annealing and as polymorph II upon solvent-vapor annealing with tetrahydrofuran. Subsequent solvent-vapor annealing of polymorph I converts it to polymorph II; thermal annealing polymorph II transforms HBC to yet a different crystal structure, denoted polymorph II'. Though the crystal structure can be tuned through sequential processing, the preferred out-of-plane molecular orientation adopted by HBC is determined primarily by the first processing step. By imposing different processing sequences, we can access films having different polymorphs but the same molecular orientation, and also films having the same polymorph but different molecular orientations, thereby allowing us to decouple the relative contributions of polymorphism and preferential orientation to charge transport. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G22.00003: High Modulus, High Conductivity Nanostructured Polymer Electrolyte Membranes via Polymerization-Induced Phase Separation Lucas McIntosh, Morgan Schulze, Marc Hillmyer, Timothy Lodge Solvent-free, solid-state polymer electrolyte membranes (PEMs) will play a vital role in next-generation electrochemical devices such as Li-metal batteries and high-$T$ fuel cells. The primary challenge is that these applications require PEMs with substantial mechanical robustness, as well as high ionic conductivity. The key to optimizing orthogonal macroscopic properties is to use a heterogeneous composite with well-defined nanoscopic morphology---specifically, long-range co-continuity of high modulus and ion transport domains, which has proven difficult to achieve in commonly-studied diblock copolymer-based electrolytes. We report a simple synthetic strategy to generate PEMs via polymerization-induced phase separation, where the delicate balance between controlled addition of styrene onto a poly(ethylene oxide) macro-chain transfer agent and simultaneous chemical crosslinking by divinylbenzene results in a disordered structure with domain size of order 10 nm. Crucially, both domains exhibit long-range continuity, which results in PEMs that are glassy solids (modulus $\approx$ 1 GPa) owing to the isotropic network of stiff, crosslinked polystyrene, and are highly conductive ($>$ 1 mS/cm at 70 $^{\circ}$C) because ions migrate in channels of low $T_{\rm g}$ poly(ethylene oxide). [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G22.00004: Physical Aging of Thin and Ultrathin Free-Standing Polymer Films: Effect of Stress and Reduced Glass Transitions Justin Pye, Connie Roth While great effort has been made in elucidating the effect of confinement on the glass transition (Tg) in polymers, considerably less work has been done on physical aging. Starting with supported films, we have previously shown that the reduced physical aging rates in ultrathin polystyrene (PS) films can be linked to the reduced Tg near the free surface [Macromolecules 2010, 43, 8296]. We then showed that high molecular weight (MW) free-standing PS films have two reduced Tgs suggesting that two separate mechanisms are acting simultaneously to propagate enhanced mobility at the free surface deeper into the film [PRL 2011, 107, 235701]. To help determine the mechanisms of these two reduced Tgs, we performed physical aging measurements on these high MW free-standing PS films. For thick films (220-1800 nm) in which there are no Tg reductions, we find that the physical aging rate depends strongly on stress caused by thermal expansion mismatch between film and support. This stress, applied to the films as they are quenched into the glassy state, can nearly double the physical aging rate when changing the frame material from polycarbonate to silicon [Macromolecules 2013, DOI:10.1021/ma401872u]. Finally, ultrathin high MW PS films held at a temperature between the two Tgs do exhibit physical aging, indicating that at least some of the film is glassy between these two transitions. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G22.00005: The Origin of Hyperdiffusive Relaxations in Soft Glasses Samanvaya Srivastava, Donald Koch, Lynden Archer Small particles suspended in fluids move randomly over long length- and time-scales. This motion is the expected response of weakly interacting particles to uncoordinated bombardments from the fluid molecules. This feature of suspensions is considered a fundamental characteristic of their equilibrium state and, over long-enough observation times, leads to universal diffusive particle motions. We report on the motions of particles in single component suspensions in which the suspended (particle) and suspending (fluid) phases are chemically linked. We find that even in equilibrated systems these motions are hyperdiffusive. Our observations add to a large number of recent reports, which show that diffusive motion is not the norm in soft matter such as colloidal gels, nanoemulsions and soft nanoparticle glasses. In such systems, particle motions can be highly correlated over long distances and time, belying long-lived, directed forces thought to arise from out-of-equilibrium, metastable states that can drive sudden irreversible structural re-arrangements. We show that hyperdiffusive motion in soft matter does not require such states and can arise naturally from volume fluctuations brought about by thermal forces. We further show that the simplicity of the force dipoles produced by volume fluctuations in our single-component suspensions leads to a physical origin for hyperdiffusion as fundamental as that commonly thought to produce diffusion of particles in dilute suspensions. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G22.00006: On the crossover from Odijk to de Gennes in tube-confined semiflexible polymers Douglas Tree, Kevin Dorfman The problem of a semiflexible polymer confined in a tube was considered solved almost 30 years ago. However, the need to manipulate single molecules of the semiflexible biopolymer DNA for emerging genomic mapping technologies led to measurements of the extension of DNA in nanochannels, which challenged the classic results of Odijk and de Gennes. We have investigated the behavior of semiflexible polymers using an off-lattice implementation of the pruned-enriched Rosenbluth method (PERM), enabling simulation of confined chains that are more than two orders of magnitude longer than chains used in conventional Metropolis methods. Our simulations suggest the presence of additional universal regimes, which arise due to the competing effects of stiffness and excluded volume for polymers in moderate confinement. We have also examined previous interpretations of these regimes in the light of recent results for both the mean polymer extension and extension fluctuations of very long chains. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G22.00007: Polymer Structure and Dynamics under Cylindrical Confinement: Experiments, Simulations and Theory Wei-Shao Tung, Daniel Sussman, Nigel Clarke, Russell Composto, Kenneth Schweizer, Robert Riggleman, Karen Winey Polymer structure and dynamics are perturbed under confinement, especially when the dimension of the confinement is smaller than the polymer's radius of gyration (R$_{\mathrm{g}})$. While most studies focus on thin film confinement, we study cylindrical confinement using experiments, simulations and theory. Our MD simulations study the change in R$_{\mathrm{g}}$, local dynamics, entanglement molecular weight (N$_{\mathrm{e}})$, and diffusion coefficient (D) for polymers in cylindrical confinement with different diameters (d/2R$_{\mathrm{g}}$ $\sim$ 0.4 -- 6). We found increased N$_{\mathrm{e}}$ and D in cylindrical confinement for d/2R$_{\mathrm{g}}$\textless 1.5. Moreover, R$_{\mathrm{g}}$ decreases in the direction of confinement and increases along the cylinder axis. We developed an analytical theory to relate the transformation of chain conformations to the preferential orientation of primitive path steps, and further predicted the increase of N$_{\mathrm{e}}$. Experimentally, we infiltrated 400 kg/mol polystyrene (2R$_{\mathrm{g}}$ $\sim$ 35nm) into anodized aluminum oxide (AAO) membranes (d $\sim$ 18-150nm) to confine polymers into cylindrical nanopores (d/R$_{\mathrm{g}}$ $\sim$ 0.5--4). We use SANS to probe R$_{\mathrm{g}}$, QENS to probe the local dynamics, and elastic recoil detection to measure D of deuterated PS inside PS-filled AAO nanopores. Values obtained from our experiments and literature are quantitatively compared with our simulation results. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G22.00008: Dynamics of entangled rod-coil block copolymers Muzhou Wang, Ksenia Timachova, Alfredo Alexander-Katz, Alexei E. Likhtman, Bradley D. Olsen Polymer science is exploring advanced materials which combine functional domains such as proteins and semiconducting polymers with traditional flexible polymers onto the same molecule. While many studies have focused on equilibrium structure-property relationships, little is known about how the conformational restrictions of rigid domains affect dynamical phenomena such as mechanical properties, processing pathways, and self-assembly kinetics. We have recently introduced a reptation theory for entangled rod-coil block copolymers as a model for this wider class of functional polymeric materials. The theory hypothesizes that the motion of rod-coils is slowed relative to rod and coil homopolymers because of a mismatch between the curvature of the rod and coil entanglement tubes. This effect leads to activated reptation and arm retraction as two relaxation mechanisms that govern the short and long rod regimes, respectively. These results were verified by tracer diffusion measurements using molecular dynamics simulation and forced Rayleigh scattering in both the rod-coil diblock and coil-rod-coil triblock configurations. The tracer diffusion results were then compared to experimental self-diffusion measurements which require a consideration of the motion of the surrounding chains. [Preview Abstract] |
Session G23: Invited Session: Fuels From Sunlight: Computational Studies of Photo-Electrodes and Catalysts
Sponsoring Units: DCOMPChair: Giulia Galli, University of California, Davis
Room: 505-507
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G23.00001: Electronic Excitations in Light Absorbers for Photoelectrochemical Energy Conversion: First Principles Calculations Based on Many-Body Perturbation Theory Invited Speaker: Yuan Ping The efficiency of photo-electrochemical cell for water splitting relies on the availability of Earth abundant, stable light absorbers with band gaps in the visible range, and band edges properly aligned with water redox potentials. We will present several ab initio calculations aimed at understanding and predicting the electronic properties of candidate photo-electrode materials [1]. Our calculations were carried out at different levels of theory, including density functional and many body perturbation theory (MBPT). We focused on WO$_3$ and CuW$_{1-x}$Mo$_x$O$_4$, and on functionalized Si wires for the photo-anode and photo-cathode, respectively. In particular, we will discuss how to decrease the band gap of WO3 by small molecule and rare gas atom intercalation [2,3], and by forming copper tungstate solid solutions [4]; in addition we will discuss how to improve band alignments with water redox potentials by considering phases of WO3 stable at high temperature [5]. Finally we will present calculations of absorption spectra of WO3 and silicon wires obtained using MBPT, by solving the Bethe Salpeter Equation, and we will present comparisons with recent experiments. \\[4pt] [1] Y. Ping, D. Rocca, G. Galli, Chem. Soc. Rev. 42, 2437 (2013).\\[0pt] [2] Y. Ping, Y. Li, F. Gygi, G. Galli, Chem. Mater., 24, 21, (2012).\\[0pt] [3] Q. Mi, Y. Ping, Y. Li, B. Cao, B. Brunschwig, P. Khalifah , G. Galli, H. Gary and N. Lewis, J.Am.Chem.Soc., 134, 44, (2012).\\[0pt] [4] J. Hill, Y. Ping, G. Galli and K. Choi, Energy Environ. Sci., 6, 2440 (2013).\\[0pt] [5] Y. Ping and G. Galli, submitted, 2013. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G23.00002: Oxide/Water Interfaces: How the Surface Chemistry Modifies the Electronic Energy Alignment Invited Speaker: Michiel Sprik The minimum of the $d$-electron conduction band of an aqueous transition metal oxide electrode is typically no more than a few 100 mV away from the standard hydrogen electrode (SHE). Because of this favourable alignment of the electronic energy levels (near) metallic transition metal oxides with partly filled $d$ bands can be used as electrocatalysts while the compounds with finite electronic gap can be used as photocatalysts. However, because of their ionic character, transition metal-oxide surfaces also show amphiphilic acid-base activity. At low pH the basic sites are protonated and at high pH the acidic sites deprotonated creating an electrical double layer with corresponding surface potential. The alignment of the electronic energy levels, and by implication their redox activity, is therefore pH dependent. In fact, even in absence of protonic surface charge, the coordination with water molecules is already capable of shifting the electronic energy levels of the oxide by 1 eV or more. Computation of the electronic energies in transition metal oxide electrodes requires therefore a detailed modeling of their aqueous surface chemistry. The solvation energy of the proton is the common energy reference for both redox potentials on the SHE scale and acidity constants (pKa). Computation of the H$^+$ solvation energy is therefore a key component in a unified treatment of redox and acid-base chemistry. In this talk we outline the Density Functional Theory based Molecular Dynamics (DFTMD) method we have developed for this purpose [1,2]. The central tool of our approach is a method for reversible insertion of protons in the aqueous part of the DFTMD model system. As an illustration we discuss the application to the rutile TiO$_2$/water and MnO$_2$/water interface. \\[4pt] [1] Cheng, J.; Sulpizi, M.; VandeVondele, J.; Sprik, M. ChemCatChem 4 (2012) 636.\\[0pt] [2] Cheng, J.; Sprik, M. Phys. Chem. Chem. Phys 14 (2012) 11245. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G23.00003: Cobalt, nickel/iron, and titanium oxide electrodes for water oxidation Invited Speaker: Annabella Selloni Water splitting on metal oxide surfaces has attracted enormous interest for more than forty years. While a great deal of work has focused on titanium dioxide (TiO$_{\mathrm{2}})$, recently cobalt and mixed Ni-Fe oxides have also emerged as promising electrocatalysts for water oxidation due to their low cost and high activity. In this talk I shall discuss various aspects of water oxidation on cobalt (hydro-)oxides, pure and mixed nickel and iron (hydro-)oxides, and TiO$_{\mathrm{2\thinspace }}$surfaces. Using DFT$+$U calculations, I shall examine the composition and structure of cobalt and Ni-Fe oxides under electrochemical conditions, and present studies of the oxygen evolution reaction (OER) on the relevant stable compounds. I shall also present hybrid functional calculations of the first proton-coupled-electron transfer at the water/TiO$_{\mathrm{2}}$ interface in the presence of a photoexcited hole. Our results provide evidence that the proton and electron transfers are not concerted but rather represent two sequential processes. They also suggest that the OER is faster at higher pH, as indeed observed experimentally. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G23.00004: From Electronic Structure to Catalytic Activity: Descriptors for water splitting reactions Invited Speaker: Aleksandra Vojvodic |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G23.00005: Nonadiabatic Dynamics of Photoinduced Proton-Coupled Electron Transfer Processes in Solution Invited Speaker: Alexander Soudackov Theoretical approaches developed to elucidate the fundamental principles underlying the nonequilibrium dynamics of photoinduced proton-coupled electron transfer (PCET) processes in solution will be presented [1-3]. These processes are simulated by propagating nonadiabatic surface hopping trajectories on electron-proton vibronic surfaces that depend on the solute and solvent nuclear coordinates. The solvent is represented either as explicit solvent molecules or as a dielectric continuum. In the latter case the solvent dynamics is described in terms of two collective solvent coordinates corresponding to the energy gap coordinates associated with electron and proton transfer. Calculations on model systems reveal two distinct solvent relaxation timescales, where the faster timescale relaxation corresponds to librational motions of solvent molecules in the first solvation shell and the slower time scale corresponds to the bulk solvent dielectric response. These calculations illustrate that the charge transfer dynamics, solvent dynamics, and vibrational relaxation processes are strongly coupled. Extensions of the methodology and applications to experimentally relevant systems will also be discussed [4]. \\[4pt] [1] Hazra, A.; Soudackov, A. V. and Hammes-Schiffer, S. J. Phys. Chem. B 114, 12319--12332 (2010). \newline [2] Soudackov, A. V.; Hazra, A. and Hammes-Schiffer, S. J. Chem. Phys. 135, 144115 (2011). \newline [3] Auer, B.; Soudackov, A. V. and Hammes-Schiffer, S. J. Phys. Chem. B 116, 7695--7708 (2012). \newline [4] Ko, C.; Solis, B. H.; Soudackov, A. V. and Hammes-Schiffer, S. J. Phys. Chem. B 117, 316--325 (2013). [Preview Abstract] |
Session G24: Focus Session: Advances in Scanned Probe Microscopy II: High Frequencies and Optical Techniques
Sponsoring Units: GIMSChair: Sebastian Loth, Max Plank Institute for Structure and Dynamics
Room: 504
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G24.00001: Nanoscale cantilevers with integrated optomechanical readout: increasing speed and sensitivity Invited Speaker: Vladimir Aksyuk Decreasing a mechanical probe size and mass into the nanoscale and sub-picogram range offers a way to increase the transduction bandwidth while maintaining the desired mechanical stiffness and, ideally, maintaining or lowering the mechanical damping and the associated fundamental thermal force noise. Such transducers require a new approach to low noise and fast motion readout that is also stable, practical, low power and capable of operating over a wide temperature range. We are using on-chip cavity optomechanical sensing for realizing fast, sensitive and practical integrated AFM probes. Integrated micrometer-scale silicon microdisk high Q optical cavities evanescently couple to sense motion of suspended silicon beams with nanoscale cross sections (e.g. 100 nm x 260 nm). The sensors achieve sub- fm/Hz$^{0.5}$ motion sensitivity, which is near the standard quantum limit for these beams, with dissipated optical power under 300 $\mu $W and the readout bandwidth of approximately 1 GHz. The mechanical properties of the beams are broadly adjustable by design, covering four decades of mechanical stiffness (0.01 N/m to 290 N/m) and frequencies from 250 kHz to 110 MHz, with similar motion readout sensitivity across the range. Combining the low mass mechanical transducer with the ultraprecise readout potentially opens up new regimes of operation while also posing design tradeoffs in gain, bandwidth and dynamic range. The mechanical probe can be excited and the dynamics can be tuned by optomechanical effects as well as application of optical and electrostatic forces via feedback. Effective stiffness modification, regenerative oscillation as well as optomechanical and feedback damping can be useful in different modalities of probe operation. Unresolved sideband operation gives the readout bandwidth larger than the mechanical frequency, which is particularly useful for broadband feedback actuation, e.g. to modify the transfer function, extend the useful measurement bandwidth and cool the sensor motion to reduce nonlinearity and mechanical backaction of the mechanical probe on the sample. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G24.00002: Detection of thin film NMR spectrum by Magnetic Resonance Force Microscopy Seung-Bo Saun, Sungmin Kwon, Soonchil Lee, Soonho Won NMR is widely used in many fields due to its powerful advantages such as nondestructive, chemically selective detection, and local probing. However, because of its low sensitivity, it is difficult to investigate thin film samples by conventional NMR. MRFM is the combined technic of NMR and Scanning Probe Microscopy (SPM), and it enabled exceptional sensitivity increasement of NMR detection. We succeeded in detecting general thin film NMR spectrum for the first time by modifying the MRFM. CaF$_{2}$ 34nm thin film NMR was detected and we observed 20 Gauss spectrum in proximity to bulk spectrum which is about 10 Gauss. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G24.00003: Implementation of NMR pulse sequences for Magnetic Resonance Force Microscopy Bradley Moores, Alexander Eichler, Christian Degen Magnetic resonance force microscopy (MRFM) is a scanning microscopy technique that allows measuring nuclear spin densities with a resolution of a few nanometers. Ongoing efforts are aiming at improving this resolution, which might ultimately facilitate non-destructive 3D scans of complex molecules or solid state systems with atomic resolution. Here, we review our current efforts to utilize in an MRFM experiment pulsing techniques borrowed from the nuclear magnetic resonance community. The use of advanced pulsing schemes may improve signal-to-noise ratio, imaging resolution, and allow the investigation of novel phenomena. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G24.00004: Development of Low Temperature Nuclear Magnetic Resonance Force Microscopy (NMRFM) Experiments for Probing Nanoscale Films and Microcrystals Jeremy Paster, Daniel Tennant, Shirin Mozaffari, John Markert Force detection of nuclear spins is accomplished by coupling NMR spin-flip sequences to a mechanical oscillator. A thin ferromagnet deposited on the tip of the oscillator sets up a large gradient magnetic field in the vicinity of the spins. This provides a magnetic force signature which we can distinguish from the thermal noise of the oscillator. The gradient field also traces out a slice in space in which spins are resonantly tuned to the RF field. We review the advantages of various strategies for inducing nuclear spin flips including cantilever-driven and RF-modulation techniques. We also report on the current state of the project, highlighting important developments and experimental results. In particular, we've adapted a low temperature NMRFM probe for easy transition between thin-film and microcrystal experiments. In one configuration, we orient the oscillator perpendicular to the sample plane so we can work in the region where the ferromagnet's field gradient is largest. Conversely, we can rotate the oscillator 90 degrees to change the geometry of the gradient field. With this orientation we maximize resolution in one dimension by using the flat part of the resonance slice to pick up as many in-plane nuclei as we can. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G24.00005: Scanning capacitance microscopy of atomically-precise donor devices in Si Ezra Bussmann, M. Rudolph, S.M. Carr, G. Subramania, G. Ten Eyck, J. Dominguez, M.P. Lilly, M.S. Carroll Recently, a scanning tunneling microscopy (STM) technique to fabricate atomically-precise dopant-based nanoelectronics in Si has been developed. Phosphorus donors are placed via an atomic-precision template formed by STM H-depassivation lithography, then capped with epi-Si and lastly metal contacts are made to the buried donor layer using conventional microfabrication. New challenges are introduced with this approach that center around difficulties to locate and characterize the pattern of buried donors. In this talk, we show that scanning capacitance microscopy (SCM) can image these buried donor nanostructures with sub-100-nm tip-limited resolution. The technique is used to successfully locate and characterize buried donor nanostructures relative to surface alignment marks. This approach relaxes alignment requirements for the STM lithography step and can offer improved alignment of subsequent metallization steps. The SCM technique is also used to nondestructively image the shape of the electronic carrier distribution and characterize the relative doping levels. This work, performed in part at the Center for Integrated Nanotechnologies, a U.S. DOE Office of Basic Energy Sciences user facility, was supported by Sandia's Lab Directed Research and Development Program. Sandia is a multi-program lab operated by Sandia Corp, a Lockheed-Martin Company, for U. S. DOE under Contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G24.00006: Quantum Many-Body Dynamics in Luminescence from Molecular Exciton and Plasmon Induced by Scanning Tunneling Microscopy Kuniyuki Miwa, Mamoru Sakaue, Branko Gumhalter, Hideaki Kasai In scanning-tunneling-microscope (STM)-induced light emission (STM-LE) from clean and molecule-covered metal surfaces, surface plasmons localized near the tip-substrate gap region play important roles in electronic excitations and radiative decays of molecules. A recent experiment succeeded to observe that the dynamics of the molecule (e.g., luminescence and energy absorption) have an influence on the luminescence spectral profiles of surface plasmons. To understand this from a microscopic point of view, there is a need to investigate the dynamics of the molecule and surface plasmons within the framework of quantum many-body theory. In this study, we construct the effective model of the system and investigate the effects of coupling between a molecular exciton and a surface plasmon (exciton-plasmon coupling) on the luminescence properties using the nonequilibrium Green's function method. It is found that in addition to the dynamics of the molecule, the dynamics of surface plasmons plays an essential role in determining the luminescence spectral profiles of surface plasmons. Prominent peak and dip structure observed in recent experiments are interpreted by the developed theory. The details of exciton-plasmon coupling on the luminescence properties will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G24.00007: Enhancing Stimulated Emission based Fluorescence Detection with Interferometric Setup Fu-Jen Kao Stimulated emission, being spatially coherent, supports unattenuated fluorescence detection at extended distance with low NA optics. We have demonstrated stimulated emission (SE) imaging in a long-working distance configuration. Additionally, the corresponding fluorescence lifetime imaging is realized by electronically controlling the time delay between the excitation and the SE pulses in the nanosecond ranges through pump-probe configuration. The fluorescence lifetime of selected fluorophores is accurately determined through the pump-probe configuration. However, the sensitivity of SE based fluorescence detection is usually limited by the dynamic range and saturation of photodetectors. We are showing that interferometric setup can greatly enhance the detection sensitivity by reducing the DC level of the stimulation beam with destructive interference. The results show that there are many interesting possibilities by combining interferometric techniques with stimulated emission based fluorescence detection. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G24.00008: Sub-Hz Linewidth harmonics in a microwave frequency comb generated by focusing a mode-locked ultrafast laser on the tunneling junction of a scanning tunneling microscope Mark Hagmann, Frank Stenger, Dmitry Yarotski A microwave frequency comb (MFC) with hundreds of measurable harmonics superimposed on the DC tunneling current is generated by optical rectification when focusing a mode-locked ultrafast laser on the tip-sample junction of a scanning tunneling microscope (STM). Using a Kerr-lens passively mode-locked Ti:Sapphire laser (CompactPro, Femtolasers) having a pulse repetition frequency of 74.25 MHz with a STM (UHV700, RHK Technology) operated in air, 200 harmonics from 74.25 MHz to 14.85 GHz have reproducible measured linewidths equal to the 1 Hz resolution bandwidth (RBW) of the spectrum analyzer. At the 200$^{\mathrm{th}}$ harmonic the signal-to-noise ratio is 20 dB. When the RBW exceeds 1 Hz the measured linewidth increases to remain equal to the RBW. However, for a RBW of 0.1 Hz the measured linewidth is distributed from 0.1 Hz to 1.2 Hz which we attribute to the stochastic behavior of the pulse repetition frequency in the unstabilized laser. Measurements of drift in the pulse repetition frequency and a derivation showing the effects of timing jitter support this hypothesis. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G24.00009: Variable temperature nano-optics in correlated electronic systems Adrian Gozar, Rainer Held, Darrell Schlom We report on the development and performance of instrumentation designed to study nano-scale optical properties of correlated electronic systems in a cryogenic environment. The main capability of our Variable-Temperature scattering-based Scanning Near-Field Optical Microscope (VT-SNOM) is to measure the complex dielectric function with a spatial resolution of 20-30 nm in a 10 K - 300 K temperature range. VT-SNOM measurements around the metal-insulator transition on 20 nm thick subsurface EuO films will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G24.00010: Imaging and quantification of electrical properties at the nanoscale using Scanning Microwave Impedance Microscopy (sMIM) Stuart Friedman, Oskar Amster Scanning Microwave Impedance Microscopy (sMIM) is a novel mode for AFM-enabling imaging of unique contrast mechanisms and measurement of local permittivity and conductivity at the 10's of nm length scale. We will review the state of the art, including imaging studies of microelectronic devices as well as novel materials and nanostructures, such as graphene and patterned optical crystals and ferro-electrics. In addition to imaging, the technique is suited for a variety of metrology applications where specific physical properties are determined quantitatively. We will present research results on quantitative measurements of dielectric constant (permittivity) and conductivity (e.g. dopant concentration). For samples where properties such as dielectric constant are known the technique can be used to measure film thickness. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G24.00011: Scanning near field microwave microscopy based on an active resonator Naser Qureshi, Oleg Kolokoltsev, Cesar Leonardo Ordonez-Romero A large number of recent implementations of near field scanning microwave microscopy (NFSMM) have been based on the perturbation of a resonant cavity connected to a sharp scanning probe. In this work we present results from an alternative approach: the perturbation of a microwave source connected to a scanning tip. Based on a yittrium iron garnet (YIG) cavity ring resonator this scanning probe system has a quality factor greater than 10$^{6}$, which allows us to detect very small frequency shifts, which translates to a very high sensitivity in sample impedance measurements. Using a selection of representative semiconductor, metal and biological samples we show how this approach leads to unusually high sensitivity and spatial resolution. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G24.00012: Spectral frustration and coherence in thermal near-field spectroscopy Brian O'callahan, William Lewis, Andrew Jones, Markus Raschke The thermal near-field is characterized by fundamentally distinct spatial, spectral, and coherence properties compared to far-field thermal radiation. Scattering scanning near-field microscopy (s-SNOM) has recently opened spectroscopic access to the enhanced electromagnetic local density of states associated with electronic and vibrational resonances. We study the influence of the tip on the scattered near-field spectral response due to the frustration of the evanescent thermal field by the tip. With the example of the extrinsic resonance of the surface phonon polariton (SPhP) in SiC we demonstrate redshifts by $0$ cm$^{-1}$ to $50$ cm$^{-1}$ of the unperturbed 948 cm$^{-1}$ resonance. We model the behavior as a result of tip-sample coupling or effective medium change due to the presence of the tip. We show that the effect is most significant for momentum dependent and strongly dispersive resonances. In addition, distance dependence measurements demonstrate a competition between scattering of the near-field associated with the thermally driven stochastically fluctuating optical polarization and that of the spatially coherent SPhP which is excited. The results indicate the possibility for local tuning of SPhP resonant conditions via evanescent thermal near-field coupling. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G24.00013: Highest resolution Confocal Raman-AFM-SNOM: Advantages and new insights for Graphene characterization Wei Liu, Ute Schmidt, Thomas Dieing An important goal of graphene study is precisely determining the number of layers forming the graphene flake. The aim of this contribution is to show how the confocal Raman AFM - SNOM can contribute to the characterization of graphene. In the past two decades, AFM (Atomic Force Microscopy)was one of the main techniques used to characterize the morphology of nano-materials. From such images it is possible to gain information about the physical dimensions of the material, but not their chemical composition, crystallinity or stress state. On the other hand, Raman spectroscopy is known to be used to unequivocally determine the chemical composition of a material. By combining the chemical sensitive Raman spectroscopy with high resolution confocal optical microscopy, the analyzed material volume can be reduced below 0.02 $\mu$m$^{3}$, thus leading to the ability to acquire diffraction limited resolution Raman images. Furthermore, using SNOM (Scanning Near-field Optical Microscopy) technology, it will be shown for the first time how the transparency of different graphene sheets is changing as a function of the number of layers. The combination of confocal Raman microscopy with AFM and SNOM is a breakthrough in microscopy. Using such a combination, the topographic information obtained with an AFM can be directly linked to the chemical information provided by confocal Raman and transparency properties obtained with SNOM. [Preview Abstract] |
Session G25: Focus Session: Organic Electronics and Photonics - Novel Devices and Fabrication
Sponsoring Units: DMPChair: David Gundlach, National Institute of Standards and Technology
Room: 503
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G25.00001: Solution Processable Electrochemiluminescent Ion Gels for Flexible, Low Voltage, Emissive Displays on Plastic Hong Chul Moon, Timothy P. Lodge, C. Daniel Frisbie We have expanded the functionality of ion gels and successfully demonstrated low voltage, flexible electrochemiluminescent (ECL) devices using patterned ECL gels. An ECL device composed of only an emissive gel and two electrodes was fabricated on an ITO-coated substrate by solution casting the ECL gel and brush-painting the top silver electrode. The device turned on at an AC voltage as low as 2.6 V ($-$1.3 V $\sim$ $+$1.3 V) and showed a relatively rapid response (sub-ms). Also, we varied the mechanical properties of the ECL gel simply by substituting polystyrene-block-poly(methyl methacrylate)-block-polystyrene (SMS) with commercially available poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)), enabling the fabrication of flexible ECL devices on any target substrate by the ``cut-and-stick'' strategy. This simple, rubbery ECL gel should be attractive for flexible electronics applications such as displays on packaging. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G25.00002: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G25.00003: Charge-extraction analysis of organic and inorganic dielectrics for organic field-effect transistors Josue Martinez Hardigree, Thomas Dawidczyk, Mathias Nyman, Ronald Osterbacka, Howard Katz Organic field-effect transistors (OFETs) offer the promise of low-power, inexpensively-processed electronic devices. However, high threshold voltages (Vt) required for operation and poor Vt stability due to gate bias stress in OFETs has limited their adoption in high duty-cycle applications such as display technology. Herein we employ the charge extraction in a linearly-increasing voltage (CELIV) method to investigate the Vt stability of polarized gate dielectrics consisting of pristine and polarized polystyrene (PS) and perfluorinated polystyrene (F-PS). CELIV measurements were carried out on representative gate stacks analogous to previously-investigated p-type OFETs (J. Appl. Phys. doi: 10.1063/1.1427136). We compare CELIV transients of pristine and polarized polystyrene (PS) and perfluorinated polystyrene (F-PS) dielectrics in representative pentacene OFET gate stacks, correlating transient differences and Vt stability to the observed surface potential measurements of lateral OFETs fabricated using a recently-developed method (Appl. Phys. Lett., doi: 10.1063/1.3684977). [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G25.00004: Enhanced performance of ferroelectric-based all organic transistors and capacitors through choice of solvent Grant Knotts, Anagh Bhaumik, Kartik Ghosh, Suchismita Guha We examine the role of solvents in the performance of pentacene devices using the ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFe) as a gate insulating layer. High dipole moment solvents such as dimethyl sulfoxide used to dissolve the copolymer for spin casting increase the charge carrier mobility in field-effect transistors by nearly an order of magnitude as compared to lower dipole moment solvents. The polarization in Al/PVDF-TrFe/Au metal-ferroelectric-metal devices is also investigated. An increase in remnant polarization of $\sim$ 20{\%} is observed in the sample using dimethyl sulfoxide as the ferroelectric solvent. Interestingly, at low applied electric fields of $\sim$ 100 MV/m a remnant polarization is seen in the high dipole moment device that is nearly 3.5 times larger than the value observed in the lower dipole moment samples, suggesting that the degree of dipolar order is higher at low operating voltages for the high dipole moment device. Detailed analysis of the capacitance characteristics of metal-insulator-semiconductor structure is performed. The density of interface trap states is nearly an order of magnitude lower for the high dipole moment device. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G25.00005: Micro-processing of Hybrid Field-Effect Transistor Arrays using Picosecond Lasers Robert Ireland, Yu Liu, Josef Spalenka, Supriya Jaiswal, Shingo Oishi, Kenshi Fukumitsu, Mochizuki Ryosuke, Padma Gopalan, Paul Evans, Howard Katz We use a solid-state picosecond laser to pattern thin-film semiconductors that completely cover a substrate and utilize an array of top-contact electrodes, particularly for materials with high chemical sensitivity or resistance. Picosecond laser processing is fully data-driven, both thermally and mechanically non-invasive, and exploits highly localized non-linear optical effects. We investigate FETs comprised of p-channel tellurium and organic semiconductor molecules sequentially vapor-deposited onto Si/SiO$_{2}$ substrates. Secondly, zinc oxide and zinc-tin oxide are used for high mobility n-channel FETs, cast onto Si/SiO$_{2}$ by sol-gel method. Finally, zinc oxide FETs are prepared as photomodulatable devices using rhenium bipyridine as a light-sensitive electron-donating molecule. The laser effectively isolates FETs while charge carrier mobility is maintained, but leakage currents through the FET dielectric are drastically reduced, and other functions are enhanced. For instance, the ratio of measured gate current to photocurrent for photomodulatable FETs drops from a factor of five to zero after laser isolation, in both forward and reverse bias. We also observe a threshold voltage shift in organic semiconductors after laser isolation, possibly due to local charging effects. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G25.00006: Investigating Organic Field Effect Transistors with Reduced Graphene Oxide Electrodes of Different Reduction Efficiency Narae Kang, Saiful I. Khondaker Organic field-effect transistors (OFETs) have received much attention owing to their flexibility, transparency, and low-cost of fabrication. One of the major limiting factors in fabricating high-performance OFET is the large injection barrier at metal electrodes/organic semiconductor interface, which results in low charge injection from metal electrodes to organic semiconductor. Graphene has been suggested as an alternative electrode material due to its high work function, extraordinary electronic properties and strong $\pi $-$\pi $ interaction with organic molecule; all of which can reduce the injection barrier at the electrode/organic interface. In particular, due to its solubility, large scale production, and its chemical functionality, reduced graphene oxide (RGO) has been introduced as a promising electrode for OFETs. Its tunability of electrical and optical properties can make RGO a highly desired electrode material because the work function match is essential for better charge injection at electrode/organic interface. In this talk, we will discuss the fabrication of OFETs with RGO of different reduction efficiency as an electrode material. We will also present the electrical transport properties fabricated devices. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G25.00007: Effect of device geometry on organic electrochemical transistor switching speeds Jacob Friedlein, Robert McLeod, Sean Shaheen It has been demonstrated that redox switching of PEDOT:PSS can be used as the basis for the modulation of channel current in organic electrochemical transistors (OECTs). In this work, we examine the response time of OECTs with different geometries and electrolyte compositions. In particular, we demonstrate which OECT dimensions (thickness, channel length, channel width, and gate-channel distance) have the strongest influence on switching speed and how the switching speed scales with these dimensions. We fabricate our OECTs on glass with evaporated gold electrodes. The PEDOT:PSS is spin-cast to form an approximately 100 nm film. OECT channels are defined by subtractively patterning the PEDOT:PSS film using a microcutter, and the electrolyte is printed from solution using a microcontact printing platform (for solid-state electrolytes) or drop cast (for liquid electrolytes). We characterize these devices with drain voltages $\sim$ 100 mV and gate voltages of $\sim$ 2 V, and we typically obtain drain currents of $\sim$ 50 $\mu$A with ON/OFF ratios up to 100 and switching times of 10 -- 1000 s. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G25.00008: A Facile Approach for P3HT/ZnO hybrid Synthesis for Solar Cell Application Chi-An Dai, Yi-Huan Lee, Yu-Ping Lee, Yang-Hui Yang, Leeyih Wang The fabrication of organic/inorganic hybrid materials based on conducting polymers and semiconducting nanoparticles has gathered great attentions recently due to its potential applications in renewable energy such as solar cells. However, problems occur as the two pre-synthesized materials are mixed since the resulting hybrids typically undergo macrophase separation with increasing nanoparticle loadings, leading to reduced charge separation and transport. To this end, a number of methods have been pursued to achieve a favorable dispersion of nanoparticles in hybrid solar cells. In this study, we have developed an in-situ synthesis method followed by a heating treatment to grow highly elongated P3HT/ZnO nanofibrils. The optoelectronic property and the solar cell performance of the resulting hybrid will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G25.00009: Assembly of Hybrid Solar Cells: Polythiophene Wrapped CdSe Nanorods Sirinya Chantarak, Todd Emrick, Thomas P. Russell We prepared cadmium selenide nanorods (CdSe NRs) covered with three types of polythiophenes: poly(3-hexylthiol thiophene), poly(3-hexylamine thiophene), and poly(3-hexylphosphonate thiophene) with thiol, amine, and phosphonate functional groups, respectively, to anchor to the nanorods. This led to a thin layer of p-type conducting polymer covering the n-type inorganic nanorods. A vertically-oriented assembly of polythiophene-functionalized CdSe NRs on a conducting substrate was obtained with the use of an applied electric field. Ternary nanocomposites of CdSe-polythiophene-graphene were obtained via $\pi $-$\pi $ stacking. These oriented CdSe NRs-polythiophenes nanocomposites y potential applications in hybrid photovoltaic devices. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G25.00010: Morphology control of phase separated ferroelectric-semiconductor polymer blends for organic memory Gregory Su, Andrew Jacobs, Edward Kramer, Michael Chabinyc The ability to store memory is essential for many electronic applications. All-organic memory devices based on a blend of a ferroelectric polymer and a semiconducting polymer have recently shown great promise for low-cost memory technology based on ferroelectricity. The thin film morphology of the phase separated ferroelectric-semiconductor polymer blend is critically important for working devices and improved operation. However, precise morphology control has so far been relatively unattainable. Here, we report on a new semiconducting polythiophene with a modified side chain structure (PEPT) that demonstrates a greatly improved phase separated morphology with the well-studied ferroelectric polymer poly[(vinylidenefluoride-co-trifluoroethylene] (PVDF-TrFE). Thin film surface and bulk characterization via microscopy, soft X-ray spectroscopy, and X-ray scattering experiments reveal that PEPT:PVDF-TrFE blends exhibit domain sizes that are easily tunable through simple parameters such as blend ratio. These results demonstrate progress toward achieving organic ferroelectric-semiconductor memory with optimized morphology and the techniques required for thorough thin film surface and bulk characterization. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G25.00011: Progress Toward Tunable White Light-Emitting Electrochemical Cells Tyko Shoji, Amanda Norell Bader, Janelle Leger The high photoluminescence efficiency, narrow emission peaks, and size-tunable band gaps of quantum dots (QDs) make them attractive for application to light emitting devices. However, charge injection barriers due to the insulating surface ligands of QDs often result in undesired emission from the polymer host material. Additionally, typical QD devices have also suffered from voltage-dependent emission color, most likely caused by shifts in the emission zone under different applied voltages. One promising approach to addressing these issues is through the incorporation of QDs in a single layer light-emitting electrochemical cell (LEC). In the generally accepted model of LEC operation, an analog of a self-assembled p{\-}i{\-}n junction forms under an applied bias. The homogenous blend of QDs throughout the polymer ensures a consistent concentration of QDs in the emission zone despite recombination zone shifts during operation. Light emission occurs within a thin intrinsic region, facilitating QD emission and limiting emission from the polymer host. Our group has demonstrated precise color-tunable emission in QD LECs by adjusting the mass ratios of two different quantum dots blended in a single LEC. We discuss our progress in extending these results to the development of white light-emitting QD LECs. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G25.00012: Normally-ON and Normally-OFF Carbon Nanotube-based Ionic-Liquid Supercapacitor-Gated Vertical Organic Field-Effect Transistors Jonathan Yuen, Alexander Cook, Julia Bykova, Vidisha Srivastav, Joseph Micheli, Anvar Zakhidov We report on novel implementations of the vertical organic field effect transistor (VOFET). Instead of a typical capacitor below the organic diode, a carbon nanotube (CNT) based ionic-liquid supercapacitor (or ionic gate) is on top. The present work has been motivated by the discovery that the conductivity and work function of carbon nanotubes can be strongly modified by electric double layer charging (EDLC) in an electrolyte as much as $+$/-0.7eV. The conductivity of EDLC CNT is enhanced by a factor of two. By coupling the ionic gate with an organic diode, charge injection into the diode can be controlled via modulation of the workfunction of the CNT electrode, resulting in transistor-like behavior. Additionally, the high capacitance of the supercapacitor will enable the VOFET to be operated at low voltages. The entire device is processed under ambient conditions with no vacuum equipment used. We have tested VOFETs with two different materials, p-type P3HT and n-type PC$_{70}$BM. The polarity of the charge transported in the material determines the charge injection rate and whether the device is a normally-ON or a normally-OFF transistor. Both devices have high current transport, excellent output characteristics, good on-off ratios and low operation voltages. We believe that these novel VOFETs will have exciting potential for various future electronic applications. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G25.00013: Studies on Structure Property Relations in Printed Polymer Semiconductors Nikhila Mahadevapuram, Saeed Ahmadi Vaselabadi, David Reza Shakarisaz, Joseph Strzalka, Paul Ruchhoeft, Gila Stein Printed polymer semiconductors can be used in systems which require precise control on domain placements and for sequential casting like in sensors, multi color light-emitting diodes or tandem solar cells. Morphology in polymer semiconductors places an important role on carrier mobility. Polymer crystals help in charge transport. In this work, we used helium ion beam lithography to irradiate polymer films and study crystallinity and carrier mobility. Thin films of poly (3-hexylthiphene) P3HT were irradiated with helium ion beam and light absorption properties were measured using UV-Vis spectroscopy. Crystal orientations in irradiated P3HT films were investigated using grazing incidence wide angle X-ray scattering (GIWAXS). Degree of crystallinity in irradiated polymer films were estimated by constructing pole figures. Charge mobility was estimated from device measurements. It was observed that the light absorption properties were retained in irradiated polymer films. Irradiation can influence both crystal orientations and charge mobility as a function of exposure dose. In summary, polymer crystallinity can be independently varied using this technique and a better understanding of the charge transport and device function can be established. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G25.00014: Transfer Printing Controlled by Substrate Thickness Michael Bartlett, Alfred Crosby Transfer printing techniques have played an important role in electronics, biology and other fabrication processes. These techniques have been demonstrated to control interfacial adhesion through prescribed actuation mechanisms, surface patterns, or by changing the interfacial adhesion energy through chemical treatments or kinetic control. Here we present a simple transfer printing mechanism which is governed by the geometric confinement of soft, polymeric substrates. As the substrate thickness decreases the adhesive force capacity increases, allowing objects to be printed to thinner substrates without any specific actuation, chemical treatment, or surface topography. This functionality is experimentally demonstrated by printing millimeter and centimeter-scale silicon wafers to progressively thinner substrates. We further show the selective transfer of objects based on position and how these techniques can be used in roll-to-roll processes. We support these experiments with a theoretical model which demonstrates how interfacial confinement enables the precise control of adhesive force capacity, as well as a mechanism to increase interfacial strength. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G25.00015: Fast Printing and In-Situ Morphology Observation of Organic Photovoltaics using Slot-Die Coating Feng Liu, Sunzida Ferdous, Cheng Wang, Alexander Hexamer, Thomas Russell The solvent-processibility of polymer semiconductors is a key advantage for the fabrication of large area, organic bulk-heterojunction (BHJ) photovoltaic devices. Most reported power conversion efficiencies (PCE) are based on small active areas, fabricated by spin-coating technique. In general, this does not reflect device fabrication in an industrial setting. To realize commercial viability, devices need to be fabricated in a roll-to-roll fashion. The evolution of the morphology associated with different processing parameters, like solvent choice, concentration and temperature, needs to be understood and controlled. We developed a mini slot-die coater, to fabricate BHJ devices using various low band gap polymers mixed with phenyl-C71-butyric acid methyl ester (PCBM). Solvent choice, processing additives, coating rate and coating temperatures were used to control the final morphology. Efficiencies comparable to lab-setting spin-coated devices are obtained. The evolution of the morphology was monitored by in situ scattering measurements, detecting the onset of the polymer chain packing in solution that led to the formation of a fibrillar network in the film. [Preview Abstract] |
Session G26: Focus Session: Materials in Extremes: Phase Transitions
Sponsoring Units: GSCCM DCOMP DMPChair: Renata Wentzcovich, University of Minnesota
Room: 502
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G26.00001: Computation of temperature induced phase transitions at high pressure Invited Speaker: Anatoly Belonoshko Phase transitions at high pressure and temperature is perhaps one of the most controversial topics in high pressure science. There is a number of theoretical methods developed recently to address this problem. Unfortunately, none of them is fully satisfactory, considering stringent requirements to accuracy of the computed free energies of the involved phases. This, in part, explains the variety of the suggested methods. The experimental data is rather controversial as well, probably because the experiments at extreme conditions are difficult. I will provide overview of the theoretical methods that are applied for the computation and simulation of T-induced phase transitions. Both liquid-solid and solid-solid transitions will be covered. Possible sources of the disagreement between theoretical methods as well as between theory and experiment will be illustrated by examples. Insight from simulations will be used to suggest alternative explanations of experimental data. The optimal, at present, method to compute the transitions will be suggested. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G26.00002: Ab-inition melting curve of titanium Vincent Stutzmann, Johann Bouchet, Francois Bottin Thermodynamical properties of titanium are of great interest for aerospace and aviation industries and many studies are done in order to understand its behaviour under pressure (P) and temperature (T) : phase transitions at low T, melting curve at high T and P. In this work we compute the first \textit{ab-initio} melting curve of titanium. This one is obtained with the \textsc{Abinit} package using DFT, in the GGA approximation, and in the framework of the projector augmented wave method (PAW). At first, we perform ground state calculations and study the five allotropic phases of titanium. Two PAW atomic data are generated with two different cutoff radius. The larger one gives results near previews \textit{ab-initio} calculations, whereas the smaller one gives results near all electron calculation. Using the second PAW atomic data and performing \textit{ab-initio} molecular dynamic simulations, we then compute the melting curve of titanium with three different methods. Results show relevance of our calculations, but also discrepencies with experimental data. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G26.00003: Is Orthorhombic Iron Tetraboride Superhard? Bo Xu, Qianqian Wang, Julong He, Yongjun He Millimeter-sized FeB$_{4}$ bulks were synthesized under high pressure of 15 GPa and 1600 $^{\circ}$C. Multiple analysis methodologies including XRD, SEM, TEM, and Raman spectroscopy verified this new phase with an oP10 crystal structure. A relatively low hardness value of 15.4 GPa, which is consistent with both macroscopic and microscopic hardness models, excludes FeB$_{4}$ as a superhard material. Resistance and magnetization measurements do not indicate superconductivity in FeB$_{4}$for temperature as low as 2.5 K. Previously reported superconductivity in FeB$_{4}$ needs a further investigation, especially for structural imperfections, unidentified phases, and/or contaminations. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G26.00004: Solidification and fcc- to metastable hcp- phase transition in krypton under modulating dynamic pressures Jing-Yin Chen, Choong-Shik Yoo, Minseob Kim, Hanns-Peter Liermann, Hyunchae Cynn, Zsolt Jenei, William Evans We describe high-pressure kinetic studies of the solidification, melting and fcc-hcp transitions of Krypton under dynamic loading conditions, using a \textit{dynamic}-diamond anvil cell ($d$-DAC) coupled with time-resolved x-ray diffraction. The time-resolved diffraction patterns and dynamic pressure responses show compression-rate dependencies associated with both the decay and growth time constants of the liquid-solid and solid-liquid transitions. According to the Avrami equation, both the solidified and melting processes are spontaneous nucleation and a rod-like (1-D) growth. Furthermore, under dynamic loading conditions, Kr-hcp forms from fcc close to the melting line. The nucleation time of fcc and hcp are very fast, with little dependence of compression rates or shorter than the time resolutions. The threshold pressure for the formation of Kr-hcp is $\sim$ 0.8 GPa at the dynamic loadings of 0.004-13 GPa/s. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G26.00005: Compositional changes upon compression of sodium azide predicted using density functional theory Brad Steele, Aaron Landerville, Ivan Oleynik The pressure induced phase transitions in sodium azide, which include a potential polymeric nitrogen phase transition, are investigated using evolutionary crystal structure prediction methods coupled with density functional theory calculations. Two new phases are predicted to be stable above 53 GPa that have an inequivalent ratio of sodium to nitrogen atoms as compared to sodium azide. The Raman spectrum is calculated from 0-100 GPa using these newly predicted structures, as well as the newly discovered I4/mcm phase of sodium azide. The predicted Raman spectrum is shown to give good agreement to experimental data above 30 GPa and below 15 GPa. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G26.00006: Phase transition in liquid hydrogen at high pressure and temperature Katsuya Shimizu, Kenji Ohta, Kota Ichimaru, Takahiro Matsuoka, Yasuo Ohishi, Naohisa Hirao The heating rate measurements indicated the existence of a transition in the liquid phase of hydrogen. A gold thin foil was loaded into the sample chamber of DAC with liquid hydrogen and compressed up to 100 GPa. The gold foil was heated by IR laser and the temperature was measured by the radiation. The anomaly was found on the temperature curve as a function of the laser power which indicates a transition at the temperature. The absorptance change indicates the heat flow rate change from the gold foil to the diamond surface through the hydrogen. The condition of the pressure and temperature of the transition is good agreement with the previous theoretical and experimental reports. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G26.00007: New Phase Transition in High Pressure Molecular Oxygen Yanier Crespo, Sandro Scandolo, Erio Tosatti Oxygen, one of the most common and important elements in nature, has an exceedingly well explored phase diagram under pressure, up and beyond 100 GPa. We propose a subtle, yet surprisingly undetected pressure-induced phase transition of molecular O$_2$ within the $\epsilon$ phase, near 20 GPa. While thus far in the whole $\epsilon$ phase region from about 8 to 96 GPa each individual O$_2$ molecule was generally believed to be in an S=0 state, we provide theoretical results, strongly connecting with existing vibrational and structural evidence, showing that this holds only above 20 GPa, whereas there is a collective switch from S=0 to an S=1 molecular state at and below 20 GPa. The former $\epsilon$ phase thus breaks up into two: a spinless $\epsilon_0$ (20-96 GPa), and $\epsilon_1$ (8-20 GPa) where the molecules possess spin with at least short range antiferromagnetic correlations. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G26.00008: Novel Si networks in Ca/Si phase diagram under pressure Guoying Gao, Neil Ashcroft, Roald Hoffmann In the Ca/Si phase diagram, many compositions are known. In these calcium silicides, silicon atoms form many different organizations, for example, at low pressure silicons are isolated silicon atoms in Ca$_{\mathrm{2}}$Si, Si chains in CaSi and corrugated hexagonal Si layers and a three-dimensional network of \textit{sp}$^{\mathrm{2}}$ bonds in CaSi$_{\mathrm{2}}$. The crystal structures for these silicides under pressure have not been studied completely, and we are very interested in the new chemical and physical behavior of Si in these silicides under pressure. Therefore, we take a theoretical study of Ca$_{\mathrm{2}}$Si, CaSi and CaSi$_{\mathrm{2}}$ under pressure. We predicted many interesting Si networks in the calcium silicides under pressure. Si atoms form Si chains in Ca$_{\mathrm{2}}$Si, flat quadrangular and hexagonal Si layers in CaSi, and 6-coordinated Si tetrahedrons and 4, 8-coordinated Si octahedrons in CaSi$_{\mathrm{2}}$ at high pressure. All of these predicted structures are dynamically stable. Moreover, these calcium silicides are all metals. Some of them are good candidates to be superconductors. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G26.00009: High pressure Raman spectroscopy of phase change materials Wen-Pin Hsieh, Peter Zalden, Matthias Wuttig, Aaron Lindenberg, Wendy Mao We used high-pressure Raman spectroscopy to study the evolution of vibrational frequencies of the phase change materials (PCMs) Ge$_{2}$Sb$_{2}$Te$_{5}$, GeSb$_{2}$Te$_{4}$, and SnSb$_{2}$Te$_{4}$. We found that the critical pressure for triggering amorphization in the PCMs decreases with increasing vacancy concentration, demonstrating that the presence of vacancies, rather than differences in the atomic covalent radii, is crucial for pressure-induced amorphization in PCMs. Compared to the \textit{as-deposited} amorphous phase, the \textit{pressure-induced} amorphous phase has a similar vibrational spectrum, but requires much lower laser power to transform into the crystalline phase, suggesting different kinetics of crystallization, which may have implications for applications of PCMs in non-volatile data storage. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G26.00010: Pressure-Induced Amorphization in Single-Crystal Ta$_{2}$O$_{5}$ Nanowires: A Kinetic Mechanism and Improved Electrical Conductivity Xujie Lu, Qingyang Hu, Wenge Yang, Ligang Bai, Howard Sheng, Lin Wang, Fuqiang Huang, Jianguo Wen, Dean Miller, Yusheng Zhao Pressure-induced amorphization (PIA) in single-crystal Ta$_{2}$O$_{5}$ nanowires is observed at 19 GPa and the obtained amorphous Ta$_{2}$O$_{5}$ nanowires show significant improvement in electrical conductivity. The phase transition process is unveiled by monitoring structural evolution with in-situ synchrotron XRD, PDF, Raman spectroscopy and TEM. The first principles calculations reveal the phonon modes softening during compression at particular bonds, and the analysis on the electron localization function also shows bond strength weakening at the same positions. Based on the experimental and theoretical results, a kinetic PIA mechanism is proposed and demonstrated systematically that amorphization is initiated by the disruption of connectivity between polyhedra at the particular weak-bonding positions along the a-axis in the unit cell. The one-dimensional morphology is well preserved for the pressure-induced amorphous Ta$_{2}$O$_{5}$ and the electrical conductivity is improved by an order of magnitude compared to traditional amorphous forms. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G26.00011: Nanowire Ice of Phase VI and Distorted VII in Mesoporous Silica Nanotorus Superlattice Jinlong Zhu, Jianzhong Zhang, Yusheng Zhao The motivation of nano H$_{2}$O realization and characterization is the highly polarized nature of H$_{2}$O molecules and the spatial hydrogen bonded networks both in liquid and solid form. The hydrogen bonding character of water molecules results in a remarkably rich phase diagram in the pressure-temperature space. Water/Ice confined in nanochannels showed novel structures and properties as results of hydrophobic and hydrophilic interactions and hydrogen bonding interaction between water molecule and the surface of nanochannel. Studies on nano H$_{2}$O can provide potential pathway to understand the complicated structure evolutions of ice in the $P$-$T$ space, because the interplay between nano-confinement and strong intermolecular hydrogen interactions can lead to even richer ice structures which were not found in the none-confined bulk form. The high pressure experiment indicated that the pressure of nanowire ice VI and VII shifted up to 1.7 GPa and 2.5 GPa, and about $\sim$ 0.65 GPa and 0.4 GPa higher than that of normal ice. The nano size effect and the strength of mesoporous silica nanotorus are responsible for the pressure shifts of ice phase regions. More pronounced, the cubic ice VII changed into a tetragonal distorted ``psuedocubic'' structure of the nanowire ice when confined in the mesoporous tubes. The degree of tetragonality increased with increasing pressure, which is resulted from the uniaxial pressure nanowire ice felt, and the anisotropic hydrogen bonding interactions including the H$_{2}$O-H$_{2}$O hydrogen bonds in the bulk of the ice and the H$_{2}$O-silica --OH hydrogen bonds between the interface of nanowire ice and mesoporous silica. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G26.00012: Giant pressure-induced volume collapse in the pyrite mineral MnS2 Simon Kimber, Ashkan Salamat, Shaun Evans, Harald Jeschke, Kaliappan Muthukumar, Milan Tomic, Francesc Salvat-Pujol, Roser Valenti, Maria Kaisheva, Ivo Zizak, Tapan Chatterji Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high temperature superconductivity. In transition metal materials, collapses are usually driven by so-called ``spin state'' transitions- the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S= 5/2 mineral Hauerite (MnS2) undergoes an unprecedented 22\% volume collapse driven by a conceptually new magnetic mechanism. Using synchrotron x-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic ground state using in-situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S= 1/2 moments are quenched by dimerisation. Our results show how the emergence of metal-metal bonding can stabilise giant spin-lattice coupling in Earth's minerals. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G26.00013: Nonhysteretic superelasticity of shape memory alloys at nanoscale Zhen Zhang, Xiangdong Ding, Xiaobing Ren, Kazuhiro Otsuka Superelasticity with recoverable strains about 8{\%} is known to appear in shape memory alloys (SMAs) possessing a spontaneous martensitic transformation (MT). However, it is unknown whether such a property can still exist below the critical size where MT disappears. We perform molecular dynamics simulations to show that SMA nanoparticles below the critical size not only demonstrate superelasticity but also exhibit features such as absence of hysteresis, continuous nonlinear elastic distortion, and high blocking force. Atomic level investigations show that this nonhysteretic superelasticity results from a continuous transformation from the parent phase to martensite under external stress. This aspect of SMAs is attributed to a surface effect; i.e., the surface locally retards the formation of martensite and then induces a critical-end-point-like behavior when the system is below the critical size. Our work potentially broadens the application of SMAs to the nanoscale. It also suggests a method to achieve nonhysteretic superelasticity in conventional bulk SMAs. [Preview Abstract] |
Session G27: Focus Session: Petascale Science and Beyond: Applications and Opportunities for Materials Science II
Sponsoring Units: DCOMPChair: Thomas Schulthess, ETH Zurich/ Swiss National Supercomputing Center (CSCS)
Room: 501
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G27.00001: Multi-Million-Atom Molecular Dynamics Simulations of Polymer Nanoparticle Composites using Explicit Solvent Treatment Sanket Deshmukh, Ganesh Kamath, Derrick Mancini, Subramanian Sankaranarayanan Poly(N-isopropylacrylamide) (PNIPAM) is a thermosensitive polymer that is well-known for its lower critical solution temperature (LCST) around 305K. Below the LCST, PNIPAM is soluble in water, and above this temperature, polymer chains collapse and transform into a globule-state. Our simulations of systems consisting of single polymer chains in presence of explicit water molecules ($\sim$ 50 K atoms) predicted the LCST of PNIPAM close to the observed experimental value of $\sim$ 305 K. This study also suggested the importance of using an explicit water model in studying the coil-to-globule transition in thermo-sensitive polymers. In the current studies, we are carrying out MD simulations of composites of PNIPAM inorganic nanoparticles in the aqueous solution using an explicit solvent treatment. We study the effect of grafting density on the coil-to-globule transition of the PNIPAM brushes. We graft PNIPAM polymer chains consisting of 60 monomer units onto a gold nanoparticle with varying grafting densities. Studied system consisted of $\sim$3 million atoms. All the simulations were carried out below (275K) and above (325K) the LCST of PNIPAM. Simulation trajectories are analyzed for structural and dynamical properties. In particular, we look at the morphology of the uncollapsed and collapsed structures, and relate this to observation in scattering measurements. Future work will expand this approach to studying the dynamics of agglomeration of such brush structures to form self-assembled nanocomposites. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G27.00002: Experiment-scale molecular simulation study of liquid crystal thin films Trung Dac Nguyen, Jan-Michael Y. Carrillo, Michael A. Matheson, W. Michael Brown Supercomputers have now reached a performance level adequate for studying thin films with molecular detail at the relevant scales. By exploiting the power of GPU accelerators on Titan, we have been able to perform simulations of characteristic liquid crystal films that provide remarkable qualitative agreement with experimental images. We have demonstrated that key features of spinodal instability can only be observed with sufficiently large system sizes, which were not accessible with previous simulation studies. Our study emphasizes the capability and significance of petascale simulations in providing molecular-level insights in thin film systems as well as other interfacial phenomena. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G27.00003: Petascale Molecular Dynamics Simulations of Thermal Annealing of P3HT:PCBM Active Layers in Bulk Heterojunctions Jan-Michael Carrillo, Rajeev Kumar, Monojoy Goswami, S. Michael Kilbey II, Bobby Sumpter, W. Michael Brown Using petascale coarse-grained molecular dynamics simulations, we have investigated the thermal annealing of poly(3-hexylthiophene) (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM) blends in the presence of a silicon substrate. The simulations were run on the Titan supercomputer using 21{\%} of the capacity of the machine. This is in contrast to recent studies, which were unable to obtain results representative of the entire thermal annealing process because of limited simulation time and size. The simulations are in agreement with neutron reflectivity (NR) and near edge X-ray fine structure (NEXAFS) experiments and reveal a vertical composition profile of the bulk heterojunction normal to the substrate with enrichment of PCBM near the substrate. We demonstrate that the addition of short P3HT chains, as a third component of the blend, can be used to alter the morphology of the active layer. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G27.00004: Petascale resources and CP2K: enabling sampling, large scale models or correlation beyond DFT Invited Speaker: Joost VandeVondele Already with modest computer resources, GGA DFT simulations of models containing a few hundred atoms can contribute greatly to chemistry, physics and materials science. With the advent of petascale resources, new length, time and accuracy scales can be explored. Recently, we have made progress in all three directions: \begin{enumerate} \item A novel Tree Monte Carlo (TMC) algorithm introduces a further level of parallelism and allows for generating long Markov chains. Sampling 100'000s of configurations with DFT, the dielectric constant and order-disorder transition in water ice Ih/XI has been studied.[1] \item The removal of all non-linear scaling steps from GGA DFT calculations and the development of a massively parallel GPU-accelerated sparse matrix library make structural relaxation and MD possible for systems containing 10'000s of atoms.[2] \item A well parallelized implementation of a novel algorithm to compute four center intergrals over molecular states (RI-GPW), allows for many-body perturbation theory (MP2, RPA) calculations on a few hundred atoms. Sampling liquid water at the MP2 level yields a very satisfying model of liquid water, without empirical parameters.[3,4] \end{enumerate} References:\\[0pt] [1] Mandes Sch\"onherr, Ben Slater, J\"rg Hutter, and Joost VandeVondele, submitted. \newline [2] Joost VandeVondele, Urban Borstnik, J\"urg Hutter, JCTC 8, 3565 (2012) \newline [3] Mauro Del Ben, Mandes Sch\"onherr, J\"urg Hutter, and Joost VandeVondele JPCL 4, 3753 (2013) \newline [4] Mauro Del Ben, J\"urg Hutter, and Joost VandeVondele, JCTC 9, 2654 (2013) \newline [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G27.00005: Optimizing GW for Petascale HPC and Beyond Jack Deslippe, Andrew Canning, Yousef Saad, James Chelikowsky, Steven Louie The traditional GW-Bethe-Salpeter (BSE) approach has, in practice, been prohibitively expensive on systems with more than 50 atoms. We show that through a combination of methodological and algorithmic improvements, the standard GW-BSE approach can be applied to systems with hundreds of atoms. We will discuss the massively parallel GW-BSE implementation in the BerkeleyGW package (on-top of common DFT packages) including the importance of hybrid MPI-OpenMP parallelism, parallel IO and library performance. We will discuss optimization strategies for and performance on many-core architectures. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G27.00006: Computing quasiparticle energies and band offsets for large systems Marco Govoni, Giulia Galli We present a massively parallel implementation $[1]$ of a method $[2]$ recently proposed for the calculations of quasiparticle energies of molecules and solids, which does not require the explicit evaluation of single particle virtual states. Explicit inversion and storage of large dielectric matrices are also avoided and frequency integration is explicitly carried out, without resorting to plasmon pole models. We present application to complex semiconducting interfaces, inclusive of order and disordered systems, with more than one thousand electrons. \\ $[1]$ M. Govoni and G. Galli, in preparation.\\ $[2]$ H-V. Nguyen et al. Phys. Rev. B 85, 081101(R) (2012); T.A. Pham et al. Phys Rev. B 87, 155148 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G27.00007: An accurate and scalable O(N) algorithm for First-Principles Molecular Dynamics computations on petascale computers and beyond Daniel Osei-Kuffuor, Jean-Luc Fattebert We present a truly scalable First-Principles Molecular Dynamics algorithm with O(N) complexity and fully controllable accuracy, capable of simulating systems of sizes that were previously impossible with this degree of accuracy. By avoiding global communication, we have extended W. Kohn's condensed matter ``nearsightedness'' principle to a practical computational scheme capable of extreme scalability. Accuracy is controlled by the mesh spacing of the finite difference discretization, the size of the localization regions in which the electronic wavefunctions are confined, and a cutoff beyond which the components of the overlap matrix can be omitted when computing selected elements of its inverse. We demonstrate the algorithm's excellent parallel scaling for up to 100,000 atoms on 100,000 processors, with a wall-clock time of the order of one minute per molecular dynamics time step. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G27.00008: Accelerating Hybrid Density Functional Theory Molecular Dynamics William Dawson, Francois Gygi For many systems, accurate First-Principles Molecular Dynamics (FPMD) simulations require the use of hybrid density functional theory. Molecular Dynamics requires short wall clock times and thus highly scalable parallel algorithms. The Qbox[1] code implements the recursive subspace bisection algorithm[2,3] which accelerates hybrid density functional theory calculations by creating a set of localized orbitals to reduce the number of exchange integrals computed. This approach allows for controlled accuracy and requires no a priori assumptions about localization. We discuss heuristic algorithms for improving the scalability and performance of this approach. We then demonstrate these improvements in applications to aqueous solutions and water-metal interfaces. \\{} [1] http://eslab.ucdavis.edu/software/qbox \\{} [2] F.~Gygi, Phys.~Rev.~Lett.~{\bf 102}, 166406 (2009).\\{} [3] F.~Gygi and I.~Duchemin, J.~Chem.~Theory Comput.~{\bf 9}, 582 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G27.00009: Phonon Quasi-Particles and Anharmonic Free Energy in Complex Systems Dong-Bo Zhang, Tao Sun, Renata Wentzcovitch We use a hybrid strategy to obtain anharmonic frequency shifts and lifetimes of phonon quasi-particles from first principles molecular dynamics simulations in modest size supercells. This approach is effective irrespective of crystal structure complexity and facilitates calculation of full anharmonic phonon dispersions, as long as phonon quasi-particles are well defined. We validate this approach to obtaining anharmonic effects with calculations in MgSiO$_{\mathrm{3}}$-perovskite, the major Earth forming mineral phase. First, we reproduce irregular temperature induced frequency shifts of well characterized Raman modes. Second, we combine the phonon gas model (PGM) with quasi-particle frequencies and reproduce free energies obtained using a direct approach such as thermodynamic integration. Using thoroughly sampled quasi-particle dispersions with the PGM we then obtain first-principles anharmonic free energy in the thermodynamic limit (N $\to \infty )$. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G27.00010: Million atom DFT calculations using coarse graining and petascale computing Don Nicholson, Kh. Odbadrakh, G.D. Samolyuk, R.E. Stoller, X.G. Zhang, G.M. Stocks Researchers performing classical Molecular Dynamics (MD) on defect structures often find it necessary to use millions of atoms in their models. It would be useful to perform density functional calculations on these large configurations in order to observe electron-based properties such as local charge and spin and the Helmann-Feynman forces on the atoms. The great number of atoms usually requires that a subset be ``carved'' from the configuration and terminated in a less that satisfactory manner, e.g. free space or inappropriate periodic boundary conditions. Coarse graining based on the Locally Self-consistent Multiple Scattering method (LSMS) and petascale computing can circumvent this problem by treating the whole system but dividing the atoms into two groups. In Coarse Grained LSMS (CG-LSMS) one group of atoms has its charge and scattering determined prescriptively based on neighboring atoms while the remaining group of atoms have their charge and scattering determined according to DFT as implemented in the LSMS. The method will be demonstrated for a one-million-atom model of a displacement cascade in Fe for which 24,130 atoms are treated with full DFT and the remaining atoms are treated prescriptively. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G27.00011: Atomic Structure Prediction with Large-Scale High Performance Computing Cai-Zhuang Wang, Bruce Harmon, Manh Cuong Nguyen, Xin Zhao, Kai-Ming Ho Many unknown binary or ternary materials for energy applications have very complex crystal structures, containing large number of atoms in their unit cells and possible uncertainty in composition. Computational prediction for atomic structures of such complex materials is a highly demanding work. Advances in modern large-scale high performance computational resources and computational algorithms now make it feasible to perform an efficient crystal structure prediction. We developed an adaptive genetic algorithm to perform large-scale structure search on high performance supercomputer. Examples of successful structure prediction/solving of complex materials will be presented. Further applications of the adaptive genetic algorithm to aid material discoveries will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G27.00012: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G27.00013: Dissipative Particle Dynamics Simulations at Extreme Scale: GPU Algorithms, Implementation and Applications Yu-Hang Tang, George Karniadakis \noindent We present a scalable dissipative particle dynamics simulation code, fully implemented on the Graphics Processing Units (GPUs) using a hybrid CUDA/MPI programming model, which achieves 10-30 times speedup on a single GPU over 16 CPU cores and almost linear weak scaling across a thousand nodes. A unified framework is developed within which the efficient generation of the neighbor list and maintaining particle data locality are addressed. Our algorithm generates strictly ordered neighbor lists in parallel, while the construction is deterministic and makes no use of atomic operations or sorting. Such neighbor list leads to optimal data loading efficiency when combined with a two-level particle reordering scheme. A faster \textit{in situ} generation scheme for Gaussian random numbers is proposed using precomputed binary signatures. We designed custom transcendental functions that are fast and accurate for evaluating the pairwise interaction. Computer benchmarks demonstrate the speedup of our implementation over the CPU implementation as well as strong and weak scalability. A large-scale simulation of spontaneous vesicle formation consisting of 128 million particles was conducted to illustrate the practicality of our code in real-world applications. [Preview Abstract] |
Session G29: Focus Session: Artificial Gauge Fields and Systems with Long Range Interactions I
Sponsoring Units: DAMOPChair: Victor Galitski, University of Maryland at College Park
Room: 603
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G29.00001: Spin Waves and Dielectric Softening of Polar Molecule Condensates Brandon Peden, Ryan Wilson, Charles Clark, Seth Rittenhouse We consider an oblate Bose-Einstein condensate of heteronuclear polar molecules in a weak applied electric field. This system supports a rich quasiparticle spectrum that plays a critical role in determining its bulk dielectric properties. In particular, in sufficiently weak fields, the system undergoes a polarization wave rotonization, leading to the development of textured electronic structure and a dielectric instability that is characteristic of the onset of a negative static dielectric function. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G29.00002: Measuring dipolar spin exchanges in ultracold polar KRb molecules Steven Moses, Bo Yan, Bryce Gadway, Jacob Covey, Kaden Hazzard, Ana Maria Rey, Deborah Jin, Jun Ye By encoding spin in rotational states, we have observed spin exchanges of ultracold polar KRb molecules that are confined in a deep three dimensional optical lattice [Yan \textit{et al.}, Nature \textbf{501}, 521 (2013)]. The interactions manifest as a density dependent decay of the spin coherence of the system, which is probed via Ramsey spectroscopy. In addition to decaying, there are oscillations in the contrast, with frequency components that are consistent with the dipolar interaction energies. By adding additional pulses, we can suppress pairwise dipolar interactions. We have studied these spin exchanges for two different pairs of rotational states, which differ by a factor of two in interaction strength, and find the decay and oscillations to be roughly twice as fast in the case of stronger interactions. This work lays the foundation for future studies of quantum magnetism with polar molecules in optical lattices. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G29.00003: New tools for far-from-equilibrium quantum spin dynamics inspired by ultracold molecule experiments Kaden Hazzard, Michael Foss-Feig, Bryce Gadway, Bo Yan, Steven Moses, Jacob Covey, Deborah Jin, Jun Ye, Ana Maria Rey We describe new numerical techniques based on a type of cluster expansion and analytic solutions for treating far-from-equilibrium dynamics in quantum many-body spin models. Specifically, we apply them to dynamics following a quantum quench that is routinely implemented in experiments with Ramsey spectroscopy. For many observables, these new approaches converge extremely rapidly compared to existing techniques, which are unable to converge using any feasible computational resources. We describe the theoretical methods and our understanding of their superior convergence. These calculations are motivated by recent experiments with ultracold molecules in optical lattices [ Yan \textit{et al.}, Nature \textbf{501}, 521 (2013) ] and trapped ions [ Britton \textit{et al.}, Nature \textbf{484}, 489 (2012) ], which are described by spin models with long-range interactions in appropriate limits. We will compare theoretical predictions with experimental observations in these systems. We expect the novel methods developed to describe ultracold matter to also have applications to solid state systems, for example in the dynamics of nitrogen-vacancy centers in diamond or energy transfer in complicated molecules. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G29.00004: Dipolar interactions of lattice-confined polar molecules Invited Speaker: Deborah Jin Long-range dipolar interactions can be used to realize lattice spin models for exploring quantum magnetism. I will discuss experiments where we observe dipolar spin-exchange interactions for ultracold KRb molecules confined in a deep three-dimensional optical lattice. The long-range dipolar interactions exist even in the absence of tunneling and extend beyond nearest neighbors. This enables coherent spin dynamics even for gases with relatively low lattice filling. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G29.00005: Propagation of information in long-range interacting lattice systems Zhe-Xuan Gong, Michael Foss-Feig, Spyridon Michalakis, Alexey V. Gorshkov Propagation of information in short-range interacting lattice systems is restricted to within a linear ``light cone,'' as demonstrated by the well-known Lieb-Robinson bound, thus ensuring a well defined notion of maximum propagation velocity. Whether long-ranged interactions can lead to a different shape of this light cone, and the divergence of the associated velocity, is an important but largely unexplored question. We prove that for a wide class of long-range interacting lattice systems, a linear light cone still exists for certain regions of space and time, and for some experimentally relevant classes of models this linear light cone persists in the entire space-time. We then give counter-examples showing that, for well-engineered lattice system, long range interactions can indeed give rise to a sub-linear ``light cone,'' and thus a divergent speed of information propagation. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G29.00006: Generation of many-body entanglement in long-range interacting systems Michael Foss-Feig, Zhe-Xuan Gong, Alexey Gorshkov, Charles Clark The existence of long-ranged interactions generally complicates the description of many-body systems. However, in the limit where the interactions become infinitely long-ranged---i.e. independent of distance---the emergence of extra conserved quantities typically makes the behavior quite simple. Such infinite-ranged interactions are often assumed in the description of experiments aiming to produce large scale entangled states, for instance via spin-squeezing, but of course ``infinite'' in this context is an idealization. We consider the generation of entanglement in Ising models with long (but not infinite) ranged interactions, which are relevant to the description of a variety of quantum information/simulation platforms including trapped ions, polar molecules, Rydberg atoms, and nitrogen vacancy centers in diamond. We demonstrate that there exists a notion of sufficiently long-ranged interactions, for which the scaling of entanglement with system size expected from the infinite-range idealization is completely unmodified. Our results have direct applications to experimental protocols aiming to achieve quantum-enhanced metrology. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G29.00007: Spin dynamics and entanglement growth with trapped ions, atoms \& molecules Johannes Schachenmayer, Ben Lanyon, Christian Roos, Andrew Daley, Bihui Zhu, Ana Maria Rey Trapped ions and systems of cold atoms or molecules in optical lattices offer controlled environments to experimentally study non-equilibrium dynamics of many-body quantum spin-models with interactions of varying range. Theoretically calculating dynamics of observables for these experiments is a major challenge both analytically and numerically. In 1D, the growth behavior of the entanglement entropy between different blocks of a many-body state determines whether the evolution of the system can be efficiently simulated on a classical computer or not. In return, the study of entanglement growth can guide experiments to regimes where a quantum simulator can outperform a numerical simulation. Here we present results on the entanglement growth behavior in 1D strings of ions after a quench, and show how the growth depends on the range of the interactions. Furthermore we report on progress on methods for higher dimensional systems. These can be used to model Ramsey-dynamics for current experiments with alkaline earth atoms or polar molecules in optical lattices, or for systems with Rydberg atoms. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G29.00008: Engineered long-range interactions on a 2D array of trapped ions Joseph W. Britton, Brian C. Sawyer, John J. Bollinger, James K. Freericks Ising interactions are one paradigm used to model quantum magnetism in condensed matter systems. At NIST Boulder we confine and Doppler laser cool hundreds of $^9$Be$^+$ ions in a Penning trap. The valence electron of each ion behaves as an ideal spin-1/2 particle and, in the limit of weak radial confinement relative to axial confinement, the ions naturally form a two-dimensional triangular lattice. A variable-range anti-ferromagnetic Ising interaction is engineered with a spin-dependent optical dipole force (ODF) through spin-dependent excitation of collective modes of ion motion. We have also exploited this spin-dependent force to perform spectroscopy and thermometry of the normal modes of the trapped ion crystal. The high spin-count and long-range spin-spin couplings achievable in the NIST Penning trap brings within reach simulation of computationally intractable problems in quantum magnetism. Examples include modeling quantum magnetic phase transitions and propagation of spin correlations resulting from a quantum quench. The Penning system may also be amenable to observation of spin-liquid behavior thought to arise in systems where the underlying lattice structure can frustrate long-range ordering. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G29.00009: Effects of the anisotropic dipole-dipole interaction in few-particle quantum systems Gunnar Eriksson, Jonas C. Cremon, Stephanie M. Reimann Recent realizations of condensates of atoms with large magnetic dipole moments, ultracold gases of dipolar molecules and Rydberg atoms have made it possible to study systems with a long range and spatially anisotropic interaction. These special properties of the interaction is presumed to give rise to new and exotic phases and other many-body phenomena and they can be altered by changing the direction of the dipoles with e.g. an external field. For ultracold gases there is now also the possibility to access the few-particle regime. Combining these techniques, few-particle systems with the dipole-dipole interaction, resembling e.g. electrons in a quantum dot or nucleons in a nuclei, could be studied. We have performed exact diagonalization to systems of a few fermions interacting with the dipole-dipole interaction in a quasi-2D system. For such a system under rotation, the anisotropy of the interaction can be used to reveal the vortex structure in the particle density and the probability current (Phys. Rev. A 86, 043607). The anisotropy also allows the system to reduce the interaction energy by more exotic particle arrangements, which leads to new behavior that can be externally tuned. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G29.00010: Effective mass and superfluid fraction in a periodic ultracold Bose gas with long-range interaction Che-Hsiu Hsueh, Yu-Ching Tsai, Wen-Chin Wu The scheme that superfluid fraction $f_s=\rho_s/\rho$ is equal to the ratio of bare to effective mass $m/m^*$ of the particles is applied to investigate the superfluid-supersolid (SF-SS) transition in a periodic ultracold Bose gas with long-range interaction. We consider both the Rydberg-dressed Bose gas with tunable blockade radius and coupling constant and the dipolar Bose gas with controllable coupling constant and polarization. Both the strong and week lattice limits are investigated. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G29.00011: Strongly interacting photons Mohammad Maghrebi, Ofer Firstenberg, Alexey Gorshkov We develop a quantum theory of light strongly interacting via long-range Rydberg-Rydberg interactions within the framework of the electromagnetically induced transparency. We elaborate on the interaction between photons, and discuss attractive as well as repulsive regimes. Specifically, we find solitonic bound states of photons, and explain their propagation inside the medium. Finally, we discuss the possibilities of many-body phases of strongly interacting light. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G29.00012: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G29.00013: Cavity Optomechanics with synthetic Landau levels of ultra cold Fermi gas Sankalpa Ghosh, Bikash Padhi Ultra cold fermionic atoms placed in a synthetic magnetic field arrange themselves in Landau levels. We theoretically study the optomechanical interaction between the light field and collective excitations of such fermionic atoms in synthetic magnetic field by placing them in side a Fabry Perot cavity. We derive the effective hamiltonian for particle hole excitations from a filled Landau level using a bosonization technique and obtain an expression for the cavity transmission spectrum. Using this we show that the cavity transmission spectrum demonstrates cold atom analogue of Subnikov de Hass oscillation in electronic condensed matter systems. We discuss the experimental consequences for this oscillation for such system and the related optical bistability. Ref. Bikash Padhi and Sankalpa Ghosh, Physical Review Letters, Vol 111, 043603 (2013) [Preview Abstract] |
Session G30: Focus Session: Graphene Devices: Fabrication, Characterization and Modeling: Solitons in Few Layer Graphene
Sponsoring Units: DMPChair: James Hone, Columbia University
Room: 605
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G30.00001: Interacting wrinkles in graphene on patterned substrates Zoe Budrikis, Alessandro L. Sellerio, Zsolt Bertalan, Stefano Zapperi The wrinkling of graphene on patterned substrates is interesting both because graphene is an exemplary thin sheet with effective mechanical thickness less than 1 angstrom, and because of the importance of strain for graphene's electronic properties. We present a combination of atomistic and large-scale coarse-grained numerical simulations of graphene on top of a substrate of size $\sim 1 \mu$m$^2$ decorated with nanoparticles of diameter $\sim 10$nm. We are able to reproduce previous experimental results in which substrate protrusions are connected by a network of long narrow wrinkles [1], and we clarify the role of substrate-graphene interactions in determining the morphology of these. Our simulations also allow us to explore in further detail a previously-overlooked feature, namely the possibility for interacting wrinkles to form stable ``avoiding'' configurations, in a manner reminiscent of interacting cracks [2]. By nucleating and growing wrinkles in a controlled way, we are able to characterize the role of long-range stress fields in determining whether two wrinkles will avoid or merge. \\[4pt] [1] M. Yamamoto et al, Phys. Rev. X 2, 041018 (2012).\\[0pt] [2] M. L. Fender, F. Lechenault, and K. E. Daniels, Phys. Rev. Lett. 105, 125505 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G30.00002: Ripple domains on graphene on a SiO$_2$ substrate Sungjong Woo, Jin Sik Choi, Young Jun Chang, Young-Woo Son, Yeonggu Park, Mi Jung Lee, Ik-Su Byun, Jin-Soo Kim, Choon-Gi Choi, Aaron Bostwick, Eli Rotenberg, Bae Ho Park Out-of-plane lattice distortions in two-dimensional materials are prevalent among structural movements at finite temperature. Graphene's negative thermal expansion coefficient is a direct consequence of such an intrinsic property. In our recent work, we have shown that friction measurements on graphene exfoliated on a silicon oxide substrate exhibit an anomalous anisotropy whose origin is attributed to the formation of ripple domains. We further uncover the atomistic origin of the observed friction domains using a newly developed method called cantilever torsion microscopy (CTM) together with angle-resolved photoemission spectroscopy (ARPES) measurements. We experimentally demonstrate that ripples on graphene are formed along the zigzag direction of the hexagonal lattice. We have also calculated theoretically the bending stiffness of carbon-carbon bonds and adhesive interactions between graphene and the surface underneath it that are consistent with our experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G30.00003: Transport in Strained Graphene with Applied Magnetic Fields Juan Aguilera-Servin, Marc Bockrath Strain in graphene layers produces synthetic gauge fields that may be used to modify the properties of its electron system [1,2]. We study single layers of graphene transferred over Ti/Au electrical contacts on oxidized Si wafers with etched triangular holes in the oxide. The layers are strained by applying pressure either electrostatically from a gate voltage or hydrostatically from an external inert gas. We investigate electronic transport in this suspended variable-strain graphene system under applied magnetic fields and find that the device conductance is modulated by the external pressure [3] as well as by the Hall effect. We will discuss our latest results.\\[4pt] [1] Guinea, F., Katsnelson, M. I., Geim, A. K. Energy gaps and a zero-field quantum Hall effect in graphene by strain engineering. Nat. Phys. 6, 30-33 (2009).\\[0pt] [2] Levy, N., et al. Strain-induced pseudo--magnetic fields greater than 300 tesla in graphene nanobubbles. Science, 329 544-547 (2010).\\[0pt] [3] Smith, A. D., et al., Pressure sensors based on suspended graphene membranes. Solid-State Electron. 88, 89-94 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G30.00004: Interlayer strain solitons in bilayer graphene Invited Speaker: Paul McEuen The interlayer registry between graphene layers can have dramatic effects on the physical and electronic properties of few-layer graphene. For example, in the presence of a perpendicular electric field, a band gap appears in the electronic spectrum of so-called Bernal-stacked graphene. This band gap is intimately tied to a structural spontaneous symmetry-breaking, where one of the graphene layers shifts by an atomic spacing with respect to the other. This shift can happen in multiple directions, resulting in stacking domains with soliton-like structural boundaries between them. Theorists have recently proposed that novel electronic states exist at these boundaries, but very little is known about their structural properties. Here we use electron microscopy to measure with nanoscale and atomic resolution the widths, motion, and topological structure of soliton boundaries and related topological defects in bilayer graphene [1]. We find that each soliton consists of an atomic-scale registry shift between the two graphene layers occurring over 6-11 nm. We infer the minimal energy barrier to interlayer translation and observe soliton motion during in-situ heating. The abundance of these structures across a variety samples, as well as their unusual properties, suggests that they will have substantial effects on the electronic and mechanical properties of bilayer graphene. \\[4pt] [1] Jonathan S. Alden, Adam W. Tsen, Pinshane Y. Huang, Robert Hovden, Lola Brown, Jiwoong Park, David A. Muller, and Paul L. McEuen, PNAS 110, 11256-11260 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G30.00005: Electronic properties of strained graphene: A new Dirac Hamiltonian that solves several issues in previous ones Maurice Oliva-Leyva, Gerardo Naumis In the literature, strained graphene is usually described using a combination of tight-binding model and linear elasticity theory. This leads to the appearance of pseudomagnetic fields. However, as we will discuss, the resulting equations contains some problems. The most important is that the Fermi energy does not appears in the high-symmetry points of the reciprocal lattice as assumed in previous theories. In this work we consider the particular case of a uniform strain, which can be solved in some cases without any approximation. Then the dispersion relation is simply shifted and deformed. Among our results highlights [1], we present analytical expressions for the shift of the Dirac points and the corresponding Dirac-like equation, which cannot be derived from the usual theory of the strain-induced pseudomagnetic field. The obtained Dirac-like equation is tested against tight-binding computer simulations showing a good agreement. As an application, we generalize the frequency-dependent conductivity expression of graphene under uniform strain, which is calculated by using the Kubo formula. Finally, we discuss a generalization of our results, for the case of nonuniform strain. \\[4pt] [1] M. Oliva-Leyva, and G.G.Naumis, PRB 88, 08543 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G30.00006: Strain Effects in Graphene Transport Measurements on Micropatterned Substrates J. Henry Hinnefeld, Stephen Gill, Nadya Mason Since its initial isolation in 2004, graphene has been the subject of intense study due to its extraordinary electrical and mechanical properties. However, the interplay between these properties remains comparatively unexplored. Here we present transport and scanning probe microscopy measurements of graphene devices on micropatterned substrates, where the interactions between surface adhesion, mechanical strain, and electrical conductivity can be observed. We find a positive correlation between strain applied via the substrate and electrical resistivity, and explore the mechanisms responsible for this increase, including surface delamination, microcrack formation, and mechanical strain in the graphene lattice structure, using atomic force microscopy measurements. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G30.00007: Effect of homogeneous strain on the Landau levels and the Klein tunneling in graphene Yonatan Betancur, Romeo de Coss We analyze the effect of homogeneous strain on the Landau levels (LLs) spectra and Klein tunneling of graphene. Using the tight-binding approach to first nearest neighbors, we study the electron dynamics in graphene under homogeneous strained and uniform perpendicular magnetic field. We obtain an analytical expression for the energy of the LLs in function of strain for low magnetic fields. For Klein tunneling, we study how can change the tunneling transmission when the graphene layer is under a homogeneous strain. In particular, we analyze the effects of uniaxial, shear, and isotropic strain and combinations of these deformations. Our results show that homogeneous deformations in graphene induce changes in the LLs spectra or Klein tunneling, due to the linear dependence of the effective Fermi velocity with the tensile strain. The effects of homogeneous strain on conductivity, Hall resistance, and others electronic properties of graphene, are discussed. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G30.00008: Controlling thermal and electrical properties of graphene by strain-engineering its flexural phonons Hiram Conley, Ryan Nicholl, Kirill Bolotin We explore the effects of flexural phonons on the thermal and electrical properties of graphene. To control the amplitude of flexural phonons, we developed a technique to engineer uniform mechanical strain between 0 and 1{\%} in suspended graphene. We determine the level of strain, thermal conductivity and carrier mobility of graphene through a combination of mechanical resonance and electrical transport measurements. Depending on strain, we find significant changes in the thermal expansion coefficient, thermal conductivity, and carrier mobility of suspended graphene. These changes are consistent with the expected contribution of flexural phonons. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G30.00009: Excitonic Pairing between Strained Graphene Layers Peter Harnish, Anand Sharma, Valeri Kotov It is well known that inter-layer electron-hole (excitonic) pairing, caused by the inter-layer Coulomb interaction, can occur between graphene sheets appropriately doped with electrons and holes. However in such a system the energy of the excitonic condensate, as well the corresponding critical temperature, are very small due to the effective screening of the inter-layer interaction potential. We study pairing between two uniaxially- strained graphene layers, focusing particularly on the dependence of the pairing gap on the applied strain. The graphene layers are modeled as anisotropic Dirac fermion systems. We find a strong dependence on the strain, particularly in the weak-coupling regime where screening is not very relevant. In this case the condensate energy is enhanced due to the increase of the density of states as a function of anisotropy. At moderate and strong coupling the pairing becomes less sensitive to strain because of the subtle interplay between density of states effects and the strain-modified screening of the inter-layer Coulomb potential. We also analyze the possibility of higher angular momentum pairing (beyond the conventional s-wave) for the strained graphene layers. In addition, we propose that pairing can be further enhanced at the Lifshitz transition point when a relatively large strain is applied in the zig-zag direction. At such topological transition the electronic dispersion deviates strongly from the simplest anisotropic (elliptical) Dirac cone and becomes quadratic in one of the lattice directions while remaining Dirac-like along the other. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G30.00010: Graphene Calisthenics: Straintronics of Graphene with Light-Reactive Azobenzene Polymer Kacey Meaker, Peigen Cao, Mandy Huo, Michael Crommie Although a promising target for next-generation electronics, graphene's lack of a band gap is a severe hindrance. There are many ways of opening a gap, and one controllable way is through application of specific non-uniform strains which can produce extremely large pseudomagnetic fields. This effect was predicted and verified experimentally, but so far there have been few methods developed that reliably control the size, location, separation and amount of strain in graphene. We have used a layer of light-reactive azobenzene polymer beneath the graphene to produce strained monolayer graphene with light exposure. Using Raman spectroscopy, we have measured a shift of up to 20 cm$^{\mathrm{-1}}$ in the 2D peak when the graphene and polymer sample was exposed to 532 nm laser illumination indicating that the graphene is undergoing a strain from deformation of the azobenzene layer below. AFM topographic measurements and COMSOL simulations were used to verify this assertion. Use of polymeric materials to reliably strain graphene in non-uniform ways could result in controllable production of large pseudomagnetic fields in graphene and more control over graphene's low-energy charge carriers. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G30.00011: Strain Engineering of Graphene: Atomistic Simulation of Y-junctions and Nanobubbles Zenan Qi, Dario Bahamon, Harold Park, Vitor Pereira, David Campbell, Antonio Castro Neto We have studied the effects of two and three-dimensional states of strain on electronic transport in monolayer graphene. Using a combined atomistic simulation approach with molecular mechanics, molecular dynamics, tight binding exact diagonalization and Landauer-B\"{u}ttiker formulism, we have explored how various deformation patterns induce tunable pseudomagnetic field (PMF) distributions and furthermore how the Landau levels arising from PMF affect the quantum transport properties. Specifically, graphene Y-junction structures are found, under triaxial strains, to behave like pseudomagnetic quantum dots that selectively guide electron movement; valley degeneracy is broken when both strain-induced PMF and external real magnetic fields are present. Furthermore, graphene nano-bubbles with different geometries obtained by gas pressure can also be controlled as functional blocks due to PMF-restricted quantum transport by manipulation of strain. The simulation results show the promising potential to utilize graphene as a tunable building block for electronic NEMS/MEMS devices by strain engineering. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G30.00012: First-Principles Calculations on the Effect of Doping and Biaxial Tensile Strain on Electron-Phonon Coupling in Graphene Chen Si, Zheng Liu, Wenhui Duan, Feng Liu Graphene has exhibited a wealth of fascinating properties, but is also known not to be a superconductor. Remarkably, we show that graphene can be made a conventional Bardeen-Cooper-Schrieffer superconductor by the combined effect of charge doping and tensile strain. While the effect of doping obviously enlarges the Fermi surface, the effect of strain profoundly increases the electron-phonon coupling. At the experimental accessible doping ($\sim4\times10^{14}$ cm$^{-2}$) and strain ($\sim16$\%) levels, the superconducting critical temperature $T_{c}$ is estimated as high as $\sim30$ K, the highest for a single-element material above the liquid hydrogen temperature. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G30.00013: Unconventional magnetic ground state in strained graphene: theory of global anti-ferromagnetic phase Fakher Assaad, Bitan Roy, Igor Herbut An unconventional magnetic ground state is proposed for the Hubbard Hamiltonian in strained graphene. We show that when the chemical potential lies close to the Dirac point, strained graphene supports magnetic ordering that simultaneously gives rise to anti-ferromagnetic and ferromagnetic orders, even for weak onsite interaction. Whereas the anti-ferromagnetic order parameter is of the same sign in the entire system, the ferromagnetic order at the boundary has the opposite sign from the bulk. The spatially-integrated ferromagnetic order parameter is this way zero, and the magnetic ground state is therefore a spin-singlet. This peculiar magnetic ordering results from the nature of the strain-induced (near) zero energy states, which have support on one sublattice in the bulk, and on the other sublattice near the boundary of a finite system. We support our claim with the self-consistent numerical mean-field calculation of the magnetic order parameters, and with a Monte-Carlo simulations of the Hubbard model in a strained honeycomb lattice. [Preview Abstract] |
Session G31: Focus Session: Van der Waals Interactions in Complex Materials: Bridging Theory and Experiment III
Sponsoring Units: DMPChair: Anthony Reilly, Fritz Haber Institute, Germany
Room: 607
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G31.00001: Non-additivity of molecule-surface van der Waals potentials from force measurements Invited Speaker: Stefan Tautz Van der Waals (vdW) forces act ubiquitously in condensed matter. Their description as an inherently quantum mechanical phenomenon was developed for single atoms and homogeneous macroscopic bodies by London, Casimir, and Lifshitz. For intermediate-sized objects like organic molecules an atomistic description is required, but explicit first principles calculations are very difficult since correlations between many interacting electrons have to be considered. Hence, semi-empirical correction schemes are often used that simplify the vdW interaction to a sum over atom-pair potentials. A similar gap exists between successful measurements of vdW and Casimir forces for single atoms on the one hand and macroscopic bodies on the other, as comparable experiments for molecules are absent. I will present experiments in which long-range vdW potentials between a series of related molecules and a metal surface have been determined experimentally. The experiments rely on the extremely sensitive force detection of an atomic force microscope in combination with its molecular manipulation capabilities. The results allow us to confirm the asymptotic force law and to quantify the non-additive part of the vdW interaction which is particularly challenging for theory. In the present case, cooperative effects account for 10{\%} of the total interaction. This effect is of general validity in molecules and thus relevant at the intersection of chemistry, physics, biology, and materials science. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G31.00002: Determination of Surface-Substrate Adsorption Energy usin the Exchange-Hole Dipole Moment Matthew Christian, Alberto Otero de la Roza, Erin Johnson Calculated surface-substrate binding energies are usually underestimated because conventional density functionals do not include dispersion, which is necessary to capture the van der Waals interactions that lead to weak physiadsorption. The exchange-hole dipole moment (XDM) model is a non-empirical density-functional approach to model dispersion. Adsorption energies for several aromatic molecules and nuclebases on noble metal surfaces were calculated using B86bPBE-XDM. In this talk, I compare the calculated adsorption energies with experiment and present implications for future applications to modeling surface interactions. \\[4pt] [1] A.\ Otero-de-la-Roza and E.\ R.\ Johnson, \textsl{J.\ Chem.\ Phys.} \textbf{138} 204109 (2013).\\[0pt] [2] A.\ Otero-de-la-Roza and E.\ R.\ Johnson, \textsl{J.\ Chem.\ Phys.} \textbf{137} 054103 (2012).\\[0pt] [3] A.\ Otero-de-la-Roza and E.\ R.\ Johnson, \textsl{J.\ Chem.\ Phys.} \textbf{136} 204109 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G31.00003: Trends in Adsorption Characteristics of Organic Molecules on Transition Metal Surfaces: Role of Surface Chemistry and van der Waals Interactions Handan Yildirim, Abdelkader Kara The accurate description of interface characteristics between organic molecules and metal surfaces has long been debated in theoretical studies. A well-founded description of interface geometry and adsorption energy is highly desirable for these systems. Using first principles calculations with the inclusion of van der Waals interactions, we examine the adsorption characteristics of a few organic molecules on several transition metal surfaces. Our aim is to obtain insights into the role of vdW interactions in the adsorption characteristics as well as to build an understanding on how these functionals treat the adsorption on varying surface chemistries. Furthermore, the comparisons made between the results obtained using different vdW functionals for each organic molecule type provide the means to assess their performance. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G31.00004: Adsorption characteristics of Thiophene on Cu and Ni(100): role of van der Waals Tomas Rojas, Jeronimo Matos, Abdelkader Kara We apply density functional theory, with and without the inclusion of self-consistent van der Waals (vdWs) interactions (optB86, optB88, optPBE, revPBE, rPW86), to study the adsorption of thiophene (C$_{\mathrm{4}}$H$_{\mathrm{4}}$S) on Cu(100) and Ni(100). Our calculations reveal that the C$_{\mathrm{4}}$H$_{\mathrm{4}}$S molecule adsorbs, on either substrate, with its molecular plane parallel to the surface with the sulfur close to the bridge site. The inclusion of vdWs interactions results in a significant increase in the binding energy of thiophene on Cu(100) (from 0.12 eV to up to 0.77 eV), while the adsorption height is also modified from 3.2 A down to, at most, 2.38 A, depending on the functional used. The Ni(100) case presents a similar behavior for the binding energy (enhancement from 1.56 eV to up to 2.34 eV), but the adsorption heights increase from 2.12 {\AA} up to 2.32 {\AA}. In addition to adsorption geometry and energetics, we present the results and analysis of the electronic properties (charge transfer, changes in the d-band of the substrate, and change in the work function) of these two systems to complement our understanding of the molecule-substrate bonding. Our results suggest that the adsorption characteristics are dependent on the type of functional used; opt-type functionals (optB86, optB88, optPBE) are found to produce stronger bonding as compared to PBE, revPBE and rPW86. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G31.00005: The Effect of van der Waals Interactions on the Sexithiophene Adsorption on Ag(110) Jeronimo Matos, Tomas Rojas, Abdelkader Kara We use density functional theory to study the adsorption of Sexithiophene (6T) on Ag(110). Special attention is given for exploring the effects of van der Waals interactions on the adsorption geometry and energy using vdW-DF family functionals. The 6T molecule is found to bind to the Ag(110) surface via two orientations, with the long molecular axis parallel to the [001] and [110] directions. Including van der Waals interactions resulted in a substantial increase in the binding energy (from 0.6 eV to 4 eV), while the binding height is slightly modified (from 3.1 {\AA} to 2.75 {\AA}). Both the binding energies and heights show significant variations depending on the vdW functional used: the opt-type functionals (optB86, optB88, optPBE) further enhance the adsorption energy when compared to those obtained using PBE, revPBE, or rPW86 functionals. Upon adsorption, there is a small, however, noticeable broadening and a shift (towards higher binding energy) in the position of the d-band center of the substrate surface atoms is observed. Note that, the absence of charge transfer, interfacial states, changes in the atomic structure of the molecule or the substrate suggests that the bonding characteristic of the 6T/Ag(110) system can be categorized as weak chemisorption or strong physisorption. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G31.00006: Van der Waals quantum friction and fluctuation theorems Diego Dalvit, Francesco Intravaia, Ryan Behunin We use general concepts of statistical mechanics to compute the quantum frictional force on an atom moving at constant velocity above a planar surface. We derive the zero-temperature frictional force using a non-equilibrium fluctuation-dissipation relation, and show that in the large-time, steady-state regime quantum friction scales as the cubic power of the atom's velocity. We also discuss how approaches based on Wigner-Weisskopf and quantum regression approximations fail to predict the correct steady-state zero temperature frictional force, mainly due to the low frequency nature of quantum friction. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G31.00007: Probe the Corrugation and Friction of Cu(111) toward Ne and Xe: First Principles Studies Yanning Zhang, Virginio Bortolani, Ruqian Wu The interaction between rare-gas (RG) and metal surface is typically described as the sum of two contributions: van der Waals attraction at large RG-metal distances, and Pauli repulsion at short distances. In the repulsive range, RG atoms can see a corrugated or anticorrugated potential surface, depending on the change of charge density profile of the surface atoms. The probe of the corrugation effects near the attractive part is also important since the corrugated or anticorrugated charge distribution at the surface can significantly change the physical properties of the whole system. In this letter, we show that also near the negative potential well of Ne and Xe monolayers on Cu(111), we can observe different surface corrugations: while the potential surface of Ne on Cu(111) is corrugated, it is anticorrugated for Xe/Cu(111). The analyses of electronic properties reveal that the weak hybridization of RG p- and substrate d-states is critical for the surface anticorrugation. Studies of the activation energies along sliding paths imply that Ne motion is much faster than Xe on Cu(111). Density functional calculations with self-consistent nonlocal van der Waals functional were used throughout our studies. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G31.00008: Water films on transition metal surfaces: A physical disclosure of adsorption energy Guillem Revilla-L\'opez, Nuria L\'opez Our work reports novel physical models derived from DFT calculations including van der Waals forces for the adsorption of different water motifs: ice bilayer, $\surd $37 x$\surd $37-R 25.3$^{\circ}$ and rosette on transition metal surfaces. This energy decomposition scheme is obtained by analyzing the two driving energies of adsorption: water-water and water-metal interactions. The former explained by single water adsorption and the latter by ice resonance stabilization. These two magnitudes drive, to different extent, the adsorption of ice bilayer and $\surd $37 whereas rosette motif lacks the resonance contribution. The equations successfully reproduce and predict the experimental results for the wettability and the dissociation of water films on the fcc(111) and hcp(0001) facets of Pd, Pt, Ru Ir, Rh, Au, and Ag. So happens for the temperature of the hydrophobic/hydrophilic water film transition and for the effect of the surface roughness on it. Furthermore, the metastability and the wettability of other water films like $\surd $39 x$\surd $39-R 16.1$^{\circ}$ can be anticipated by the rationalization of their geometry and their dissociation state. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G31.00009: Probing Adsorption Interactions In Metal-Organic Frameworks Using X-ray Spectroscopy and Density Functional Theory Walter Drisdell, Roberta Poloni, Thomas McDonald, Jeffrey Long, Berend Smit, Jeffrey Neaton, David Prendergast, Jeffrey Kortright Metal-organic frameworks (MOFs) are currently among the most promising materials for gas separation applications such as carbon capture. We explore the local electronic signatures of molecular adsorption at coordinatively unsaturated binding sites in the metal-organic framework Mg-MOF-74 using X-ray spectroscopy and first principles calculations. \textit{In situ} measurements at the Mg $K$-edge reveal distinct pre-edge absorption features associated with the unique, open coordination of the Mg sites. These spectral features are suppressed upon adsorption of CO$_{2}$ and $N,N'$-dimethylformamide. Density functional theory shows that these spectral changes arise from modifications of local symmetry around the Mg sites upon gas uptake and are strongly dependent on the metal-adsorbate binding strength. Similar sensitivity to local symmetry is expected for any open metal site, making X-ray spectroscopy an ideal tool for examining adsorption in such MOFs. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G31.00010: Design of a metal-organic framework with enhanced back bonding for the separation of N2 and CH4 Kyuho Lee, William Isley, Allison Dzubak, Pragya Verma, Samuel Stoneburner, Li-Chiang Lin, Joshua Howe, Eric Bloch, Douglas Reed, Matthew Hudson, Craig Brown, Jeffrey Long, Jeffrey B. Neaton, Berend Smit, Christopher J. Cramer, Donald G. Truhlar, Laura Gagliardi Removing dinitrogen, an omnipresent but noncombustible contaminant, from natural gas or other methane-rich gases is an extraordinarily difficult separation based on physical properties alone, as both gases lack a permanent dipole and have similar polarizabilities, boiling points, and kinetic diameters. In this work, by using dispersion-corrected density functionals and wavefunction approaches, we predict a new metal-organic framework (MOF) of potential utility for the highly selective and efficient separation of dinitrogen from methane, a particularly challenging separation of critical value for utilizing natural gas. Selective back bonding interactions from the vanadium(II) cation centers in V-MOF-74 to the unoccupied $\pi$* orbitals of N2 can be used to separate N2/CH4 mixtures. We compare our calculations with the experimentally characterized Fe-MOF-74. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G31.00011: First-Principles Prediction of Small Molecule Adsorption in MOF-74 Variants Joshua Howe, Kyuho Lee, Li-Chiang Lin, Berend Smit, Jeffrey Neaton Using density functional theory (DFT), we predict binding energies of flue gas molecules (CO, CO$_{2}$, H$_{2}$O, H$_{2}$S, N$_{2}$, NH$_{3}$, SO$_{2}$, and H$_{2})$ and small hydrocarbons (CH$_{4}$, C$_{2}$H$_{2}$, C$_{2}$H$_{4}$, C$_{2}$H$_{6}$, C$_{3}$H$_{6}$, and C$_{3}$H$_{8})$ in a variety of ``MOF-74'' variants.~ Using a harmonic approximation to compute quantum zero-point and thermal corrections, we compute binding enthalpies for comparison with experimental heats of adsorption.~ Our study is performed using vdW-DF2, a fully nonlocal dispersion-corrected density functional along with Hubbard U corrections on 3$d$-orbital electrons as appropriate.~ We study MOF-74 variants, ``M-MOF-74'', where ``M'' is chosen to be any divalent third-row metal cation (M$=$ Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn).~ Additionally, we study ``M-MOF-74'' systems with ``meta-dobdc'' as the linker (as compared to the traditional ``para-dobdc'').~ We compare with experiment when available and find reasonable agreement. ~We identify trends, and compare with experiment where available, finding excellent agreement. This work supported by DOE through the EFRC on Gas Separations for Clean Energy Technologies; computational resources provided by NERSC.~ [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G31.00012: High-throughput screening of small-molecule adsorption in MOF-74 T. Thonhauser, P. Canepa Using high-throughput screening coupled with state-of-the-art van der Waals density functional theory, we investigate the adsorption properties of four important molecules, H$_2$, CO$_2$, CH$_4$, and H$_2$O in MOF-74-$\mathcal{M}$ with $\mathcal{M} =\;$Be, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Zr, Nb, Ru, Rh, Pd, La, W, Os, Ir, and Pt. We show that high-throughput techniques can aid in speeding up the development and refinement of effective materials for hydrogen storage, carbon capture, and gas separation. The exploration of the configurational adsorption space allows us to extract crucial information concerning, for example, the competition of water with CO$_2$ for the adsorption binding sites. We find that only a few noble metals---Rh, Pd, Os, Ir, and Pt---favor the adsorption of CO$_2$ and hence are potential candidates for effective carbon-capture materials. Our findings further reveal significant differences in the binding characteristics of H$_2$, CO$_2$, CH$_4$, and H$_2$O within the MOF structure, indicating that molecular blends can be successfully separated by these nano-porous materials. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G31.00013: The DeNO$_x$ process and NO$_2$ adsorption in MOF74 S. Zuluaga, T. Thonhauser, K. Tan, Y. Chabal Due to the harmful character of NO$_2$ and its slow decomposition rate, the use of catalytic materials for its removal (DeNO$_x$ process) has attracted a lot of attention. The high porosity and highly reactive uncoordinated metal centers of MOF74 have led us to investigate the use of Mg- and Zn-MOF74 as materials for trapping NO$_2$ with resistance to poisoning by SO$_2$. In this combined theoretical and experimental study, we investigate the interaction between the unsaturated metal centers of the MOF and the NO$_2$ guest molecules. For our theoretical modeling we use ab initio calculations at the DFT level, utilizing vdW-DF to capture the significant van der Waals component of the interaction between NO$_2$ and the MOF. We present detailed first-principle results concerning the adsorption energies and geometries, as well as vibration frequencies of the NO$_2$ molecule adsorbed in the MOF. Our experimental efforts (IR and Raman spectroscopy) have shown a blue shift to 1684 cm$^{-1}$ in the vibration stretching mode of the NO$_2$ upon adsorption and a thermal stability up to 150$^{\circ}$C. Our first-principle calculations and experimental results show a remarkable agreement, allowing us to give a complete picture of the adsorption of NO$_2$ molecules in the MOF74 structure. [Preview Abstract] |
Session G32: Invited Session: Superconducting Qubits
Sponsoring Units: GQI DCMPChair: Matthias Steffen, IBM
Room: 708-712
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G32.00001: Steps Toward Fault-Tolerant Quantum Computing with 2D and 3D Superconducting Qubits Invited Speaker: Douglas McClure Recent improvements in superconducting qubit coherence times, entangling gates, and measurement techniques have set the stage for high-fidelity demonstrations of multi-qubit operations needed for performing error correction in architectures such as the surface code. Using a planar network of transmon qubits and superconducting resonators, we benchmark a complete set of high-fidelity single- and two-qubit gates on a three-qubit sub-section of the surface code. Combining these gates with high-fidelity individual single-shot readouts, we deterministically entangle two non-nearest-neighbor qubits to implement a parity check operation, an essential component of surface code error correction. A complementary system consisting of three-dimensional cavities linked by individually placed transmon qubits provides an additional platform for the investigation of loss mechanisms and entangling schemes. Using this architecture, we demonstrate high-fidelity entanglement between arbitrary qubit pairs in a three-qubit, four-cavity network. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G32.00002: Deterministic Quantum Teleportation with Feed-Forward in a Solid State System Invited Speaker: Andreas Wallraff Using modern micro and nano-fabrication techniques combined with superconducting materials we realize electronic circuits the dynamics of which are governed by the laws of quantum mechanics. Making use of the strong interaction of photons with superconducting quantum two-level systems realized in these circuits we investigate both fundamental quantum effects of light and applications in quantum information processing. In this talk I will discuss the deterministic teleportation of a quantum state in a macroscopic quantum system with near unit success probability at a rate of 10 kHz [1]. Teleportation is useful for distributing entanglement between distant qubits in a quantum network and for realizing universal and fault-tolerant quantum computation. Previously, we have demonstrated the implementation of a teleportation protocol up to the single-shot measurement step [2]. Now we have realized a new device in which four qubits are coupled pair-wise to three resonators. Making use of parametric amplifiers [3] coupled to the output of two of the resonators we are able to perform high-fidelity single-shot read-out. Based on a close to ideal Bell-measurement identifying all four Bell-states in a single joint two-qubit measurement we have implemented fast feed-forward to complete the teleportation process. In this setup we have demonstrated teleportation by individually post-selecting on any Bell-state, by deterministically and simultaneously distinguishing between all four Bell states, and by implementing the feed-forward step to have the protocol succeed with near unit probability [1]. In all instances, we demonstrate that the fidelity of the teleported states and the fidelity of the teleportation process are above the thresholds imposed by classical physics. The presented experiments are expected to contribute to the realization of quantum communication over small and medium scale distances with microwave photons in the foreseeable future. \\[4pt] [1] L. Steffen et al., Nature 500, 319 (2013). \newline [2] M. Baur et al., Phys. Rev. Lett. 108, 040502 (2012). \newline [3] C. Eichler et al., Phys. Rev. Lett. 107, 113601 (2011). [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G32.00003: Two-qubit parity meters in 3D and 2D circuit QED Invited Speaker: Leonardo DiCarlo Non-demolition measurements of multi-qubit observables and feedback control conditioned on their outcomes are essential for quantum error correction. We present two implementations of two-qubit parity meters in circuit QED. In 3D, we match the dispersive coupling of two qubits to a common cavity to encode parity in the transmission of an applied microwave pulse. In 2D, we first encode the parity of two data qubits in the computational state of an ancillary qubit using resonant interactions, and subsequently project the ancilla using a dedicated, dispersively-coupled resonator. A key advantage of this second scheme is the protection of data qubits from dephasing by measurement photons. First applications of these parity meters include probabilistic entanglement by measurement, and deterministic entanglement using digital feedback control. Current efforts target the implementation of measurement-based bit-flip error correction. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G32.00004: Applications of superconducting circuits to quantum computing Invited Speaker: Frank Wilhelm Superconducting circuits containing Josephson junctions are strong contenders for the implementation of a quantum computer. At its 15 year mark, the field has seen tremendous progress with an increase of coherence by six orders of magnitude and it is now taking off from the few- to the multi-qubit level. I will present highlights of research on the level of single devices, in particular the strong increase of coherence that counter-intuitively comes with a growth in device size, as well as readout. I will cover challenges related to multi-qubit systems focusing on precise multi-qubit control and calibration, and present an outlook on future architectures dictated by the requirements of fault tolerance. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G32.00005: Fault-tolerant quantum computing with superconducting qubits Invited Speaker: John Preskill I review and assess the theory of fault-tolerant quantum computing, emphasizing applications to superconducting circuits. Topic considered include topological codes, protection against biased noise, and error-resistant hardware. [Preview Abstract] |
Session G33: Quantum Entanglement I
Sponsoring Units: GQIChair: Josh Combes, University of New Mexico
Room: 706
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G33.00001: Universal entanglement crossover of coupled quantum wires Romain Vasseur, Jesper Jacobsen, Hubert Saleur We consider the entanglement between two one-dimensional quantum wires (Luttinger Liquids) coupled by tunneling through a quantum impurity. The physics of the system involves a crossover between weak and strong coupling regimes characterized by an energy scale $T_B$, and methods of conformal field theory therefore cannot be applied. The evolution of the entanglement in this crossover has led to many numerical studies, but has remained little understood, analytically or even qualitatively. This is, in part, due to the fact that the entanglement in this case is non-perturbative in the tunneling amplitude. We argue that the correct universal scaling form of the entanglement entropy $S$ (for an arbitrary interval containing the impurity) is $\partial S/\partial \ln L = f(L T_B)$. In the special case where the coupling to the impurity can be refermionized, we show how the universal function $f(L T_B)$ can be obtained analytically using recent results on form factors of twist fields and a defect massless-scattering formalism. Our results are carefully checked against numerical simulations. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G33.00002: Rapid creation of distant entanglement by multi-photon resonant fluorescence Guy Z. Cohen, L.J. Sham We study a simple, effective and robust method for entangling two separate stationary quantum dot spin qubits with high fidelity using multi-photon Gaussian state. The fluorescence signals from the two dots interfere at a beam splitter. The bosonic nature of photons leads, in analogy with the Hong-Ou-Mandel (HOM) effect, to selective pairing of photon holes (photon absences in the fluorescent signals). By the HOM effect, two photon holes with the same polarization end up at the same beam splitter output. As a result, two odd photon number detections at the outgoing beams, which must correspond to two photon holes with different polarizations, herald entanglement creation. The robustness of the Gaussian states is evidenced by the ability to compensate for photon absorption and noise by a moderate increase in the number of photons at the input. We calculate the entanglement generation rate in the ideal, non-ideal and near-ideal detector regimes and find substantial improvement over single-photon schemes in all three regimes. Fast and efficient spin-spin entanglement creation can form the basis for a scalable quantum dot quantum computing network. Our predictions can be tested using current experimental capabilities. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G33.00003: From remote entanglement between solid state qubits to deterministic quantum teleportation Hannes Bernien, Bas Hensen, Wolfgang Pfaff, Gerwin Koolstra, Suzanne van Dam, Machiel Blok, Lucio Robledo, Tim Taminiau, Matthew Markham, Daniel Twitchen, Lilian Childress, Ronald Hanson Quantum networks enable the distribution of quantum information that is processed and stored in local nodes [1]. Setting up a quantum network requires the generation of entanglement between widely separated qubits combined with local long-lived quantum registers. Here we present our recent results towards the realization of scalable quantum networks with solid-state qubits. We have entangled two spin qubits, each associated with a nitrogen vacancy center in diamond [2]. The two diamonds reside in separate setups three meters apart from each other. With no direct interaction between the two spins to mediate the entanglement, we make use of a scheme based on quantum measurements: we perform a joint measurement on photons emitted by the NV centers. The detection of the photons projects the spins into an entangled state. We verify the generated entanglement by single-shot readout of the spin qubits in different bases and correlating the results. We will present these experiments along with our latest results towards deterministic quantum teleportation between distant qubits. [1] H. J. Kimble, Nature, 453, 1023 (2008) [2] H. Bernien et al., Nature 497, 86 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G33.00004: Shadow Lattice Stabilization Program for Strongly Correlated States of Light Eliot Kapit, Mohammad Hafezi, Steven Simon Recent progress in nanoscale quantum optics and superconducting qubits has made the creation and quantum simulation of strongly correlated, and even topologically ordered, states of photons a real possibility. Many of these states are gapped and exhibit anyon excitations, which could be used for quantum information processing. However, the question of how to stabilize the many-body ground states of photonic quantum simulators against decays remains largely unanswered. We here propose a simple mechanism which achieves this goal. Our construction uses a uniform two-photon drive field to entangle the qubits of the primary lattice with an auxiliary ``shadow" lattice of qubits with a much faster loss rate than the primary qubits. This entanglement raidly refills hole states created by photon losses, and a many-body gap prevents further photons from being added once the strongly correlated ground state is reached. We present a set of general guidelines for designing the shadow lattice and coupling Hamiltonians to stabilize the ground state of a given primary Hamiltonian. We then provide an explicit construction which stabilizes abelian and non-abelian fractional quantum Hall states of light. The device parameters needed for our scheme to work are within reach of current technology. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G33.00005: Demonstration of Quantum Entanglement in an Analog Electronic Device Brian La Cour, Granville Ott, Gary Wilson, Marti Barlett There are a surprising number of classical analogs to phenomena generally regarded as uniquely quantum in nature. We consider one such analog that uses complex basebanded signals in a classical electronic device to represent a multi-qubit quantum state. Formally, such a representation is capable of reproducing the mathematical structure of a tensor-product Hilbert space. In particular, entangled bipartite states can be represented in this manner. As a demonstration of this concept, we describe an experiment using an analog electronic device capable of emulating an arbitrary two-qubit system. We then show how, using a pair of independent ``Alice'' and ``Bob'' analog measurement devices, one can achieve correlations in violation of Bell's inequality, much as those found in optical tests of quantum nonlocality. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G33.00006: Experimental distribution of entanglement with separable carriers Alessandro Fedrizzi, Margherita Zuppardo, Geoff Gillett, Matthew Broome, Marcelo de Almeida, Mauro Paternostro, Andrew White, Tomasz Paterek Quantum networks will allow us to overcome distance limitations in quantum communication, and to share quantum computing tasks between remote quantum processors. The key requirement for quantum networking is the distribution of entanglement between nodes. Surprisingly, entanglement can be generated across a network without directly being communicated between nodes. In contrast to information gain, which cannot exceed the communicated information, the entanglement gain is bounded by the communicated quantum discord, a more general measure of quantum correlation that includes but is not limited to entanglement. Here we report an experiment in which two communicating parties who share three initially separable photonic qubits are entangled by exchange of a carrier photon that is not entangled with either party at all times. We show that distributing entanglement with separable carriers is resilient to noise and in some cases becomes the only way of distributing entanglement over noisy environments. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G33.00007: Entanglement of formation in the Kondo model at finite temperature Seung-Sup Lee, Jinhong Park, H.-S. Sim We compute the entanglement of formation, a generalization of the entanglement entropy quantifying any bipartite entanglement in mixed states, in the Kondo model at finite temperature $T$, using a new approach [1,2] based on the optimal entanglement witness operator and the numerical renormalization group method. We focus on the entanglement between the Kondo impurity spin and the electrons lying inside distance $L$ from the impurity. The entanglement characterizes the thermal suppression and the spatial profile of the macroscopic quantum correlations in the Kondo systems, providing the quantum information perspective of the Kondo cloud. We reveal the universal scaling behaviors of the entanglement at low $T$ and large $L$. [1] S.-S. B. Lee and H.-S. Sim, Phys. Rev. A \textbf{85}, 022325 (2012). [2] S. Ryu, S.-S. B. Lee, and H.-S. Sim, Phys. Rev. A \textbf{86}, 042324 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G33.00008: Generation of Massive Entanglement Through Adiabatic Quantum Phase Transition in a spinor condensate Zhen Zhang, Luming Duan We propose a method to generate massive entanglement in a spinor Bose-Einstein condensate from an initial product state through adiabatic sweep of magnetic field across a quantum phase transition induced by competition between the spin-dependent collision interaction and the quadratic Zeeman effect. The generated many-body entanglement is characterized by the experimentally measurable entanglement depth in the proximity of the Dicke state. We show that the scheme is robust to practical noise and experimental imperfection and under realistic conditions it is possible to generate genuine entanglement for hundreds of atoms. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G33.00009: Deterministic generation of many-photon GHZ states using quantum dots in a cavity Michael Leuenberger, Mikhail Erementchouk, Ahmed Elhalawany We propose a novel theoretical scheme based on the off-resonant interaction of $N$ photons with four InAs/GaAs semiconductor quantum dots (QDs) in an GaAs microdisk cavity to create many-photon GHZ states deterministically in the polarization degree of freedom at a wavelength of 1.3 $\mu$m with probability $p=1$ for $N$ up to 60, without the need of any projective measurement or local unitary operation. Taking advantage of off-resonant interaction, the time evolution of the $N$-photon state is robust against decoherence due to exciton-phonon and hyperfine interactions. However, decoherence due to leakage of the photons out of the cavity is not negligible and is therefore considered. Remarkably, by taking advantage of a cascaded multi-level Landau-Zener transition, we are able to reduce the GHZ state generation time to below 100 ps for $N$ up to 60, which allows for the creation of GHZ states with $N$ up to 60 in cavities with $Q=10^6$ with fidelity above 70\% including decoherence due to leakage. Our method paves the way to the miniaturization of many-photon GHZ state sources to the nanoscale regime, with the possibility to integrate them on a computer chip based on semiconductor materials. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G33.00010: Entanglement Generation in a Two-Qubit System Coupled to Vacuum Electromagnetic Field Saeed Pegahan The entanglement generation in a two-qubit system interacting with electromagnetic vacuum field and an external local magnetic field is investigated in the framework of the master equation. The time-evolution for the most general density matrix of the two-qubit system is obtained and solved. It is shown that the two-qubit system ends up in an entangled stationary state independent on the initial separable state. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G33.00011: Many-body localization: Local integrals of motion, area-law entanglement, and quantum dynamics Dmitry Abanin, Maksym Serbyn, Zlatko Papic We demonstrate that the many-body localized phase is characterized by the existence of infinitely many local conservation laws. We argue that many-body eigenstates can be obtained from product states by a sequence of nearly local unitary transformation, and therefore have an area-law entanglement entropy, typical of ground states. Using this property, we construct the local integrals of motion in terms of projectors onto certain linear combinations of eigenstates. The local integrals of motion can be viewed as effective quantum bits which have a conserved z-component that cannot decay. Thus, the dynamics is reduced to slow dephasing between distant effective bits. For initial product states, this leads to a characteristic slow power-law decay of local observables, which is measurable experimentally, as well as to logarithmic in time growth of entanglement entropy. We support our findings by numerical simulations of random-field XXZ spin chains. Our work shows that the many-body localized phase is integrable, reveals a simple entanglement structure of eigenstates, and establishes the laws of dynamics in this phase. [1] M. Serbyn, Z. Papic, D. A. Abanin, Phys. Rev. Lett. 111, 127201 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G33.00012: Entanglement propagation and typicality of measurements in a quantum version of the Kac ring Johannes Oberreuter, Ingo Homrighausen, Stefan Kehrein Time development in quantum many body systems poses serious challenges to our understanding of classical statistical mechanics. Exact results are very rare due to the large Hilbert spaces and the resulting complexity involved. We propose a pedagogical approach with a very tractable toy model, in which questions of entanglement creation, propagation and destruction between a system and an environment can be studied explicitly. Comparing this quantum model with its classical counterpart [1], we find an intriguing correspondence between the typical result of repeated measurements on a classical ensemble and the repeated measurements of a quantum system in an appropriate superposition. \\[4pt] [1] G.A. Gottwald, M. Oliver, Boltzmann's Dilemma: An Introduction to Statistical Mechanics via the Kac Ring, SIAM Review (2009) Vol. 51, No. 3, pp. 613-635. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G33.00013: Universal slow growth of entanglement in interacting strongly disordered systems Maksym Serbyn, Zlatko Papic, Dmitry Abanin Localized phase of interacting particles has recently been shown to have a slow, logarithmic in time, growth of the entanglement entropy for initial product states. This property has been conjectured to distinguish the many-body localized phase from the ordinary Anderson insulator. In the present work we put this assumption on solid ground by uncovering the underlying mechanism of the entanglement generation. We show that the entanglement arises from dephasing due to exponentially small interaction-induced corrections to the eigenenergies of different states. For weak interactions, we find that the entanglement entropy grows as $\xi \ln (Vt/\hbar)$ with time $t$, where $V$ is the interaction strength and $\xi$ is the single-particle localization length. The saturated value of the entanglement entropy at long times is determined by the participation ratios of the initial state over the eigenstates of the subsystem. Our work shows that the logarithmic entanglement growth is a universal phenomenon characteristic of the many-body localized phase in any number of spatial dimensions, and reveals a broad hierarchy of dephasing time scales present in such a phase. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G33.00014: Violation of the Entanglement Area Law in Bosonic Systems with Bose Surfaces: Possible Application to Bose Metals Hsin-Hua Lai, Kun Yang, Nicholas Bonesteel We show the violation of the entanglement-area law for bosonic systems with Bose surfaces. For bosonic systems with gapless factorized energy dispersions on a $N^d$ Cartesian lattice in $d$-dimension, e.g., the exciton Bose liquid in two dimension, we explicitly show that a belt subsystem with width $L$ preserving translational symmetry along $d-1$ Cartesian axes has leading entanglement entropy $(N^{d-1}/3)\ln L$. Using this result, the strong subadditivity inequality, and lattice symmetries, we bound the entanglement entropy of a rectangular subsystem from below and above showing a logarithmic violation of the area law. For subsystems with a single flat boundary we also bound the entanglement entropy from below showing a logarithmic violation, and argue that the entanglement entropy of subsystems with arbitrary smooth boundaries are similarly bounded. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G33.00015: Environmental Entanglement Caused by a Qubit: Multipolaron Ansatz for Biased Spin-Boson Model Soumya Bera, S. Florens, H. Baranger, A. Nazir, A. Chin We show that a qubit interacting with its environment produces highly entangled states within the environment with emerging non-adiabatic features (Schrodinger-cat-like states of the environment). The model consists of a two-level system coupled to a continuum of bath modes in the presence of a bias field, which can be realized, for instance, by a qubit coupled to a high impedance superconducting transmission line. We develop a systematic coherent state expansion for the many-body ground state of this model. Comparisons to accurate numerical renormalization group calculations and the exact Bethe Ansatz solution of the model demonstrate the rapid convergence of our variationally-optimized multi-polaron expansion. This coherent state Ansatz captures all the essential features of the biased model such as the formation of low-energy antipolarons, peaks seen in the quantum tomography of the environment, and the stabilization of spin coherence. [Preview Abstract] |
Session G34: Atoms in Reduced Dimensions and Novel Geometries
Sponsoring Units: DAMOPChair: Lincoln Carr, Colorado School of Mines
Room: 704
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G34.00001: Magnetism on the Lieb Lattice Fr\'ederic H\'ebert, Vladimir Iglovikov, Richard Scalettar, George Batrouni The fermionic Hubbard model on a square lattice is known to exhibit antiferromagnetism at half-filling for arbitrarily weak interactions, due to the nesting of its Fermi surface and the divergence of the density of states. This talk presents a determinant Quantum Monte Carlo study of the magnetic properties of the Hubbard model on the ``Lieb lattice,'' which is obtained from the square lattice by removing 1/4 of the sites in a regular pattern. This model exhibits a flat band at half-filling, surrounded by two dispersive bands. The non interacting states at half-filling are localized, which allows us to study the magnetic properties of a system that does not have a Fermi surface, although there is an infinite density of states at half-filling. Other magnetic phases may also appear away from half-filling. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G34.00002: Quantum Monte Carlo Study of Superconductivity on the Lieb Lattice Vladimir Iglovikov, Richard Scalettar, Fr\'ederic H\'ebert, George Batrouni The attractive fermion Hubbard model on a square lattice is known to have a finite temperature (Kosterlitz-Thouless) transition to a superconducting ground state away from half-filling. This talk presents determinant Quantum Monte Carlo results for pairing correlations on a ``Lieb lattice,'' which is obtained from the square lattice by regular pattern removal of 1/4 of the sites. The Lieb lattice has two dispersing bands, and one flat band. We will show how superconductivity differs when the chemical potential lies in the flat band. For large values of the on-site attractive interaction, the fermions form tightly bound bosonic pairs. In that limit, our work addresses the nature of Bose-Einstein condensation when the non-interacting band structure has no wave-vector which minimizes the energy. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G34.00003: FFLO pairing correlations in a trapped quasi one-dimensional Fermi gas Dominique Gautreau, Stephen Kudla, Daniel Sheehy Recent work has pursued the possibility of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase of imbalanced Fermi gases in one dimension, in which an imposed population imbalance between two species of interacting fermion leads to spatially-modulated local pairing correlations. While experiments at Rice (Liao et al, Nature 467, 567 (2010)) show consistency with the expected bulk phase diagram using the local density approximation, little is known about how the FFLO pairing correlations will be revealed experimentally. Using a simple variational wavefunction ansatz for the FFLO state of a trapped 1D gas, in which the population imbalance leads to an imbalance in pairing among harmonic oscillator states, we compute predicted experimental signatures of the FFLO phase in a trapped one-dimensional fermionic atomic gas. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G34.00004: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G34.00005: Heat capacity and sound velocities of low dimensional Fermi gases P. Salas, M.A. Solis We report the heat capacity ratio and sound velocities for an interactionless Fermi gas immersed in periodic structures such as penetrable multilayers or multitubes created by one (planes) or two perpendicular (tubes) external Dirac comb potentials. The isobaric specific heat of the fermion gas presents the dimensional crossover previously observed in the isochoric specific heat [1] - from 3D to 2D or to 1D -. The quotient between the two quantities has a prominent bump related to the confinement, and as the temperature increases, it goes towards the monoatomic classical gas value 5/3. We present the isothermal and the adiabatic sound velocities of the fermion gas which show anomalous behavior at temperatures below $T_F$ due to the dimensionality of the system, while at higher temperatures again we recover the behavior of a classical Fermi gas. Furthermore, as the temperature goes to zero the sound velocity has a finite value, as expected.\\[4pt] [1] P. Salas and M.A. Sol\'is, ``Trapping effect of periodic structures on the thermodynamic properties of a Fermi gas,'' J. Low Temp, Phys. (2013) DOI 10.1007/s10909-013-0939-x. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G34.00006: Spin imbalance in 1D trapped attractive Fermi atoms: T\textgreater 0 quantum Monte Carlo results ChangMo Yang, D.M. Ceperley Spin imbalance in a one-dimensional system of fermions with short-ranged attractive interactions is studied with continuous-space path-integral Monte Carlo simulation. Following closely the experiment with ${}^6$Li atoms~[1], the pair momentum distribution is calculated at various spin polarizations. FFLO-type pairing is confirmed at the experimental temperature and coupling strength. We compare our results to those of other numerical methods~[2-4] and discuss the prospects for experimental detection.\\[4pt] [1] Y.-A. Liao et al.,~\textit{Nature},~467, 567 (2010).\\[0pt] [2] H. Lu et al.,~\textit{Phys.~Rev.~Lett.},~108, 225302 (2012).\\[0pt] [3] F. Heidrich-Meisner et al.,~\textit{Phys.~Rev.~A},~81, 023629 (2010).\\[0pt] [4] M. Takahashi,~\textit{Prog.~Theor.~Phys,}~46, 1388 (1971).\\[0pt] [5] M. Casula et al.,~\textit{Phys.~Rev.~A},~78, 033607, (2008). [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G34.00007: Quasi-two-dimensional Fermi gases Meera Parish, Andrea Fischer, Jesper Levinsen We consider a two-component gas of fermionic atoms confined to a quasi-two-dimensional geometry by a harmonic trapping potential in the transverse direction. We construct a mean field theory of the BCS-BEC crossover that allows us to extrapolate to an infinite number of transverse harmonic oscillator levels. Even when the interactions are weak and the Fermi energy is less than the confinement frequency, we find that the higher transverse levels can substantially modify fermion pairing. We also investigate the effect of confinement on few-body correlations, and we demonstrate the existence of a universal tetramer for mass ratios $> 5.0$. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G34.00008: Pairing correlations in a trapped quasi one-dimensional Fermi gas Stephen Kudla, Dominique Gautreau, Daniel Sheehy We utilize a BCS-type variational wavefunction to study attractively-interacting quasi one-dimensional fermionic atomic gases, motivated by cold-atom experiments that access this regime using a anisotropic harmonic trapping potential (characterized by $\omega_x = \omega_y\gg \omega_z$) that confines the gas to a cigar-shaped geometry. To handle the presence of the trap along the $z$ direction, we construct our variational wavefunction from the harmonic oscillator Hermite functions that are the eigenfunctions of the single-particle problem. Using an analytic determination of the effective interaction among Hermite function states along with a numerical calculation of the resulting variational equations, we make specific experimental predictions for how local pairing correlations will be revealed in experimental probes like the local density, the momentum distribution, and the momentum correlation function. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G34.00009: Finite-temperature dynamical structure factor of the one-dimensional Bose gas: From the Gross-Pitaevskii equation to the Kardar-Parisi-Zhang universality class of dynamical critical phenomena Austen Lamacraft, Manas Kulkarni We study the finite-temperature dynamical structure factor S(k,$\omega$) of a one-dimensional Bose gas using numerical simulations of the Gross-Pitaevskii equation appropriate to a weakly interacting system. The line shape of the phonon peaks in S(k,$\omega$) has a width proportional to $|$k$|^{3/2}$ at low wave vectors. This anomalous width arises from resonant three-phonon interactions, and reveals a remarkable connection to the Kardar-Parisi-Zhang universality class of dynamical critical phenomena. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G34.00010: Casimir interaction between mobile impurities in one-dimensional quantum liquids Michael Schecter, Alex Kamenev At zero temperature virtual phonons of a quantum liquid scatter off impurities and mediate a long-range interaction, analogous to the Casimir effect. At finite temperature, moving impurities also experience a correlated friction due to coherent exchange of real phonons. In one dimension the effect is universal and the induced interaction decays as $1/r^3$, much slower than the van der Waals interaction $\sim1/r^6$ where $r$ is the impurity separation. The magnitude of the effect is characterized by the product of impurity-phonon scattering amplitudes, which are seen to vanish for the class of integrable impurity models. By tuning the parameters near integrability one can thus observe an attractive interaction turned into a repulsive one. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G34.00011: Spreading of correlations in the XXZ chain at finite temperatures Lars Bonnes, Andreas L\"auchli In a quantum quench, for instance by abruptly changing the interaction parameter in a spin chain, correlations can spread across the system but have to obey a speed limit set by the Lieb-Robinson bound. This results into a causal structure where the propagation front resembles a light-cone. One can ask how fast a correlation front actually propagates and how its velocity depends on the nature of the quench. This question is addressed by performing global quenches in the XXZ chain initially prepared in a finite-temperature state using minimally entangled typical thermal states (METTS). We provide numerical evidence that the spreading velocity of the spin correlation functions for the quench into the gapless phase is solely determined by the value of the final interaction and the amount of excess energy of the system. This is quite surprising as the XXZ model is integrable and its dynamics is constrained by a large amount of conserved quantities. In particular, the spreading velocity seems to interpolate linearly from a universal value at $T=\infty$ to the spin wave velocity of the final Hamiltonian in the limit of zero excess energy for $\Delta_{\mathrm{final}} > 0$. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G34.00012: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G34.00013: Order-by-disorder of interacting bosons on the dice lattice under a synthetic gauge field Matjaz Payrits, Ryan Barnett We consider a gas of interacting bosons in the two-dimensional dice lattice in the presence of a half-elementary magnetic flux threading each plaquette. The single particle spectrum of the system consists of three doubly-degenerate completely flat bands, which indicates a large ground state degeneracy. It is shown how this degeneracy is partially lifted in the superfluid regime at the mean-field level. Furthermore, it is shown how quantum and thermal fluctuations conclusively remove the remaining accidental degeneracy between the mean field states, thus selecting a unique state up to overall symmetries. This can be elegantly described by means of the distribution of condensate vortices in the Kagom\'{e} vortex lattice, which is dual to the dice lattice. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G34.00014: Collective Excitations in Quasi-2D and 3D Condensates Dan Lobser, Andrew Barentine, Heather Lewandowski, Eric Cornell Collective motion of a Bose-Einstein condensate has been a system of interest since the discovery of BEC. Boltzmann proved that the monopole mode of a thermal gas in an isotropic, harmonic and 3D trap is undamped [1, 2]. BECs are not classical gases and their weakly interacting nature causes damping in a 3D monopole mode. However, experimental limitations have precluded studies of this behavior in very spherical traps. Quantum gases confined to lower dimensions exhibit remarkable physical properties such as the Berezkinskii-Kosterlitz-Thouless transition or the Tonks-Girardeau gas. Confinement effects in a quasi-2D condensate are predicted to shift the frequency of the monopole mode [3]. One correction in particular connects a small frequency shift with certain quantum corrections [4]. Current results of our studies in 2D and 3D will be presented. \\[4pt] [1] ``Transverse Breathing Mode of an Elongated Bose-Einstein Condensate,'' F. Chevy, V. Bretin, P. Rosenbusch, K. W. Madison, and J. Dalibard, Phys. Rev. Lett. 88 250402 (2002). \\[0pt] [2] L. Boltzmann, Wissenschaftliche Abhandlungen, edited by F. Hasenorl (Barth, Leipzig, 1909), Vol. II \\[0pt] [3] Olshanii et al., Phys. Rev. Lett. 105, 095302 (2010) \\[0pt] [4] Hu, et al., Phys. Rev. Lett. 107, 110401 (2011) [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G34.00015: Universal high frequency high momentum behavior of dynamic structure factor in one dimensional interacting boson gas Ran Qi, Michael Pustilnik, Shina Tan We study the short-distance and short-time structure of density-density correlation in one dimensional repulsively interacting boson gas. A compact universal formula is obtained for the high frequency high momentum asymptotic behavior of dynamic structure factor. We observe non-monotonous behavior in the dependence of DSF on interacting strength and qualitative change in the singular behavior in different region of $q^2/\omega$. Possible experimental applications are discussed. [Preview Abstract] |
Session G35: Quantum Communication, Decoherence, & Cryptography
Sponsoring Units: GQIRoom: 702
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G35.00001: An Exactly Solvable Model of Quantum Communications Graeme Smith, John Smolin Information theory establishes the ultimate limits on performance for noisy communication systems. Accurate models of physical communication devices must include quantum effects, but these typically make the theory intractable. As a result, communication capacities are not known, even for transmission between two users connected by an electromagnetic waveguide with gaussian noise[6]. We present an exactly solvable model of communications with a fully quantum electromagnetic field. This gives explicit expressions for all point-to-point capacities of noisy quantum channels, with implications for quantum key distribution and fiber optical communications. We also develop a theory of quantum communication networks by solving some rudimentary networks for broadcasting and multiple access. We compare the predictions of our model with the orthodox gaussian model and in all cases find agreement to within a few bits. At high signal to noise ratios (SNRs) our simple model captures the relevant physics while remaining amenable to exact solution. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G35.00002: Strong converse rates for classical communication over thermal bosonic channels Bhaskar Roy Bardhan, Mark Wilde We prove that several known upper bounds on the classical capacity of thermal bosonic channels are actually strong converse rates. Our results strengthen the interpretation of these upper bounds, in the sense that we now know that the probability of correctly decoding a classical message rapidly converges to zero in the limit of many channel uses if the communication rate exceeds these upper bounds. In order for these theorems to hold, we need to impose a maximum photon number constraint on the states input to the channel (the strong converse property need not hold if there is only a mean photon number constraint). Our first theorem demonstrates that a capacity upper bound due to Koenig and Smith is a strong converse rate, and we prove this result by utilizing their structural decomposition of a thermal channel into a pure-loss channel followed by an amplifier channel. Our second theorem demonstrates that an upper bound due to Giovannetti {\it et al.} corresponds to a strong converse rate, and we prove this result by relating success probability to coding rate and bosonic entropies. Both bounds are within 1.45 bits of the known lower bound on capacity that arises from a coherent-state coding scheme. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G35.00003: Entropic error-disturbance relations Patrick Coles, Fabian Furrer We derive an entropic error-disturbance relation for a sequential measurement scenario as originally considered by Heisenberg, and we discuss how our relation could be tested using existing experimental setups. Our relation is valid for discrete observables, such as spin, as well as continuous observables, such as position and momentum. The novel aspect of our relation compared to earlier versions is its clear operational interpretation and the quantification of error and disturbance using entropic quantities. This directly relates the measurement uncertainty, a fundamental property of quantum mechanics, to information theoretical limitations and offers potential applications in for instance quantum cryptography. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G35.00004: Mismatched quantum filtering and entropic information Mankei Tsang Quantum filtering is a signal processing technique that estimates the posterior state of a quantum system under continuous measurements and has become a standard tool in quantum information processing, with applications in quantum state preparation, quantum metrology, and quantum control. If the filter assumes a wrong model due to assumptions or approximations, however, the estimation accuracy is bound to suffer. In this talk I shall present formulas that relate the error penalty caused by quantum filter mismatch to the relative entropy between the true model and the nominal model, with one formula for Gaussian measurements, such as homodyne detection, and another for Poissonian measurements, such as photon counting. These formulas generalize recent seminal results in classical information theory and provide new operational meanings to relative entropy, mutual information, and channel capacity in the context of quantum experiments. See http://arxiv.org/abs/1310.0291 for details. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G35.00005: Action principle for continuous quantum measurement and quantum trajectories with pre and post-selection Areeya Chantasri, Justin Dressel, Steven Weber, Kater Murch, Irfan Siddiqi, Andrew Jordan We apply an action principle to continuous quantum measurement by introducing a joint probability density function of measurement outcomes and quantum state trajectories in a path integral form. Using a modified principle of least action, we find the paths of maximum likelihood connecting boundary states between any two points in time, at which we call the most-likely paths. We present, as an example, the most-likely paths for a continuous qubit measurement with pre and post-selected states, along with a preliminary comparison to data from a superconducting qubit coupled to a microwave cavity. We, furthermore, introduce other interesting statistical characterizations of the quantum trajectories such as mean paths, variances and most-likely times, that can be derived from our path integral formalism. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G35.00006: Long-Distance Continuous-Variable Quantum Key Distribution with Scalar Reconciliation and Gaussian Adaptive Multicarrier Quadrature Division Laszlo Gyongyosi, Sandor Imre The two-way Continuous-Variable Quantum Key Distribution (CVQKD) systems allow higher key rates and improved transmission distances over standard telecommunication networks in comparison to the one-way CVQKD protocols. To exploit the real potential of two-way CVQKD systems a robust reconciliation technique is needed. It is currently unavailable, which makes it impossible to reach the real performance of a two-way CVQKD system. We propose an efficient logical layer-based reconciliation method for two-way CVQKD to extract binary information from correlated Gaussian variables. We demonstrate that by operating on the raw-data level, the noise of the quantum channel can be corrected in the scalar space and the reconciliation can be extended to arbitrary high dimensions. The results allow to significantly improve the currently available key rates and transmission distances of two-way CVQKD. We show that by exploiting the proposed adaptive multicarrier modulation scheme, two-way CVQKD can be extended to a range of 160 km over optical fiber with improved tolerable loss and excess noise. The proposed scalar reconciliation can also be applied in one-way systems as well, and can be extended for multiuser communication. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G35.00007: Dynamics of the spin in slowly rotating magnetic field Amrit Poudel, Canran Xu, Maxim G. Vavilov We study the dynamics of a spin coupled to its environment in a slowly rotating magnetic field. We show that once rotation starts abruptly, the spin exhibits precession around rotating magnetic field. This precession is suppressed due to the decoherence of the spin induced by the environment. At longer times, the spin rotates with the magnetic field and has a component perpendicular to the plane of rotation of the field, which is proportional to the product of the Berry curvature and the angular velocity of the rotation. Finite temperature environment causes thermalization of the spin and, in particular, effectively reduces the magnitude of the spin in the direction perpendicular to the plane of rotation. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G35.00008: Semiclassical Decoherence in He-Surface Scattering Matthew Schram The field of Helium-surface scattering has been recently reexamined with great fervor thanks to recent technological advances and a more focused interest in studying issues surrounding decoherence and the quantum to classical transition. Recent work (Schuller et al 2007, Bundaleski et al 2008) has unexpectedly observed diffraction peaks in He-LiF and He-Ag interactions. This raises fundamental questions about the degree of elasticity of these high-energy collisions, and moreover what the scattering particle is coherent ``with.'' We present results using semiclassical gaussian wavepackets to simulate surface scattering and report on the relative contributions on coherence from different types of inelastic interactions. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G35.00009: Exact non-Markovian two-time correlation functions and current noise spectrum of electron transport through a quantum dot Chung-Chin Jian, Hsi-Sheng Goan Two-time correlation functions (CF's) of the electric currents through nanostructure devices are important in the study of the transport properties of current fluctuations and noise spectra. In the Markovian case, an extremely useful procedure to calculate the two-time (multiple-time) CF's is the so-called quantum regression theorem (QRT). For transport problems, a widely used method to calculate the steady-state current noise spectrum (i.e., Fourier transform of the steady-state current-current two-time CF's) is the MacDonald's formula which can be shown to be equivalent to QRT. However, similar to the QRT where only the evolution equations of the single-time expectation values are required to evaluate two-time CF's, the MacDonald's formula involves also only the single-time expectation values. Thus the MacDonald's formula, in our opinion, may not be applicable to calculate the current noise spectrum for transport problems that involves processes with non-Markovian (memory) effects. Here we develop a correct method to calculate the non-Markovian two-time CF's and finite-frequency noise power spectra based on the approach of the non-Markovian quantum state diffusion (NMQSD) or diffusive stochastic Schrodinger equation. This powerful NMQSD method allows us to calculate the exact current-current two-time CF and thus the exact current noise power spectrum for electron transport through a quantum dot. Our exact results reduce to those obtained by QRT or the MacDonald's formula in the Markovian limit. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G35.00010: Uncertainty Principle Consequences on Thermal Equilibrium Thermodynamics Johan Triana Galvis, Leonardo Pachon Contreras, David Zueco, Paul Brumer In the framework of classical mechanics, it is shown that the thermal equilibrium distribution of a system interacting via central forces with a non self-interacting environment, irrespectively of the interaction strength, is exactly characterized by the canonical Boltzmann distribution. In the framework of quantum mechanics, we show that the fundamental constraints on the contraction of the phase-space volume, imposed by the uncertainty principle, not only inhibits the system thermal-equilibrium-state to be described by the canonical Boltzmann distribution but also it is the responsible of the failure of the Onsager's regression hypothesis and a violation of the KMS condition. Furthermore, as a consequence of this analysis, we discuss the emergence of an \emph{effective coupling} to the environment that depends on all the energy scales involved in the system and reservoir interaction. This effective coupling defines a new quantum limit and has immediately consequences: (i) For the case of strong effective coupling, the system thermal equilibrium state does not match into the canonical distribution and (ii) For the case of weak effective coupling, quantum fluctuations are able to maintain, e.g., stationary entanglement at higher temperatures. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G35.00011: Quantum-limited amplification via reservoir engineering A. Metelmann, A.A. Clerk We describe a new kind of phase-preserving quantum amplifier which utilizes dissipative interactions in a parametrically-coupled three-mode bosonic system [1]. The use of dissipative interactions provides a fundamental advantage over standard cavity-based parametric amplifiers: large photon number gains are possible with quantum-limited added noise, with no limitation on the gain-bandwidth product. Our approach is related to reservoir engineering, where one constructs a non-trivial dissipative reservoir that relaxes the system to a desired target state. We instead realize a dissipative amplification process mediated via an engineered reservoir. The proposed scheme is simple enough to be implemented both in optomechanical systems and in superconducting microwave circuits.\\ \noindent \begin{footnotesize} \hfill [1] A.Metelmann and A.A. Clerk, ArXiv e-prints (2013), 1311.0273. \end{footnotesize} [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G35.00012: Autonomous Fock state stabilization by reservoir engineering E. Holland, B. Vlastakis, R. Heeres, U. Vool, Z. Leghtas, L. Frunzio, G. Kirchmair, M. Mirrahimi, R.J. Schoelkopf Quantum computing requires the ability to create and maintain quantum states. However, due to persistent coupling to the environment a quantum state suffers from decoherence. In order to fight decoherence physicists have come up with different approaches such as circuit based quantum error correction and reservoir engineering. Here we present a reservoir engineering scheme which autonomously stabilizes Fock states in a superconducting waveguide cavity. We will discuss how a vertical transmon qubit is used as a nonlinear coupler between two superconducting waveguide cavities. This nonlinear coupling creates a direct, strong-dispersive interaction between the two cavities. We utilize this interaction to autonomously stabilize Fock states by applying classical continuous wave drives. We present preliminary experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G35.00013: Decay of the Loschmidt echo in an open, out of equilibrium quantum system Silvia Viola Kusminskiy, Mark Thomas, Torsten Karzig, Felix von Oppen The dynamics of a classical heavy particle moving in a quantum environment is determined by a Langevin equation which encapsulates the effect of the environment-induced reaction forces on the particle. For an open quantum system these include a Born-Oppenheimer force, a dissipative force and a stochastic force due to shot and thermal noise. Recently it was shown that these forces can be expressed in terms of the scattering matrix of the system by considering the classical heavy particle as a time-dependent scattering center. At the same time, it is well known that small changes in a scattering potential can have a profound impact on a fermionic system due to the Anderson orthogonality catastrophe. A useful tool to study this effect on the dynamics of the quantum system is the Loschmidt echo. In this work we study the decay of the Loschmidt echo due to a small change in a scattering potential, for an open quantum system which is out of equilibrium due to an applied bias potential. With methods of scattering theory, and relying on the expressions obtained previously for the environment-induced forces on a heavy particle, we determine the decay of the Loschmidt echo in terms of the fluctuations and dissipation of the system. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G35.00014: Landau-Zener Transition for a qubit coupled to an Ohmic environment Maxim G. Vavilov, Canran Xu, Amrit Poudel We study dynamics of a qubit coupled to an Ohmic environment using the Bloch-Redfield approach. We first discuss how Bloch-Redfield equations can be modified to describe a quantum system with a slowly varying Hamiltonian. We apply this method to the Landau--Zener problem in the presence of environment at zero and finite temperatures. We show that the environment causes relaxation and excitation processes with time--dependent transition rates and the transition probability is greatly affected by these processes. In particular, the transition probability is reduced for environment at zero temperature, when only relaxation is present. At finite temperatures, the competition between relaxation and excitation give rise to non--monotonic dependence of the transition probability on the coupling strength. We also discuss the applicability of the Lindblad formalism to this problem. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G35.00015: Dynamical and thermodynamical control of Open Quantum Walks Francesco Petruccione, Ilya Sinayskiy Over the last few years dynamical properties and limit distributions of Open Quantum Walks (OQWs), quantum walks driven by dissipation, have been intensely studied [S. Attal et. al. J. Stat. Phys. 147, Issue 4, 832 (2012)]. For some particular cases of OQWs central limit theorems have been proven [S. Attal, N. Guillotin, C. Sabot, ``Central Limit Theorems for Open Quantum Random Walks,'' to appear in Annales Henri Poincar\'e]. However, only recently the connection between the rich dynamical behavior of OQWs and the corresponding microscopic system-environment models has been established. The microscopic derivation of an OQW as a reduced system dynamics on a 2-nodes graph [I. Sinayskiy, F. Petruccione, Open Syst. Inf. Dyn. 20, 1340007 (2013)] and its generalization to arbitrary graphs allow to explain the dependance of the dynamical behavior of the OQW on the temperature and coupling to the environment. For thermal environments we observe Gaussian behaviour, whereas at zero temperature population trapping and ``soliton''-like behaviour are possible. Physical realizations of OQWs in quantum optical setups will be also presented. [Preview Abstract] |
Session G36: Invited Session: Condensed Matter Physics in China
Sponsoring Units: FIPChair: Esen Alp, Argonna National Laboratory
Room: 703
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G36.00001: History, Present and Future of IoP and Solving the unsolvable integrable models Invited Speaker: Yupeng Wang The talk contains two parts. In the first part, I'll give a brief introduction of the Institute of Physics, Chinese Academy of Sciences, including the history, present and future. The main missions and the achievements will also be introduced. In the second part, I'll talk about a newly proposed theoretical method which has been successfully used to solving some long-standing problems in the field of integrable models. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G36.00002: Surface Studies of Ice Invited Speaker: Enge Wang Despite ice being a ubiquitous and well-studied substance, it is surprising that some basic questions about its surface properties are still debated. Here computer simulations are used to study the unusual structure and dynamics of ice surface at atomic scale. An order parameter, which defines the ice surface energy, is identified for the first time. A classical electrostatic model proves useful to explain the physics inside. We predict that the proton order-disorder transition, which occurs in the bulk at $\sim$ 72 K, will not occur at the surface at any temperature below surface melting. In addition, we find that the surface of crystalline ice exhibits a remarkable variance in vacancy formation energies that is more characteristic of an amorphous material. A fraction of surface molecules are bound by less than the strength of a single hydrogen bond, yet other sites are more strongly bound than those in the crystal interior. Vacancy energies are found to be as low as $\sim$ 0.1eV at the surface, leading to a higher than expected concentration of vacancies at the external layer. Once a surface vacancy is formed, the energetic cost of forming neighbouring vacancies is greatly reduced, facilitating pits on the surface and other processes that may contribute to the phenomenon of pre-melting and quasi-liquid layer formation. Finally, we show that the distribution of local arrangement of dangling atoms, characterized by a surface proton order parameter, is also of crucial importance for the adsorption of water monomer on ice surface. The positive correlation of adsorption energy of water monomer with surface proton ordering suggests that the adsorption may prefer to firstly occur in the inhomogeneous surface, which sheds light on our understanding of the ice nucleation and growth as well as other physical/chemical reactivity in high altitude clouds. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G36.00003: Pressure tuned novel states of new quantum materials Invited Speaker: Changqing Jin High Pressure plays significant role in shaping quantum materials states. Pressure can effectively modify spin, charge or orbital features that in turn change physical properties of quantum matters. We will introduce our recent works [1-8] along the direction in this presentation based on wide international collaborations. We thank our collaborators for their significant contributions.\\[4pt] [1] C. Q. Jin \textit{et al};\textbf{ Proc. Natl. Acad. Sci. USA}\textbf{\textit{,}}\textbf{ 105}, 7115 (2008).\\[0pt] [2] X. C. Wang et al, \textbf{Solid State Communications 148}, 538 (2008).\\[0pt] [3] D. Haskel et al., \textbf{Phys. Rev. Letts. 109}, 027204 (2012).\\[0pt] [4] J.G. Zhao \textit{et al}, \textbf{J. Am. Chem. Soc.130}, 13828 (2008).\\[0pt] [5] J. L. Zhang et al., \textbf{Proc. Natl Acad. Sci. 108}, 24 (2011).\\[0pt] [6] Z. Deng et al \textbf{Nature Communications 2}, 422 (2011).\\[0pt] [7] J. G. Cheng et al., \textbf{Phys. Rev. Letts. 108,} 236403 (2012).\\[0pt] [8] K. Zhao et al.\textbf{ Nature Communications 4}: 1442 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G36.00004: Introduction of BASIC (Beijing Advanced Sciences and Innovation Centre) Invited Speaker: Hong Ding In this talk I will review the goal, planning, and current status of Beijing Advanced Sciences and Innovation Centre (BASIC), which will become the first multidisciplinary basic science laboratory within Chinese Academy of Sciences. I will mainly focus on some of large-scale scientific facilities which may be built inside BASIC in the near future. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G36.00005: Half-Century of Chinese Condensed Matter Physics--from humble beginnings Invited Speaker: Lu Yu Half-century ago, modern condensed matter physics was almost non-existing in China. During the past 30 years, especially since the beginning of the 21 century, the situation has changed drammatically. A number of outstanding young physicists from China, with cutting edge research output, appeared on the World arena. How did this quantal transition occur? From a personal perspective, I will discuss some early attempts to build-up research capacities, interruptions during the ``cultural revolution,'' survival of scientific work, opening-up to the World and revival of research. The transition from total isolation to close international exchange and collaboration is a crucial factor contributing to this success. [Preview Abstract] |
Session G37: Focus Session: Quantum Hall Effect in Graphene
Sponsoring Units: DMPChair: Zhigang Jiang, Georgia Institute of Technology
Room: 705/707
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G37.00001: Quantum Hall effect in epitaxial graphene - a metrological perspective Invited Speaker: Alexander Tzalenchuk Although the QHE has been used successfully for more than two decades to realise the resistance scale, graphene has potential to supersede conventional semiconductors as the material of choice for quantum electrical metrology. The physical mechanisms giving graphene, grown on SiC in particular, an edge over the conventional semiconductors include the pinning of the filling factor over an extraordinarily broad range of magnetic field, large inter-Landau level spacing and a very short energy relaxation time. Together they lead to a very robust quantum Hall state opening an opportunity to realise the quantum resistance standard of greatly reduced cost and complexity operating at high temperatures, low magnetic fields and high signal-to-noise ratio. I will review the progress achieved in graphene engineering, physical understanding and metrology from the first accurate QHE measurements performed on exfoliated samples (with precision of $15$ parts in $10^6$) and on graphene on SiC ($3$ parts in $10^9$) to a direct comparison between graphene on SiC and GaAs demonstrating equivalence of the quantised values of the Hall resistance with a relative uncertainty of $8.6$ parts in $10^{11}$. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G37.00002: Transport Properties of Topological 1D Zero-Line Mode in Graphene Zhenhua Qiao, Jeil Jung, Chungwei Lin, Allan MacDonald, Qian Niu When the inversion symmetry of graphene systems is broken, e.g. graphene subjected to a staggered sublattice potential or bilayer under an applied interlayer potential difference, a bulk band gap opens to support the quantum valley-Hall state. When the potential varies spatially, a topological one-dimensional conducting channel is formed along the zero-line of the potential. We find that such a state shows the property of zero bend resistance. And if two straight zero lines crosses, we show that the splitting of the zero line mode obeys a counterintuitive current partition law. We provide a theory to understand the physics behind these novel characteristics. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G37.00003: Hall effect in triangular antidot graphene under a weak magnetic field Yuan Zheng In triangular antidot graphene (TAG),a quasi-gap is formed by enhanced electron-electron interaction when the charge carrier density is very low, in conjunction with the appearance of a small effective mass for the TAG, owing to the altered dispersion relation. In the gap, a very long de-phasing length (10micron) has been observed at 2K. It means that there is an enlarged mesoscopic region in TAG. Within the quasigap, the inelastic scatterings are exponentially suppressed at low temperatures. Physics of electrons transport can therefore be treated with only elastic scatterings in the low temperature regime. Classical Hall effect is an effective way to probe the type of charge carriers and charge carrier density in semiconductor. When charge carrier density is low and the geometric size of the scattering features is also small, the Fermi wavelength of electrons can become comparable to the size of the scattering features. In the weak magnetic field regime, the giant Hall effect as well as the vanishing Vxy are both possible under certain conditions. In this talk we present both theoretical simulations and experimental results for the Hall effect in the antidot graphene. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G37.00004: Quantum Hall effect in tunable superlattice in graphene Sudipta Dubey, Mandar Deshmukh Superlattice in graphene is created by a bottom gate and an array of top gates pinned to the same potential. The difference in charge density between region with and without top gate creates the amplitude of superlattice potential and thus can be tuned by the gate voltage. The superlattice period is 150 nm. We study the effect of magnetic field in this array of p-n junctions when the magnetic length is smaller than the superlattice period. Depending on the gate voltage applied in the top and bottom gate, the edge states circulate in the same or opposite direction. As the filling fraction in the adjacent region can be controlled, we can tune the backscattering between alternate regions by changing the charge density in the region between them. When the gate voltages are so tuned that we have electrons and holes in the adjacent region, the edge states circulating in the opposite direction in the p and n region bring electrons and holes at the p-n interface. In this regime, we observe a large longitudinal resistance. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G37.00005: Spectroscopy of snake states using a graphene Hall bar Slavisa Milovanovic, Massoud Ramezani Masir, Francois Peeters A novel approach to observe snake states in a graphene Hall bar containing a pn-junction is proposed. The magnetic field dependence of the bend resistance in a ballistic graphene Hall bar structure containing a tilted pn-junction oscillates as a function of applied magnetic field. We show that each oscillation is due to a specific snake state that moves along the pn-interface. Furthermore depending on the value of the magnetic field and applied potential we can control the lead in which the electrons will end up and hence control the response of the system. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G37.00006: Canted magnetism and edge transport in tunable quantum Hall phases in graphene Jose Lado, Joaquin Fernandez-Rossier Motivated by recent experimental results [1] we study theoretically the quantum Hall effect in graphene in the presence of strong in plane magnetic field considering short range electron electron interactions. The experiments show a variety of phase transitions that change the bulk spin order order between different states, including antiferromagnetic (AF), ferromagnetic (FM) and canted antiferromagnetic (CAF), resulting in dramatically different edge states that control the conductivity. Here we model the non-trivial phase diagram of this system using a Hubbard model for a wide ribbon in a non-collinear mean field approximation. Our theory is able to account for the main experimental findings and provides a comprehensive phase diagram with at least 4 different electronic phases: AF, FM, CAF and a ferrimagnetic phase. Specifically, our model describes the presence of counter-propagating spin-filtered edge states in the FM phase at half filling, as well as a fully polarized single edge channel when the FM phase is doped into a ferrimagnetic phase with an electron-hole gap. [1] A. F. Young, J. D. Sanchez-Yamagishi, B. Hunt, S. H. Choi, K. Watanabe, T. Taniguchi, R. C. Ashoori, P. Jarillo-Herrero, arXiv:1307.5104 [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G37.00007: Broken SU(4) Symmetry and The Fractional Quantum Hall Effect in Graphene Inti Sodemann, Allan MacDonald We describe a simple variational approach to understand the spin-valley broken symmetry states in the fractional quantum Hall regime of graphene. Our approach allows to predict the incompressible ground states and their charge gaps and is able to explain the observed differences between filling factor ranges $|\nu|<1$ and $1<|\nu|<2$. We find that in the SU(4) invariant case the incompressible ground state at $|\nu|=1/3$ is a three-component incompressible state, not the Laughlin state, and discuss the competition between these two states in the presence of SU(4) spin-valley symmetry breaking terms. We find that the lowest energy fractionally charged quasi-particles involve spin/valley flips in several prominent fractions. We discuss the expected behavior of the gaps under tilting the magnetic field away from normal which allows to tune the relative strength of Zeeman and valley symmetry breaking interactions. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G37.00008: Quantum Hall effect in polycrystalline CVD graphene: grain boundaries impact Rebeca Ribeiro-Palau, Fabien Lafont, Felicien Schopfer, Wilfrid Poirier, Vincent Bouchiat, Zhen Han, Alessandro Cresti, Aron Cummings, Stephan Roche It was demonstrated by Janssen et al. (New J. Phys. 2011) that graphene could surpass GaAs for quantum Hall resistance standards with an accuracy better than $10^{-10}$. Graphene should render possible the realization of a standard operating at $T>4$ K and $B<4$ T, easing its dissemination towards industry. To materialize this goal scalable graphene with outstanding electronic transport properties is required. We present measurements performed in large area Hall bars made of polycrystalline CVD graphene on Si/SiO$_2$, with a carrier mobility of 0.6 T$^{-1}$. Even at 20.2 T and 300 mK, the Hall resistance plateaus are insufficiently quantized at $\nu=\pm2$ and $\pm6$. This is due to a high dissipation manifested by a longitudinal resistance which does not drop to zero. We pointed out unusual power-law temperature dependencies of $R_{xx}$ and an exponential magnetic field dependence. We do not observe the common thermally activated or VRH behaviors. This can be attributed to the grain boundaries in the sample that short-circuit the edge states, as supported by our numerical simulations. This reveals new and peculiar aspects of the quantum Hall effect in polycrystalline systems. Another unexpected feature is the observation of the $\nu=0$ and 1 states in such low mobility systems. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G37.00009: Proximity-Induced Anomalous Hall Effect in Graphene Zhiyong Wang, Chi Tang, Raymond Sachs, Yafis Barlas, Jing Shi Pre-patterned graphene devices are transferred from SiO$_{2}$/Si to atomically flat magnetic insulator thin films, yttrium iron garnet (YIG) deposited by a laser molecular beam epitaxial system on gadolinium gallium garnet (GGG) substrate. Room temperature Raman spectroscopy reveals both single-layer graphene and YIG characteristic peaks. In addition to the ordinary Hall effect, there is a clear non-linear Hall component correlated with the magnetization of the YIG films, which we attribute to the anomalous Hall effect (AHE). The magnitude of AHE in graphene/YIG devices decreases as temperature increases. With device-to-device variations, in some devices, AHE persists to room temperature, indicating a strong proximity-induced exchange interaction. By sweeping top gate voltages, one can tune the carrier density across the Dirac point. We also find that the carrier mobility is not significantly different in graphene/YIG. As the graphene is tuned from the electron- to hole-type, the ordinary Hall changes the sign as expected, but the sign of the AHE contribution remains the same. It suggests that AHE does not simply originate from the carrier density change which is responsible for the ordinary Hall effect, but is related to the spin-orbit interaction in the system. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G37.00010: Real space imaging of quantum hall edge states in graphene Yongtao Cui, Georgi Diankov, Eric Yue Ma, Francois Amet, Yongliang Yang, Michael Kelly, David Goldhaber-Gordon, Cory Dean, Zhi-Xun Shen At integer quantum hall filling factors in a two-dimensional electron gas, electrons in the bulk are localized, while near the edge it remains conductive as energy bands bend and cross the Fermi level. These conductive channels, known as the ``edge states,'' are immune to back scattering, giving rise to quantized resistance values -- the hallmark of the quantum hall effect. Here we use microwave impedance microscope to study the quantum hall edge states in graphene devices. Scanning images clearly show dividing regions of insulating bulk and conductive edges. We study the evolution of the edge patterns as the carrier density is tuned through multiple filling factors. Correlation between real space images and transport measurement demonstrates the robustness of the quantum hall effect -- even though the real space patterns are strongly affected by disorder, the quantization of resistance is retained due to the topological nature of the edge states. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G37.00011: Observation of new fractional quantum Hall states in the first Landau level in graphene Francois Amet, Andrew Bestwick, James Williams, Kenji Watanabe, Takahashi Taniguchi, David Goldhaber-Gordon We report on transport studies of the fractional quantum Hall effect in the n=0 and n=1 Landau level of monolayer graphene. The quality of the devices studied here -with mobilities up to 400 000cm2/Vs, and magnetic fields up to 45T- allows us to observe a variety of fractional quantum hall states following the composite fermion sequence, with denominators up to 9. The presence of odd numerator fractions between nu=1 and nu=2 is attributed to the breaking of the valley symmetry and correlates with a zero field insulating state observed at charge neutrality. We discuss the in-plane field dependence of the gaps, which is not trivial and shows dramatic differences between the n=0 and the n=1 Landau levels. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G37.00012: Broken SU(4) symmetry in quantum hall states in graphene: an exact diagonalization study Fengcheng Wu, Inti Sodemann, Yasufumi Araki, Thierry Jolicoeur, Allan MacDonald Electrons in graphene have four flavors due to low-energy spin and valley degrees of freedom. Long-range Coulomb interactions are SU(4) symmetric in spin and valley space, providing an experimental realization of the SU(4) fractional quantum hall effect. However, weak short-range electron-electron and electron-phonon interactions break the valley symmetry, and act as a source of isospin anisotropy. Using an exact diagonalization method that takes all four flavors into account, we study the SU(4) fractional quantum Hall effect, identifying singlet and broken symmetry ground states and low lying excitations at integer and fractional filling factors within the N=0 Landau level. We also account for the presence of valley-isospin anisotropy and Zeeman fields. For the quantum Hall states at neutrality we assess the impact of quantum fluctuations that are beyond the mean-field theory of quantum Hall ferromagnets. For the fractional quantum Hall states, we compute the energies of novel multi-component states and evaluate their prospects for experimental realization. A systematic symmetry analysis based on the SU(4) multiplet structure of the many body spectrum will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G37.00013: Quantum anomalous Hall effect in Co or Rh doped graphene Jun Hu, Ruqian Wu The recent discovery of topological insulators (TIs)---that act as insulator in the bulk yet possess quantized conducting edge or surface states---has triggered extensive interests in the field of condensed matter physics and materials science. One of the most interesting phenomena related to TIs is the quantum anomalous Hall effect (QAHE). Although there are a lot of theoretical predictions about the existence of the QAHE in different materials, the QAHE has been observed only in Be2Se3 so far, in an extreme experimental condition bellow 0.1 K due to the tiny TI gap. On the contrary, it was found that huge TI gaps can be induced in graphene by transition metal adatoms. In the present work, we predict that deposition of sparse Co or Rh adatoms on graphene can produce a TI gap of 37 or 100 meV around the Fermi level. Furthermore, we demonstrate that the QAH state is very robust, due to the strong perpendicular magnetic anisotropies. [Preview Abstract] |
Session G38: Invited Session: Graduate Education: Sustaining Thriving Programs by Embracing Challenges and Opportunities in the 21st Century
Sponsoring Units: FEdChair: Theodore Hodapp, American Physical Society
Room: 709/711
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G38.00001: Highlights From the Second Conference on Graduate Education in Physics Invited Speaker: Renee Diehl The Second Conference on Graduate Education in Physics was held in January 2013 with more than 100 participants from 74 different institutions. The participants comprised a diverse group faculty from large and small departments, staff from industry and national labs, and graduate students and postdocs. The conference was aimed at fostering innovation and creativity in our approach to graduate education in physics. Because the majority of physics PhDs ultimately find permanent employment outside academia, and because of the many competing demands on new faculty, many departments are reviewing their graduate programs. The presentations and discussions at the conference included the increasing attention being paid to broader and more flexible graduate curricula, forming industrial partnerships, strategies to increase diversity, professional skills training for graduate students and postdocs, and improving mentoring practices and instituting family-friendly policies for graduate students. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G38.00002: Increasing Diversity in Physics at the PhD Level and Beyond Invited Speaker: Keivan Stassun We briefly review the current status of underrepresented minorities in physics: The underrepresentation of Blacks, Hispanics, and Native Americans is an order of magnitude problem. We then describe the Fisk-Vanderbilt Masters-to-PhD Bridge program as a successful model for addressing this problem. Since 2004 the program has admitted 67 students, 60 of them underrepresented minorities (50\% female), with a retention rate of 90\%. Already, the program is the top producer of African American master's degrees in physics, and is the top producer of minority PhDs in astronomy, materials science, and physics. We summarize the main features of the program including its core strategies: (1) replacing the GRE in admissions with indicators that are better predictive of long-term success, (2) partnering with a minority-serving institution for student training through collaborative research, and (3) using the master's degree as a deliberate stepping stone to the PhD. We show how misuse of the GRE in graduate admissions may by itself in large part explain the ongoing underrepresentation of minorities in PhD programs, and we describe our alternate methods to identify talented individuals most likely to succeed. We describe our mentoring model and toolkit which may be utilized to enhance the success of all PhD students. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G38.00003: The Landscape of Graduate Admissions: Surveying Physics Programs about Doctoral Admissions Practices Invited Speaker: Geoff Potvin Sustaining or improving the best graduate programs as well as increasing the diversity of the physics community requires us to better understand the critical gatekeeping role played by graduate admissions. Admissions processes determine not only who is allowed to begin graduate study but can also influence who chooses to even consider applying. Recently, in concert with some of the activities of the APS Bridge Program, a survey was conducted of directors of graduate admissions and associated faculty in doctoral-granting departments about their admissions practices. Receiving responses from over 75\% of departments that award PhDs in physics, respondents were probed about their admissions decisions with special attention on the criteria used in admissions and their relative importance, and how student representation considerations are dealt with in the admissions process (if at all). Results indicate a number of important issues for future students, faculty, and administrators to consider including the importance placed on GRE scores. Results also indicate a sizable number of departments express a latent demand for greater numbers of students from traditionally-underrepresented backgrounds (including women) but simultaneously report a dearth of such students who even apply to their doctoral programs. Implications of these and other findings will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G38.00004: The future of the graduate physics curriculum and exam structure Invited Speaker: Michael Thoennessen Although the need for a ``core'' knowledge that all graduate student in physics should master is generally accepted, many departments have begun to modify their curriculum and exam structure in order to focus more on research skills and accommodate interdisciplinary degrees. There is no ``one size fits all'' solution and each department has to define itself and highlight the unique aspects of its programs. A summary of the discussion on the graduate physics curriculum and exam structure at the recent APS/AAPT Graduate Education Conference will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G38.00005: Preparing Graduate Students for Non-Academic Careers Invited Speaker: Lawrence Woolf One of the primary topics discussed at the conference concerned career development, since most graduate students will not have the academic careers of their advisors. Goals included reviewing the primary functions of physicists in industry, evaluating how students are currently prepared for these careers, and identifying how to fill gaps in preparation. A number of non-academic physicists provided insight into meeting these goals. Most physics graduate programs in general do not purposely prepare students for a non-academic career. Strategies for overcoming this shortcoming include advising students about these careers and providing training on broadly valued professional skills such as written and verbal communication, time and project management, leadership, working in teams, innovation, product development, and proposal writing. Alumni and others from industry could provide guidance on careers and skills and should be invited to talk to students. Academic training could also better prepare students for non-academic careers by including engineering and cross disciplinary problem solving as well as incorporating software and toolsets common in industry. [Preview Abstract] |
Session G39: Invited Session: Advanced First-Principles Methods for Complex Oxides
Sponsoring Units: DCMP DCOMPChair: James Rondinelli, Drexel University
Room: Mile High Ballroom 2A-3A
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G39.00001: Formal Valence, $3d$ Occupation, and Charge Ordering Transitions Invited Speaker: Warren Pickett The metal-insulator transition (MIT), discovered by Verwey in the late 1930s, has been thought to be one of the best understood of MITs, the other ones being named after Wigner, Peierls, Mott, and Anderson. Continuing work on these transitions finds in some cases less and less charge to order, raising the fundamental question of just where the entropy is coming from, and just what is ordering. To provide insight into the mechanism of charge-ordering transitions, which conventionally are pictured as a disproportionation, I will (1) review and reconsider the charge state (or oxidation number) picture itself, (2) introduce new theoretical results for the rare earth nickelates (viz. YNiO$_3$), the putative charge ordering compound AgNiO$_2$, and the dual charge state insulator AgO, and (3) analyze cationic occupations of actual (not formal) charge, and work to reconcile the conundrums that arise. Several of the clearest cases of charge ordering transitions involve no disproportion; moreover, the experimental data used to support charge ordering can be accounted for within density functional based calculations that contain no charge transfer The challenge of modeling charge ordering transitions with model Hamiltonians will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G39.00002: Controlling structural complexity as a path towards new multifunctional correlated materials Invited Speaker: Craig Fennie Perovskite ABO$_3$ oxides display an amazing variety of phenomena that can be altered by subtle changes in the chemistry and internal structure. Most undergo non-polar structural distortions associated with a rotation of the BO$_6$ octahedra about one or more of the crystal axes. These distortions are well known to control the charge/orbital, magnetic and electronic degrees of freedom. This strong coupling represents an opportunity to understand and create new functional materials that respond to an external perturbation in a useful way. For example, if octahedral rotations can be designed to induce ferroelectricity, an applied electric field would be able to directly couple to the BO$_6$ octahedra, thereby controlling emergent phenomena such as magnetism, and possibly controlling metal/insulator transition. In this talk I will discuss our recent work in this area, highlighting the opportunities and the challenges to realizing such materials. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G39.00003: Controlling the two-dimensional electron gas at complex oxide interfaces Invited Speaker: Anderson Janotti Heterostructures of complex oxides have attracted great interest since the demonstration of a high-density two-dimensional electron gas (2DEG) at the SrTiO$_3$/LaAlO$_3$ (STO/LAO) interface. Still, the density of the 2DEG is only one tenth of what was expected from simple electron counting, i.e., 1/2 electron per unit-cell area. Since then, the origin and amount of the charge, the electrical properties of the 2DEG, the role of native defects, and the abrupt variation of the electron density with the thickness of the LAO top layer have been the subject of numerous theoretical and experimental studies. More recently, a 2DEG with the full density of 1/2 electron per unit cell area has been observed at the interface between the band insulator STO and the Mott insulator GdTiO$_3$ (GTO) [1], shedding additional light on the origin of the 2DEG, and raising important questions on the differences between the STO/LAO and STO/GTO heterostructures. Here we will discuss the similarities of the 2DEG at the STO/LAO and STO/GTO heterostructures from the perspective of first-principles simulations. We will address the differences in band alignments in the STO/LAO and STO/GTO heterostructures, and how the 2DEG is affected by the surface of the LAO top layer in the STO/LAO, but apparently not in the STO/GTO case [2]. Finally, we will also discuss how heterostructures can be used to drastically alter the electronic structure of STO, transforming it from a band insulator into a Mott insulator. \\[4pt] [1] P. Moetakef, T. A. Cain, D. G. Ouellette, J. Y. Zhang, D. O. Klenov, A. Janotti, C. G. Van de Walle, S. Rajan, S. J. Allen, and S. Stemmer, App. Phys. Lett. {\bf 99}, 232116 (2011). \newline [2] A. Janotti, L. Bjaalie, L. Gordon, and C. G. Van de Walle, Phys. Rev. B {\bf 86}, 241108(R) (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G39.00004: One-Dimensional Electron Gas at the Steps of a LaAlO3-SrTiO3 Interface Invited Speaker: Emilio Artacho Thin films of LaAlO$_3$ (LAO) epitaxially grown on SrTiO$_3$ (STO) substrates give rise to a two-dimensional electron gas that has spurred lots of interest and activity, given its application possibilities (if, for instance, coupled to any of the many interesting effects displayed by perovskite materials as LAO and STO), and the fundamental questions it poses. The gas originates as a consequence of the polarisation discontinuity at the interface: Although both materials are centrosymmetric, one (STO) belongs to the category of such insulators with zero bulk dielectric polarisation, while LAO is a member of the family that displays half a polarisation quantum along its $\langle 001 \rangle$ direction. The lattice of polarisation values associated to each material allows the prediction of the different polarisation discontinuities for different interface directions. This includes vicinal interfaces, which would be expected to display a structure of terraces of lower index interfaces separated by steps. This enables predicting the effect of steps in the electrostatics across these films, and the possibility of charge carriers being attracted to them. Large-scale first-principles calculations based on density functional theory were performed to substantiate such predictions, allowing to expect the formation of one-dimensional electron gases associated to steps at well-chosen interfaces of the two materials. Such gases should prove to be of fundamental interest since they are expected to show highly correlated electron carriers. The polarisation analysis and the results of the calculations will be presented along related and supporting results. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G39.00005: DFT+DMFT calculations in oxide superlattices Invited Speaker: Chris Marianetti While density functional theory (DFT) is a useful tool for understanding transition metal oxides and their heterostructures, it can break down qualitatively for certain properties or in specific transition metal oxides. We present a dual variable theory, density functional theory plus dynamical mean-field theory (DFT+DMFT), which remedies many of the shortcomings of DFT. Our full implementation of this method uses a plane wave basis and maximally localized Wannier functions to create the correlated subspace. We will demonstrate the ability to compute not only spectra and low energy properties, but also fully charge self-consistent total energies. We will explain the role of the double-counting correction, and introduce a more optimal approach. The method will be applied to various nickelate superlattices, and we will identify a new class of polar Mott insulators. Additionally, we will address the ``thinness'' driven metal-insulator transition which is observed in nickelates with very few layers. [Preview Abstract] |
Session G40: Invited Session: The Physics of Climate
Sponsoring Units: GPCChair: James Brasseur, Pennsylvania State University
Room: Mile High Ballroom 2B-3B
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G40.00001: Sea ice, climate, and multiscale composites Invited Speaker: Kenneth Golden In September of 2012, the area of the Arctic Ocean covered by sea ice reached its lowest level ever recorded in more than three decades of satellite measurements. In fact, compared to the 1980's and 1990's, this represents a loss of more than half of the summer Arctic sea ice pack. While global climate models generally predict sea ice declines over the 21st century, the precipitous losses observed so far have significantly outpaced most projections. I will discuss how mathematical models of composite materials and statistical physics are being used to study key sea ice processes and advance how sea ice is represented in climate models. This work is helping to improve projections of the fate of Earth's ice packs, and the response of polar ecosystems. A brief video of a recent Antarctic expedition where sea ice properties were measured will be shown. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G40.00002: Ice sheet-ocean interactions and sea level change Invited Speaker: Patrick Heimbach Mass loss from the Greenland and Antarctic ice sheets has increased rapidly since the mid-1990s. Their combined loss now accounts for about one-third of global sea level rise. In Greenland, a growing body of evidence points to the marine margins of these glaciers as the region from which this dynamic response originated. Similarly, ice streams in West Antarctica that feed vast floating ice shelves have exhibited large decadal changes. We review observational evidence and present physical mechanisms that might explain the observed changes, in particular in the context of ice sheet-ocean interactions. Processes involve cover 7 orders of magnitudes of scales, ranging from mm boundary-layer processes to basin-scale coupled atmosphere-ocean variability. We discuss observational needs to fill the gap in our mechanistic understanding. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G40.00003: The Role of Clouds in Climate Change Invited Speaker: Daniel Cziczo The role of greenhouse gases, predominantly CO$_{2}$, on climate has been understood since the work of Arrhenius in the late 1800's. The role of clouds on the Earth's radiative balance is far more uncertainty. It is known that small atmospheric particles act as the seeds on which water droplets and ice crystals form but projecting laboratory and field measurements of particles and clouds to climate projections remains the most uncertain aspect of climate change. Laboratory and field studies show that cloud formation occurs when the ambient water vapor exceeds the equilibrium saturation value. When the ambient temperature is above 0$^{\circ}$C and the relative humidity above 100{\%}, liquid water condenses on aerosol particles, known as cloud condensation nuclei. The spontaneous formation of ice within aqueous droplets of the size commonly found in the atmosphere does not occur until a level of supercooling exceeding -38$^{\circ}$C, and a saturation near that of liquid water, is reached. Consequently, ice nucleation from 0 to -38$^{\circ}$C requires the presence of a special particle, known as an ice nucleus. This talk will describe the current state of knowledge of cloud formation and what aspects remain uncertain. Information gained from laboratory and field studies will be compared to our understanding of Earth's current state and how climate is projected to change in the future. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G40.00004: Causes and consequences of time-varying climate sensitivity Invited Speaker: Kyle Armour While constraining climate sensitivity has long been a focus of climate science, this global and equilibrium metric provides only limited understanding of transient and regional changes over the coming centuries. Indeed, pronounced spatial and temporal variability of climate change has been observed, and climate models diverge strongly in projections of future warming. This intermodel spread is due, in part, to different representations of how global climate sensitivity (set by feedbacks linking surface warming to top-of-atmosphere radiative response) will vary in time as the Earth warms. Here I discuss mechanisms governing the time variation of climate sensitivity, and consider its implications for future climate prediction. I show that climate sensitivity depends fundamentally on the respective geographic patterns of local radiative feedbacks and surface warming, and thus it naturally varies in time as the pattern of surface warming evolves, activating feedbacks of different strengths in different regions. Further, the pattern of surface warming and the strength of local radiative feedbacks themselves (shortwave clouds feedbacks in particular) depend on regional ocean circulations and the resulting time-varying geographic pattern of ocean heat uptake. These results imply that equilibrium climate sensitivity cannot be reliably estimated from transient climate observations. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G40.00005: Causes and implications of the growing divergence between climate model simulations and observations Invited Speaker: Judith Curry For the past 15+ years, there has been no increase in global average surface temperature, which has been referred to as a 'hiatus' in global warming. By contrast, estimates of expected warming in the first several decades of 21st century made by the IPCC AR4 were 0.2C/decade. This talk summarizes the recent CMIP5 climate model simulation results and comparisons with observational data. The most recent climate model simulations used in the AR5 indicate that the warming stagnation since 1998 is no longer consistent with model projections even at the 2\% confidence level. Potential causes for the model-observation discrepancies are discussed. A particular focus of the talk is the role of multi-decadal natural internal variability on the climate variability of the 20th and early 21st centuries. The ``stadium wave'' climate signal is described, which propagates across the Northern Hemisphere through a network of ocean, ice, and atmospheric circulation regimes that self-organize into a collective tempo. The stadium wave hypothesis provides a plausible explanation for the hiatus in warming and helps explain why climate models did not predict this hiatus. Further, the new hypothesis suggests how long the hiatus might last. Implications of the hiatus are discussed in context of climate model sensitivity to CO2 forcing and attribution of the warming that was observed in the last quarter of the 20th century. [Preview Abstract] |
Session G41: Focus Session: Flexoelectrics, Electrocalorics and Perovskites on Semiconductors
Sponsoring Units: DMP DCOMPChair: Alexander Tagantsev, EPFL
Room: Mile High Ballroom 3C
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G41.00001: Electrocaloric energy efficiency and cooling power Invited Speaker: Neil Mathur How much energy is required to drive electrocaloric effects near ferroelectric phase transitions? I will compare electrocaloric ceramic and polymer films with each other, with magnetocaloric materials (exploited in over 40 prototype refrigerators), and with elastocaloric materials. I will also discuss the cooling power that could be achieved in electrocaloric heat pumps based on multilayer capacitors in which heat flow is modelled using finite element analysis. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G41.00002: Thermodynamics of Multicaloric Effects in Multiferroics Antoni Planes, Teresa Castan, Avadh Saxena Ferroic and multiferoic materials thermally respond to externally driven changes of ferroic properties. Usually these changes are induced by application or removal of the field thermodynamically conjugated to a specific property. The isothermal change of entropy and the adiabatic change of temperature are commonly used in order to quantify the caloric response of a given material. From this perspective we provide a general thermodynamic framework to study multicaloric effects in multiferroic materials. This is applied to the case of a magnetoelectric multiferroic, which is described by means of a Landau free energy with a biquadratic coupling between polarization and magnetization. We obtain a phase diagram, the isothermal entropy change and the adiabatic temperature change across different continuous and first order transitions as the applied electric and magnetic fields are varied. The obtained multicaloric effects are suitably decomposed into the corresponding electrocaloric and magnetocaloric contributions. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G41.00003: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G41.00004: First principles investigation of the electro-caloric effect in BaTiO$_3$ Claude Ederer, Madhura Marathe The electro-caloric effect, a change of temperature or entropy under the application of an electric field, is very promising for future applications in solid state cooling devices [1]. It has been shown that temperature changes of several Kelvin can be achieved in thin films close to the ferroelectric transition temperature [2]. However, to utilize this effect within an actual device, a good control over the caloric properties at different operating temperatures as well as a good understanding of materials-specific trends is very important. Here, we use first principles-based effective Hamiltonians [3] to study the electro-caloric effect in the prototypical ferroelectric material BaTiO$_3$. In particular, we assess the effect of epitaxial strain, which is likely to occur in thin film devices, on the caloric properties, and we show that the electro-caloric effect is quite sensitive to such epitaxial strain. We also compare direct and indirect determination of the adiabatic temperature change. The latter uses a Maxwell relation that relates the electro-caloric and pyroelectric effects. [1] J. F. Scott, Annu. Rev. Mater. Res. 41, 229 (2011). [2] Mischenko et al., Science 311, 1270 (2006). [3] Nishimatsu et al., Phys Rev. B 78, 104104 (2008). [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G41.00005: Flexoelectricity via coordinate transformations Invited Speaker: Massimiliano Stengel Flexoelectricity describes the electric polarization that is linearly induced by a strain gradient, and is being intensely investigated as a tantalizing new route to converting mechanical stimulation into electrical signals and vice versa [1]. While several breakthough experiments have been reported in the past few years, progress on the theoretical front has been comparatively slow, especially in the context of first-principles electronic-structure theory. The main difficulty with calculating the flexoelectric response of a material is the inherent breakdown of translational periodicity that a strain gradient entails, which at first sight questions the very applicability of traditional plane-wave pseudopotential methods. In this talk I will show how these obstacles can be overcome by combining density-functional perturbation theory with generalized coordinate transformations [2,3], gaining access to the full microscopic response (in terms of electronic charge density, polarization and atomic displacements) of a crystal or nanostructure to an arbitrary deformation field. As a practical demonstration, I will present results on the full flexoelectric response of a SrTiO$_3$ film, including atomic relaxations and surface effects. \\[4pt] [1] P. Zubko, G. Catalan, and A. K. Tagantsev, Annu. Rev. Mater. Res. {\bf 43}, 387-421 (2013).\\[0pt] [2] M. Stengel, Phys. Rev. B, in press. (arXiv:1306.4240).\\[0pt] [3] M. Stengel, Nature Communications {\bf 4}, 2693 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G41.00006: Contribution of polar nanorregions to the giant flexoelectricity of relaxor ferroelectrics Gustau Catalan, Jackeline Narvaez We have studied the bending-induced polarization of single crystal relaxor ferroelectrics close to a morphotropic phase boundary. Anomalously large flexoelectric and flexocoupling coefficients were registered, with values well in excess (up to 10 times bigger) than theoretical expectations based on Kogan's theory [1] below critical temperature. The temperature dependence of the effective flexoelectric coefficients shows that this anomalous enhancement persists in the temperatures up to T* $\sim$ 250$^{\circ}$C, above which the values fall back in line with theoretical expectation for pure flexoelectricity. Cross-correlation between flexoelectric and elastic measurements indicates that the anomalous enhancement of bending-induced polarization is caused by the flexoelectric reorientation of non-percollating polar nanotwins that exist in the temperature range between Tc and T*. \\[4pt] [1] S. M. Kogan, Soviet Physics Solid State, 5, (1964) 2069. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G41.00007: Structural and Magnetotransport Study of SrTiO$_{3-\delta}$/Si Films Grown by Molecular Beam Epitaxy Alex Currie, Ryan Cottier, Oscar Villarreal, Jesus Cantu, Arturo Ponce, Nikoleta Theodoropoulou SrTiO$_{3}$ (STO) films were grown on p-Si (001) substrates using molecular beam epitaxy (MBE). Oxygen vacancies were introduced by controlling the Oxygen resulting in SrTiO$_{3-\delta}$ with $\delta$ $\sim$ 0.02{\%} for the lowest pressure. The single phase STO/Si films were of high crystalline quality as verified by x-ray diffraction, transmission electron microscopy, and had an rms roughness of less than 0.5nm measured by atomic force microscopy. Transport measurements were performed on the STO/Si structures in a Van der Pauw configuration. We measured resistance as a function of temperature, T $=$ 3K-300K and as a function of an applied magnetic field , H$=$0 to $\pm$ 9T. The resistivity decreased from 1 Ohm cm to 3x10$^{-2}$ Ohm cm as the film thickness increased (3nm-60nm) for all temperatures. The magnetoresistance (MR) shows a reproducible trend for all films, the MR is positive at 300K, becomes negative between 200K and 100K and at low temperatures T$=$3-20K the MR is positive at low H$=$0 to $\pm$ 2T but at high fields, it starts decreasing again. The MR behavior combined with the Hall effect data indicates the presence of localized electrons that delocalize with H and T. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G41.00008: BaTiO$_3$/GaAs heterostructures: a possible route to reconfgurable III-V nanoelectronics Dongyue Yang, Giriraj Jnawali, Lu Cheng, Feng Bi, Patrick Irvin, Jeremy Levy, Borzoyeh Shojaei, Chris Palmstrom, Rocio Contreras-Guerrero, Ravi Droopad Ferroelectric field effect device concepts have existed since the early days of the transistor.\footnote{J. F. Scott , \textit {Ferroelectric Memories}, Springer-Verlag , Berlin \textbf {2000}} The challenges have been more materials-based rather than conceptual. Recent advances in oxide-molecular-beam epitaxy have allowed high quality interfaces between complex oxides and compound semiconductors. Here we focus on heterostructures between GaAs and BaTiO$_3$, which are well lattice-matched and have atomically sharp interfaces. These structures can be configured both in the III-V layer, by growing GaAs/AlGaAs heterostructures and quantum wells; and they can be ferroelectrically patterned in the BaTiO$_3$ layer using a scanning probe microscope. We discuss current efforts to develop this material for both optical and transport-based experiments. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G41.00009: Ab initio study of the epitaxial BaTiO3/Ge interface Mehmet Dogan, Divine Kumah, Charles Ahn, Frederick Walker, Sohrab Ismail-Beigi Growing thin films of crystalline metal oxides on silicon or germanium has been of great research interest for decades because of the possible applications of such systems in electronic devices. An example is provided by the ferroelectric oxide BaTiO$_{3}$: if it remains ferroelectric on a semiconductor, one may be able to realize non-volatile electronic devices based on the interfacial field effect where the state of the system is encoded in the polarization direction of the oxide. Thanks to recent advances in epitaxial growth methods, one can explore such interfaces in parallel with the experiment. Here, we use density functional theory to study the interface between BaTiO$_{3}$ and Ge. We describe how the structure of the interface depends on the oxygen content of the interface and compare to X-ray diffraction experiments. We show how the polarization of the BaTiO$_{3}$ thin film changes compared to the bulk. We analyze the electronic structure of the interface and illustrate how valence and conductance bands are aligned. We explore the energetics of oxygen vacancies in BaTiO$_{3}$ both in terms of positional and concentration dependence. We also discuss dynamics of oxygen vacancies by computing the energy barriers for oxygen vacancy diffusion. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G41.00010: Atomic and electronic structure of the BaTiO3-Ge (001) interface Kurt Fredrickson, Patrick Ponath, Agham Posadas, Martha McCartney, Toshihiro Aoki, David Smith, Alexander Demkov There is tremendous interest in putting perovskite oxides, such as SrTiO$_{\mathrm{3}}$ (STO) or BaTiO$_{\mathrm{3}}$ (BTO), on semiconductors due to their very high permitivitties. BTO can be grown directly on Ge using an approach similar to the growth of STO in Si. To date, very little is known about the atomic and electronic structure of the BTO-Ge interface. We use molecular beam epitaxy to grow BTO with in-plane polarization directly on Ge(001) using a Sr Zintl buffer layer. This results in an atomically flat, oxygen- and carbon-free Ge surface with very sharp (2x1) reconstruction as observed with reflection high energy electron diffraction. Using scanning transmission electron microscopy, we are able to precisely determine the atomic geometry of the interface, with the exception of the exact positions of the oxygen atoms. \textit{In situ }x-ray photoemission spectroscopy is used to analyze the oxidation state of the interfacial Ge and to determine the valence band offset at the interface. We use density functional theory to determine placement of interfacial O and calculate the valence band offset. The theoretical valence band offset is in good agreement with the photoemission data, strongly suggesting the correctness of the interface geometry. We calculate the effect of O vacancies and ionic substitution at the interfacial layer on the valence band offsets. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G41.00011: Epitaxial BaTiO$_{3}$ on Ge (001) Patrick Ponath, Agham Posadas, Keji Lai, Dave Smith, Alex Demkov Germanium, with its higher hole and electron mobility, might become an attractive candidate to replace silicon as a channel material in a field effect transistor. The ferroelectric high-k dielectric barium titanate (BTO) can be integrated on germanium (001) due to the small lattice mismatch between BTO and Ge and could therefore be a potential candidate for a ferroelectric memory if the problem of relatively high leakage could be solved. We report the epitaxial growth of BTO on a germanium (001) substrate with a thin buffer layer, which causes the BTO to be out of plane polarized. The BTO film crystallizes as-deposited which is monitored by reflection high energy electron diffraction. X-ray diffraction measurements of the BTO film indicate an out of plane ferroelectric polarization. [Preview Abstract] |
Session G42: Focus Session: Novel Effects in Topological Insulator Films
Sponsoring Units: DMPChair: Hsin Lin, National University of Singapore
Room: Mile High Ballroom 4A
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G42.00001: Surfaces, interfaces, and ultrathin films of topological insulators Invited Speaker: Tai Chiang Three-dimensional topological insulators are characterized by an inverted bulk band gap caused by a strong spin-orbit coupling. This gap must close at the surface and reopen outside in vacuum where the gap is noninverted (and infinite). The resulting metallic surface states, or topological states, are spin-polarized and span the bulk gap. They carry a spin current, independent of the details of the surface, which is a feature of strong interest for spintronic applications. This talk will focus on thin films of topological materials (Sb, Bi$_{2}$Se$_{3}$, and Bi$_{2}$Te$_{3})$. Thin films are basic building blocks of devices, which typically involve multilayers of various materials. As the thickness of a topological insulator film is reduced to the nanoscale, the bulk bands are reduced to discrete quantum well states, and the surface/interface states associated with the two faces of the film can interact, resulting in spin mixture and formation of a tunneling gap. The detailed atomic bonding at each face of the film can also affect the overall electronic structure of the system. The interplay of quantum confinement, topological order, spin polarization, and surface/interface bonding and chemistry will be discussed. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Grant No. DE-FG02-07ER46383. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G42.00002: Bulk-Insulating Bi$_{2}$Se$_{3}$ Thin Films and Decoupled Topological Surface States Matthew Brahlek, Nikesh Koirala, Maryam Salehi, Namrata Bansal, Seongshik Oh By applying the simple criteria given by Mott and Ioffe-Regel it is easily seen that even the best TIs are not true insulators in the Mott sense, but at best are weakly-insulating bad metals. However, band-bending effects contribute significantly to the TI transport properties, and we show that utilization of this band-bending effect can lead to a Mott insulating bulk state in the thin regime. This is realized in transport experiments on compensation doped Bi$_{2}$Se$_{3}$ thin-films, where the bulk-insulating picture is supported by enhanced surface mobilities, Hall effect, Shubnikov de-Haas oscillations as well a clear signature of a thickness dependant decoupling of surface states by analyzing the weak anti-localization effect. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G42.00003: Electrodynamics of the topological insulator (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}}$)$_{2}$Se$_{3}$ tuned to the brink of quantum criticality Liang Wu, Rolando Valdes Aguilar, Andreas Stier, Christopher Morris, Yuval Lubashevsky, Peter Armitage, Matthew Brahlek, Nikesh Koirala, Namrata Bansal, Seongshik Oh We have utilized time-domain terahertz (THz) spectroscopy to investigate the low frequency optical conductivity in (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}}$)$_{2}$Se$_{3}$ through its topological phase transition from the pure compound (x$=$0) to the topologically trivial strongly insulating material (x$=$0.27). Above a thickness dependent doping threshold we observe a sudden collapse in the transport lifetime that indicates the destruction of the topological phase. We associate this with the doping where the states from opposite surfaces hybridize. As a function of thickness this threshold asymptotically approaches the doping x $\sim$ 0.06 of a maximum in the mid-infrared absorption, which can be identified with the bulk band gap closing and change in topological class. The realization of a topological quantum critical point allows the possible realization of other novel phenomena including the Weyl semi-metal. I will discuss our results on the THz response of these systems in a new generation of materials with greatly suppressed bulk carrier density levels. Reference: Wu, \textit{et al}, \textbf{Nature Physics} 9, 410-414 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G42.00004: Probing Bi2Te3 thin nanoplates by Raman Spectroscopy Rui He, Conor Delaney, Ben Beck, Tim Kidd, Cliff Chancey, ZhenHua Wang, Richard Qiu, Xuan Gao Two infrared (IR)-active vibrational modes centered at 93 and 113 cm$^{-1}$ are observed in Raman spectra from as-grown thin nanoplates (NPs) of topological insulator Bi$_2$Te$_3$. The presence of IR modes in Raman scattering reveals a breakdown of inversion symmetry in thin NPs grown on SiO$_2$. Both Raman and IR modes are preserved after typical device fabrication processes, suggesting the robustness of surface properties. In NPs transferred to another SiO$_2$ substrate, the IR modes are absent, and the Raman spectra are similar to those from bulk samples. These differences could be attributed to interactions between the SiO$_2$ substrate and the as-grown NPs. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G42.00005: Effect of film thickness on ultrafast carrier relaxation rates in thin-films of the topological insulator Bi$_{2}$Se$_{3}$ Yuri D. Glinka, Sercan Babakiray, Trent A. Johnson, Alan D. Bristow, Mikel B. Holcomb, David Lederman Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in thickness, of the topological insulator Bi$_{2}$Se$_{3}$ reveal a strong dependence of the ultrafast carrier relaxation rate on the film thickness. We exploit this behavior to distinguish between the contributions from the bulk 3D states and the 2D gapless surface states. Based on experimental observations we conclude that there is a crossover between two carrier relaxation mechanisms associated with the polar phonon (Frohlich) interaction in the bulk insulating phase and the electron-lattice interaction in the surface metallic phase. It is suggested that this crossover could be a result of hybridization of Dirac cone states at the opposite surfaces of the thinnest films. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G42.00006: Bi$_{2}$-Bi$_{2}$Se$_{3}$ Superlattice Materials Studied by Photoemission Spectroscopy Andrew Weber, Ivo Pletikosic, Quinn Gibson, Huiwen Ji, Leslie Schoop, Jurek Sadowski, Anthony Caruso, Elio Vescovo, Alexei Fedorov, Robert Cava, Tonica Valla Although searches are carried out independently for new 2D quantum spin Hall or 3D topological materials for their robust, spin-polarized edge or surface conduction states, little has been done to experimentally show that both phases can coexist in a single material or explore how they may interact. The superlattice series Bi$_{\mathrm{2m}}$(Bi$_{2}$Se$_{3})_{\mathrm{n}}$, featuring stacked layers of Bi$_{2}$ and Bi$_{2}$Se$_{3}$, may contain systems where a combination of 2D and 3D topological phenomena should be at play, the latter of which can be identified by combined computational and spin-and angle-resolved photoemission spectroscopy studies. We find that several members of the series, (m$=$0, n$=$1), (m$=$1, n$=$1) and (m$=$2, n$=$1) have spin-chiral surface states at the center of the surface Brillouin zone, a trait of strong topological insulators. The characterization of the topological surface states will be discussed for these series members. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G42.00007: Nontrivial topological electronic structures in a single Bi(111) bilayer on different substrates Feng-Chuan Chuang, Zhi-Quan Huang, Chia-Hsiu Hsu, Yu-Tzu Liu, Hua-Rong Chang, Hsin Lin, Arun Bansil Electronic structures, minimum energy configurations, and band topology of strained Bi(111) single bilayers placed on a variety of semiconducting and insulating substrates are investigated using first-principles calculations [1]. A topological phase diagram of a free-standing Bi bilayer is presented to help guide the selection of suitable substrates. Numerous substrates were studied to determine whether they are able to support 2D TIs. The insulating hexagonal-BN is identified as the best candidate substrate material for supporting nontrivial topological insulating phase of Bi bilayer thin films. A planar hexagonal Bi layer is predicted under tensile strain, which we show could be realized on a SiC substrate. The Bi bilayer becomes metallic under the compressive strain induced by Si and Ge substrates. [1] Zhi-Quan Huang, Feng-Chuan Chuang, Chia-Hsiu Hsu, Yu-Tzu Liu, Hua-Rong Chang, Hsin Lin, and Arun Bansil, Phys. Rev. B 88, 165301 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G42.00008: Quantum Interference Control of Currents in Bi$_{2}$Se$_{3}$ Topological Insulators Derek Bas, Kevin Vargas, Sercan Babakiray, Trent Johnson, Yuri Glinka, Mikel Holcomb, David Lederman, Alan Bristow Quantum interference control of bulk and surface currents are investigated in Bi$_{2}$Se$_{3}$ films ranging from 6 to 40 quintuple layers in thickness. The samples are grown with a two-step method on sapphire substrates and protected with an MgF$_{2}$ capping layer that prevents oxidation. Co-polarized harmonically related pulses excite a population of carriers through interference of single- and two-photon absorption pathways. Dependences of the relative phase between the two pulses and intensity of each pulse show the correct signatures confirming the third-order nonlinear optical process. We observe an increase in the strength of the injected currents with decreasing thickness and a peak at 10 quintuple layers. It is believed that the peak coincides with the onset of hybridization of the Dirac cone on opposite surfaces of the sample. The increase in signal strength is related to an increase in the expected spin-polarized surface currents, which begin to dominate over the bulk pure charge currents. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G42.00009: Investigations of Topological Surface States in Sb (111) Ultrathin Films by STM/STS Experiments and DFT Calculations Ziyu Luo, Guanggeng Yao, Wentao Xu, Yuanping Feng, Xue-sen Wang Bulk Sb was regarded as a semimetal with a nontrivial topological order. It is worth exploring whether the Sb ultrathin film has the potential to be an elementary topological insulator [1]. In the presence of quantum confinement effect, we investigated the evolution of topological surface states in Sb (111) ultrathin films with different thickness by the scanning tunneling microscopy/ spectroscopy (STM/STS) experiments and density functional theory (DFT) calculations [2]. By comparing the quasiparticle interference (QPI) patterns obtained from Fourier-transform scanning tunneling spectroscopy (FT-STS) and from DFT calculations, we successfully derive the spin properties of topological surface states on Sb (111) ultrathin films. In addition, based on the DFT calculations, the 8BL Sb (111) ultrathin film was proved to possess up to 30{\%} spinseparated topological surface states within the bandgap. Therefore, the highquality 8BL Sb (111) ultrathin film could be regarded as an elementary topological insulator. [1] F.C. Chuang et al., App. Phy. Lett.102, 022424 (2013) [2] G Yao et al., Sci. Rep. 3, 2010 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G42.00010: Tuning the Surface States of Ultra-thin Topological Insulator Films Jianxin Zhong In this talk, I will introduce our recent progress on tuning the surface states of ultra-thin topological insulator films. Using first-principles methods, we explain the puzzling band-topology difference between Sb$_{2}$Se$_{3}$ and Bi$_{2}$Se$_{3}$ and propose an approach to tuning the topological phase by strain [1]. We demonstrate that Sb$_{2}$Se$_{3}$ can be converted into a topological insulator by applying compressive strain while the tensile strain can turn Bi$_{2}$Se$_{3}$ into a normal insulator. I will also show that the separation distance between quintuple layers (QL) in ultra-thin Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ films have a large increase after relaxation, leading to gap-opening at the surface Dirac cone, in good agreement with the experimental observation [2]. I will further show that Pb adlayers on Bi$_{2}$Se$_{3}$ result in splitting of the Dirac cones and large Rashba spin splitting of the quantum well states [3]. Most importantly, the quantum size effect of Pb adlayers leads to an oscillatory behavior of the Rashba splitting.\\[4pt] [1] W. L. Liu et al., Phys. Rev. B 84, 245105 (2011);\\[0pt] [2] W. L. Liu et al., Phys. Rev. B 87, 205315 (2013);\\[0pt] [3] H. Yang et al., Phys. Rev. B 86, 155317 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G42.00011: Electrical and structural properties of elemental Sb quantum wells grown by molecular beam epitaxy Kaushini Wickramasinghe, Chomani Gaspe, Shayne Cairns, Lin Lei, Nolan Teasdale, Tetsuya Mishima, Joel Keay, Sheena Murphy, Michael Santos Elemental Sb has gained attention recently because calculations indicate that the inherently large spin-orbit coupling enables topological insulator behavior. Because the band structure of bulk elemental Sb is semi-metallic, transport measurements will be dominated by bulk conduction. Our goal is to suppress the bulk conductivity by quantum confinement in thin Sb layers, to enable transport measurements of topological surface states. A growth procedure was developed to realize ultra-thin layers of Sb with thickness of $\sim$ 1nm to 10nm. Field-emission scanning electron microscopy and transmission electron microscopy measurements of ultra-thin Sb QWs show good crystalline quality with a suppression of the bulk conductivity at 20K by as much as 400x. We will discuss the epitaxial growth procedure for Sb quantum wells with GaSb barriers grown on GaAs(111)A and GaSb(111)A substrates. We will also discuss characterization of the structural and electrical properties of the ultra-thin films. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G42.00012: Current-Voltage Characteristics Along Terraces in MBE-Grown Bi$_{2}$Te$_{3}$ Rita Macedo, Sara Harrison, Tatiana Dorofeeva, James Harris, Richard Kiehl Capturing the novel, but elusive, physics of topological insulators for electronic devices will require a baseline characterization of their surface electronic properties in an ambient environment. We report on the current-voltage (I-V) characteristics observed along terraces in MBE-grown Bi$_{2}$Te$_{3}$ by room-temperature conductive atomic force microscopy (C-AFM). The films were grown on sapphire by a two-step MBE process, leading to large-area films with micrometer-sized domains formed by wide concentric atomically flat terraces with a typical width of 170 nm and a step height of 1 nm. Control samples comprised of spherical nanoparticles and HOPG terraces were also examined for comparison. The Bi$_{2}$Te$_{3}$ C-AFM measurements consistently showed well-behaved, nearly symmetric exponential I-V characteristics with similar ideality factors on terraces and in the transition regions between terraces. Notably, current in the 25-nm transition regions was 10X higher than on the terraces. Negligible current increase was observed for the controls, indicating that this behavior is not an artifact of the tip-sample contact, but rather is due to a difference in the conductance along the Bi$_{2}$Te$_{3}$ terrace edges. Possible mechanisms including oxide, doping, defect, strain and topological effects will be discussed. These results motivate further edge conduction studies in these materials as essential background for studying topological insulator physics and devices. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G42.00013: Growth of topological insulator Bi$_{2}$Se$_{3}$ thin films on amorphous for multi-channel structure Sahng-Kyoon Jerng, Yong Seung Kim, Jae Hong Lee, Youngwook Kim, Jun Sung Kim, Kisu Joo, Euijoon Yoon, Sang-Moon Yoon, Miyoung Kim, Seung-Hyun Chun A topological insulator exhibits the topologically protected gapless Dirac surface states in bulk band gap which was predicted in Bi$_{2}$Se$_{3}$. Thin layered films of Bi$_{2}$Se$_{3}$ have been heteroepitaxially grown on the crystalline substrate by molecular beam epitaxy (MBE). Here, we show the growth of Bi$_{2}$Se$_{3}$ thin films on amorphous SiO$_{2}$ substrate by MBE. In order to achieve the growth on amorphous surface, van der Waals epitaxy method with the selenium passivation was adopted. Bi$_{2}$Se$_{3}$ films are grown along [001] direction with periodical structure in spite of lattice mismatched amorphous substrate. Low-temperature transport measurement revealed the weak anti-localization effect with electrical gating, which suggest that surface transport properties can be comparable to those of epitaxially grown Bi$_{2}$Se$_{3}$ films on crystalline substrate. In addition, we demonstrate the growth of multi-channel Bi$_{2}$Se$_{3}$ films separated by amorphous insulating layer. These results provide a potential of growth of layered topological insulator films on amorphous materials and junctions. [Preview Abstract] |
Session G43: Weyl Semimetals: Theory
Sponsoring Units: DCMPChair: James Analytis, University of California, Berkeley
Room: Mile High Ballroom 4B
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G43.00001: Coulomb scattering of Weyl fermions through a potential barrier Mahtab Khan, Michael Leuenberger We investigate the effects of the Coulomb interaction on the two-dimensional relativistic quantum-mechanical scattering of two Weyl fermions, injected on the opposite sides of a potential barrier. We consider the Coulomb interaction in the standard two-body problem and evaluate the corresponding scattering amplitude. We apply our formalism to describe the scattering of Weyl fermions in two-dimensional materials exhibiting Dirac cones, such as graphene and the surface of 3d topological insulators. We obtain a complex shape for the scattering amplitude due to the angle-dependent Klein tunneling through a potential barrier. We show that the Coulomb interaction leads to shifts and broadenings of the transmission peaks. . [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G43.00002: Crystalline Topological Insulators and Semimetals with $C_{nv}$ Symmetry A. Alexandradinata, Chen Fang, Matthew J. Gilbert, B. Andrei Bernevig We explore a class of 3D materials with $C_{nv}$ symmetry. For $n =3,4$ and $6$, we find the first-known 3D topological insulators with robust surface modes, but $without$ spin-orbit coupling, and $not$ $needing$ time-reversal symmetry; the relevant symmetries are purely crystalline. To describe these $C_{nv}$ systems, we introduce the notion of a mirror chirality: an integer invariant which characterizes half-mirror-planes in the 3D Brillouin zone. In the evolution between two gapped phases with distinct mirror chiralities, we find that the intermediate gapless phase is a Weyl semimetal. Applications are discussed in the context of photonic crystals. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G43.00003: Quantum transport in Weyl semimetals Yuya Ominato, Mikito Koshino Quantum transport in 3D Weyl (massless Dirac) electron system with long-range Gaussian impurities is studied theoretically using a self-consistent Born approximation (SCBA). We find that the conductivity significantly changes the behavior at a certain scattering strength which separates the weak and strong disorder regimes. In the weak disorder regime, the SCBA conductivity mostly agrees with the Boltzmann conductivity, while the agreement fails near the Weyl point where the SCBA conductivity drops to zero linearly to the Fermi energy. In the strong disorder regime, the conductivity is smooth and finite near the Weyl point, and the minimum conductivity becomes larger in increasing the disorder potential, contrary to the usual metallic behavior. We also study the charged impurities, and argue the qualitative difference from the Gaussian case. The theory applies to three dimensional gapless band structures, including Weyl semimetals. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G43.00004: Local Electronic Structure around a Single Impurity in an Anderson Lattice Model for Topological Kondo Insulators Cheng-Ching Joseph Wang, Jean-Pierre Julien, Jian-Xin Zhu Shortly after the discovery of topological band insulators, the topological Kondo insulators (TKIs) have also been theoretically predicted. The latter has ignited renewed interest in the properties of Kondo insulators. By starting with a minimal-orbital Anderson lattice model, we explore the local electronic structure in a Kondo insulator. We show for the first time that the two strong topological regimes sandwiching the weak topological regime give rise to a dual location of Dirac cone on the surface of TKI. We further find that, when a single impurity is placed on the surface, low-energy resonance states are induced in the weak scattering limit for the strong TKI and the resonance level moves monotonically across the hybridization gap with the strength of impurity scattering potential; while low energy states can only be induced in the unitary scattering limit for the weak TKI, where the resonance level moves universally toward the center of the hybridization gap. These impurity induced low-energy quasiparticles will lead to characteristic signatures in scanning tunneling microscopy/spectroscopy, which has recently found success in probing exotic properties in heavy fermion systems. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G43.00005: Chiral magnetic effect of Weyl fermions and its applications to cubic noncentrosymmetric metals Sumanta Tewari, Pallab Goswami When the right and the left handed Weyl points are separated in energy, they give rise to a non-dissipative charge current along the direction of a uniform applied magnetic field, even in the absence of an external electric field. This effect is known as the chiral magnetic effect and is a hallmark of the underlying chiral anomaly of the Weyl fermions. According to the linearized continuum theory of Weyl fermions, the induced current is proportional to the magnetic field strength and the energy separation with a universal coefficient $e^2/h^2$. By considering a generic tight binding model for the cubic non-centrosymmetric metals, we show that such a system naturally supports a set of Weyl points, which are separated in energies. We also show the existence of the chiral magnetic effect for generic band parameters, and recover the universal result of the continuum Weyl fermions for a restricted parameter regime. Our work proves that the cubic non-centrosymmetric metals can serve as suitable platforms for realizing Weyl fermions and the exotic chiral elctrodynamic phenomena, which have promising technological applications. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G43.00006: Line node semimetals Vivek Aji, Michael Phillips Weyl semimetals are remarkable states of matter that are have chiral surface states, despite being gapless in the bulk. They are characterized by the touching of two non degenerate bands at an even number of points in the Brillouin zone. A variant of these is a semimetal with line nodes, rather than points, which is realized in a heterostructure made up of alternating layers of of topological and magnetic insulators. In this talk we explore the properties of this system such as the low energy density of states, conductivity and expected oscillatory signatures in magnetic fields. In particular we focus on the parametric dependence on magnetization which offers a knob to tune the properties of the system. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G43.00007: Exotic Chiral Anomaly of Double-Weyl Fermions Cheung Chan, Hong Yao Double-Weyl points in a 3D topological semimetal are protected by crystallographic point-group symmetries, as predicted in Hg$_{2}$Cr$_{2}$Se$_{4}$. The dispersion of fermions around double-Weyl points is highly anisotropic in momentum space, namely quadratic along $x$- and $y$-directions but linear in $z$-direction in the low energy effective Hamiltonian, which has intriguing physical consequences. For instances, we show that the double-Weyl fermions give rise to anisotropic chiral anomaly, qualitatively different from the usual chiral anomaly of (linear) Weyl fermions. We also discuss how the anisotropic dispersions of double-Weyl fermions affect their transport behaviors. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G43.00008: Collective modes in Weyl semimetals Dmytro Pesin, Ivan Panfilov, Anton Burkov Weyl semimetals are three-dimensional crystalline systems where pairs of bands touch at points in momentum space, termed Weyl nodes, that are characterized by a de?nite topological charge: the chirality. Consequently, they exhibit the Adler-Bell-Jackiw (chiral) anomaly. We consider new plasmon modes that appear in Weyl semimetals in a magnetic field due to the existence of the chiral anomaly in such systems. We show that due to the $\sqrt{B}$ dependence of their frequency on the magnetic field magnitude at low levels of doping, these modes couple effectively to the acoustic vibrations, leading to the existence of new hybrid plasmon-phonon modes. We discuss the implications of the existence of such modes for the sound absorption in Weyl semimetals. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G43.00009: Beyond the semi-Dirac, semi-Weyl dispersion: extending the tight binding model Yundi Quan, Warren Pickett Weyl semimetals with ``Dirac points'' have gained widespread notice due to the many unusual features they display, including topological characteristics. The discovery of the 2D semi-Dirac semimetal (more precisely, semi-Dirac, semi-Weyl) in thin VO$_2$ slabs with dispersion that is massless in one direction but massive perpendicular, provides an additional peculiarity introduces its own distinct behavior. We have generalized the two-band tight binding model for this system, obtaining new types of extreme bands and density of states. A specific feature is that the bottom of the upper band terminates at a contour (rather than at a point) such that electron doping leads to a pair of large, slightly separated Fermi lines and the DOS becomes 1D-like. Other unusual features will be described. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G43.00010: Magnetic Oscillations in Weyl Semimetals Phillip Ashby, Jules Carbotte Weyl semimetals are a three-dimensional phase containing band touchings at isolated points in the Brillouin zone. A Weyl semimetal can be thought of as a higher dimensional generalization of graphene. We study the thermodynamic and transport properties of a Weyl semimetal subject to an applied magnetic field. We examine the quantum oscillations in the magnetization to look for signatures that distinguish the Weyl semimetal from conventional phases of matter. We find distinctive sawtooth-like oscillations in the magnetization that reflect the relativistic nature of the bulk bands. The effect of impurities on these signatures will also be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G43.00011: Weyl semimetal phase in solid-solution zero-gap semiconductors Daichi Kurebayashi, Kentaro Nomura Weyl semimetals are recently found novel magnetic materials with the pseudo-relativistic linear dispersions. Near the band-touching points, the excitations are described by the Dirac-Weyl Hamiltonian. Quasiparticles, Weyl fermions, are assigned by a chirality, and the bulk gap opens only if two Weyl fermions with opposite chirality meet each other. This topological behavior originates in the nonzero Berry curvature enclosing a Weyl point. In TlBi(Se$_{1-x}$S$_x$)$_2$, it was recently found that the bulk gap closes as substituting sulfur by selenium. This vanishing of bulk gap is considered as the topological phase transition, and it is expected that the Weyl semimetal phase can be realized in this regime by breaking time-reversal symmetry. We determine the condition for the Weyl semimetal caused by a magnetic transition in zero-gap semiconductors doped with magnetic impurities. As a model, we use the Wilson Hamiltonian and the s-d exchange Hamiltonian within the mean-field approximation. We calculate the magnetization by solving the Hamiltonian self-consistently and obtain the topological phase diagram. Consequently, we find the Weyl semimetal phase with the finite anomalous Hall conductivity can be realized below the Curie temperature depending on the impurity concentration. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G43.00012: Electric field induced spin dynamics and polaritons in Weyl semimetal Jimmy Hutasoit, Chao-Xing Liu In Weyl semimetal, magnetization acts like a ``chiral magnetic'' field that, unlike the conventional magnetic field, couples differently to the left-handed and right-handed Weyl fermions. Integrating out the Weyl fermions, we find a non-local effective theory that describes the interaction between the magnetization and the electromagnetic field. In particular, we find that the system exhibits non-trivial spin dynamics controllable by the external electric field. Furthermore, the coupling between the magnetization and electromagnetic waves give rise to polaritons. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G43.00013: Topological States in Ferromagnetic CdO/EuO Quantum Well Haijun Zhang, Jing Wang, Gang Xu, Yong Xu, Shou-Cheng Zhang The quantum anomalous Hall (QAH) effect exhibits a quantized hall conductance without the external magnetic field and the associated landau levels. The topologically protected chiral edge states in a QAH state conducts electric current without dissipation and could be used for interconnects of semiconductor devices. In this talk, based on \textit{ab-initio} calculations, we demonstrate that the ferromagnetic CdO/EuO superlattice is a simple Weyl semimetal with two linear Weyl nodes in the Brillouin zone. The corresponding CdO/EuO quantum well realizes the stichometric quantum anomalous Hall (QAH) state without random magnetic doping, and its working temperature is expected to be close to bulk EuO's Curie temperature (around 70K). In addition, a simple effective model is presented to describe the basic mechanism of spin polarized band inversion in this system. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G43.00014: Quantum Oscillations from Fermi-Arcs in Weyl and Dirac Semimetals Andrew Potter, Itamar Kimchi, Ashvin Vishwanath Weyl semi-metals exhibit unusual surface-states whose Fermi-``surface" is not actually a surface, but rather consists of disjoint line segments. Such Fermi-arcs are a fingerprint of the topological aspects of the bulk band-structure. Magnetic field induced quantum oscillations of the density of states have traditionally enabled one to experimentally map out a material's Fermi-surface. On their own, the disjoint nature of surface Fermi-arcs does not permit closed semi-classical orbits in a magnetic field, naively rendering them inaccessible to quantum oscillatory probes. However, a slab of Weyl semi-metal has counter-propagating Fermi-arcs on both the top and bottom surfaces, which together could support closed orbits. Can such orbits which span a a non-local Fermi-surface give rise to quantum oscillations? If so, what happens to these oscillations as the top and bottom surfaces are increasingly isolated in progressively thicker slabs? This talk will address these questions, and apply the results to closely related and recently discovered 3D Dirac semi-metals. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G43.00015: Topological protection, disorder, and interactions: Life and death at the surface of a topological superconductor Matthew Foster, Hongyi Xie, Yang-Zhi Chou The key attribute of a 3D topological phase of matter is the prediction of robust, gapless surface states. These are said to be protected from the effects of disorder, in the sense that these states escape Anderson localization. Sufficiently weak interactions also have negligible effect, at least for surface states doped to the Dirac point. Here we consider the \emph{combined} effects of disorder and interactions on the surface states of 3D topological superconductors. Generalizing previous work [Foster and Yuzbashyan, PRL 109, 246801 (2012)], we study the enhancement of interactions due to disorder-mediated wavefunction multifractality, and the suppression of the Altshuler-Aronov correction to the surface quasiparticle spin conductance, due to the topology. We construct global surface state phase diagrams employing numerics, perturbative Finkel'stein non-linear sigma model calculations, and exact conformal field theory results. We establish the restrictive conditions under which surface states can be robust to both disorder and interactions. [Preview Abstract] |
Session G44: Focus Session: Defects in Semiconductors: Characterization
Sponsoring Units: DMP FIAPChair: Yong Zhang, University of North Carolina at Charlotte
Room: Mile High Ballroom 4C
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G44.00001: Angstrom resolved imaging of charge percolation through the interface between phosphorous doped crystalline silicon and silicon dioxide Kapildep Ambal, Philipp Rahe, Clayton C. Williams, Christoph Boehme Using a high resolution ($\approx$100fm/$\sqrt{\mathrm{Hz}}$ spectral noise density) scanning probe at T$\approx$4K, we measure currents through the interface between phosphorus doped ([P] $\approx$ 10$^{17}$-10$^{18}$ cm$^{-3}$) crystalline silicon and a native silicondioxide layer as a function of either the lateral cantilever position or the applied cantilever bias voltage (c-AFM imaging). These measurements visualize the percolation of charge through the interface and they show that local current maxima exist in patch-like structures of $\approx$30nm diameter, randomly distributed with an average distance between the centers of 30-40 nm. We associate these with P donor electron states. Within the patch-like structures, we observe additional, extremely localized ($\approx$5{\AA}), current maxima. We associate those to silicon dangling bonds at the interface or within the silicondioxide. The hypothesized association of these very reproducible features is tested by current-voltage (I-V) measurements. For any randomly chosen surface position, these measurements reveal one of only four qualitatively distinct I-V responses, each of which is identified with charge percolation from P donors to the cantilever either with or without different kinds of silicon dangling bond involvement. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G44.00002: Imaging of the native inversion layer in Silicon-On-Insulator wafers via Scanning Surface Photovoltage: Implications for RF device performance Daminda Dahanayaka, Andrew Wong, Philip Kaszuba, Leon moszkowicz, James Slinkman Silicon-On-Insulator (SOI) technology has proved beneficial for RF cell phone technologies, which have equivalent performance to GaAs technologies. However, there is evident parasitic inversion layer under the Buried Oxide (BOX) at the interface with the high resistivity Si substrate. The latter is inferred from capacitance-voltage measurements on MOSCAPs. The inversion layer has adverse effects on RF device performance. We present data which, for the first time, show the extent of the inversion layer in the underlying substrate. This knowledge has driven processing techniques to suppress the inversion. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G44.00003: Investigating individual arsenic dopant atoms in silicon using low-temperature scanning tunnelling microscopy Neil Curson, Kitiphat Sinthiptharakoon, Steven Schofield, Philipp Studer, Veronika Brazdova, Cyrus Hirjibehedin, David Bowler We study sub-surface arsenic dopants in a hydrogen terminated Si(001) sample at 77 K, using STM and STS. We observe a number of different dopant related features that fall into two classes, which we call As1 and As2. The As1 features are consistent with buried dopants that are in the electrically neutral (D0) charge state when imaged in filled states, but become positively charged (D$+)$ through electrostatic ionisation when imaged under empty state conditions. DFT calculations predict that As dopants in the third layer of the sample induce two states lying just below the conduction band edge, which hybridize with the surface structure creating features with the surface symmetry consistent with our STM images. The appearance of the As2 features surprisingly suggests they are negatively charged at all biases. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G44.00004: Optical and electrical manipulation of a single bi-stable Si-atom in GaAs Paul Koenraad, Erwin Smakman We will show that a Si atom in the outermost layer of GaAs has a bi-stable character much alike the well-known DX-center in Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As. In the ground state the Si atom is negatively charged and in the excited metastable state it is positively charged. These two charge states are related to a modification of the bond configuration of the Si atom in the GaAs surface layer. The voltage dependence of this bi-stable character can be used to bring the Si atom in either of the two states while probing it with an STM tip. The electrical excitation and relaxation processes were studied by analyzing the current and voltage dependence of the observed Random Telegraph Noise. We have successfully used this to create a memory element based on a single impurity atom. Our low T STM setup allows to illuminate the tunneling area and/or to collect tunneling induced photons from the area below the STM tip. We will show our recent results with the optical manipulation of the bond configuration and corresponding charge state of a single bi-stable Si atom as a function of the excitation wavelength (E.P. Smakman et al. PRB \textbf{87} 085414 (2013)). This allowed us to unravel different pathways for the excitation and relaxation processes that are involved in this optical manipulation. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G44.00005: Observation of the exciton in low-temperature-grown GaAs using four-wave mixing Daniel Webber, Luke Hacquebard, Murat Yildirim, Sam March, Reuble Mathew, Angela Gamouras, Xinyu Liu, Margaret Dobrowolska, Jacek Furdyna, Kimberley Hall Low-temperature-grown (LT) semiconductors are the materials of choice in optoelectronic devices such as fast photodetectors and THz sources and detectors owing to their unique photoconductive properties tied to the presence of antisite defects . Recent experiments have provided insight into the carrier trapping processes responsible for the subpicosecond recovery times in these systems, as well as the relevance of band-tail transitions in the vicinity of the band gap; However, little is known about the coherent interband response in low-temperature-grown systems. Here we report the application of femtosecond four-wave mixing techniques to LT-GaAs. Our experiments reveal a clear response associated with bound excitons despite the absence of any such feature in linear spectroscopy studies on LT-GaAs. Experiments performed over a wide range of conditions indicate that carrier-carrier scattering dominates dephasing for carriers above the band gap, and that the exciton response tied to excitation-induced dephasing may be quenched in the presence of a prepulse with a sufficiently high fluence. Our findings provide new insight into the optical response of LT-GaAs in the ultrafast nonlinear regime applicable to the operating conditions of optoelectronic devices. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G44.00006: Evolution of transition metal dopant properties near the GaAs surface Anne Benjamin, Jay Gupta As electronic devices get smaller, individual defects become more important, and surface layers and interfacial regions constitute a greater proportion of the material in the device. We use scanning tunneling microscopy (STM) to examine the layer dependence of electronic properties of near-surface level 3d transition metals in gallium arsenide. Some transition metal dopants have been shown to have layer-dependent properties; manganese atoms are deep defects in surface layers and shallow defects in the bulk, while surface-layer zinc atoms can be shallow or deep defects depending on the proximity of other, subsurface zinc defects. By depositing a selected metal on our GaAs sample, and annealing to diffuse defects into the first few layers, we can study the layer dependence of a wide range of defects. At present, the different surface and subsurface properties of defects in semiconductors can only be studied after doping during growth or through individual placement of atoms. A diffusion method will allow for greater flexibility in the type and density of defects able to be studied with STM. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G44.00007: Sub-surface minority carrier lifetime mapping in photovoltaic materials Invited Speaker: P. James Schuck The minority carrier lifetime is considered one of the most critical and variable parameters in photovoltaic materials. However, accurately measuring its value is one of the great challenges in evaluating unconventional semiconductor materials for PV applications. I will describe our two-photon time-resolved photoluminescence decay measurements, which allow us to decouple surface and bulk recombination processes even in unpassivated samples. We demonstrate how the traditional one-photon technique can underestimate the bulk lifetime in a CdTe crystal by 10X and show that two-photon excitation more-accurately measures the bulk lifetime. I will finish by discussing how this technique enables the generation of three-dimensional minority carrier lifetime and charge collection efficiency maps that will be useful in identifying efficiency bottlenecks for new and conventional (e.g. CdTe {\&}~CIGS) thin film PV materials. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G44.00008: Imaging Long-Range Carrier Diffusion Across Grains in Polycrystalline CdTe Kirstin Alberi, Brian Fluegel, Helio Moutinho, Ramesh Dhere, Jian Li, Angelo Mascarenhas The use of polycrystalline semiconductors in electronic devices enables low cost fabrication on large area substrates. Understanding the extent to which structural defects and impurities influence carrier transport in these materials is increasingly important as device performance is maximized, but most conventional characterization techniques often cannot directly probe their effects. We have applied a novel photoluminescence imaging technique to directly observe carrier diffusion in the presence of grain boundaries and impurities in poly-CdTe films. Our results show that the grain boundaries in this material are relatively transparent to free carrier and exciton diffusion as compared to poly-GaAs. Furthermore, a network of inhomogeneously distributed impurity states is found to mediate hole transport across multiple grains to distances greater than 10 microns from the point of photogeneration. These results underscore the importance of controlling the concentration and distribution of impurity states in poly-CdTe thin film solar cells. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G44.00009: Atomic-resolution study of dislocation structures and interfaces in poly-crystalline thin film CdTe using aberration-corrected STEM Tadas Paulauskas, Eric Colegrove, Chris Buurma, Moon Kim, Robert Klie Commercial success of CdTe-based thin film photovoltaic devices stems from its nearly ideal direct band gap which very effectively couples to Sun's light spectrum as well as ease of manufacturing and low cost of these modules. However, to further improve the conversion efficiency beyond 20 percent, it is important to minimize the harmful effects of grain boundaries and lattice defects in CdTe. Direct atomic-scale characterization is needed in order identify the carrier recombination centers. Likewise, it is necessary to confirm that passivants in CdTe, such as Cl, are able to diffuse and bind to the target defects. In this study, we characterize dislocation structures and grain boundaries in poly-crystalline CdTe using aberration-corrected cold-field emission scanning transmission electron microscopy (STEM). The chemical composition of Shockley partial, Frank and Lomer-Cottrell dislocations is examined via atomic column-resolved X-ray energy dispersive (XEDS) and electron energy-loss spectroscopies (EELS). Segregation of Cl towards dislocation cores and grain boundaries is shown in CdCl2 treated samples. We also investigate interfaces in ultra-high-vacuum bonded CdTe bi-crystals with pre-defined misorientation angles which are intended to mimic grain boundaries. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G44.00010: Defect Energy Levels in GaAsBi and GaAs Grown at Low Temperatures Patricia Mooney, Keelan Watkins, Zenan Jiang, Alberto Basile, Ryan Lewis, Vahid Bahrami-Yekta, Mostafa Masnadi-Shirazi, Daniel Beaton, Thomas Tiedje GaAs$_{1-x}$Bi$_x$ alloys have the potential to extend conventional III-V semiconductor devices to longer infrared wavelengths. The bandgap energy decreases as the Bi fraction is increased, but with a small increase in lattice constant, thus reducing lattice mismatch constraints for GaAsBi/GaAs heterostructures. However, Bi is incorporated into GaAs films grown by molecular beam expitaxy (MBE) only at T$_G$ $<$400 $^{\circ}$C, making defects a concern. DLTS measurements show that trap concentrations in Si-doped (n-type) GaAs layers grown at standard temperatures are $<$4x10$^{13}$ cm$^{-3}$. They increase to 2x10$^{16}$ cm$^{-3}$ when T$_G$ is 390 $^{\circ}$C and to $\sim$10$^{18}$ cm$^{-3}$ when T$_G$ is 330 $^{\circ}$C, where the energy level of the dominant defect is E$_C$-0.40 eV. When only 0.3\% Bi is incorporated into n-type GaAs at 330 $^{\circ}$C, formation of the E$_C$-0.40 eV trap is suppressed. Other electron traps, including the dominant traps having energy levels at E$_C$-0.66 eV and E$_C$-0.80 eV, are present in similar concentrations in both GaAs and GaAsBi layers grown at 330 $^{\circ}$C and, therefore, result from the low growth temperature. The dominant traps are both point defect complexes involving an arsenic atom on a gallium lattice site (AsGa). [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G44.00011: A Single Molecule Approach to Defect Studies in ZnO N.R. Jungwirth, Y.Y. Pai, H.S. Chang, E.R. MacQuarrie, G.D. Fuchs Single molecule investigations are a powerful tool for understanding molecular systems with inhomogeneous behavior that is either broadened or completely washed out of ensemble measurements. Here we apply single molecule microscopy methods to defects in ZnO. In addition to its status as an emerging optoelectronic material, ZnO hosts point defects which may have useful quantum properties akin to those of nitrogen-vacancy centers in diamond, which are promising as single photon sources and solid-state qubits. We present confocal fluorescence measurements of single defects in ZnO nanoparticles and sputtered films that are selectively excited by sub-bandgap light. The resulting 560-720 nm emission often exhibits two broad spectral peaks separated by approximately 100 meV. Photon correlation measurements yield both antibunching and bunching, indicative of single photon emission from isolated defects with a metastable shelving state. Excited state lifetimes span 1-13 ns and are uncorrelated with doping concentration. We report discrete jumps in the fluorescence intensity between a bright and dark state. The dwell times are exponentially distributed in each state and the average dwell time in the bright (dark) state does (may) depend on the power of the excitation laser.~ [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G44.00012: Room-Temperature Plasticity in ZrC: Role of Crystal Anisotropy Christian Ratsch, S. Kiani, A.M. Minor, S. Kodambaka, J.M. Yang NaCl structure Group IV and V transition-metal carbides are hard, stiff, and high-melting solids with excellent wear, abrasion, and corrosion resistances, and are commonly used in advanced high-temperature structural applications. In this talk, we report results obtained from in situ transmission electron microscopy (TEM) studies and density functional theory calculations of uniaxial compression of ZrC(100) and ZrC(111) single crystals. In situ TEM observations show that dislocation motion and tangling lead to plastic deformation in ZrC(111), while slip along \textbraceleft 110\textbraceright \textless 1-10\textgreater is dominant in ZrC(100). We find that the yield strengths of ZrC crystals increase with decreasing size. Interestingly, yield strengths of uniaxially compressed ZrC(111) crystals are lower than those of ZrC(100), unexpected for NaCl-structured compounds. Based upon density-functional theory calculations, we attribute the orientation-dependent yield strengths to relatively lower energy barrier for shear along \textbraceleft 001\textbraceright \textless 1-10\textgreater compared to \textbraceleft 110\textbraceright \textless 1-10\textgreater . Our results provide important insights into the effects of crystal size and orientation on room-temperature plasticity. We expect that similar phenomena are likely to exist in other cubic-structured transition-metal carbides and nitrides. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G44.00013: Carrier dynamics in sulfur-hyperdoped silicon studied by time-resolved terahertz spectroscopy Meng-Ju Sher, Christie Simmons, Austin Akey, Mark Winkler, Daniel Recht, Tonio Buonassisi, Michael Aziz, Aaron Lindenberg Intermediate-band photovoltaics have been proposed to enhance efficiencies of solar cells by harvesting additional energy from sub-bandgap photons. One proposed method for fabricating an IB material is by introducing deep-level dopants at concentrations above the insulator-to-metal transition (IMT). Theory suggests that as the dopant states become delocalized, the non-radiative recombination is suppressed and the lifetime recovery enables photo-generated carriers to be harvested. We use optical-pump/terahertz-probe spectroscopy to study carrier dynamics of sulfur-hyperdoped silicon and test whether lifetime recovery is possible in this material system. S-hyperdoped silicon exhibits strong sub-bandgap light absorption and IMT at S concentration above $2 \times 10^{20}$ cm$^{-3}$. Previous photoconductivity study suggests the lifetime is less than 130 ps for samples at concentrations below IMT. Time-resolved THz spectroscopy is suitable for studying carrier dynamics on short time scales. We use a 400-nm fs-laser pulse to generate carriers and by monitoring the transmission of the THz probe as a function of time, we extract the carrier dynamics and mobility all-optically. [Preview Abstract] |
Session G45: Fractional Quantum Hall Effect: Second Landau Level
Sponsoring Units: FIAPChair: Gabor Csathy, Purdue University
Room: Mile High Ballroom 4D
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G45.00001: Low-Energy Excitations in the Second LL: Fundamental Insights from Inelastic Light Scattering Ursula Wurstbauer, Aron Pinczuk, Antonio L. Levy, John Watson, Sumit Mondal, Michael J. Manfra, Ken West, Loren Pfeiffer The competition between quantum phases that dictates the physics in the second Landau level (SLL) results in striking phenomena. Our work explores this fascinating interaction physics by measurements of low-lying neutral excitation modes in the SLL from resonant inelastic light scattering experiments. We focus here on the marked differences of the low-lying collective excitation spectra of the even-denominator state at $\nu$=5/2 with those in the range 5/2>$\nu$>2. Filling factor 5/2 is characterized by the presence of gapped modes, a spin mode exactly at $E_{Z}$ and the absence of a continuum of low-lying excitations. In contrast, a continuum of low-lying excitations and gapped modes are coexistent at $\nu$=2+1/3, 2+3/8 and 2+2/5 and the spin-modes appear significantly below $E_{Z}$. All observed modes weakens with smallest variations in filling factor substantiating the transition from an incompressible quantum Hall fluid to compressible states. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G45.00002: Optical Emission from quantum phases of the second Landau level Antonio Levy, Ursula Wurstbauer, Aron Pinczuk, John Watson, Sumit Mondal, Michael Manfra, Ken West, Loren Pfeiffer Optical emission across the host semiconductor bandgap has proven a powerful tool in examining the properties fractional quantum Hall sates (fqhs). While the luminescence of fqhs in the first (N$=$0) Landau Level has been extensively studied, there are significantly fewer studies of the optical emission in the N$=$1 Landau Level. We report studies of luminescence in the filling factor range 4\textgreater nu\textgreater 2 N$=$1 Landau level. The marked dependence on filling factor suggests that optical emission is here linked to competing quantum phases. A comparison of luminescence in a range about $\nu =$7/3 with extensively studied optical emission near $\nu =$1/3 creates venues to explore the competing quantum phases of the second Landau levels. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G45.00003: Disorder matters in tilt magnetic field induced anisotropy in the $\nu =5/2$ fractional quantum Hall effect W. Pan, Xiaoyan Shi, K.W. Baldwin, K.W. West, L.N. Pfeiffer, D.C. Tsui It is known that under a moderate in-plane magnetic field ($B_{\parallel}$) the even-denominator fractional quantum Hall effect (FQHE) at the Landau level filling $\nu=5/2$ is destroyed and becomes anisotropic. However, in recent two reports, it was observed that this tilt magnetic field induced anisotropy depends on GaAs crystallographic directions. Electronic transport becomes anisotropic when $B_{\parallel}$ is parallel to [$\bar{1}10$] but remains isotropic if $B_{\parallel}$ parallel to [110]. In this talk, we report a systematic tilt-magnetic field study of the $\nu=5/2$ FQHE in a series of high quality GaAs/Al$_x$Ga$_{1-x}$As heterostructure samples, in which the level of disorder is varied continuously by changing the setback distance (\textit{d}) between the modulation doping layers and the GaAs quantum well. We observed that in highly disordered samples electronic transport is anisotropic in one crystallographic direction but remains more or less isotropic in the other direction, consistent with the recent two reports. In contrast, in less-disordered samples, where \textit{d} is large, electronic transport is anisotropic in both crystallographic directions. Our results clearly show that disorder matters in tilt magnetic field induced anisotropy in the 5/2 FQHE. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G45.00004: The $\nu=5/2$ Fractional Quantum Hall State in presence of Alloy Disorder Nianpei Deng, Geoffrey Gardner, Sumit Mondal, Ethan Kleinbaum, Michael Manfra, Gabor Csathy Disorder plays a prominent role in the formation and the strength of all fractional quantum Hall states, including the one forming at filling factor $\nu=5/2$. Many aspects of the disorder are, however, poorly understood. At the root of this lack of understanding one often finds our inability to control and characterize disorder. We have investigated the effect of a specific type of disorder, alloy disorder, on the $\nu=5/2$ state. The alloy disorder is controllably introduced into the electron channel by growing a series of Al$_{0.24}$Ga$_{0.76}$As/Al$_x$Ga$_{1-x}$As/Al$_{0.24}$Ga$_{0.76}$As quantum well samples with different aluminum molar fraction $x$ using Molecular Beam Epitaxy techniques. We find a suppression of the energy gap of $\nu=5/2$ state with increasing $x$. To our surprise, we observe a fully quantized $\nu=5/2$ state in an extremely low mobility regime in which, based on existing data, we did not expect the $\nu=5/2$ state to develop. Such a result indicates that $\nu=5/2$ state is unusually robust to the short-ranged alloy disorder. This work was supported by the DOE BES contract no. DE-SC0006671. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G45.00005: Development of high quality, density-tunable two-dimensional electron gases for the study of the quantum Hall effect in the 2$^{\mathrm{nd}}$ Landau level John Watson, Sumit Mondal, Michael Manfra We report on progress in state-of-the-art high mobility two-dimensional electron gases (2DEGs) in 30 nm GaAs/AlGaAs quantum wells in which the density is modulated by an in-situ grown back-gate. Such in-situ gates can be grown close to the 2DEG ($\sim$ 1 $\mu$m) and without doping layers between the 2DEG and gate, resulting in non-hysteretic gating with a very uniform electric field and large gate capacitance. We discuss heterostructure design parameters and device processing conditions leading to low gate leakage currents, low ohmic contact resistances, high electron mobilities (17 x 10$^{6}$ cm$^{2}$/Vs), and large fractional quantum Hall energy gaps in the second Landau level. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G45.00006: Proliferation of upstream neutral modes in the fractional quantum Hall regime Hiroyuki Inoue, Anna Grivnin, Yuval Ronen, Moty Heiblum, Vladimir Umansky, Diana Mahalu The fractional quantum Hall effects (FQHE) are canonical examples of topological phases, resulting from correlations among planar electrons under strong perpendicular magnetic field. Chargeless energy transport, in the form of upstream (anti-chiral) neutral edge modes, were recently observed in the \textit{hole-conjugate} FQHEs (filling $\nu $ of the $n^{\mathrm{th}}$ Landau level in the range $n+$1/2\textgreater $\nu $\textit{\textgreater n}$+$1, with $n=$0, 1, 2) as well as $\nu =$5/2. These modes had been predicted to appear due to edge reconstructions by Coulomb interaction and random tunneling among multiple channels. Here we report highly sensitive shot noise measurements that reveal, unexpected theoretically, the presence of such upstream neutral modes in \textit{electron-like} FQHEs such as $\nu =$1/3, 2/5, etc$.$ Furthermore, we also found neutral bulk modes that propagate through the incompressible bulk; though weaker than the edge modes. The proliferation of such neutral modes detected only in FQHEs drastically changes the accepted picture of the transport therein. Moreover, our observation may shed new light on a source of decoherence, which prevented thus far a definite observation of quantum interference of fractional quasiparticles. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G45.00007: Anomalous Insulating States in Landau Levels $N \geq 1$ Talbot Knighton, Jian Huang, Zhe Wu, Loren Pfeiffer, Ken West Quantum Hall measurements are performed for a rectangular two-dimensional (2D) hole system confined to a 20 nm quantum well in $\langle 100 \rangle$ GaAs. Quantum oscillations reveal a density of $4.3 \cdot 10^{10}$ cm$^{-2}$ with mobility $\mu = 1.9\cdot10^{6}$ cm$^2$/V$\cdot$s. For temperatures less than $\sim$150 mK, anomalous insulating peaks are observed near integer fillings 1,2, and 3 for which both in-phase and out-of-phase signals rise substantially to be near or well above the quantum resistance. They differ from usual re-entrant insulating phases such as that observed before $\nu = 1/3$ where the out-of-phase signal remains less than 3\% of the in-phase signal. The relationship between in-phase and out-of-phase signals of the magnetoresistances resembles that of the orthogonal components $\rho_{xx}$ and $\rho_{yy}$ previously observed for collective anisotropic states in $\langle 100 \rangle$ and $\langle 311 \rangle$ GaAs 2D systems. These non-monotonic phase shifts will be discussed in relation to possible stripe phases. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G45.00008: Flip Chip: Keep pristine materials clean Arjan Beukman, Fanming Qu, Leo Kouwenhoven We introduce the Flip Chip setup, a new platform which allows nanoscale electrical gating of a material without exposing it to invasive nanoprocessing. The conventional fabrication of metallic gate structures on pristine materials degrades their interesting properties, e.g., ultra-high mobility in GaAs heterostructures. This research takes a new approach to keep the material unaffected. The gate structure is fabricated on a separate chip which is flipped and brought close (\textless 100 nm) to the sample under research. A vacuum gap between the gates and material, acting as an insulating layer, solves the problem of leakage and trapped charges in traditional gate-dielectrics. In addition, this approach allows more freedom in fabrication methods, as the `dirty' processing is done on a separate chip. With the Flip Chip setup we intend to study the 5/2 FQH state in GaAs heterostructures which is expected to have non-Abelian statistics. I will present measurement results for an interferometer at integer quantum Hall states using the Flip Chip technique. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G45.00009: Theoretical Investigation of Edge Reconstruction in $\nu=5/2$ and $7/3$ Fractional Quantum Hall States Yuhe Zhang, Yinghai Wu, Jimmy Hutasoit, Jainendra K. Jain We study the possibility of edge reconstruction for the $\nu=5/2$ fractional quantum Hall (FQH) state in a realistic geometry which includes the background positive charge at a distance $d$ and the lower filled Landau level. For this purpose, we diagonalize the second Landau level Coulomb interaction within the Pfaffian basis of edge excitations, and find the range of setback distance d where edge reconstruction occurs. We also study the edge of the $\nu= 7/3$ FQH system with composite fermion diagonalization method and find the edge reconstruction occurs more easily at $\nu= 7/3$ than at $\nu= 1/3$. We also ask how edge reconstruction affects the exponent associated with tunneling into the edge. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G45.00010: Study of Quasiholes in FQHE for different Landau Levels R.N. Bhatt, Sonika Johri, Zlatko Papic, Peter Schmitteckert We study the sizes of elementary quasiholes in $\nu=1/3$ and $\nu=7/3$ quantum Hall states. Calculations are carried out by exact diagonalization and density matrix renormalization group method for spherical and cylindrical geometries. We use both short- and long-range pinning potentials to localize the quasihole [Johri et. al arXiv:1310.2263]. The size of the quasihole in the model Laughlin state is estimated to be around $\approx 2.5\ell_B$, where $\ell_B$ is the magnetic length . In contrast, the size of the quasihole in the Coulomb ground state at filling factor $\nu=1/3$ is $\approx 4\ell_B$, while that at $\nu=7/3$ is $\approx 7\ell_B$. Our results support the earlier findings by Balram et. al [PRL {\bf 110}, 186801 (2013)] that the $e/3$ quasihole in the first excited Landau level is significantly larger than in the lowest Landau level. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G45.00011: Probing non-unitarity in fractional quantum Hall states Nicolas Regnault, Benoit Estienne, Andrei Bernevig Recent developments[1,2,3] have proposed an exact matrix product state representation of a large series of fractional quantum Hall states. The model states include all paired $k=2$ Jack polynomials, such as the Moore-Read and Gaffnian states, as well as the Read-Rezayi $k=3$ state. We will discuss how this approach provides some new insight on the pathological features on the non-unitarity states such as the Gaffnian. [1] M.P. Zaletel and R.S. K. Mong, Phys. Rev. B 86, 245305 (2012). [2] B. Estienne, Z. Papic, N. Regnault, B. A. Bernevig, Phys. Rev. B 87, 161112(R) (2013). [3] B. Estienne, N. Regnault, B. A. Bernevig, arXiv:1311.2936. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G45.00012: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G45.00013: New topological excitations in quantum Hall systems Yuli Lyanda-Geller, Tsuging Lin, George Simion, John D. Watson, Michael J. Manfra, Gabor Csathy, Leonid Rokhinson We discover new topological excitations of two dimensional electrons in the quantum Hall regime. The strain dependence of resistivity observed experimentally is shown to change sign upon crossing filling-factor-specified boundaries of reentrant integer quantum Hall effect (RIQHE) states. This observation violates the known symmetry of electron bubbles thought to be responsible for the RIQHE. We demonstrate theoretically that electron bubbles become elongated in the vicinity of charge defects and form textures of finite size. Calculations confirm that textures lower the energy of excitations. In the two-electron bubble crystal these textures form two-dimensional hedgehogs around defects having one extra electron, and vortices around defects lacking one electron. Strain affects vortices and hedgehogs differently, explaining striking strain-dependent resistivity. The sharp transition from insulating RIQHE state to conducting state is caused by melting of Abrikosov crystal comprised of the defects. The proposed physical mechanism of conductivity due to topological defects is shown to lead to an unusually large magnitude of the strain effect on resistivity in the range of RIQHE filling factors, in agreement with experiment. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G45.00014: New melting transition in Quantum Hall systems George Simion, Tsuging Lin, John D. Watson, Michael J. Manfra, Gabor Csathy, Leonid Rokhinson, Yuli Lyanda-Geller We discover a new melting transition caused by topological excitations of two dimensional electrons in the quantum Hall regime. Experimentally, strain dependence of resistivity changes sign upon crossing filling-factor-specified boundaries of reentrant integer quantum Hall effect (RIQHE) states. This observation violates the symmetry of electron bubble crystal, whose melting was thought to be responsible for insulator to metal transition in the range of RIQHE filling factors. We demonstrate theoretically that electron bubbles become elongated in the vicinity of charge defects and form textures of finite size. Textures lower the energy of excitations. In the two-electron bubble crystal these textures form hedgehogs (vortices) around defects having (lacking) one extra electron. At low density these textures form an insulating Abrikosov lattice. At densities sufficient to cause the textures to overlap, their interactions are described by the XY-model and the defect lattice melts. This explains the sharp metal-insulator transition observed in finite temperature conductivity measurements. In this regime, melting is a function of several variables and forms a continuous phase boundary in the field-temperature ($B-T$) plane. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G45.00015: Superconducting analogue of the parafermion fractional quantum Hall states Abolhassan Vaezi Read and Rezayi $Z_k$ parafermion wavefunctions describe $\nu= 2 + k/(kM+2)$ fractional quantum Hall (FQH) states. These states support non-Abelian excitations from which protected quantum gates can be designed. However, there is no experimental evidence for these non-Abelian anyons to date. In this talk, we discuss the $\nu=2/k$ FQH-superconductor heterostructure and through analytical and numerical calculations we argue that it can yield the superconducting analogue of the $Z_k$ parafermion FQH state. The resulting topological state has a gapless chiral edge state with $Z_k$ parafermion conformal field theory description. For instance, we find that a $\nu= 2/3$ FQH in proximity to a superconductor produces a $Z_3$ parafermion superconducting state. This state can host Fibonacci anyons capable of performing universal quantum computation through braiding operations. We finally discuss our experimental proposal for realizing parafermion superconductors. Reference: arXiv:1307.8069 [Preview Abstract] |
Session G46: Ce- and Pu-Based 115 Materials and Related Systems
Sponsoring Units: DCMPChair: Johnpierre Paglione, University of Mayland
Room: Mile High Ballroom 4E
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G46.00001: Phase-Sensitive Bogoliubov Quasi-Particle Interference Spectroscopy in CeCoIn$_5$ John Van Dyke, Freek Massee, Milan Allan, J.C. Davis, Cedomir Petrovic, Dirk Morr Recent scanning tunneling spectroscopy experiments [1] have provided unprecedented insight into the momentum structure of the superconducting gap in CeCoIn$_5$ using quasi-particle interference (QPI) spectroscopy. In this talk, we demonstrate that the symmetry of the superconducting gap in CeCoIn$_5$ can be determined via phase-sensitive quasi-particle interference (PQPI) spectroscopy. This method is based on the insight that the intensity of the QPI spectrum is different for potential and magnetic defects. Using this idea, we present a theory for phase-sensitive QPI spectroscopy in heavy fermion materials. We demonstrate that a variation in the phase of the superconducting gap along the Fermi surface can be identified by comparing QPI spectra in zero and finite magnetic fields. Analysing recent experimental QPI results on CeCoIn$_5$ in $H=0$ and 3 Tesla magnetic fields, we show that the resulting PQPI spectrum provides strong evidence for a $d_{x^2-y^2}$ symmetry of the superconducting gap [2]. [1] M.P. Allan et al., Nature Physics 9, 468-473 (2013) [2] J. Van Dyke, F. Massee, M.P. Allan, J.C. Davis, C. Petrovic, D.K. Morr, submitted. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G46.00002: High Pressure NMR study of Knight Shift Anomaly on the heavy electron material CeRhIn5 Ching Lin, Nicholas Curro, Kent Shirer, John Crocker, Adam Dioguardi, Abigail Shockley, Matthew Lawson We have measured the Nuclear Magnetic Resonance Knight Shift in the heavy f-electron material CeRhIn5 in order to investigate the coherence temperature T$^{\mathrm{\ast }}$ as a function of pressure up to 2GPa. We find that the Knight Shift of both In(1) and In(2) sites changes significantly with pressure. Our results are consistent with the phase diagram proposed by Yang and Pines. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G46.00003: Evolution of the Hyperfine Couplings with Pressure in CeRhIn$_5$ Nicholas Curro, Ching Lin, Kent Shirer, John Crocker, Adam Dioguardi, Abigail Shockley, Matthew Lawson Measurements of the Knight shift in CeRhIn$_5$ under pressure reveal several changes to the hyperfine coupling constants at both the In(1) and In(2) sites. We discuss these changes, both to the on-site contact term as well as the transferred term to the local moments. Our data suggest that the changes we observe reflect changes in the hybridization of the Ce 4f moments as the system is tuned from an antiferromagnetic ground state to superconducting. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G46.00004: Fermi surface collapse, gap, coherence: an ARPES study of the hybridization in Ce$_2$RhIn$_8$ Fanny Rodolakis, Cris Adriano, Francisco Restrepo, Priscila F.S. Rosa, Pascoal Pagliuso, Juan Carlos Campuzano The crossover of localized magnetic moments at high temperatures into itinerant states of heavy mass at low temperatures in some metals containing \emph{f} electrons, first addressed by Kondo, is a fundamental problem in condensed matter physics involving a temperature dependent hybridization between \emph{f} levels and conduction electrons (\emph{ce}). Here we present an extensive angular resolved photoemission spectroscopy study performed in Ce$_2$RhIn$_8$\ as a function of temperature. Our experiments reveal the presence of three energy scales, differing by an order of magnitude from each other: first at room temperature, where the \emph{f} levels are localized, we observe a small Fermi surface (FS), which undergoes dramatic topological changes toward a large FS near the minimum in the resistivity around 200K; the opening of a spectral gap below 30K without a change in topology of the Fermi surface; and finally, below 5K, composite quasiparticles form, as the resistivity suddenly decreases. The expectation that hybridization, spectral gap, and \emph{f} electron coherence go hand in hand should be expanded to include the possibility of separate energy scales for each of these phenomena. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G46.00005: CePt$_{2}$In$_{7}$: Shubnikov-de Haas measurements on micro-structured samples under high pressures J. Kanter, P. Moll, S. Friedemann, P. Alireza, M. Sutherland, S. Goh, F. Ronning, E.D. Bauer, B. Batlogg CePt$_{2}$In$_{7}$ belongs to the Ce$_{m}$M$_{n}$In$_{3m+2n}$ heavy fermion family, but compared to the Ce$M$In$_{5}$ members of this group, exhibits a more two dimensional electronic structure. At zero pressure the ground state is antiferromagnetically ordered. Under pressure the antiferromagnetic order is suppressed and a superconducting phase is induced, with a maximum T$_{c}$ above a quantum critical point around 31 kbar. To investigate the changes in the Fermi Surface and effective electron masses around the quantum critical point, Shubnikov-de Haas measurements were conducted under high pressures in an anvil cell. The samples were micro-structured and contacted using a Focused Ion Beam (FIB). The Focused Ion Beam enables sample contacting and structuring down to a sub-micrometer scale, making the measurement of several samples with complex shapes and multiple contacts on a single anvil feasible. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G46.00006: Doping study of the heavy fermion superconductor CePt$_{2}$In$_{7}$ Nirmal Ghimire, Filip Ronning, J. Thompson, Eric Bauer The CeMIn$_{5}$ (M$=$Co, Rh, Ir) materials are prototypical heavy fermion superconductors close to antiferromagnetism, making them ideal candidates to investigate the interplay of unconventional superconductivity and magnetism and to explore quantum criticality. CeRhIn$_{5}$ displays all of the signatures of a material close to an antiferromagnetic quantum critical point (QCP): 1) the antiferromagnetism at T$_{N} =$ 3.8 K is suppressed under applied pressure at Pc$=$2.5 GPa, 2) non-Fermi liquid behavior in the electrical resistivity and specific heat is observed near Pc, and 3) a dome of unconventional superconductivity appears with Tc max$=$2.6 K. To investigate the nature of the quantum criticality in the Ce$_{m}$M$_{n}$In$_{3m+2n}$ family, we focus attention on the newest member, CePt$_{2}$In$_{7}$, with m$=$1 and n$=$2, where m and n are CeIn$_{3}$ and MIn$_{2}$ layers. Similar to its cousin CeRhIn$_{5}$ (m$=$1, n$=$1), it shows a dome of superconductivity and signatures of quantum criticality under pressure in the vicinity of where the Neel temperature is suppressed at Pc$=$3 GPa. As an alternative to the application of pressure to access the QCP, we present the magnetic, thermal and transport properties of doped CePt$_{2}$In$_{7}$. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G46.00007: Dispersion of Spin Resonance mode in Yb Doped CeCoIn5 Yu Song, Mengshu Liu, Benjamin White, Brian Maple, Pengcheng Dai It is commonly believed superconductivity in CeCoIn5 has a d wave symmetry, given such symmetry the spin exciton interpretation of the spin resonance which has been widely applied to many systems where such a mode is observed, predicts a downward dispersion of the resonance mode (PRL 101, 187001 (2008)). Here we discuss our neutron scattering results on Yb doped CeCoIn5, our results can clearly differentiate whether the resonance mode is indeed a spin exciton or if it's a magnon-like exciton as argued in another work (PRL 101, 087001 (2008)). [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G46.00008: Theory of Composite Paring in Yb doped CeCoIn$_5$ Onur Erten, Piers Coleman Recent experiments on the $R$ ($R$= La, Yb) doped CeCoIn$_5$ have yielded fascinating results. La, which acts as an inert cation kills superconductivity rapidly[1], whereas superconductivity is much more robust in the case of Yb doping[2]. Experiments also show that unlike La, Yb is in a mixed valent state for all concentrations of doping. Motivated with these experiments, we investigate the effects of doping and disorder on composite paring by diluting two channel Kondo lattice model. This talk will discuss the doping dependence of the coherence temperature and $T_c$ and various possibilities for the discrepancies between thin films and bulk samples. [1] S. Nakatsuji {\it et al.} Phys. Rev. Lett. {\bf 89}, 106402 (2002) [2] L. Shu {\it et al.} Phys. Rev. Lett {\bf 106}, 156403 (2011) [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G46.00009: Low Temperature Magnetometry Measurements of the Heavy Fermion Superconductor Nd1$-x$Ce$x$CoIn5 with x $=$ 0.98, 0.95, and 0.90 Kevin Storr, Kenneth Purcell, Torrance Rasco, Sarah Schwartz, Cedomir Petrovic The Nd$_{\mathrm{1-x}}$Ce$_{x}$CoIn$_{5}$ alloys evolve from local moment magnetism $x=$0 to heavy fermion superconductivity $x=$1, as the Nd substitution alters the level of 4f-conduction electron coupling. Superconductivity has been shown to exist in Nd concentrations between x $=$ 0 and x $=$ 0.22. We report the temperature and angular dependence of the critical field of the superconducting state of the x $=$ 0.98, 0.95, and 0.90 doping levels at temperatures ranging from 20 -- 500 mK, investigating the evolution of the phase diagram for different concentrations of Nd at these previously unexplored low temperatures. No evidence of a low temperature mixed superconducting and magnetic mixed state was observed such that as that seen in CeCoIn5. The suppression of the critical field is more dramatic than the application of pressure and was observed to be rather anisotropic in line with the higher temperature measurements. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G46.00010: Incommensurate Magnetic Order in Ce(1-x)Yb(x)RhIn(5) Steven Disseler, S. Jang, B.D. White, Yang Zhao, Jeff Lynn, M.B. Maple We present a detailed study of a series of Yb-doped CeRhIn5 single crystals through measurements of the bulk magnetic properties and elastic neutron scattering. We find that all samples up to x $=$ 0.8 undergo a magnetic ordering transition below 4 K, despite observations that the Yb-valence rapidly decreases toward a non-magnetic state at high concentrations. Furthermore, we find that this magnetically ordered state is described by an incommensurate structure similar to the parent compound, and with a propagation wave vector that is weakly dependent on concentration. The authors acknowledge funding source US DOE DE-FG02-04-ER46105. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G46.00011: Controllable Rashba spin-orbit interaction in artificially engineered superlattices CeCoIn$_{5}$/YbCoIn$_{5}$ Masaaki Shimozawa, Swee Goh, Ryota Endo, Ryo Kobayashi, Tatsuya Watashige, Yuta Mizukami, Hiroaki Ikeda, Hiroaki Shishido, Youichi Yanase, Takahito Terashima, Takasada Shibauchi, Yuji Matsuda Recently the inversion symmetry breaking (ISB) together with strong spin-orbit interaction is suggested to affect the electron pairing in superconductivity, leading to various physical phenomena. However, it is hard to tune the degree of ISB in bulk crystals because the degree is determined by the crystal structure itself. Here, by using the molecular beam epitaxy technology, we fabricate artificial heavy fermion superlattices with the alternating layers of heavy fermion CeCoIn$_{5}$ and nonmagnetic metal YbCoIn$_{5}$ with atomic scale thicknesses. We demonstrate that the Rashba spin-orbit interaction arising from ISB is largely tunable by introduction of the thickness modulation in YbCoIn$_{5}$ block-layers, which leads to profound changes in the nature of the superconductivity. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G46.00012: Many-body coherence in $f$-electron Ce$_{\mathrm{1-x}}$Yb$_{\mathrm{x}}$CoIn$_{5}$ alloys Y.P. Singh, D.J. Haney, X.Y. Huang, M. Dzero, I. Lum, B.D. White, M.B. Maple, C.C. Almasan We investigated experimentally the onset of the many-body coherence in the $f$-orbital single crystalline alloys Ce$_{\mathrm{1-x}}$Yb$_{\mathrm{x}}$CoIn$_{5}$ (0.00 $\le $ x $\le $ 0.775). Specifically, we performed thermodynamic and magneto-transport measurements to study the evolution of the many-body electronic state as the Kondo lattice of Ce moments is transformed into an array of Ce impurities. Our analysis of the residual resistivity data unveils the presence of correlations between Yb ions for x \textgreater 0.50. For concentrations around 50{\%} of Yb we observe the smooth crossover from the predominantly localized moment regime to the predominantly itinerant regime. Indeed, our analysis of specific heat data reveals that for 0.65 $\le $ x $\le $ 0.775, Yb $f$-electrons strongly interact with the conduction electrons while the Ce moments remain completely decoupled. The sub-linear temperature dependence of resistivity across the whole range of Yb concentrations suggests the presence of a nontrivial scattering mechanism for the conduction electrons. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G46.00013: NQR Study of the Heavy-Fermion Pu-115 Superconductors G. Koutroulakis, H. Yasuoka, P.H. Tobash, J.N. Mitchell, E.D. Bauer, J.D. Thompson We present $^{115}$In nuclear quadrupolar resonance (NQR) measurements on the heavy-fermion superconductors Pu$M$In$_5$ ($M$=Co, Rh; $T_c$=2.5K, 1.6K, respectively), in the temperature range \mbox{$0.29{\rm K}\leq T\leq 100{\rm K}$}. From the identified spectral lines, we deduce the quadrupolar parameters for the two inequivalent In sites, which are found to be qualitatively similar to those for other Ce- and Pu-115s. The quadrupolar frequency $\nu_Q$ varies with temperature in the normal state as per the empirical formula for conventional metals. As superconductivity develops, however, $\nu_Q$ exhibits a sharp, albeit small shift, which is a key prediction of the theory of composite superconducting (SC) pairing [1]. The temperature variation of the nuclear spin-lattice relaxation rate $T_1^{-1}$ delineates distinctive regimes of dynamic behavior. An excess of strong in-plane antiferromagnetic spin fluctuations is observed in the vicinity of $T_c$, which are believed to be playing a central role in the formation of the SC condensate. Analysis of the $T_1^{-1}$ data in the SC state suggests that these compounds are strong-coupling $d$-wave superconductors. [1]R. Flint, A. H. Nevidomskyy, and P. Coleman, Phys. Rev. B {\bf 84}, 064514 (2011). [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G46.00014: Probing the Nature of Superconductivity in the Heavy Fermion PuMGa$_{5}$ and PuMIn$_{5}$ (M$=$Co, Rh) Compounds Eric Bauer, Paul Tobash, Jeremy Mitchell, Joe Thompson The discovery of superconductivity in PuCoGa$_{5}$ with a T$_{\mathrm{c}} =$ 18.5 K has generated renewed interest in Pu-based compounds. PuCoGa$_{5}$, and its superconducting cousin PuRhGa$_{5}$ (T$_{\mathrm{c}} =$ 8.7 K), have the same crystal structure as the tetragonal CeMIn$_{5}$ (T$=$Co, Rh, Ir) heavy fermion superconductors, suggesting that the structure plays a key role in generating superconductivity in these materials. While a variety of measurements have firmly established that the CeTIn$_{5}$ compounds are unconventional $d$-wave superconductors, most probably mediated by antiferromagnetic spin fluctuations, it is less clear what drives the high transition temperature in PuCoGa$_{5}$, which is an order of magnitude larger than all other know Ce- or U-based heavy fermion superconductors. The physical properties of two new members of this ``115'' family of superconductors, PuRhIn$_{5}$ and PuCoIn$_{5}$, indicate that they reside close to an antiferromagnetic quantum critical point, while the smaller effective masses and much smaller unit cell volumes of PuCoGa$_{5}$ and PuRhGa$_{5}$ suggest that they may be near a T$=$0 valence instability or that the Pu 5f electrons couple to conduction electrons in multiple channels to form ``composite'' superconducting pairs. The nature of superconductivity in these four Pu115 materials will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G46.00015: Possibility of nodal $s^\pm$ pairing symmetry in plutonium-based 115 superconductors Matthias J. Graf, Tanmoy Das, Jian-Xin Zhu We performed RPA calculations to investigate the role of spin-fluctuations in the superconducting state of Pu-115 compounds. First-principles electronic structure calculations were used as input, combined with the spin-fluctuation exchange approximation, to compute within the RPA method the nesting conditions on the Fermi surface. Of special interest for superconductivity are hot spots caused by nesting near the wave vectors ($\pi,\pi,q_z$) connecting the four Fermi surfaces. Surprisingly, in this multiband material the normal-state instability toward superconductivity is dominated by a nodal gap with $s^\pm$ symmetry over a more typical $d_{x^2-y^2}$-wave gap. We will discuss the possibility of magnetic resonances in the superconducting state and how to differentiate between these closely competing pairing symmetries in inelastic neutron scattering and point-contact spectroscopy. [Preview Abstract] |
Session G47: Metal-Insulator and Other Electronic Phase Transitions: Theory
Sponsoring Units: DCMPChair: Andre-Marie Tremblay, Universite de Sherbrooke
Room: Mile High Ballroom 4F
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G47.00001: Charge-Spin Mutual Frustration in Hubbard Model and Quantum Phase Diagram Long Zhang, Zheng-Yu Weng The charge-spin mutual frustration shapes the electron and spin correlations in the moderate coupling regime of the Hubbard model. We propose that it can be captured by the mutual semion statistics between chargons and spinons, i.e., they view each other as $\pm \pi$ gauge fluxes. Gapless spin liquid phases emerge on a square lattice with a $\pi$-flux in each plaquette and an anisotropic triangular lattice, consistent with numerical simulations and experiments on the organic material EtMe$_3$Sb[Pd(dmit)$_2$]$_2$. We find that the quasiparticle decoherence due to severe phase fluctuations can cause Mott transition without fully suppressing charge fluctuations, suggesting the concept of ``weak Mott insulators.'' [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G47.00002: The nature of quantum criticality in the Hubbard model on honeycomb lattice Igor Herbut, Fakher Assaad Hubbard model on graphene's honeycomb lattice at the filling one half and at zero temperature exhibits the semi-metallic phase at weak coupling, and the insulating Neel ordered phase at strong coupling. The nature of the phase transition between these two phases has been a contentious issue in literature. We will present evidence from recent quantum Monte Carlo calculations in favor of the direct, continuous transition, without an intermediate spin-liquid phase. Both the staggered magnetization and the single-particle gap display excellent finite-size scaling, with the same scaling function, and with the critical exponents which are in accord with the dimensional expansion that was devised for the problem near three spatial dimensions. We will discuss the effective Gross-Neveu-Yukawa low-energy theory for this quantum phase transition, with the new ``fermionic'' critical point, at which Dirac fermions are fully coupled, and cannot be simply ``integrated out''. Some new universal amplitudes which characterize this interesting phase transition with massless excitations on both sides will be mentioned. References: F. Assaad and I. F. Herbut, Physical Review X, vol. 3, 031010 (2013); I. F. Herbut, V. Juricic, and O. Vafek, Physical Review B, vol. 80, 075432 (2009). [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G47.00003: The effect of geometric frustration on the charge and spin correlations of the 2D Hubbard model near the metal-insulator transition Matthew Enjalran We investigate the effect of geometric frustration on the low temperature charge and spin correlations of interacting electrons on the triangular and kagome lattices near the metal-insulator transition. We consider the half-filled single band Hubbard model in the unrestricted Hartree-Fock approximation and study the evolution of the low temperature phases on each lattice as function of geometric frustration, which is achieved by tuning the ratio of the hopping parameters $t^{\prime}/t$. We present results for the mean-field phase diagram of the anisotropic triangular lattice as the relative hopping strength changes the lattice topology from the square lattice to the fully frustrated triangular lattice to weakly coupled chains. Preliminary results for mean-field phases on the anisotropic kagome lattice are also presented. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G47.00004: Mott-Hubbard vs Charge-Transfer Insulating Behavior in the CuO$_2$ Plane Giovanni Sordi, Patrick Semon, A.-M. S. Tremblay High temperature superconductivity emerges in the CuO$_2$ plane upon doping a Mott insulator. The Mott insulating state occurs in different regimes, Mott-Hubbard insulator and charge-transfer insulator. The latter is relevant especially for hole-doped cuprates. Within a three-band model we can explore both charge-transfer and Mott-Hubbard systems. Here we study the metal-insulator transitions in a three-band copper oxide model within cluster dynamical mean-field theory with continuous-time quantum Monte Carlo as an impurity solver. As a function of doping, charge-transfer energy and interaction strength, the normal state of this model shows the two types of metal-insulator transitions, metal to charge-transfer insulator and metal to Mott-Hubbard insulator. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G47.00005: Spatial Correlation in the Three-band Copper Oxide Model: Dynamical Mean-field Study with Configuration Interaction Based Impurity Solver Ara Go, Andrew J. Millis The three-band copper oxide model is studied using the single-site and four-site dynamical mean-field theory with configuration interaction based impurity solver. Comparison of the single and four site approximations shows that short ranged antiferromagnetic correlations are crucial to the physics. In the undoped case, they increase the gap size, shift the metal-insulator phase boundary and enhance the conductivity at the gap edge. The relation of antiferromagnetism and the pseudogap is discussed for the doped case. The new solver permits the inclusion of more bath orbitals which are crucial for accurate studies of spectral properties near the gap edge. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G47.00006: Mottness-induced healing in strongly correlated superconductors S. Tang, V. Dobrosavljevi\'c, E. Miranda We study impurity healing effects in models of strongly correlated superconductors. We show that in general both the range and the amplitude of the spatial variations caused by nonmagnetic impurities are significantly suppressed in the superconducting as well as in the normal states. We explicitly quantify the weights of the local and the non-local responses to inhomogeneities and show that the former are overwhelmingly dominant over the latter. We find that the local response is characterized by a well-defined healing length scale, which is restricted to only a few lattice spacings over a significant range of dopings in the vicinity of the Mott insulating state. We demonstrate that this healing effect is ultimately due to the suppression of charge fluctuations induced by Mottness. We also define and solve analytically a simplified yet accurate model of healing, within which we obtain simple expressions for quantities of direct experimental relevance, such as the healing length. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G47.00007: Effects of Hund's coupling on the Mott transition in multiorbital systems Aaram Joo Kim, Gun Sang Jeon, MooYoung Choi We study the phase transitions in the two-orbital Hubbard model having different orbital bandwidths, with emphasis on the effects of the Ising-type Hund's coupling. Within the dynamical mean-field theory combined with the continuous-time quantum Monte Carlo method, we confirm the existence of a non-Fermi-liquid for intermediate interactions. In contrast to the paradigmatic Mott transition in the single-band Hubbard model, a metallic phase is dominant over a localized Mott insulator at finite temperatures, resulting in the opposite slope of the phase boundary. We also investigate how the nature of the Mott transition between the non-Fermi liquid and the Mott insulator is affected by the variations in the Hund's coupling strength. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G47.00008: Phase separation in doped Mott insulators Chuck-Hou Yee, Leon Balents Motivated by the commonplace observation of Mott insulators away from integer filling, we construct a simple thermodynamic argument for phase separation in first-order doping-driven Mott transitions. The theory predicts the transition is percolative and should exhibit Coulomb frustration. As an application, we consider the titanate family of perovskites, an ideal test case since both the doping and correlation strength can be tuned. We compute the critical dopings required to drive the Mott transition using first-principles methods combined with dynamical mean-field theory, finding good agreement with experiment. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G47.00009: Random Field Driven Spatial Complexity at the Mott Transition in Vanadium Dioxide Shuo Liu, Benjamin Phillabaum, Erica Carlson, Karin Dahmen, Mumtaz Qazilbash, Dimitri Basov, Vidhyadhiraja Sudhindra Scanning near-field infrared microscopy on vanadium dioxide (VO$_2$) reveals the complex pattern formation associated with the temperature driven metal-insulator transition [1]. We apply recently developed cluster techniques [2] to the observed multiscale patterns of inhomogeneous local conductivity, quantifying the statistics of the sizes and shapes of the geometric metallic and insulating clusters through several measures characterized by critical exponents in the power law scaling, such as the cluster size distribution $\tau$, volume fractal dimension $d_v$ and hull fractal dimension $d_h$. These quantitative measures show power-law behavior over multiple decades, and the values of the extracted critical exponents indicate that the Mott critical end point is in the universality class of the random field ising model, revealing a delicate interplay between interactions and disorder in the material. The cluster techniques employed here can readily be applied to 2D image data in the context of other strong correlated systems and microscopy techniques for the study of critical behavior.\newline \par \noindent [1] M. M. Qazilbash {\it et al.}, {\it Science} {\bf 318}, 1750 (2007).\newline [2] B. Phillabaum, E. W. Carlson, and K. A. Dahmen, {\it Nat. Commun.} {\bf 3}, 915 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G47.00010: Nature of the Mott transition in the one- and two-dimensional Hubbard models Masanori Kohno The relationship between the single-particle excitation in the metallic phase and the spin excitation in the Mott insulating phase is discussed, based on the results for the one- and two-dimensional Hubbard models obtained by using the Bethe ansatz, dynamical density-matrix renormalization group method, and cluster perturbation theory [1,2]. By noting that the dispersion relation of the single-particle excitation in the zero-doping limit is directly related to that of the spin-wave excitation of the Mott insulator and that the spectral weight of the single-particle excitation in the electron addition spectrum gradually disappears toward the Mott transition, the nature of the Mott transition can be considered as freezing of the charge degrees of freedom, reflecting the spin-charge separation in the Mott insulator [1,2]. This feature is contrasted with the feature of a Fermi liquid and that of the transition between a band insulator and a metal. \\[4pt] [1] M. Kohno, Phys. Rev. Lett. 105, 106402 (2010). \\[0pt] [2] M. Kohno, Phys. Rev. Lett. 108, 076401 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G47.00011: Effect of electron-electron interactions on density of states singularities found in the Anderson model Rachel Wortis, Jayanayana Perera After Anderson first predicted localization in 1958, significant effort went into demonstrating that there is no singularity in the density of states associated with the mobility edge. It therefore came as a surprise when Johri and Bhatt[PRL {\bf 109} 076402 (2012)] recently uncovered the existence of a non-analyticity in the density of states near the band edge for systems with bounded disorder, in an energy range outside that captured by previous work. Moreover, they found that the singularity marks a transition to an energy range in which the DOS contributions come primarily from resonant states: states associated with clusters of sites of similar potential. While the work of Johri and Bhatt considers the traditional Anderson model without electron-electron interactions, there is currently significant interest in the effect of interactions on disordered systems. We therefore explore the effect a Hubbard $U$ interaction on the DOS feature found by Johri and Bhatt. We find that the original singularity persists at low values of $U$ but loses its sharpness at intermediate values, while new singularities associated with different types of resonance appear elsewhere in the spectrum. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G47.00012: Landau theory of Anderson localization and STM spectra in $Ga_{1-x} Mn_x As$ S. Mahmoudian, V. Dobrosavljevic, E. Miranda The recently developed Typical Medium Theory\footnote{V. Dobrosavljevi\'c, Int. J. Mod. Phys. B {\bf 24}, 1680 (2010).} provides the conceptually simplest order parameter description of Anderson localization by self-consistently calculating the geometrically-averaged (typical) local density of states (LDOS). Here we show how spatial correlations can also be captured within such a self-consistent theory, by utilizing the standard Landau method of allowing for (slow) spatial fluctuations of the order parameter, and performing an appropriate gradient expansion. Our theoretical results provide insight into recent STM experiments, which were used to visualize the spatially-fluctuating electronic wave functions near the metal insulator transition in $Ga_{1-x} Mn_x As$.\footnote{A. Richardella {\em et al.}, Science {\bf 327}, 665 (2010).} We show that, within our theory, all features of the experiment can be accounted for by considering a model of disorder renormalized by long-range Coulomb interactions. This includes the pseudogap formation, the $C(R)\sim 1/R$ form of the LDOS autocorrelations function, and the $\xi \sim 1/E$ energy dependence of the correlation length at criticality. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G47.00013: Strong disorder renormalization group study of Anderson localization H. Javan Mard, V. Dobrosavljevi\'c, J.A. Hoyos, E. Miranda We formulate a Strong Disorder Renormalization Group (SDRG) approach, to investigate $1D$ tight-binding models with simultaneous presence of random site energies and random hopping elements. We show that the beta function (describing the scaling properties of the conductance) can, under certain conditions, be obtained from an analytical solution of the appropriate SDRG flow equations, and we find excellent agreement with the results obtained from (numerically) exact transfer matrix calculations. We also show that, for the purposes of calculating the conductance, current conservation assures that the SDRG decimation represents an exact procedure for any amount of disorder. Our study demonstrates that the particle-hole symmetric model (no site disorder) represents an unstable but universal fixed point of the SDRG flows. In contrast, for the generic model where both disorder types are present, the system flows toward a {\em line of fixed points}, corresponding to different amounts initial (site) disorder, thus implying a non universal form of the beta function. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G47.00014: Dynamics close to the many-body localization transition Yevgeny Bar Lev (Krivolapov), David R. Reichman It has recently been suggested that in a generic class of disordered and (short-ranged) interacting quantum systems a dynamical metal-insulator transition may occur at finite temperatures. This proposed phenomenon is called many-body localization (MBL). In this work we study the real-time dynamics of this transition for a range of parameters where the transition should manifest according to theory and recent numerical studies. For this purpose, we numerically solve the non-equilibrium quantum kinetic equations in the self-consistent second-Born approximation, the same approximation used in the original prediction of MBL. For accessible times, we observe a complex sequence of dynamical regimes. Surprisingly we find little change of behavior upon crossing the putative dynamical phase boundary as determined by previous numerical studies. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G47.00015: Asymptotically Exact Scenario of Strong-Disorder Criticality in One-Dimensional Superfluids Lode Pollet, Nikolay Prokof'ev, Boris Svistunov We present a controlled rare-weak-link theory of the superfluid-to-Bose/Mott glass transition in one-dimensional disordered systems. The transition has Kosterlitz-Thouless critical properties but may occur at an arbitrary large value of the Luttinger parameter $K$. In contrast to the scenario by Altman {\it et al.} [Phys. Rev. B {\bf 81}, 174528 (2010)], the hydrodynamic description is valid under the correlation radius and defines criticality via the renormalization of microscopically weak links, along the lines of Kane and Fisher [Phys. Rev. Lett. {\bf 68}, 1220 (1992)]. The hallmark of the theory is the relation $K^{(c)}=1/\zeta$ between the critical value of the Luttinger parameter at macroscopic scales and the microscopic (irrenormalizable) exponent $\zeta$ describing the scaling $\propto 1/N^{1-\zeta}$ for the strength of the weakest link among the $N/L \gg 1$ disorder realizations in a system of fixed mesoscopic size $L$. [Preview Abstract] |
Session G48: Focus Session: Spin Transport and Magnetization Dynamics in Metal-Based Systems: Skyrmion I
Sponsoring Units: DMP FIAP GMAGChair: J.-Y. Lin, National Chiao Tung University
Room: Mile High Ballroom 1A
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G48.00001: The skyrmion phase in Mn$_{x}$Si revealed by the specific heat J.-Y. Lin, F.C. Chou We have measured the low temperature specific heat of Mn$_{x}$Si single crystals with two manganese contents, $x$ = 0.998 and 0.945. The skyrmion phase is manifested in the specific heat data. More importantly, it was found that the skyrmion phase is stabilized at $H$= 0 for $x$ = 0.998. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G48.00002: Electrical Detection of a Skyrmion State in Sputter Deposited MnSi Thin Films Brian Youngblood, Ilya Krivorotov We report a simple method of growing highly ordered B20-phase MnSi films on Si(111) by magnetron sputtering of a stoichiometric target and demonstrate that these films support a skyrmion phase. By measuring the DC magnetoresistance as a function of temperature and applied magnetic field we are able to map a portion of the phase diagram of the MnSi film. For a ~15nm film we find an extended region of stability (20K-35K) above 400G for the skyrmion phase, similar to that reported for MBE deposited films. The critical (Curie) temperature (~30K) at zero magnetic field is greater than that of thin free-standing single crystal samples and comparable to that of bulk MnSi. We also report measurements of the transverse conductivity in these films, including the topological Hall contribution due to the skyrmions. We further report the observation of temperature hysteresis of the resistance, indicating frustrated order coexisting with the skyrmion phase. Our work simplifies the fabrication of spintronic devices based on MnSi and the helimagnons and skyrmions it hosts. The skyrmions in particular are highly technologically relevant due to the ease with which they can be manipulated by injected angular momentum and their toplogically protected stability. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G48.00003: Magnetotransport properties of Fe doped MnSi under pressure Benjamin Chapman, Maxwell Grossnickle, Thomas Wolf, Minhyea Lee In an exotic magnetic phase known as a skyrmion lattice, the coupling between spin texture and conduction electrons produces enormous emergent magnetic fields, experimentally detectable with electrical transport measurements as evidenced in MnSi and similar B20 structured magnets. We present magnetotrasport data from a helimagnet Mn$_1-x$Fe$x$Si ($x$ = 0.09) under hydrostatic pressure. In this system, both pressure and Fe doping suppress long-range magnetic order while their effects on the saturated magnetic moment and helical pitch length are quite different. We discuss the size and characteristics of the topological Hall effect, a signature of these emergent magnetic fields, in pure and 9\% Fe doped MnSi under pressure, and compare to other samples with different Fe compositions at ambient pressure. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G48.00004: Non-Fermi liquid phase in metallic Skyrmion crystals Haruki Watanabe, Siddharth Parameswaran, Srinivas Raghu, Ashvin Vishwanath Motivated by reports of a non-Fermi liquid state in MnSi, we examine the effect of coupling phonons of an incommensurate skyrmion crystal (SkX) to conduction electrons. We find that non-Fermi liquid behavior emerges in both two and three dimensions over the entire phase, due to an anomalous electron-phonon coupling that is linked to the net skyrmion density. A small parameter, the spiral wave vector in lattice units, allows us to exercise analytic control and ignore Landau damping of phonons over a wide energy range. At the lowest energy scales the problem is similar to electrons coupled to a gauge field. The best prospects for realizing these effects is in short period skyrmion lattice systems such as MnGe or epitaxial MnSi films. We also compare our results with the unusual $T^{3/2}$ scaling of temperature dependent resistivity seen in high pressure experiments on MnSi. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G48.00005: Elastic properties of skyrmion crystal in MnSi Yoichi Nii, Akiko Kikkawa, Yasujiro Taguchi, Yoshihiro Iwasa, Yoshinori Tokura Recently magnetic skyrmion, discovered in several chiral magnets, has attracted strong attention. The particle-like objects crystallize in the form of triangular lattice, in analogy with magnetic flux lattice in type-II superconductors. Here, we report the elastic properties of MnSi by means of ultrasonic measurement. We have succeeded in detecting the skyrmion crystal (SkX) phase from elastic anomalies. A clear elastic hardening in the SkX phase signifies the elastic stiffness of skyrmion lattice on the background of crystal lattice stiffness. Moreover large ultrasonic absorption was confirmed at phase boundaries between SkX and intermediate phases. From these data, we mapped out characteristic phase diagram in the skyrmion system. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G48.00006: Thermally Driven Ratchet Motion of Skyrmion Microcrystal and Topological Magnon Hall Effect Masahito Mochizuki, Xiuzhen Yu, Shinichiro Seki, Naoya Kanazawa, Wataru Koshibae, Jiadong Zang, Maxim Mostovoy, Yoshinori Tokura, Naoto Nagaosa By means of the Lorentz TEM, we have found that micron-sized crystals of topologically nontrivial spin textures so called skyrmions in thin specimens of MnSi and Cu2OSeO3 show unidirectional rotations. Our numerical simulation based on a stochastic Landau-Lifshitz-Gilbert equation demonstrates that these rotations are driven sheerly by thermal fluctuations in the presence of temperature gradient. We show that hidden behind the skyrmion rotation is the rotational flow of magnons deflected by the effective magnetic field of skyrmions. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G48.00007: Theoretical Studies on Dynamical Phenomena of Magnetic Skyrmions Invited Speaker: Masahito Mochizuki We discuss our recent results of theoretical studies on the dynamical phenomena of magnetic skyrmions in chiral-lattice magnets. The topics are; \begin{enumerate} \item electric-current-induced dynamics of skyrmions \item resonant excitations and microwave diode effect of skyrmions \item electric-field generation of skyrmions. \end{enumerate} [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G48.00008: Current-induced skyrmion dynamics in constricted geometries Junichi Iwasaki, Masahito Mochizuki, Naoto Nagaosa Skyrmion is a vortex-like swirling spin structure with quantized topological number realized in chiral magnets, which can be driven by ultralow current density. For the skyrmion-based spintronic devices, we studied skyrmion motions in confined geometries by micromagnetic simulations. We found that the current-driven motion of skyrmions under the influence of geometrical boundaries is completely different from that in an infinite plane. In a channel of finite width, the confinement transverse to the direction of the current gives the steady-state characteristics of the skyrmion velocity as a function of current that are similar to those of domain walls in ferromagnets, whereas the transient behaviour depends on the initial distance of the skyrmion from the boundary and is distinct to skyrmion. Furthermore, we show that the creation of a single skyrmion can be controlled by an electric current in a simple constricted geometry comprising a plate-shaped specimen of suitable size and geometry.\\[4pt] [1] J. Iwasaki, M. Mochizuki and N. Nagaosa, Nature Nanotech. 8, 742 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G48.00009: Skyrmion motion induced by the spin Seebeck effect and ac current generation in chiral magnetic insulators Avadh Saxena, Shi-Zeng Lin, Cristian Batista, Charles Reichhardt Stable topological excitations such as domain walls, and vortices are ubiquitous in condensed matter systems and are responsible for many emergent phenomena. Recently a new mesoscopic spin texture called skyrmion with radius about 10 $\sim$ 100 nm was discovered experimentally in certain conducting as well as insulating chiral magnets. In the temperature-magnetic field phase diagram, skyrmions form a triangular lattice in the low temperature and intermediate magnetic field regime in thin films. Because of the low dissipation and the existence of magnetoelectric coupling, skyrmions in insulators have attracted considerable interests. In this work, we show that a thermal gradient can be used to move magnetic skyrmions in insulating chiral magnets: the induced magnon flow from the hot to the cold region drives the skyrmions in the opposite direction via a magnonic spin transfer torque. We also show that a temperature gradient induces an ac electric current in multiferroic insulators when the sample is embedded in a circuit. Both results are combined to compute the effect of skyrmion motion on the ac current generation. We demonstrate that skyrmions in insulators are a promising route for spin caloritronics applications. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G48.00010: Magnetization process of chiral magnet: creation and annihilation of skyrmions and anti-skyrmions Wataru Koshibae, Naoto Nagaosa By applying a magnetic field to the single-$q$ helical state in the chiral magnet, the skyrmions appear. The topology of the skyrmion is characterized by the skyrmion number $N_{sk}$ defined as $N_{sk} = \int \frac{d^2 r}{4 \pi} {\vec n}_{\vec r} \cdot [ ({ \partial {\vec n}_{\vec r}}/{\partial x})\times ({ \partial {\vec n}_{\vec r}}/{\partial y}) ],$ where ${\vec n}_{\vec r}$ is the unit vector along magnetic moment at ${\vec r}$, assuming the two-dimensional configuration. The single-$q$ helical state is a topologically trivial magnetic texture, i.e., $N_{sk}=0$. Therefore, within the continuous deformation, there is no way to realize the skyrmions from the single-$q$ helical state. We find, by a numerical simulation of Landau-Lifshitz-Gilbert equation, the pair nucleation of skyrmion and anti-skyrmion occurs, and they annihilate to reach the skyrmion crystal-like state or the ferromagnetic state. We show the lives of skyrmion and anti-skyrmion in the dynamics of a quenched chiral magnet. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G48.00011: Skyrmions: a showcase for non-Newtonian kinematics Aron Beekman, Naoto Nagaosa Consisting of hundreds or thousands of spins, skyrmions in magnets can nevertheless be regarded as individual particles that keep their identity due to topological protection. These particles interact with externally imposed waves like electric current or magnons. We have recently shown that they do this in a completely counterintuitive, non-Newtonian way. For instance, elastic scattering causes the skyrmion to move in the direction opposite to the incoming wave. The underlying reason is that the skyrmion momentum is descendant from the ferromagnetic dynamics, such that the skyrmion center of mass coordinates are each other's canonical conjugates. Here we argue that this is a general feature of dynamics of excitations in media with broken time-reversal symmetry, and is strongly related to the existence of Berry phases. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G48.00012: Topological Hall Effect in Skyrmions: A Nonequilibrium Coherent Transport Approach Gen Yin, Jiadong Zang, Roger Lake Skyrmion is a topological spin texture recently observed in many materials with broken inversion symmetry. In experiments, one effective method to detect the skyrmion crystal phase is the topological Hall measurement. At adiabatic approximation, previous theoretical studies show that the Hall signal is provided by an emergent magnetic field, which explains the topological Hall effect in the classical level. Motivated by the potential device application of skyrmions as digital bits, it is important to understand the topological Hall effect in the mesoscopic level, where the electron coherence should be considered. In this talk, we will discuss the quantum aspects of the topological Hall effect on a tight binding setup solved by nonequilibrium Green's function (NEGF). The charge distribution, Hall potential distribution, thermal broadening effect and the Hall resistivity are investigated in detail. The relation between the Hall resistance and the DM interaction is investigated. Driven by the spin transferred torque (SST), Skyrmion dynamics is previously studied within the adiabatic approximation. At the quantum transport level, this talk will also discuss the non-adiabatic effect in the skyrmion motion with the presence of the topological Hall effect. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G48.00013: Spin-Orbit Torques and Anisotropic Magnetization Damping in Skyrmion Crystals Kjetil Hals, Arne Brataas We theoretically study the effects of reactive and dissipative homogeneous spin-orbit torques and anisotropic damping on the current-driven skyrmion dynamics in cubic chiral magnets. Our results demonstrate that spin-orbit torques play a significant role in the current-induced skyrmion velocity. The dissipative spin-orbit torque generates a relativistic Magnus force on the skyrmions, whereas the reactive spin-orbit torque yields a correction to both the drift velocity along the current direction and the transverse velocity associated with the Magnus force. The spin-orbit torque corrections to the velocity scale linearly with the skyrmion size, which is inversely proportional to the spin-orbit coupling. Consequently, the reactive spin-orbit torque correction can be the same order of magnitude as the non-relativistic contribution. More importantly, the dissipative spin-orbit torque can be the dominant force that causes a deflected motion of the skyrmions if the torque exhibits a linear or quadratic relationship with the spin-orbit coupling. In addition, we demonstrate that the skyrmion velocity is determined by anisotropic magnetization damping parameters governed by the skyrmion size. [Preview Abstract] |
Session G49: Focus Session: Manganite and Cuprate Heterostructures
Sponsoring Units: DMPChair: Bernhard Keimer, Max Planck Institute for Solid State Research
Room: Mile High Ballroom 1C
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G49.00001: Unraveling electronic and magnetic structure at cuprate-manganite interfaces Invited Speaker: John Freeland Oxide interfaces offer a rich variety of physics and a pathway to create new classes of functional oxide materials. The interface between the cuprate high-temperature superconductors and ferromagnetic manganites is of particular interest due to the strongly antagonistic nature of the superconducting and ferromagnetic phases. Advancements in the synthesis of oxide heterostructure offers the opportunity to merge these two dissimilar oxides with atomic precision to understand the fundamental limits of bringing such states into close proximity. However, the main challenge is to understand the physical framework that describes the behavior of strongly correlated electrons near oxide interfaces. One aspect that will be addressed here is the use of advanced tools to gain detailed electronic and magnetic information from the boundary region[1-3]. In this talk, recent work will be addressed both in connection to visualizing the interface with spatially resolved tools [3] as well as harnessing layer-by-layer growth to explore the limits in ultrathin superlattices. These insights allow us to better understand the physics behind the interfacial spin and orbital reconstruction observed in this system [1,2]. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.\\[4pt] [1] J. Chakhalian et. al. Nature Physics 2, 244 (2006). \\[0pt] [2] J. Chakhalian et. al. Science 318, 1114 (2007).\\[0pt] [3] Te-Yu Chien et al. Nature Communications 4 2236 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G49.00002: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G49.00003: Microstructural mechanism for attenuation of superconductivity in manganite/cuprate thin-film heterostructures J.Y.T. Wei, H. Zhang, N. Gauquelin, G.A. Botton Anomalously long-ranged proximity effects have recently been reported in manganite/cuprate heterostructures, and attributed to spin-triplet correlations for odd-frequency pairing. To elucidate this exotic scenario microscopically, we studied multilayer $\mathrm{La_{2/3}Ca_{1/3}MnO_3/YBa_2Cu_3O_{7-\delta}}$ (LCMO/YBCO) thin films using scanning transmission electron microscopy (STEM), x-ray diffraction (XRD) and electrical transport [1]. The atomic-scale STEM data revealed double CuO-chain intergrowths which effectively form regions with the 247 lattice structure in the YBCO layer. These nanoscale 247 regions do not show up in XRD, but can physically account for the reduction in superconducting critical temperature ($T_c$) as a function of YBCO thickness. We also observed similar $T_c$ reduction in $\mathrm{LaNiO_3/YBCO}$ heterostructures, where $\mathrm{LaNiO_3}$ is also epitaxially-matched with YBCO but is not ferromagnetic. These results suggest that microstructural defects, rather than magnetism, are responsible for the attenuation of superconductivity occuring in manganite/cuprate heterostructures. [1] H. Zhang \textit{et al.}, Appl. Phys. Lett. \textbf{103}, 052606 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G49.00004: Induced Ferromagnetism at Interfaces between BiFeO$_3$ and YBa$_2$Cu$_3$O$_7$ Jian-Xin Zhu, Xiao-Dong Wen, J.T. Haraldsen, C. Panagopoulos, E.E.M. Chia Transition metal oxides (TMOs) exhibit many emergent phenomena ranging from high-temperature superconductivity and giant magnetoresistance to magnetism and ferroelectricity. In addition, when TMOs are interfaced with each other, new functionalities can arise, which are absent in individual components. Here, we report results from first-principles calculations on the magnetism at the BiFeO$_3$/YBa$_2$Cu$_3$O$_7$ interfaces. By comparing the total energy for various magnetic spin configurations inside BiFeO$_3$, we are able to show that the ferromagnetism is induced near the interface. We further develop an interface exchange-coupling model and place the extracted exchange coupling interaction strengths from the first-principles calculations, into a resultant generic phase diagram. The emergence of interfacial ferromagnetism should have implications to electronic and transport properties. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G49.00005: Emergent Interfacial Ferromagnetism in CaMnO$_{3}$-based Superlattices Invited Speaker: Alexander Grutter Interfaces of complex oxide materials provide a rich playground not only for the exploration of properties not found in the bulk constituents but also for the development of functional interfaces to be incorporated in spintronic applications. Emergent interfacial magnetic phenomena have been of great interest but surprisingly there have been few examples of emergent interfacial ferromagnetism. In this talk, I will describe our recent work on the stabilization of ferromagnetism in CaMnO$_{3}$-based superlattices. We have demonstrated ferromagnetism at the interface between the antiferromagnetic insulator CaMnO$_{3}$ and a paramagnetic metallic layer, including CaRuO$_{3}$ and LaNiO$_{3}$. Theoretically the ferromagnetism has been attributed to an interfacial double exchange interaction among the interfacial Mn ions that is mediated by itinerant electrons from the paramagnetic metallic layer. Through polarized neutron reflectivity and observation of exchange bias, we have demonstrated that the ferromagnetism comes from Mn ions in a single unit cell at the interfaces just as theory has predicted. We have also demonstrated that the metallicity of the paramagnetic layer is critical in stabilizing ferromagnetism at the interface and that the interfacial ferromagnetism can be suppressed by suppressing the metallicity of the paramagnetic layer. Despite the agreement with theory, there remain open questions as to the magnetic interactions among the interfacial ferromagnetic layers. For example, the saturated magnetic moment modulates as a function of the thickness of both the CaMnO$_{3}$ and paramagnetic metal layers. The origins of this oscillation are not well understood and may stem from either structural effects or long-range oscillatory magnetic coupling interactions reminiscent of RKKY interactions. Evidence of the doubling of the unit cell and long range antiferromagnetic correlations support these speculations. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G49.00006: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G49.00007: Fabrication and Crystal Structure of [REMO$_{3}$ /ABO$_{3}$] (A$=$Ca, La, B$=$Fe, Mn, RE$=$Bi, La, M$=$Fe, Fe$_{0.8}$Mn$_{0.2})$ Superlattices Grown by Pulsed Laser Deposition Method K. Takase, Y. Watabe, N. Iwata, T. Oikawa, T. Hashimoto, M. Huijben, G. Rijnders, H. Yamamoto The superlattices of [REMO$_{3}$/ABO$_{3}$] (RE$=$Bi, La, M$=$Fe, Fe$_{0.8}$Mn$_{0.2}$ A$=$Ca, La, B$=$Fe, Mn) were prepared by Pulsed laser deposition (PLD) method grown on SrTiO$_{3}$(STO)(100) for the novel materials which show ferromagnetic and ferroelectric properties with giant magnetoelectric effect at room temperature. When the superlattices were prepared, seven units LaFeO$_{3}$(LFO) film was deposited first, and the required pulses for other materials to grow seven units were calculated using the growth rate ratio and the growth rate of the last three units of LFO. One of the superlattices, [7 units - BiFe$_{0.8}$Mn$_{0.2}$O$_{3}$(BFMO) / 7 units - CaMnO$_{3}$(CMO)] stacking for 14 times, the satellite peaks from -2 to $+$1 were observed. From the fitting to the X-ray reflection spectra, thickness of BFMO and CMO in [BFMO/CMO] one cycle was 2.139nm (5.3 units) and 2.042nm (5.5 units). Although the deposited number of units was definitely less than seven, the satellite peaks are derived from the superstructure. Reciprocal space mapping shows the \textit{in-plain} lattice constant of [BFMO/CMO] superlattices was not fitted to that of substrate. The calculated \textit{in-plain} lattice parameter was 0.382 nm longer than the value of 0.3732 nm, which is the bulk CMO and \textit{in-plain} lattice parameter of CMO thin film grown on STO(001) substrate. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G49.00008: Interface-Induced Magnetic Coupling in Multiferroic/Ferromagnetic Bilayer: An Ultrafast Pump-Probe Study Elbert Chia, Chan La-o-vorakiat, Y.F. Tian, Tom Wu, Christos Panagopoulos, Jian-Xin Zhu, Haibin Su By use of optical pump-probe measurement, we study the relaxation dynamics of a muliferroic-ferromagnetic TbMnO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ bilayer. The relaxation dynamics of both layers are well separated in time allowing us to investigate the magnetic coupling across the bilayer. We observe that the relaxation dynamics of the individual layers in the bilayer sample are the result of the interplay between the intrinsic magnetic order and the induced interfacial effect. Our data suggest the existence of induced ferromagnetic order in the TbMnO$_{3}$ layer, and antiferromagnetic order in the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ layer. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G49.00009: Control of magnetism by tuning interfacial octahedral rotations in atomic-layer superlattices Xiaofang Zhai, Long Cheng, Yang Liu, Christian Schleputz, Hui Li, Xiaoqiang Zhang, Shengqi Chu, Lirong Zheng, Jing Zhang, Aidi Zhao, Hawoong Hong, Anand Bhattacharya, James Eckstein, Changgan Zeng It was recently predicted that structural reconstructions caused by discontinuities in the oxygen octahedral rotation (OOR) patterns at complex oxides interfaces are capable of shaping interface magnetic properties. However, experimental evidences for this interrelation are scarce. By combining state-of-the-art laser molecular-beam-epitaxy and synchrotron X-ray diffraction techniques, we demonstrate that interfacial OOR are closely linked to the strongly modified ferromagnetism (FM) in (LaMnO3plusd)N/(SrTiO3)N superlattices. The maximized FM moment in the N$=$6 superlattice is accompanied by charge-transfer and a metastable structure (Imcm) featuring minimal octahedral rotations. Quenched FM for N smaller than 4 superlattices is attributed to a substantially enhanced c-axis octahedral rotation (about 8 deg), a phenomenon that has been predicted theoretically but never observed experimentally. Our study demonstrates that engineering superlattices with controllable interfacial structures may be a new route in realizing functional magnetic materials. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G49.00010: What EELS Spectra Tell Us About Manganite/ferroelectric Interfaces: Ab Initio Results Alexandru Bogdan Georgescu, M.S.J. Marshall, A. Gulec, P.J. Philips, R.F. Klie, F.J. Walker, C.H. Ahn, Sohrab Ismail-Beigi The interplay of structure, electronic states, and magnetism is a rich field of research for transition metal oxides. Manganese oxides are well known for the dependence of their magnetic state and resistivity on doping (colossal magnetoresistance) which can be modified by chemical alloying or dynamically via the ferroelectric field effect. We focus on the interface between an oxide ferroelectric and the manganite La$_x$Sr$_{1-x}$MnO$_3$ where it has already been established that there is a dramatic coupling of atomic geometry, electronic structure and magnetism which leads to a very large magnetoelectric coupling. In this work, we use first principles theory to understand what observed atomically-resolved EELS spectra on such systems tell us about the interfacial structure and electronic properties. By understanding the link between EELS and atomic-scale structure at an oxide interface, one can then more confidently interpret experimental results to understand interfaces of novel materials, particularly correlated electron systems. [Preview Abstract] |
Session G50: Focus Session: Plasmon Enhanced Light: Matter Interactions
Sponsoring Units: DMPChair: Xiaoqin (Elaine) Li, University of Texas at Austin
Room: Mile High Ballroom 1D
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G50.00001: Metal Nanostructure-Multiexciton Interactions: From Emission Enhancement to Modification of Photon Statistics Invited Speaker: Han Htoon In the past decade, a tremendous amount of research efforts has been invested in the study of metal nanostructure (NM)-nanoemitter interactions. However, most of these studies have been conducted in the context of MNs interacting with single excitons. In contrast to these studies, we ventured into the realm of multi-exciton-MN interactions by performing low temperature photoluminescence (low-T PL) and photon-correlation spectroscopy studies on individual core/ultra-thick-shell NQDs (``giant''-NQDs or g-NQDs) deposited on nano-roughened silver films. Our low-T PL study show that (1) the multiexciton (MX) emissions in g-NQD coupled to silver films were enhanced mainly through the direct modification on the competition between the radiative and nonradiative recombination processes of MXs; and (2) strong enhancement in absorption is not necessary for a strong multiexciton emission.\footnote{\textit{J. Phys. Chem. Letts.} \textbf{4}, 1465-1470, (2013).} Our room temperature photon-correlation spectroscopy studies reveal that the MN-g-NQD interaction can transform sub-Poissonian photon emission statistics of individual g-NQDs to strong super-Poissonian statistics (photon-bunching).\footnote{\textit{Phys. Rev. Lett.} \textbf{110}, 117401, (2013).} We further derived the conditions required for the manifestation of this phenomenon and show that it can also manifest in other nanoemitters such as epitaxially grown QDs and single walled carbon nanotubes. The understandings attained in this work could open a new plasmonic route for manipulation of important multiexciton processes such as optical amplification, lasing and entangled-photon-pair generation. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G50.00002: Plasmon enhanced photoluminescence in dye doped films coupled to random metal aggregates Shikhadeep Gill, Miriam Deutsch We present a study of plasmon enhanced light emission from metal/dielectric composites comprising aggregated nanocrystalline silver islands deposited onto thin polymer films which have been doped with the amplifying dye Rhodamine 6G. We address the dependence of dye photoluminescence on the morphology of the metal aggregates as additional silver is added to the films. A frequency doubled Nd:YAG laser is used as the pump for exciting both the gain medium as well as localized surface plasmons in the metal aggregates. We exploit the intense plasmon fields localized to the metal islands to obtain greatly enhanced luminescence signals. In addition, the large scattering cross sections sustained by the metal nanoparticles serve to further increase light-molecule interaction in this system. We discuss the dependence of luminescence enhancement factors on the material's structural properties, and show that maximal signals are obtained when metal islands begin to coalesce. Using films in the coalescence regime, we then proceed to discuss the dependence of dye emission on pump power, and address the prospects for achieving plasmon-enhanced random lasing in these materials. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G50.00003: Giant fluorescence enhancement of fluorophores coupled to nanopatch antennas Maiken H. Mikkelsen, Alec Rose, Thang B. Hoang, Felicia McGuire, Jack J. Mock, Cristian Cirac\`i, David R. Smith Plasmonic cavities and nanoantennas have proven to be particularly attractive candidates for modifying the excitation and decay rates of nearby emitters. Here, we demonstrate giant enhancement of fluorescence in planar nanoparticles electromagnetically coupled to a metallic film, resembling nanopatch antennas. The antennas consist of colloidally synthesized silver nanocubes deposited over a 50 nm silver film. The cubes and film are separated by a $\sim 5$ nm selfassembled polyelectrolyte spacer layer, coated with a dilute layer of fluorophores (sulfo-cy5 carboxylic acid). By varying the size of the nanocubes, we tune the plasmonic resonance throughout the excitation and emission spectra of the embedded fluorophores, demonstrating a seamless transition between fluorescence enhancement and quenching. The experimentally observed behavior agrees well with performed finite-element simulations. Using this tunable platform, design rules for optimal enhancement are revealed, allowing us to demonstrate giant fluorescence enhancements and a significantly increased spontaneous emission rate. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G50.00004: Single particle optical investigation of gold shell enhanced upconverted fluorescence emission Kory Green, Shuang Fang Lim, Hans Hallen Upconverting nanoparticles (UCNPs) excited in the near IR offer novel advantages as fluorescent contrast agents, allowing for background free bio-imaging. However, their fluorescence brightness is hampered by low quantum efficiency due to the low absorption cross section of Ytterbium and Erbium ions in the near IR. We enhance the efficiency of these particles by investigating the plasmonic coupling of 30nm diameter core NaYF4: Yb, Er upconverting particles (UCNPs) with a gold shell coating. An enhancement of green emission by a factor of five and a three times overall increase in emission intensity has been achieved for single particle spectra. UV-Vis absorption has confirmed the surface plasmon resonance (SPR) of the gold shell to the near IR and transmission electron microscope (TEM) images demonstrates successful growth of a gold shell around the upconversion particle. Time-resolved spectroscopy shows that gold shell coupling changes the lifetime of the energy levels of the Erbium ion that are relevant to the emission process. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G50.00005: Extraordinary SPP propagation distance in epitaxially grown silver film Yanwen Wu, Chengdong Zhang, Jisun Kim, Matt Zhang, Liuyang Sun, Chih-Kang Shih, Xiaoqin Li, Yang Zhao, N. Mohammadi Estakhri, Xing Xiang Liu, Andrea Al\`u, Greg Pribil We measured greatly enhanced propagation distances of surface plasmon polaritons (SPPs) on an atomically-smooth epitaxial silver \textit{(Ag}) film beyond what was previously considered possible. These extraordinary propagation lengths approach the fundamental limit determined by the new optical constants measured in these films. We excited and detected the SPPs in reflection geometry. Light incident on a single groove launched the SPPs, which were subsequently detected at a series of output coupling slits with increasing distance from the launching site. We used incident wavelengths of 632nm and 880nm and extracted propagation distances of 22$\mu$m and 42$\mu$m, respectively. Calculations using the optical constants measured on the same film predict distances of 42$\mu$m at 63nm and 155$\mu$m at 880nm. The discrepancy is mainly due to scattering from the 1-2 monolayer fluctuations at the \textit{Ag} surface. The propagation distance extrapolated from our measurements far exceeds the speculated theoretical limit ($\sim$ 5X) in template stripping \textit{Ag} films. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G50.00006: Plasmonic Magnetic Nanostructure Farbod Shafiei, Francesco Monitcone, Khai Q. Le, Xing-Xiang Liu, Thomas Hartsfield, Andrea Alu, Xiaoqin Li AFM manipulation technique has been used to position individual 100 nm gold nanospheres into a subwavelength plasmonic metamolecule nanoring consisting of four closely spaced nanoparticles. This structure supported a strong magnetic response coupled to a broad electric responce in the visible range. Asymmetries in the assembled nanoring enable the interaction between electric and magnetic modes, leading to the first observation of a magnetic-based Fano scattering resonance at optical frequencies. Such a metamolecule is suitable building block for negative-index metamaterials. AFM manipulation technique gave us ability to modify the nanostructure in real time while we were monitoring the elctric and magnetic responses of the nanostructure. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G50.00007: Assembling Three-Dimensional Optical Stereo-Nanocircuits Jinwei Shi, Sarah Elias, Francesco Monticone, Yanwen Wu, Daniel Ratchford, Xiaoqin Li, Andrea Alu The development level of integration of photonic devices is lagging behind compared with microelectronics, due to diffraction limit and the difficulty of realizing basic functionalities with lumped photonic circuit elements at the nanoscale and achieving versatile operations by combining these elements in large circuits. Here we demonstrate the design, assembly and characterization of various 3D photonic nanocircuits with increasing complexity by accurately positioning a number of metallic and dielectric nanoparticles (NPs) in a reconfigurable way with atomic force microscope (AFM) manipulation, in analogy to what an electrical engineer does when putting together an electronic circuit. The NP clusters are shown to produce the designed spectral response, qualitatively predicted by simple circuit rules, with fixed optical lumped impedance value of each NP for different nanocircuit configurations. Additionally, such nanophotonic circuits exhibit stereo-functionality, i.e., a response that can be controlled by the polarization of impinging light. Our work represents an important step toward transplanting and extending the powerful design tools of electronic circuits to nanophotonic systems. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G50.00008: An ``optical diode'' based on surface plasmon cavity modes Michael J. Burns, Fan Ye, Michael J. Naughton We present the discovery and systematic study of a novel optical phenomenon that works like an ``optical diode,'' where the center of an optically thick circular Ag disk surrounded by step gap looks dark when observing in the far field from the top side, and appears bright when seeing from the bottom side. In both cases, the circular step gap circumference appears bright. We call the effect when observing from the top side the ``plasmonic halo,'' and the effect from the bottom side the ``reverse halo.'' In our previous work, we have demonstrated the physical nature of the ``plasmonic halo'' effect: modulation of transmission by the surface plasmon polariton (SPP) drumhead modes.\footnote{F. Ye, M. J. Burns, M. J. Naughton, \textit{Nano Lett}. \textbf{13}, 519-523 (2013).} Here we will explain the ``reverse halo'' effect by a three-step process: coupling from photons to SPPs, interference of SPPs forming cavity modes, and out coupling from SPPs to photons. Full-wave electromagnetic simulations based on finite element method support our theory. We have thus arrived at a thorough understanding of this ``optical diode'' effect, which could have potential applications in biomedical plasmonics, dielectric constant sensing, discrete optical filtering, and photonic logic, among others. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G50.00009: Modification of electric and magnetic dipole emission near plasmonic metal David Keene, Rabia Hussain, Natalia Noginova, Maxim Durach Strongly different behavior of magnetic and electric dipole spontaneous emission is observed near plasmonic metal at wavelengths close to the plasmon resonance range. Results are related to different coupling of the dipoles with plasmonic modes, and can be used to study and map modifications of local optical fields in plasmonic systems. We visualize the effects using a simple microscope setup and provide a theoretical description of the effects observed in a planar geometry, based on the dyadic Green's function approach for a layered medium. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G50.00010: Control of plasmonic coupling and radiative emission in tip-enhanced photoluminescence Vasily Kravtsov, Samuel Berweger, Joanna M. Atkin, Markus B. Raschke We study plasmon enhanced photoluminescence (PL) as a probe of local fields and plasmonic coupling in metallic nanostructures and nanogaps, and to gain microscopic insight into the mechanisms of tip--sample coupling in tip-enhanced spectroscopy in particular, and the behavior of radiative emitters in the proximity of interfaces in general. For that purpose we measure the laser induced PL response of a nanogap formed by a sharp Au tip and a flat Au sample surface, with distance precisely controlled using shear force feedback. We find three different distance regimes for the PL spectral behavior, characterized by non-monotonic changes in the PL intensity and linewidth, as well as the shift of the emission peak position, on a scale of several nanometers. Through relating the PL signal to the underlying mechanism of plasmonic enhancement, we describe the behavior of the plasmonic resonance of a nanogap of a varying size, where the weak dipole coupling gradually transforms into the higher order multipole and charge transfer coupling modes. The role of the plasmon nonlocality and the influence of the dielectric surface layer are discussed. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G50.00011: Experimental demonstration of photon-dipole interactions in quantum dot emission Yikuan Wang, Tianyu Yan, Mark T. Tuominen Light emission occurs spontaneously when electrons at excited states transit to ground state. One may wonder if there is an interaction at play between the atom and the radiation field during light emission. Although theoretical attempts were made as early as 1927, so far this interaction has not been taken into account in the calculation of emission properties of matter because of the lack of recognizable photon-atom interaction in a decisive experiment. Here we show that photoluminescence decay rates of semiconductor quantum dots affected by surface plasmons are dependent on detection angle and polarization of photons, as a result of photon-dipole interactions. Our results demonstrate how a dipole emitter interact with the photon field in the spontaneous emission process, thus provide a basis for controlling light emission through dipole orientation of molecules. This work will directly influence the future design of molecular emitting devices. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G50.00012: Dynamics of Excitons in Bare and Organic/Metal coated InP Nanowires Masoud Kaveh, Qiang Gao, Chaennupati Jagadish, Gerd Duscher, Hans-Peter Wagner We investigate the exciton dynamics in bare and organic/metal coated wurzite/zincblende (WZ/ZB) InP nanowires (NW) by temperature-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL). Aluminum quinoline (Alq$_{\mathrm{3}})$ as well as Alq$_{\mathrm{3}}$/Mg:Ag covered NW heterostructures are fabricated by organic molecular beam deposition. PL measurements on bare InP nanowires at 15 K reveal two emission bands at 1.45, and 1.48 eV originating from indirect WZ/ZB and point-defect (PD) trapped excitons, respectively. TR PL traces show an approximately single exponential decay for PD trapped excitons with a lifetime of 2 ns and biexponential decay for indirect WZ/ZB excitons with lifetimes of 3.3 ns and 14 ns. In Alq$_{\mathrm{3}}$ covered NWs we observe a stronger emission from both exciton transitions and longer decay times for indirect excitons indicating surface state passivation at the Alq$_{\mathrm{3}}$/NW interface. In Alq$_{\mathrm{3}}$/Mg:Ag NWs the PD trapped exciton emission is notably reduced which is attributed to a fast energy-transfer from free excitons in the WZ segments to plasmon oscillations in the metal film. The emission from indirect excitons is still comparable to the PL yield of bare NWs. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G50.00013: Broadband wide angle nearly perfect absorption with polarization-independence in plasmonic absorber based on multiple surface plasmon resonances Sze Fung Lee, King Chun Lai, Kin Wah Yu Nearly perfect absorption (NPA) of electromagnetic waves is useful in building photovoltaic cells, sensors and filters etc. Therefore designs of nearly perfect absorber based on different geometries have been proposed to yield NPA, with different operating frequency bandwidths, sensitivities to angle of incidence (AOI) and polarization of incident light. In this work, a design of broadband, wide AOI and polarization-independent nearly perfect absorber is proposed. The absorber is composed of a composite layer which generates multiple resonance, coated on a reflecting metal. The absorbance is computed to confirm NPA for TM and TE modes. The absorbance depends also on the thickness of the composite layer and this dependence is explained by the hybridized surface plasmon polariton (HSPP) formed inside the absorber. Particularly for TE mode, the fast HSPP wave (with phase velocity larger than the speed of light in vacuum) which coupled efficiently with incident light, can only be generated for proper thickness of layer. The proposed absorber can be one of the candidates for building light harvesting devices because of its efficient energy collection. [Preview Abstract] |
Session G51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties V
Sponsoring Units: DMPChair: Justin Song, Harvard University
Room: Mile High Ballroom 1E
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G51.00001: Revealing the atomic, electronic and optical properties of two-dimensional Van der Waals heterostructures Leonardo Basile, Juan-Carlos Idrobo In this talk, we will present a study of the optical absorption of graphene on hexagonal boron nitride (h-BN) using a combination of first-principles calculations and aberration-corrected scanning transmission electron microscopy (STEM). We will show the emergence of a novel interesting electron-optical phenomenon present on 2D heterostructures. Specifically, the absorption spectrum of a graphene layer on a h-BN layer under illumination with a dichroic signal was calculated. The results indicate that the rotation angle between graphene and h-BN layers can be used as a tuning variable to achieve valley polarization, that is, to localize electrons to specific momentum valleys. We will discuss how the emergent field of valleytronics, in 2D heterostructures, can be accessed at the atomic scale using a monochromated aberration-corrected STEM and novel vortex electron probes carrying orbital angular momentum. This research was supported by the National Secretariat of Higher Education, Science, Technology and Innovation of Ecuador (SENESCYT) (LB), and the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy (JCI). [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G51.00002: Probing Pseudospin-mixing Potential in Graphene/Boron Nitride Moire Superlattice by Infrared Spectroscopy Zhiwen Shi, Chenhao Jin, Wei Yang, Long Ju, Jason Horng, Guangyu Zhang, Feng Wang Graphene/boron nitride (BN) Moire superlattice, where the slow superlattice period is superimposed on a fast B-N oscillation, provides an attractive approach to engineer the electron pseudospin in graphene that goes beyond an electrostatic. Here we perform micro-infrared spectroscopy on graphene/BN superlattice, and find that the BN substrate effect in the context of pseudo-spin is much richer than an electrostatic potential. We further show that the BN substrate effect can be modified through electrostatic gating. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G51.00003: Landau level spectroscopy of incompressible quantum hall states in BN/bilayer graphene/NbSe2 heterostructures Dmitri Efetov, Clevin Handschin, Lei Wang, Cory Dean, James Hone, Philip Kim Inducing Superconductivity (SC) via proximity effect into the topological edge states of a 2-dimensional (2D) conductor in the Quantum Hall Regime (QHE) has been a long standing proposition which has recently reinvigorated attention. Such devices would allow to study the proximity effect in the ballistic 2D limit, where predictions go as far as specular Andreev Reflections and formation of Andreev Edge States in strong magnetic fields. Here we present a new route of fabrication of such devices made entirely out of cleanly stacked layered van der Waals materials BN/Graphene/NbSe$_{\mathrm{2}}$. Electric contact between NbSe$_{\mathrm{2\thinspace }}$and high mobility BN/graphene channels allows us to perform the Andreev reflection spectroscopy in the fully developed Quantum Hall states. We find that the NbSe2/graphene superconductor-normal metal interface (SN) has a very high transparency with extremely low electrical resistances of R$\sim $100Ohm and gives rise to Andreev reflections in graphene below the critical superconducting transition temperature. The high mobility of the graphene on h-BN and the relatively high upper critical magnetic field of NbSe2 provide a wide magnetic field range where the SC and the QHE coexist. We observe a clear enhancement of the Andreev Reflection probability when Cooper Pairs are injected into the incompressible Quantum Hall states. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G51.00004: Landau levels of graphene on h-BN probed by magneto-optics Zhiguo Chen, Zhiwen Shi, Wei Yang, Guangyu Zhang, Feng Wang, Zhiqiang Li Hexagonal boron nitride (h-BN) is an ideal substrate for achieving high-mobility graphene devices due to its atomically flat and clean surface. Moreover, the coupling between h-BN and graphene at small twist angles gives rise to a long-range moire supperlattice potential, which can significantly modify the electronic properties of graphene. Here, we will present infrared transmission measurements on graphene on h-BN in high magnetic fields applied perpendicular to the samples. Several inter-Landau-level transitions of graphene on h-BN were observed in fields, which exhibit pronounced deviations from the SQRT(B) field dependence for Landau levels of bare graphene. We will discuss possible mechanisms for the modifications of Landau levels of graphene by h-BN. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G51.00005: Hofstadter's Butterfly in a Lithographically Defined Super-Lattice Carlos Forsythe, Diego Scarabelli, Patrick Maher, Kenji Watanabe, Takashi Taniguchi, Shalom Wind, Cory Dean, Philip Kim We will present magnetotransport measurements of a gated superlattice formed on hBN encapsulated graphene in which a local gate provides a lattice-like external potential. Recent improvements in lithography have allowed for the patterning of periodic lattice structures with lattice constants of 40 nm or less. While the Hofstadter fractal energy spectrum has been studied previously through the Moire interference pattern between graphene and hBN, it has not been observed in such a tunable environment. Our multi-gate structure allows us to independently set carrier density and lattice strength, allowing us to investigate the competition between multiple allowed states associated with different lattices in our 2DEG. By comparing different lattice geometries we will also present a richer understanding of how lattice symmetry alters the Hofstadter energy spectrum. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G51.00006: Substrate Coupling, Transport, and Exchange Interactions of Graphene on Boron Nitride Ashley DaSilva, Jeil Jung, Shaffique Adam, Allan MacDonald Boron nitride has proven to be a nearly ideal substrate for high quality graphene devices. Graphene sheet electrons are nevertheless coupled to the boron nitride by Coulomb interactions with boron and nitrogen ions, and by weak distortions of the graphene sheet bonding structure. Because the lattice constants of graphene and boron nitride differ and because the hexagonal lattices of the two sheets are usually misoriented, the substrate interaction produces a long-period moire pattern. We report on a theory of how the substrate interactions influence the electronic structure and the transport properties of the graphene sheet. Our theory is based on a low-energy effective model [1] of the graphene sheet which accounts for both electrostatic interactions and bonding pattern variations. We find that the conductivity has a minimum when the number of carriers per spin per moire period is an integer, and that exchange interactions have a large influence on the gap at the Dirac point. [1] Ab initio theory of moire bands in layered two-dimensional materials, J. Jung, A. Raoux, Z. H. Qiao and A. H. MacDonald, (submitted). [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G51.00007: Anomalous Topological Currents in Graphene Superlattices Polnop Samutpraphoot, Justin Song, Leonid Levitov Berry's phases naturally arise from the spinor structure of Dirac systems, yet observation of non-trivial Berry's phase effects in the transport characteristics of Dirac systems, such as the Valley Hal effect, has proved elusive. Recently, layered graphene heterostructures have emerged as a promising setting to observe novel electron dynamics. We will discuss how novel features in Berry's curvature arise in Graphene/h-BN superlattices to allow long range topological currents to develop. Non-intuitively, we find superlattice mini-bands that have non-trivial Valley Chern number even though the sub-lattice asymmetric potential oscillates in sign. This results in clear non-local transport signatures for the topological character of the bands formed in Graphene/h-BN heterostructures. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G51.00008: Fermi velocity renormalization in misoriented graphene on hexagonal boron nitride Mahesh Neupane, Darshana Wickramaratne, Supeng Ge, Gen Yin, Roger Lake The electronic structure, Fermi velocity, and bandgap are calculated for graphene on BN as a function of misorientation angle. The Fermi velocity of Bernal stacked graphene on BN increases to 1.6 $v_0$ where $v_0$ is the velocity of single-layer graphene. For misorientation angles ranging from 5 to 27 degrees, the Fermi velocities of the Dirac electrons in graphene are relatively insensitive to the angle with values ranging between 0.85 and 0.9 $v_0$. In addition, the bandgap at the Dirac point for rotated graphene on BN decreases by an order of magnitude compared to that of perfectly registered graphene on h-BN. This suggests a reduction in the interlayer coupling between the graphene and BN layers due to the rotation. Calculations are performed using density functional theory. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G51.00009: Moire bands in twisted double layer graphene separated by a hBN monolayer Luis Brey We consider double layer graphene separated by a hBN monolayer. The three layers have a relative twist between them. We obtain that although a single monolayer graphene is only slightly perturbed by a hBN layer, in this sandwich structure the two monolayers graphene become strongly coupled. At small twist angle, the Fermi velocity is significantly reduced with respect the isolated single layer value. We study the velocity renormalization as function of the band gap of hBN and the band offset with respect the graphene Dirac point. The coupling between the graphene monolayers is reduced when they are separated by two hBN layers. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G51.00010: Engineering Electronic Band Structure in Graphene Superlattices on Hexagonal Boron Nitride Guorui Chen, Mengqiao Sui, Yijun Yu, Wei Yang, Kenji Watanabe, Takashi Taniguchi, Guangyu Zhang, Yuanbo Zhang When subjected to a periodic potential, the Dirac fermion spectrum in graphene undergoes dramatic transformation. This makes it possible to engineer electronic band structure in graphene through the formation of Moir\'e patterns on hexagonal Boron Nitride (hBN) substrate. By varying the angle between graphene and hBN substrate, we are able to produce graphene superlattices with different period. We further probe the electronic structure of the graphene superlattices through electronic transport measurements. Vastly different band structures are observed in graphene superlattices with different Moir\'e wavelength, which is in agreement with our theoretical model. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G51.00011: Magneto-phonon resonances in exfoliated graphene on hexagonal boron nitride Christoph Neumann, Sven Reichardt, Marc Droegeler, Kenji Watanabe, Takashi Taniguchi, Slava V. Rotkin, Bernd Beschoten, Christoph Stampfer Raman microscopy has become a powerful and widespread tool in graphene research. An interesting scenario emerges when Raman microscopy is combined with magnetic fields, as transitions between distinct Landau levels can couple to the optical phonon modes responsible for the graphene G-Line, forming magneto-phonon resonances (MPRs). Here, we investigate exfoliated graphene flakes partly deposited on SiO$_{2}$ and partly on hexagonal boron nitride (hBN). Employing a confocal Raman setup with 500 nm spot size and variable magnetic field of up to 9 T, we compare the regions with different substrates. Distinct MPRs occur only in the graphene on hBN area. From the dominant MPR at around 3.7 T we extract an increased Fermi velocity of above 1.15 x 10$^{6}$ m/s, owing to very low doping in our samples. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G51.00012: Graphene ``butterflies'' G.L. Yu, A. Mishchenko, R. V. Gorbachev, L.A. Ponomarenko, R. Jalil, J.S. Tu, C. R. Woods, D.C. Elias, K.S. Novoselov, A.K. Geim By stacking different 2D crystals on top of each other, some astonishingly properties and new phenomena may be shown. Typically, when graphene is transferred onto atomically flat boron nitride substrate with a certain angel, a moire pattern may come into being. Within this superlattice structure, elections will rearrange themselves to make multiple clones of Dirac fermions. At higher field even more clones would be created, accordingly, the pattern of Hofstadter butterfly can turn out. Here, both of the resistive and capacitive measurements are used to research the Hofstadter spectrum experimentally. Resistive measurement shows the longitudinal conductivity has 1/B oscillations independent of carrier density, while the Hall Effect repeatedly changes its sign with B. Quantum capacitance measurement is employed to examine directly the density of states (DoS) in graphene superlattice devices and its evolution into a clear Hofstadter spectrum. In both case, self-similarity could be observed at the fractions where the magnetic sates begin to entwine, forming a Hofstadter-like pattern. While many more minigaps are observed in the capacitance measurement. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G51.00013: Scanning Tunneling Microscopy and Landau Level Spectroscopy of twisted graphene double layers on SiO$_{2}$ and hBN substrates Chih-Pin Lu, Eva Andrei, Guohong Li, Adina Luican, T. Taniguchi, K. Watanabe Electrons in Graphene, being confined within a one-atom thick crystal, are very sensitive to environmental disturbances. This makes it possible to engineer vertical heterostructures with designer electronic properties by stacking graphene together with other thin layers. In particular superposing two graphene layers twisted away from Bernal stacking mitigates the effect of substrate-induced random potential fluctuations and provides access to the intrinsic electronic properties near the Dirac point. We studied samples consisting of two stacked Graphene layers deposited on SiO$_{2}$ or BN and configured in a device geometry which allows varying the carrier density by gating across a 300nm layer of SiO$_{2}$. Using low temperature high-field Scanning Tunneling Microscopy and Landau level spectroscopy, we demonstrated that the random potential is significantly weakened compared to the case of single layer graphene deposited on the same substrate. As a result we were able to observe high quality Landau level spectra, comparable to those seen in graphene on graphite, starting at fields as low as 1T. We will report on the effect of isolated impurities on the Landau level spectra and on the evolution of Landau levels into edge states. Supported by DOE-FG02-99ER45742 and NSF DMR 1207108. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G51.00014: Hofstadter Butterfly Formation for Modulated Graphene Godfrey Gumbs, Andrii Iurov, Danhong Huang, Paula Fekete, Liubov Zhemchuzhna A two-dimensional (2D) periodic array of scatterers has been introduced to monolayer graphene in the presence of a uniform perpendicular magnetic field. The corresponding eigenvalue equation has been solved numerically to display the mixing of Landau orbits to form minibans. Comparison of the Hofstadter butterfly in graphene is made with that in modulated 2D electron gas (EG). Additionally, we calculated the \textit{density-of-states} in the low, intermediate and high magnetic field regimes. The results reflect the effect of Landau level structure in the three regimes and specifically the fractal structure at intermediate magnetic fields. [Preview Abstract] |
Session G52: Focus Session: Films and Crystals: Understanding Causes of Superconductivity
Sponsoring Units: DMPChair: Minghu Pan, Oak Ridge National Laboratory
Room: Mile High Ballroom 1F
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G52.00001: Infrared probe of the pseudogap in the BaFe$_{2}$As$_{2}$ high-$T_{c}$ superconductors Invited Speaker: Soonjae Moon The nature of the pseudogap of high-$T_{c}$ cuprate superconductors is one of the most significant problems in condensed matter physics. The central issue concerns whether the pseudogap is associated with the precursor superconductivity or other broken symmetry state. Regardless of the origin, the pseudogap is considered as an essential part of physics of unconventional cuprate superconductors. There is mounting evidence that the superconductivity of the Fe-based materials is also quite exotic. However, the spectroscopic manifestations of the pseudogap in the Fe-based superconductors remained elusive. We present the \textit{ab}-plane and $c$-axis infrared data of the prototypical pnictide system: the BaFe$_{2}$As$_{2}$ family. Our experiments have identified the hallmarks of the pseudogap that mirror the manifestations of the pseudogap in the underdoped cuprates. The evolution of the charge dynamics across the phase diagram suggests that the pseudogap is not directly related to precursor superconductivity but may be linked to antiferromagnetism. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G52.00002: Pressure induced metallic state in LaMnPO revealed via Infrared spectroscopy Kirk Post, Alex Goncharov, Jack Simonson, Daniel McNally, Zhiping Yin, Brian Chapler, Gabriel Kotliar, Meigan Aronson, Dimitri Basov We investigated the energy gap (E$_{Gap}$) of the antiferromagnetic insulator LaMnPO$_{1-x}$F$_{x}$ for x = 0.0, 0.04, as a function of temperature and pressure using infrared spectroscopy. The results obtained from these measurements show that the band gap shows no discontinuous change upon crossing the Ne\'el temperature of 375 K and is therefore, likely unrelated to the antiferromagnetic ordering. Despite the resilience of the band gap to temperature, the band gap is dramatically reduced with the application of pressure and fully collapses by 28 GPa for both samples. These measurements confirm theoretical work predicting the collapse of the band gap with pressure. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G52.00003: Strain and strain relaxation analysis of superconducting Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2}$As$_{2}$ films on various substrates Artemis Rafti, Q.Y. Lei, M. Golalikhani, W.K. Withanage, J. Qiu, M. Hambe, F. Williams, Q. Yang, D. Temple, E.D. Bauer, F. Ronning, Q.X. Jia, X.F. Wang, X.H. Chen, J.D. Weiss, E.E. Hellstrom, X.X. Xi We have grown high quality, optimally doped superconducting Ba(Fe$_{0.92}$Co$_{0.08})_{2}$As$_{2}$ films on SrTiO$_{3}$, (La, Sr)(Al, Ta)O$_{3}$, LaAlO$_{3}$, CaF$_{2}$ and BaF$_{2}$ substrates. The variation in lattice mismatch allows the study of epitaxial strain effects on the structural and transport properties of the films. Reciprocal space mapping has been employed for detailed strain and strain relaxation analysis of the Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2}$As$_{2}$ films on the different substrates. We observed large substrate dependant changes in both in plane and out of plane lattice parameters. Furthermore, the crystallinity of the grown films, the lattice constant and lattice volume evolution with strain and strain relaxation were investigated, revealing an epitaxial strain and strain relaxation dependence on the superconducting transition temperature. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G52.00004: ARPES investigations of single unit cell iron selenide James Lee, Felix Schmitt, Robert Moore, Steve Johnston, Yongtao Cui, Wei Li, Ming Yi, Zhongkai Liu, Makoto Hashimoto, Yan Zhang, Donghui Lu, Tom Devereaux, Dung-Hai Lee, Zhi-Xun Shen Recent spectroscopic measurements on single unit cell iron selenide (1UC FeSe) films have indicated the opening of a superconducting-like gap at temperatures near 65K. ~A current goal is to understand the cause of such a high gap-opening temperature in this system and its relation to superconductivity. Here we present in-situ angle-resolved photoemission studies of 1UC FeSe films grown via molecular beam epitaxy. We find signatures of strong coupling between the electrons in the FeSe and the phonons in the substrate, which manifest as replica bands in the spectra. The implications of this electron-phonon coupling on the Cooper-pairing interaction strength are discussed. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G52.00005: Interfacial Interplay Between Superconducting FeSe Films and Underlying Substrate Rob Moore The recent discovery of superconducting single unit cell iron selenide (FeSe) films on strontium titante (STO) substrates with significantly enhanced transition temperatures (Tc) has created a flurry of activity. Understanding the influence of the underlying substrate is paramount for fundamental understanding of the superconducting phenomena with a potential for breaking current Tc records. We have investigated the influence of different substrates and the interfacial structure utilizing thin films grown via molecular beam epitaxy (MBE) with in situ angle-resolved photoemission spectroscopy (ARPES) and low energy electron diffraction (LEED-IV) characterization. We will discuss the implications of substrates on the electronic and crystalline structure of the single FeSe unit cell. Our results help illuminate the interfacial coupling between these degrees of freedom and suggest mechanisms for Tc enhancement. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G52.00006: RIXS-probed spin excitations in one individual unit cell of ``214'' and ``123'' cuprate superconductors Giacomo Ghiringhelli, G. Dellea, L. Braicovich, M. Minola, M. Le Tacon, F. Baiutti, G. Cristiani, G. Logvenonv, B. Keimer, M. Salluzzo High quality ultrathin epitaxial films of nominally optimally doped cuprates preserve their transport properties down to few unit cells (uc). However at 2 uc the critical temperature ($T_c$) drops, and at 1 uc superconductivity is lost. The substrate and the protecting overlayer can induce strain and structural modifications, doping can be modified by charge transfer across the interfaces and oxygen content can be altered. We have used Cu L$_3$ resonant inelastic x-ray scattering (RIXS) to map the spin excitation spectra of optimally doped La$_{2-x}$Sr$_x$CuO$_4$ and NdBa$_2$Cu$_3$O$_7$ ultrathin individual films down to 1 uc. Paramagnons are present even in the thinnest films, but their energy and dispersion are significantly different than in thick films and bulk crystals. These results complete the recent findings on the robustness of paramagnons up to very high doping levels in Y123, Tl2201 [1] and La214 [2], and in CaCuO$_2$/SrTiO$_3$ superconducting superlattices [3].\\[4pt] [1] M. Le Tacon et al, Phys. Rev. B 88, 020501 (2013).\\[0pt] [2] P.M.P. Dean et al, Nat. Mater. 12, 1019 (2013).\\[0pt] [3] G. Dellea et al, unpublished. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:27PM |
G52.00007: Superconductivity and Critical Current of Iron-Based Superconductors in High Field Invited Speaker: Qiang Li Although high-temperature superconducting cuprates have been discovered for more than 26 years, high-field applications are still based on low-temperature superconductors (LTS), such as Nb$_{\mathrm{3}}$Sn. The high anisotropies, brittle textures and high manufacturing costs limit the applicability of the cuprates. Recently, we demonstrated that the iron superconductors, without most of the drawbacks of the cuprates, have a superior high-field performance over LTS at 4.2 K [Nat. Commun. \textbf{4}:1347 (2013); Rep. Prog. Phys. \textbf{74} 124510 (2011)]. In this presentation, I will discuss recent progress aimed at understanding the relationships between superconductivity, critical current, and nano-scaled structure defects in iron-based superconductors, with emphasis on the properties of superconducting iron chalcogenide films. Critical current densities $J_{\mathrm{c}}$ $\sim$ 10$^{\mathrm{7}}$ A/cm$^{\mathrm{2}}$ were observed in FeSe$_{\mathrm{0.5}}$Te$_{\mathrm{0.5}}$ films grown on CeO$_{\mathrm{2}}$ buffered single-crystalline and flexible metal substrates. These films are capable of carrying $J_{\mathrm{c}}$ exceeding 10$^{\mathrm{5}}$ A/cm$^{\mathrm{2}}$ under 30 T magnetic fields. Furthermore, we found that these films have significantly higher $T_{\mathrm{c}}$ (\textgreater 20K) as compared to bulk samples (bulk $T_{\mathrm{c}}$ $\sim$ 15 K) for the entire doping regime of FeSe$_{\mathrm{1-x}}$Te$_{\mathrm{x}}$. Structural analysis revealed that these films generally have significantly smaller c-axis and a-axis lattice constant than the bulk value, suggesting that the crystal structure changes have a dominating impact on the superconducting transition in iron-based superconductors. Large $J_{\mathrm{c}}$ enhancement can also be realized in iron based superconductors by irradiation with proton and heavy ions that opens a new avenue for a tailored landscape of effective vortex pinning defects. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G52.00008: Superconductivity in ultra-thin FeSe films L.Z. Deng, Y.Y. Xue, B. Lv, Z. Wu, L.L. Wang, X.C. Ma, Q.K. Xue, C.W. Chu The recent discovery of a high Tc above 50 K in FeSe unit-cell film in comparison with the 8 K in bulk FeSe has attracted much attention, which is proposed to be related to interface superconductivity. Meissner effect and zero resistivity are two critical evidence for the existence of superconductivity. Unfortunately, the Tc has mostly indirectly obtained from the energy gap measurements, and preliminary resistive and magnetic measurements. There has not been report of the observation of Meissner effect to provide the sufficient proof of superconductivity to date. This motivates our systematic magnetic investigation here. We have observed in the 1-4 unit-cell FeSe-films: 1) Meissner effect with extensive weak-links up to $\sim$ 20 K; 2) unconnected small superconducting patches up to $\sim$ 40 K; and 3) an unusual relaxation of the diamagnetic signal of unknown nature up to 80 K, all are consistent with our resistance results. Their implications on the high Tc superconductivity and the film growth will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G52.00009: A transparent superconductor: LiTi$_{2}$O$_{4}$ epitaxial films Taro Hitosugi, Takeo Ohsawa, Tsutomu Nojima, Ryota Shimizu, Naoomi Yamada, Susumu Shiraki A framework is presented for the transparent conducting mechanism of transparent conductor LiTi$_{2}$O$_{4}$. Within the Bardeen-Cooper-Schrieffer (BCS) theory, achieving high superconducting transition temperature ($T_{\mathrm{c}})$ requires large carrier density at Fermi energy. This requirement prohibits the emergence of transparent superconductivity at high temperature, since the large carrier density leads to the optical absorption in visible. However, we here demonstrate high optical transmittance in superconducting LiTi$_{2}$O$_{4}$(111) epitaxial with $T_{\mathrm{c}}$ exceeding 13 K. Photoemission studies, electron transport measurements and optical analysis reveal the key role of electron effective mass, shifting a plasma frequency to infrared region. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G52.00010: Fabrication and Transport Properties of FeSe Thin Films on CaF$_2$ Substrates with Increased $T_{\rm c}$ Fuyuki Nabeshima, Yoshinori Imai, Masafumi Hanawa, Ataru Ichinose, Ichiro Tsukada, Atsutaka Maeda Fe(Se,Te) has the simplest crystal structure among Fe-based superconductors. Superconducting transition temperature, $T_{\rm c}$, is strongly dependent on the applied pressure. Indeed, strained thin films of FeSe$_{0.5}$Te$_{0.5}$ have higher $T_{\rm c}$ than that of bulk crystals[1,2]. On the other hand, an end member, FeSe, shows large increase in $T_{\rm c}$ under pressure compared with Te-doped ones. However there is no report on increased $T_{\rm c}$ of FeSe thin films except for the interface-induced superconductivity[3]. In the presentation we will report on the first successful introduction of compressive strain in FeSe thin films using CaF$_2$ substrates. As a result, $T_{\rm c}^{\rm {zero}}$ reaches 11.4 K, which is about 1.5 times higher than that of bulk crystals[4]. We will also report on the transport properties of FeSe thin films on CaF$_2$ in the normal state including the THz conductivity and the Hall resistivity comparing them with the results of FeSe$_{0.5}$Te$_{0.5}$ films. [1] E. Bellingeri $et\ al$., APL {\bf 96} (2010) 102512. [2] I. Tsukada $et\ al$., APEX {\bf 4} (2011) 053101. [3] Q.-Y. Wang $et\ al$., Chin. Phys. Lett. {\bf 29} (2012) 037402. [4] F. Nabeshima $et\ al$., APL {\bf 103} (2013) 172602. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G52.00011: Magnetotransport and structure of Ba(Fe $_{\mathrm{1-x}}$ Co$_{\mathrm{x}})_{2}$As$_{2}$ ultrathin films Adele Ruosi, Sanghan Lee, T. Hernandez, Yanjun Ma, M.S. Rzchowski, C.B. Eom Since the discovery of superconductivity in iron-based materials significant progress has been made in the fabrication of high quality bulk and thin film materials to explore their intrinsic properties and evaluate novel device applications. For both pathways, the best crystalline quality and optimal superconducting properties are required. Here Co-doped Ba-122 thin films grown on various substrates and thicknesses down to 6 nm, have been investigated. Crystal structure analysis was used to investigate the Fe-As-Fe bond angle and the Fe-As distance, and magnetotransport measurements were used to evaluate the electronic characteristics of the thin films. In particular, we observe an anomalous Hall effect that depends on temperature and film thickness. Success in very thin film fabrication involving pnictides will serve to spur progress in heterostructured systems exhibiting novel interfacial phenomena and device applications. [Preview Abstract] |
Session G53: Focus Session: Diffusion, Quantum Size Effects, and Growth of Metals, Islands, and Grain Boundaries on Surfaces
Sponsoring Units: DMPChair: Harald Brune, Ecole Polytechnique Federale de Lausanne
Room: Mile High Ballroom 2C
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G53.00001: Collective Mass Transport in Ag/Ge(110) 1D Nanoisland Growth Shirley Chiang, Cory Mullet, Michael Tringides, Marshall van Zijll, Bret Stenger, Emilie Huffman, Dylan Lovinger The growth of Ag deposited on Ge(110) was studied with low energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). The LEEM studies showed the formation of long one-dimensional (1D) multi-height islands over the temperature range 430C-530C. During deposition, the length of the islands increases at a constant rate ($\sim$ 106 atoms/sec reaching $\sim$ 20 microns) and constant width (100-200nm) for 9ML total deposition. Stochastic diffusion cannot account for these very high island growth rates. Similarly when island decay is observed, it happens exceedingly fast and cannot be explained by uncorrelated detachment of Ag atoms. Both processes indicate a more collective mass transport, which must be related to the mobility of the wetting layer. STM images show the crystalline structure of the 1D Ag islands and also that the reconstructed regions between the islands consist of bare Ge; thus they confirm that the wetting layer provides the material for the islands to grow at these high rates. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G53.00002: Growth of fcc(111) Dy multi-height islands on 6H-SiC(0001) graphene Matthew Hershberger, Myron Hupalo, Patricia Thiel, Michael Tringides Graphene based spintronic devices require understanding the growth of magnetic metals. Rare earth metals have large bulk magnetic moments so they are good candidates for such applications, and it is important to identify their growth mode. Dysprosium was deposited on epitaxial graphene, prepared by thermally annealing 6H-SiC(0001). The majority of the grown islands have triangular instead of hexagonal shapes. This is observed both for single-layer islands nucleating at the top of incomplete islands and for fully completed multi-height islands. We analyze the island shape distribution and stacking sequence of successively grown islands to deduce that the Dy islands have fcc(111) structure, and that the triangular shapes result from asymmetric barriers to corner crossing. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G53.00003: QSE induced oscillatory electric field on stepped Pb(111) film and its influence on surface reactivity Xiaojie Liu, Cai-Zhuang Wang, Myron Hupalo, Hai-Qing Lin, Kai-Ming Ho, Michael Tringides When the thickness of ultrathin metal films is comparable to the Fermi wavelength , significant effects on the structure stability and the electronic properties emerge due to electron confinement. Using first-principles calculations, we showed that quantum size effects (QSE) can induce oscillatory electrostatic potential and thus alternating electric field on the surface of wedge-shaped Pb(111) films. The alternating electric field has significant influence adatom diffusion, leading to selective even or odd layer nucleation preference depending on the charge state of the adatom. This QSE induced alternatively modulated electric field is confirmed in growth experiments with the odd-layer preference of Mg adsorption on wedge-shaped Pb/Si(111) films. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G53.00004: 2014 Beller Lectureship: Quantum Size Effects: surface morphology and the stability of low dimensional structures Invited Speaker: Bene Poelsema Quantum Size Effects (QSE) play an important role in determining the surface morphology of certain epitaxial metal films on metal and semiconductor substrates. These give rise to distinct preferred film heights, imposed by a favourable relationship between the film's Fermi wave length and its interlayer spacing. QSE in thin films are usually observed on substrates with a surface projected band gap. We obtained evidence for QSE in Pb and Bi films deposited on Ni(111) [1]. For Pb on Ni(111) a classic QSE induced stabilization of specific Pb film thicknesses is obtained in a way very similar to Pb/Si(111). Slow heating of large QSE-stabilized Pb mesas leads to their ultrafast collapse and evidences collective motion of giant numbers of Pb atoms, resulting in mass transport rates much higher than expected form the activation energies derived from STM observations and DFT calculations for individual processes. For ultrathin Bi fims on Ni(111), the QSE lead to the evolution of distinctly different crystalline structures. No longer, a sequence of preferred heights with one unique crystal structure is observed, but rather the emergence of different crystalline structures with increasing thickness. This remarkable observation is attributed to the establishment of specific favourable relationships between the Fermi wave length and the interlayer spacing. The film's crystal structure is imposed by QSE, facilitated by the inclination of Bi towards allotropism. QSE not only lead to preferred thicknesses of thin layers (2D), as reported for various systems in the recent past, but can also lead to quantized nanowire lengths (1D), as we reported for Ir/Ge(001). The preferred Ir-nanowire lengths correspond to multiples of six unit cells and SPS measurements support the QSE induced nature of the length quantization. \\[4pt] [1] T. R. J. Bollmann, R. van Gastel, H. J. W. Zandvliet, and B. Poelsema, Phys. Rev. Let. 107, 136103 (2011); PRL 107, 176102 (2011). \\[0pt] [2] T. F. Mocking, P. Bampoulis, N. Oncel, B. Poelsema, Nature Commun. 4, 2387 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G53.00005: Long range repulsive interactions in Fe on epitaxial graphene Myron Hupalo, Xiaojie Liu, Steven Binz, Cai-Zhuang Wang, Wen-Cai Lu, Patricia Thiel, Kai-Ming Ho, Edward Conrad, Michael Tringides The understanding of metal nucleation on graphene is essential for promising future applications, especially of magnetic metals which can be used in spintronics. A common method to study the grown morphology is to measure the nucleated island density n as a function of growth parameters. Surprisingly the growth of Fe on graphene is found not to follow classical nucleation: n is unexpectedtly high, it increases continuously with the deposited amount $\theta $ and shows no temperature dependence. These unusual results indicate the presence of long range repulsive interactions. Kinetic Monte Carlo simulations and DFT calculations support this conclusion. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G53.00006: Tuning Fe nucleation morphology via charge doping of graphene substrate Wenmei Ming, Feng Liu Graphene with Fe deposition can be potentially used as magnetic storage device when Fe atoms assume island morphology, or as magnetic electric contact for spin injection in spintronics device when Fe atoms assume a uniform thin-film morphology. We propose that the Fe initial growth morphology on graphene can be tuned in a controllable manner via charge doping of graphene. On one hand, charge doping may either increase or decrease the bonding strength between Fe adatom and graphene, affecting Fe adsorption; on the other hand, it may modulate the Fe adatom--adatom interaction, affecting the Fe island nucleation. Using first principles calculations, we have investigated the following diverse effects of charge doping on Fe deposition on graphene as a function of doping concentration: (1) adatom adsorption energy; (2) local magnetic moment; (3) dipole moment; (4) elastic deformation energy; (5) adatom diffusion barrier; (6) adatom-adatom interaction. Furthermore, using kinetic Monte Carlo simulations augmented with first-principles parameters, we have studied the nucleation morphology of Fe deposition on the charge doped graphene. Our results shed new light on understanding and control of the growth morphology of metal atoms on graphene. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G53.00007: Diffusion of small Cu islands on the Ni(111) surface: Results of the self learning kinetic Monte Carlo(II) simulations Shree Ram Acharya, Syed Islamuddin Shah, Talat S. Rahman We have examined the diffusion of two dimensional Cu islands (up to 10 atoms) on the Ni(111) surface using Self-Learning Kinetic Monte Carlo (SLKMC-II)[1] method which allows occupancy of both fcc and hcp sites on the fcc(111) surface for the identification of local neighborhood of a diffusing atom.The SLKMC-II reveals various single-atom , multi-atoms and concerted processes on the fly and stores them in a database. Energy barriers for these processes are calculated using semi-empirical embedded-atom method potential. Here we discuss some of the novel processes and their energy barriers found during the simulations and compare them with those found for the diffusion of Cu islands on Cu(111). We also report temperature dependence of the diffusion constants and frequency of occurrence of single-atom, multi-atom and concerted processes for these islands. The size dependence of effective energy barriers derived from the Arrhenius plots is also discussed. [1].S.I.Shah, et al., J.Phys.: Condens. Matter 24(2012)354004 [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G53.00008: Further Developments in Characterizing Capture Zone Distributions (CZD) in Island Growth T.L. Einstein, Alberto Pimpinelli, Diego Luis Gonz\'alez As argued previously, analysis of the distribution of the areas of capture zones (i.e. proximity polygons [or Voronoi tesselations] with respect to island centers) is often the best way to extract the critical nucleus size in studies of epitaxial growth. For non-Poisson deposition (i.e. when island nucleation is not fully random) the areas of these Voronoi cells can be well described by the generalized Wigner distribution (GWD), particularly in the central region around the mean area where the distribution is largest. We discuss several recent applications to experimental systems, catelogued in a recent minireview,\footnote{TLE, AP, \& DLG, arXiv 1311.xxxx} showing how this perspective leads to insights about the critical nucleus size. In contrast, several (but not all) studies have shown that the GWD may not describe the numerical data from painstaking simulations in both tails.$^2$ We discuss some refinements that have been proposed, as well as scaling forms. Finally, we comment on applications to social phenomena. Emphasis is on very recent developments. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G53.00009: Inhibition of surface instabilities by combined action of electric fields and thermal gradients Lin Du, Dwaipayan Dasgupta, Georgios I. Sfyris, Dimitrios Maroudas Surface instabilities, such as the Asaro-Tiller-Grinfeld (ATG) and the Stranski-Krastanow (SK) instabilities, originating due to the competition between surface free energy and elastic strain energy, pose serious reliability problems for device fabrication. Elastic strain energy is stored in bulk-like crystalline solids due to externally applied or process-induced stress and in epitaxial thin films on substrates due to the lattice mismatch between the film and substrate materials. We demonstrate that proper application of sufficiently strong external field(s) can eliminate ATG and SK instabilities based on linear stability analysis according to a fully nonlinear three-dimensional model of driven surface morphological evolution. We find that the simultaneous action of an electric field and a thermal gradient, in conjunction with substrate engineering, is capable of reducing the critical external field strength requirement by several orders of magnitude. We also derive the conditions for the synergy or competition between the two external fields toward surface stabilization. We validate the linear stability theory by comparisons of its predictions with results of self-consistent dynamical simulations of electrically and thermally driven surface evolution. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G53.00010: Dislocation mechanisms in stressed crystals with surface effects Chi-Chin Wu, Joshua Crone, Lynn Munday Understanding dislocation properties in stressed crystals is the key for important processes in materials science, including the strengthening of metals and the stress relaxation during the growth of hetero-epitaxial structures. Despite existing experimental approaches and theories, many dislocation mechanisms with surface effects still remain elusive in experiments. Even though discrete dislocation dynamics (DDD) simulations are commonly employed to study dislocations, few demonstrate sufficient computational capabilities for massive dislocations with the combined effects of surfaces and stresses. Utilizing the Army's newly developed FED3 code, a DDD computation code coupled with finite elements, this work presents several dislocation mechanisms near different types of surfaces in finite domains. Our simulation models include dislocations in a bended metallic cantilever beam, near voids in stressed metals, as well as threading and misfit dislocations in as-grown semiconductor epitaxial layers and their quantitative inter-correlations to stress relaxation and surface instability. Our studies provide not only detailed physics of individual dislocation mechanisms, but also important collective dislocation properties such as dislocation densities and strain-stress profiles and their interactions with surfaces. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G53.00011: Unraveling micro-mechanisms of grain boundary migration using molecular dynamics simulation and reaction path techniques Shijing Lu, Donald Brenner Understanding grain boundary (GB) migration mechanisms plays a key role in understanding the deformation mechanics of nano-crystalline materials. Despite the many theories have been proposed, there still exists widespread disagreement in the research community. For instance, the normal direction diffusion model is often assumed in conventional grain growth models, but recent studies have suggested that shear-coupled grain boundary migration is dominate for high angle structures during stress driven dynamics. This study addresses the competition between the two mechanisms by using molecular dynamics simulations to characterize symmetric tilt grain boundary migration in response to an external driving force. The fundamental idea is to first determine an order parameters using principal coordinate analysis and then find the reaction pathways under different simulation conditions by minimum free energy path (MFEP) search techniques. Once the MFEP is found, the free energy profile for GB migration can be computed from thermodynamic integration. Our preliminary results show that migration behavior of a symmetric tilt grain boundary with various misorientation angles can be well represented by two order parameters, and surprisingly the MFEP for most misorientation GBs has a zigzag shape instead of the commonly observed a smoothed interface. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G53.00012: Investigation of Grain Boundary Grooving Using Phase Field Crystal Model Shang-Chun Lin, Kuo-An Wu Dynamics of Grain Boundary (GB) grooving plays an important role in microstructure evolution. Classical theory on GB grooving assumes the solid-solid interface as a homogeneous boundary where details of GB structures are ignored. This assumption clearly requires certain modifications for cases such as low angle GB. The advantage of phase field crystal (PFC) method is its capability to describe materials with atomic resolutions. We investigate how dislocations influence dihedral angle in low angle GB. Furthermore, we find interesting phenomena occur during GB grooving, such as grain rotation and dislocation translation, which provide an alternative way to control grain growth at the nanoscale. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G53.00013: Energetics of Cs in 3 grain boundary of 3C-SiC Pushpa Raghani Energetics of Cs defects at 3 grain boundaries of 3C-SiC has been studied using density functional theory to understand the role of the grain boundaries in Cs diffusion and its eventual release from the tristructural isotropic fuel particles (TRISO). Cs is shown to be much more stable at the 3 grain boundary than in bulk of SiC with a significant decrease (7 - 17 eV) in the formation energies at grain boundaries than in bulk. It is found to have even lower formation energies than those of Ag at the 3 grain boundaries, while this trend was opposite in the bulk SiC as demonstrated previously from similar density functional theory calculations. Based on these results, a possible route to control Cs release from SiC layer via grain-boundary-engineering is suggested. [Preview Abstract] |
Session G54: Theoretical Methods in Correlated Electron Magnetism
Sponsoring Units: GMAGChair: Yi Nina Li, Princeton University
Room: Mile High Ballroom 1B
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G54.00001: Exact Results of Itinerant Ferromagnetism in Multi-orbital Hubbard Systems Yi Nina Li, Elliott H. Lieb, Congjun Wu We study itinerant ferromagnetism in multi-orbital Hubbard models in certain two-dimensional square and three dimensional cubic lattices. In the strong coupling limit where doubly occupied orbitals are not allowed we prove that the fully spin-polarized states are the unique ground states, apart from the trivial spin degeneracies, for any generic filling factor {\$}0 \textless n \textless 2 (0 \textless n \textless 3){\$} in two (three) dimensions. This theorem applies to both certain d-orbital transition-metal oxide layer systems and the p-orbital bands with ultra-cold fermions in optical lattices [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G54.00002: Sign-problem free quantum Monte-Carlo simulations on itinerant ferromagnetism in multi-orbital band systems Shenglong Xu, Yi Nina Li, Congjun Wu In a recent paper by Li, Lieb and Wu, it has been proved recently that certain multiorbital Hubbard models exhibit ferromagnetic ground states in strong coupling limit at zero temperature and any generic fillings. In a suitably defined basis, it can be proved that the sign problem of quantum Monte-Carlo simulations is absent. Quantum Monte Carlo simulations are performed to investigate the nature of itinerant ferromagnetism in such systems. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G54.00003: Observation of Block Magnetic States within the Orbital-Selective Mott Regime of Multiorbital Hubbard Models Julian Rincon, Gonzalo Alvarez, Adriana Moreo, Elbio Dagotto The orbital-selective Mott phase (OSMP) of multiorbital Hubbard models has been extensively analyzed using static and dynamical mean-field approximations. In parallel, the existence of Block states (antiferromagnetically coupled ferromagnetic spin clusters) in Fe-based superconductors has also been much discussed. This effort uses numerically exact techniques in one-dimensional systems to show that the OSMP remains stable even in the presence of full quantum fluctuations. Our main result is the observation of Block states within the OSMP regime, connecting two seemingly independent areas of research, and providing analogies with the physics of Double-Exchange models. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G54.00004: Ultrasoft pseudopotentials and Hubbard U values for rare-earth elements (Re=La-Lu) guided by HSE06 calculations Mehmet Topsakal, Koichiro Umemoto, Renata Wentzcovitch The lanthanide series of the periodic table comprises fifteen members ranging from La to Lu - the rare-earth (Re) elements. They exhibit unique (and mostly unexplored) chemical properties depending on the fillings of 4f-orbitals. Due to strong electronic correlation, 4f valence electrons are incorrectly described by standard DFT functionals. In order to cope with these inefficiencies, the DFT+U method is often employed where Hubbard-type U is introduced into the standard DFT. Another approach is to use hybrid functionals. Both improve the treatment of strongly correlated electrons. However, DFT+U suffers from ambiguity of U while hybrid functionals suffer from extremely demanding computational costs. Here we provide Vanderbilt type ultrasoft pseudopotentials for Re elements with suggested U values allowing efficient plane-wave calculations. Hubbard U values are determined according to HSE06 calculations on Re-nitrides (ReN). Generated pseudopotentials were further tested on some Re-cobaltite (Re-CoO3) perovskites. Alternative pseudopotentials with f-electrons kept frozen in the core of pseudopotential are also provided and possible outcomes are addressed. We believe that these new pseudopotentials with suggested U values will allow further studies on rare-earth materials. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G54.00005: Atomic-scale magnetism of Fe and Co on a complex surface Barbara Jones, Shruba Gangopadhyay, Oliver Albertini Miniaturization is one of present challenges for development of future spintronic devices. Our goal is to exploit the unusual properties of magnetism of transition metal atoms on complex surfaces. In collaboration with Almaden's Scanning Tunneling Microscopy team, we use DFT+U to calculate the properties of transition atoms on nanolayers of insulator on top of a metal such as silver. In this talk we report the results of detailed calculations of Fe and Co on MgO/Ag. MgO is a common spintronic insulator, but in a nanolayer on metallic Ag, its behavior is not that of the bulk. We find that Fe and Co have very different local spin and charge interactions with this surface. Using an onsite Hubbard U parameter which we determine from first principles, we are able to study the variability of the magnetic moment and nature of bonding. The magnetic adatoms affect the surrounding interface layer in unexpected ways. We are able to obtain interesting insights which help us understand how magnetism propagates along surfaces as well as between interfaces. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G54.00006: Quantum Monte Carlo calculations of magnetic couplings in cuprates Kateryna Foyevtsova, Jaron Krogel, Jeongnim Kim, Fernando Reboredo Spin excitations are generally believed to play a fundamental role in the mechanism of high temperature superconductivity in cuprates. However, accurate description of the cuprates' magnetic properties and, in particular, calculation of spin exchange couplings have been a long-standing challenge to the electronic structure theory. While the quantum-mechanically more rigorous cluster methods suffer from finite-size effects, the density functional theory approach, on the other hand, is ambiguous due to a rich variety of approximations to the exchange-correlation functional available which often give very different numbers for the spin exchange constants. For example, in some cuprates the theoretically predicted values of the nearest-neighbor superexchange range from 1 eV (local density approximation) to 0.05 eV (periodic unrestricted Hartree Fock) [C. de Graaf \textit{et al}, PRB \textbf{63} 014404 (2000)]. We compute spin exchange constants with the fixed-node diffusion Monte Carlo method (FN-DMC). In one-dimensional cuprates, we find that the FN-DMC computed nearest-neighbor spin superexchange is in an excellent agreement with experiment. This both demonstrates that FN-DMC is capable of describing properly the magnetism of strongly correlated oxides as well as positions this technique as the method of choice for theoretical parameterization of spin models. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G54.00007: Valence-Bond Monte Carlo Study of the 1D t-J Model Julia Wildeboer, Nicholas Bonesteel, Anders Sandvik We show that the valence-bond Monte Carlo (VBMC) method [1] can be applied to the one-dimensional $t$-$J$ model. In this projector Monte Carlo approach, the ground state of the model is sampled directly from a generalized valence-bond basis consisting of states with fixed hole configurations and electron spins singlet correlated in pairs to form valence bonds. For $n < 0.6$, where $n$ is the number of electrons per site, as $J/t$ is increased from 0, the 1D $t$-$J$ exhibits a quantum phase transition at which a spin-gap opens, followed by a transition to a phase separated state for large $J/t$ [2]. Using VBMC, we calculate the valence-bond entanglement entropy [3] (roughly, the average number of valence bonds leaving a block of size $L$) as the system is tuned through the transition to the spin gap phase. [1] A. Sandvik, PRL 95, 207203 (2005). See, e.g., A. Moreno, A. Muramatsu, and S.R. Manmana, PRB 83, 205113 (2011). [3] F. Alet, S. Capponi, N. Laflorencie, and M. Mambrini, PRL 99, 117204 (2007). [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G54.00008: Perturbation energy as an alternative to the total energy calculations Andrey Kutepov, Vladimir Antropov, Mark van Schilfgaarde, Victor Antonov We analyze different approaches to determine the energy from a perturbation using modern electronic structure methods. We compare the energy of perturbation from standard perturbation theory with what is obtained directly in self consistent band structure methods. The method is applied for studies such perturbations as internal magnetic field and spin orbital coupling in solids. This method is further compared with integration over the coupling constant. Numerical tests have been performed for magnetic Fe and Gd systems using the local density approximation. The main advantage of present scheme is its usefulness in methods for strongly correlated electronic systems studies where total energy calculations are not always possible. Specific calculations are performed using self consistent quasiparticle GW and LDA+U calculations for MnBi where the right value of magnetic moment and sign/value of magnetic anisotropy as a function of temperature have been obtained. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G54.00009: Double expansion with respect to $U$ and $1/(N-1)$ for an SU($N$) impurity Anderson model Akira Oguri, Miyuki Awane We apply a new large-$N$ scheme for an SU($N$) impurity Anderson model [1,2] to the Green's function for finite frequency $\omega$ and finite Coulomb interaction $U$. This approach is essentially different from the conventional large-$N$ theories, such as the non-crossing approximation and its extensions which are based on a perturbation expansion in the hybridization strength $V$. Our expansion scheme, which uses $1/(N-1)$ and the scaled interaction $u \equiv (N-1)U$ as a set of two independent variables, gives the Hartree-Fock (HF) results at zeroth order. Then, to leading order in $1/(N-1)$ it describes the Hartree-Fock random phase approximation (HF-RPA). The higher-order corrections systematically describe the fluctuations beyond the HF-RPA. It was shown that the renormalized local-Fermi-liquid parameters, calculated up to order $1/(N-1)^2$, agree closely with the exact NRG results at $N=4$ where the degeneracy is still not so large [1,2]. We discuss the $\omega$ dependence of the Green's function to clarify both the low- and high-energy features. \\[4pt] [1] A.O., R.\ Sakano, and T.\ Fujii, PRB {\bf 84}, 113301 (2011).\\[0pt] [2] A.O., PRB {\bf 85}, 155404 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G54.00010: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G54.00011: Critical Exponents of Strongly Correlated Fermion Systems from Diagrammatic Multi-Scale Methods Andrey Antipov, Stefan Kirchner, Emanuel Gull The dynamical mean field theory (DMFT) has become the standard tool in describing strongly correlated electron materials. While it captures the quantum dynamics of local fields, it neglects spatial correlations. To describe e.g. anti-ferromagnetism, unconventional superconductivity or frustration a proper treatment of non-local correlations is necessary. Diagrammatic multi-scale approaches offer an elegant option to accomplish this: the difficult correlated part of the system is solved using a non-perturbative many-body method, whereas 'easier', 'weakly correlated' parts of the problem are tackled using a secondary perturbative scheme. Here we employ such a method, the dual fermion approach, to problems of charge ordering in Falicov-Kimball model [1] by constructing a systematic diagrammatic extension on top of DMFT. Near the critical point of the Falicov-Kimball model we study the interplay between charge excitations and long-range fluctuations. We show that such multi-scale approach is indeed capable of capturing the non mean-field nature of the critical point of the lattice model and correctly describes the transition to mean-field like behavior as the number of spatial dimensions increases. [1] A. Antipov, S. Kirchner, E. Gull, arXiv:1309.5976 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G54.00012: Representing highly excited eigenstates of many-body localized systems using matrix product states Bryan Clark, David Pekker Many-body localization remains a mysterious topic largely due to the lack of tools for describing highly excited eigenstates of interacting quantum systems. Matrix Product States (MPS) are a family of low-entanglement variational ansatz. Typically, excited states of many-body systems exhibit volume law scaling of the entanglement entropy and therefore cannot be efficiently described by an MPS of low bond-dimension. In many-body localized systems, though, the eigenstates generically have area law scaling suggesting the existence of an efficient MPS representation. Here we investigate how to to find these states. Our achievement opens a new numerical window on many-body localization. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G54.00013: The spin Drude weight in the spin-1/2 XXZ chain: a combined exact diagonalization and time-dependent DMRG study Fabian Heidrich-Meisner, Christoph Karrasch, Johannes Hauschild, Stephan Langer Various theoretical approaches predict a finite Drude weight for spin transport in the gapless phase of the spin-1/2 XXZ chain, suggesting ballistic transport properties. We compute the Drude weight at finite temperatures with two approaches: Time-dependent density matrix renormalization group simulations and exact diagonalization. For the latter, we present a detailed comparison of different schemes of evaluating finite-size data, namely either in a grand-canonical ensemble or in a canonical one. We argue that the grand-canonical data, obtained from averaging over all subspaces with different magnetizations, have a more systematic finite-size dependence than the canonical one. The results for D(T) from exact diagonalization and tDMRG are in good quantitative agreement in the massless phase [1]. Financial support from the DFG through FOR 912 is gratefully acknowledged. \\[4pt] [1] Karrasch, Hauschild, Langer, HM, Phys. Rev. B 87, 245128 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G54.00014: Magnetic properties of FeO: a DFT+DMFT study Peng Zhang, R.E. Cohen, Kristjan Haule The magnetic properties of the transition metal oxide FeO greatly effect its equation of state and elasticity, and thus have been of great interest [1-3]. But FeO is not treated well by density functional theory which makes it a metal, instead of an insulator at low pressures. Employing the newly developed method of the density functional theory plus dynamical mean field theory, the magnetic properties of FeO within a wide range of pressure and temperature are investigated. Relative to density functional theory, the local correlations of the Fe d-electrons is exactly included in the new method in a fully self-consistent way. Adopting the hybridization expansion continuous time quantum Monte Carlo method as the impurity solver, the ab initio calculated impurity magnetic susceptibility is inserted in the Bethe-Salpeter equation, to derive the bulk magnetic susceptibility. By exploring the antiferromagnetic ordering and the Neel temperature as a function of pressure and temperature, the magnetic phase diagram of FeO is plotted. Our preliminary results indicate $T_{N}=203.091K$ at V=540 $b.a.u.^{3}$ and $T_{N}=223.345K$ at V=520 $b.a.u.^{3}$. \\[4pt] [1] J. Badro et al, PRL, 83, 4101(1999). [2] M.P. Pasternak et al, PRL, 79, 5046 (1997). [3] A. Mattinla et al, PRL, 98, 196404(2007). [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G54.00015: Dynamical Mean-Field Approach to Core-Level Spectroscopy of NiO and its Insulating Character Atsushi Hariki, Takayuki Uozumi Core-level X-ray photoemission spectroscopy (XPS) is a powerful tool to investigate electronic structure of strongly correlated electron systems, such as 3$d$ transition metal oxides. In the Ni2$p $XPS of NiO, a characteristic double-peak structure has been observed in the 2$p_{3/2}$ main line, [1] which is considered to be related with the insulating property of NiO. However, previous studies contradicts each other for the spectral assignment of the double peaks. [1,2] Thus, a further investigation about the microscopic origin of the double peaks from a different viewpoint is required. In this talk, we investigate the double-peak structure using a framework, which was recently proposed in our research group, based on the dynamical mean-field theory (DMFT) under realistic crystal structure. We show that, besides the so-called nonlocal screening indicated by Van Veenendaal et al., [2] the antiferromagnetic ordering of NiO plays a crucial role of the formation of the double peaks. We conclude from the spectral analysis that the lowest first ionization state of NiO is given by an electron removal from the Zhang-Rice doublet band. \\[4pt] [1] M. Taguchi et al.: Phys. Rev. Lett. {\bf 100} (2008) 206401.\\[0pt] [2] M. A. van Veenendaal and G. A. Sawatzky: Phys. Rev. Lett. {\bf 70} (1993) 2459. [Preview Abstract] |
Session G55: Invited Session: Proximity Induced Ferromagnetism in Topological Insulators
Sponsoring Units: DCMPChair: Ilya Eremin, Ruhr University Bochum
Room: Four Seasons Ballroom 1
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G55.00001: Broken lattice-symmetry influence on electronic anisotropy and interface exchange-coupling-induced ferromagnetic state in TI thin films and heterostructures Invited Speaker: Jagadeesh Moodera Inducing an exchange gap locally on the Dirac surface states of a topological insulator (TI) is ideal for observing the predicted unique features such as the quantized topological magnetoelectric effect, half-integer quantized Hall effect, as well as to con?ne Majorana fermions. Our work experimentally demonstrated the proximity-induced interface ferromagnetism in a heterostructure combining a ferromagnetic insulator EuS layer with Bi2Se3, without introducing defects [1]. An exchange gap was observed to be induced on the surface of the TI. Extensive magnetic and magneto-transport (magnetoresistance and anomalous Hall effect) investigation of the heterostructures, including synchrotron interfacial (XAS and XMCD measurements) studies have shown the emergence of a ferromagnetic phase in TI, which is a step forward to unveiling the above exotic properties. Also, to understand the intrinsic properties of TI it is necessary to correlate structure with the exotic electronic properties as well as interaction with other materials. Molecular beam epitaxy (MBE) ideally allows us to engineer the system whereas using synchrotron and electron diffraction based experimental techniques helps us to investigate with atomic resolution. We will elucidate our studies on well-defined TI films and heterostructure, and the role of imperfections on the symmetry of the material that leads to internal atomic ordering by the decoration of the defects. Charge transport and mobility are seen to relate with film growth strain and relaxation as well as display strong directional dependence on the defect geometry. Work done in collaboration with Peng Wei, Ferhat Katmis and others. \\[4pt] [1] P. Wei, F. Katmis, B. A. Assaf, H. Steinberg, P. Jarillo-Herrero, D. Heiman, and J. S. Moodera, PRL, 110, 186807 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G55.00002: Emerging weak localization effects on a topological insulator--insulating ferromagnet (Bi$_{2}$Se$_{3}$-EuS) interface Invited Speaker: Qi Yang A topological insulator (TI) has a full energy gap in the bulk, and possesses gapless Dirac-like surface states. Because of time reversal symmetry, the surface states cannot be back-scattered by non-magnetic impurities [1]. When a thin magnetic layer is applied on the surface, a full insulating gap is opened, and an electric charge close to the surface is predicted to induce an image magnetic monopole [2]. To further elucidate the uniqueness of transport in the surface state of TI materials, and to investigate such predicted interplay with magnetic materials, we studied the interface between a thin film TI (Bi$_{2}$Se$_{3})$ and an insulating ferromagnet (EuS). While above the Curie temperature ($T_{C})$ of the EuS we observed positive magnetoresistance (MR), which is obtainted ubiquitously in similar TI thin films and interpreted as weak antilocalization (WAL) effects [3], below $T_{C}$ the MR becomes negative near zero field, clearly indicating a proximity effect between the TI and the IF [4]. This phenomenon is consistent with recent theories that predict weak localization (WL) effects in TIs resulted from gap-opening at their surface state Dirac point [5,6].\\[4pt] [1] X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. \textbf{83}, 1057 (2011).\\[0pt] [2] J. E. Moore, Nature \textbf{464}, 194 (2010). \\[0pt] [3] H.-T. He, G. Wang, T. Zhang, I.-K. Sou, G. K. L. Wong, J.-N. Wang, H.-Z. Lu, S.-Q. Shen, and F.-C. Zhang, Phys. Rev. Lett. \textbf{106}, 166805 (2011).\\[0pt] [4] Q. I. Yang, M. Dolev, L. Zhang, J. Zhao, A. D. Fried, E. Schemm, M. Liu, A. Palevski, A. F. Marshall, S. H. Risbud, and A. Kapitulnik, Phys. Rev. B \textbf{88}, 081407 (2013).\\[0pt] [5] I. Garate and L. Glazman, Phys. Rev. B \textbf{86}, 035422 (2012).\\[0pt] [6] H.-Z. Lu, J. Shi, and S.-Q. Shen, Phys. Rev. Lett. \textbf{107}, 076801 (2011). [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G55.00003: Weak localization and antilocalization in topological insulator thin films with coherent bulk-surface coupling Invited Speaker: Ion Garate This talk will review the theory and experiments concerning quantum corrections to conductivity in thin films of three dimensional topological insulators. In particular, I will discuss how the magnitude and sign of the low-field magnetoresistance are influenced by bulk-surface coupling, gate voltage, size quantization effects and magnetic order. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G55.00004: Proximity-induced magnetization dynamics, interaction effects, and phase transitions on a topological surface Invited Speaker: Flavio Nogueira When a ferromagnetic or antiferromagnetic insulating layer is grown on the surface of a three-dimensional topological insulator, ferromagnetic order is induced on its surface. In the case of an out-of-plane magnetization, the Dirac fermion surface states become gapped. Quantum fluctuations of the Dirac fermions significantly affect the dynamics of the magnetization at the interface between the topological insulator and the magnetic material. In this talk we will discuss different aspects of the proximity-induced effective action on the topological surface and show that the Coulomb interaction between the Dirac fermions play a crucial role. In the case of a ferromagnet inducing an out-of-plane magnetization, the magnetization dynamics is modified due to a fluctuation-induced Chern-Simons term in the effective action. Such a topological term leads to a topological magnetoelectric torque in the Landau-Lifshitz-Gilbert equation, which is coupled to a non-local Poisson equation for the fluctuating electric field associated to the Coulomb interation. At finite temperature this leads to a downwards shift of the Curie temperature on the topological surface relative to the Curie temperature of the ferromagnet in the absence of the topological insulator. We also analyze the influence of the chemical potential on the magnetization dynamics and phase transitions at finite temperature and show that a thermoelectric screening takes place depending on the magnitude of the fermionic gap. For the case of an in-plane magnetization, a quantum phase transition occurs as the strength of the Coulomb interaction is varied, leading in this way to a semimetal-insulator transition on the topological surface. A Chern-Simons term is generated only when the Coulomb interaction is large enough. In this case parity and time-reversal symmetries are spontaneously broken. A semimetal insulator transition also happens when the magnetic layer is antiferromagnetic. In this case there is quantum criticality with unconventional critical exponents, and the magnetic susceptibility features a large anomalous dimension. \\[4pt] [1] F. S. Nogueira and I. Eremin, Phys. Rev. Lett. {\bf 109}, 237293 (2012).\\[0pt] [2] F. S. Nogueira and I. Eremin, Phys. Rev. B {\bf 88}, 085126 (2013).\\[0pt] [3] F. S. Nogueira and I. Eremin, arXiv:1309.3451. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G55.00005: Thin-Film Magnetization Dynamics on the Surface of a Topological Insulator Invited Speaker: Yaroslav Tserkovnyak We theoretically study the magnetization dynamics of a thin ferromagnetic film exchange coupled with a surface of a strong three-dimensional topological insulator. We focus on the role of electronic zero modes imprinted by domain walls (DWs) or other topological textures in the magnetic film. Thermodynamically reciprocal hydrodynamic equations of motion are derived for the DW responding to electronic spin torques, on the one hand, and fictitious electromotive forces in the electronic chiral mode fomented by the DW, on the other. An experimental realization illustrating this physics is proposed based on a ferromagnetic strip, which cuts the topological insulator surface into two gapless regions. In the presence of a ferromagnetic DW, a chiral mode transverse to the magnetic strip acts as a dissipative interconnect, which is itself a dynamic object that controls (and, inversely, responds to) the magnetization dynamics. [Preview Abstract] |
Session G58: APS Corporate Reform, Open Access Publishing, and the Future of Open Data
Sponsoring Units: APSRoom: 601
Tuesday, March 4, 2014 11:30AM - 11:50AM |
G58.00001: APS Corporate Reform Malcolm Beasley |
Tuesday, March 4, 2014 11:50AM - 12:50PM |
G58.00002: Town Hall Meeting: Open Access and Open Data: What They Mean for APS Journals Joseph Serene, Michael Lubell For a number of years Congress and the White House have been exploring mechanisms to make scientific literature more available to the public. A mandate for achieving that goal appeared in the 2010 America COMPETES Act, and since then Executive Branch Agencies have been actively developing plans to provide such access. We will review the status of the federal plans and discuss the implications for authors and publishers. We will also report on the status of plans for requiring authors to make available publicly data upon which their published results are based. [Preview Abstract] |
Session G60: Graduate Student Lunch With The Experts
Sponsoring Units: SPSRoom: Four Searsons Ballroom 2-3
Tuesday, March 4, 2014 12:30PM - 2:00PM |
G60.00001: Graduate Students Lunch with the Experts Graduate students may sign up to enjoy a complimentary box-lunch while participating in an informal discussion with an expert on a topic of interest to them. [Preview Abstract] |
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