Bulletin of the American Physical Society
APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012; Boston, Massachusetts
Session L1: Focus Session: Imaging and Modifying Materials at the Limits of Space and Time Resolution
Sponsoring Units: DMP GIMSChair: Bernd Kabius, Pacific Northwest National Laboratory
Room: 203
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L1.00001: In-Situ Transmission Electron Microscopy with Nanosecond Temporal Resolution Invited Speaker: Nigel Browning The dynamic transmission electron microscope (DTEM) can obtain both high spatial ($\sim $1nm or better) and high temporal ($\sim $1$\mu $s or faster) resolution. The high temporal resolution is achieved by using a short pulse laser to create the pulse of electrons through photo-emission. This pulse of electrons is propagated down the microscope column in the same way as in a conventional high-resolution TEM. The only difference is that the spatial resolution is limited by the electron-electron interactions in the pulse (a typical 10ns pulse contains $\sim $10$^{9}$ electrons). To synchronize this pulse of electrons with a particular dynamic event, a second laser is used to ``drive'' the sample a defined time interval prior to the arrival of the laser pulse. The important aspect of the DTEM is that one pulse of electrons is used to form the whole image, allowing irreversible transitions and cumulative phenomena such as nucleation and growth, to be studied directly in the microscope. The use of the drive laser for fast heating of the specimen presents differences and several advantages over conventional resistive heating \textit{in-situ} TEM -- such as the ability to drive the sample into non-equilibrium states. So far, the drive laser has been used for \textit{in-situ} processing of nanoscale materials, rapid and high temperature phase transformations, and controlled thermal activation of materials. In this presentation, a summary of the development of the DTEM and in-situ stages to control the environment around the specimen will be described. Particular attention will be paid to the potential for gas stages to study catalytic processes and liquid stages to study biological specimens in their live hydrated states. The future potential improvements in spatial and temporal resolution that can be expected through the implementation of upgrades to the lasers, electron optics and detectors will also be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L1.00002: Design and performance of a Near Ultra High Vacuum Helium Ion Microscope Bene Poelsema, Raoul van Gastel, Gregor Hlawacek, Harold J.W. Zandvliet The advent of He Ion Microscopy (HIM) as a new nanoscopic technique to image materials has enabled a new look at materials that is based on the interaction of swift light ions with matter. Initial HIM instruments have demonstrated high-resolution imaging, combined with great surface sensitivity, the ability to neutralize charge very efficiently, and with enhanced materials contrast when ion induced secondary electrons are used for imaging. To achieve ultimate performance, the chamber vacuum of the existing platform may be improved. For instance, carbon deposits due to beam interaction are readily seen due to the surface sensitivity of the technique. At high current densities the sharply focused beam may very efficiently decompose residual hydrocarbons. Not only can this obscure a clear view of the sample, thereby negating the benefits of the small spot size, it also limits the available acquisition time. This has proven extremely useful for nanopatterning for sensors, and other device fabrication applications at the sub-10nm level. However, it is undesirable when the instrument is used for materials characterization. We will discuss the basic considerations that went into the design of a Near-UHV He Ion Microscope [1]. First applications that the instrument was used for will be highlighted and its impact in surface physics and other research areas that require increased imaging sensitivity will be discussed. \\[4pt] [1] R.van Gastel et al, \textit{Microscopy and Microanalysis} \textbf{17}, 928-929 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L1.00003: Instrumentation Development for Dynamic Atom Probe Tomography Brian Gorman, David Diercks Atom probe tomography (APT) is a materials characterization technique widely recognized as having the highest combined spatial (sub-nm) and chemical (less than 10$^{17}$ atoms / cm$^3$) resolution. This imaging time of flight mass spectrometry technique typically utilizes laser pulsed field emission in semiconductors and dielectrics. Laser pulsed field emission is widely considered to be a thermal effect, with specimen temperatures from 100 K to greater than 1500 K observed. Following the thermal spike, specimens typically cool to their base temperature in less than 5 ns, depending upon their thermal transport and geometry. Combining the laser pulsed atom probe experiment with pulsed transmission electron diffraction will enable thermal annealing and quenching experiments at atomic resolution with nanosecond temporal resolution. The combined instrument, titled a Dynamic Atom Probe, will be used to monitor solid state processes including solute drag effects on grain boundary motion, atomic scale kinetics of crystallization in amorphous semiconductors and oxides, atomic scale nucleation and spinodal decomposition kinetics in oxides, and phase transformations in metallic alloys. This discussion will include the specifics of the instrumentation currently under development as well as proof of principle first experiments. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L1.00004: Simulation of ultrashort photoelectron pulses as a guide for developing reliable ultrafast electron diffraction systems Jenni Portman, He Zhang, Zhensheng Tao, Chong-yu Ruan, Martin Berz, Phillip Duxbury The development of a reliable experimental ultrafast electron diffraction and imaging system requires a theoretical understanding of the underlying physical phenomena and an accurate modeling of the optical elements present in the beam column. To achieve this goal, we have developed two types of computer simulations: a mean-field Gaussian approximation, in which the linear effects of realistic optical elements are incorporated. Due to the limitations inherent in the theory, it fails to capture the intricate behavior of a real system but it is computationally very inexpensive and offers valuable information on the relevant parameter ranges. The second type of simulation considered is an explicit N particle model which uses a O(N) method for calculating the space charge effects, enabling simulations of over one million electrons in a pulse. While being computationally very expensive, it offers the advantage of incorporating realistic descriptions for the electromagnetic fields of the optical elements, along with their linear and non-linear contributions. The results of this work show the limits of the validity of the mean-field approach and offer a detailed and highly accurate physical description of the beam dynamics under different operational regimes. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L1.00005: Charged nanoparticle dynamics in water induced by scanning transmission electron microscopy E.R. White, Matthew Mecklenburg, Brian Shevitski, S.B. Singer, B.C. Regan Using scanning transmission electron microscopy we image $\sim 4$~nm platinum nanoparticles deposited on an insulating membrane, where the membrane is one of two electron-transparent windows separating an aqueous environment from the microscope's high vacuum. Upon receiving a relatively moderate dose of $\sim 10^4\,e^-$/nm$^2$, initially immobile nanoparticles begin to move along trajectories that are directed radially outward from the center of the field of view. As the dose rate is increased the particle motion becomes increasingly dramatic. These observations demonstrate that even under mild imaging conditions, the \emph{in situ} electron microscopy of aqueous environments can produce charging effects that dominate the dynamics of nanoparticles under observation. Such effects provide a new tool for modifying \emph{in vitro} environments such as those used for TEM studies of wet biological systems. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:30PM |
L1.00006: Coherent X-ray microscopy of real materials at 10nm resolution Invited Speaker: Andreas Menzel Coherent X-ray microscopy replaces image-forming optics by mathematical algorithms to ``reconstruct'' the micrograph from the distribution of scattered light. ~In case of X-rays, its popularity is largely based on its ability to alleviate limitations of resolution and efficiency of available optics. Various methodologies have been developed, and high resolving power could be demonstrated. Yet, stringent constraints on sample preparation and data quality had to be addressed before the technique could advance from proof of principle studies to application. About a decade after the first demonstration using X-rays [J.W. Miao et al., Nature 400 (1999) 342] coherent diffractive imaging (CDI) seems to have matured into a microscopy technique that proves useful in real-life scientific investigations. The major coherent diffractive imaging methods will be reviewed, i.e., its original implementation using plane waves, the so-called Fresnel CDI that exploits curvature in the illuminating wavefront, and ptychographic CDI, which is a scanning microscopy method using multiple exposures. Being fully compatible with spectroscopic contrast channels makes quantitative structural, chemical, and magnetic information accessible, and the penetration power of X-rays can be exploited by tomographic or ab initio reconstruction methods in order to yield three-dimensional reconstructions with high resolution, high specificity, and high dose efficiency. These recent advances have turned coherent X-ray microscopy into a reliable and robust method that is attractive for scientists independent on how (un-)familiar they are with X-ray microscopy. Applications from the materials and life sciences will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L1.00007: Time-Resolved Near-Edge Coherent Diffractive Imaging Fabian Weise, Daniel Neumark, Stephen R. Leone, Oliver Gessner Coherent diffractive imaging (CDI) with x-rays is a well-established technique that provides structural information beyond the limitations of optical microscopy. Free electron lasers provide ultrashort x-ray pulses with sufficiently high peak brightness to facilitate single-shot imaging and the extension of CDI into the time-domain. Recent progress in the generation of spatially coherent ultrashort x-ray pulses by high harmonic generation (HHG) using tabletop lasers lead to the emergence of a new field of laboratory-based CDI. While a relatively low photon flux and limited photon energies result in lower imaging resolution compared to x-ray studies at large-scale facilities, the significantly greater availability makes laboratory-based experiments well suited for developing new CDI techniques. We present a new apparatus for CDI, which provides ultrashort XUV pulses with tunable photon energies. By implementing a monochromator in a HHG-based CDI setup, the photon energy can be tuned to the inner-shell absorption edges of different elements in the sample. The wavelength-dependence of the x-ray optical constants close to the resonances facilitates to exploit the element selectivity and chemical sensitivity of x-ray transitions in time-domain CDI experiments. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L1.00008: In-situ proton radiography of solidification in Sn-Bi, Al-Cu, Al-In and Bi-Ga melts Jason Cooley, Amy Clarke, Christopher Morris, Brian Hollander, Tim Tucker, Thomas Ott, Robert Field, David Korzekwa, Duncan Hammon, Kester Clarke, Patrick Kennedy, Frank Merill, Fessaha Mariam, Martha Barker, James Foley, Robert Aikin, Joshua Hill, Dan Thoma, Finian O'neill, Megan Emigh, Bo Folks In-situ observation of the solidification phenomena in metals can lead to better understanding and control of microstructure evolution. Proton radiography offers the ability to image thick samples of high z material. Recently, in-situ proton radiography was used to directly observe dynamic processes during melting and solidification in bulk binary alloy systems. The spatial resolution was $\sim $65 microns for a 44 x 44 mm2 field of view. The time scale of each experiment was 2 to 6 hours. The data collected allowed for the determination of solidification front velocities, captured the changes in solid/liquid densities and showed evidence of convective fluid flow in the melt. Microstructural features larger than approximately 100 microns in the solid phase were observed. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L1.00009: True Attosecond Imaging of Inhomogeneous Systems with Standing-Wave Inelastic X-Ray Scatterin Yu Gan, Peter Abbamonte Inelastic X-ray scattering (IXS) has recently been used to image electron dynamics at the attosecond scale, but it has been shown that these images are spatially averaged. The problem is that the existing technique can only access the ``diagonal'' ($k_1=-k_2$) elements of the electron density response $\chi(k_1,k_2,\omega)$. It has been shown, however, that inelastic X-ray scattering at the Bragg position is sensitive to the entire response $\chi(k_1,k_2,\omega)$. With this method, a standing wave field is established in the sample by exciting a Bragg condition, allowing access to all the off-diagonal ($k_1\ne- k_2$) elements of $\chi$. In this talk I will present a simple model demonstrating that, in principle, this approach can be used to map the entire density response. In particular, I show that a one-dimensional system is experimentally impossible to probe, a two-dimensional system is experimentally accessible but typically difficult to measure in practice, and a three-dimensional system is experimentally both plausible and practicable. Finally, I propose possible experimental realizations of this technique. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L1.00010: Sub-Picosecond Dynamics of Displacement Cascades Bennett Larson, Jon Tischler, Roger Stoller, Yuri Osetskiy, Christian David Sub-picosecond x-ray pulses produced by the Linac Coherent Light Source (LCLS) now enable real time experimental measurements of atomic displacement cascade structure and dynamics on sub-picosecond time scales. Such measurements will make possible the first direct experimental test of molecular dynamics (MD) displacement cascade simulations. Here we will discuss the potential to use single, seeded, 100-fs LCLS hard x-ray pulses focused to $\sim $100 nm diameter by diamond-based Fresnel zone plate optics to make real-time diffuse scattering measurements on 50 keV Ar-ion-induced cascades in thin single-crystal samples. We will present x-ray Bragg diffuse scattering calculations based on $\sim $ 4M atom, 25 keV primary knock-on energy MD cascade simulations demonstrating that temporally-random, 100 fs LCLS x-ray pulse measurements of diffuse scattering near low index Bragg reflections can be time-ordered from sub-picoseconds to a few picoseconds. Time ordering is made possible in this regime by the distinct nature of diffuse scattering profiles as a function of time that are produced by shock-induced pressure waves according to MD cascade simulations in Fe. The expected results and the experimental challenges anticipated to perform such measurements will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L1.00011: Nanostructure Formation on Sb$_{2}$Te$_{3}$ Thin Films Induced by Femtosecond Laser Irradiation Yuwei Li, Vladimir. A. Stoica, Lynn Endicott, Guoyu Wang, Huarui Sun, Kevin. P. Pipe, Ctirad Uher, Roy Clarke Sb$_{2}$Te$_{3}$ has applications in thermoelectrics, phase-change memory devices and topological insulators. In the case of thermoelectricity, nanostructure formation in this type of material has been predicted to enhance its figure of merit for thermal energy conversion. Here, we present our results on modification of the surface morphology of Sb$_{2}$Te$_{3}$ thin films after femtosecond laser irradiation. Under a narrow range of laser fluence and irradiation time, long and highly-aligned nanotracks were formed in the plane of the film, having a periodicity 10 times smaller than the irradiation laser wavelength. The laser fluence and irradiation time can result in different surface nanostructure morphologies with varying degrees of order. Finally, using an optical pump-probe technique, we find that the laser-irradiated nanostructured areas of the film have a lower thermal conductivity compared to that of the reference smooth areas not irradiated by the laser. Such Sb$_{2}$Te$_{3}$ nanostructures can be important for thermoelectric applications as well as for further studies of femtosecond laser interaction with opaque materials. [Preview Abstract] |
Session L2: Invited Session: Andrei Sakharov Prize Session
Sponsoring Units: DPB FIPChair: W. Barletta, Massachusetts Institute of Technology
Room: 204AB
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L2.00001: A. Sakharov and Fusion Research Invited Speaker: Bruno Coppi In the landmark paper by Tamm and Sakharov [1], a controlled nuclear fusion reactor based on an axisymmetric magnetic confinement configuration whose principles remain valid to this day, was proposed. In the light of present understanding of plasma physics the virtues (e.g. that of considering the D-D reaction) and the shortcomings of this paper are pointed out. In fact, relatively recent results of theoretical plasma physics (e.g. discovery of the so called second stability region) and advances in high field magnet technology have made it possible to identify the parameters of meaningful experiments capable of exploring D-D and D-$^3$He burn conditions. At the same time an experimental program (IGNIR) has been undertaken through a (funded) collaboration between Italy and Russia to investigate D-T plasmas close to ignition conditions based on an advanced high field toroidal confinement configuration. A. Sakharov envisioned a bolder approach to fusion research than that advocated by some of his contemporaries. The time taken to design and decide to fabricate the first experiment capable of reaching ignition conditions is due in part to the problem of gaining an adequate understanding the expected physics of fusion burning plasmas. However, most of the relevant financial effort has gone in the pursuit of slow and indirect enterprises complying with the ``playing it safe'' tendencies of large organizations or motivated by the purpose to develop technologies or maintain a high level of expertise in plasma physics to the expected benefit of other kinds of endeavors. The creativity demonstrated by A. Sakharov in dealing with civil rights and disarmament issues is needed, while maintaining our concerns for energy and the environment on a global scale, to orient the funding for fusion research toward a direct and well based scientific effort on concepts for which a variety of developments can be envisioned. These can span from uncovering new physics relevant, for instance, to high energy astrophysics to the feasibility of new neutron sources.\\[4pt] [1] A. Sakharov, Collected Scientific Works (Publ. Marcel Dekkes, Inc., New York, N.Y., 1982). [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:42PM |
L2.00002: CP Violation and the Matter Anti-Matter Asymmetry of the Universe Invited Speaker: Robert Cahn There is no scientific question more fundamental than ``Why are we here?'' or as we physicists might phrase it ``Why is there more matter than anti-matter?'' Because, as Andrei Sakharov first showed, CP violation is necessary to any explanation of the matter anti-matter asymmetry, CP violation is the focus of much of the international experimental program in particle physics. CP conservation was what could be salvaged after parity was overthrown in 1956, but it survived only until 1964 when K mesons were found not to respect it. While parity violation was a large effect in weak interactions, CP violation seemed small and confined to the kaons. When the Standard Model of particle physics emerged in early 1970's, Kobayashi and Maskawa observed that if there were three families of quarks, CP violation would arise quite naturally. The Standard Model suggested that CP violation could be large in decays of B mesons. Nonetheless, no matter what parameters are used in the Standard Model, CP violation among quarks cannot be large enough to explain the matter anti-matter asymmetry. Major experiments in the U.S. and Japan were undertaken to explore CP violation in B mesons to search for signs of CP violation outside the Standard Model, which might explain the dominance of matter over anti-matter. Neither experiment found such a discrepancy, but new programs will continue this search with much higher statistics. While the three families of leptons are in many ways analogous to the three families of quarks, the neutrinos have a unique character. As neutral particles, it is possible that they are their own antiparticles. If this is so, there may be additional, very heavy, neutrinos beyond those we know already. If they violate CP they may be the source of the matter anti-matter asymmetry. But do neutrinos experience CP violation? Experiments around the world are just now setting out to answer this question. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L2.00003: Sakharov and his Times Invited Speaker: Tatiana Yankelevich |
Tuesday, February 28, 2012 4:18PM - 4:54PM |
L2.00004: Sakharov Prize Lecture Invited Speaker: Richard Wilson |
Tuesday, February 28, 2012 4:54PM - 5:30PM |
L2.00005: Scientists and Human Rights Invited Speaker: Yousef Makdisi The American Physical Society has a long history of involvement in defense of human rights. The Committee on International Freedom of Scientists was formed in the mid seventies as a subcommittee within the Panel On Public Affairs ``to deal with matters of an international nature that endangers the abilities of scientists to function as scientists'' and by 1980 it was established as an independent committee. In this presentation I will describe some aspects of the early history and the impetus that led to such an advocacy, the methods employed then and how they evolved to the present CIFS responsibility ``for monitoring concerns regarding human rights for scientists throughout the world''. I will also describe the current approach and some sample cases the committee has pursued recently, the interaction with other human rights organizations, and touch upon some venues through which the community can engage to help in this noble cause. [Preview Abstract] |
Session L3: Invited Session: Fermiology of Electron and Hole Doped Cuprates - A Guide to High Temperature Superconductivity
Sponsoring Units: DCMPChair: Suchitra Sebastian, University of Cambridge
Room: 205AB
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L3.00001: Hidden Fermi Liquid: Self-Consistent Theory for the Normal State of High-Tc Superconductors Invited Speaker: Philip Anderson |
Tuesday, February 28, 2012 3:06PM - 3:42PM |
L3.00002: Quantum oscillations and nodal pockets from Fermi surface reconstruction in the underdoped cuprates Invited Speaker: Neil Harrison Fermiology in the underdoped high $T_{\rm c}$ cuprates presents us with unique challenges, requiring experimentalists to look deeper into the data than is normally required for clues. Recent measurements of an oscillatory chemical potential affecting the oscillations at high magnetic fields provide a strong indication of a single type of carrier pocket. When considered in conjunction with photoemission and specific heat measurements, a Fermi surface comprised almost entirely of nodal pockets is suggested. The mystery of the Fermi surface is deepened, however, by a near doping-independent Fermi surface cross-sectional area and negative Hall and Seebeck coefficients. We explore ways in which these findings can be reconciled, taking an important hint from the diverging effective mass yielded by quantum oscillations at low dopings. The author wishes to thank Suchitra Sebastian, Moaz Atarawneh, Doug Bonn, Walter Hardy, Ruixing Liang, Charles Mielke and Gilbert Lonzarich who have contributed to this work. The work is supported by the NSF through the NHMFL and by the DOE project ``Science at 100 tesla.'' [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L3.00003: Phase competition in trisected superconducting dome Invited Speaker: Inna Vishik The momentum-resolved nature of angle-resolved photoemission spectroscopy (ARPES) has made it a key probe of emergent phases in the cuprates, such as superconductivity and the pseudogap, which have anisotropic momentum-space structure. ARPES can be used to infer the origin of spectral gaps from their distinct phenomenology---temperature, doping, and momentum dependence, and this principle has been used to argue that the pseudogap is a distinct phase from superconductivity, rather than a precursor [1]. We have studied Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi-2212) using laser-ARPES, and our data give evidence for three distinct quantum phases comprising the superconducting ground state, accompanied by abrupt changes at p$\sim $0.076 and p$\sim $0.19 in the doping-and-temperature dependence of the gaps near the bond-diagonal (nodal) direction [2]. The latter doping likely marks the quantum critical point of the pseudogap, while the former represents a distinct competing phase at the edge of the superconducting dome. Additionally, we find that the pseudogap advances closer towards the node when superconductivity is weak, just below T$_{c}$ or at low doping, and retreats towards the antinode well below T$_{c}$ and at higher doping. This phase competition picture together with the two critical doping are synthesized into our proposed phase diagram, which also reconciles conflicting phase diagrams commonly used in the field. Our results underscore the importance of quantum critical phenomena to cuprate superconductivity, provide a microscopic picture of phase competition in momentum space, and predict the existence of phase boundaries inside the superconducting dome which are different from simple extrapolations from outside the dome. \\[4pt] [1] I. M. Vishik, W. S. Lee, R.-H. He, M. Hashimoto, Z. Hussain, T. P. Devereaux, and Z.-X. Shen. \textit{New J. Phys. }\textbf{12}, 105008 (2010). \\[0pt] [2] I. M. Vishik, M. Hashimoto, R.-H. He, W. S. Lee, F. Schmitt, D. H. Lu, R.G. Moore, C. Zhang, W. Meevasana, T. Sasagawa, S. Uchida, K. Fujita, S. Ishida, M. Ishikado, Y. Yoshida, H. Eisaki, Z. Hussain, T. P. Devereaux, and Z.-X. Shen, \textit{Submitted} (2011). [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:54PM |
L3.00004: Novel Magnetism in the Pseudogap Phase of the Cuprates Invited Speaker: Martin Greven Magnetic correlations might cause the superconductivity in the cuprates and are generally believed to be antiferromagnetic. Following our success in growing sizable crystals of the tetragonal compound HgBa$_2$CuO$_{4+\delta}$ [1], we used polarized neutron diffraction to demonstrate that the unusual magnetic order previously observed in YBa$_2$Cu$_3$O$_{6+\delta}$ [2] is a universal property of the pseudogap phase [3]. Subsequent inelastic neutron scattering experiments revealed several accompanying, weakly-dispersive magnetic excitation branches in HgBa$_2$CuO$_{4+\delta}$ [4]. Unlike antiferromagnetism, the novel magnetic order does not break the lattice translational symmetry. Nevertheless, the excitations mix with conventional antiferromagnetic fluctuations. Our results point toward the need for a multi-band description of the cuprates, and they are consistent with the notion that the phase diagram is controlled by an underlying quantum critical point [5]. The neutron scattering results will be discussed together with new dc resistivity data for the pseudogap phase of HgBa$_2$CuO$_{4+\delta}$ [6].\\[4pt] [1] X. Zhao {\it et al.}, Adv. Mat. {\bf 18}, 3243 (2006).\\[0pt] [2] B. Fauque {\it et al.}, Phys. Rev. Lett. {\bf 96}, 197001 (2006).\\[0pt] [3] Y. Li {\it et al.}, Nature {\bf 455}, 372 (2008).\\[0pt] [4] Y. Li {\it et al.}, Nature {\bf 468}, 283 (2010), and unpublished results.\\[0pt] [5] C. Varma, Nature {\bf 468}, 184 (2010).\\[0pt] [6] N. Barisic {\it et al.}, unpublished results. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:30PM |
L3.00005: From Spin Liquid to High T$_{c}$ Superconductors Invited Speaker: Patrick A. Lee The discovery of high T$_{c}$ superconductors has revived interest in Anderson's resonating valence bond theory (RVB) of a quantum spin liquid, first proposed in 1973. In the past few years, several examples of quantum spin liquids have been discovered experimentally. The organic spin liquid has been studied most thoroughly and shows strong evidence for emergent fermionic spinons. I shall review some of the data and argue that theories based on slave particles and gauge fields have been successful in accounting for these remarkable data. The question remains as to whether a similar formulation of fermionic spinon and bosonic holes can form the basis for a theory of high T$_{c}$ superconductors. I shall show that a recent modification\footnote{T. Senthil and P.A. Lee, Phys. Rev. Lett. \textbf{103}, 076402 (2009).} of the mean field RVB phase diagram can explain a lot of the phenomenology. I shall also attempt to put this theory in the context of recent discoveries concerning symmetry breaking in the pseudogap phase. [Preview Abstract] |
Session L4: Focus Session: Numerical Methods for Studying Nonequilibrium Many-Body Dynamics; DMRG, DMFT, Truncated Wigner Approximation, Exact Diagonalization
Sponsoring Units: DAMOPRoom: 205C
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L4.00001: Finite temperature DMRG and the Drude weight of spin 1/2 Heisenberg chains Christoph Karrasch, Jens Bardarson, Joel Moore We propose an easy-to-implement approach to study time-dependent correlation functions of one dimensional systems at finite temperature $T$ using the the density matrix renormalization group (DMRG). If the auxiliary degrees of freedom which purify the statistical operator are time-evolved with the physical Hamiltonian but reversed time, the entanglement blow-up inherent to any time-dependent DMRG calculation is dramatically reduced. The numerical effort of finite temperature DMRG becomes comparable to that at $T=0$, and thus significantly longer timescales can be reached. We exploit this to investigate current correlation functions of the XXZ spin $1/2$ Heisenberg chain. At intermediate to large $T$, we can explicitly extract the Drude weight $D$ from the long-time asymptotics. For the isotropic chain, $D$ is finite. At low temperatures, we establish an upper bound for the Drude weight. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L4.00002: Universal non-equilibrium quantum dynamics in imaginary time Claudia De Grandi, Anatoli Polkovnikov, Anders Sandvik We propose a method to study the dynamical response of a quantum systems by evolving it with an imaginary-time dependent Hamiltonian. The leading non-adiabatic response of the system driven to a quantum-critical point is universal and characterized by the same exponents in real and imaginary time. For a linear quench protocol, the fidelity susceptibility and the geometric tensor naturally emerge in the response functions. Beyond linear response we extend the finite-size scaling theory of quantum phase transitions to non-equilibrium setups. Imaginary-time dynamics is also amenable to quantum Monte Carlo simulations, which we apply here to quenches of the transverse-field Ising model to quantum critical points in one and two dimensions. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L4.00003: The Approach To Typicality in Many-Body Quantum Systems Sai Vinjanampathy, Shawn Dubey, Luciano Silvestri, Kurt Jacobs The recent discovery that for large Hilbert spaces, almost all (that is, typical) Hamiltonians have eigenstates that place small subsystems in thermal equilibrium, has shed much light on the origins of irreversibility and thermalization. Here we present numerical evidence that many-body lattice systems generically approach typicality as the number of subsystems is increased, and thus provide further support for the eigenstate thermalization hypothesis. We will present our results that indicate that the deviation of many-body systems from typicality scales as an inverse power of the number of systems, and we compare this with the equivalent scaling for random Hamiltonians. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L4.00004: Quantum Dynamics of Population-Imbalanced Fermi Mixture in One-dimensional Optical Lattices Bin Wang, Erhai Zhao, W. Vincent Liu We study the (pseudo-)spin dynamics of two-component Fermi mixture with population imbalance in one-dimensional (1D) optical lattices within the framework of 1D Hubbard model, utilizing the time-evolving block decimation (TEBD) algorithm. We consider the situation in which the center region of the system is initially fully occupied and the left-over fully polarized fermions are initially localized on the wings. It is found that for strong interaction the (pseudo-)spin polarization diffuses throughout the system qualitatively in the same way as the diffusion of non-interacting particles in 1D optical lattices. We further look into the time evolution of correlation functions such as pairing correlation and single particle green functions. According to our simulations, the partially polarized inhomogeneous Fermi gas appears to show no tendency of dynamically developing into a FFLO-type of state, contrary to what might have been expected. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L4.00005: Non-equilibrium dynamics of the driven Hubbard model Adriano Amaricci, Cedric Weber, Massimo Capone, Gabriel Kotliar We investigate the dynamics of a two-dimensional Hubbard model in a static electric field in order to identify the conditions to reach a non-equilibrium stationary state. For a generic electric field, the convergence to a stationary state requires the coupling to a thermostating bath absorbing the work done by the external force. Following the real-time dynamics of the system, we show that a non-equilibrium stationary state is reached for essentially any value of the coupling to the bath. We map out a phase diagram in terms of dissipation and electric field strengths and identify the dissipation values in which steady current is largest, and correspondingly a suitable entropy function is smallest, for a given field. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L4.00006: Probing Phases and Quantum Criticality using Deviations from the Local Fluctuation-Dissipation Theorem Eric Duchon, Yasuyuki Kato, Naoki Kawashima, Nandini Trivedi One of the major open questions in the study of ultracold atom systems is how to obtain the finite temperature phase diagram of a given Hamiltonian directly from experiments.\footnote{Q. Zhou, et al., {\it Phys. Rev. Lett}. {\bf 103}, 085701 (2009).} Previous work in this direction required quantum Monte Carlo simulations to directly model the experimental situation in order to extract quantitative information, clearly defeating the purpose of an optical lattice emulator. We propose a new method that utilizes deviations from a local fluctuation dissipation theorem to construct a finite temperature phase diagram, for the first time, from local observables accessible by {\it in situ} experimental observations. Our approach extends the utility of the fluctuation-dissipation theorem from thermometry to the identification of quantum phases, associated energy scales and the quantum critical region. We test our ideas using large-scale quantum Monte Carlo simulations of the two-dimensional Bose Hubbard model.\footnote{Y. Kato, et al., {\it Nature Physics} {\bf 4}, 592 - 593 (2008).} [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L4.00007: A new theoretical method to describe nonequilibrium cold atoms in optical lattices Karlis Mikelsons, Jim Freericks, H. R. Krishnamurthy We use perturbation theory in the hopping (strong-coupling expansion) to describe the nonequilibrium dynamics of strongly correlated fermions. Our expansion is a self-consistent expansion for the self-energy which goes beyond the RPA and allows for damping and relaxation effects. We apply this method to solve the homogeneous Fermi - Hubbard model driven by an external field. We investigate the damping of Bloch oscillations (for a uniform dc field) and show results for the current, the nonequilibrium density of states and the momentum distribution. We carefully benchmark the technique using the exact sum rules to determine its accuracy and we discuss regions of parameter space where the method no longer converges. This technique is quite competitive with other methods (such as DMFT) in the regions where it converges. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L4.00008: A Slave Spin Impurity Solver for Non Equilibrium Dynamical Mean Field Theory Marco Schiro The non equilibrium dynamics of strongly correlated electronic systems represents a challenging theoretical problem in condensed matter physics with applications ranging from pump probe experiments in correlated materials to dynamics in ultracold atomic gases. Dynamical Mean Field Theory (DMFT) has emerged in recent years as a powerful theoretical framework to deal with strong correlations in a non pertubative way. Its extension to the out of equilibrium case requires the solution of an auxiliary quantum impurity model in a non equilibrium bath. Here we present an impurity solver for Non Equilibrium DMFT based on a slave spin representation of the fermionic degrees of freedom. We apply this method to study the quench dynamics in the single band fermionic Hubbard model and compare the results with the time dependent Gutzwiller method. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L4.00009: A multi-site mean-field theory for cold bosonic atoms in optical lattices Pawel Pisarski, Thomas McIntosh, Robert Gooding, Eugene Zaremba Mean-field theory is one of the most commonly used approximate methods in condensed matter physics. As applied to the Bose-Hubbard model it provides a simple, qualitative explanation of the Mott insulator -- superfluid transition. In its usual form, one invokes a superfluid order parameter to decouple the Bose-Hubbard Hamiltonian into a sum of independent site Hamiltonians. Within this single-site mean-field theory (SSMFT) the equilibrium state is determined by minimizing the grand potential with respect to the order parameter. To improve on this procedure we have developed a multi-site mean-field theory (MSMFT), whereby the lattice is partitioned into small clusters which are decoupled by means of the usual mean-field method. The most general decoupling procedure necessitates the assignment of site-dependent order parameters to the sites bounding the cluster. This leads to a non-trivial topology of the grand potential and one finds, in general, that the equilibrium state is a saddle point. As a result, one cannot use a variational principle to locate the equilibrium states of interest. In this talk, we outline the MSMFT we have developed and give an example of its application. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L4.00010: Feynman diagrams versus Feynman quantum emulator Kris Van Houcke, F\'elix Werner, Evgeny Kozik, Nikolay Prokof'ev, Boris Svistunov, Mark Ku, Ariel Sommer, Lawrence Cheuk, Andr\'e Schirotzek, Martin Zwierlein Precise understanding of strongly interacting fermions, from electrons in modern materials to nuclear matter, presents a major goal in modern physics. However, the theoretical description of interacting Fermi systems is usually plagued by the intricate quantum statistics at play. Here we present a cross-validation between a new theoretical approach, Bold Diagrammatic Monte Carlo (BDMC), and precision experiments on ultra-cold atoms. Specifically, we compute and measure with unprecedented accuracy the normal-state equation of state of the unitary gas, a prototypical example of a strongly correlated fermionic system. Excellent agreement demonstrates that a series of Feynman diagrams can be controllably resummed in a non-perturbative regime using BDMC. This opens the door to the solution of some of the most challenging problems across many areas of physics. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 5:06PM |
L4.00011: Non-equilibrium dynamics, heating, and thermalization of cold atoms in optical lattices Invited Speaker: Andrew Daley In recent years, out of equilibrium many-body dynamics have become accessible in a controlled way in experiments with ultracold quantum gases. Time-dependent processes in these systems are not only intrinsically interesting, but also extremely important for understanding many-body state preparation, heating, and thermalization as they arise in experiments. They also offer new connections to phenomena studied in solid-state systems. This interest has been complemented by the development of a range of numerical methods, including time-dependent density matrix renormalization group (t-DMRG) methods and matrix product state approaches, which have been applied to study dynamics in 1D lattice systems and spin chains. We have extended and applied these methods to study the non-equilibrium dynamics of cold atoms in optical lattices arising from different heating mechanisms, especially due to spontaneous emissions from incoherent scattering of the lattice light, or via classical noise on the optical potential. Understanding how these heating mechanisms affect the properties of different many-body states is crucial in addressing current experimental challenges in the preparation of interesting quantum phases at low temperatures. The resulting non-equilibrium dynamics typically depend strongly on the properties of the many-body state, with different states being more or less sensitive to different heating mechanisms. Moreover, there is often a separation of timescales between some excitations that thermalize rapidly, and some that do not properly thermalize in the duration of an experimental run, which can strongly modify, and even reduce the overall effects of the heating processes. Part of this work involves the treatment of open many-body quantum systems, where we derive many-body master equations to describe the dynamics and solve these numerically by combining t-DMRG methods with quantum trajectory techniques from quantum optics. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L4.00012: Pattern formation in time-of-flight images of heavy-light mixtures of atoms undergoing Bloch oscillations James Freericks Nonequilibrium dynamical mean-field theory is employed to solve for the response of a light-heavy Fermi-Fermi mixture of atoms to the presence of a uniform electric field (via ``pulling'' the lattice through the cloud of atoms). When we express the (momentum-dependent) light atom distribution functions as functions of the band energy and the projection of the velocity along the direction of the artificial electric field, the system develops characteristic spiral patterns that become more complex as the system evolves, but remain stable for a long period of time. These patterns typically show a contrast of about 10-20\% fluctuations about the mean density, so they are challenging but possible to observe in a time-of-flight measurement. We also show a characteristic change of character of the system between small fields and large fields. The best candidate system for examining these patterns is a Li$^6$-K$^{40}$ mixture on a two-dimensional optical lattice. We expect similar results should occur for light Fermi-heavy Bose mixtures as well, but it is likely this behavior will not be seen in Bose-Bose mixtures. [Preview Abstract] |
Session L5: Focus Session: Interfaces in Complex Oxides - Nickelates
Sponsoring Units: DMPChair: John Freeland, Argonne National Laboratory
Room: 206A
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L5.00001: Exchange Bias in LaNiO$_{3}$-based heterostructures Marta Gibert, Raoul Scherwitzl, Pavlo Zubko, Jorge I\~niguez, Jean-Marc Triscone The wide spectrum of exotic properties exhibited by transition-metal based oxides is triggered by the interplay between the spin, charge, orbital and lattice degrees of freedom. In this context, interface engineering in complex oxide heterostructures enables not only further tuning of the exceptional properties of these materials, but also gives access to hidden phases and emergent physical phenomena. Here, we show how interface engineering can induce a complex magnetic structure in a non-magnetic material. We specifically show that exchange bias phenomenon can unexpectedly emerge in (111)-oriented heterostructures involving materials a priori non-candidates for the development of such behavior, namely paramagnetic LaNiO$_{3}$ (LNO) and ferromagnetic LaMnO$_{3}$ (LMO). The observation of the exchange bias in LNO/LMO superlattices not only implies the development of interface-induced magnetism in the paramagnetic LNO layers, but also provides us with a very subtle tool for probing the interfacial coupling between the LNO and the LMO layers. First-principles calculations indicate that this interfacial interaction may give rise to an unusual spin order resembling a spin density wave within the LNO layers. Other possible magnetic orders are also discussed. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L5.00002: Resonant x-ray reflectivity study of interface reconstructions in LaCoO(3) and other transition-metal-oxide heterostructures Vladimir Hinkov, Woo-Seok Choi, Jorge Hamann-Borrero, George A. Sawatzky, Ho-Nyung Lee, Sebastian Macke, Abdullah Radi Transition-metal-oxide (TMO) heterostructures offer the opportunity to combine the study of unconventional physical properties with the design of novel functionalities, which are not observed in simple semiconductor or metal heterostructures. In my talk I will concentrate on electronic and chemical reconstructions observed at the interfaces of heterostructures based on LaCoO(3) and other correlated oxides. After briefly summarizing the properties of the novel resonant x-ray technique of orbital reflectometry which we recently used to study the orbital reconstruction in LaNiO(3) heterostructures (Benckiser et al., Nature Materials 10, 189, (2010)), I will discuss the application of this technique, and of usual resonant x-ray reflectometry, to study the electronic and orbital properties at the interfaces and at the surface of LaCoO(3) / LaAlO(3) heterostructures. Finally, I will offer an outlook to the application of orbital reflectometry to other systems. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L5.00003: Quantum confinement of correlated e$_{g}^{1}$ electrons in rare earth nickelate heterostructures Jian Liu, M. van Veenendaal, S. Okamoto, M. Kareev, B. Gray, P. Ryan, J.W. Freeland, J. Chakhalian Complex oxide heterostrutures have emerged as a new playground for controlling the mutually coupled charge, spin, orbital and lattice degrees of freedom, and a promising route to stabilize unusual phases not existing in the bulk. In particular, quantum well structures have recently attracted attention due to the potential in creating novel two-dimensional systems with confined correlated electrons. To this end, we have studied the e$_{g}^{1}$ system based on the 3d$^{7}$ low-spin state in perovskite rare earth nickelates which are artificially confined by wide-gap dielectrics LaAlO$_{3}$. The combination of transport measurements and dynamical-mean-field calculations indicate that, a Mott-type metal-insulator transition can be induced by confinement via dimensionality-control. X-ray absorption spectroscopy reveals that the electronic modification in proximity to the confining interfaces is caused by modulated covalency, which is in good agreement with cluster calculations. J.C. was supported by DOD-ARO under the Contract No. 0402-17291 and NSF Contract No. DMR-0747808. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L5.00004: Epitaxial Stabilization of Ultrathin Rare-Earth Nickelates D.J. Meyers, E.J. Moon, M. Kareev, I.C. Tung, B.A. Gray, Jian Liu, M.J. Bedzyk, J.W. Freeland, J. Chakhalian The nickelate family of perovskite materials has attracted great attention in recent years due to the interesting range of properties they exhibit. In this talk we report on the successful synthesis of EuNiO$_{3}$, YNiO$_{3}$, and PrNiO$_{3}$ films grown by interrupted pulse laser epitaxy on various substrates. Investigation of the phase space of nickelate thin film epitaxy revealed a linear trend between the optimized growth temperature and the Goldschmidt tolerance factor. This correlation is explained through epitaxial stabilization and the increase of the lattice energy via distortion of the ideal perovskite cell. This explanation offers the additional benefit of not being restricted only to the nickelate perovskite family, giving it possible applicability for a wide range of perovskite-structured materials. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L5.00005: Unipolar Field-Effect Diode Based on a Complex Oxide Weng Lee Lim, Sergei Urazhdin, Eun Ju Moon, Michael Kareev, Derek Meyers, Jak Chakhalian, John Freeland We demonstrate rectifying behavior in a field-effect device structure fabricated from thin NdNiO$_{3}$ films grown on SrTiO$_{3}$ substrates by the pulsed laser deposition technique. In contrast to the conventional three-terminal field effect devices, the device has only two terminals with the field gate electrode connected to one of the terminal electrodes. The active device area is a $10\mu$m$\times$$10\mu$m square with a Au/Al$_{2}$O$_{3}$/NdNiO$_{3}$/SrTiO$_{3}$ structure, where Au and Al$_{2}$O$_{3}$ are the gate and the gate insulator, respectively. At small bias voltages, the device exhibits a metal-insulator transition near T=150K, similar to extended NdNiO$_{3}$ films. I-V measurements reveal a strong dependence of device characteristics on temperature, applied bias, and both thermal and applied bias histories. We analyze the IV characteristics by using a modified charge-control model based on accumulation of charges in the channel near the gate oxide interface. We deduce the temperature dependence of the effective zero-field charge carrier mobility for the channel by including a field-dependence mobility. The observed hysteretic effects can be utilized in complex oxide devices that combine together both the diode and the memory functionalities. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L5.00006: Asymmetric Orbital-Lattice Interactions in Ultrathin Correlated Oxide Films Jak Chakhalian Epitaxial control of strongly correlated electrons offers opportunities to push beyond the bulk phase diagram and access new ground states. Using resonant x-ray spectroscopies combined with density functional calculations, we report the discovery of an asymmetric biaxial strain-induced 3d-orbital response in ultrathin films of the correlated metal LaNiO3 that are not accessible in the bulk [J. Chakhalian et al., PRL, 107, 116805 (2011)]. Compressive strain results in an orbital polarization due to structural induced changes in the crystal field, but tensile strain shows no orbital response. This is accompanied by a strong change in the oxygen hole states due a systematic change of the charge transfer energy as a function of strain. We suggest that knowledge of this asymmetric orbital-lattice interaction is fundamental to the rational design of quantum materials with exotic correlated phases and enhanced critical temperatures. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L5.00007: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L5.00008: \textit{in situ} studied correlated oxide LaNiO$_{3}$ ultra thin film by angle resolved photoemission spectroscopy H.K. Yoo, Y.J. Chang, K.S. Kim, L. Moreschini, D.W. Jeong, Y.S. Kim, A. Bostwick, E. Rotenberg, T.W. Noh Recently, $R$NiO$_{3}$ ($R$: rare earth) attracted increasing attention due to the possible realization of the electronic band structure similar to the high-temperature superconductor cuprates. Among them, LaNiO$_{3}$ based heterostructures have shown various fascinating physical properties such as dimensionality controlled electronic phase transitions. Theoretical works on confined LaNiO$_{3}$ through heterostructuring predicted cuprate-like band structure and magnetic properties. Here, we reports \textit{in-situ} angle resolved photoemission spectroscopy results on the LaNiO$_{3}$ films grown by pulsed laser deposition method. We carefully controlled the thickness of LaNiO$_{3}$ films from one to 30 unit cells and measured the thickness dependent band dispersions. First, we will discuss the strong electronic correlation effect in bulk-like band structure of thick LaNiO$_{3}$ films comparing to the previously reported LDA+DMFT calculation. Moreover, we will discuss the thickness dependent band structure. As decreasing the film thickness, we observed the charge redistribution of two Ni $e_{g}$ orbitals at the Fermi surface. The origins of thickness dependent electronic structure will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L5.00009: Influence of Symmetry on the Octahedral Rotations of Epitaxial RNiO$_3$ Thin Films I.C. Tung, Jian Liu, B. Gray, J. Chakhalian, J.M. Rondinelli, E.A. Karapetrova, J.H. Lee, M.J. Bedzyk, J.W. Freeland Understanding the structural and electronic behavior of ABO$_3$ thin films subjected to confinement, lattice misfit and broken symmetry at the interface in the ultra-thin limit is fundamentally important for the rational design of new materials [1]. However, the epitaxial strain will not only due to a change the in-plane lattice constants but also the octahedral rotations connected to bond angles and crystallographic symmetry. Here we present a study of the effect of the bulk lattice symmetry on octahedral rotations under epitaxial strain in thin films of RNiO$_3$ (R=La, Pr, Nd) grown on various substrates by pulsed laser deposition. A combination of high-resolution x-ray diffraction, polarization-dependent soft x-ray absorption spectroscopy, and first-principles density functional calculations has been applied to elucidate structural and electronic properties of the samples. Work at the Advanced Photon Source, 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., Phys. Rev. Lett., 107, 116805 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L5.00010: Probing the Nickelate Ground State in NdNiO3 Superlattices Ankit Disa, Divine Kumah, Joseph Ngai, Fred Walker, Charles Ahn The rare-earth nickelates of the type RNiO$_3$ exhibit tunable, sharp metal-insulator transitions as a function of R size, film thickness, and external fields. The nature of these transitions has been the subject of much study, including examination of the insulating ground state. NdNiO$_3$ has garnered particular interest due to a concomitant magnetic crossover to an antiferromagnetic state occurring at the metal-insulator transition temperature. Several previous studies have focused on thin films; in this work, we examine NdNiO$_3$ layers confined in heterostructures. The metal-insulator transition temperature can be manipulated based on the thickness of the NdNiO$_3$ layers within the heterostructure, and a suppression of the high-temperature metallic phase leads to a crossover to localization related to a change in dimensionality. The electronic structure of these phases probed using x-ray absorption spectroscopy enables us to elucidate the link between potential charge ordering and dimensionality in the ground state of this system. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L5.00011: Electrostatic control of the metal-insulator transition of ultrathin NdNiO$_{3}$ films Junwoo Son, Bharat Jalan, Adam P. Kajdos, Leon Balents, S. James Allen, Susanne Stemmer Rare earth nickelates (RNiO$_{3}$) exhibit a first order metal insulator transition upon cooling. Bulk studies on chemical doping indicated that both divalent and quatrovalent ions were effective in shifting T$_{MIT}$ to lower temperatures by $\sim$ 50 to 25 K for 1 \% hole and electron doping, respectively. However, separating the influence of structural distortions from band filling is particular important for the nickelates. Here we present a new approach to control the band-filling in nanoscale NdNiO$_{3}$ thin films by modulation doping. NdNiO$_{3}$ is remotely doped by interfacing it with a degenerately doped conventional band insulator, La-doped SrTiO$_{3}$. We show that the remote doping approach allows for purely electronic modulation of a carrier density in the absence of other structural changes. The proposed approach is experimentally tested using ultrathin (2.5 nm) NdNiO$_{3}$ films grown on La-doped SrTiO$_{3}$ films with different carrier concentrations. We show that remote doping systematically changes the charge carrier density in the NdNiO$_{3}$ film and causes a moderate shift ($\sim$ 20 K) in the metal-insulator transition temperature. These results will be discussed in the context of theoretical models of the materials exhibiting a metal-insulator transition. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L5.00012: Tuning Electronic Conductivity in Nickelate Heterostructures Divine Kumah, Joseph Ngai, Hanghui Chen, Ankit Disa, Karyn LeHur, Sohrab Ismail-Beigi, Charles Ahn, Fredrick Walker Bulk LaNiO$_{3}$ (LNO) is a metallic material, but for thin films below a critical thickness of about 5-6 unit cells of LNO, a transition occurs from metallic to insulating transport behavior. This transition is suppressed in superlattice structures with LaAlO$_{3}$ (LAO) spacers which are found to be metallic down to 3 unit cell thick LNO. To understand the differences between thin film and superlattices, we have identified structural differences in thin films and superlattice structures of LNO and LAO using a combination of first principles theory and synchrotron x-ray diffraction. Distortions are observed in the Ni-O-Ni bond angle in LNO epilayers grown on LAO resulting from relaxations at the vacuum-LNO interface. These distortions are suppressed in superlattice structures. Based on our observations, we propose that the electronic properties of nickelate superlattices can be designed by carefully selecting spacer layers that result in specific structural distortions in the LNO conducting layers. By tuning the observed distortions in this fashion, we identify a new pathway for controlling the electronic properties of rare earth nickelate compounds. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L5.00013: Composition spread studies of (RE)NiO$_{3}$-LaNiO$_{3}$ thin films Richard Suchoski, Ichiro Takeuchi, Kui Jin, Richard Greene Epitaxial thin films of La$_{x}$Nd$_{1-x}$NiO$_{3}$ and La$_{x}$Pr$_{1-x}$NiO$_{3}$ were grown via pulsed laser deposition on SrTiO$_{3}$ (001) with an automated~moving shutter system to create 100 nm thick composition spreads. These films have the advantage of providing a continuous spectrum of doping compositions at the same deposition conditions, such as temperature and oxygen pressure, without the need to prepare different stoichiometric targets or perform different depositions for desired compositions.~Composition evolution is verified via wavelength dispersive spectroscopy, and x-ray diffraction mapping shows that the lattice constant of the film varies continuously from LaNiO$_{3}$ to NdNiO$_{3}$. Simultaneous measurements of resistance versus temperature down to 4.2 K across the spreads show evolution in transport behavior. The details will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L5.00014: Conduction mechanisms in epitaxial and polycrystalline SmNiO$_{3}$ thin films Sieu D. Ha, Shriram Ramanathan Correlated oxides that exhibit metal-insulator phase transitions are emerging as potential candidates for switching devices and their fundamental physical properties are of interest. One such material is SmNiO$_{3}$, which has a transition temperature above room temperature ($\sim$400 K in bulk crystals). We present temperature- and bias-dependent conduction mechanisms in epitaxial and polycrystalline SmNiO$_{3}$ thin films. In both cases, at low electric field we observe thermally assisted hopping conduction through defect states. At high electric field the conduction transitions to a space-charge limited regime controlled by an exponential trap distribution. The trap decay parameter in epitaxial films does not have the expected 1/T temperature dependence, which may be a signature of band gap narrowing at high temperature due to the insulator to metal transition. The role of defects in affecting charge transport parameters will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L5.00015: Many-body effects in the capacitance of multilayers composed of Mott insulators Simon Hale, Jim Freericks Inhomogeneous dynamical mean-field theory is employed to investigate the non-linearity of the capacitance of multilayer nanostructures. The multilayer nanostructures are constructed with semi-infinite metallic leads coupled via a strongly correlated (Mott insulating) dielectric barrier. Results on the effects of varying barrier thickness temperature, potential difference, screening length, and chemical potential will be presented. In addition, we also intend to examine phase separation effects which have been experimentally measured to enhance the many-body capacitance over the geometric capacitance. [Preview Abstract] |
Session L6: Focus Session: van der Waals Bonding in Advanced Materials - Metal Oxide Frameworks & Active Materials
Sponsoring Units: DMPChair: Yves Chabal, University of Texas at Dallas
Room: 206B
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L6.00001: Trends in CO$_{2}$-MOF Binding from First Principles: Implications for Gas Separations Joshua Howe, Berend Smit, Jeffrey Neaton Metal-organic frameworks (MOFs) are a class of highly ordered, highly customizable nanoporous materials that are attractive for use in energy-relevant gas separations. MOF-253 Al(OH)(bpydc) can be post-synthetically modified by introduction of metal cations and charge-stabilizing anions [1]. These post-synthetically modified MOF-253 samples have been shown to exhibit enhanced CO$_{2}$-N$_{2}$ selectivity over the unmodified framework [1]. Here we use van der Waals-corrected density functional theory (DFT) to study CO$_{2}$ binding energy trends in this series of modified frameworks. Particular focus is paid to examining the predictive power of our calculations on modified bipyridine fragments as a proxy for the full framework, as well as the suitability of binding energy trends to predict measured gas selectivity trends [1]. We focus on the following series of 10 post-synthetic modifications: CoCl$_{2}$, CuCl$_{2}$, FeCl$_{2}$, NiCl$_{2}$, PdCl$_{2}$, Co(BF$_{4})_{2}$, Cu(BF$_{4})_{2}$, Fe(BF$_{4})_{2}$, Ni(BF$_{4})_{2}$, Pd(BF$_{4})_{2}$.\\[4pt] [1] E. Bloch, et. al, J. Am. Chem. Soc., 132, 14382-14384, 2010. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L6.00002: Ligand-Assisted Enhancement of CO$_{2}$ Capture in Tunable MOFs: Balancing Electrostatic and van der Waals Interactions Roberta Poloni, Berend Smit, Jeffrey B. Neaton Metal-organic frameworks (MOFs) are promising nanoporous materials for CO$_2$ separation technologies. Here, we use first-principles van der Waals (vdW)-corrected calculations to identify and understand how CO$_2$ binds to a novel ``BTT-type'' MOF [1] featuring open metal centers. Our study indicates that CO$_2$ binds to the open metal cation sites, but with an adsorption energy that can be enhanced by more than a factor of two depending on the choice of the bridging ligand. Judicious choices for metal cations and bridging ligands are shown to lead to a maximum binding energy of 0.67 eV for MgBTT. In all cases, the binding can be attributed to a combination of electrostatics and dispersion, both critically sensitive to the local environment, and contributing nearly equally to the overall binding strength. The possibility to independently tailor these energetics in a manner optimal for CO$_2$ capture is discussed in the context of recent experiments. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L6.00003: Simulations of physical adsorption of gases in IRMOFs Osaro Harriott, Silvina Gatica We performed grand canonical Monte Carlo simulations to study the adsorption of noble gases, H$_{2}$ and CO$_{2}$ in IRMOF-1. The IRMOF is modeled as a simple structure where a cubic lattice is composed of adsorption centers that reproduce the strength of the metallic corners and organic linkers in the real structure. From the adsorption isotherms obtained in our simulations we calculated the isosteric heat of adsorption and compare with available experimental results. Research supported by NSF and ACS, PRF. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:42PM |
L6.00004: Graphene Oxide Derived Carbons (GODC); High-Surface Area NanoPorous Materials for Hydrogen Storage and Carbon Capture Invited Speaker: Taner Yildirim Even though there has been extensive research on gas adsorption properties of various carbon materials based on activated carbon and nanotubes, there has been little work done on the gas adsorption properties of graphite oxide (GO). In this study [1], we show that one-and-a-half-century-old graphite oxide can be easily turned into a potentially useful gas storage material. In order to create high-surface nanoporous materials from GO, we used two different approaches. In the first approach, we have successfully synthesized graphene-oxide framework materials (GOFs) by interlinking GO layers by diboronic acids. The resulting GOF materials have well defined pore size and BET surface area up to 500 m2/g with twice larger heat of adsorption of H$_{2}$ and CO$_{2}$ than those found in other physisorption materials such as MOF5. In the second approach, we synthesized a range of high surface area GO derived carbons (GODC) and studied their applications toward H$_{2}$, CO$_{2}$ and CH$_{4}$ gas storage. The GODCs, with wide range of pore structure, have been prepared by chemical activation with potassium hydroxide (KOH). We obtain largely increased surface areas up to nearly 1900 m$^{2}$/g for GODC samples from 10 m$^{2}$/g for initial GO. A detailed experimental study of high pressure excess sorption isotherms on GODCs reveal an increase in both CO$_{2}$ and CH$_{4}$ storage capacities compared to other high surface area activated carbons. Finally, we compared the gas sorption properties of our GO-based matarials with other systems such as MOFs, ZIFs, and COFs. \\[4pt] [1] See http://www.ncnr.nist.gov/staff/taner for references and more information. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L6.00005: Isosteric heat of adsorption of CO$_{2}$ in bundles of carbon nanotubes. Mamadou Mbaye, Sidi Maiga, Silvina Gatica Using the grand canonical Monte Carlo method, we have evaluated the adsorption isotherms of CO$_{2}$ on the exterior of a bundle of carbon nanotubes. The isosteric heat is a property of the adsorbate that can be calculated from experimental or simulation data, and gives hints of the energy of adsorption and the structure of the adsorbate. From the simulated adsorption isotherms we calculated the isosteric heat of CO$_{2}$, Ar and CH$_{4}$ and compare with existing experimental data. Research supported by NSF and ACS, PRF. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L6.00006: Gas Adsorption and Selectivity in Zeolitic Imidazolate Frameworks from First Principles Calculations Keith Ray, David Olmsted, Ning He, Yao Houndonougbo, Brian Laird, Mark Asta Zeolitic Imidazolate Framework (ZIFs) are excellent candidate materials for carbon capture and gas separation. Here we employ the van der Waals density functional (vdW-DF) [1] in an analysis of the binding energetics for CO2, CH4 and N2 molecules in a set of ZIFs featuring different chemical functionalizations. We investigate multiple low-energy binding sites, which differ in their positions relative to functional groups on the imidazole linkers. In all cases an accurate treatment of van der Waals forces appears essential to provide reasonable binding energy magnitudes. We report results obtained from different parameterizations of the vdW-DF, providing comparisons between calculations and experimental values of the heat of adsorption [2]. This research is supported by the Energy Frontier Research Center ``Molecularly Engineered Energy Materials,'' funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001342. [1] M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, B. I. Lundqvist, Phys. Rev. Let. 92, 246401 (2004) [2] W. Morris, B. Leung, H. Furukawa, O. K. Yaghi, N. He, H. Hayashi, Y. Houndonougbo, M. Asta, B. B. Laird, O. M. Yaghi, J. AM. CHEM. SOC. 2010, 132, 11006-11008 [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L6.00007: A theoretical study of the hydrogen-storage potential of (H$_2$)$_4$CH$_4$ in nanotubes and MOFs Timo Thonhauser, Qi Li The material (H$_2$)$_4$CH$_4$, also called H4M, has exceptional hydrogen-storage potential of up to 33.3~mass\%, not including the hydrogen in CH$_4$.\footnote{W.L. Mao et al., Physics Today {\bf 60}, 42 (2007).} But, unfortunately, H4M is not stable under ambient conditions. For hydrogen storage near ambient pressure, it needs to be cooled to 65~K, and ambient temperature requires a pressure of 5--6~GPa.\footnote{W.L. Mao et al., Chem. Phys. Lett. {\bf 402}, 66 (2005).} In this study we use \emph{ab initio} methods based on van der Waals DFT\footnote{M. Dion et al., Phys. Rev. Lett. {\bf 92}, 246401 (2004).}$^,$\footnote{T. Thonhauser et al., Phys. Rev. B {\bf 76}, 125112 (2007).} to investigate the possibility of creating such pressures through external agents such as metal organic framework (MOF) materials and carbon nanotubes. We find that MOFs can create considerable pressure for H4M in their cavities, but not the required 5--6~GPa, and therefore moderate cooling is still necessary. On the other hand, carbon nano\-tubes can create these high pressures for H4M, but the fact that this pressure exists only inside the nanotubes---and not in-between tubes in e.g.\ a bundle---lowers the volumetric storage density and makes this option less favorable for practical applications. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L6.00008: The Role of Many-Body Dispersion Interactions in Molecular Crystal Polymorphism Leslie Leiserowitz, Noa Marom, Robert A. DiStasio, Jr., Viktor Atalla, Sergey Levchenko, Sergey Kapishnikov, James R. Chelikowsky, Alexandre Tkatchenko Molecular crystals often have several polymorphs that are close in energy (few meV per molecule), but possess very different physical and chemical properties. Treating polymorphism from first principles has been a long standing problem because conventional density-functional theory (DFT) lacks a proper description of long-range dispersion interactions that govern the structure and energetics of molecular crystals. Here we assess the effect of the many-body dispersion (MBD) energy on the structure and relative energies of the polymorphs of benchmark molecular crystals: glycine, alanine, and para-diiodobenzene. This is accomplished by using the recently developed first-principles DFT+MBD method [A. Tkatchenko, R.A. DiStasio Jr., R. Car, M. Scheffler, submitted], based on the earlier Tkatchenko-Scheffler (TS) dispersion correction [PRL 102, 073005 (2009)]. We show that the non-additive MBD energy plays a crucial role in making qualitatively and quantitatively accurate predictions for the structure and relative energies of polymorphs. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L6.00009: Interaction of endohedral molecular hydrogen with C$_{60}$: infrared study Toomas R{\~o}{\~o}m, Min Ge, D. H\"uvonen, U. Nagel, S. Mamone, M.H. Levitt, M. Carravetta, Y. Murata, K. Komatsu, Xuegong Lei, N.J. Turro We report on the dynamics of isotopically different hydrogen molecules, H$_2$, D$_2$ and HD, trapped in the molecular cages of a fullerene C$_{60}$[Min Ge et al., J. Chem. Phys. {\bf 134}, 054507 (2011), {\bf 135}, 114511 (2011)]. The infrared spectra were measured at temperatures from 5K to 300K and analyzed using a model of a vibrating rotor trapped in a spherical potential. The interaction potential was determined in the ground and in the first excited vibrational state of a hydrogen molecule. The isotropic part of the potential is similar for all three molecules studied. In HD@C$_{60}$ we observe the mixing of rotational states and an interference effect of the dipole moment terms due to the displacement of the HD rotation center from the fullerene cage center. A three-site Lennard-Jones potential in the pairwise additive five-dimensional potential energy surface reproduces the hydrogen IR spectrum with great accuracy[M. Xu et al., J. Chem. Phys.{\bf 130}, 224306 (2009)]. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L6.00010: ABSTRACT HAS BEEN MOVED TO D28.00007 |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L6.00011: The role of dispersion interactions in the formation of near-surface oxygen defects in CeO$_2$(111): Comparing DFT$+U$ with HF/DFT hybrid functionals using plane-waves as a basis set Joachim Paier, Christopher Penschke, Joachim Sauer Cerium oxide is of paramount importance in the field of heterogeneous catalysis and in solid oxide fuel cells. Its ability to easily release and store oxygen goes together with the distinct facility to form and heal oxygen vacancies. Upon its formation, the electrons left need to localize, but conventional semilocal density functionals (LDA, GGA) are known to fail here. Viable solutions to this problem are the DFT$+U$ approach and hybrid Hartree-Fock/DFT functionals being computationally more demanding [1]. However, common implementations of aforementioned approaches do not incorporate long-range dispersion interactions. Given that relaxation effects upon defect formation lead to more open structures and given the relatively high polarizability of Ce-atoms, dispersion interactions are supposed to be none-neglible. We apply Grimme's dispersion correction [2] and carefully cross-check results. Furthermore, we will give thorough estimates on dynamic effects and try to shed some light on the effects induced by the defect concentration. [1] M. V. Ganduglia-Pirovano, J. L. F. Da Silva, and J. Sauer, Phys. Rev. Lett. {\bf 102}, 026101 (2009). [2] S. Grimme, J. Comput. Chem. {\bf 27}, 1787 (2006). [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L6.00012: Interaction of W(CO)$_6$ with SiO$_2$ Surfaces Juan Shen, Kaliappan Muthukumar, Harald O. Jeschke, Roser Valenti The interaction of tungsten hexacarbonyl W(CO)$_6$ precursor molecules with SiO$_2$ substrates is investigated by means of density functional theory calculations with and without inclusion of long range van der Waals interactions. We consider two different surface models, a fully hydroxylated and a partially hydroxylated SiO$_2$ surface, corresponding to substrates under different experimental conditions. For the fully hydroxylated surface we observe only a weak interaction between the precursor molecule and the substrate with physisorption of W(CO)$_6$. Inclusion of van der Waals corrections results in a stabilization of the molecules on this surface. In contrast, we find a spontaneous dissociation of the precursor molecule on the partially hydroxylated SiO$_2$ surface where chemisorption of a W(CO)$_5$ fragment is observed upon removal of one of the CO ligands from the precursor molecule. Irrespective of the hydroxylation, the precursor molecule prefers binding of more than one of its CO ligands. In the light of these results, implications for the initial growth stage of tungsten nano-deposits on SiO$_2$ in an electron beam induced deposition process are discussed. [1] K. Muthukumar et al. Phys. Rev. B (in press) (2011) [Preview Abstract] |
Session L7: Carbon Nanotube Hybrid Structures - Properties, Synthesis, Characterization
Sponsoring Units: DMPChair: Apparao Mohan Rao, Clemson University
Room: 207
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L7.00001: Self-assembly and electrostatic interactions of carbon nanotubes in nonpolar dielectric liquids Luciana Oliveira, Ramakrishna Podila, Jay Gaillard, Steven Serkiz, Apparao Rao We report on the self-assembly of carbon nanotubes (CNTs) in nonpolar dielectric liquids under the influence of DC-generated electric fields. This process gives rise to electrostatic and hydrodynamic interactions of the CNTs in insulating nonpolar solvents. While some studies of the self-assembly of carbon nanotubes in response to a DC-field have been carried out in conductive solvents, the analysis of the self-assembly process is complicated by the current flow temporally affecting the particle charge and an assembly timescale of tenths to hundredths of seconds. In contrast, experiments in insulating liquids allow for the investigation of self-assembly processes where: the particle charge is not expected to change as a function of time and at a timescale of seconds this allows for an investigation of the transient states of the assembly process. In the presence of an electric-field, CNTs present in the solution experience an electrophoretic force due to their surface charges. When a DC field is applied across the electrodes, CNT bundles move according to their electrophoretic mobility. We find that the threshold voltage, above which the insulator-to-conductor transition occurs, varies sensitively as a function of zeta potential and hydrodynamic particle size. In addition, a percolation power law supports the observed threshold voltage as a function of CNT concentration and zeta potential. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L7.00002: A Group Theoretic Approach to Nonlinear and Gradient Elastic Terms for Graphene and Carbon Nanotubes Alexander Alemi, James Sethna Nonlinear and gradient corrections to the elastic theory of graphene and carbon nanotubes are important for determining the thermal conductivity, mechanical properties under large deformations, dissipation in nanotube oscillators and other mechanical properties. By applying group theory, we can systematically and efficiently work out all nonlinear elastic terms that can appear in a Ginzburg-Landau formulation of the elastic free energy in terms of the material displacement gradient tensor. Graphene serves as a perfect example material, having an interesting hexagonal symmetry and being a material in which nonlinear elastic terms are important. This approach allows us to demonstrate the usual cubic corrections. Additionally, we find higher order strain corrections which correspond to bending and rolling the graphene sheet, as well as gradient terms which are important for describing the long wavelength phonon dispersion. We will enumerate the most important contributions to the free energy, and measure the corresponding elastic constants with simulation. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L7.00003: A Signature of Spatial Correlations between rare earth ions and single-wall nanotubes wrapped with DNA in their mixed solution Tetyana Ignatova, Slava V. Rotkin We propose that the fluorescence resonance energy transfer (FRET) between the rare earth ions (REI) and single-wall nanotubes (SWNT) can be used to measure their Coulomb correlation in solution. As a calibration experiment the FRET between two different REIs, being the energy donor and the acceptor, in their mixed solution has been used. From the photoluminescence decay time we were able to extract the characteristic distance between unlike REIs. Our study revealed negative correlation (the repulsion) for Tb-Eu solution. In the case of the solution containing the REI and the SWNTs wrapped with DNA we observed a significant positive correlation (the attraction and the complex formation). The data is in a good agreement with the theoretical estimates and allows to propose REIs and their FRET as a sensitive tool for detecting kinetics of interaction of SWNTs in aqueous solutions. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L7.00004: Computational Modeling of Cancer Treatment Using Carbon Nanotubes Feng Gong, Hongyan Zhang, Jin Wen Tan, Dimitrios V. Papavassilliou, Vincent B.C. Tan, Swee Hin Teoh, Hai Minh Duong Laser thermal therapy selectively kills cancer cells without harming surrounding cells due to the high-energy absorbance of the functionalized carbon nanotubes (CNTs). However, the technique application is still very limited due to lack of experimental and computational works to understand how to kill cancer cells effectively and how the CNTs are heated by the external laser. The goal of this work is to present integrated computer models to capture changes in CNT heat transfer characteristics due to variations in the properties of CNTs and tissues during laser surgery. Numerical results show that the models are able to characterize variations of tissue properties for laser surgical procedures (by three dimensional finite element models) and predict anisotropic temperature fields within the CNTs (by finite different models). The current model is made more realistic and accurate than previous models by taking into account the anisotropic properties of CNTs and the energy loss from the CNT surface. The effects of the CNT concentration, morphology, orientations, and the power and heating duration of the laser are also investigated. The developed models help experimentalists to understand cancer treatment mechanisms and optimize operating conditions of the laser thermal therapy. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L7.00005: Synthesis and Electronic Properties of Silicon-Nitrogen Hetero-doped Single Walled Carbon Nanotubes Martha Audiffred, Ana Laura Elias, Humberto R. Gutierrez, Florentino Lopez-Urias, Humberto Terrones, Mauricio Terrones, Gabriel Merino We investigated the stability and electronic properties of hetero-doped carbon nanotubes using first-principles density functional theory. Silicon, silicon-nitrogen, and silicon-oxygen were incorporated within the lattice of different types of single-walled carbon nanotubes. The structural stability, electronic density of states, doping energy, band structure, HOMO and LUMO were analyzed. When silicon and nitrogen are bonded and inserted in the nanotube lattice, non-dispersive bands appear around the Fermi level. The Nitrogen arranged in a pyridine-like fashion together with a silicon atom placed inside the vacancy was also studied. The latter configuration becomes more stable than the substitutional nitrogen embedded in the (9,0) and (5,5) nanotubes. We have also succeeded in the synthesis of Si and SiN-doped single-walled carbon nanotubes (CSixNy -SWNTs) by chemical vapor deposition. We carried out Raman spectroscopy, high-resolution electron microscopy, electron energy loss spectroscopy, energy-dispersive X-Ray spectroscopy, Auger spectroscopy and X-Ray photoelectron spectroscopy, in order to identify the presence of both dopants within the nanotube lattice. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L7.00006: Magneto-photoluminescence in lanthanide-bearing endohedral metallofullerenes with various cage symmetries Travis Merritt, Harry Dorn, Giti A. Khodaparast, Steve McGill Taken as a family, endohedral metallofullerenes (EMF) nanomaterials provide opportunities for exquisite functional tunability at the nanoscale, enabling a wide range of synthetic nanoparticles with diverse sizes, symmetries, electronic, optical and, especially, magnetic properties. In particular, metallofullerenes incarcerating lanthanide ions will permit endohedral luminescence due to the 4f optically-active electrons being uninvolved in the stabilizing charge transfer between the endohedral guest and cage. In addition, if those lanthanide ions possess optical transitions beyond the absorption onset of the cage, a well-defined optical spectrum may be observed for the metallofullerene system. In this talk, several magneto-optical and time-resolved studies at high magnetic fields on lanthanide-based EMFs with different cage symmetries will be presented, where the residual magnetic degeneracies in the lanthanide ion energy levels are lifted and observed in the optical spectrum with magnetic field strengths in excess of 10 T. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L7.00007: Exploring the Fundamental Properties of Carbon Nanotubes using Electromechanical Resonators Mehmet Aykol, William Branham, Shun-Wen Chang, Rohan Dhall, Stephen B. Cronin By monitoring the nanoelectromechanical (NEM) response of suspended individual carbon nanotubes (CNT), we measure the expansion coefficient of individual suspended single-walled CNTs. Here, the temperature dependence of the mechanical resonance frequency of NEM CNT resonator is used to determine the change in tension on the device caused by the expansion on the nanotube. We also measured the effects of gas adsorption on the CNT surface, by measuring change in conductivity and its response to gate doping. By monitoring the mass loading on the surface of the resonator we calculate the adsorption energy of the specific gasses on the surface of the nanotube. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L7.00008: Fano Resonances in Mid-Infrared Spectra of Single-Walled Carbon Nanotubes Fran\c{c}ois Lapointe, \'{E}tienne Gaufr\`{e}s, Isabelle Tremblay, Nathalie Tang, Patrick Desjardins, Richard Martel We show that optical phonon modes in single-walled carbon nanotubes (SWNTs) become observable in mid-infrared (MIR) spectroscopy by the means of Fano resonances. The scattering of a low energy electronic continuum over phonon discrete states yields anti-resonances that are recognizable in the spectra by their characteristic asymmetric line shape. Experimentally, we control the charge carrier density in SWNTs by \emph{p}~doping with different molecular oxidizers at saturation and compare the spectra of doped and intrinsic samples. The only measurable feature in the intrinsic state is a kink at $\sim865$~cm$^{-1}$. Kinks at $\sim1600$ and $\sim1250$~cm$^{-1}$ appear upon doping. We find no significant differences between the dopants; hence the bands belong to the SWNTs. Fitting of the band at $\sim1600$~cm$^{-1}$ yields good agreement with a phenomenological Fano resonance model. Finally, SWNTs mats are functionalized with bromophenyls, which are known to increase the number of defects. We find that upon \emph{p}~doping, the Fano resonances' cross sections of damaged SWNTs increase compared to that of \emph{p}~doped pristine SWNTs. Hence, we conclude that defects lower the symmetry of the lattice and activate optical phonon modes in MIR spectroscopy. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L7.00009: Computational modeling of electron transfer in hydrogenase and carbon material complexes Kwiseon Kim, Hai Long In biohybrid and biomimetic devices for energy conversion, the electron transfer between the enzyme and the electrode plays a central role. We use hydrogenase and carbon material as model systems and investigate the binding and electron transfer configurations between hydrogenase and carbon materials, including single-wall carbon nanotubes and graphene surfaces. We use Brownian dynamics simulations to sample the hydrogenase/carbon material phase-space. The results provide an atomistic picture of how enzyme interacts with the electrode materials. We find that the optimal enzyme/electrode binding configurations are not optimal for electronic tranfer. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L7.00010: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L7.00011: Theory of coherent phonons in carbon nanoribbons Gary Sanders, Chris Stanton We have developed a microscopic theory for the generation and detection of coherent phonons in armchair and zigzag carbon nanoribbons using an extended tight-binding model for electrons and a valence force field model for the phonons. Coherent phonons are generated through the electron-phonon deformation potential interaction and we use Heisenberg's equation to obtain a driven oscillator equation for the coherent phonon amplitudes. We find that the driving function depends explicitly on the time-dependent photoexcited carrier distribution functions. We simulate the generation and detection of coherent phonons in coherent phonon spectroscopy experiments. We consider coherent phonon oscillations of the lowest lying radial breathing like mode (RBLM) in zigzag and armchair nanoribbons as a function of ribbon width and pump/probe polarization angle. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L7.00012: Biomimetic Carbon Nanotube for Catalytic Hydrolysis of CO$_{2}$: First Principles Investigation of Role of Oxidation State and Metal Substitution Donghwa Lee, Yosuke Kanai Reducing the amount of carbon dioxide (CO$_{2})$ in the atmosphere is one of the most important challenges we face in this century. Metallo-enzyme, carbonic anhydraze (CA), is known for its catalytic activity of CO2 hydrolysis, and a number of research groups have been experimentally working to mimic this activity in small molecules for the CO2 collection processes. Using accurate first principles electronic structure calculations, we investigate how the catalytic hydrolysis reaction of CA can be mimicked in a metal-porphyrin carbon nanotube system. Our work shows that the two-step catalytic process can be improved remarkably by controlling the oxidation state and also through the metal substitution in the porphyrin unit. Our work shows the feasibility of CO$_{2}$ hydrolysis in the metal-porphyrin carbon nanotube and also how the catalytic activity could be improved. This work is Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L7.00013: Coupling of carbon nanotubes and graphene nanoribbons by the titanium and vanadium nanowires: First-principles study Chi-Hsuan Lee, Chih-Kai Yang We investigate the combined structure of a carbon nanotube (CNT) and graphene nanoribbon (GNR) through the adsorption of a titanium or vanadium nanowire (NW), using first-principles calculations. The binding energy depends upon the stacked configuration and is much larger than that between the two subsystems without the nanowire. The band structure reveals strong hybridization between {\$}d{\$} orbitals of the transition metal and {\$}p{\$} orbitals of the carbon atoms. Furthermore, if the CNT is deposited near the border of GNR, structural stability is enhanced and magnetic moments of the edge atoms are reduced. The result points to possible application for synthesizing nanowires in nanoelectronic devices. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L7.00014: Synthesis and phase behavior of AgI atomicwires in carbon nanotubes Shin-ichi Ito, Ryo Kitaura, Yasumitsu Miyata, Hisanori Shinohara $\alpha -$phase silver iodide($\alpha $-AgI) has been well-known as a solid state ionic conductor due to its superionic conductivity and is one of the promising candidates for solid-state electrolytes for various electrochemical devices. Below 420 K, $\alpha $-AgI undergoes a phase transition into the poorly conducting $\beta $- and $\gamma $-polymorphs, thereby limiting their applications. Recently, we have found that AgI nanowires with a diameter of 10 nm can retain $\alpha $-phase even at 313 K where size and morphology of AgI presumably plays a great role in this $\alpha $-AgI stabilization [1]. To investigate the effect further, we have focused on low dimensional nanostructure of AgI with a diameter of 5 - 10 nm. For this purpose, one-dimensional (1D) nanospace of carbon nanotubes (CNTs) has been utilized. CNTs have unique 1D nanospace ranging in diameter from 0.4 to 50 nm, which can stabilize otherwise unstable nanomaterials. We have synthesized AgI@CNTs by the sublimation method already reported [2]. In the presentation, we will discuss detailed characterization of structure and properties based on electron beam diffraction and HR-TEM. \\[4pt] [1] R. Makiura et al., Nature.Mater.8, 476 (2009). \\[0pt] [2] R. Kitaura, et al., Nano Res.1, 152 (2008). [Preview Abstract] |
Session L8: Focus Session: Frustrated Magnetism - 1D
Sponsoring Units: DMP GMAGChair: Oleg Starykh, University of Utah
Room: 208
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L8.00001: Magnon pairing in Quantum Spin Nematic Invited Speaker: Mike Zhitomirsky The phenomenon of the Bose-Einstein condensation is inherent not only to superfluid He-4 and cold atomic gases but also describes in a unified way a variety of field-induced transitions in quantum magnets. In this talk we discuss a novel example of the Bose-Einstein condensation of bound magnon pairs. The binding mechanism is based on a competition between ferro- and antiferromagnetic exchange bonds in a frustrated quantum spin system. As a result bound magnon pairs are formed in the fully polarized magnetic state at high fields. Upon decreasing field magnon pairs undergo a condensation into a state which is bosonic analog of a BCS superconductor [1]. The magnon-pair condensate lacks a conventional transverse magnetic order and is described instead by a quadrupolar or spin-nematic order parameter. We consider in detail magnon-pairing mechanism for two spin models: frustrated chains weakly coupled by interchain interactions and frustrated square-lattice antiferromagnet, which exhibit high-field spin-nematic states. Our theory predicts existence of the long-range spin-nematic phase in the frustrated chain material LiCuVO4. We also review recent experimental evidences, which support presence of a new phase in this material. Work done in collaboration with H. Tsunetsugu (ISSP, University of Tokyo) \\[4pt] [1] M. E. Zhitomirsky and H. Tsunetsugu, Europhys. Lett. 92, 37001 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L8.00002: Competition between Spin Nematic and Spin Density Wave Orders in Spatially Anisotropic Frustrated Magnets in Magnetic Fields Masahiro Sato, Toshiya Hikihara, Tsutomu Momoi Magnetic multipolar order including spin nematic order is one of the current topics in frustrated magnetism. Recently, frustrated spin chains with ferromagnetic nearest-neighbor coupling J1 and antiferromagnetic next-nearest-neighbor one J2 have been theoretically shown to exhibit multipolar quasi long-range orders in the wide region of J1/J2 as an external magnetic field is applied. In addition, it is known that several kinds of quasi one-dimensional cuprates can be described by this J1-J2 spin chain. Particularly, a recent experiment shows that LiCuVO4, one of the cuprates, possesses a new phase near saturation and it is expected to be a spin nematic ordered phase. Motivated by these results, we have completed the field-temperature phase diagram for spatially anisotropic magnets consisting of weakly coupled J1-J2 spin chains, by making use of accurate results of the single J1-J2 spin chain. The phase diagram contains spin nematic and spin-density-wave ordered phases, and these two orders compete with each other. We will discuss some universal features of the phase diagram and the relevance of our result to LiCuVO4. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L8.00003: Unusual interchain coupling effects in frustrated edge-shared chain cuprates with ferromagnetic NN in-chain coupling Stefan-Ludwig Drechsler, Satoshi Nishimoto, Jeroen van den Brink, Jiri Malek, Roman Kuzian, Johannes Richter, Miriam Schmitt, Helge Rosner We consider the effect of weak antiferromagnetic interchain coupling (AFM IC) on the saturation field, the magnetization curve, the phase diagram of multipolar phases at high magnetic fields, the dynamical magnetic structure factor, as well as the pitch angle at ambient fields applying the DMRG-technique to clusters of coupled long chains and the hard-core boson method to quasi-1D spin nematics at T=0 The critical AFM IC couplings for various multipolar phases and types of IC are determined. The results are applied to Li$_{2}$CuO$_{2}$, LiVCuO$_{4}$, Ca$_{2}$Y$_{2}$Cu$_{5}$O$_{10}$, as well as to linarite. The multipolar phases can be stabilized by easy-axis spin anisotropy. Linarite and LiVCuO$_{4}$, are found to be good candidates for the detection of mulipolar phases. Microscopic considerations based on the extended five-band Hubbard model and L(S)DA+$U $calculations provide exchange integrals which support the empirically found values for the main exchange integrals. We discuss the applicability of spin-wave theory and the role of quantum fluctuations for a correct description of magnetic excitations. Cases when a weak IC coupling dominates solely or predominantly the saturation field and/or the pitch angle are emphasized. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L8.00004: Frustrated 1D Spin-S chain Below Saturation Magnetization Teimuraz Vekua, Alexei Kolezhuk, Fabian Heidrich-Meisner, Sebastian Greschner, Marcelo Arlego, Gerardo Rossini, Andreas Honecker Ground states of frustrated spin-S chains in strong magnetic field in the immediate vicinity of saturation are mapped out. For ferromagnetic nearest-neighbor and frustrating antiferromagnetic next-nearest-neighbor exchange interactions generic feature is metamagnetic behavior under the influence of an external magnetic field for small S, in the form of a first-order transition to the fully polarized state. The magnetization jump increases gradually starting from an S-dependent critical value of exchange couplings and takes a maximum in the vicinity of a ferromagnetic Lifshitz point. The metamagnetism results from resonances in the dilute magnon gas caused by an interplay between quantum fluctuations and frustration. For antiferromagnetic nearest neighbour interactions generic feature is emergence of two-component Luttinger liquid phase and series of phase transitions between that phase and chiral phase. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L8.00005: Quasi-one-dimensional antiferromagnetism and multiferroicity in CuCrO$_4$ Reinhard K. Kremer, J.M. Law, P. Reuvekamp, R. Glaum, C. Lee, J. Kang, M.-H. Whangbo The bulk magnetic properties of the new quasi-one-dimensional Heisenberg antiferromagnet, CuCrO$_4$, were characterized by magnetic susceptibility, heat capacity, optical spectroscopy, EPR and dielectric capacitance measurements and density functional evaluations of the intra- and interchain spin exchange interactions. We found type-II multiferroicity below the N\'{e}el temperature of 8.2(5)~K, arising from competing antiferromagnetic nearest-neighbor ($J_{\rm nn}$) and next-nearest-neighbor ($J_{\rm nnn}$) intra-chain spin exchange interactions. Experimental and theoretical results indicate that the ratio $J_{\rm nn}$/$J_{\rm nnn}$ is close to 2, putting CuCrO$_4$ in the vicinity of the Majumdar-Ghosh point. First low-temperature neutron powder diffraction data are consistent with a canted magnetic structure below $\sim$8 K. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L8.00006: Magnetic field induced ferroelectric transition of quantum spin chain system Rb$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$ Yukio Yasui, Yudai Yanagisawa, Ryuji Okazaki, Ichiro Terasaki, Yasuhiro Yamaguchi, Tsuyoshi Kimura Dielectric and magnetic properties have been studied for Rb$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$, which includes quasi one-dimensional spin $1/2$ chains formed of edge-sharing CuO$_{4}$ square planes called CuO$_{2}$ ribbon chains. The system does not exhibit a magnetic transition above temperature $T>2$K owing to quantum fluctuation and low dimensionality. We have observed anomalous increase of dielectric constant $\varepsilon$ with decreasing $T$ below $\sim 50$ K, which is originated from growing a short range ordering of a helical magnetic structure. For an external magnetic field $H>0.5$T, a peak structure is observed in the $\varepsilon -T$ curves at $T_{c} \sim 8$K and the ferroelectric polarization has been observed below $T_{c}$. However, the magnetic susceptibility and specific heat do not have anomaly at $T_{c}$ for $H>0.5$T. The anomalous increase of $\varepsilon$ and field-induced ferroelectric transition are found to be suppressed by impurity doping such as Zn and Ni atoms to the Cu sites. These results indicate that the ferroelectric transition is found to be induced by applying field without magnetic transition which strongly suggests a new type of ferroelectric transition triggered by the magnetism of frustrated quantum spin systems. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L8.00007: On the onset of incommensurate behavior in the $J_1$-$J_2$-chain with an odd number of sites Andreas Deschner, Erik S. Sorensen The anti-ferromagnetic spin-$1/2$-$J_1$-$J_2$-chain is one of the most researched spin-systems. For a chain with an even number of sites the ground-state is analytically known at $J_2/J_1 = 0.5$ and at $J_2/J_1 > 0.5$ spin-spin-correlations become incommensurate. Very little is known about how this incommensurability is manifested in chains with an odd number of sites. In this presentation we show results of variational calculations for $J_1$-$J_2$-chains with an odd number of sites and open boundary conditions in this incommensurate regime. The results indicate that the system becomes gapless at $J_2/J_1 \approx 0.53$. We show results for the on-site magnetization, the entanglement as well as correlation functions and discuss how the incommensurability dramatically affects them. In particular we show how the usual well defined single soliton excitation breaks up at $J_2/J_1 \approx 0.53$. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L8.00008: Broken vector spin chirality in biatomic Fe chains on Ir(001) S. Blugel, Y. Mokrousov, M. Menzel, R. Wieser, K. von Bergmann, E. Vedmedenko, A. Kubetzka, R. Wiesendanger, S. Heinze We investigate from {\em ab initio} the magnetism of biatomic Fe chains, which form due to self-organization on the (5$\times$1)-reconstructed Ir(001) surface [1,2]. Using the {\tt FLEUR} code [3], we calculate the magnetic properties and exchange interactions in this system, finding a very small Heisenberg exchange along the chain of the order of 10 meV/Fe-atom. Upon including spin-orbit coupling we obtain the contribution from the Dzyaloshinskii-Moriya interaction and find that it leads to a 120$^{\circ}$ spin-spiral ground state of the Fe chains with a unique rotational sense. The results of the Monte-Carlo simulations based on the parameters from {\it ab initio} are in a very good agreement to STM experiments on the system. Moreover, simulations indicate a robustness of the spin chiral order parameter, which decays with temperature much slower than the scalar spin correlation, in analogy to a vector spin chiral liquid state. We discuss possible applications of the magnetism in these chains with respect to the transfer of information on the nanoscale.\newline [1] L. Hammer {\it et al.}, Phys. Rev. B {\bf 67}, 125422 (2003). [2] Y. Mokrousov \textit{et al.}, Phys. Rev. B \textbf{80}, 195420 (2009). [3] www.flapw.de [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L8.00009: Highly frustrated quantum magnetism in the mineral azurite: multi-step approach from first-principles computations to experimental data Harald O. Jeschke, Ingo Opahle, Roser Valenti, Hena Das, Tanusri Saha-Dasgupta, Michael Lang, Shijie Hu, Xiaoqun Wang, Robert Peters, Andreas Honecker The natural mineral azurite Cu$_3$(CO$_3$)$_2$(OH)$_2$ is a frustrated magnet displaying unusual and controversially discussed magnetic behavior. We perform a theoretical study based on density functional theory as well as state-of-the-art numerical many-body calculations [1]. We propose an effective generalized spin-1/2 diamond chain model which provides a consistent description of experiments: low-temperature magnetization, inelastic neutron scattering, nuclear magnetic resonance measurements, magnetic susceptibility as well as new specific heat measurements. With this study we demonstrate that the balanced combination of first principles with powerful many-body methods successfully describes the behavior of this frustrated material. \newline [1] H. O. Jeschke {\it et al.}, Phys. Rev. Lett. {\bf 106}, 217201 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L8.00010: Magnetic anisotropy in the frustrated spin-ladder system BiCu$_{2}$PO$_{6}$ from magnetostriction in pulsed fields Atsuko Uchida, Yoshimitsu Kohama, Shuang Wang, Marcelo Jaime, Christian R\"uegg The spin interactions in BiCu$_{2}$PO$_{6}$ have been studied by inelastic neutron scattering, magnetic susceptibility, and numerical calculation. There is strong frustration between magnetic interactions along the ladder leg, $J_{Leg}$ and $J_{NNN}$, and it has been pointed out that a spin gap persists in this frustrated system. Both $J_{leg}$ and $J_{NNN}$ are intra-ladder and two-leg ladder is always gapped. Longitudinal magnetostriction (MS) measurements were performed using a fiber optic strain gauge in a 60 T pulsed magnet [1] . Specific heat ($C_{p})$ and magnetocaloric effect (MCE) measurements were performed in a 35 T DC magnet. $C_{p}$ vs $T$ was obtained using both a thermal relaxation time and dual slope techniques. We have in this way determined the (H,T) phase diagram of BiCu$_{2}$PO$_{6}$ up to 45 T. The 3D character of phase transitions is suggested by the observation of sharp anomalies in CM and \textit{$\Delta $}L/L. Our MCE and MS data provides direct evidence of first-order phase transitions for H//c, while phase transitions for H//a and H//b are characterized as second-order phase transitions. The anisotropic and complex phase boundaries will be discussed. \\[4pt] [1] Daou R et al., \textit{Rev. Sci. Instrum}. \textbf{81}, 033909 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L8.00011: Magnetism of a Frustrated Four-Spin-Tube Wolfram Brenig, Marcelo Arlego We report on the magnetism of a frustrated four-leg spin-$1/2$ tube (FFST) [1]. Using a combination of series expansion, based on the continuous unitary transformation method and density-matrix renormalization group we analyze the ground-state correlations, and the one-, and the two-particle excitations in the regime of strong rung-coupling. We find that frustration destabilizes the spin-gapped quadrumer singlet-phase of the FFST, leading to first order quantum phase transitions. Apart from the well-know triplon branch of two-leg spin-ladders, the FFST is shown to sustain additional elementary excitations, including a singlon, and additional triplons. Finally, in the two-particle sector the FFST exhibits several collective (anti)bound states. Frustration has significant impact on the FFST leading to a flattening of the ground-state energy landscape, a mass-enhancement of the excitations, and a relative enhancement of the (anti)binding strength.\\[4pt] [1] Marcelo Arlego and Wolfram Brenig Phys. Rev. B {\bf 84}, 134426 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L8.00012: Pursuit of a spin Bose-metal phase in Hubbard-type models on the two-leg triangular strip Ryan V. Mishmash, Ivan Gonzalez, Roger Melko, Olexei I. Motrunich, Matthew P.A. Fisher Motivated by recent experiments on the organic materials $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ and EtMe$_3$Sb[Pd(dmit)$_2$]$_2$, we numerically investigate the Mott metal-insulator transition in a system of interacting, itinerant electrons at half-filling on the two-leg triangular strip (i.e., zigzag chain). Previous work [1] has revealed that an exotic ``spin Bose-metal'' (SBM) phase with three gapless modes is stabilized on the zigzag strip in a pure spin model of Heisenberg exchange supplemented with four-site cyclic ring exchange, a model appropriate for describing weak Mott insulators near the Mott transition. Indeed, a physically appealing picture of the realized SBM phase is to view it as a particular Mott insulating instability out of a two-band metal of interacting electrons. Guided by this idea, we perform large-scale DMRG calculations across the Mott transition in various Hubbard-type models (e.g., with on-site repulsion, longer-ranged repulsion, and/or explicit spin exchange terms). We focus on the successes and failures of describing the insulating phase near the transition within the SBM framework. Finally, the implications of our findings to the full 2D triangular lattice will be discussed.\\[4pt] [1] D. N. Sheng \emph{et al.}, PRB {\bf 79}, 250112 (2009). [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L8.00013: Symmetry protected topological phases for 1-dimensional spin-1 antiferromagnets Zheng-Xin Liu, Xie Chen, Xiao-Gang Wen Symmetry protected topological phases for 1-dimensional S=1 antiferromagnets respecting D2+T symmetry (D2 is a point group and T is the time reversal symmetry) are studied. There are 15 different nontrivial topological phases, all of them are non-symmetry-breaking. One of these phases is the usual Haldane phase, and the others are new. We find that four of the nontrivial SPT phases can be realized in spin-1 chains and the rest can be realized in spin-1 ladders. We propose experimental methods to distinguish all of these phases. [Preview Abstract] |
Session L9: Focus Session: Complex Bulk Oxides: Theoretical techniques for oxides
Sponsoring Units: DMP GMAGChair: James Rondinelli, Drexel University
Room: 209
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L9.00001: Thermodynamics of Ferrotoroidic Materials Avadh Saxena, Teresa Castan, Antoni Planes The three primary ferroics, namely ferromagnets, ferroelectrics and ferroelastics exhibit corresponding large (or even giant) magnetocaloric, electrocaloric and elastocaloric effects when a phase transition is induced by the application of an appropriate external field ($E$, $H$ and stress). Recently the suite of primary ferroics has been extended to include ferrotoroidic materials in which there is an ordering of toroidic moments in the form of magnetic vortex-like structures, examples being LiCo(PO$_4$)$_3$ and Ba$_2$CoGe$_2$O$_7$. We formulate the thermodynamics of ferrotoroidic materials. Within a Landau free energy framework we calculate the toroidocaloric effect by quantifying isothermal entropy change (or adiabatic temperature change) in the presence of an applied toroidic field ($G=E\times H$) when usual magnetization and polarization may also be present simultaneously. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L9.00002: Theory of K-edge Resonant Inelastic X-ray Scattering for Perovskite Manganites Tsezar F. Seman, Xuerong Liu, John P. Hill, Michel van Veenendaal, Keun Hyuk Ahn We present calculations of K-edge resonant inelastic x-ray scattering (RIXS) spectrum for layered and three dimensional perovskite manganites with charge, orbital, and spin orderings. We extend the approach in Ref. [1] to the tight binding model for the manganites, calculate RIXS intensity in momentum and energy space, and compare with experiment data. The results show strong dependence of the RIXS intensity on momentum, which agrees well with experimental observation. We discuss its implications on the material properties and the RIXS process. \\[4pt] [1] K. H. Ahn, A. J. Fedro, and M. van Veenendaal, Phys. Rev. B 79, 045103 (2009). [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L9.00003: Nonreciprocal Directional Dichroism and Toroidalmagnons in Helical Magnets Shin Miyahara, Noubo Furukawa We investigate a dynamical magnetoelectric effect owing to a magnetic resonance in helical spin structures through a coupling between magnetization and electric polarization via a spin current mechanism [1]. We show that the magnon has a dynamical magnetic moment dM and an electric moment dP which is perpendicular to dM, simultaneously, i.e., a dynamical toroidal moment T\^{}d = dM x dP under the external magnetic fields, and thus named it as a toroidalmagnon. The toroidalmagnon exists in most conical spin structures due to generality of the spin current mechanism. In absorption of electromagnetic wave, the toroidalmagnon excitation process generally induces nonreciprocal directional dichroism as a consequence of an interference of the magnetic and the electric responses. The nonvreciprocal directional dichroism should be experimentally observed in various cycloidal multiferroic materials,$ e.g. \quad R$MnO{\_}3. [1] H. Katsura, A.V. Balatsky, and N. Nagaosa: Phys. Rev. Lett.\textbf{ 98} 027203 (2007) [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:42PM |
L9.00004: First-principles Wannier function based methods for disordered materials and applications to studies of oxides and Fe-superconductors Invited Speaker: Wei Ku This talk will discuss recently developed first-principles methods for materials with disordered impurities, and their applications to case studies of correlated oxides and Fe-based superconductors containing vacancies, substitutions and intercallants. Simplified via the use of Wannier functions, the first method [1] is to unfold the one-particle spectral function from the reduced Brillouin zone of a broken symmetry state back to the regular Brillouin zone of the normal state. This unfolding not only allows a clearer visualization of the physical effects of the broken translational symmetry, but also connects directly to the experimental spectral weight of angular resolved photo emission spectroscopy. The second method [2] is to reduce the computational expense of configuration-averaged spectral function of disordered materials by orders of magnitude, to allow inclusion of large length scale required for weakly localized states and short-range orders. This Wannier function based method is systematically improvable, beyond-mean-field, and not perturbation limited. Case studies to be discussed include dilute magnetic semiconductors [3], transition metal oxides[1,2], and Fe-based superconductors [4,5]. \\[4pt] [1] Wei Ku et al, PRL 104, 216401 (2010)\\[0pt] [2] T. Berlijn et al, PRL 106, 077005 (2011)\\[0pt] [3] T. S. Herng et al, PRL 105, 207201 (2010)\\[0pt] [4] C.-C. Lee et al, PRL 103, 267001 (2009)\\[0pt] [5] C.-H. Lin et al, arXiv:1107.1485 [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L9.00005: Incommensurate spin excitations stabilized multiferroic phase in CuO Lixin He Cupric oxide is a unique magnetoferroelectrics with transition temperature way above the boiling point of liquid nitrogen. However, the mechanism of high T$_c$ multiferroicity in CuO is still puzzling. In this paper, we clarify the mechanism of high T$_c$ multiferroicity in CuO using combined first-principles calculations and an effective Hamiltonian model. We find that CuO contains two magnetic sublattices, with strong intra-sublattice interactions and weakly frustrated inter-sublattice interactions, which might be one of the main reasons that the compound has a high ordering temperature. The weak spin frustration leads to incommensurate spin excitations that dramatically enhances the entropy of the mutliferroic phase, and eventually stabilize the mutliferroic phase in CuO. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L9.00006: Pressure-driven high-spin to low-spin and orbital-selective insulator to metal transition in cubic CoO Li Huang, Xi Dai, Yilin Wang We studied the magnetic and spectral properties for cubic para-magetic phases of CoO under high pressures by using \emph{ab initio} many-body method which combining local density approximation with dynamical mean-field theory. Experimentally observed metal-insulator transition at high pressure is successfully reproduced in calculations. Our calculation predicts CoO as a Mott insulator at ambient pressure and metal at extreme high pressure. In the intermediate pressure regime, our results indicate that there is an orbital selective Mott phase with $t_{2g}$ orbitals being metallic and $e_{g}$ orbitals being insulating. In contrast with MnO and Fe$_{2}$O$_{3}$ ($d^5$ configuration) where metal-insulator transition is accompanied by a high-spin to low-spin transition, we found that the local moment of CoO ($d^7$ configuration) decreases gradually from 2.8 ($S = 3$ states) to 1.4 ($S = 1$ states) with increasing pressure, which is in agreement with experimental data. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L9.00007: Mott physics in multi-band Hubbard model with strong spin-orbit interaction Liang Du, Li Huang, Xi Dai Spin-orbit coupling and electron correlation both play very crucial roles in the Mott physics of $4d$ and $5d$ transition-metal oxides. By studying three band Hubbard model with full Hund's rule coupling and spin-orbit coupling, we show that spin-orbit coupling intends to strongly enhance the Mott transition. By means of generalized Gutzwiller variational method and dynamical mean field (DMFT) with continuous time quantum Monte Carlo (CTQMC) as impurity solver, we obtain the complete phase diagram for this problem, which can be divided into metal, Mott insulator and band insulator phases. At mean while, we have also studied the effect of the Coulomb interaction on the strength of the spin-orbital coupling in the metallic phase. Our conclusion is that the correlation effect on the spin-orbital coupling is far beyond the mean field treatment even in the intermediate coupling regime. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:54PM |
L9.00008: Covalency, Excitons, Double Counting and the Metal-Insulator Transition in Transition Metal Oxides Invited Speaker: Xin Wang We present single-site dynamical mean-field studies of realistic models of transition metal oxides, including the cuprate superconductors and rare earth nickelates (in bulk and superlattice form). We include orbital multiplet effects and hybridization to ligands. We explicitly calculate the d-d exciton spectra for cuprates, finding sharp exciton lines in both metallic and insulating phases, which should be visible in experiments. We also find that the additional $d_{3z^2-r^2}$ orbital does not contribute to an additional Fermi surface at any reasonable doping, in contradiction to previous slave-boson studies. The hybridization to ligands is shown to have crucial effects, for example suppressing the ferro-orbital order previously found in Hubbard model studies of nickelates. Hybridization to ligands is shown to be most naturally parametrized by the d-orbital occupancy. For cuprates and nickelates, insulating behavior is found to be present only for a very narrow range of d-occupancy, irrespective of the Coulomb repulsion. The d-occupancy predicted by standard band calculations is found to be very far from the values required to obtain an insulating phase, calling into question the interpretation of these materials as charge transfer insulators. \\[4pt] This work is done in collaboration with A.J. Millis, M.J. Han, C.A. Marianetti, L. de' Medici, and H.T. Dang, and is supported by NSF-DMR-1006282, the Army Office of Scientific Research, and the Condensed Matter Theory Center and CNAM at University of Maryland. \\[4pt] [1] X. Wang, H. T. Dang, and A. J. Millis, Phys. Rev. B 84, 014530 (2011).\\[0pt] [2] X. Wang, M. J. Han, L. de' Medici, C. A. Marianetti, and A. J. Millis, arXiv:1110.2782.\\[0pt] [3] M. J. Han, X. Wang, C. A. Marianetti, and A. J. Millis, Phys. Rev. Lett. 107, 206804 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L9.00009: Ab initio study of the anti-ferromagnetic, non-collinear CuB$_2$O$_4$ crystal Yiing-Rei Chen, P.-R. Lee, J.-Y. Lin, J.-M. Chen, A.N. Vasiliev The spiky features in the crystal absorption spectrum, and the distinct differences in the directional oxygen K-edge absorption spectroscopy of the non-collinear anti-ferromagnetic, incommensurate CuB$_2$O$_4$, had led us to this LDA+U study of the crystal, although in the commensurate phase, due to the instrumental limitation. The calculated band structure matches the spiky features in the absorption spectrum, while the orbital analyzed DOS data explain the differences in the directional oxygen K-edge absorption spectroscopy. The two groups of dispersion-less bands, immediately above the gap, come from different groups of plaquettes, of Cu(A) and Cu(B), and are responsible for the spiky features observed experimentally. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L9.00010: Exact results on magnon-mediated pairing of spin-polarons Mirko Moeller, Mona Berciu The motion of a charged particle in a magnetically ordered background determines the electronic behavior of weakly doped, magnetically ordered insulators and semiconductors. This problem can be solved exactly for a single charge carrier in a ferromagnetic background at zero-temperature. The solution is a spin-polaron, {\em i.e} a dressed quasiparticle consisting of a charge carrier and a bound magnon which is dynamically emitted and reabsorbed by the charge carrier. If the exchange interaction between the charged particle and the ferromagnetic background is antiferromagnetic, then the spin polaron describes the low-energy states. We generalized the exact solution to the case of two charge carriers. This allows us to characterize the conditions (what ranges of parameters and for what types of lattices) under which magnon-mediated pairing occurs, so that spin-bipolarons describe the low-energy states. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L9.00011: Understanding the interplay between crystal structures and magnetic states of RCo$_{2}$ ( R = heavy rare earths) Durga Paudyal, Y. Mudryk, V.K. Pecharsky, K.A. Gschneidner, Jr. The RCo$_{2}$ compounds with R = heavy lanthanides are well known model systems for both experimentalists and theorists because of the complex nature of the magnetism of these materials. Better understanding of the magnetism can be achieved from parameter-free first principles calculations as well as carefully executed experiments. From first principles calculations we show that the indirect 4$f $- 4$f$ exchange polarizes the 5$d$ spins and the spin up 5$d$ and spin down 3$d$ hybridization gives rise to ferrimagnetism, i.e. antiparallel 5$d$ and 3$d$ itinerant magnetic moments at low temperature. The itinerant electron metamagnetism is known to support first order phase transformations in some of the RCo$_{2}$ compounds. However the clear understanding of this mechanism is lacking and, therefore, we clarify this mechanism from first principles calculations and experimentally confirm the nature of phase transformation of TbCo$_{2}$. The interrelation between the crystal structure and the magnetic states has also been investigated considering TbCo$_{2}$ as an example. [Preview Abstract] |
Session L10: Invited Session: Quantum Optics in Condensed Matter
Sponsoring Units: DLSChair: Nick Vamivakas, University of Rochester
Room: 210A
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L10.00001: Two-photon interference from quantum dot and parametric-down conversion single photons Invited Speaker: Glenn Solomon |
Tuesday, February 28, 2012 3:06PM - 3:42PM |
L10.00002: Quantum spectroscopy of semiconductors with Schr\"odinger's cat states Invited Speaker: Mackillo Kira Quantum investigations on simple systems --- such as atoms or quantized light modes --- have reached a level where one can read and write information directly onto the density matrix itself. Currently, the same level of quantum-information control seems inconceivable in condensed-matter systems simply because the many-body states involved are unimaginably complicated. In this talk, I will present the first steps in realizing targeted access of many-body states within condensed-matter systems by combining quantum-optics and many-body theory [1] with classical high-precision laser spectroscopy. The light--matter interaction has an inherent capability to directly excite targeted many-body states through the light source's quantum fluctuations [2]. The related quantum-optical responses can be projected from the classical data set by applying the cluster-expansion transformation [3] (CET). As a proof of principle, we CET project the measured nonlinear absorption of semiconductor quantum wells [4] into the quantum absorption generated by Schr\"{o}dinger's cat-state sources. The results expose a completely new level of many-body physics that remains otherwise hidden. Especially, the investigations reveal an anomalous reduction of Coulomb scattering of excitons, the excitation-induced narrowing of the exciton-molecule resonance, and the formation of electron--hole complexes (multi-exciton clusters) [5]. \\[4pt] [1] M. Kira and S.W. Koch, {\it Semiconductor quantum optics}, (Cambridge University Press, 2011). \\[0pt] [2] M.~Kira and S.W.~Koch, Phys.~Rev.~A {\bf 73}, 013813 (2006); S.W.~Koch, M.~Kira, G.~Khitrova, and H.M.~Gibbs, Nature Mat.~{\bf 5}, 523 (2006); M.~Kira and S.W.~Koch, Prog.~Quantum Electr.~{\bf 30}, 155 (2006). \\[0pt] [3] M.~Kira and S.W.~Koch, Phys.~Rev.~A {\bf 78}, 022102 (2008). \\[0pt] [4] R.P.~Smith {\it et al.}, Phys.~Rev.~Lett.~{\bf 104}, 247401 (2010). \\[0pt] [5] M. Kira {\it et al.}, Nature Physics {\bf 7}, 799-804 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L10.00003: Quantum optics with solid-state atom-like systems Invited Speaker: Mikhail Lukin |
Tuesday, February 28, 2012 4:18PM - 4:54PM |
L10.00004: Quantum Dot Spins and Photons Invited Speaker: Mete Atature Self-assembled semiconductor quantum dots are interesting and rich physical systems. Their inherently mesoscopic nature leads to a multitude of interesting interaction mechanisms of confined spins with the solid state environment of spins, charges and phonons. In parallel, the relatively clean spin-dependent optical transitions make quantum dots strong candidates for stationary and flying qubits within the context of spin-based quantum information science. The recently observed quantum dot resonance fluorescence has become a key enabler for further progress in this context. I will first discuss the real-time optical detection (or single-shot readout) of quantum dot spins, and then I will discuss how resonance fluorescence allows coherent generation of single photons suitable (and tailored) for linear-optics quantum computation and for establishing a high-efficiency spin-photon quantum interface within a distributed quantum network. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:30PM |
L10.00005: Dynamics of vortices in polariton quantum fluids : From full vortices, to half vortices and vortex pairs Invited Speaker: Benoit Deveaud-Pl\'edran Polariton quantum fluids may be created both spontaneously through a standard phase transition towards a Bose Einstein condensate, or may be resonantly driven with a well-defined speed. Thanks to the photonic component of polaritons, the properties of the quantum fluid may be accessed rather directly with in particular the possibility of detained interferometric studies. Here, I will detail the dynamics of vortices, obtained with a picosecond time resolution, in different configurations, with in particular their phase dynamics. I will show in particular the dynamics the dynamics of spontaneous creation of a vortex, the dissociation of a full vortex into two half vortices as well as the dynamics of the dissociation of a dark soliton line into a street of pairs of vortices. Work done at EPFL by a dream team of Postdocs PhD students~and collaborators: K. Lagoudakis, G. Nardin, T. Paraiso, G. Grosso, F. Manni, Y L\'{e}ger, M. Portella Oberli, F. Morier-Genoud and the help of our friend theorists V, Savona, M. Vouters and T. Liew. [Preview Abstract] |
Session L11: Focus Session: Graphene Structure, Stacking, Interactions: Magnetism and Interactions
Sponsoring Units: DMPChair: Enrico Rossi, College of William and Mary
Room: 210B
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L11.00001: Kondo Effect in Bilayer Graphene Diego Mastrogiuseppe, Sergio Ulloa, Nancy Sandler Because of the linear dependence of the density of states (near Dirac points), the physics of localized magnetic moments on graphene exhibits unique characteristics [1]. From the experiments by Mattos et al [2], where a possible observation of the 2-channel Kondo effect was reported, to recent studies on vacancies [3], the role of local moments on graphene remains poorly understood. The technical difficulties to determine the nature of the origin of the local moment add to the complexity of the problem. To gain insight into this problem, we have undertaken a study of a bilayer graphene system with Bernal stacking and an intercalated magnetic impurity. We model the system with a multiband Anderson impurity model and obtain the effective Kondo Hamiltonian via a Schrieffer-Wolff transformation. Although several conducting channels couple to the impurity, the standard 1-channel Kondo regime is recovered at low temperatures. The effective Kondo exchange couplings depend on the interlayer hopping giving rise to tunable Kondo temperatures. \\[4pt] [1] P. S. Cornglia et al., PRL 102, 046801 (2009); B. Uchoa et al., PRL 106, 016801 (2011)\\[0pt] [2] L. Mattos et al. (unpublished)\\[0pt] [3] J. -H. Chen et al., Nature Phys. 7, 535(2011); M. M. Ugeda et al., PRL 104, 096804 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L11.00002: Mid-gap states and Kondo effect in disordered graphene Caio Lewenkopf, Vladimir Miranda, Luis Dias da Silva Recent experiments on graphene flakes with short range scattering defects have stengthen the interest on Kondo physics in graphene systems. The experimental data show a temperature dependence of the resistivity consistent with the low-temperature Kondo screening of local magnetic moments. While the linear dispersion in the density of states in graphene justify a pseudogap Kondo model showing a rich variety of quantum critical behavior as a function of the gate-controlled chemical potential, the presence of disorder can alter this effect in favor of the ``standard'' Kondo model, with a Fermi-liquid ground state. We study these effects with different numerical methods. Tight-binding calculations for diluted scattering defects show the appearance of quasi-localized midgap states in the local density of states at the vicinity of the charge neutrality point. This leads to the formulation a Anderson-like model of localized states within the graphene matrix, which may lead to a Kondo screening consistent with the experiments. To verify this hypothesis, we perform numerical renormalization group (NRG) calculations to study the gate-dependence of the Kondo temperature and the transport properties of this model. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L11.00003: Dynamic RKKY interaction in graphene Mauro Ferreira, Stephen Power, Filipe Guimaraes, Antonio Costa, Roberto Muniz The growing interest in carbon-based spintronics has stimulated a number of recent theoretical studies on the RKKY interaction in graphene, based on which the energetically favourable alignment between magnetic moments embedded in this material can be calculated. The general consensus is that the strength of the RKKY interaction in graphene decays as $1/D^3$ or faster, where $D$ is the separation between magnetic moments. Such an unusually fast decay for a 2-dimensional system suggests that the RKKY interaction may be too short ranged to be experimentally observed in graphene. Here we show in a mathematically transparent form that a far more long ranged interaction arises when the magnetic moments are taken out of their equilibrium positions and set in motion. We not only show that this dynamic version of the RKKY interaction in graphene decays far more slowly but also propose how it can be observed with currently available experimental methods. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L11.00004: Formation of Localized Magnetic States on Adatoms in Uniaxially Strained Graphene Anand Sharma, Valeri Kotov We investigate the effect of applied uniaxial strain on the formation of localized magnetic states on adatoms in graphene. In the framework of the single impurity Anderson model, we systematically analyze the interplay between the anisotropic (strain-induced) nature of the Dirac fermions and the on-site Hubbard interaction. We numerically calculate the polarization of the electrons in the localized orbital within the mean-field self-consistent scheme. A phase diagram is obtained, containing non-magnetic as well as large magnetic regions, which can find prospective applications in the field of carbon-based spintronics. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L11.00005: Spin and band ferromagnetism in trilayer graphene Ralph van Gelderen, Lih-king Lim, Cristiane Morais Smith We study the ground state properties of an ABA-stacked trilayer graphene. The low energy band structure can be described by a combination of both a linear and a quadratic particle-hole symmetric dispersion, reminiscent of monolayer- and bilayer-graphene, respectively. The multi-band structure offers more channels for instability towards ferromagnetism when the Coulomb interaction is taken into account. Indeed, if one associates a subband-index degree of freedom to the bands (parabolic/linear), it is possible to realize also a band-ferromagnetic state, where there is a shift in the energy bands, since they fill up differently. By using a variational procedure, we compute the exchange energies for all possible variational ground states and identify the parameter space for the occurrence of spin- and band-ferromagnetic instabilities as a function of doping and interaction strength. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L11.00006: Magnetic order in graphene with broken inversion symmetry J. Fernandez-Rossier, D. Soriano We study the effect of sublattice symmetry breaking on the magnetic and transport properties of two dimensional graphene as well as zigzag terminated one dimensional graphene nanostructures. The systems are described with the Hubbard model within the collinear mean field approximation. In the case of 2D and zigzag ribbons we compute the phase diagram, at half-filling, defined by the normalized interaction strength U/t and the sublattice potential V/t, where t is the first neighbor hopping. In the case of 2D graphene we find that the system is always insulating, except at the transition between the antiferromagnetic (AF) and the non-magnetic (NM) phase where the system is half-metallic. In the case of zigzag ribbons, at finite V we find that the system undergoes a phase transition from non-magnetic insulator for U$<$Uc (V) to a phase with ferromagnetic order in the edges and antiferromagnetic inter-edge coupling. The conduction properties of the magnetic phase depend on V and can be insulating, conducting and even half-metallic, yet the total magnetic moment in the system is zero. In the latter case, we find a strong spin filter effect. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L11.00007: Magnetic Properties of Rhombohedral Graphene Thin Films Thanh Cuong Nguyen, Minoru Otani, Susumu Okada Ever since the fabrication of single and few layers graphene, the graphene thin films have been attracting so much attention in the field not only of low-dimensional sciences but also of nano-scale technologies due to their perfect two-dimensional network. One of fascinating issues in this carbon allotrope is the intrinsic magnetism that is inherent in their topological properties. We have demonstrated that the (0001) surfaces of graphene thin film with rhombohedral-stacked arrangement exhibit ferrimagnetic spin ordering induced by flat dispersion band associated with the peculiar surface localized electron states classified as the ``edge state'' [1]. In this work, we systematically investigate how the electronic and magnetic properties of the rhombohedral-stacked graphene thin films depend on the number of graphene layers, BN substrate, and uniaxial pressure using first-principles total-energy calculations in the framework of density functional theory [2]. \\[4pt] [1] M. Otani, M. Koshino, Y. Takagi, and S. Okada, Phys. Rev. B 81 (2010) 161403(R). \\[0pt] [2] N. T. Cuong, M. Otani, and S. Okada, Surf. Sci. (2011), doi:10.1016/j.susc.2011.10.001 [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L11.00008: Emergence of atypical magnetic and electronic properties in graphitic nanowiggles Vincent Meunier, Eduardo Costa-Girao, Liangbo Liang, Eduardo Cruz-Silva, Antonio Gomes Souza Filho Graphitic nanowiggles (GNWs) are periodic repetitions of non-aligned finite-sized graphitic nanoribbon domains seamlessly stitched together without structural defects. These complex nanostructures have been recently fabricated using the self-assembly and subsequent fusion of small aromatic compound (Nature \textbf{466}, 470 (2010)). The structures are predicted to possess unusual properties, such as tunable bandgaps and versatile magnetic behaviors (Phys. Rev. Lett. \textbf{107}, 135501 (2011)). First-principles theory was used to highlight the microscopic origin of the emerging electronic and magnetic properties of the main subclasses of GNWs, thereby establishing a road-map for guiding the design and synthesis of specific GNWs with targeted nanoelectronic, optoelectronic, and spintronic properties. We will show the unusual versatility of GNWs' magnetic properties, we will highlight the variation of electronic properties with the details of the structures and how these structures can be used to transport electrons. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L11.00009: Chiral superconductivity from repulsive interactions in doped graphene Rahul Nandkishore, Leonid Levitov, Andrey Chubukov We present a model wherein repulsive interactions unambiguously lead to superconductivity with enhanced Tc. The superconducting state is the chiral d + id superconducting state, which has no known experimental realizations. Intriguingly, our model has a natural realization in graphene that is doped to the M point of the Brilliouin zone. At this doping level, the Fermi surface nesting and the divergent density of states can produce interaction driven instabilities to exotic phases with high energy scales. Analyzing the competition between various ordering tendencies within a renormalisation group framework, we find that the leading instability is to d-wave superconductivity, for any choice of weak repulsive interactions. The instability develops simultaneously in two distinct d-wave channels, which are degenerate by lattice symmetries. Analysis of the pairing below Tc reveals that both orders co-exist to produce d + id superconductivity, with the phase of the order parameter winding by $4\pi$ as we go around the Fermi surface. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L11.00010: Competing many-body instabilities and unconventional superconductivity in graphene Christian Platt, Maximilian Kiesel, Werner Hanke, Dmitry A. Abanin, Ronny Thomale The band structure of graphene exhibits van Hove singularities (VHS) at doping x = $\pm$1/8 away from the Dirac point. Near the VHS, interactions effects, enhanced due to the large density of states, can give rise to various many-body phases at experimentally accessible temperatures. We study the competition between different many-body instabilities in graphene using functional renormalization group (FRG). We predict a rich phase diagram, which, depending on long range hopping as well as screening strength and absolute scale of the Coulomb interaction, contains a d + id-wave superconducting (SC) phase, or a spin density wave phase at the VHS. The d + id state is expected to exhibit quantized charge and spin Hall response, as well as Majorana modes bound to vortices. In the vicinity of the VHS, we find singlet d + id-wave as well as triplet f -wave SC phases. \\[4pt] [1] arXiv:1109.2953v1 [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L11.00011: Power law Kohn anomaly in graphene induced by Coulomb interactions Fernando de Juan, Herbert A. Fertig Phonon dispersions generically display non-analytic points, known as Kohn anomalies, due to electron-phonon interactions. We analyze this phenomenon for a zone boundary phonon in graphene. When electron-electron interactions with coupling constant $\beta$ are taken into account, one observes behavior demonstrating that the electrons are in a critical phase: the phonon dispersion and lifetime develop power law behavior with $\beta$ dependent exponents. The observation of this signature would allow experimental access to the critical properties of the electron state, and would provide a measure of its proximity to an excitonic insulating phase. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L11.00012: Effect of electron-phonon coupling on energy and density of states renormalizations of dynamically screened graphene J.P.F. LeBlanc, J.P. Carbotte, E.J. Nicol Motivated by recent tunneling and angle-resolved photoemission (ARPES) work [1,2], we explore the combined effect of electron-electron and electron-phonon couplings on the renormalized energy dispersion, the spectral function, and the density of states of doped graphene. We find that the plasmarons seen in ARPES are also observable in the density of states and appear as structures with quadratic dependence on energy about the minima. Further, we illustrate how knowledge of the slopes of both the density of states and the renormalized dispersion near the Fermi level can allow for the separation of momentum and frequency dependent renormalizations to the Fermi velocity. This analysis should allow for the isolation of the renormalization due to the electron-phonon interaction from that of the electron-electron interaction. \\[4pt] [1] Brar et al. Phys. Rev. Lett. 104, 036805 (2010) [2] Bostwick et al. Science 328, p.999 (2010) [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L11.00013: Screening and electrostatic doping in multilayer graphene systems Marcelo Kuroda, J. Tersoff, Razvan Nistor, Glenn Martyna We study the electrostatic screening in multilayer graphene systems using ab initio calculations and analytical models. First principles calculations reveal that oriented (Bernal) and turbostratic graphene multilayers in contact with a metal slab show only small differences in their charge distribution despite their dissimilar electronic structure. In the turbostratic systems the layer decoupling enables the identification of the Dirac point for each individual layer. We then measure the shift of each Dirac point relative to the Fermi level of the system and compute the charge transfer to each layer. Results are compared with an analytical model considering discrete layers. The model shows that at T = 0 charge screening is highly nonlinear due to the vanishing density of states at the Fermi level. More importantly a strong dependence on charge and temperature results in a change of the screening length by more than an order of magnitude depending on the experimental conditions, reconciling the large range of screening lengths previously reported in experiments. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L11.00014: Screening in Chiral Multilayer Graphene Hongki Min, EuyHeon Hwang, Sankar Das Sarma We calculate the static polarization function of multilayer graphene and study the effects of stacking arrangement, carrier density and onsite energy difference. At low densities, the energy spectrum of multilayer graphene is described by a set of chiral two-dimensional electron systems and the associated chiral nature determines the screening properties of multilayer graphene showing very different behavior depending on whether chirality indices are even or odd. As the density increases, the energy spectrum follows that of monolayer graphene thus the polarization function approaches that of monolayer graphene. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L11.00015: Unusual electron self-energy in graphene Choongyu Hwang, David Siegel, Sung-Kwan Mo, William Regan, Ariel Ismach, Yuegang Zhang, Alex Zettl, Alessandra Lanzara Electron-Electron interactions bear important information on fundamental electronic properties such as electron effective mass, conductivity, and charge mobility. By using angle-resolved photoemission spectroscopy, we study unusual electron self-energy in graphene induced by the electron-electron interactions, which are distinguished from those of an ordinary Fermi liquid. Our findings provide a new route for two-dimensional electron systems toward device applications. [Preview Abstract] |
Session L12: Focus Session: Graphene: Growth, Mechanical Exfoliation, and Properties - Growth on Single Crystals
Sponsoring Units: DMPChair: James Hannon, IBM T.J. Watson Research Center
Room: 210C
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L12.00001: Graphene growth on coinage-metal substrates Invited Speaker: Kevin McCarty The low solubility of carbon in Cu and Au gives these coinage metals advantages as substrates for graphene growth. Namely, growth occurs exclusively by surface processes, avoiding the complications of C segregating from the bulk of the metal substrate. However, the relatively weak interactions of Cu and Au with graphene can lead to mosaic films having large ranges of in-plane orientations. This talk will emphasize understanding the relationship between the microstructure of graphene sheets and the mechanisms of island nucleation and growth. We use low-energy electron microscopy (LEEM) to observe growth. We find that bunches of substrate steps on Cu(111) can generate misorientation boundaries in a graphene sheet as it overgrows the steps [1]. Thus, growth on rough Cu(111) leads to large rotational disorder. Optimized growth on smooth Cu(111) and Au(111), however, produces islands all in close registry to a single in-plane orientation. On Cu(100), the most abundant grain orientation of commercial Cu foils, graphene islands align around two equivalent in-plane Cu directions [2]. This inherent source of disorder from symmetry mismatch is further compounded by large spreads of orientation around the equivalent directions. The substrate choice also affects the microscopic growth mechanism. The rate that C diffuses to the graphene islands limits growth on Cu(111) [and likely on Au(111)]. The sheet edges are then morphologically unstable, with dendritic islands at low temperature and six-fold loped islands at higher temperature. In contrast, growth on Cu(100) is limited by the rate of C attaching to the graphene edge. This mechanism, combined with the symmetry mismatch, produces two-fold islands. Finally, the coinage metals will be compared to other transition metal substrates. \\[4pt] [1] Phys. Rev. B 84, p. 155425 (2011). \\[0pt] [2] Nano Lett. 10, p. 4890 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L12.00002: Growth and Characterization of Graphene on Single Crystal Cu Substrates Z.R. Robinson, P. Tyagi, H. Geisler, C.A. Ventrice, Jr., A.A. Bol, J.B. Hannon One of the key issues for the use of CVD graphene in device applications is the influence of defects on the transport properties of the graphene. Therefore, it is important to understand the influence of the substrate on the orientation of the graphene. Growth of graphene films on Cu(111) has the potential for producing films with a low defect density because of the hexagonal symmetry of the substrate and relatively small lattice mismatch, whereas growth on Cu(100) is expected to result in multi-domain growth because of its square symmetry. In this study, graphene films were grown on Cu single crystal substrates, and characterized with LEEM, LEED, SEM, AFM, and Raman spectroscopy. The clean Cu substrates were prepared by sputtering and annealing in UHV. For the initial growth studies, the samples were transferred to a tube furnace for graphene growth using a technique optimized for Cu foils. The UHV system has recently been modified with a button heater compatible with the conditions needed for graphene growth to enable in-situ growth and characterization. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L12.00003: Suppression of Grain Boundaries in Graphene Growth on Superstructured Mn-Cu(111) Surface Wei Chen, Hua Chen, Haiping Lan, Ping Cui, Tim Schulze, Wenguang Zhu, Zhenyu Zhang A standing obstacle in epitaxial graphene growth on metal substrates is the prevalence of undesirable grain boundaries (GB) that severely degrade the electronic, transport and mechanical properties of graphene. Employing density functional theory calculations, we demonstrate that the inherent multi-orientational degeneracy of the graphene islands on Cu(111) is the underlying reason for the prevalence of GB. We propose a possible solution, by invoking a functionalized Cu(111) surface to lift the orientational degeneracy of graphene islands and consequently suppress the creation of GB. We have identified the candidate substrate---a superstructured Mn-Cu(111) alloyed surface, which is experimentally achievable and ensures a single orientation for the graphene islands. The proposed approach promises to drastically improve the quality of epitaxial graphene without compromising on efficiency and yield. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L12.00004: Rotational homogeneity in graphene grown on Au(111) Joseph Wofford, Elena Starodub, Andrew Walter, Shu Nie, Aaron Bostwick, Norman Bartelt, Konrad Th\"urmer, Eli Rotenberg, Kevin McCarty, Oscar Dubon The set of properties offered by the (111) surface of gold makes it intriguing as a platform on which to study the fundamental processes that underpin graphene growth on metals. Among these are the low carbon solubility and an interaction strength with graphene that is predicted to be smaller than most transition metals. We have investigated this synthesis process using low-energy electron microscopy and diffraction to monitor the sample surface in real time, and found that the resulting graphene film possesses a remarkable degree of rotational homogeneity. The dominant orientation of the graphene is aligned with the Au lattice, with a small minority rotated by 30 degrees. The origins of this in-plane structuring are puzzling because angularly resolved photo-emission spectroscopy and scanning tunneling microscopy experiments both suggest only a relatively small interaction between the two materials. Finally, the implications of these findings for the growth of high structural-quality graphene films are discussed. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L12.00005: Fundamental growth mechanisms of graphene on nickel surfaces Invited Speaker: Matthias Batzill CVD growth of graphene over transition metal surfaces has become a main approach for synthesizing large area graphene wafers. The low carbon solubility in copper makes a good material for monolayer graphene synthesis, while carbon dissolution and re-segregation from the bulk of other transition metals, namely nickel, make these materials more demanding for controlling graphene growth. However, lower growth temperatures and defined graphene orientation relative to the substrate are some benefits for graphene synthesis on nickel compared to copper. Here we thoroughly characterize the fundamental growth processes of graphene on nickel substrate with the aim to find growth procedures that enable controlled graphene synthesis. We also identify defect structures in graphene that are formed as a consequence of the nickel substrate. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L12.00006: DFT studies of Graphene on Ni(111) and Surface Nickel Carbide Ni$_{2}$C Andreas Garhofer, Peter Jacobsen, Bernhard St\"oger, Gareth S. Parkinson, Michael Schmid, Roman Caudillo, Florian Mittendorfer, Josef Redinger, Ulrike Diebold Graphene with its unique transport properties supported on ferromagnetic materials is a promising candidate for the fabrication of spin-filtering devices. In order to study the growth of graphene on metal surfaces, graphene on Ni(111) is a perfect system from a structural point of view. The CVD growth of graphene on Ni(111) was studied with STM. The experiments showed not only perfect aligned (1x1) structures, but also several moir\'{e} patterns are observed. They are due to grain rotations of graphene on both Ni(111) and also surface nickel carbide Ni$_{2}$C. During CVD growth carbon atoms segregate into the Ni bulk. With an increasing carbon concentration first the surface carbide and then a graphene layer on top is built. We studied the systems using the DFT program package VASP with the DFT-D2 method of Grimme to include van der Waals interactions. In our theoretical analysis we tried to understand the formation of the graphene and Ni$_{2}$C on Ni(111). We calculated the stability of the surface carbide phase and the binding energies of rotated and unrotated graphene on Ni(111) as well as on Ni$_{2}$C in order to understand the experimental findings. Graphene and Ni$_{2}$C have no epitaxial relationship due to their incommensurate lattices, leading to a spread in grain rotations. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L12.00007: Graphene nucleation and growth on the transition metal surfaces: the role of pentagon, metal step and magic carbon clusters Junfeng Gao, Jijun Zhao, Feng Ding The nucleation behavior of graphene on transition metal surfaces, either on a terrace or near a step edge, is systematically explored using density functional theory calculations. The supported carbon clusters, CN~(N=1$\sim $24), on the Ni(111) surface are carefully optimized [1,2]. A structural transformation from a C chain to a sp$^{2}$~C network at~C$_{12}$ and the most stable structures of sp$^{2}$ graphene islands contain one to three pentagons. In agreement with experimental observations, our calculations show that graphene nucleation near a metal step edge is superior to that on a terrace. Besides, ground state structures of supported CN (N = 16$\sim $26), clusters on four selected transition metal surfaces: (Rh(111), Ru(0001), Ni(111) and Cu(111)) are explored [3]. A core-shell structured of C$_{21}$ stands out as a magic cluster, which is one of the dominating carbon precursors in graphene CVD growth and has been observed in experimental STM images. The energy barrier of two C$_{21}$ clusters' coalescence is computed to illustrate their influence on the kinetics of graphene CVD growth at different temperatures. \\[4pt] [1] J. Gao, et al,. J. Am. Chem. Soc. 133, 5009 (2011). \\[0pt] [2] J. Gao, et al., J. Phys. Chem. C 115, 17695 (2011). \\[0pt] [3] Q. Yuan, et al., J. Am. Chem. Soc. (accepted). [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L12.00008: Optimizing Epitaxial Cu and Ni Films on Al$_2$O$_3$(0001) for Uniform Graphene Growth David L. Miller, Mark W. Keller, Robert R. Keller, Justin M. Shaw, Ann N. Chiaramonti Copper and nickel are the most commonly used substrates for the growth of graphene by chemical vapor deposition. While cold-rolled polycrystalline foils are most often selected for their commercial availability and ability to withstand the high temperatures required for graphene growth, (111) crystal faces have been shown to offer better growth characteristics on both materials. We deposited Cu and Ni films onto single crystal Al$_2$O$_3$(0001) using magnetron sputtering at temperatures between $250 ^{\circ}$C and $700 ^{\circ}$C. This gave films with pure (111) texture but with two epitaxial in-plane orientations as measured by x-ray diffraction and electron backscatter diffraction. Upon heating to graphene CVD temperatures ($900 ^{\circ}$C to $1000 ^{\circ}$C), the grain boundaries widen and deepen into trenches that prevent the growth of uniform graphene over large areas. Reactive sputtering of a thin layer of Al$_2$O$_3$ before depositing the metal results in a single in-plane orientation over $>90\%$ of the film for Ni. In addition, gradually increasing the temperature during metal deposition suppresses the formation of deep trenches under graphene CVD conditions. We compare CVD graphene grown on the optimized films with that grown on commercial foils. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L12.00009: Kinetic Pathways towards Fabrication of Narrow Graphene Nanoribbons on Stepped Metal Surfaces Ping Cui, Jin-Ho Choi, Hua Chen, Wei Chen, Wenguang Zhu, Changgan Zeng, Zhenyu Zhang Graphene nanoribbons (GNRs) are predicted to exhibit intriguing electronic transport properties that strongly depend on their widths. To this end, one standing challenge is controlled fabrication of narrow GNRs with sizeable band gaps. In this study, we use first-principles approaches to explore the possibility of growing narrow GNRs along the step edges of catalytic metal surfaces. By minimizing the lattice mismatches of the growing graphene islands, optimizing their adsorption geometries, and exploiting the diffusion anisotropy of selective precursor monomers on the stepped surfaces, we identify the growth conditions under which the precursor monomers can be nucleated at the steps and then grow along the steps. Our studies point to kinetic pathways towards controlled fabrication of some of the narrowest GNRs with zigzag or armchair edges, and are expected to stimulate experimental efforts to realize these predictions. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L12.00010: Molecular Beam Epitaxy of graphene nanocrystals Jorge M Garcia, Ulrich Wurstbauer, Theanne Schiros, Annette S. Plaut, Loren N. Pfeiffer, Antonio Levy, Cherno Jaye, Daniel Fisher, Abhay Pasupathy, Aron Pinczuk The ability to produce large-area graphene films on commonly used dielectric substrates can lead to many technological applications. We demonstrate the fabrication of large area conducting graphene nanocrystalline films on arbitrary dielectric substrates by MBE (molecular beam epitaxy) using a solid carbon source that can offer the integration of capable graphene production with high flexibility and variety in ultra-high vacuum environment together with state of the art thin film technology. Synchrotron x-ray and Raman spectroscopies show that the films consist on graphene nanocrystals oriented parallel to the sample surface. The growth rate is a key parameter that determines the bonding environment. Careful control of the growth conditions results in the production of predominantly sp$^{2}$-bonded carbon thin films on arbitrary substrates, with the potential of growing large graphene grains on epitaxial substrates. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L12.00011: ABSTRACT WITHDRAWN |
Session L13: Focus Session: Low-Dimensional and Molecular Magnetism - 1D Magnetism/Single-Chain Magnets - Experiment
Sponsoring Units: DMP GMAGChair: Roberta Sessoli, University of Florence - Italy
Room: 211
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L13.00001: Magnetic Order in the Spin Chain Antiferromagnet Ca3Co2O6 Martin Lees, O.A. Petrenko, C.L. Fleck, S. Agrestini, C. Mazzoli, A. Bombardi, L.C. Chapon We have used powder neutron diffraction to investigate the magnetic structure of the Ising spin chain compound Ca$_{3}$Co$_{2}$O$_{6}$. Our investigation focuses on the low-temperature regime (T $<$ 14 K $<<$ $T_{N}$ = 25 K) where previous neutron diffraction studies have shown that there is an increasing instability in the spin density wave (SDW) order within this material. The results of this work reveal that there is an order-order transition from the SDW structure to a new commensurate antiferromagnetic phase. The extraordinary time dependence of the magnetic reflections demonstrates that this transition occurs via a very slow transformation process. As the temperature is reduced the characteristic time of the transition process increases rapidly and at low temperatures the magnetic states become frozen. We have also investigated the stability of the low-temperature commensurate phase in an applied magnetic field. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L13.00002: Density functional calculation of the electronic and magnetic properties of $\alpha$-CoV$_2$O$_6$ Andres Saul, Guillaume Radtke In this work, the magnetic properties of the low dimensional $\alpha$-CoV$_2$O$_6$ system have been investigated using density-functional calculations. This system is constituted of CoO$_6$ octahedra connected by the edges and forming one dimensional linear chains. The experimental magnetization curves recorded at very low temperature show a surprising magnetization plateau at one-third of the saturation magnetization and a strong anisotropy. The estimated Co magnetic moment is large reaching a value of 4.5 $\mu_B$ suggesting a large orbital contribution. Our calculations show that three different magnetic configurations for the Co are possible, the lowest energy one being a high spin configuration in agreement with the S=3/2 character of the Co$\sp{+2}$ ion observed in this compound. Spin-orbit interactions have been included in order to calculate the magnetic anisotropy and the orbital contribution to the magnetic moment. The results are discussed in terms of crystal field splitting of the $3d$ orbital and a tight-binding Hamiltonian. Using a broken-symmetry formalism we have evaluated the effective exchange interactions of the Heisenberg Hamiltonian. They allow us to propose the magnetic structures corresponding to the ground state and to the observed magnetization plateaus. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L13.00003: Heat capacity and magnetization of CoNb$_2$O$_6$ near quantum critical point Tian Liang, Seyed Koohpayeh, Jason Krizan, Sian Dutton, Tyrel McQueen, Robert Cava, N. Phuan Ong CoNb$_2$O$_6$ is a quasi-1D quantum magnet in which magnetic Co$^{2+}$ ions are ferromagnetically arranged into nearly isolated chains along the c axis with the magnetic moment confined in the ac-plane. By applying transverse magnetic field along b-axis, quantum phase transition from magnetically ordered phase to paramagnetic phase occurs. Evidence for emergent E$_8$ symmetry was recently obtained by neutron scattering near the quantum critical point (QCP) in an applied transverse magnetic field of 5.5 T We will report on experiments to investigate the behavior of the heat capacity and torque magnetization in the vicinity of the QCP and discuss their implications. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L13.00004: Terahertz excitations in the 1D Ising chain quantum magnet CoNb$_2$O$_6$ Christopher M. Morris, R. Vald\'es Aguilar, S. Koopayeh, C. Broholm, N.P. Armitage The one-dimensional magnet CoNb$_2$O$_6$ was recently demonstrated to be an excellent realization of a one-dimensional quantum Ising spin chain. It has been shown to undergo a quantum phase transition in a magnetic field oriented transverse to its ferromagnetically aligned spin chains. Low energy spin-flip excitations in the chains were recently observed via inelastic neutron scattering.\footnote{R. Coldea, \textit{et al}, Science \textbf{327}, 177 (2010)} The energy spectrum of these excitations was shown to have a interesting energy scaling governed by symmetries of the E8 exceptional Lie group. Here, time-domain terahertz spectroscopy (TDTS) is used to investigate optically active low energy excitations in CoNb$_2$O$_6$. We take advantage of the polarization sensitivity of this technique to characterize both electric and magnetic dipole active excitations in this compound. A connection is made from the $q=0$ response observed here to the excitations observed by neutron scattering. In addition, we will show preliminary data on the terahertz spectra of this material as it undergoes the magnetic field-tuned quantum phase transition. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L13.00005: Thermal Transport in CuSb$_2$O$_6$ single crystals Narayan Prasai, Joshua L. Cohn, Michael G. Smith, Alwyn Rebello, John J. Neumeier CuSb$_2$O$_6$ behaves as a uniform, one-dimensional (1D) $S=1/2$ Heisenberg spin chain with long-range, antiferromagnetic ordering below $T_N\simeq 8.5$~K.\footnote{A. Nakua {\it et al.}, J. Solid State Chem. {\bf 91}, 105 (1991); B. J. Gibson {\it et al.}, J. Magn. Magn. Mater. {\bf 272-276}, 927 (2004).} Unusual for cuprates, the Cu$^{2+}$ ions lie within quite regular CuO$_6$ octahedra and 1D magnetism appears to arise from orbital ordering driven by correlation effects.\footnote{Deepa Kasinathan, Klaus Koepernik, and Helge Rosner, Phys. Rev. Lett. {\bf 100}, 237202 (2008).} We will report the results of thermal conductivity measurements on single crystals over the temperature range $5{\rm K}\leq {\rm T}\leq 330 {\rm K}$. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L13.00006: Static and dynamic properties of Single-Chain Magnets with broad domain walls Alessandro Vindigni, Thomas Michaels, Orlando Billoni, Danilo Pescia It is well-known that long-range order cannot occur in 1d magnetic systems with short-range interactions. Remanent magnetization may, however, be observed in anisotropic spin chains due to slow dynamics. The physics of such systems -- called Single-Chain Magnets (SCMs) -- is mainly dictated by the temperature dependence of the relaxation time ($\tau$) and the correlation length ($\xi$). The behavior of $\tau$ and $\xi$ is, in turn, determined by domain-wall (DW) excitations. Both statics and dynamics are dependent on whether DWs extend over more than one lattice distance (\textit{broad}) or not (\textit{sharp}). The transition from one regime to the other is controlled by the strength of the magnetic anisotropy energy with respect to the exchange interaction. For broad domain walls, we found that the interplay between localized excitations and spin waves turns crucial at finite temperatures. Moreover, all the relevant quantities display universal behaviour, provided that temperature is measured in units of DW energy and distance in units of DW widths. These facts allowed us to explain the experimental behavior of a class of Mn-based SCMs with broad DWs and may also be relevant to the study of metallic nanowires. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L13.00007: Light induced kickoff of magnetic domain walls in Ising chains Invited Speaker: Lapo Bogani Controlling the speed at which systems evolve is a challenge shared by all disciplines, and otherwise unrelated areas use common theoretical frameworks towards this goal. A particularly widespread model is Glauber dynamics, which describes the time evolution of the Ising model and can be applied to any binary system. Here we show, using molecular nanowires under irradiation, that Glauber dynamics can be controlled by a novel domain-wall kickoff mechanism. Contrary to known processes, the kickoff has unambiguous fingerprints, slowing down the spin-flip attempt rate by several orders of magnitude, and following a scaling law. The required irradiation power is very low, a substantial improvement over present methods of magnetooptical switching: in our experimental demonstration we switched molecular nanowires with light, using powers thousands of times lower than in previous optical switching methods. This manipulation of stochastic dynamic processes is extremely clean, leading to fingerprint signatures and scaling laws. These observations can be used, in material science, to better study domain-wall displacements and solitons in discrete lattices. These results provide a new way to control and study stochastic dynamic processes. Being general for Glauber dynamics, they can be extended to different kinds of magnetic nanowires and to a myriad of fields, ranging from social evolution to neural networks and chemical reactivity. For nanoelectronics and molecular spintronics the kickoff affords external control of molecular spin-valves and a magnetic fingerprint in single molecule measurements. It can also be applied to the dynamics of mechanical switches and the related study of phasons and order-disorder transitions. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L13.00008: On-Surface Design and Characterization of Magnetic Macromolecules Shih-Hsin Chang, Andrew DiLullo, Kendal Clark, Jan-Peter Kloeckner, Marc-Heinrich Prosenc, Roland Wiesendanger, Germar Hoffmann, Saw-Wai Hla The formation of molecular chains from basic magnetic molecular building blocks is addressed on various surfaces. Via a surface catalyzed reaction multi-spin macromolecules are synthesized in ultra-high vacuum and then investigated by scanning tunneling microscopy (STM) at low temperatures. In my presentation, I will discuss the impact of the surface on the catalytic step and present a systematic comparison between Cu(111), Cu(100), NaCl/Cu(111), Co/Cu(111), and Au(111) surfaces. Depending on the substrate system, either unwanted sideproducts or spin-active macromolecules are created. In the latter case, scanning tunneling spectroscopy measurements are performed revealing an intermolecular spin-spin interaction through the analysis of the Kondo resonance. Our approach shades new light on molecular magnetism where magnetically coupled complexes will be synthesized from basic units on surfaces for future spintronic applications. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L13.00009: NMR Study of the Spin-Peierls transition in TiPO$_4$ Raivo Stern, Ivo Heinmaa, Enno Joon, Joseph Law, Reinhard Kremer, Robert Glaum We investigated the magnetic and structural properties of the quasi-one dimensional 3$d^1$-quantum chain system TiPO$_4$ ($J \sim$ 965 K) by NMR measurements. TiPO$_4$ undergoes two magnetostructural phase transitions, one at 111 K and the other at 74 K. Below 74 K, NMR detects two different $^{31}$P signals and the magnetic susceptibility vanishes, while DFT calculations evidence a bond alternation of the Ti\ldots Ti distances within each chain. Thus, the 74~K phase transition is a spin-Peierls transition which evolves from an incommensurate phase existing between 111~K and 74~K. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L13.00010: Magnetic exchange studies of one-dimensional Co(II) molecular chains Asma Amjad, G.M. Espallargas, J.M. Clemente-Juan, R. Klemm, E. del Barco, E. Coranado, M. Evangelisti We present a detailed experimental and theoretical study of a 1D spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ Co-based molecular chain, trans-[CoCl$_{2}$(3,5-Br$_{2}$py)$_{2}$]. Our results show distinct features that are associated to both intra-molecular interactions (along the chain) and inter-molecular interactions (perpendicular to the direction of chains). The hysteresis observed at low temperature (230mK), indicate presence of 3D ordering attributed to the exchange interactions between the chains. Measurements done at different angles from the chains axial direction (c-axis), restricted within the a-c and b-c planes, reveal uniaxial anisotropy along the c-axis. The experimental data are explained using the mean field approximation, focusing on the behavior of inter-chain interactions and g-tensor anisotropy at the Co sites in the presence of a static magnetic field. A transition between two different relaxation regimes encountered for variable sweep rates will also be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L13.00011: Magnetic structure and stability of 1D Co/Fe wires Jessica Bickel, Matthias Menzel, Kirsten von Bergmann, Andre Kubetzka, Roland Wiesendanger As device scales continue to decrease low dimensional structures are needed to take the place of wires and thick films. Particularly in the field of spintronics, it is important to develop 1D structures that can transmit information via the spin. This work realizes one method of spin transport via mixed Co/Fe 1D chains on Ir(001). Pure chains and mixed chains were examined by spin-polarized scanning tunneling microscopy (SPSTM) and spectroscopy. Both Fe and Co self-assemble into bi-atomic chains on the Ir(001)-(5x1) surface reconstruction. The Fe deposits as a single stacking chain while the Co exhibits two different stackings. When co-deposited, the materials can be differentiated from one another by spectroscopy due to differences in the local density of states. Spin-resolved measurements of pure Fe chains show a periodic spin-spiral along the entire length of the chain which is stabilized in an applied field but fluctuates at a speed greater than the time resolution of the STM in zero applied field. The Co, however, exhibits a ferromagnetic ground state that is stable at 8K. When the materials are co-deposited on the surface, the Co stabilizes the Fe spin-spiral and information about the magnetic state of the Co can be transmitted via the Fe spin-spiral. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L13.00012: Magnetic diamond chains of $Cu_3O_4$ Brigitte Leridon, Willem Rischau, Philippe Monod, Doroth\'ee Colson We present magnetization measurements on a satellite phase of $YBa_2Cu_3O_x$ namely $BaCu_3O_4$. This two-dimensional material is composed of alternate layers of Ba and $Cu_3O_4$. In the latters, the Cu and O atoms are one-dimensionaly ordered in diamond-shaped chains of formula unit $Cu_3O_4$. We will show that this material encounters a magnetic phase transition at around 336 K which is due to intra-ordering through superexchange coupling of the spins born by the Cu atoms in these diamond chains. We will discuss the possibility of either ferrimagnetic or antiferromagnetic inter-ordering of these weakly coupled one-dimensional objects and the possible role of Dzyaloshinskii-Moriya interactions. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L13.00013: Competing $^1$H Spin Relaxation Mechanisms in Low-Dimensional $Per_2Pt[mnt]_2$ Elizabeth Green, J.S. Brooks, P.L. Kuhns, A.P. Reyes, J.A. Wright, S.E. Brown, M. Almeida, M.J. Matos, R.T. Henriques $Per_2[Pt(mnt)_2]$ is a low-dimensional organic conductor consisting of parallel conducting ($perylene$) and magnetic chains ($Pt[mnt]_2$) which undergo a charge density wave (CDW) and spin-Peierls (SP)-transition, respectively. The conducting chain has been studied extensively, however fundamental questions about the spin-dynamics of the magnetic chain in the SP-state remained. By using $^1$H NMR, we discovered the low temperature nuclear relaxation rates (T$_1^{-1}$) display an anomalous upturn at the SP-transition which differs from classical SP-systems. This ``bump'' is suppressed by magnetic field and coincides with the Curie tail, seen in susceptibility measurements. The field-dependent activation energies, extracted from NMR and susceptibility, reveal two distinct behaviors evidencing coexisting spin systems. At low fields, the spin relaxation mechanism derived from a paramagnetic contribution, possibly unpaired Pt spins, is dominant, but is suppressed above 10T. Hence, the intrinsic SP behavior is recovered for high fields. Furthermore, spectra in the field induced (FICDW) state, up to 33T, reveal an increase in the electronic spin polarization. \textit{DMR-NSF-1005293} [Preview Abstract] |
Session L14: Focus Session: Spins in Semiconductors - Magnetic Semiconductors II
Sponsoring Units: GMAG DMP FIAPChair: George Kioseoglou, University of Crete
Room: 212
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L14.00001: The intra-gap electromagnetic response of gated GaMnAs in an electric double-layer field effect Brian Chapler, T.W. Elson, S. Mack, D.D. Awschalom, E. Namdas, J. Yuen, A.J. Heeger, L. Ju, F. Wang, D.N. Basov We have fabricated field effect transistors (FETs) utilizing an electrolyte oxide gate insulator for monitoring electrostatic doping in GaMnAs via infrared (IR) spectroscopy. Previous studies of gated GaMnAs have been confined primarily to transport measurements. IR experiments are able probe the electric field induced changes to the optical conductivity spectrum, providing direct insight into electronic structure of GaMnAs. The IR spectra show an enhancement of the Drude response and mid-IR resonance of GaMnAs upon hole accumulation, with symmetric decrease in these features in the depletion mode. A sum rule analysis of the IR spectra, combined with charge accumulation/depletion information from transport measurements for the same gated structures, enables accurate evaluation of the effective masses of mobile holes without any underlying assumptions about the level of compensation and/or disorder in the films. The low bound for the effective mass is several $m_{e}$, where $m_{e}$ is the free electron mass. We therefore conclude that electronic states in the vicinity of the Fermi energy retain significant impurity band character. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L14.00002: Effect of As vacancies on the binding energy and exchange splitting of Mn impurities on a GaAs surface M.R. K. Mahani, C.M. Canali, A.H. MacDonald State-of-the-art STM spectroscopy is nowadays able to manipulate and probe the magnetic properties of individual magnetic impurities located near the surface of a semiconductor. A recent advance of these technique employs the electric field generated by a As vacancy in GaAs to affect the environment surrounding substitutional Mn impurities in the host material [1]. Here we calculate the binding energy of a single Mn dopant in the presence of nearby As vacancies, by using a recently-introduced tight-binding method [2] that is able to capture the salient features of Mn impurities near the (110) GaAs surface. The As vacancies, modeled by the repulsive potential they produce, are expected to decrease the acceptor binding energy in agreement with experiment [1]. Within this theoretical model, we investigate the possible enhancement of the exchange splitting for a pair of ferromagnetically ordered Mn impurities, observed experimentally when As vacancies are present [3]. We also calculate the response of the Mn-impurity---As-vacancy complex to an external magnetic field. \\[4pt] [1] H. Lee and J. A. Gupta, Science, 1807-1810, (2010). \\[0pt] [2] T. O. Strandberg, C. M. Canali, A. H. MacDonald, Phys. Rev. B 80, 024425, (2009). \\[0pt] [3] J.A. Gupta, private communication. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L14.00003: Observation of a Sharp Band Edge Response in GaMnAs Using Nonlinear Spectroscopy Tristan de Boer, Angela Gamouras, Sam March, Vit Novak, Kimberley Hall Diluted magnetic semiconductors (DMS) have gained considerable interest over the last decade owing to their potential use in magneto-sensitive electrical and optical devices, in which the carrier-mediated nature of the ferromagnetism allows the magnetic properties to be controlled using optical or electrical gating techniques. The position of the Fermi level, which may lie within the valence band or within an impurity band, plays a critical role in current theories of ferromagnetism in DMS and has been the subject of intense controversy in recent years. Linear spectroscopy techniques are unable to address this issue due to strong band tailing in the vicinity of the fundamental band gap. Here we present results of time- and spectrally-resolved differential reflection measurements on GaMnAs, as well as low-temperature-grown GaAs and semi-insulating GaAs. Our results indicate a sharp band edge response due to the diminished contributions of band tail states in the nonlinear optical regime. We observe a blue shift of the band gap in GaMnAs, supporting a valence band model of ferromagnetism in DMS. Numerical simulations of the measured nonlinear response using an effective mass model support our conclusions. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L14.00004: Tuning the magnetic interaction between Mn dopants in GaAs David Gohlke, Jay Gupta Manganese can be used as a dopant in gallium arsenide to create a ferromagnetic semiconductor. We use low-temperature scanning tunneling microscopy to study these magnetic properties. The magnetic coupling between Mn dopants in GaAs(110) changes between ferromagnetic and antiferromagnetic depending on the orientation of the acceptors due to the zincblende crystal structure of the surface [Kitchen et al, Nature, 2006]. We have recently reported tuning of the resonance energy for a single Mn acceptor by moving charged atomic point defects [Lee and Gupta, Science, 2010]. Here, we tune the magnetic interaction between surface-layer Mn atoms in the same way. Funding for this research was provided by the Center for Emergent Materials at the Ohio State University, an NSF MRSEC (Award Number DMR-0820414). http://www.physics.ohio-state.edu/$\sim$jgupta/ [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L14.00005: Spin-Seebeck Effect in III-V Based Semiconductors Christopher M. Jaworski, Roberto C. Myers, Joseph P. Heremans The spin-Seebeck effect has now been observed in metals$^{1}$ (NiFe), semiconductors$^{2}$ (GaMnAs), and insulators$^{3}$ (YIG). It consists of a thermally generated spin distribution that is phonon driven. Here we extend our measurements of the spin-Seebeck effect to other group III-V based magnetic semiconductors and present measurements of conventional thermomagnetic and galvanomagnetic properties as well as the spin-Seebeck effect. Work supported by the National Science Foundation, NSF-CBET-1133589 1. K. Uchida, et al., Nature \textbf{455} 778 (2008) 2. C.M. Jaworski et al., Nature Materials \textbf{8} 898 (2010), Phys. Rev. Lett. \textbf{106} 186601 (2011) 3. K. Uchida, et al., Nature Materials \textbf{8} 893 (2010) [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L14.00006: High Resolution Magneto-Optic Measurements in GaAs using a Sagnac Interferometer Alexander Fried, Aharon Kapitulnik The Sagnac Interferometer is a tool which measures the Polar Kerr effect--a direct indicator of magnetism. ~Using 820 nm light from a superluminescent diode, we probe GaAs structures and measure the Kerr angle with sub-microradian resolution. ~By utilizing diffraction limited optics and a piezoelectric scanner, we also achieve high spatial resolution. ~Our measurements are performed at cryogenic temperatures and offer a way to measure the Spin Hall Effect in the DC regime along with other forms of magnetic order. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L14.00007: All-optical four-state magnetization reversal in (Ga,Mn)As ferromagnetic semiconductors Myron Kapetanakis, Ilias Perakis, Jingang Wang, Carlo Piermarocchi The emerging field of femtomagnetism has revealed the central role of non-equilibrium interactions and transient optical coherence in determining photoinduced spin dynamics. However the many-body theory of such effects remains controversial. A microscopic theory that engages the elements of coherence, correlation and nonlinearity on an equal footing is needed. We propose here such a theory, based on density matrix equations of motion and a tight-binding band calculation. We prepare the system within 100fs, via coherent nonlinear photoexcitation close to the strong peak of the density of states for interband transitions along the eight equivalent directions {\{}111{\}} of the GaAs BZ in the vicinity of 3eV. It then selectively relaxes to one of the four local minima of the magnetic free energy with biaxial anisotropy. We thus propose a non--thermal mechanism for all-optical switching between four metastable magnetic states, initiated non-thermally within 100fs and completed within 100ps. Our predicted switching comes from magnetic nonlinearities triggered by a femtosecond magnetization tilt that is sensitive to un-adiabatic light-induced spin interactions and controlled via the optical and the external magnetic field. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L14.00008: First-Principle Calculation of The Effective Hamiltonian for (Ga,Mn)As and (Ga,Mn)N Ryky Nelson, Anh Ngo, Wei Ku, Juana Moreno, Mark Jarrell Most of the models used to study (Ga,Mn)As have failed to explain the experimental results of (Ga,Mn)N especially its ferromagnetic critical temperature $T_c$. The need for a consistent and comprehensive model for the dilute magnetic semiconductors (DMS) motivates our study. We obtain the effective Hamiltonian for (Ga,Mn)As and (Ga,Mn)N using a Wannier function based first-principles method. We use density functional theory to calculate the band structure of a range of disordered supercell configurations of (Ga,Mn)As and (Ga,Mn)N and Wannier functions to obtain downfolded Hamiltonians. Those are then disorder averaged to get an effective Hamiltonian. We solved this effective model using the dynamical mean field approximation. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L14.00009: Polaronic trapping in magnetic semiconductors Hannes Raebiger GaN doped with iron is an interesting candidate material for magnetic semiconductors, as p-d coupling between the localized Fe-d and extended N-p hole states is expected to facilitate long-range ferromagnetic alignment of the Fe spins [1]. This picture of extended states in GaN:Fe, however, falls apart due to a polaronic localization of the hole carriers nearby the Fe impurities. To elucidate the carrier localization in GaN:Fe and related iron doped III-V semiconductors, I present a systematic study using self-interaction corrected density-functional calculations [2]. These calculations predict three distinct scenarios. (i) Some systems do sustain extended host-like hole states, (ii) some exhibit polaronic trapping, (iii) and some exhibit carrier trapping at Fe-d orbitals. These behaviors are described in detail to give an insight as to how to distinguish them experimentally. I thank T. Fujita, C. Echeverria-Arrondo, and A. Ayuela for their collaboration.\\[4pt] [1] T. Dietl et al, Science, 287, 1019 (2000).\\[0pt] [2] S. Lany and A. Zunger, Phys. Rev. B, 80, 085202 (2009). [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:54PM |
L14.00010: Magnetoresistance in antiferromagnet-based spin tunnel junctions Invited Speaker: Tomas Jungwirth To date spintronics research and applications of magnetically ordered systems have focused on ferromagnets (FMs). There are, however, fundamental physical limitations for FM materials which may make them impractical to realize the full potential of spintronics. Metal FMs offer high temperature operation but the large magnetic stray fields make them unfavorable for high-density integration and metals are unsuitable for transistor and information processing applications. FM semiconductors on the other hand do not allow for high-temperature operation. We present a concept in which these limitations are circumvented in spintronics based on antiferromagnets. The concept is based on relativistic magnetic and magneto-transport anisotropy effects in nanodevices whose common characteristics is that they are an even function of the microscopic magnetic moment vector, i.e., can be equally strong in AFMs as in FMs. As a demonstration we present our experimental observation of $>$100\% tunneling anisotropic magnetoresistance in a device with an IrMn AFM tunnel electrode [1]. We will also discuss candidate materials for high-temperature AFM semiconductor spintronics [2].\\[4pt] [1] B.~G. Park, J.Wunderlich, X.Marti, V.Holy, Y.Kurosaki, M.Yamada, H.Yamamoto, A.Nishide, J.Hayakawa, H.Takahashi, A.B.Shick, T.Jungwirth, Nature Mat. \textbf{10}, 347 (2011). \\[0pt] [2] T.Jungwirth, V.Nov\'{a}k, X.Marti, M.Cukr, F.M\'{a}ca, A.B. Shick, J.Ma\v{s}ek, P.Horodysk\'a, P.N\v{e}mec, V.Hol\'y, et~al., Phys. Rev. \textbf{B 83}, 035321 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L14.00011: Numerical studies of non-Drude ac-conductivity and infrared magneto-optics in (Ga$_{1-x}$,Mn$_{x}$As Huawei Gao, Jairo Sinova Optical absorption experiments on (III,Mn)V diluted magnetic semiconductors (DMS's) show that the ac-conductivity has non-Drude behavior at low frequency. The numerical simulation of this problem has been done previously using the effective Hamiltonian model with various treatments of the disorder effects. We are re-examining the previous works with a similar numerical method to establish the nature of the transitions in the low to the high doped regime. We use the effective Hamiltonian k.p model to describe the holes introduced by Mn impurities and treat the Mn impurities exactly using the envelope function approximation. With this technique we are able in principle to analyze spectrally the origin of the mid-infrared absorption peak, its trends, and the nature of the states near the Fermi energy as well as the excitation states. We will also report on numerical results of the magneto-optical response with this more accurate treatment of the effect of disorder. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L14.00012: Electric field manipulation of room temperature ferromagnetism in anatase Ti$_{1-x}$Co$_{x}$O$_{2-\delta}$ Yoshinori Yamada, Kazunori Ueno, Tomoteru Fukumura, Hongtao Yuan, Hidekazu Shimotani, Yoshihiro Iwasa, Lin Gu, Susumu Tsukimoto, Yuichi Ikuhara, Masashi Kawasaki Ferromagnetic semiconductor is one of the most attractive materials for semiconductor spintronics because of the controllability of both charge and spin degrees of freedom. Electric field effect of magnetism in the ferromagnetic semiconductors such as (Ga,Mn)As has been demonstrated only at low temperature due to their low Curie temperatures. In this study, we report the electric field manipulation of ferromagnetism in a ferromagnetic semiconductor Ti$_{1-x}$Co$_{x}$O$_{2-\delta }$ at room temperature [1]. Anatase Ti$_{1-x}$Co$_{x}$O$_{2-\delta }$ (001) epitaxial film was deposited on TiO$_{2}$ buffer 5 nm / LaAlO$_{3}$ (100) substrate in various oxygen pressures in order to vary an electron density by pulsed laser deposition method. An electric double layer transistor was fabricated on a paramagnetic film with an electron density of 1x10$^{19}$ cm$^{-3}$. With increasing gate voltage, the electron density was increased to 7x10$^{19}$ cm$^{-3}$. Ferromagnetic hysteresis loop was observed for $V_{G}$ above 3.0 V in an anomalous Hall resistivity, which is proportional to a magnetization of the film. This result represents that the ferromagnetism was induced at room temperature by an electrostatic charge accumulation, indicating that the ferromagnetism in this compound is mediated by the electron carriers. \\[4pt] [1] Y. Yamada et al., Science \textbf{332}, 1065 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L14.00013: BaMn$_{2}$Sb$_{2}$: A New Semiconducting Ferromagnet Jianneng Li, S. Stadler, A. Karki, Y. Xiong, R. Jin We have grown high-quality single crystals of BaMn$_{2}$Sb$_{2}$, which possesses the ThCr$_{2}$Si$_{2}$ structure as determined by X-ray powder diffraction technique. Magnetization measurements indicate that BaMn$_{2}$Fe$_{2}$ is ferromagnetic below $T_{C}$ = 580K. On the other hand, the temperature dependence of electrical resistivity shows semiconducting behavior, which can be described by thermally-activated resistivity formula with thermal activation energy about 0.25 eV . While the Hall coefficient has positive sign between 2 and 300 K, the Seebeck Coefficient undergoes sign change from positive at high temperatures to negative at low temperatures, reaching -260 $\mu $V/K at 70 K. The implication will be discussed. [Preview Abstract] |
Session L15: Focus Session: Spins in Metals - Resonance Phenomena I, Spin Wave Excitation and Spin Torque Oscillators
Sponsoring Units: DMP FIAP GMAGChair: Andrew Kent, New York University
Room: 213
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L15.00001: Spin-torque excited spin waves revealed by micro-focused Brillouin light scattering Invited Speaker: Marco Madami Since the discovery of the spin transfer torque (STT) effect [1,2] a great effort has been devoted to the realization and study of spin torque oscillators (STOs) because of their potential applications as spin waves injectors in magnonic devices or current-tunable broad-band microwave sources. More recently the possibility to synchronize multiple STOs [3,4] via the emitted spin waves, propagating in the magnetic ``free'' layer, envisioned a way to overcome their main limitation in the output power. For these reasons it's now crucial to obtain a detailed knowledge and understanding of the emitted spin waves properties like: their spatial distribution, their propagating or localized character, their decay length, wavelength and group velocity. In the last two years micro-focused Brillouin light scattering ($\mu $-BLS) revealed to be a powerful tool in order to investigate several of this properties [5,6]. In this presentation we discuss the potentialities of $\mu $-BLS to the study of emitted spin waves in STOs systems with particular focus on the results of our latest work [6]. Here we took advantage of our $\mu $-BLS setup in order to study spin waves emitted by an out-of-plane magnetized nano-contact STO. Performing a ``wave-vector resolved'' $\mu $-BLS experiment we provided the first direct experimental evidence of the \textit{propagating} nature of SWs emitted from an out-of-plane magnetized STO. The decay of the propagating SW intensity up to several microns away from the nano-contact position showed great potential for STT based magnonic devices. We also investigated the STO tunability measuring the emitted SW frequency as a function of both the applied direct current and external field intensities. Micromagnetic simulations provided the theoretical support to quantitatively reproduce the results. \\[4pt] [1] Slonczewski, J. C. J. Magn. Magn. Mater. 159, L1 (1996).\\[0pt] [2] Berger, L. Phys. Rev. B 54, 9353 (1996).\\[0pt] [3] Kaka, S. et al. Nature 437, 389 (2005).\\[0pt] [4] Mancoff, F. B., Rizzo, N. D., Engel, B. N., Tehrani, S. Nature 437, 393 (2005).\\[0pt] [5] Demidov, V. E., Urazhdin, S., and Demokritov, S. O. Nature materials, 9(11), (2010).\\[0pt] [6] Madami, M., Bonetti, S. et al. Nature Nanotechnology, 6, 635 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L15.00002: Probing spin wave excitations using magnetic tunnel junction structures Xin Fan, Chong Bi, John Xiao We propose a quantitative method based on the use of high tunneling magnetoresistance and small lateral dimension of magnetic tunnel junction (MTJ) to detect spin excitations in magnetic films with promising high spatial resolution and sensitivity. One ferromagnetic (FM) layer of the MTJ is pinned by an antiferromagnetic layer and the other one is free to rotate in response to an external magnetic field. In the presence of microwave magnetic fields, the free layer will precess, leading to the average resistance of MTJ change. By applying a constant dc current bias to the MTJ, a time dependent voltage can be introduced and measured which is related is the time dependent magnetization along the external static field direction. We have demonstrated the usefulness of this method by studying the spin wave excitations in a single elliptical Permalloy thin film (50 $\mu $m $\times $30 $\mu $m $\times $40 nm). At low microwave power, a uniform linear ferromagnetic resonance behavior has been observed. Surprisingly, above the spin wave instability threshold, the experimental results show a linear response of to the microwave field over a large range, which is followed by a phase limiting behavior. The linear behavior can be described by the theoretical model describing subsidiary resonance. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L15.00003: Generating Damon-Eshbach Spin Waves in Py using a Conducting Diffraction Grating J. Sklenar, V.S. Bhat, L. DeLong, J.B. Ketterson We have patterned silver hole arrays directly on top of uniform permalloy (Py) films. Typical Py and Ag film thicknesses are 25nm and 40 nm respectively; the holes in the Ag have a 500nm diameter and are patterned on a 1 micron lattice constant. We have measured resonant modes arising from a quasi-uniform microwave excitation field, applied in the plane of the sample, as a function of the in-plane external field and the in-plane field orientation relative to the principal axes of the array. Measurements were done using our broadband meanderline-based ferromagnetic resonance (FMR) spectrometer.\footnote{C. C. Tsai, J. Choi, S. Cho, B. K. Sarma, C. Thompson, O. Chernyashevskyy, I. Nevirkovets, and J. B Ketterson, Rev. of Sci. Instr. \textbf{80}, 023904 (2009).} In addition to a uniform FMR mode we observe satellite modes that correspond to the Damon-Eshbach spin waves\footnote{R. W. Damon and J. R. Eshbach J. Phys. Chem. Solids \textbf{19}, 308 (1961).} with wave vectors having Fourier components of the reciprocal lattice of the silver array. Hence, in an otherwise uniform magnetic film the silver array acts as a \textit{diffraction grating} which excites spin waves with k $\ne $ 0 from the dynamic k $\approx $ 0 microwave magnetic field. The observed spin wave angular dispersion is in excellent agreement with a magnon dispersion relation for spin waves in a uniform film given by Kriesel et al.\footnote{A. Kreisel, F. Sauli, L. Bartosch, and P. Kopietz, Eur. Phys. J. B \textbf{71}, 59 (2009).} [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L15.00004: Time-dependent spin-wave theory Andreas Kreisel, Andreas R\"uckriegel, Peter Kopietz We generalize the spin-wave expansion in powers of the inverse spin to time-dependent quantum spin models describing rotating magnets or magnets in time-dependent external fields. We show that in these cases the spin operators should be projected onto properly defined rotating reference frames before the spin components are bosonized using the Holstein-Primakoff transformation. As a first application of our approach, we calculate the re-organization of the magnetic state due to Bose-Einstein condensation of magnons in the magnetic insulator yttrium-iron garnet; we predict a characteristic dip in the magnetization which should be measurable in experiments. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L15.00005: Autonomous and driven dynamics of spin torque nano-oscillators Invited Speaker: Sergei Urazhdin Understanding the dynamical properties of autonomous spin torque nano-oscillators (STNO) and their response to external perturbations is important for their applications as nanoscale microwave sources. We used spectroscopic measurements to study the dynamical characteristics of nanopillar- and point contact-based STNOs incorporating a microstrip in close proximity to the active magnetic layer. By applying microwave current at frequency $f_{ext}$ to the microstrip, we were able to generate large microwave fields of more than 30 Oe rms at the location of STNO. We demonstrate that for a wide range of $f_{ext}$, STNO exhibits multiple synchronization regimes with integer and non-integer rational ratios between $f_{ext}$ and the oscillation frequency $f$. We show that the synchronization ranges are determined by the symmetry of the oscillation orbit and the orientation of the driving field relative to the symmetry axis of the orbit. We observe synchronization hysteresis, i.e. a dependence of the synchronization limits on the dynamical history caused by the nonlinearity of STNO. We also show that the oscillation can be parametrically excited in the subcritical regime of STNO by a microwave field at twice the frequency of the oscillation. By measuring the threshold and the frequency range of parametric excitation, we determine damping, spin-polarization efficiency, and coupling to the microwave signal. In addition, by measuring the frequency range of parametric synchronization in the auto-oscillation regime, we determine the dynamic nonlinearity of the nanomagnet. Thus, analysis of the driven oscillations provides complete information about the dynamical characteristics of STNO. Finally, we discuss several unusual dynamical behaviors of STNO caused by their strong nonlinearity. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L15.00006: Improved coherence of a quasi-linear spin-torque nano-oscillator OukJae Lee, Vlad Pribiag, Dan Ralph, Robert Buhrman We have fabricated tapered nanopillar spin-valve devices, $\sim $ 50 x 145 nm2, from a Py(5)/Cu(12)/Py(20) multilayer (thickness in nm) for spin torque nano-oscillator (STNO) studies. When biased with electron flow from the thick layer to the thin layer multiple, high power, but broad ($\Delta $f $>$50 MHz), spin torque excitation modes are obtained at hard-axis fields Hy $<$ 500 Oe. For Hy $\sim $ 700 Oe we obtain much more coherent ST oscillations, $\Delta $f $<$ 10 MHz, close to that predicted for a linear STNO at 300 K. A macrospin model successfully explains the optimum field bias as being where the amplitude-dependent red shift effect due to the demagnetization field is closely balanced by the blue shift effect due to the in-plane anisotropy field. We have also modeled the internal field within the free layer as a function of Hy and conclude that the multiple modes at lower fields originate from the relatively broad spatial distribution of the internal field, or equivalently from the broad natural frequency distribution of the individual magnetic elements. Our results suggest pathways for further enhancements in STNO performance. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L15.00007: Time-Domain Measurements of Real-Space Magnetization Trajectories in Spin Torque Oscillators Graham E. Rowlands, Jian Zhu, Jordan A. Katine, Juergen Langer, Pedram Khalili Amiri, Kang L. Wang, Ilya N. Krivorotov We make time-domain measurements of the microwave signal emitted by spin torque nano-oscillators (STNOs) based on magnetic tunnel junctions with Fe-rich free layers. The perpendicular magnetic anisotropy of the free layer nearly cancels its easy-plane shape anisotropy, allowing the magnetization to undergo large amplitude precession. The microwave power emitted by such STNOs reaches values approaching 0.4 $\mu$W. We employ a high-gain low-noise amplifier to further amplify the emitted signal, thereby bringing it to a level ($\sim$ 0.5 V rms) far exceeding the noise floor (5mV rms) of a 12 GHz, 40 Gs/s storage oscilloscope used for time-domain measurements. Relying on the assumption that extrema of the measured voltage versus time trace correspond to the magnetization crossing the sample plane, we use these time-domain traces to reconstruct the statistical distributions of the azimuthal angles at which the magnetization vector of the free layer crosses the plane of the sample. We measure the evolution of these crossing angle distributions as a function of current density and compare to theoretical predictions. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L15.00008: Network Analyzer Measurements of Spin-Torque Dynamics Lin Xue, Chen Wang, Yongtao Cui, R.A. Buhrman, D.C. Ralph A microwave current flowing through a magnetic tunnel junction (MTJ) produces an oscillating spin torque. This oscillating spin torque is able to excite resonant magnetic dynamics and produce an oscillating resistance. The oscillating resistance combined with an applied DC current can generate a microwave voltage signal at the same frequency as the input microwave signal. We show that a network analyzer measurement of the amplitude and phase of this signal provides a simple way to make a quantitative measurement of the strength and direction of the spin transfer torque vector in MTJs at non-zero biases, the regime of primary interest for applications. Compared with a previous time-domain technique for measuring the spin torque vector, this technique requires no specialized equipment and provides roughly similar sensitivity. Compared to dc-detected spin-torque ferromagnetic resonance, the network-analyzer method is free of artifacts at high bias. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L15.00009: Spin-transfer-driven parametric resonance in magnetic nanodomains C. Wang, H. Seinige, T. Staudacher, M. Tsoi We study experimentally the parametric excitation of a magnetic nanodomain by spin-transfer-torque (STT). In our experiments, we use a nanoscale point contact to inject high-density ac (microwave frequency) and dc currents into an exchange-biased IrMn/NiFe/Cu/NiFe spin valve (EBSV) and to produce STT [1] on NiFe moments in a small contact region. Here a time-dependent STT associated with the microwave current produces a time-dependent modulation of the effective damping parameter which, in turn, drives the magnetic moments into parametric resonance [2]. The resonance was detected electrically by measuring a small rectified dc voltage which appears across the contact at resonance [3]. We study this resonance signal as a function of frequency and power of the applied microwaves. As expected for parametric excitation, this resonance has an ac threshold and occurs at double the natural frequency of magnetic precession (FMR frequency). We found that both the excitation threshold and the width of the resonance depend on the applied dc bias. Detailed dc bias dependent measurements of the resonance signal provide a means to characterize instability regions in parameter space known as Arnold tongues. The parametric excitation can be potentially used in magnetic memory technology for reducing power and increasing speed of logic and memory devices. [1] J. C. Slonczewski (1996); L. Berger (1996); M. Tsoi et al. (1998). [2] M. Faraday (1831). [3] T. Staudacher and M. Tsoi (2011). [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L15.00010: Spin-orbit coupling and spin excitations in nanoscopic strucutres Antonio Costa, Samir Lounis, Roberto Muniz, Douglas Mills We have developed a formalism to calculate the spectra of spin excitations of structures of nanoscopic dimensions that takes into account spin-orbit coupling. We study structures composed by magnetic units (adatoms, clusters, ultrathin films) deposited on metallic substrates. The reduced symmetry of the magnetic units enhance the effects of spin-orbit coupling and activate mechanisms such as the Dzyaloshinskii-Moriya anti-symmetric exchange coupling. We are also able to predict anysotropic g-factors. In the case of ultrathin films, our formalism can span the entire Brillouin zone, being able to describe within the same framework FMR results and spectra obtained with SPEELS. We can also calculate the spectra that would be obtained by local probes such as inelastic scanning tunneling spectroscopy. We will present results for Fe ultrathin films on W(110) and for several transition metal adatoms on metallic substrates. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L15.00011: Spin orbit driven ferromagnetic resonance and torques in single ferromagnetic layers Ferran Maci\'{a}, Charles P\'{e}pin, A.D. Kent The coupling of spin and charge may convert electrical currents into spin currents in non-magnetic metals. In non-magnetic metals with strong spin orbit (SO) interaction in combination with magnetic metals one can also us the effect to excite magnetization dynamics; electrical currents in the non-magnetic metal transform to spin currents and the spin currents diffuse to the magnetic metal interacting with the magnetic moments. The combination of non-magnetic metals and magnetic metals has been recently used to determine spin hall angles. Here we demonstrate that spin currents in a ferromagnetic layer associated with SO interactions can excite ferromagnetic precession in the same layer. We have studied Co|Ni multilayers with both in-plane anisotropy and weak out-of-plane anisotropy. Results show that the samples have strong SO interactions. We have injected microwaves into patterned samples with several geometries and measured the mixed voltage in the same leads. Oscillatory currents drive FMR in the thin-film layer. We show that SO torques are primarily responsible for the magnetic excitations in samples with strong SO interactions, whereas samples with a weaker SO barely respond to the injected microwaves and show asymmetric components from charge current induced Oersted fields. [Preview Abstract] |
Session L16: Heavy Fermions - Theory and Experiment
Sponsoring Units: DCMPChair: Jianxin Zhu, Los Alamos National Laboratory
Room: 251
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L16.00001: $\beta-YbAlB_4$: a critical vortex metal Aline Ramires, Piers Coleman, Alexei Tsvelik $\beta-YbAlB_4$ is the first $Yb$ based heavy fermion superconductor and has a non Fermi Liquid behavior in the normal state that develops without external tuning by pressure or doping, making it intrinsically quantum critical [1]. Application of a magnetic field is found to drive the development of a Fermi Liquid in which the Fermi temperature is determined by the Zeeman energy [2]. Here we present a theory for the intrinsic quantum criticality in which the main ingredient is an anisotropic hybridization matrix with line nodes in momentum space that carry a vorticity and resemble topological defects. Our theory predicts that the application of a field induces a novel Lifshitz transition, in which a quasi-two dimensional Fermi Liquid with density of states $N^*(B) \propto 1/\sqrt{B}$ nucleates around the line node in momentum space. We also discuss how the vortex metal picture can account for the ESR anomalies observed in this system [3]. \vskip 0.3truein \noindent[1] S. Nakatsuji \textit{et al.}, Nature Phys. 4, 603 (2008). \noindent[2] Y. Matsumoto \textit{et al.}, Science 21, 316 (2011). \noindent[3] L. M. Holanda \textit{et al.}, Phys Rev. Lett. 107, 026402 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L16.00002: Correlated disorder in Kondo lattice Maxim Dzero, Xinyi Huang Motivated by recent experiments on Yb-doped CeCoIn$_5$, we study the effect of correlated disorder in Kondo lattice. Correlations between the impurities are considered at the two-particle level. We use mean-field theory approximation for the Anderson lattice model to calculate how the emergence of coherence in the Kondo lattice is impacted by correlations between impurities. We show that the rate at which disorder suppresses coherence temperature depends on the length of impurity correlations. As impurity concentration increases, we generally find that the suppression of coherence temperature is significantly reduced. The results are discussed in the context of available experimental data. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L16.00003: High-energy dispersion anomalies in actinide compounds T. Das, T. Durakiewicz, J.-X. Zhu, J.J. Joyce, Matthias J. Graf The observation of a prominent peak-dip-hump feature in the spectral weight in number of actinide compounds including Pu-115 superconductors and non-superconducting U-115 remains an unsolved problem. We have developed a first-principles intermediate coupling model to show that most aspects of these strong correlation features can be understood from the spin-fluctuation interaction.[1] The results show that a strong peak in the spin-fluctuation dressed self-energy is present around 0.5 eV in all these materials, which is mostly created by spin-orbit split 5f bands. These fluctuations couple to the single-particle spectrum and give rise to a peak-dip-hump feature, characteristic of the coexistence of itinerant and localized electronic states. Results are in quantitative agreement with photoemission spectra. Finally, we show that the studied actinides can be understood within the rigid-band filling approach, in which the spin-fluctuation coupling constant follows the same materials dependence as the superconducting transition temperature Tc. Work is supported by US DOE. \\[4pt] [1] T. Das. J.-X. Zhu, and M. J. Graf, arXiv:1108.0272 [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L16.00004: The Electronic Correlation Strength of Pu R.C. Albers, Jian-Xin Zhu, A. Svane, N.E. Christensen, Mark van Schilfgaarde Many materials have strong electron-electron correlation effects that can cause large deviations in electronic structure and materials properties from those predicted by conventional band-structure theory based on the local density approximation. We present a new method or scale to quantify electronic correlations in strongly correlated electron systems and apply it to the different phases of elemental Pu. Using the GW approximation, we show that the f-electron band-width reduction due to correlation effects scales as a function of the initial LDA bandwidth. This relationship is a universal relationship in that it is independent of crystal structure and atomic volume. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L16.00005: The Role of the Kondo and Phonon Correlations in the Cerium Volume Collapse Peter Reis, Peng Zhang, Ka Ming Tam, Juana Moreno, Mark Jarrell, Fakher Assaad, Andy Mcmahan We review the most recent experimental and theoretical progress of the $\alpha \leftrightarrow \gamma$ volume collapse occurring in the element cerium. We discuss the experimental results which have been important in differentiating the various electronic and phononic properties of the volume collapse of cerium, these experiments illustrate that drastically different electronic and phononic properties exist between the small volume phase $\alpha$ and large volume phase $\gamma$ of cerium. After interpreting the physical data and realizing that the volume collapse in cerium is primarily the result of different electronic and vibronic correlations in the $\alpha$ and $\gamma$ phases we set up a Hamiltonian which encompasses the physical properties of the collapse. To model the electronic and phononic properties of the $\alpha \leftrightarrow \gamma$ transition of cerium we use the Periodic Anderson Model (PAM) + Holstein Model, with the aim that the Kondo like electronic correlations of the $\alpha$ phase is encoded in the PAM where the large vibronic physics of the $\gamma$ phase is encompassed in the Holstein Model which incorporates a conduction electron-phonon mediated interaction. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L16.00006: Numerical Study of the Electron-Phonon coupling in the Cerium Volume Collapse Peng Zhang, Pete Reis, Ka-ming Tam, Fakher Assaad, Juana Moreno, Andy McMahan, Mark Jarrell Rare earth elements, for example Cerium, will experience a volume collapse with increasing pressure. Researchers have struggled for six decades to discover the mechanism behind this unusual first order phase transition. Although different models have been proposed that provide some qualitatively correct results, there is no theory that completely captures the volume collapse. Notably, some recent experiments show that lattice oscillations play an important role, contrary to the previous consensus that this phase transition is mainly driven by the contributions from spins and electrons. In this talk, we will discuss our study which employs the DMFA method to explore the role of electron-phonon coupling in the Periodic Anderson Model. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L16.00007: Lifshitz transition with interactions in high magnetic fields: Application to CeIn$_3$ Pedro Schlottmann The N\'eel ordered state of CeIn$_3$ is suppressed by a magnetic field of 61 T at ambient pressure. There is a second transition at $\sim$45 T, which has been associated with a Lifshitz transition [1,2]. Skin depth measurements [2] indicate that the transition is discontinuous as $T \to 0$. Motivated by this transition we study the effects of Landau quantization and interaction among carriers on a Lifshitz transition. The Landau quantization leads to quasi-one-dimensional behavior for the direction parallel to the field. Repulsive Coulomb interactions give rise to a gas of strongly coupled carriers [3]. The density correlation function is calculated for a special long-ranged potential [4]. It is concluded that in CeIn$_3$ a pocket is being emptied as a function of field in a discontinuous fashion in the ground state. This discontinuity is gradually smeared by the temperature [4] in agreement with the skin depth experiments [2]. \vskip 0.05in \par\noindent [1] S.E. Sebastian {\it et al}, PNAS {\bf 106}, 7741 (2009). \par\noindent [2] K.M. Purcell {\it et al}, Phys. Rev. B {\bf 79}, 214428 (2009). \par\noindent [3] P. Schlottmann and R. Gerhardts, Z. Phys. B {\bf 34}, 363 (1979). \par\noindent [4] P. Schlottmann, Phys. Rev. B {\bf 83}, 115133 (2011); J. Appl. Phys., in print. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L16.00008: Coexistence of antiferromagnetism and superconductivity induced by Pauli-paramagnetic pair breaking Kazushi Aoyama, Ryusuke Ikeda The heavy-fermion material CeCoIn$_5$ is a spin singlet $d$-wave superconductor with a strong Pauli-paramagnetic pair-breaking (PPB) effect. In the case with a magnetic field parallel to the basal plane of this material , there exists a distinct high field and low temperature (HFLT) superconducting (SC) phase in which a Fulde-Ferrell-Larkin-Ovchinnikov vortex lattice may be realized as a result of the strong PPB. Recently, it has been clarified that an antiferromagnetic (AFM) order exists inside the HFLT SC phase in spite of the absence of AFM order in the nonsuperconducting state [1]. Considering that AFM and SC orders are competing with each other in zero field, it is surprising that the AFM ordering is enhanced in the SC state. We theoretically study the stability of an AFM order in a $d$-wave and paramagnetic superconductor. We show that the PPB enhanced strongly by increasing the magnetic field induces an AFM order inside the SC phase and that, in contrast to the competitive nature of AFM and SC orders, the induced AFM order is not localized in vortex cores but coexistent with the SC order [2]. [1] M. Kenzelmann et al., Science 321, 1652 (2008). [2] K. Aoyama and R. Ikeda, arXiv:1107.5577. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L16.00009: Current-Carrying States in Fulde-Ferrell-Larkin-Ovchinnikov superconductors Jin An, Chin-Sen Ting, Chia-Ren Hu Fulde-Ferrell-Larkin-Ovchinnikov(FFLO) superconducting state is believed to be favorable in Pauli-limited heavy-fermion superconductors such as CeCoIn5. Based on Bogliubov de-Gennes equations, we present a theoretical study of current-carrying FFLO states, including the stability and characterization of a superconducting order parameter for a current-carrying FFLO state. Inhomogeneous and anisotropic current density distribution and quasi-particle current contribution related to the order parameter modulation and sign change are found in FFLO state, which are expected to provide an easy way to detect the existence of an FFLO state. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L16.00010: CeRu2Al2B: A new local moment 4f magnet with a complex T-H phase diagram Ryan E. Baumbach, X. Lu, K. Gofryk, F. Ronning, J.D. Thompson, H. Chudo, Hiroshi Yasuoka, C.H. Wang, V.O. Garlea, A.D. Christianson, E.D. Bauer There is ongoing interest in the study of Ce-based compounds that are derivatives of certain structure types (e.g., ThCr$_{2}$Si$_{2}$and PbClF), as they are often associated with correlated electron phenomena. We will report results for a new system that falls into this category, CeRu$_{2}$Al$_{2}$B. This compound crystallizes in a filled variant of the layered/tetragonal CeMg$_{2}$Si$_{2}$ structure. Contrary to what is often observed for Ce-based compounds, we find pronounced local moment behavior of the Ce ions, resulting in complicated magnetic ordering at strikingly high temperatures: i.e., antiferromagnetism (AFM) at T$_{N}$ = 14.1 K followed by ferromagnetism (FM) at T$_{C}$ = 12.8 K, which is first order in character. We also find a temperature-magnetic field phase diagram that consists of three distinct ordered phases: (1) AFM, (2) spin reoriented, and (3) FM. Since this type of behavior is unusual for Ce-based compounds, we will discuss prospects for suppressing the ordered state toward T = 0 in order to produce a Doniach-like phase diagram, which may provide a route towards a FM quantum critical point. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L16.00011: T-P phase diagram for the new local moment 4f ferromagnet CeRu$_{2}$Ga$_{2}$B Eric D. Bauer, R.E. Baumbach, X. Lu, T. Shang, F. Ronning, J.D. Thompson, C.H. Wang, V.O. Garlea, A.D. Christianson We will report results for single crystals of a new Ce-based local moment ferromagnet, CeRu$_{2}$Ga$_{2}$B. Electrical resistivity, magnetization, and magnetic susceptibility measurements reveal ferromagnetism and hysteresis around T$_{C}$ = 16.1 K, while specific heat measurements uncover a huge anomaly at the phase transition, which results in a large discontinuity in the magnetic entropy ($\Delta $S$_{mag }$= 1.7 J/mol-K). Taken together, these data show that CeRu$_{2}$Ga$_{2}$B undergoes a first order ferromagnetic phase transition at a surprisingly high temperature. Since this type of behavior is unusual for Ce-based compounds, we additionally undertook an effort to tune the magnetic state using pressure. We will present electrical resistivity measurements under applied pressures and the resulting temperature-pressure phase diagram, with an emphasis on implications for possible nearness to a ferromagnetic quantum critical point. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L16.00012: Superconductivity and Magnetic Ordering in $RE$$_2$Pt$_3$Ge$_5$ ($RE$ = La and Pr) Single Crystals Nakheon Sung, C.J. Roh, B.K. Cho Superconductivity and magnetic properties of rare-earth ternary germanide intermetallic compounds, $RE$$_2$Pt$_3$Ge$_5$ ($RE$ = La and Pr), are investigated. La$_2$Pt$_3$Ge$_5$ and Pr$_2$Pt$_3$Ge$_5$ single crystals were synthesized by high temperature metal flux method with Ge self flux. These two compounds were formed in U$_2$Co$_3$Si$_5$-type orthorhombic structure (space group $Ibam$) and the lattice parameters were determined using XRD of pulverized single crystals. La$_2$Pt$_3$Ge$_5$ exhibits an onset of superconducting phase transition at $T_c$ = 8.1 K, which, to the best of our knowledge, is the highest $T_c$ in U$_2$Co$_3$Si$_5$-type superconductors. Pr$_2$Pt$_3$Ge$_5$ shows both superconducting phase transition at $T_c$ = 7.9 K and antiferromagnetic transition at $T_N$ = 4.4 K. In addition, Pr$_2$Pt$_3$Ge$_5$ reveals strong magnetic anisotropy with an easy magnetic axis perpendicular to the $c$-axis, due to crystalline electric field effect. Including these results, we will discuss the nature of $RE$$_2$Pt$_3$Ge$_5$ single crystals in detail. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L16.00013: Multi-orbital Kondo physics of Co in Cu hosts Alexander Lichtenstein, Tim Oliver Wehling, Aljoscha Wilhelm, Brigitte Surer, Matthias Troyer, Philipp Werner, Andreas L\"{a}uchli We investigate the electronic structure of cobalt atoms on a copper surface and in a copper host by combining density functional calculations with a numerically exact continuous-time quantum Monte Carlo treatment of the five-orbital impurity problem. In both cases we find low energy resonances in the density of states of all five Co $d$-orbitals. The corresponding self-energies indicate the formation of a Fermi liquid state at low temperatures. Our calculations yield the characteristic energy scale -- the Kondo temperature -- for both systems in good agreement with experiments. We quantify the charge fluctuations in both geometries and suggest that Co in Cu must be described by an Anderson impurity model rather than by a model assuming frozen impurity valency at low energies. We show that fluctuations of the orbital degrees of freedom are crucial for explaining the Kondo temperatures obtained in our calculations and measured in experiments. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L16.00014: Origin of the Heavy-Fermion behavior in Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ Naoya Arakawa, Masao Ogata We investigate the electronic states in Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ for $0.5\leq x\leq2.0$ on the basis of the three-orbital Hubbard model in the Gutzwiller approximation. We take the dominant effects of the Ca-substitution into account as the changes of the hybridizations, whose squares are proportional to the nearest-neighbor hopping integrals of the Ru-$t_{2g}$-orbitals, between the Ru-$4d$- and O-$2p$-orbitals. In this presentation, we show the renormalization factors for the Ru $t_{2g}$-orbitals as a function of the intraorbital Coulomb interaction or an angle of the rotation of RuO$_{6}$-octahedra and discuss the origin of the heavy-fermion behavior around $x=0.5$. Our calculation suggests that moderately strong Coulomb interaction and the orbital-dependent modifications of the electronic structures for the Ru-$t_{2g}$-orbitals due to the Ca-substitution cause the heavy-fermion behavior in Ca$_{2-x}$Sr$_{x}$RuO$_{4}$. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L16.00015: Far-infrared optical properties of pyrochlore heavy fermion superconductor Cd$_2$Re$_2$O$_7$ in the normal and superconducting states M. Reedyk, M. Hajialamdari, D.A. Crandles, F.S. Razavi, R.K. Kremer Cd$_2$Re$_2$O$_7$ is a pyrochlore oxide which exhibits superconductivity with a transition temperature \textit{T$_C$} near 1 K. The far-infrared optical properties of Cd$_2$Re$_2$O$_7$ will be presented at temperatures above and below \textit{T$_C$}. Superconductivity induced changes in the phonon structure are observed. Thermal reflectance spectra show two absorption features, near 9.6 and 19.3 cm$^{-1}$ which arise in the superconducting state. Optical conductivity spectra reveal a softening ($\sim$ 1 cm$^{-1}$) of the phonon mode at 35 cm$^{-1}$ in the superconducting state. Analysis of the frequency dependent optical effective mass and scattering rate support the classification of this material as a modest heavy electron system at low temperatures. [Preview Abstract] |
Session L17: Focus Session: Thermoelectrics - Electronic Structure and Transport
Sponsoring Units: DMP GERA FIAPChair: Oded Rabin, University of Maryland
Room: 252A
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L17.00001: Potassium is a resonant level in Bi$_{1-x}$Sb$_{x}$ alloys Joseph P. Heremans, Hyungyu Jin, Bartlomiej Wiendlocha Bi$_{1-x}$Sb$_{x}$ alloys are the most promising thermoelectric materials for cryogenic Peltier cooling. Resonant impurity levels are known to increase the thermoelectric power and figure of merit of semiconductors and known examples are Tl:PbTe [1] and Sn:Bi$_{2}$Te$_{3}$ [2]. Here we add K:Bi$_{1-x}$Sb$_{x}$ to that list. Band structure calculations show that substitutional potassium in bismuth can form sharp density of states peaks, suggesting the presence of a resonant level. Single crystal samples were synthesized by a modified horizontal Bridgeman-Hor method. Cryogenic thermoelectric transport data will be presented as a function of K and Sb concentrations. At certain concentrations, the addition of potassium has a large effect on the thermopower, and increases the figure of merit to reach zT = 0.7 at 100K with zT $>$ 0.5 at all temperatures from 50K up to room temperature, numbers are valid along the trigonal axis of single crystals. [1] J. P. Heremans {\&} al. Energy Environ. Sci. DOI:10.1039/C1EE02612G; Science \textbf{321}, 554 (2008); [2] C.M. Jaworski {\&} al., Phys. Rev. B \textbf{80}, 233201, (2009) [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L17.00002: Fermi level pinning in Ti doped PbTe Yi-Bin Gao, Jan Koenig, Michele D. Nielsen, Bartlomiej Wiendlocha, Harald Boettner, Joseph P. Heremans The doping of PbTe attracts much interest due to the possible improvement of the thermoelectric properties by forming resonant levels. Here we report on doping of PbTe with Ti by molecular beam epitaxy, and back up the results with band structure calculations that show that Ti is a resonant level in the conduction band of PbTe. Ti is found to be a donor, leading to electron concentrations up to 1x10$^{19}$cm$^{-3}$, above which it pins the Fermi level at about 52 meV above the conduction band edge, and further increase of Ti shows no effect. At higher Ti contents the concentration of free electrons starts to rise again. However, Ti doping does not enhance the thermopower above that of similarly-doped PbTe, suggesting that the electrons on Ti are localized. We propose a model for Fermi level pinning due to different ionization states of the donor impurity which is similar to a chemical buffer. Further electronic structure calculations for Ti:PbTe confirm existence of the quasi-localized Ti states in the conduction band of PbTe and predict a local magnetic moment on Ti atom of 1.8 Bohr magnetons. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L17.00003: Enhancement of Thermoelectric Figure-of-Merit by Resonant States of Aluminum Doping in Lead Selenide Qinyong Zhang, Hui Wang, Weishu Liu, Hengzhi Wang, Bo Yu, Qian Zhang, Zhiting Tian, George Ni, Sangyeop Lee, Keivan Esfarjani, Gang Chen, Zhifeng Ren By adding aluminium (Al) into lead selenide (PbSe), we successfully prepared n-type PbSe thermoelectric materials with a figure-of-merit (\textit{ZT}) of 1.3 at 850 K. Such high \textit{ZT} is achieved by a combination of high Seebeck coefficient caused by very possibly the resonant states in the conduction band created by Al dopant and low thermal conductivity from nanosized phonon scattering centers. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L17.00004: Thermoelectric properties of indium-doped PbSe Eric Evola, Michele Nielsen, Joseph Heremans P-type [1,2] and n-type [3] PbSe have recently exhibited good thermoelectric properties without using the relatively uncommon element Tellurium. Here we report thermal conductivity, galvanomagnetic and thermomagnetic properties of bulk samples of n-type PbSe doped with indium at varying concentrations within the solid solution solubility range. A figure of merit zT value well in excess of 1 has been achieved. Resonant level effect and changes in the dimensionless figure of merit will be discussed although data indicate that the thermoelectric properties do not lie above the Pisarenko relation. \\[4pt] [1] D.J. Parker et al, Phys. Rev. B 82, 035204 (2010) \\[0pt] [2] H. Wang et al, Adv. Mater. 23 1366-1370 (2011) \\[0pt] [3] J. Androulakis et al, Phys. Rev. B 83, 195209 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L17.00005: Study on thermoelectric properties of n-type PbSe doped with B, Ga, and In Qian Zhang, Qinyong Zhang, Bo Yu, Dezhi Wang, George Ni, Gang Chen, Zhifeng Ren We report here systematic study of the thermoelectric properties of n-type PbSe with B, Ga, and In doping. The comparison of the electrical resistivity, Seebeck coefficient, and thermal conductivity is conducted. Room temperature Hall measurements show the effective increase of carrier concentration by both Ga and In doping to $\sim $10$^{20}$ cm$^{-3}$. The high power factor $\sim $ 2.4$\times $10$^{-3}$Wm$^{-1}$K$^{-2}$ is obtained when B is doped, however, it is decreased with increasing temperature, which is inversed with the other dopants. No resonant state is found in all these three materials. A figure of merit, ZT $>$1.2 at 873 K is achieved in 0.5{\%} In doped PbSe. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L17.00006: Electronic structure of ferroelectrically distorted PbTe Mal-Soon Lee, Mercouri G. Kanatzidis, S.D. Mahanti PbTe is one of the most promising thermoelectric materials. The electronic transport properties of the p-doped system are generally explained based on the nearly degenerate pockets near $L$ and along $\Sigma$ at high temperatures, where the energy at the $L$ point decreases with increasing temperature and approaches the $\Sigma$ point maxima. Recently, Bozin et al. showed that structure of PbTe is ferroelectrically distorted at high temperatures.\footnote{E. S. Bozin, C. D. Malliakas, P. Souvatzis, T. Proffen, N. A. Spaldin, M. G. Kanatzidis, and S. J. L. Billinge, Science 330, 1660 (2011).} Following this experiment, a lattice dynamics study using first-principle molecular dynamics simulations have shown an increase of the band gap with increased temperature as seen experimentally.\footnote{Y. Zhang, X. Ke, P. R. C. Kent, J. Yang, and C. Chen, Prys. Rev. Lett. 107, 175503 (2011).} Motivated from these studies, we have performed electronic structure calculations to investigate the effect of structural distortion and expansion on the band structure. We observe that the ordering of energy levels changes dramatically with distortion. In addition to these results we will also discuss how this can affect the electronic transport properties at high temperatures. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L17.00007: Potential thermoelectric performance from optimization of hole-doped Bi$_{2}$Se$_{3}$ David Parker, David Singh We present an analysis of the potential thermoelectric performance of hole-doped Bi$_2$Se$_3$, which is commonly considered to show inferior room temperature performance when compared to Bi$_2$Te$_3$. We find that if the lattice thermal conductivity can be reduced by nanostructuring techniques (as have been applied to Bi$_2$Te$_3$) the material may show optimized {\it ZT} values of unity or more in the 300 - 500 K temperature range and thus be suitable for cooling and moderate temperature waste heat recovery and thermoelectric solar cell applications. Central to this conclusion are the larger band gap and the relatively heavier valence bands of Bi$_2$Se$_3$. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L17.00008: Electronic and Thermoelectric properties of RuIn$_{3-x}A_{x}$ ($A$=Sn, Zn) from first principles Deepa Kasinathan, Klaus Koepernik, Helge Rosner Recently, substitution derivatives of the intermetallic compound RuIn$_{3-x}A_{x}$ ($A$ = Sn, Zn) have been shown to exhibit relatively high Seebeck coefficients. Substitution by Sn results in n-type behavior while p-type is the norm for substitution of In by Zn. We discuss in detail the electronic structure of the parent compound and the substitution derivatives obtained from density functional theory (DFT) based calculations using the Full Potential Local Orbital (FPLO) code. The substitution effects have been studied using three different approximations: the simple virtual crystal approximation (VCA), the ordered supercell approach and the disordered coherent potential approximation (CPA). Both Sn and Zn prefer different site symmetry positions in the unit cell. While the parent compound RuIn$_{3}$ is a semiconductor, the substitution derivatives are not. For small doping concentrations, we observe a rather rigid-band-like behavior due to the parabolic nature of the bands forming the valence band maximum and the conduction band minimum. Transport properties calculated using the semi-classical Boltzmann transport equations (BoltzTraP) based on the constant scattering approximation are consistent with the experiments. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L17.00009: Electronic structure and thermopower of Cu$_3$SbSe$_4$ Dat Do, Vidvuds Ozolins, S.D. Mahanti, Mal-Soon Lee, YongSheng Zhang, Chris Wolverton Cu$_3$SbSe$_4$ (Se4), a ternary derivative of the II-VI zincblende semiconductors, is a narrow band gap semiconductor (band gap $\sim$0.1 -- 0.4 eV) and a promising thermoelectric. Recently, Skoug et al. [1] have measured transport properties of pure and doped Se4 (Ge and Sn substituting for Sb). They find that p-doping by 2\% Sn results in optimized value of ZT=0.72 at 630 K. To understand the electronic structure and transport properties of Se4 we have carried out ab initio density functional electronic structure calculations. LDA/GGA/GGA+U approximations do not show that Se4 is a standard semiconductor. They give a resonance-like peak near the top of the valence band of width $\sim$0.5 eV. The Fermi energy for the undoped system lies below the peak, making it a pseudo-gap system, in disagreement with experiment. Non-local exchange with relaxation of Sb-Se bonds lead to the opening of a gap (0.26 eV), its origin being intimately related to the valency of Sb. Transport calculations show that Se4 is an excellent p-type thermoelectric, in agreement with experiment. \\[4pt] [1] Skoug et al., Sci. Adv. Mater., 3, 602 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L17.00010: Thermoelectric Properties of CoSb3-xSnx Michele D. Nielsen, Si Hui, Ctirad Uher, Janusz Tobola, Joseph P. Heremans We substitute Sn for Sb in CoSb3. Band structure calculations predict that Sn should be a resonant level with and energy near the top of the heavy (Co-3d) valence band. This is confirmed experimentally. Indeed, heavily Sn-doped samples show a low-temperature anomaly in their thermopower consistent with this prediction. The hole concentration, however, is too high for this materials to have a high thermopower and figure of merit. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L17.00011: Is the high frequency thermopower useful for predicting properties of strongly correlated materials? Jesus Cruz, James Freericks Shastry has proposed that the high-frequency thermopower can be used for meaningful predictions of the dc thermopower in many strongly correlated materials. If true, this will make it much easier to screen strongly correlated materials for useful thermoelectric properties because it is much easier to calculate the high-frequency limit. By solving this problem exactly for the Falicov-Kimball model using dynamical mean-field theory, we find that this approach often does not work. We also compare with usual approximations for the thermopower, we find that the Heikes formula gives a good description of the high frequency thermopower while the Kelvin formula is equivalent at high temperature. Unfortunately, the only accurate way to find the dc thermopower is with the conventional Kubo formula. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L17.00012: Electrical and thermal transport properties of the substituted defect manganese silicides Mn$_{1-x-y}$Cr$_{x}$Ru$_{y}$Si$_{\delta \sim 1.74}$ Vijayabarathi Ponnambalam, Donald T. Morelli Crystallizing in the TiSi$_{2}$ structure with considerable amount of random vacancies at the Si site, defect manganese silicides MnSi$_{\delta }(\delta \quad \sim $ 1.72-1.74) are unusual in many respects. One of them is their structural stability which is determined by the electron concentration. In addition, MnSi$_{\delta }$ is known for unusually low thermal conductivity $\sim $ 3 - 3.5 W/m K at 300 K. We have substituted MnSi$_{\delta }$ with Cr and Ru simultaneously and studied the electrical and thermal transport properties of the resulting alloys. Both resistivity and Seebeck coefficient are less sensitive to substitutions and maintain robust values as high as that of MnSi$\sim _{1.74}$. Hall measurements indicate that the carrier concentration remains high around 10$^{21}$/cm$^{3}$ and more or less same for all compositions. Thermal conductivity is decreased further and interestingly increases with T reaching values $\sim $ 2.5 W/m K at 300 K. These results will be presented and discussed. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:30PM |
L17.00013: Transport in Old and New Thermoelectric Materials Invited Speaker: David Singh There is increasing interest in thermoelectric materials motivated in part by recent progress and in part by the potential of these materials in various energy technologies. Thermoelectric performance is a multiply contra-indicated property of matter. For example, it requires (1) high thermopower and high electrical conductivity, (2) high electrical conductivity and low thermal conductivity and (3) low thermal conductivity and high melting point. The key is finding an optimal balance. In this talk, I discuss some of the issues involved in the context of recent results. These include the surprising doping dependence of the thermopower in PbTe and PbSe, and the interplay between acoustic and optical phonons in PbTe. The potential of some new materials is discussed. This work was done in collaboration with David Parker, Olivier Delaire and Mao-Hua Du. [Preview Abstract] |
Session L18: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Quantum Dots and Related Structures
Sponsoring Units: DMPChair: Latha Venkataraman, Columbia University
Room: 252B
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L18.00001: The Role of Surface Ligands in Electronic Charge Transport in Semiconductor Nanocrystal Arrays Invited Speaker: Cherie Kagan The long, insulating ligands commonly used in the synthesis of colloidal semiconductor nanocrystals (NCs) inhibit strong interparticle coupling and charge transport once NCs are assembled in the solid state into NC arrays. We introduce ammonium thiocyanate (NH$_{4}$SCN) and its derivatives, ammonium selenocyanate and selenourea to exchange the long, insulating ligands commonly used in the synthesis of colloidal semiconductor NCs. NCs may be exchanged with the new ligand in solution to form dispersions from which NC arrays are deposited or NC arrays with the long, insulating ligands may be exchanged in the solid state with the new ligands. The new compact ligands enhance interparticle coupling and charge transport in thin film, NC arrays as seen by red-shifts in the optical absorption and concomitant increases in carrier mobilities. Thiocyanate-capped CdSe thin film, NC arrays form sensitive photodetectors and n-type field-effect transistors with electron mobilities of $\sim $10 cm$^{2}$/Vs and current modulation of $>$10$^{6}$, while preserving NC quantum confinement. Temperature-dependent transport measurements reveal band-like transport in NC arrays, overcoming carrier hopping that has typified transport in NC arrays until recently. The non-caustic, chemically benign nature of the ammonium thiocyanate treatment enables the fabrication of NC thin film devices and circuits on flexible plastics. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L18.00002: n and p Type HgTe Colloidal Quantum Dot Film Heng Liu, Sean Keuleyan, Philippe Guyot-Sionnest HgTe colloidal quantum dots (CQDs) are a new system that can be stably charged n- and p-type by electrochemistry and it exhibits carrier-dependent photoresponse and magneto-resistance. For both electron or hole injection, interband bleach and intraband absorption confirm that the charges can be injected in the delocalized quantum states. Both carrier types lead to conductivity and show similar mobilities ($\sim$ 0.1 cm$^2$/Vs). However p-type films show good photoresponse and long photocurrent lifetime of hundreds of microseconds, while n-type films show 100 times shorter lifetimes and little or no photoresponse. With these samples, p-type will therefore be better for photovoltaic response. The magneto-resistance of n-type films is also found to be large while that of p-type films is negligible. This is consistent with the heavier hole mass compared to electrons in HgTe. The HgTe CQDs share similar features with the II-VI CdSe but with the advantage of allowing both n- and p-type charging. While this property is shared with the narrow gap IV-VI PbSe, HgTe has a simpler electronic structure and better stability in ambient conditions. The stability of the HgTe CQDs for n- and p-type charging opens many further investigations. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L18.00003: Tailoring Quantum Dot Assemblies to Extend Exciton Coherence Times and Improve Exciton Transport Kenton Seward, Zhibin Lin, Mark Lusk The motion of excitons through nanostructured assemblies plays a central role in a wide range of physical phenomena including quantum computing, molecular electronics, photosynthetic processes, excitonic transistors and light emitting diodes. All of these technologies are severely handicapped, though, by quasi-particle lifetimes on the order of a nanosecond. The movement of excitons must therefore be as efficient as possible in order to move excitons meaningful distances. This is problematic for assemblies of small Si quantum dots (QDs), where excitons quickly localize and entangle with dot phonon modes. Ensuing exciton transport is then characterized by a classical random walk reduced to very short distances because of efficient recombination. We use a combination of master equation (Haken-Strobl) formalism and density functional theory to estimate the rate of decoherence in Si QD assemblies and its impact on exciton mobility. Exciton-phonon coupling and Coulomb interactions are calculated as a function of dot size, spacing and termination to minimize the rate of intra-dot phonon entanglement. This extends the time over which more efficient exciton transport, characterized by partial coherence, can be maintained. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L18.00004: Efficient Exciton Transport Between Strongly Quantum-Confined Silicon Quantum Dots Mark Lusk, Zhibin Lin, Alberto Franceschetti First-order perturbation theory and many-body Green function analysis are used to quantify the influence of size, surface reconstruction and surface treatment on exciton transport between small silicon quantum dots. Competing radiative processes are also considered in order to determine how exciton transport efficiency is influenced. The analysis shows that quantum confinement causes small ($\sim$ 1 nm) Si quantum dots to exhibit exciton transport efficiencies far exceeding that of their larger counterparts. We also find that surface reconstruction significantly influences the absorption cross-section and leads to a large reduction in both transport rate and efficiency. Exciton transport efficiency is higher for hydrogen-passivated dots as compared with those terminated with more electronegative ligands. This is because such ligands delocalize electron wave functions towards the surface and result in a lower dipole moment. This work [1] is a first step in the development of a framework for the design of quantum dot assemblies with improved exciton transfer efficiency. \vskip 2mm \noindent [1] Z. Lin, A. Franceschetti and M. T. Lusk, arXiv:1110.6456v1 [cond-mat.mes-hall] [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L18.00005: Finite Element Modeling of Current-Induced Filaments in Nanocrystalline Silicon Sean Fischer, Christian Osorio, Nicholas Williams, Helena Silva, Ali Gokirmak Rapid, heat induced phase transitions in a mixed phase semiconductor may lead to current percolation along a highly conductive preferred path, or filament. In this study, we use 2-D, finite element simulations to model the time dependent evolution of current-induced filaments in nanocrystalline silicon (nc-Si) wires. Nc-Si wires are modeled as isolated crystalline silicon (c-Si) circles randomly distributed in an amorphous silicon (a-Si) wire 500 nm in length and 75 nm in width. Simulations include temperature dependent material parameters for electrical conductivity, thermal conductivity, heat capacity, and account for latent heat of fusion during phase transition from solid to liquid silicon. Field dependent material parameters are neglected to improve simulation convergence. Voltage pulses of amplitude 300 V and rise time less than 1 ns produce molten filaments $\sim $ 5 nm in width extending the length of the wire. The resistance of each wire decreases by four orders of magnitude during formation and filament current density exceeds 500 MA/cm$^{2}$. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L18.00006: Ultrashort channel length quantum-dot photodetectors Ferry Prins We present an efficient photodetector based on a one-dimensional array of parallel contacted PbSe quantum dots. In this device-architecture, the electrodes act as optical nano-antennae which concentrate incident light into the nanoparticle junction where they are converted into electron-hole pairs. The excitons are extracted with high efficiency due to the fact that the quantum dots are in direct contact with both source and drain electrodes, in contrast to previous studies which employed assemblies of quantum dots. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L18.00007: The Influence of Phonons and Surface Termination on Optical Transitions in Small Silicon Quantum Dots and Implications for Exciton Transport Zhibin Lin, Alberto Franceschetti, Mark Lusk Bulk silicon is an indirect-gap semiconductor, and radiative recombination can proceed only through phonon assistance. Recent experiments suggest, though, that fast, zero-phonon, pseudo-direct transitions occur in silicon quantum dots (Si QDs) as a result of quantum confinement. On the other hand, previous theoretical studies based on tight-binding and effective mass methodologies yield contradictory conclusions on the degree to which phonons play a role in radiative recombination within Si QDs. The resolution of this issue has important repercussions for ways in which Si QDs can be incorporated into future photovoltatic designs. This also has implications as to how QD size, phonons, and surface ligands collectively influence exciton transport. Density functional theory, in concert with a phonon-corrected version of Fermi's Golden Rule, is used to investigate the degree to which phonons influence the rate of optical transitions in Si QDs and to elucidate how the role of phonons changes with dot size and surface termination (H, CH3, and OH). The results are compared with available experimental data and those of previous calculations. In addition, the implications for phonon-assisted exciton transport dynamics within Si QD assemblies will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L18.00008: Dual MOSFET Charge Sensing in PbS Nanocrystal Quantum Dots Nirat Ray, Tamar Mentzel, Kenneth MacLean, Darcy Wanger, Moungi Bawendi, Marc Kastner We use nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs) as charge sensors for measuring transport in a nearby nanocrystal array. While our technique enables a high resistance measurement, and enables us to probe a wide range of conductance, the main limitations of using single MOSFET charge sensors is the step-like switching of the current caused by electrons tunneling into and out of traps, presumably in the oxide. This makes it difficult to distinguish events that originate in the film from those that originate in the MOSFETs. We use two MOSFETs as simultaneous charge sensors to perform a correlation analysis and distinguish these events. We pattern a 80 nm wide ordered array of PbS nanocrystals, approximately 50 nm away from each sensor, to maximize the signal in the MOSFETs from charge fluctuations in the nanocrystal film. This configuration then enables us to probe electron transport in the nanocrystal array. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L18.00009: Measuring electron transport in nano-patterned films of PbS nanocrystals using a charge sensor Tamar Mentzel, Darcy Wanger, Nirat Ray, Brian Walker, Moungi Bawendi, Marc Kastner The ability to form nanoscale patterns of semiconductor nanocrystal films and to align those patterns to a substrate with nanoscale precision opens the possibility of novel electronic measurements and optoelectronic devices. We demonstrate a novel method for patterning nanoscopic films of semiconductor nanocrystals with electron-beam lithography. The resulting films are ordered and do not suffer from the cracking that arises when annealing or exchanging the capping ligand of the film. The patterning method is effective for a wide range of nanocrystal materials and capping ligands. We pattern a film of PbS nanocrystals approximately 80 nm wide, and position it within 100 nm of charge sensors made from nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs). The charge sensors, which can measure the fluctuations of individual electrons in the nearby electrostatic environment, are used to measure electron transport in the films as a function of temperature and applied field. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L18.00010: Photoinduced modification of surface states in nanoporous InP observed by terahertz spectroscopy James Lloyd-Hughes, Susanne Mueller, Giacomo Scalari, Hugh Bishop, Alison Crossley, Mihai Enachi, Lilian Sirbu, Ion Tiginyanu A precise control of the surface properties of semiconductor nanomaterials is vital for their functionality and use in many opto-electronic applications. Terahertz time-domain spectroscopy allows the non-contact investigation of electron transport in semiconductor nanomaterials, without the complication of contact fabrication. The technique allows the photoconductivity to be determined on picosecond timescales, under the assumption that the material's properties are not permanently altered by photoexcitation. Here we demonstrate that this assumption is not always valid. We report an investigation of nanoporous honeycombs of n-type InP using terahertz time-domain and X-ray photoemission spectroscopies. After photoexcitation the dark conductivity was found to increase quasi-irreversibly, recovering only after several hours in air. The calculated electron density for different surface pinning energies suggests that photoexcitation may reduce the density of surface states. The photoinduced modification of porous semiconductors may be useful in material processing as it is a clean, dry, and area-selective method to increase the conductivity. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L18.00011: Delayed photoluminescence in three-dimensional silicon/silicon germanium nanostructures Leonid Tsybeskov, Nikhil Modi, Selina Mala, J.-M Baribeau, X Wu, D. J. Lockwood In three-dimensional (3D), SiGe nano-island multi-layers separated by nanometer-thick Si layers with the enhanced local strain field visualized by transmission electron microscopy (TEM), we find unusual low temperature photoluminescence (PL) dynamics. The PL detected at 1350 nm rises practically instantly and decays with a lifetime faster than 20 nanoseconds. In contrast, the PL detected at 1550 nm has a rise time of longer than 3-4 microseconds, and it decays with a characteristic lifetime which changes from 10 microseconds to milliseconds. The proposed model considers recombination of excitons bound to SiGe/Si interface as a mechanism responsible for the fast PL at 1350 nm. The observed slow rising PL with a peak near 1550 nm is associated with Auger ionization of SiGe clusters and separation of electrons and holes followed by carrier/exciton diffusion within Si layers toward a longer wavelength luminescence sites. These sites are associated with SiGe cluster areas containing a higher Ge concentration, and they are detected by analytical TEM. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L18.00012: Transport of exciton-polariton condensation in semiconductor microcavity at BEC-BCS crossover Jung-Jung Su, Yoshihisa Yamamoto We study the transport properties of exciton-polariton condensation in the microcavity at the BEC-BCS crossover. Exciton-polariton (EP) is the quasiparticle of exciton and photon that can condense at a temperature as high as room temperature due to its extreme lightness. So far intense studies of this intriguing condensation have been limited to the photoluminescence (PL) measurement, mostly at the lower density BEC regime. When increasing density, the condensate enters the EP BCS regime in which the fermionic nature of electron and hole inside of exciton becomes important. This electron-hole nature is more prominent in the transport than in the PL measurements. We propose a transport measurement scheme of creating indirect-excitons in semiconductor bilayer which is embedded in the planar microcavity filled with photons. Leads are attached to the two sides of the bilayers to perform transport measurements. We present the different features of tunneling conductance signature for EP condensation in the BCS and in the BEC regime. [Preview Abstract] |
Session L19: Invited Session: Spin Liquids with Disorder
Sponsoring Units: DCMP GMAGChair: Leon Balents, University of California, Santa Barbara
Room: 253AB
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L19.00001: The spin liquid ground state of the $S=1/2$ Heisenberg model on the kagome lattice Invited Speaker: Steven White Condensed matter physicists have long sought a realistic two-dimensional (2D) magnetic system whose ground state is a {\it spin liquid}---a zero temperature state in which quantum fluctuations have melted away any form of magnetic order. The nearest-neighbor $S=1/2$ Heisenberg model on the kagome lattice has seemed an ideal candidate, but in recent years some approximate numerical approaches to it have yielded instead a valence bond crystal. We have used the density matrix renormalization group to perform very accurate simulations on numerous cylinders with circumferences up to 12 lattice spacings, finding instead of the valence bond crystal a spin liquid, gapped in both the singlet and triplet sectors, with substantially lower energy. [Simeng Yan, David A. Huse, and Steven R. White, {\sl Science} 332, 1173 (2011). {\it Cover article}] Our results, through a combination of very low energy, short correlation lengths and corresponding small finite size effects, a new rigorous energy bound, and consistent behavior on many cylinders, provide strong evidence that the 2D ground state of this model is a gapped spin liquid. One key feature of this spin liquid is that the predominant valence bond resonances occur on eight site loops rather than the shortest six site loops. The eight site loops allow a greater fraction of the bonds in the lattice to resonate. Our recent studies including a next nearest neighbor interaction $J_2$ reveal that the $J_2=0$ point is near the edge of a substantial spin liquid phase centered near $J_2=0.05-0.1$. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:42PM |
L19.00002: Structure, disorder, and magnetism in Herbertsmithite, a kagom\'{e} Heisenberg antiferromagnet Invited Speaker: Tyrel McQueen Geometric frustration of magnetic ordering on triangle-based lattices is thought to be one avenue to inducing macroscopic quantum states in electron systems. Due to the triangular arrangement of ions, it is impossible to satisfy all nearest-neighbor interactions simultaneously. This ``frustration'' suppresses classical magnetic long-range order and is thought to be capable of resulting in novel quantum states such as various resonating-valence-bond or ``spin-liquid'' ground states for a two-dimensional (2D) S = 1/2 antiferromagnet. However, ``structurally perfect'' frustrated materials are rare; frequently, materials undergo a structural distortion at low temperature, relieving the magnetic frustration and giving rise to a classical ground state. In this talk, I will present recent structural and property studies of the 2D candidate spin-liquid material ``ZnCu$_3$(OH)$_6$Cl$_2$.'' Using X-ray scattering differences at elemental absorption edges and an improved analysis technique, I will show that there is no Zn occupation of the intralayer Cu sites within the kagom\'{e} layer, as previously reported; however there is Cu present on the Zn intersite, leading to a real structural formula of (Zn$_{0.85}$Cu$_{0.15}$)Cu$_3$(OH)$_6$Cl$_2$. Combined with recent pulsed high field magnetic measurements, the lack of Zn mixing onto the kagom\'{e} lattice sites lends support to the idea that the electronic ground state in ``ZnCu$_3$(OH)$_6$Cl$_2$'' and its relatives is non-trivial. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L19.00003: Disorder in a quantum spin liquid: the Kitaev honeycomb model with vacancies Invited Speaker: Roderich Moessner We address the effects of disorder in the Kitaev honeycomb model, focussing on the interplay of disorder and strong interactions in a quantum spin liquid. It is shown that nonmagnetic impurities bind a quantum of the emergent gauge flux. We find the formation of a local moment, which leads to a local magnetic response near the impurity which is parametrically larger than in the bulk. The magnetic response of a pair of impurities can be parametrically larger than that of an isolated one, and can in particular dominate the magnetic response of the system. The role of such impurities as diagnostic of the spin liquid state are emphasized -- most remarkably, they provide a direct signature of the emergent gauge field. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:54PM |
L19.00004: Hole doping in frustrated spinels, ZnCr$_{2}$O$_{4}$ and MgCr$_{2}$O$_{4}$, and their two dimensional analogue SCGO, SrCr$_{8}$Ga$_{4}$O$_{19}$ Invited Speaker: Sian Dutton Recent experiments on the complex geometrically frustrated magnet, $\beta $-CaCr$_{2}$O$_{4}$, clearly illustrate the divergent effect of hole and static doping on the magnetic properties [1]. Given the complex parent state of $\beta $-CaCr$_{2}$O$_{4}$ this is not an ideal system for studying perturbations to the magnetic interactions. However, the onset of ferromagnetic fluctuations and ferrimagnetic ordering in $\beta $-Ca$_{1-y}$Cr$_{2}$O$_{4}$ suggests that other hole doped Cr$^{3+/4+}$ systems may be of interest. The extreme sensitivity in the balance of competing magnetic interactions in geometrically frustrated magnets is illustrated clearly in Cr$^{3+}$ spinels, ACr$_{2}$O$_{4}$. Antiferromagnetic (AFM) ordering in ACr$_{2}$O$_{4}$ occurs at a spin-Peierls transition. Both the low temperature magnetic and structural regimes are found to be highly sensitive to the A cation. In the case of ZnCr$_{2}$O$_{4}$ we find that very fine control of the reaction conditions is necessary to make stoichiometric ZnCr$_{2}$O$_{4}$, rather than hole doped Zn$_{1+x}$Cr$_{2-x}$O$_{4}$ (x $\le $ 0.04). From analysis of magnetic measurements, specific heat and neutron diffraction we have probed the nature of the transitions at $T_{N}$ [2]. How hole doping effects the low temperature properties and the role of the $d^{2}$ Cr$^{4+}$ cations on the isotropic $d^{3}$ Cr$^{3+}$ magnetic lattice will be discussed. Our results on the more robust MgCr$_{2}$O$_{4}$ spinel will also be presented. A 2D analogue of the 3D pyrochlore magnetic lattice in the ACr$_{2}$O$_{4}$ spinels is found in SCGO, SrCr$_{8}$Ga$_{4}$O$_{19}$. In hole doped SCGO, SrCr$_{8}$M$_{x}$Ga$_{4-x}$O$_{19}$ (M = Zn, Mg, Cu), a larger fraction of the Cr$^{3+}$ can be oxidized. Hole doping is found to have a significant effect on the magnetic fluctuations, how this depends on the nature of the dopant cation will be addressed [3]. \\[4pt] [1] S. E. Dutton, C. L. Broholm, and R. J. Cava, Journal of Solid State Chemistry \textbf{183}, 1798 (2010). \\[0pt] [2] S. E. Dutton\textit{ et al.}, Physical Review B \textbf{83}, 064407 (2011). \\[0pt] [3] S. E. Dutton\textit{ et al.}, Journal of Physics-Condensed Matter \textbf{23}, 386001 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:30PM |
L19.00005: Impurity Effects in Highly Frustrated Diamond-Lattice Antiferromagnets Invited Speaker: Lucile Savary We consider the effects of local impurities in highly frustrated diamond lattice antiferromagnets, which exhibit large but non-extensive ground state degeneracies. Such models are appropriate to many A-site magnetic spinels. We argue very generally that sufficiently dilute impurities induce an {\sl ordered} magnetic ground state, and provide a mechanism of degeneracy breaking. The states which are selected can be determined by a ``swiss cheese model'' analysis, which we demonstrate numerically for a particular impurity model in this case. Moreover, we present criteria for estimating the stability of the resulting ordered phase to a competing frozen (spin glass) one. The results may explain the contrasting finding of frozen and ordered ground states in CoAl$_2$O$_4$ and MnSc$_2$S$_4$, respectively. [Preview Abstract] |
Session L20: Invited Session: STEM Outreach to Underrepresented Communities
Sponsoring Units: CSWP COMChair: Richard Peterson, Bethel University
Room: 253C
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L20.00001: LIGO: Impacting science education through gravity-wave research in the local community and beyond Invited Speaker: Stephen McGuire We describe our integration of the science teacher pre-service and in-service education programs at Southern University (SUBR) with the Laser Interferometer Gravitational-wave Observatory (LIGO) Science Education Center (SEC). Inquiry-based interactive exhibits are employed wherein we emphasize classical physics concepts of oscillations, waves, wave propagation, interference, resonance, lasers, light and Newtonian gravity. An aggressive museum docent training program is providing a means for undergraduates to learn how to effectively communicate science concepts within informal learning environments. This local educational partnership will ultimately create a science education continuum of engagement, working at multiple levels and multiple audiences to strengthen science literacy within the targeted STEM African-American community. Following a brief overview of our program of LIGO-related optical materials research, we give a detailed presentation of our K-12 science teacher preparation program with results. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:42PM |
L20.00002: STEM Outreach to the African Canadian Community - The Imhotep Legacy Academy Invited Speaker: Kevin Hewitt Like the African American community in the US, the African Canadian community is underrepresented in the Science Technology Engineering and Mathematics (STEM) fields. To serve these communities two outreach organizations emerged in Canadian cities where there is a critical mass of learners of African Descent - Toronto and Halifax. I will describe the Imhotep's Legacy Academy, which began in the Physics labs of Dalhousie University in Halifax, Nova Scotia and has grown to a province-wide program serving three-quarters of the school boards in the province with an annual budget that has grown to \$400,000 in 2011-12. It follows the learner from the time they enter grade 7 to the time they graduate from university, through three programs: (a) Weekly After-School science enrichment for junior high learners, (b) Virtual High school tutoring program and (c) Summer student internships and research scholarships for post-secondary students. This year, the program was the beneficiary of funding from TD Bank to establish scholarships for program participants to enter Dalhousie university. Modeled on the Meyerhoff scholarships the program participants are identified at an early stage and are promised a subset of funding as they meet selected criteria during participation in the program. The program enjoys support from the Department of Education and the highest levels of government. A tri-mentoring system exists where faculty of African descent train mentors, who are science students of African descent at associated universities, to deliver hands-on enrichment activities to learners of African Descent. Evidence supporting the success of the program will be highlighted. Project outcomes measured include (i) recruitment; (ii) attendance; (iii) stakeholder relationships; (iv) programming; (v) staff training; (vi) perception of ILASP's value; (vii) academic performance. The end results are new lessons and best practices that are incorporated into a strategic plan for the new project year. Teachers perceived that ILASP had a positive ripple effect on the entire academic and non-academic educational experience of the learners, crediting the project with (i) encouraging self-learning; (ii) assisting in honing learners' science and math skills; (iii) developing core skills that were applicable in learners' schoolwork; (iv) boosting learners' self-esteem; (v) improving school attendance; (vi) boosting learners' motivation to be engaged participants in all other classes. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L20.00003: Knowing your Hispanic community to improve outreach effectiveness Invited Speaker: Cristina Torres ``Know your audience,'' wise words for properly conveying information. For outreach, this can make the difference between successful connection or failure to connect to the community around us. When the target audience is the Hispanic community knowing your audience can potentially take on unexpected complexity. Like some other minority communities, the Hispanic community's culture is complex dual culture system. I will discuss my own personal observations about how this community interacts with the academic world. I also will discuss how the Hispanic community perceives itself as a member of the broader community we all live in and how non-homogeneous, our broadly defined Hispanic community really is. Discussing the Hispanic community's hidden nuances will bring to light the difficultly in getting to ``know your audience,'' when it comes to effective outreach in the rapidly growing Hispanic community. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:54PM |
L20.00004: Outreach to the Native American community through the Navajo-Hopi program Invited Speaker: Kevin Schindler |
Tuesday, February 28, 2012 4:54PM - 5:30PM |
L20.00005: Intersections of Gender and Power: Improving the Status of Women in Physics Invited Speaker: Saeqa Vrtilek Numerous problems bedevil the twin goals of increasing the numerical participation in science, technology, engineering, and mathematics (STEM) fields by women and increasing the quality of that participation. The nature of the difficulties is everywhere slightly different, but there are underlying commonalities. A wide portfolio of lessons learned that can be applied to the confluence of cultures, backgrounds, and experiences that shape any given institution have been developed and will be presented. Among these, common and dominant themes are the need for mentoring, management training, and the increased visibility of successful women scientists. These have been identified (Nelson and Rogers 2004; Sonnert and Holton 1995a; Vetter 1996) as some of the key factors in securing the encouragement and increased stability needed for more senior women scientists to thrive to their full potential and provide the example and mentoring needed for a larger and more productive new generation. [Preview Abstract] |
Session L21: Superconductivity: Mostly Mesoscopics & Low-d
Sponsoring Units: DCMPChair: Timir Datta, Univeristy of South Carolina
Room: 254A
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L21.00001: Superconducting and Critical Current Properties of NiBi$_{3}$ Microfibers and Thin Films Neel Haldolaarachchige, Yimin Xiong, Phil Adams, David Young We report the superconducting and critical current properties of thin films of NiBi$_{3}$ formed on the surface of carbon microfibers and on sapphire substrates. The NiBi$_{3}$ coated C-fibers were prepared by reacting 7-\textit{$\mu $}m diameter Ni-coated carbon fibers with Bi shot, and the thin films on sapphire were formed by exposing electron-beam deposited Ni films to Bi vapor. The fibers and films show T$_{c}$ = 4.25 K and T$_{c}$ = 4.35 K, respectively, which was slightly higher than that of bulk polycrystalline NiBi$_{3}$. The extrapolated upper critical fields of the fibers [H$_{c2}$(0) = 12 T] and films [H$_{c2}$(0) = 9 T] are higher than the reported data on polycrystalline samples. The temperature dependence of the critical current density ($J_{c})$ is well described by Ginzburg-Landau theory and gives an extrapolated value of 5.26 $\times $ 10$^{5}$ A/cm$^{2}$. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L21.00002: Proximity effect in superconductor exchange spring-superconductor junction Jiyeong Gu, Christopher Safranski, Hanming Yuan It is known that in ferromagnet/superconductor hybrid system when the magnetizations of the ferromagnetic layer are inhomogeneous superconductivity is not necessarily suppressed by the ferromagnet due to the presence of the triplet superconducting pairing. In our current work we used exchange spring magnet to produce inhomogeneous noncollinear magnetic configuration in the ferromagnet. Exchange spring trilayer structure, soft/hard/soft, was fabricated between two superconducting Nb layers; Nb/Py/SmFe(SmCo)/Py/Nb. Magnetic property of the structure was characterized using Vibrating Sample Magnetometer and Magneto Optical Kerr Effect (MOKE). MOKE was used to measure the magnetizations of the top Py and the bottom Py separately. Transition temperature of the system was measured as a function of magnitude and direction of the external magnetic field. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L21.00003: Results of Resonant Activation and Macroscopic Quantum Tunneling Experiments in Magnesium Diboride Thin Film Josephson Junctions Roberto Ramos, Steve Carabello, Joseph Lambert, Jerome Mlack, Wenqing Dai, Yi. Shen, Qi Li, Daniel Cunnane, C.G. Zhuang, Ke Chen, X.X. Xi The Josephson junction is an experimental testbed widely used to study resonant activation and macroscopic quantum tunneling. These phenomena have been observed in junctions based on conventional low-temperature superconductors such as Nb and Al, and even in high-$T_{c}$, intrinsic superconductors. We report results of superconducting-to normal state switching experiments below 1 K using MgB$_{2}$-based Josephson heterojunctions with Pb and Nb counter-electrodes. Measurements were made with and without RF excitation. With microwaves, we see evidence of a resonant peak, in addition to the primary escape (from ground state) peak -- consistent with resonant activation. We also observe features suggestive of macroscopic quantum tunneling including peaks in the escape rate enhancements and an ``elbow'' in the graph of calculated escape temperatures $T_{esc}$ versus sample temperature. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L21.00004: Carrier Transport in Heterojunction Nanocrystals Under Strain Mark Sweeney, Joel Eaves We present a theory for carrier transport in semiconducting nanoscale heterostructures that emphasizes the effects of strain at the interface between two different crystal structures. An exactly solvable model shows that the interface region, or junction, acts as a scattering potential that facilitates charge separation. As a case study, we model a Type-II CdS/ZnSe heterostructure. After advancing a theory similar to that employed in model molecular conductance calculations, we calculate the electron and hole photocurrents and conductances, including non-linear effects, through the junction at steady-state. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L21.00005: Progress towards measuring electron-electron interactions in persistent currents Manuel A. Castellanos-Beltran, Dustin Q. Ngo, Jack G.E. Harris The equilibrium persistent current (PC) in normal metal rings have been a challenge for both experimentalists and theorists. Specifically, the magnitude of the average PC's in normal metals has been a long-standing puzzle. Previous measurements of the average current ($\left\langle {I_{h/2e} } \right\rangle )$ were larger than theoretical predictions and indicated a diamagnetic sign. A possible explanation for these results is that they arise from the interplay of attractive electron-electron interactions within the metal (leading to the enhanced average PC) and trace magnetic impurities (which suppress the BCS superconductivity that would otherwise result from the attractive interactions). In this talk, I will discuss our progress towards measurements that intend to clarify the role that electron-electron interactions and magnetic impurities play in the persistent current. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L21.00006: Tuning effective pairing potential through atomic scale control of superconductor heterostructures Chendong Zhang, Jisun Kim, Jungdae Kim, Hyoungdo Nam, Qian Niu, Hongjun Gao, Chih-Kang Shih Previous experiments showed that superconductivity persists in ultrathin films of conventional superconductors, even in films that are only 1-2 atomic layers thick. However, it has also been implied that the interface and substrate can strongly influence the electronic properties in the quasi two-dimensional regime. In this work, we fabricated a heterostructure with a normal metal layer (Ag here) placed in between a superconducting film (Pb here) and an insulating substrate, and measured its superconductivity with scanning tunneling spectroscopy subsequently. The most striking observation is the requirement of an overlayer (L$_{Pb})$ to recover the superconductivity, while the recovery thickness is linearly dependent on the underlayer thickness. Considering the renormalization of pairing interaction strength for Cooper pair in the hybrid structure, the ``effective attractive potential'' model was developed in this work. It could serve as a model with predictive power to describe the general behavior of superconductor heterostructures. Moreover, the physical origin of some discrepancies in the reported transition temperature as a function of film thickness is partly elucidated by this work as well. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L21.00007: Area-dependence of spin-triplet supercurrent in ferromagnetic Josephson junctions Yixing Wang, William P. Pratt, Jr., Norman O. Birge Spin-triplet supercurrents in strong ferromagnetic Josephson junctions were reported by several groups in 2010. At the same time, the 0-$\pi $ current-phase relationship of the spin-triplet supercurrent was predicted to be controllable by the magnetization orientations of different ferromagnetic layers. Our junctions contain a series of ferromagnetic layers consisting of a synthetic antiferromagnet Co/Ru/Co sandwiched between two thin magnetic layers such as PdNi or Ni [1]. When looking along the direction of current flow, one should obtain 0 junctions if the rotation direction of magnetizations is the same from one to the next, and $\pi $ junctions when the opposite rotation direction is the case. Since our magnetic layers have multiple domains in the virgin state, we should expect 0 and $\pi $ phases to alternate randomly in different locations in the junctions. The critical current in the virgin state should scale with the square-root of the junction area. After aligning the outer ferromagnetic layers in the same direction with an external field, the current-phase relation should be uniform across the whole junction area and the critical current should be proportional to the junction area. We will present data confirming this expectation for the magnetized state, whereas the situation for the virgin state is presently unclear. \\[4pt] [1] T.S. Khaire, M.A. Khasawneh, W.P. Pratt Jr and N.O. Birge, \textit{Phys. Rev. Lett.} 104 137002 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L21.00008: Magnetic-field dependence of energy levels of superconducting nano-scale mettalic grains with strong spin-orbit scattering Konstantin Nesterov, Yoram Alhassid We study the Zeeman splitting of discrete energy levels of superconducting nano-scale metallic grains whose single-electron dynamics is chaotic [1]. In the absence of spin-orbit scattering the Zeeman splitting of a single-electron level is trivial; it is the same for all levels and linear in magnetic field. Spin-orbit coupling suppresses this splitting, induces level-to-level fluctuations and non-linear corrections to the energies. We investigate the combined effect of pairing correlations, which lead to superconductivity in the bulk limit, and spin-orbit scattering on the many-electron energy levels in a weak magnetic field. In particular, we focus our studies on the linear (g-factor) and quadratic (zero-field level curvature) corrections and their mesoscopic fluctuations. The single-electron part of the Hamiltonian follows the statistics of the Gaussian symplectic ensemble of random matrix theory, which is applicable in the limit of strong spin-orbit scattering and a large dimensionless Thouless conductance. The interaction is given by a BCS-like pairing term and the magnetic field coupling is described by a Zeeman term. [1] K. Nesterov and Y. Alhassid, to be published. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L21.00009: Temperature induced epitaxial strain and superconductivity in (Cu,C)Ba$_{2}$CuO$_{4+\delta }$ thin films F.N.U. Shipra, A. Sundaresan We have studied the effects of substrate temperature in establishing superconductivity in carbon incorporated thin films of Infinite Layered (IL) BaCuO$_{2+\delta }$. Carbon in the form of CO$_{3}^{2-}$ group modulates the IL structure into a superstructure with doubled out of plane lattice parameter, `$c$'. The superstructure, (Cu,C)Ba$_{2}$CuO$_{4+\delta }$ (Cu-1201) shows superconductivity within a narrow window of `$c$' lattice parameter varying between 8.28 {\AA} and 8.33 {\AA}. The structural analysis of these thin films using reciprocal space maps (RSMs) shows a pseudomorphic growth with an in-plane lattice parameter, `$a$', of 3.90 {\AA}, similar to that of the SrTiO$_{3}$ substrate. Growth of these films under compressive strain is obtained at substrate temperatures varying between 530 \r{ }C and 560 \r{ }C. At deposition temperatures less than 500 \r{ }C, films with a relaxed in-plane lattice parameter of 4.00 {\AA} were obtained which were non-superconducting. Deviations in the substrate temperature led to the coexistence of strained and relaxed phases. Thus the elongation along $c$ -- axis is compensated by the compression along $a $-- axis with increasing substrate temperature. Flexibility of tuning the in-plane mismatch between substrate and film also rule out the use of buffer layers. Such structural changes result in the change of bond lengths and subsequently rearrange the number of charge carriers in the CuO$_{2}$ planes. Optimum number of charge carriers lead to superconductivity. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L21.00010: Coexistence of a Triplet Nodal Order Parameter and a Singlet Order Parameter at Co / CoO / In contacts Shay Hacohen-Gourgy, Boaz Almog, Guy Deutscher We present differential conductance measurements of Cobalt / Cobalt-Oxide / Indium planar junctions, 500nm x 500nm in size. The junctions span a wide range of barriers, from very low to a tunnel barrier. The characteristic conductance of all the junctions show a V-shape structure at low bias instead of the U-shape characteristic of a s-wave order parameter. The bias of the conductance peaks is, for all junctions, larger than the gap of indium. Both properties exclude pure s-wave pairing. The data is well fitted by a model that assumes the coexistence of s-wave singlet and equal spin p-wave triplet fluids. We find that the values of the s-wave and p-wave gaps follow the BCS temperature dependance and that the amplitude of the s-wave fluid increases with the barrier strength. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L21.00011: Exploring triplet superconductivity by controlling the magnetic non-collinearity L.Y. Zhu, Yaohua Liu, F.S. Bergeret, J.E. Pearson, S.G.E. te Velthuis, S.D. Bader, J.S. Jiang Recent theories predict spin-triplet superconductivity at the interface between a singlet superconductor (SC) and a ferromagnet (FM) with \textit{inhomogeneous} magnetization [1]. Magnetic non-collinearity is a crucial but not quantitatively controlled parameter in most experiments inferring triplet superconductivity [2]. In this work, we use Nb as the SC, and an epitaxial exchange spring Py/Sm-Co bilayer with in-plane uniaxial anisotropy in Sm-Co layer as the FM. Due to the interfacial exchange coupling, a \textit{tunable} noncollinear spin spiral can be achieved by controlling the external field. At a fixed temperature within the superconducting transition, starting from a collinear magnetic configuration, as the spin spiral winding angle \textit{$\phi $} increases, the superconducting critical current ($I_{c})$ first increases. There is an optimized winding angle \textit{$\phi $}$_{o}$, which maximizes $I_{c}$, after which $I_{c}$ decreases with increasing\textit{ $\phi $}. This \textit{non-monotonic} $I_{c}$(\textit{$\phi $}) dependence cannot be explained by the short range proximity effect alone and suggests triplet pairing. More importantly, combining micromagnetic simulations with magnetoresistance measurements, we have determined the~magnetic non-collinearity and correlated it \textit{quantitatively} with the superconducting transport results. Our findings demonstrate the superconducting proximity effect can be tuned by manipulating the magnetic non-collinearity in a \textit{single} sample.\\[0pt] [1] F. S. Bergeret et al., PRL \textbf{86}, 4096. [2] M. Eschrig, Physics Today \textbf{64}, 43. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L21.00012: The Images of vortex penetration into superconducting MoGe plates observed by scanning SQUID microscope Ho Thanh Huy, Masahiko Hayashi, Tsutomu Yotsuya, Takekazu Ishida The amorphous superconducting film is very preferential as a good model in studying nanostructured superconductors because it has weakened pinning centers compared to other superconductors. The MoGe films have been deposited by a DC sputtering apparatus using a Mo$_{80}$Ge$_{20}$ target and a number of small MoGe plates were fabricated with the aid of photolithography. The vortex distribution in a MoGe circle has been investigated by means of a scanning SQUID microscope. We found that vortices form different configurations including shell structures, which evolves with the increase in applied magnetic field. Observed results are compared to theoretical studies for vortices in mesoscopic circles. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L21.00013: Image processing of scanning SQUID microscope for observing superconducting nanostructures Masahiko Hayashi, Ho Thanh Huy, Takekazu Ishida A newly developed image processing technique for the scanning superconducting quantum interference device (SQUID) microscope is presented and its application to several measurements on superconducting nanostructures, including networks and dots, is discussed. This method is based on the detailed analysis of the structures characteristic to the measurement apparatus, such as the shape and position of the pickup coil. We take account of the Meissner effect in the coil body and its influences on the obtained image are carefully removed. The separation between the coil and the sample is also considered. Actually applying this method, we present images of the superconducting networks and dots, which show clear improvement from the raw images. The results are also discussed from physical point of view. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L21.00014: Fluctuating pancake vortices revealed by dissipation of Josephson vortex lattice Itsuhiro Kakeya, Alexei Koshelev, Alexander Buzdin, Takashi Yamamoto, Kazuo Kadowaki In strongly anisotropic layered superconductors in tilted magnetic fields the Josephson vortex lattice coexists with the lattice of pancake vortices. Due to the interaction between them, the dissipation of the Josephson-vortex lattice occurs to be very sensitive to the presence of the pancake vortices. If the c-axis magnetic field is smaller then the corresponding lower critical field the pancake stacks are not formed but the individual pancakes may exist in the fluctuational regime either near surface in large-size samples or in the central region for small-size mesas. We calculate the contribution of such fluctuating pancake vortices to the c-axis conductivity of the Josephson vortex lattice and compare the theoretical result with measurements on small mesas fabricated out of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ crystals. A fingerprint of fluctuating pancakes is characteristic exponential dependence of the c-axis conductivity observed experimentally. Our results provide strong evidence of the existence of the fluctuating pancakes and their influence on the Josephson-vortex-lattice dissipation. [Preview Abstract] |
Session L22: Focus Session: Fe-based Superconductors: -- Inelastic Neutron Scattering and Magnetic Excitations
Sponsoring Units: DMP DCOMPChair: Rob McQueeny, Ames Lab/Iowa State University
Room: 254B
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L22.00001: Interstitial Iron Effects on Magnetic Excitations in Parent Phases Fe$_{1+x}$Te from Polarized and Inelastic Neutron Scattering Invited Speaker: Efrain Rodriguez One of the simplest systems of the iron-based superconducting family, Fe$_{1+x}Ch$ (where $Ch$ = S, Se, or Te) presents ample opportunity to study the relationship between antiferromagnetism and superconductivity. Several studies have demonstrated how the makeup of the $Ch$ anions changes the electronic properties drastically, but the effect of excess interstitial iron, the $x$ in Fe$_{1+x}Ch$, is not as well understood. Our previous diffraction experiments on samples varying $x$ from 4 \% to 16 \% demonstrated how the magnetic ordering changes from collinear antiferromagnetic to helical incommensurate via a spin-density wave state at the special composition of $x \approx 12\%$. We present inelastic neutron scattering measurements of the phases Fe$_{1+x}$Te for two amounts of interstitial iron in the lattice, 5\% and 14 \%. We have combined data from cold neutron triple-axis, thermal neutron triple-axis, and spallation source time-of-flight to provide a full picture of the magnetic excitations in Fe$_{1+x}$Te for $x=14 \%$ from 0.5 meV to 150 meV. In addition, we present polarized inelastic studies on this particular composition to investigate the nature of the spin waves, $i.e.$ longitudinal $vs.$ transverse. The results are compared with those found in the phase with low amounts of interstitial iron ($\approx$ 5 \%), in order to understand the nature of the exchange interactions in this important parent compound. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L22.00002: Temperature-enhanced nearly critical magnetic scattering in nearly superconducting FeTe$_{0.87}$S$_{0.13}$ Igor Zaliznyak, Cedomir Petrovic, Rongwei Hu, Andrei Savici, Karol Marty, Mark Lumsden Recent neutron scattering measurements [1] have revealed an unusual temperature-induced enhancement of dynamical magnetism in iron telluride, FeTe, the parent material of the chalcogenide family of iron-based superconductors. Here we report a study of magnetic excitations in the sulfur-doped FeTe$_{0.87}$S$_{0.13}$, where bulk measurements show presence of the filamentary superconductivity [2]. Our neutron measurements probe the bulk of the material, which is nearly critical (filamentary superconductivity emerges in a small volume fraction of the sample, which is supercritical). We observe a peculiar pattern of low-energy magnetic scattering, characteristic of critical magnetic fluctuations, and find that it is also anomalously enhanced with the increasing temperature up to $\sim 100$ K. \\[4pt] [1] I. A. Zaliznyak, \emph{et. al.}, Z. J. Xu, J. M. Tranquada, G. D. Gu, A. M. Tsvelik, M. B. Stone, Phys. Rev. Lett., in press (2011).\\[0pt] [2] Rongwei Hu, E. S. Bozin, J. B. Warren, C. Petrovic, Phys. Rev. B {\bf 80}, 214514 (2009). [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L22.00003: Friedel-like Oscillations from Interstitial Iron in Superconducting Fe$_{1+y}$Te$_{0.62}$Se$_{0.38}$ Vivek Thampy, J. Kang, J.A. Rodriguez-Rivera, W. Bao, A.T. Savici, J. Hu, T.J. Liu, B. Qian, D. Fobes, Z.Q. Mao, C.B. Fu, W.C. Chen, Q. Ye, R.W. Erwin, T.R. Gentile, Z. Tesanovic, C. Broholm Using polarized and unpolarized neutron scattering we show that interstitial Fe in superconducting Fe$_{1+y}$Te$_{1-x}$Se$_x$ induces a magnetic Friedel-like oscillation that diffracts at ${\bf Q}_{\perp}=(\frac{1}{2}0)$ and involves $>$50 neighboring Fe sites. The interstitial $>2$~$\mu_B$ moment is surrounded by compensating ferromagnetic four spin clusters that may seed double stripe ordering in Fe$_{1+y}$Te. A semi-metallic 5-band model with $(\frac{1}{2}\frac{1}{2})$ Fermi surface nesting and four fold symmetric super-exchange between interstitial Fe and two in-plane nearest neighbors largely accounts for the observed diffraction. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L22.00004: Copper-tuned magnetic order and excitations in iron-based superconductors Fe1+yTe1-xSex Jinsheng Wen, Zhijun Xu, Guangyong Xu, Mark Lumsden, Masaaki Matsuda, Patrick Valdivia, Edith Bourret, Dunghai Lee, Genda Gu, John Tranquada, Robert Birgeneau We report neutron scattering results on the Cu-substitution effects in the iron-based superconductors, Fe$_{1+y}$Te$_{1-x}$Se$_x$. In the parent compound, it is found that Cu drives the low-temperature magnetic ground state from long-range commensurate antiferromagnetic order in Fe$_{1.06}$TeCu$_{0.04}$ to short-range incommensurate order in FeTeCu$_{0.1}$. In the former sample, the structural and magnetic ordering temperature is 40~K; in FeTeCu$_{0.1}$, the structural phase transition is not obvious and a transition to the spin-glass state is found at 22~K. Cu suppresses superconductivity in FeTe$_{0.5}$Se$_{0.5}$---$T_c$ is reduced to 7~K with a 2\% Cu doping, and no superconductivity is found in the 10\% Cu-doped sample. In the meantime, the intensity and energy of the resonance mode are suppressed in the 2\% Cu-doped sample, while there is no resonance in the non-superconducting sample. Besides, the low-temperature magnetic excitation spectra are distinct for these two samples, with the superconducting one having an ``hour-glass" shape and the other one having a ``waterfall" shape. Our results provide further insights on the interplay between magnetism and superconductivity in the iron-based superconductors. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L22.00005: Neutron Scattering Study of Magnetic Excitation Spectrum on Fe1-x(Ni/Cu)xTe0.5Se0.5 Zhijun Xu, Jinsheng Wen, Guangyong Xu, Genda Gu, John Tranquada We have performed a series of neutron scattering and magnetization measurements on Fe1-x(Ni/Cu)xTe0.5Se0.5 with different Ni/Cu compositions to study the interplay between magnetism and superconductivity. Substituting 2{\%} and 4{\%} of Ni for Fe reduces Tc from 15 K to 12 K and 8 K, while 10{\%} of Cu results in lost of superconductivity. Spin resonance with lower energy are found in all superconducting samples. The overall shape of the low energy magnetic dispersion changes from two incommensurate vertical columns at T$>>$Tc to a distinctly different U-shaped dispersion at low temperature in superconducting samples. This spectral reconstruction is apparent for temperature up to 3Tc. On the other hand, no static order around (0.5,0,0.5) was found in any of these samples. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L22.00006: Magnetic excitations in Fe1.01Te0.7Se0.3 Jooseop Lee, Igor Zaliznyak, Naoyuki Katayama, Ryoichi Kajimoto, Seunghun Lee Recently, there have been intense studies on the magnetism in FeTe1-xSex, which resulted in contradicting observations making the nature of its magnetism controversial. While the Fermi surface nesting picture can well predict the position of magnetic resonance in superconducting region, it clearly fails to predict the magnetic ordering wave vector in the parent compound. To investigate the magnetism in this iron chalcogenide series, we synthesized Fe1.01Te0.7Se0.3. At this doping, it resides very close to the superconducting doping region, but is in spin glass phase. By using the Time-of-Flight neutron scattering, we obtained magnetic dispersions in this material at energies up to 257meV. We find characteristic lines of diffuse scattering in Q-space, which provide the evidence for highly frustrated interactions. These lines of degeneracy persist up to about 10meV, and start to disperse above it. Based on the shape of this quasi-degenerate manifold in Q-space, we propose a description of spin excitations using the J1-J2-J3 Heisenberg model on square lattice model near the limit of maximum frustration with weak extrinsic perturbation. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L22.00007: Temperature Dependence of Magnetic Correlations in Fe-based Parents Leland Harriger, Huiqian Luo, Mengshu Liu, Christopher Frost, Pengcheng Dai Key results from time of flight (TOF) neutron scattering measurements of BaFe$_{2}$As$_{2}$ and Fe$_{2}$Te$_{2}$ will be reviewed. These measurements were carried out over temperatures directly above and below T$_{N}$ as well as base temperature measurements deep inside the order state and intermediate temperatures between T$_{N}$ and room temperature where loosely correlated spin fluctuations persist. The large (TOF) data sets provide enough coverage of (Q, $\omega$) space to integrate out the correlated fluctuating moment, dispersion softening, and correlation length of the excitations as a function of temperature. Taken together, these results provide a systematic accounting of how the magnetic excitations in these parents evolve as they transition from long range ordered magnets to paramagnets. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L22.00008: Effect of the in-plane magnetic field on the neutron spin resonance in optimally doped FeSe0.4Te0.6 and BaFe1.9Ni0.1As2 superconductors Xingye Lu, Shiliang Li, Meng Wang, Huiqian Luo, Pengcheng Dai We use inelastic neutron scattering to study the effect of an in-plane magnetic field on the magnetic resonance in optimally doped superconductors FeSe0.4Te0.6 (Tc = 14 K) and BaFe1.9Ni0.1As2 (Tc = 20 K). While the magnetic field up to 14.5 T does not change the energy of the resonance, it partially suppresses Tc and the corresponding superconductivity-induced intensity gain of the mode. However, we find no direct evidence for the field-induced spin-1 Zeeman splitting of the resonance. Therefore it is still unclear if the resonance is the long-sought singlet-triplet excitation directly coupled to the superconducting electron Cooper pairs. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L22.00009: Itinerant Spin Excitations in SrFe$_2$As$_2$ Measured by Inelastic Neutron Scattering Russell Ewings, Toby Perring, Jack Gillett, Sitikantha Das, Suchitra Sebastian, Alice Taylor, Tatiana Guidi, Andrew Boothroyd We have performed inelastic neutron scattering (INS) measurements of the magnetic excitations in SrFe$_2$As$_2$, an iron pnictide parent compound [1]. The data extend up to energies of $\sim 260$\,meV. We find that the spectrum calculated from a local-moment J$_1$-J$_2$ model fails to describe our data in several key respects. In particular, the data can only be fitted by using different exchange parameters at high and low energies, for which there is no obvious physical justification. In both cases the nearest-neighbor exchange parameters, J$_{1\rm{a}}$ and J$_{1\rm{b}}$, are very different. Also, on warming above the magnetic/structural ordering transition, one would expect J$_{1\rm{a}}$ $\rightarrow$ J$_{1\rm{b}}$ and hence a soft mode at $\mathbf{Q}=(0.5,0.5)$, due to frustration in the local-moment model. However we find that the spectrum is largely unaltered. We find that the qualitative features of the INS spectra that cannot be described by the J$_1$-J$_2$ model are readily explained by calculations from a 5-band itinerant mean-field model. This also implies that it is not necessary to invoke additional broken symmetry, such as electronic nematic or orbital order, to explain the lack of a soft mode. [1] R. A. Ewings et al, Phys. Rev. B \textbf{83}, 214519 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L22.00010: Magnetic resonance from the interplay of frustration and superconductivity Ilya Eremin, Johannes Knolle, Joerg Schmalian, Roderich Moessner Motivated by iron-based superconductors, we develop a self-consistent electronic theory for the itinerant spin excitations in the regime of coexistence of the antiferromagnetic stripe order with wave vector {\bf Q}$_1$ = $(\pi,0)$ and $s^{+?}$ superconductivity. The onset of superconductivity leads to the appearance of a magnetic resonance near the wave vector {\bf Q}$_2$ = $(0,\pi)$, where magnetic order is absent. This resonance is isotropic in spin space, unlike the excitations near {\bf Q}$_1$, where the magnetic Goldstone mode resides. We discuss several features which can be observed experimentally. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L22.00011: Spin excitations in isovalently doped Ba(Fe1-xRux)2As2 Jun Zhao, Y. Zhao, K. Marty, C. Rotundu, M. Matsuda, E. Bourret-Courchesne, H. Yao, J. Wen, P.N. Valdivia, T.R. Forrest, J.P. Hu, Dung-Hai Lee, R.J. Birgeneau Iron based superconductors exhibit remarkably rich phase diagrams: superconductivity can be obtained through carrier doping, application of external pressure, or isovalent doping. Here we present the elastic and inelastic neutron scattering measurements on the magnetic correlations in isovalently doped Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$. For the underdoped sample the static order moment are partially suppressed and the low energy spin correlation length increases upon entering the superconducting state. The spin excitation intensity is reduced with increasing Ru doping toward the over doped regime. We also studied the transfer of the magnetic spectral weight across $T_c$ in this system. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L22.00012: Polarized Neutron Scattering Study of Over-doped BaFe$_{2-x }$Ni$_{x}$As$_{2}$ Mengshu Liu, Chris Lester, Jiri Kulda, Xingye Lu, Huiqian Luo, Stephen Hayden, Pengcheng Dai Spin excitations in over-doped superconducting BaFe$_{2-x }$Ni$_{x}$As$_{2}$ (x=0.15) were investigated using polarized neutron scattering. In a previous experiment on the optimally (x=0.1) doped sample, polarization analysis reveals that the magnetic response is highly anisotropic, at energies as high as 10 meV. Since this optimally doped sample is close to the boundary where antiferromagnetism disappears, there are still relatively strong antiferromagnetic correlations presented in the normal state. We will report our results on the over-doped sample, where the correlation is considerably weakened but still superconducting. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L22.00013: Neutron scattering studies on LiFeAs and BaFe$_{2}$As$_{2}$ M. Braden, N. Qureshi, J. Brand, P. Steffens, A. Stunault, Y. Sidis, D. Lamago, L. Harnagea, S. Wurmehl, B. B\"uchner The anisotropy of the magnetic excitations in BaFe$_{2}$As$_{2}$ was studied by polarization analysis. We find the in-plane polarized transverse magnon to lie at higher energy than the out-of-plane polarized one indicating very strong in-plane single-ion anisotropy. Superconducting LiFeAs exhibits the suppression of local susceptibility expected for spin-singlet pairing. Inelastic correlations appear in LiFeAs at the incommensurate wave vectors, they respond to the opening of the superconducting gap by a transfer of spectral weight. [Preview Abstract] |
Session L23: Focus Session: Fe-based Superconductors - First Principles, Models, and Spectroscopy
Sponsoring Units: DMP DCOMPChair: Roser Valenti, Goethe University
Room: 255
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L23.00001: Fe Based Superconductors from first Principles Frank Essenberger, Pawel Buczek, Arthur Ernst, Antonio Sanna, Leonid Sandratskii, Eberhard K.U. Gross Density functional theory (DFT) for superconductors (SCs) is, in principle, exact and contains no adjustable parameters.\footnote{\emph{Phys. Rev. Lett.}, 1988, 60, 2430} However, for real calculations an approximation to the pairing potential $\Delta_{\rm xc}$ is needed. Many-body perturbation theory can be used to derive approximations to $\Delta_{\rm xc}$. The perturbation series is developed in orders of the screened Coulomb and phonon interactions and truncated at the first order. Unfortunately, this approach fails to predict superconductivity in unconventional SCs, like Fe based compounds.\footnote{\emph{Phys. Rev. Lett.}, 2008, 101, 026403} Hence, an alternative pairing mechanism has to be present in these materials, the leading contender for which is spin fluctuations.\footnote{\emph{Phys. Rev. Lett.}, 2008, 101, 057003} We present a scheme to extend the current approximation $\Delta_{\rm xc}$ to include spin fluctuations, which involves two steps: (1) the fluctuations are calculated with time dependent DFT,\footnote{\emph{Phys. Rev. Lett.}, 2009, 102, 247206} (2) an effective electron interaction mediated by the spin fluctuations is constructed.\footnote{\emph{Phys. Rev. B, 1985, }32, 2156} We present results for FeSe. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L23.00002: First-principles study on electronic structure of Ca-Fe-Pt-As-type iron-based superconductors Hiroki Nakamura, Mashiko Machida Since the discovery of the iron-based superconductor, LaFeAsO$_{1-x}$F$_x$ whose Tc reached 26K, various types of iron-based superconductors have been fabricated to attain higher Tc. Recently, new Ca-Fe-Pt-As system of iron-based superconductors was discovered, and the highest Tc in this class becomes 38K. In this system, the crystal structure is different from those of well-known iron-based superconductors. In particular, the blocking layers of some materials in this system are expected to be metallic unlike the other types of iron-based superconductors. We perform first-principles calculations for Ca-Fe-Pt-As system and evaluate their electronic structure. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L23.00003: First-principles study of cobalt pnictides: Example compound SrCo$_{2}$N$_{2}$ Andrew O'Hara, Alexander Demkov Although it has been known for several decades that many materials with chemical form AT$_{2}$X$_{2}$ (A = alkaline earth element or lanthanide, T = transition metal, X = an element of groups IIIB-VIB) crystallize with the same structure, it was only recently that interest was renewed in these materials with the discovery that BaFe$_{2}$As$_{2}$ can be made to superconductor at temperatures as high as 38 K. Although the precise mechanism of superconductivity has yet to be determined, there is a growing interest in the study of materials that have similar electronic properties to BaFe$_{2}$As$_{2}$. In our work, we employ density functional theory in the local density approximation in order to study the isochemically substituted variant, SrCo$_{2}$N$_{2}$. In this work, we determine both the stability of SrCo$_{2}$N$_{2}$ as well as how the substitution changes the electronic properties in comparison to BaFe$_{2}$As$_{2}$. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L23.00004: Density functional study on d-orbital characters of the Fe magnetic moment in BaFe$_{2}$As$_{2}$ Hyungju Oh, Hyoung Joon Choi There have been many published papers related on the orbital characters of band structures in the iron-based superconductors. However, the orbital characters of the Fe magnetic moment still remain unrevealed. By performing first-principles calculations of the electronic and magnetic properties with constraint on the real space shape of Fe magnetic moments, we study the d-orbital characters of the Fe magnetic moment in BaFe$_{2}$As$_{2}$. We compare obtained band structures with published angle-resolved photoemission spectroscopy (ARPES) result, and propose that the Fe magnetic moment in BaFe$_{2}$As$_{2}$ has in-plane d$_{xy}$ character. This work was supported by the NRF of Korea (Grant Nos. 2009-0081204 and 2011-0018306). Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2011-C3-05) [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L23.00005: Theoretical insight on layered Mn-compounds under pressure Maria Pezzoli, Zhiping Yin, Jack Simonson, Meigan Aronson, Gabriel Kotliar Since the unexpected discovery of Fe-based superconductors, the search for new materials with a higher superconducting $T_c$ has been reinvigorated. Mn based compounds in the ThCr$_2$Si$_2$ structure contain square lattice layers of the transition metal atom, like the cuprates and the iron arsenides, but the strong antiferromagnetic correlations typical of these compounds suggest that superconductivity in the manganites is impossible. However new experiments carried under pressure show that some of these layered Mn-compounds go through an electron delocalization transition . Here we study these compounds from a theoretical point of view. Looking at the magnetic properties and the Fermi surfaces close to the electron delocalization transition, we discuss the possibility of superconductivity for these compounds. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L23.00006: Renormalization group study up to two-loop order of an effective two-band model for iron-based superconductors Hermann Freire, Vanuildo de Carvalho We perform a renormalization group (RG) study up to two-loop order of an effective low-energy two-band model to describe some of the recently discovered iron-based superconductors. Our starting point is the itinerant electronic model proposed by Chubukov \emph{et al.} [Phys. Rev. B \textbf{78}, 134512 (2008)], which displays two small, almost nested Fermi pockets with one hole pocket centered at $(0,0)$ and one electron pocket centered at $\mathbf{Q} = (\pi,\pi)$ in the folded Brillouin zone. We then proceed to implement a two-loop RG calculation for this model of four-point vertex corrections, quasiparticle weight and several order-parameter susceptibilities in order to evaluate the robustness of one-loop RG results available in the literature with respect to including self-energy effects and higher-order quantum fluctuations. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L23.00007: Microscopic derivation of GL theory for magnetism and superconductivity in multiband electronic systems Vladimir Cvetkovic, Oskar Vafek The superconductivity in pnictides arises due to interband scattering between Cooper pairs belonging to hole and particle Fermi surfaces. The amplitude of the scattering, while weak at bare level, is enhanced under RG flow, and competes with the SDW to become the leading instability when the hole and particle FS's are nearly, but not perfectly, nested. This motivates us to construct a GL theory with multiple order parameters from the microscopic action. It allows us to study the two competing orders and their interplay including a possible coexistence. The presence of both attractive and repulsive pairing terms requires a novel approach to the derivation with imaginary terms in the action as one consequence. The construction of the GL theory is a two-step process, with RG flow to an intermediate cut-off scale, determined by the deviations from the nesting, as the first step. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L23.00008: Impurity Dimer Scattering as Reflection of Band Reconstruction in Iron Pnictides Jian Kang, Zlatko Tesanovic While the impurity induced nanoscale electronic disorder has been extensively reported in the underdoped iron pnictides its microscopic origin remains a theoretical challenge. Recent STM measurements reveal a resonance in the impurity dimer scattering in cobalt-doped iron arsenides. These resonant dimers are randomly distributed but aligned with antiferromagnetic a-axis. We present a theoretical study of the impurity induced quasiparticle interference patterns in these materials, based on the five orbital model. The local density of states oscillates with the ``imperfect nesting'' wavevector encoded in the reconstructed Fermi pockets, provided one assumes the ordered pocket density wave (PDW) state along the b-axis. This anisotropic oscillation pattern breaks C$_{4}$ symmetry and is in agreement with the electronic dimmer resonance found in the STM experiments, hinting at the existence of~``hidden'' PDW order in iron-based superconductors [1]. [1] J. Kang and Z. Tesanovic, Phys. Rev. B 83, 020505(R) (2011). [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L23.00009: Jump in specific heat at the superconducting transition in iron pnictides Maxim G. Vavilov, Andrey V. Chubukov, Anton B. Vorontsov Experiments reveal that in iron-based superconductors the jump $\Delta C$ of the specific heat at the superconducting transition is not proportional to the transition temperature $T_c$, as expected in the BCS theory. Rather, the ratio $\Delta C/T_c$ varies with $T_c$, and has a peak near optimal doping and decreases at smaller and larger dopings. We show that this behavior can be naturally explained by the interplay between superconductivity and antiferromagnetism in the underdoped regime. We demonstrate that $\Delta C/T_c$ is indeed peaked at the doping where the coexistence phase with antiferromagnetism develops, and decreases at deviations from this doping in both directions. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L23.00010: Beyond anharmonicity: signature of spin-phonon coupling at the surface of BaFe$_{2}$As$_{2}$ Chen Chen, Jing Teng, Yimin Xiong, Jiandi Zhang, Rongying Jin, E.W. Plummer High Resolution Electron Energy Loss Spectroscopy (HREELS) has been used to investigate the temperature dependence of the lattice dynamics of cleaved single crystals of BaFe$_{2}$As$_{2}$, one of the parent compounds of Fe-based superconductors. Both the phonon frequency as well as phonon linewidth of the intense 32 meV out-of-plane Fe/As mode (A$_{2u})$ and the 24 meV out-of-plane As vibration mode (A$_{1g})$ show a dramatic temperature dependence and anomalous behavior below $\sim $150K. The anomalous behavior is associated with the coupled elasto-magnetic transition in the bulk but occurs appreciably higher at the surface than in the bulk ($\sim $138 K). The anharmonicity at the surface is considerably larger than that in the bulk for the orthorhombic phase, but is significantly less in the tetragonal phase. A detailed discussion is given in terms of the interplay between the spin and lattice in this novel system. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L23.00011: Low magnetic field microwave absorption and ESR studies of coexisting superconductivity and ferromagnetism in Eu-based pnictides Austin Howard, Tian Shang, Jiaowen He, Guanghan Cao, Huiqui Yuan, Myron Salamon, Anvar Zakhidov We have studied the coexistence of FM ordering and SC in the doped pnictide EuFe$_2$(As$_{1-x}$P$_x$)$_2$ system, with doping levels of $x=0.3$ and 0.27, by combined ESR and non-resonant low field microwave absorption (LFMA) methods, and magnetization in SQUID. LFMA uses an external magnetic field to create vortices in the material, which are non-resonantly excited by MW radiation. The resulting spectrum's hysteresis shape and form changes with $T$, revealing transitions around 9 and 20 K, and showing the FM onset and SC $T_c$. Two types of coexisting LFMA signals have been observed: a) a superconducting hysteretic LFMA loop, appearing below $T_c$ and changing its shape, phase and intensity with $T$, and b) the non-hysteretic, reversed phase LFMA, above SC $T_c$, which is assigned to the magnetically ordered ferromagnetic phase, since it does not depend on $T$. Additionally, in the $x=0.27$ sample, a reemergence of hysteresis is seen near 18K, corresponding with the reentrant superconductivity observed in transport measurements. LFMA has also been used to determine the position of the irreversibility line and to estimate the critical current density for the two samples. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L23.00012: ab Plane Point Contact Spectroscopy of Co-doped Iron Pnictide Superconductors John Timmerwilke, J.S. Kim, G.R. Stewart, Amlan Biswas Point contact measurements on iron-based superconductors provide valuable information about these new superconductors, including gap sizes, number of gaps, and gap symmetries. We use a point contact apparatus capable of taking measurements in the ab{\-}plane using a tungsten wire lowered mechanically onto a superconducting sample. Using this method allows us to measure the variation of superconducting gap(s) with Z, a dimensionless barrier strength parameter. Previous point contact measurements have shown Co-doped iron pnictide superconductors to be a two gapped material. Our measurements confirm the presence of two gaps in single crystal Ba(Fe$_{0.926}$Co$_{0.074)2}$As$_{2}$ and the Z variation of the measurements imply the gaps are isotropic. Acknowledgments: NSF DMR-0804452, DOE contract {\#} DE-FG02-86ER45268 [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L23.00013: Highly anisotropic Dirac fermions in a Bi square net of SrMnBi$_2$ Jun Sung Kim, Joonbum Park, Geunsik Lee, Frederik Wolff-Fabris, Yoon Young Koh, Man Jin Eom, Yeongkwan Kim, M.A. Farhan, Younjung Jo, Changyoung Kim, Ji Hoon Shim We report the highly anisotropic Dirac fermions in a Bi square net of SrMnBi$_2$, based on a first principle calculation, angle resolved photoemission spectroscopy, and quantum oscillations for high-quality single crystals. We found that the Dirac dispersion is generally induced in the (SrBi)$^{+}$ layer containing a double-sized Bi square net. In contrast to the commonly observed isotropic Dirac cone, the Dirac cone in SrMnBi$_2$ is highly anisotropic with a large momentum-dependent disparity of Fermi velocities of $\sim 8$. These findings demonstrate that a Bi square net, a common building block of various layered pnictides, provide a new platform that hosts highly anisotropic Dirac fermions. [Preview Abstract] |
Session L24: Focus Session: Dopants and Defects in Semiconductors - Chalcogenides, Oxides, and Ternaries
Sponsoring Units: DMPChair: Hartwin Peelaers, University of California, Santa Barbara
Room: 256
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L24.00001: AX centers in II-VI semiconductors: Hybrid functional calculations Koushik Biswas, Mao-Hua Du Group-V acceptors should be efficient hole producers in II-VI compounds as in ZnTe. However, good $p$-type conductivity remains elusive, for example in ZnO and ZnS. With regard to this low doping efficiency, we will discuss the dopant self-compensation in II-VI semiconductors through the formation of the AX center. These are acceptor-induced defect that acts as a donor to compensate the acceptor itself. We show that the artificially high valence band maximum in Local density approximation and Generalized gradient approximation calculations can lead to incorrect prediction on the stability of the AX center in these semiconductors. The hybrid functional calculations that correct the band gap, significantly stabilize the AX centers for selected group-V acceptor dopants in ZnO, ZnS, and ZnSe. The results on AX centers obtained by hybrid functional calculations agree well with the experimentally observed doping phenomena in ZnS and ZnSe.[1] [1] Koushik Biswas and Mao-Hua Du, Applied Physics Letters 98, 181913 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L24.00002: Magnetic Main Group Impurities in CdS Pedro Bedolla-Velazquez, Christoph Gruber, Peter Mohn With the development of magnetic semiconductors, the role of the magnetism of impurities came again into the focus of research. For \textit{d}- and \textit{f}-Âelectron impurities, the situation seems to be rather clear. A new field appears when one starts to study magnetism produced by vacancies or by atoms, which usually do not carry any magnetic moments in a bulk solid. Starting from the magnetism of carbon vacancies in graphene we will present a study of CdS where S is replaced by main group elements. On the basis of abÂinitio supercell calculations employing density functional theory (DFT) we investigate the behaviour of impurities (B, N, C, O, F, Al, Si, P, Ga, Ge) in wurtzite (w) and zincblende (zb) CdS lattices. It is found that the impurities prefer the sulfur position and most of them, depending on the concentration exhibit magnetic order. We find that for small concentrations (64zb/72w and 32zb atom supercells) a half metallic ferromagnetic behaviour is found. For a 16 atom supercell for both zb and wÂstructure partly also unsaturated magnetic moments occur. A field dependence of the magnetic moments in these materials may lead to new technological applications in these magnetic semiconductors as tunable spin injection materials. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L24.00003: Mechanisms and Kinetics of Tellurium Precipitation in CdTe-based Materials Vincenzo Lordi CdTe and related alloys are important materials for solar photovoltaic application as well as for high-resolution room-temperature gamma radiation detectors. However, the performance of devices, particularly in high-energy applications, is limited by various material defects. Among the most important defects are Te precipitates of various sizes caused by non-stoichiometric growth conditions. In this work, we study the kinetics of Te aggregation and precipitation at the atomic scale. Density functional theory is used to compute the energetics, migration rates, and binding energies of point defects involved in Te aggregation, which include various interstitials, vacancies, and anti-site defects. Kinetic Monte Carlo is then used to simulate the aggregation process leading to precipitation nuclei. The mechanisms and kinetics of formation of these Te-rich regions are analyzed for various conditions. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L24.00004: Luminescence-Based Characterization of Copper Vacancies in Optical Float Zone Refined Cuprous Oxide N. Laszlo Frazer, Kelvin Chang, Kenneth Poeppelmeier, John Ketterson Cuprous oxide (Cu$_2$O) is ideal for studying Wannier-Mott excitons which have anomalously long lifetimes in this material. However copper vacancies have a deleterious effect on the exciton lifetimes. We have measured the associated luminescence spectra in optically excited single crystals. These crystals were prepared in a radiantly-heated float-zone refiner from thermally oxidized copper rod of purity 0.999. The behavior of the vacancy luminescence can be related to exciton propagation. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L24.00005: Ab initio calculations of intrinsic defects in ZnSb Lasse Bjerg, Georg K.H. Madsen, Jeffrey C. Grossman, Bo B. Iversen Thermoelectric materials are capable of interconverting heat and electricity. The most efficient thermoelectric materials are heavily doped semiconductors, and hence they can be either n- or p-type. ZnSb has recently been predicted by theoretical calculations to be a good n-type thermoelectric material. However, synthesis produces p-type materials. Intrinsic point defects have been investigated as a possible origin using ab initio calculations. Negatively charged Zn vacancies are found to have a low formation energy, and an intrinsic p-type behavior is predicted. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L24.00006: First-principles study of defect properties of zinc blende MgTe Ji-Hui Yang, Shiyou Chen, Hongjun Xiang, Xingao Gong, Su-Huai Wei We studied the general chemical trends of defect formation in MgTe using first-principles band structure methods. The formation energies and transition energy levels of intrinsic defects and extrinsic impurities and some defect complexes in zinc blende MgTe were calculated systematically using a new hybrid scheme. The limiting factors for $p$- and $n$-type doping in MgTe were investigated. Possible solutions to overcome the doping limitation of MgTe are proposed. The best $p$-type dopant is suggested to be N with nonequilibrium growth process and the best $n$-type dopant is suggested to be I with its doping complex V$_{Mg}$ + 4I$_{Te}$. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L24.00007: Charged excitons and biexcitons bound to isoelectronic centers Gabriel Ethier-Majcher, Stephane Marcet, Clauderic Ouellet-Plamondon, Philippe St-Jean, Regis Andre, John F. Klem, Sebastien Francoeur We demonstrate that isoelectronic centers formed from two isoelectronic traps can bind, in addition to the well-studied excitons, various number of positive and negative charges. Two different systems are studied by microphotoluminescence: 1) tellurium dyads in ZnSe forming hole traps and 2) nitrogen dyads in GaAs forming electron traps. By analyzing their emission fine structure, polarization and diamagnetic shifts, we establish that Te and N dyads can bind, respectively, positively and negatively charged excitons. Using the power dependence of the emission intensity, we clearly demonstrate that both systems can also bind biexcitons. This ability to bind various charge configurations, in addition to their very low inhomogeneous broadenings and perfectly defined symmetries, further establishes isoelectronic centers as an interesting alternative to epitaxial quantum dots for a number of applications. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L24.00008: New perspective on formation energies and energy levels of point defects in non-metals Hong Zhu, Patrick Rinke, Matthias Scheffler, Rampi Ramprasad We propose a powerful scheme to accurately determine the formation energy and thermodynamic charge transition levels of point defects in non-metals. Previously unknown correlations between defect properties and the valence-band width of the defect-free host material are identified allowing for a determination of the former via an accurate knowledge of the latter. These correlations are identified through a series of hybrid density functional theory computations and an unbiased exploration of the parameter space that defines the Hyde-Scuseria-Ernzerhof family of hybrid-functionals. The applicability of this paradigm is demonstrated for point defects in several insulators, including Si, Ge, ZrO$_{2}$ and ZnO [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L24.00009: Hydrogen-related defects in SnO$_{2}$ W. Beall Fowler, Figen Bekisli, Michael Stavola Symmetry arguments along with a mass-and spring analysis of infrared absorption experiments made with polarized light on OH defects in SnO$_{2}$ yield significant insights into the possible structures of one- and two-OH defects that have been observed recently [1]. Namely, a two-OH defect must involve symmetry-equivalent OH sites, and the axes of both one-and two-OH defects are only slightly displaced from the perpendicular to the c-axis of the rutile structure. Such results cannot be explained by models involving one or two H trapped at a Sn vacancy. Rather, they are consistent with OH defects associated with either a metal atom substituting for Sn, or an interstitial metal atom (such as Sn). Results of detailed quantum-mechanical calculations [2] using CRYSTAL06 are consistent with OH structures involving a Sn interstitial. \\[4pt] [1] Figen Bekisli \textit{et al.,} Phys. Rev. B \textbf{84}, 035213 (2011). \\[0pt] [2] R. Dovesi \textit{et al.}, \textbf{\textit{Crystal06 User's Manual}}, Univ. of Torino, Torino, 2006. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L24.00010: Polarization effect in the Ionic conductor TlBr Cedric Rocha Leao, Vincenzo Lordi TlBr is an ionic crystal that in recent years has been standing out as one of the most promising materials for effective room temperature radiation detection. However, its exceptional performance invariably degrades after operation times that vary from hours to several weeks. This phenomenon, known as polarization, is assigned to the undesirable ionic current that sets in the crystal under an applied bias, leading to the accumulation of oppositely charged Tl+ and Br- ions at the electric contacts of the device. This charge build up induces a field that opposes the applied bias, impairing the collection of the photo-induced carriers. In this presentation, we use parameter free quantum mechanical simulations to discuss the possible origins of the polarization effect in TlBr, showing that ionic mobility in the intrinsic material is not enough to account for effects reported by several groups. We then discuss other possible causes for the degradation of biased TlBr and propose ways to prevent its occurrence, via careful co-doping as well as a judicious choice of the metal contacts to be employed. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L24.00011: Structural diversity and electronic properties of Cu2SnX3 (X=S, Se): A first-principles investigation Yingteng Zhai, Shiyou Chen, Jihui Yang, Hongjun Xiang, Xingao Gong, Aron Walsh, Joongoo Kang, Suhuai Wei The ternary semiconductors Cu$_2$SnX$_3$ (X=S, Se) are found frequently as secondary phases in synthesized Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$ samples, but previous reports on their crystal structures and electronic band gaps are conflicting. Here we report their properties as calculated using a first-principles approach. We find that: (i) the diverse range of crystal structures can all be derived from the zinc-blende structure. (ii) The energy stability of different structures is determined primarily by the local cation coordination around anions, which makes Cu and Sn partially disordered in the cation sublattice. (iii) The direct band gaps of the low energy compounds Cu$_2$SnS$_3$ and Cu$_2$SnSe$_3$ should be in the range of 0.8-0.9 eV and 0.4 eV respectively. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L24.00012: Fe$_{2}$MCh$_{4}$ (M=Si,Ge, Ch=S,Se): optical, electrical properties and defects Robert Kokenyesi, Liping Yu, Stephan Lany, Alex Zunger, Douglas Keszler Fe$_{2}$MCh$_{4}$ (M=Si,Ge, Ch=S,Se) are proposed as solar absorber materials, as a single phase, stable alternative to FeS$_{2}$ pyrite. Native defects have high formation energy in the band gap and no Fermi level pinning is predicted by DFT. External defect calculations, including B,Al,Ga,In, N, P, As, Sb, Bi, O, Se, predict substantial effect on the bulk electronic properties. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L24.00013: Structure, stability, and defect analysis of potential solar absorber Cu$_3$PSe$_4$ David H. Foster, Jason M. Vielma, Guenter Schneider The semiconductor Cu$_3$PSe$_4$ has recently been established to have a direct band gap of 1.4 eV and exhibit p-type conductivity [Applied Physics Letters, 99, 181903 (2011)]. Here we present density functional theory (DFT) and post-DFT results regarding the structure, stability, and dopability of Cu$_3$PSe$_4$. We a find a strong coupling between the electronic band gap and the atomic structure, clearly caused by the strong P-Se antibonding character of the conduction band. Using the Heyd-Scuseria-Enzerhof hybrid functional and $GW$ approximation methods, we find that structural relaxation using standard DFT is not sufficiently accurate to be used as input to static, post-DFT electronic structure calculations. We use the generalized gradient approximation (GGA) and the GGA+$U$ method to show a thermodynamically stable low temperature region. We calculate the formation enthalpies of intrinsic and extrinsic defects in order to understand the observed p-type behavior and to examine n-type doping mechanisms. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L24.00014: Effect of Cation Disorder on Electronic Structures and Optical Properties of Magnetoelectric Gallium Ferrite: A First-principles Study Amritendu Roy, Sushil Auluck, Rajendra Prasad, Ashish Garg Discovery of photovoltaic effects in oxide materials, especially in magnetoelectrics and multiferroics has lead to renewed interests in the optical properties of these materials. Magnetoelectrics with their transition temperature close to room temperature are of particular interest from the perspective of practical applications. Magnetoelectric gallium ferrite (GFO), a material of current interest, exhibits good optical activity and a transition temperature tunable to room temperature and above upon tailoring of cation stoichiometry. Here, we show a detailed first principle study on the electronic structure and optical properties of GFO. We have performed first-principles density functional calculations using GGA+U method to compute the electronic band structure and density of states of the ground state structure of GFO having orthorhombic \textit{Pc2}$_{1}n$ symmetry and A-type antiferromagnetic spin configuration. The calculations show that GFO possesses a direct band gap of $\sim $ 2.3 eV, making it a rather low band gap oxide. Subsequent calculations of real and imaginary parts of dielectric constants, refractive index (n), extinction coefficient (k), reflectivity (R), etc demonstrate good agreement with experimental spectra which is further improved upon introduction of cationic site disorder into the ground state structure substantiating the presence of cation site disorder in the material. We also show that major optical transitions in GFO involve transitions from valence band O 2p to conduction band Fe 3d states. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L24.00015: Radiative defects in thallium chacolgenide semiconductors J.A. Peters, N.K. Cho, Zhifu Liu, B.W. Wessels, S. Johnsen, S. Nguyen, M. Sebastian, M.G. Kanatzidis Thallium chalcohalides constitute a promising new class of semiconductor compounds for radiation detectors. Due to their wide energy bandgap, high atomic number, and high resistivity, they are being considered as potential replacement for conventional II-VI semiconductor x-ray and $\gamma $-ray detectors for room temperature operation. For these applications resistivities of $\sim $10$^{10}$ ohm-cm are required. Although defects play a major role in detector response, little is known about their nature and origin in these compounds. We have investigated Tl$_{6}$I$_{4}$Se and Tl$_{6}$I$_{4}$S compounds which have bandgaps of 1.86 and 2.03 eV, respectively. Photoluminescence (PL) spectra of single crystals were investigated in the 650-885 nm wavelength region and over a temperature range of 20-100 K. For Tl$_{6}$I$_{4}$Se we observed PL bands centered at 1.61 eV. A detailed study of the peak, as function of temperature and excitation intensity, indicates that it is due to radiative transitions from donor-acceptor pairs (DAP). The ionization energies of the donor and acceptor levels in Tl$_{6}$I$_{4}$Se were estimated at 52 and 290 meV, respectively. Similarly DAP emission in Tl$_{6}$I$_{4}$S with a peak at 1.66 eV was observed. The role of crystalline stoichiometry in DAP formation is currently under study. [Preview Abstract] |
Session L25: Focus Session: Simulation of Matter at Extreme Conditions - Warm Dense Matter
Sponsoring Units: DCOMP GSCCM DMPChair: Timothy Germann, Los Alamos National Laboratory
Room: 257A
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L25.00001: Simulation of the Correlated Electron Plasma in the Warm Dense Matter Regime by Restricted Path-Integral Molecular Dynamics Vivek Kapila, Pierre Deymier, Keith Runge Warm dense matter (WDM) can be characterized by electron temperatures of a few eV and densities an order of magnitude or more beyond ambient. This regime currently lacks any adequate highly developed class of simulation methods. Recent developments in orbital-free Density Functional Theory (ofDFT) aim to provide such a simulation method, however, little benchmark information is available on temperature and pressure dependence of simple but realistic models in WDM regime. The present work aims to fill this critical gap using the restricted path-integral molecular dynamics (rPIMD) method. Within the discrete path integral representation, electrons are described as harmonic necklaces, while, quantum exchange takes the form of cross linking between electron necklaces. The fermion sign problem is addressed by restricting the density matrix to positive values and a molecular dynamics algorithm is employed to sample phase space. Here, we focus on the behavior of strongly correlated electron plasmas under WDM conditions. We compute the kinetic and potential energies and compare them to those obtained with the ofDFT method. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L25.00002: Comparison of Finite Temperature Hartree-Fock and Density Functional Theory for Confined Systems Travis Sjostrom, S.B. Trickey, Frank E. Harris Warm dense matter (WDM) at elevated temperatures ({\it e.g.}, $T \approx 1$ to several eV) and densities ({\it e.g.} one or more orders of magnitude denser than equilibrium) is of growing importance. So far, the most detailed studies of WDM use Born-Oppenheimer molecular dynamics with ground-state density functional theory (DFT) approximations. Little, however, is known about the behavior of the free energy over the temperature and density ranges of interest. In the case of DFT, this deficiency is a barrier to assessing the validity of proposed approximate free-energy functionals. For insight into this problem, we have undertaken systematic numerical study of the thermal Hartree-Fock (THF) approximation. We report progress on application of THF to the problem of eight one-electron atoms at arbitrary positions in a hard-walled box. We discuss the physics which emerges for both high- and low-symmetry ionic arrays, including molecular binding transitions. In addition, we compare the THF results directly with approximate DFT results, including approximate finite-temperature orbital-free kinetic and exchange functionals. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L25.00003: The Korringa-Kohn-Rostoker Method Applied to Warm Dense Matter Daniel Finkenstadt, Charles E. Newnam, Brian G. Wilson The electronic structure, EOS and transport properties of warm electrons in an amorphous or disordered configuration of ions is not well described by either solid-state or plasma models. Such warm, dense systems share the characteristic of the solid state that multi-center scattering effects are of paramount importance in forming bands of valence states. Theoretical treatment of the EOS of warm, dense matter therefore requires a way to include significant occupation of higher energy and angular momentum channel continuum states. We are extending the Green's function Kohn-Korringa-Rostoker code \emph{MECCA} as an all-electron (non-pseudo potential) method that treats arbitrary mixtures of atoms on an ab-initio basis over a broad range of conditions, from cold, solid matter up to hot plasmas at extreme (ICF) compression. Specific examples of Aluminum and Boron-Nitride will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L25.00004: Density Functional versus Thermal Hartree-Fock Approximations in Warm Dense Lithium Valentin V. Karasiev, Travis Sjostrom, S.B. Trickey We compare the behaviors of the thermal Hartree-Fock (tHF) model and thermal Density Functional Theory (tDFT) using both ground-state and temperature-dependent approximate functionals. The test system is bcc Li in the temperature-density regime of warm dense matter. In the exchange-only case, we find significant qualitative differences between the exact tHF and the DFT calculations with zero-temperature local density approximation (LDA) exchange. A temperature-dependent LDA functional provides much better agreement with the tHF exchange. An underlying need is for well-characterized, reliable pseudopotentials over demanding temperature and density ranges. Thus we compare pseudopotential and all-electron results for small Li clusters of local bcc symmetry and bond-lengths appropriate to high density bulk Li. We determine the density range over which both standard projector-augmented wave(PAW) and norm-conserving pseudopotentials are reliable. Then we construct small-cutoff-radius PAW data sets (for both the local density and the generalized gradient exchange-correlation approximations) which are valid for lithium densities up to at least 80 g/cm$^3$. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L25.00005: A Different Time-Dependent Variational Principle Approach: Going Beyond Wave Packet Molecular Dynamics Paul Grabowski, Andreas Markmann, Mike Surh, Michael Murillo, Frank Graziani During inertial confinement fusion, matter evolves from a solid condensed matter phase through the warm dense matter (WDM) regime to a hot dense matter. In WDM, quantum mechanical effects are important because of both Fermi-Dirac statistics and the rate of electrons transitioning in and out of bound states is large. The time-dependent temperature and quickly changing local environment require a time-dependent quantum method. A converged dynamical quantum simulation is intractable for more than a few particles. Instead, we take as a feasible goal to match the statistical properties of a warm dense plasma. The time-dependent variational principle gives a framework for producing equations of motion. A commonly used variational form is a Hartree product of isotropic Gaussian wave packets (wave packet molecular dynamics). The resulting dynamics do not produce the right statistics. We therefore introduce a plane wave basis and discuss its advantages and test its ability to reproduce radial distribution functions produced by hyper-netted chain calculations. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L25.00006: All-Electron Path Integral Simulations of Warm, Dense Matter: Application to Water and Carbon Kevin Driver, Burkhard Militzer We develop an all-electron path integral Monte Carlo (PIMC) method for warm dense matter and apply it to study water and carbon. PIMC pressures, internal energies, and pair-correlation functions compare well with density functional theory molecular dynamics (DFT-MD) at lower temperatures and enable the construction of a coherent equation of state over a density-temperature range of 3--12 g/cm$^3$ and 10$^2$--10$^9$ K. PIMC results converge to the Debye-Huckel limiting law at high-temperatures and illuminate the breakdown of DFT pseudopotentials due to core excitations. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L25.00007: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L25.00008: Non-equilibrium Warm Dense Matter: Electron-Ion Dynamics of Pumped Nanofoils Yuan Ping, Tadashi Ogitsu, Alfredo Correa, Eric Schwegler, Gilbert Collins, Jun Zhou, Jianming Cao, Byoung-ick Cho, Kyle Engelhorn, Philip Heimann, Roger Falcone In 2006, it was reported that the dielectric function of laser-excited gold nanofoils exhibits a peculiar behavior; the interband transition peak of gold is enhanced and undergoes a clear red shift [PRL \textbf{96}, 255003 (2006)]. In 2009, based on ultrafast electron diffraction measurements on pumped gold nanofoils, it was reported that the time evolution of the Debye-Waller factor is too slow to be explained by a two-temperature model that included temperature dependent el-ph coupling. This anomaly has been attributed to a phonon hardening process caused by high electron temperatures (a few eV) [Science \textbf{323}, 1033 (2009)]. Later, it was pointed out that at such a high electron temperatures the dielectric function of gold calculated by first-principles DFT simulations does not reproduce the enhanced and red-shifted interband transition peak and an alternative explanation was proposed to reconcile the discrepancies where the effect of ejected electrons was addressed [HEDP \textbf{6}, 246 (2010)]. In this talk, we will discuss recent experimental/theoretical efforts to further examine the issues relevant to this problem, el-ph coupling, dynamics of ejected electrons, and ballistic transport of electrons [submitted to HEDP; PRL \textbf{106}, 167601(2011)]. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L25.00009: High field terahertz response of materials Dan Daranciang, John Goodfellow, Alan Fisher, Aaron Lindenberg We report on studies of the response of materials to intense ultrashort electromagnetic fields at terahertz frequencies. These are generated through coherent transition radiation using femtosecond electron bunches at the Linac Coherent Light Source and correspond to single-cycle pulses with electric field amplitudes > 20 MV/cm with a frequency centered at ~10 THz. Large amplitude nonlinear responses are observed in a range of semiconductor materials associated with field-induced ionization processes, and we show how these processes can be used to carry out nonlinear autocorrelation measurements of the pulse shape. We also discuss recent results probing the response of ferroelectric materials at high fields coupled with ultrafast x-ray probes enabling measurement of their atomic-scale response on sub-picosecond time-scales. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L25.00010: An Analytic Screening Potential for Dense, Strongly-Coupled Plasmas Liam Stanton, Michael Murillo, Frank Graziani Characterizing warm dense matter (WDM) has gained renewed interest due to advances in powerful lasers and next generation light sources. Because WDM is strongly coupled and moderately degenerate, we must often rely on simulations of WDM, which are necessarily based on molecular dynamics of ions interacting through a screened potential. Almost always, a Debye- (Yukawa-) like interaction is assumed; however, it is well known that such long wavelength models over-screen. Here, we present a new effective ion-ion interaction, which recovers the exact fermionic linear response in the long-wave limit while retaining a pair-potential functionally similar to that of the Yukawa form. This new potential not only improves the accuracy of screening effects without contributing to the computational complexity of the model, but it also adds physics entirely missing from Yukawa models (such as the onset of Friedel oscillations). Simulations of the ion structure factor are compared to XRTS data for Be and C in the WDM regime. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L25.00011: Large-Scale Reactive Simulations of Materials in Extreme Conditions Andres Jaramillo-Botero, William Goddard First-principles quantum mechanics methods are inadequate for accurately describing the effects of thermal, mechanical, chemical or radiation excitations that may occur in materials operating under extreme conditions, or impractical to use due to the prohibitive scaling cost of propagating the total Schrodinger equation for a large set of atoms. In the regime of a high number of electronic excitations, the electronic portion of the wave function contains contributions from many stationary states, and the Born-Oppenheimer approximation breaks down. We have been developing a mixed quantum-classical dynamics approach, called the Electron Force Field (eFF), to simulate the non-adiabatic dynamics of materials in extreme conditions. We have demonstrated its application to describe the: thermodynamics of dense hydrogen over 0-100,000 Kelvin; real-time dynamics of Auger fragmentation of diamond nano particles; transient electronic effects in high-strain rate silicon fracture; Coulomb explosion of carbon clusters; dynamics of cascaded valence ionizations in shocked hydrocarbons; and the dynamics of hypervelocity impact of materials. Here, we summarize our recent progress in the theory and application of eFF for modeling and simulation of materials in extreme conditions. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L25.00012: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L25.00013: First-principle Calculations of Equation of State for Metals at High Energy Density Dmitry Minakov, Pavel Levashov, Konstantin Khishchenko In this work, we present quantum molecular dynamics calculations of the shock Hugoniots of solid and porous samples as well as release isentropes and isentropic sound velocity behind the shock front for aluminum. Also we perform similar calculations for nickel and iron. We use the VASP code with ultrasoft and PAW pseudopotentials and GGA exchange-correlation functional. Up to 512 particles have been used in calculations. To calculate Hugoniots we solve the Hugoniot equation numerically. To obtain release isentropes, we use Zel'dovich's approach and integrate an ordinary differential equation for the temperature thus restoring all thermodynamic parameters. Isentropic sound velocity is calculated by differentiation of pressure along isentropes. The results of our calculations are in good agreement with experimental data at densities both higher and lower than the normal one. Thus, quantum molecular dynamics results can be effectively used for verification or calibration of semiempirical equations of state under conditions of lack of experimental information at high energy densities. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L25.00014: Color quantum simulations of strongly coupled quark-gluon plasma Vladimir Filinov, Vladimir Fortov, Mishael Bonitz, Yurii Ivanov, Pavel Levashov We propose stochastic simulation of thermodynamics and kinetic properties for quark-gluon plasma (QGP) in semi-classical approximation in the wide region of temperature, density and quasi-particles masses. In grand canonical ensemble for finite and zero baryon chemical potential we use the direct quantum path integral Monte Carlo method (PIMC) developed for finite temperature within Feynman formulation of quantum mechanics to do calculations of internal energy, pressure and pair correlation functions. The QGP quasi-particles representing dressed quarks, antiquarks and gluons interact via color quantum Kelbg pseudopotential rigorously derived for Coulomb particles. This method has been successfully applied to strongly coupled electrodynamic plasmas (EMP). A strongly correlated behavior of the QGP is expected to show up in long-ranged spatial correlations of quarks and gluons which, in fact, may give rise to liquid-like and, possibly, solid-like structures. We have done already the first calculation of the QGP equation of state, spatial and color pair distribution functions, diffusion coefficients and shear viscosity. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L25.00015: Proton crystallization and quantum melting of proton crystals in a dense hydrogen plasma Pavel Levashov, Vladimir Fortov, Vladimir Filinov, Holger Fehske, Michael Bonitz We present extensive new simulation results which allow to predict the temperature and density range for proton crystallization. We simulate a macroscopic spatially homogeneous fully ionized two-component electron-proton plasma in thermodynamic equilibrium from first principles using direct fermionic path integral Monte Carlo simulations. Our results for the phase diagram differ substantially from the previous predictions based on the one-component plasma (OCP) model: In the classical part of the phase diagram the crystal appears to be stabilized compared to the OCP predition. In contrast, in the quantum part of the phase diagram the crystal appears to be de-stabilized and vanishes at lower densities compared to the OCP prediction. Finally, the maximum temperature for the proton crystal is found to be around 40 000K, slightly below the previous prediction. Our results indicate that the OCP treatment of the liquid-solid transition in a two-component plasma has to be questioned. The OCP-assumption of a homogeneous rigid neutralizing background gives rise to substantial deviations of the critical parameters. [Preview Abstract] |
Session L26: Focus Session: Friction, Fracture and Deformation Across Length Scales - Dislocations and Fracture
Sponsoring Units: DCOMP GSNPChair: David Goldsby, Brown University
Room: 257B
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L26.00001: How dislocations and grain boundaries control wear at the nanoscale Invited Speaker: Izabela Szlufarska Ceramics show outstanding mechanical properties such as high strength and high hardness over a wide range of temperatures and are stable in harsh environments. However, the low fracture toughness of ceramics limits their practical utility for instance as wear-resistance coatings. There have been several reports of improving wear resistance of ceramics by reducing the grain sizes and/or the dimension of the cutting tools to the nanometer regime. Using SiC as a model covalent ceramic, we performed molecular dynamics (MD) simulations of wear for both single crystal and nanocrystalline material. We determined the role of dislocations and grain boundary sliding in improving wear resistance of SiC and we have quantified contributions from these mechanisms to friction and wear. We have discovered instabilities that control sliding of the amorphous-like highly disordered grain boundaries in SiC, in analogy to instabilities and deformation mechanisms that occur in bulk amorphous materials. In this talk we will also present our newly developed analytical model for plowing friction in nanoscale contacts, which model has been validated for both ceramics and metals. In order to isolate the contribution from grain boundary sliding to deformation of nanocrystalline materials, we have performed MD simulations of nanoindentation and uniaxial testing on ultrananocrystalline diamond (UNCD). We have shown that in the absence of dislocation plasticity, hardness and yield strength of nanocrystalline materials scale linearly with the grain boundary shear strength, where the latter property can be controlled by grain boundary doping. Our findings explain the experimental observations that hardness and elastic properties of UNCD decrease with an increasing H content. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L26.00002: Plastic flow and failure in metallic glass/nanocrystal composites Michael Falk, Pengfei Guan The exploitation of metallic glasses' high strength in structural applications is limited by their lack of any hardening subsequent to yield. This lack of hardening leads to plastic localization as evidenced by the spontaneous formation of shear bands. One proposed method of forestalling such instabilities is to introduce nanocrystal inclusions to disrupt shear band nucleation and propagation. We have undertaken a series of molecular dynamics simulations of glasses with different morphologies of nanocrystallites. We report the resulting plastic response, and we use various simulations of these types to test the applicability of the shear transformation zone (STZ) constitutive relation for modeling such complex nano-composite materials. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L26.00003: Atomic Origins of Friction Reduction in Metal Alloys Michael Chandross, Shengfeng Cheng Gold is a desirable material for use in high performance electrical contacts because it offers low contact resistance, does not corrode or oxidize, and can be easily made into thin sheets. However, gold contacts generally suffer from high adhesion and friction. The tribological issues are mitigated in nanocrystalline gold alloys (with, for example, Ni or Co), which can exhibit both low friction and low contact resistance. The atomic scale mechanisms responsible for the change in frictional response are poorly understood. We will present the results of large scale molecular dynamics simulations which study the tribological response of nanocrystalline films of pure gold and alloys under a variety of sliding conditions. Our results indicate that in pure metals, cold welding and microstructural reorientation lead to the formation of a commensurate sliding interface and high friction resulting from dislocation controlled plasticity. In alloys, however, differing lattice constants suppress the reorientation of grains at the contact point, which leads to grain boundary sliding and lower friction. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L26.00004: Deformation Initiation by Non-planar \{10-12\} Twinning Nucleation in Magnesium Crystal Sungho Kim, Haitham Kadiri, Mark Horstemeyer The nucleation mechanism of experimentally most commonly observed twinning in Mg crystal initiating deformation process are studied using molecular dynamic simulation. We observed nucleation of radially growing \{10-12\} twinning under tensile loading in Mg rectangular wire system without artificial creation of an twinning. The twinning nucleation mechanism is very different from the conventional twinning mechanism in that the twin nucleates from a point source rather than the fault plane following the partial dislocation line in FCC crystal. The wire axis is normal to basal plane of Mg crystal. The tensile deformation in c-axis nucleates \{10-12\} twinning starting at the corner of square of cross section of the wire. The twin boundary is spherical at the beginning and become linear boundaries in \{10-12\} planes as time goes by. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L26.00005: Softening of nanocrystalline materials at small grain size Georgios Kopidakis, Nikos Galanis, Ioannis Remediakis We examine the dependence of the mechanical properties of nanocrystalline materials on grain size. Our extensive atomistic simulations for several nanocrystalline solids show a universal softening at grain sizes of less than a few nanometers. The elastic constants decrease as the average grain size becomes smaller, in analogy with the reverse Hall-Petch effect for nanocrystalline metals. This behavior is explained by the increase of the fraction of grain boundary atoms as grain size decreases. We derive simple scaling laws for various mechanical properties as a function of the grain size by decomposing the energy into contributions from atoms in the bulk of grains and from atoms at the interfaces. Our theoretical predictions fit very well our results from atomistic simulations of different nanocrystalline materials, from nanocrystalline metals to ultrananocrystalline diamond. It is therefore argued, and quantitatively explained, that softening at small grain size is a general nanoscale effect. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L26.00006: Large scale NEMD simulations of polycrystalline Al sliding interfaces Jacqueline Milhans, J.E. Hammerberg, R. Ravelo, T.C. Germann, B.L. Holian We present the results of NonEquilibrium Molecular Dynamics (NEMD) simulations for the frictional force between polycrystalline Al samples. Polycrystalline Al samples of order 26M atoms with grain sizes from 10 - 20 nm at compressions of 15 GPa are condsidered as a function of sliding velocity . Typical sample dimensions are 58nm in the sliding and transverse directions and 116nm in the direction normal to the sliding interface. A constant temperature (300K) and constant tangential velocity boundary condition is imposed at the boundaries in the direction normal to the sliding plane. We discuss the modes of plastic deformation and polycrystalline deformation which determine the steady state frictional force and compare these results with results for defect free Al single crystals and highly defective Al single crystal samples. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L26.00007: Fracture In Disordered Media: Nucleated, Critical or Percolative? Ashivni Shekhawat, Stefano Zapperi, James Sethna Fracture is often considered to be an abrupt transition and is modeled by nucleation theory. However, the precursor events leading to macroscopic failure display scaling behavior and are understood in terms of critical phenomena. Further, the universal roughness properties of fracture surfaces have been explained by modeling fracture as a percolative process. We attempt to unify these disparate descriptions of fracture in one comprehensive theory. We study the random fuse network as a typical model of disordered brittle media. We show that in this model fracture can be nucleated, critical or percolative depending on the behavior of the tail of the distribution of fuse strengths. We explore the phase diagram by using numerical simulations as well as theoretical arguments. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L26.00008: Homogeneous Dislocation Nucleation Asad Hasan, Craig Maloney We perform atomistic computer simulations to study the mechanism of homogeneous dislocation nucleation (HDN) in a 2D hexagonal crystalline film under circular indentation. The nucleation process is governed by vanishing of energy associated with a single normal mode. For fixed film thickness, $L$, the spatial extent, $\xi$, of the critical mode grows with indenter radius, $R$. For fixed $R/L$, $\xi$ scales roughly as $\xi\sim L^{0.4}$. We perform a \emph{mesoscale} analysis to determine the lowest energy normal mode for regions of varying radius, $r_{\rm meso}$, centered on the critical mode's core. The energy of the lowest normal mode $\lambda_{\rm meso} \to 0$ rapidly as $r_{\rm meso}\to \xi$. The lowest mode shows a spatial extent, $\xi_{\rm meso}$, which increases sublinearly for $r_{\rm meso}\leq \xi$ and saturates at $r_{\rm meso} \approx 1.5\; \xi$. We demonstrate that the $\xi_{\rm meso}/ \xi$ versus $r_{\rm meso}/ \xi$ curve is \emph{universal} (independent of $L$ or $R$). Hence small regions, $r_{\rm meso}\leq \xi$, \emph{can} reveal the presence of incipient instability but give excellent estimates for the critical mode's energy and spatial extent \emph{only} for $r_{\rm meso} \geq 1.5\; \xi$. Thus HDN is a \emph{quasi-local} phenomenon. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L26.00009: Fracture Energy Issues of Brittle, Microcrack Brittle, and Dislocation Ductile Materials Ray B. Stout Somigliana elasticity models(1915) for dislocation-microcrack defect discontinuities in a material form an analog basis to relate dislocation density evolution to microcrack density evolution near an existing idealized crack-tip. Thus, a recent idealized field solution derived for stochastic dislocation density evolution near a crack-tip in a ductile material is also an analog applicable field solution for stochastic microcrack density evolution in a brittle material near a crack-tip. A non-equilibrium thermodynamic functional is derived and integrated to evaluate rates of dislocation and microcrack internal energy evolution due to the singularity terms of these crack-tip solutions in an arbitrary spatial crack-tip neighborhood and during an arbitrary fracture toughness load-up time interval of [0, t*]. At some time greater than t*, the available inter-atomic lattice(analog recoverable elastic) internal energy at a crack-tip becomes probabilistically sufficient, in an energy transfer-stability sense of Gibbs[1906], Griffith[1920], and Eshelby[Phil Trans Roy Soc, 1951], to be configurationally transported from locally recoverable internal energy at a crack-tip to non-recoverable crack-tip surface energy as a crack-tip propagates. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L26.00010: Role of interactions and damage in a cohesive fracture model Josesph Gran, John Rundle, Donald Turcotte, William Klein We study the influences of local and long range interactions in a numerical model of tensile fracture. Our model simulates fracture events on a 2D square lattice plane with a Metropolis algorithm. We chose a Hamiltonian that is written as a function of the crack separation (offset field) and includes contributions from an external field, interactions, as well as a cohesive energy across the crack surfaces. Included in our study is both a ferromagnetic-type (attractive) and antiferromagnetic-type (repulsive) interactions. We test both of these interactions individually as well as a hybrid interaction in which over a short range the interaction is antiferromagnetic and in the long range the interaction becomes ferromagnetic. This dual interaction approximates a Lennard-Jones potential. We also propose a characterization of damage and investigate the increase of damage in time for fractures occurring by a static-load as well as a time-dependent load. Damaged sites do not interact with neighboring sites and cannot hold any load. We compare our damage model to previous studies of fiber-bundle models. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L26.00011: Indentation of Graphene Membranes: Non-Linear Response, Nano-Fracture, and Crack Propagation Romain Perriot, You Lin, Vasily Zhakhovsky, Xiang Gu, Ivan Oleynik Recent indentation experiments on graphene have revealed its exceptional strength, making it an excellent candidate for the design of nano- and micro- electromechanical systems. Therefore, it is critical to understand the mechanical properties of graphene, and its response to a wide range of loading pressures beyond the elastic regime. In this work molecular dynamics (MD) simulations of indentation of circular graphene membranes were performed with a newly developed interatomic potential, specifically designed to study graphene under extreme tensile stress. The indentation curves confirmed the experimental observation of a non-linear response at large loads, as well as the brittle failure of the membranes via the generation of nano-cracks. Our MD simulations showed that the fracture process consists of two consecutive stages: an initial bond-breaking event followed by the formation and propagation of cracks. The kinetic theory of bond breaking was applied to determine the breaking strength of graphene and its dependence on the indenter radius, as well as the waiting time for failure. MD simulations were used to provide an atomic-scale description of fracture dynamics. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L26.00012: Investigation of Nonlinear Elastic Behavior of Two-Dimensional Molybdenum Disulfide Ryan Cooper, Changgu Lee, Christopher Marianetti, James Hone, Jeffrey Kysar The present study investigates the nonlinear elastic properties of a single-layer molybdenum disulfide crystal through experiment, finite element modeling, and density functional theory. Suspended single-layer molybdenum disulfide crystals are suspended over circular holes that were etched on a silicon oxide surface. Crystals are loaded at the center with an atomic force microscope until fracture occurs. The load-displacement curve is used to determine the pretension and linear-elastic response of the crystal. The force at which fracture occurs gives insight into the intrinsic strength and higher order elastic constants of the crystal. These experiments provide a platform to validate first-principles derivation of fifth-order elastic constants for in-plane stiffness using density functional theory. The derived higher order elastic constants are used in a finite element model to predict the breaking strength of two-dimensional molybdenum disulfide. The study bridges the gap between density functional theory and finite element analysis with experimental evidence. [Preview Abstract] |
Session L27: Invited Session: Quantum Design of Low-Dimensional Materials Structures for Enhanced Solar Energy Conversion
Sponsoring Units: DCMP FIAPChair: Zhenyu Zhang, University of Science and Technology of China
Room: 258AB
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L27.00001: Use of First-Principles Theory to Identify materials and nano structures for next-generation solar cells Invited Speaker: Alex Zunger Three genomic-like material design approaches are explored for finding energy relevant semiconductors:\textit{(i) Search of nanostructure combinations leading to intermediate band solar ce}lls: Within the ideology of intermediate band solar cells (IBSC) based on quantum dots, it is presently unknown which combination of dot material + matrix material + substrate satisfies the energetic criteria enabling IBSC. Using the modern theory of nano structures (based on atomistic pseudo potentials in plane waves), we examine various combinations, finding that some of the ``usual suspect'' long believed to be ideal, are, in fact inappropriate. (\textit{ii) Finding new inorganic absorbers with first-principles:} Standard compilations of inorganic compounds reveal thousands of candidate materials that were unexplored for their potential as PV absorbers, among others, because of the absence of a quantifiable ``Design Principle'' that sorts out various materials. The common Schockley - Queisser criteria gives a universal, gap vs efficiency curve which does not distinguish different types of gaps (direct-allowed vs direct-forbidden vs-indirect) nor does it account for non radiative recombination. A simple treatment, called ``Spectroscopically Limited Maximum efficiency'' (SLME) accounts for such factors and can be calculated for hundreds of compounds (using the GW approach), providing insight into previously unrecognized candidates, as well as to the mechanisms at work causing absorption enhancement.(iii) I\textit{nverse Band structure search for direct gap Si-Ge nano structures: }Using a genomic approach to pseudo potential configurational search we identify a Si-Ge nano structures that have direct band gaps, solving the long-standing dilemma of crystalline Column IV absorbers. In collaboration with L. Yu, V. Popescu, J.W. Luo and M. Davezac. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:42PM |
L27.00002: Photovoltaic effect for narrow-gap Mott insulators Invited Speaker: Efstratios Manousakis Solar cells, based on conventional band-semiconductors, have low efficiency for conversion of solar into electrical energy. The main reason is that the excess energy of the photon absorbed by an electron/hole pair beyond the band-gap becomes heat through electron-phonon scattering and phonon emission; through these processes electrons and holes relax to their band edges within a characteristic time scale of the order of $10^{-12}-10^{-13}$ secs. We will discuss that a narrow-gap Mott insulator can produce a significant photovoltaic effect and, more importantly, if appropriately chosen it can lead to solar cells of high efficiency. In this case, a single solar photon can produce multiple electron/hole (doublon/hole) pairs, an effect known as impact ionization, faster than other relaxation processes such as relaxation through phonons. It has been proposed previously that this process could lead to an efficient solar cell using band-gap semiconductors; however, the characteristic time-scale for impact ionization is comparable to that for electron-phonon relaxation in band-gap semiconductors. The reason that a Mott insulator can behave differently is that the large Coulomb repulsion present in a Mott insulator leads to a large enhancement of the impact ionization rate. Provided that this enhancement does occur in an appropriately chosen Mott insulator, it can be demonstrated that the efficiency can improve significantly over conventional band-insulators. At present, we are doing calculations on specific transition-metal-oxide based materials believed to be Mott-insulators using extensions of the density functional theory (hybrid functionals) in combination with many-body perturbation theory. Our goal is to determine a promising candidate with suitable band structure and transition matrix elements leading to fast transition rates for impact ionization to occur in a time-scale faster than other relaxation processes. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L27.00003: Quantum Dot Solar Cells Invited Speaker: Vladimir Bulovic |
Tuesday, February 28, 2012 4:18PM - 4:54PM |
L27.00004: Shaping anomalous molecular electroluminescence by resonant nanocavity plasmons Invited Speaker: Zhenchao Dong Exciton formation and decay of molecules near metallic nanostructures is important for the control of energy conversion at the nanoscale and plasmonic devices. While extensive research has been carried out to such end using photon-excited techniques, complementary insights into the optical transitions and plasmon-exciton coupling can be obtained through electron excitations by a scanning tunneling microscope [1]. In this talk, we shall describe such reversed process of light-to-electricity conversion through single-molecule electroluminescence that demonstrates the critical role of nanocavity plasmons in the generation and decay of molecular excitons. By tuning the resonant interplay between molecular excitons and plasmons in nearby metallic nanostructures, we demonstrate that plasmons can do much more beyond intensity enhancement [2]: the emission band of molecular fluorescence can be effectively tuned by resonant nanocavity plasmons. New optoelectronic effects including resonant hot-electroluminescence and upconversion electroluminescence from higher vibronic levels of singlet excited states have been realized for porphyrin molecules near metals, which breaks the Kasha's rule and conventional Franck-Condon distribution. The highly confined nanocavity plasmons can behave like a strong coherent optical source with tunable frequency, and can be used to actively control the radiative channels of molecular emitters near metals over a wide spectral range. We shall also discuss critical factors that are responsible for the realization of single-molecule electroluminescence [3]. \\[4pt] [1] Z.C. Dong, et al., Phys. Rev. Lett. 92, 086801 (2004). \\[0pt] [2] Z.C. Dong, et al., Nat. Photonics 4, 50 (2010). \\[0pt] [3] Y. Zhang, et al., Phys. Rev. Lett. (submitted). [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:30PM |
L27.00005: Quantum Photocell: Using Quantum Coherence to Reduce Radiative Recombination and Increase Efficiency Invited Speaker: Marlan Scully Laser and photocell quantum heat engines (QHEs) are powered by thermal light and governed by the laws of quantum thermodynamics. We here show how to use quantum coherence (PRL, 104, 207701 (2010)) induced by quantum noise (PNAS, 108, 15097 (2011)) to improve the efficiency of a laser or photocell QHE. Surprisingly, this coherence can be induced by the same noisy (thermal) emission and absorption processes that drive the QHE. Furthermore, this noise-induced coherence can be robust against environmental decoherence. Application of the ideas to photosynthesis (Nature, 446, 782-786 (2007)) will also be discussed. [Preview Abstract] |
Session L28: Electricity-to-Light Conversion: Solid State Lighting
Sponsoring Units: FIAP GERA DMPChair: Theodore Moustakas, Boston University
Room: 258C
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L28.00001: Auger recombination in gallium arsenide from first principles Daniel Steiauf, Emmanouil Kioupakis, Chris G. Van de Walle GaAs and its alloys are technologically important materials for solid-state optoelectronic devices such as LEDs and lasers. The internal quantum efficiency of these devices, defined as the fraction of electron-hole pairs converted to photons, is limited by loss mechanisms. Of particular importance at high carrier densities is Auger recombination, a non-radiative process where the energy and momentum of the recombining electron-hole pair is transferred to a third carrier. Here we use density functional theory to study Auger recombination in GaAs from first principles. When considering Auger recombination due to Coulomb interaction only, the calculated rate is too small and cannot account for the experimental observations. However, once additional electron-phonon interactions are included, the theoretical recombination rate increases towards the experimental value. Our work provides insight into the microscopic origins of the loss in III-V LEDs at high injected-carrier densities, and in the mechanisms governing Auger recombination rates in general. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L28.00002: Surface Energies and Cracking in GaN Cyrus E. Dreyer, Christian Carbogno, Anderson Janotti, Chris G. Van de Walle Cracking is one of the biggest limitations to growing thick GaN single crystals and films, caused by the buildup and release of strain energy. Cracking occurs along preferential crystallographic planes in the GaN wurtzite structure, and depends on the tensile stress on the given plane and the energetic balance between the strain energy released from the crack formation versus the cleaved surface area created. It is also well known that the equilibrium shape of a crystal is largely determined by its surface free energy. Therefore, to correctly predict the stresses under which certain planes will crack and aid in understanding crystal growth, knowledge of the absolute surface energy is required. We use first-principles calculations based on density functional theory and a hybrid functional to determine the surface energy for the nonpolar \{11-20\} {\it a}- and \{10-10\} {\it m}-planes, as well to as explore approximations to the surface energies of the polar \{0001\}/\{000-1\} {\it c}-planes in GaN. The effects of structure relaxations and reconstructions are fully taken into account, and the results are discussed in the light of available experimental observations. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L28.00003: Crystalline Fe films grown on non-polar GaN: theory and experiment Steven Erwin, Cunxu Gao, Claudia Roder, Jonas Laehnemann, Oliver Brandt We report an unexpected mechanism by which single crystals of Fe grow epitaxially on $M$-plane GaN substrates despite having a different crystal structure and strongly mismatched lattice constant. A simple model is proposed in which one material tilts out of the interface plane to create a coincidence-site lattice that balances two competing geometrical criteria---low residual strain and short coincidence-lattice period. We apply this model, along with complementary first-principles total energy calculations, to the interface formed by molecular-beam epitaxy of cubic Fe on hexagonal GaN and find excellent agreement between theory and experiment. The success of this model also suggests a strategy for growing non-polar GaN films on a substrate material with a suitably chosen Miller index. One very promising material is Si, which is already in widespread use as a flat substrate for GaN/Si epitaxy despite high dislocation densities. The next talk will present our predictions of the most promising Si(\textit{hhk}) substrates for growing non-polar GaN with high crystalline quality. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L28.00004: Crystalline non-polar GaN films grown on vicinal Si: theoretical predictions Alex Kutana, Steven Erwin The ability to grow highly crystalline non-polar GaN films on silicon substrates would be an important advance in optoelectronic device fabrication. We propose a simple strategy to achieve this goal: tilting the Si substrate out of interface plane to provide a good lattice match between the film and substrate. We consider epitaxial interfaces between $M$-plane GaN and arbitrary Si(\textit{hhk}) substrates. Using the model introduced in the previous talk, we downselect these substrates using two geometric criteria---low residual strain and short coincidence-lattice period. We then use density-functional theory to compute the interface formation energies of these selected candidates, which include Si(001), (112), (113), (114), and (223). We find that $M$-plane GaN films have the lowest interface formation energy when grown on the (113) and (112) surfaces. The formation energies are significantly lower than on Si(001) or (111), the substrates most often used for growing GaN. On this basis we predict that Si(113) and (112) substrates will enable growth of non-polar GaN films of higher crystalline quality than can be attained on Si(001) or (111). [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L28.00005: Hybrid functional studies of stability and diffusion of hydrogen in Mg-doped GaN Ji-Sang Park, K.J. Chang Nitride semiconductors are known to suffer from low p-type doping efficiency due to the high activation energy of Mg acceptors and the compensation of hole carriers. To enhance hole carrier concentration, the hydrogen co-doping method is widely used, in which hydrogen is intentionally doped with Mg dopants and removed by subsequent thermal annealing. In this work, we perform first-principles density functional calculations to study the stability and diffusion of hydrogen in Mg-doped GaN. For the exchange-correlation potential, we employ both the generalized gradient approximation (GGA) proposed by Perdew, Burke, and Ernzerhof and the hybrid density functional of Heyd, Scuseria, and Ernzerhof. We examine the diffusion pathways and dissociation barriers of H from the Mg-H complex using the nudged elastic band and dimer methods. We compare the results of the GGA and hybrid density functional calculations for the stability of various H interstitial configurations and the migration barriers for H diffusion. Finally, using the calculated migration barriers as inputs, we perform kinetic Monte Carlo simulations for the dissociation of the Mg-H complex and find that the Mg acceptors are activated by thermal annealing up to 700-800 $^{\circ}$C, in good agreement with experiments. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L28.00006: Electroluminescent Schottky Diodes Fabricated Using Plasma Ion Implantation Sarah Purdy, Marcel Risch, Phillip Desautels, Michael Bradley Carbon-implanted silicon light-emitting Schottky diodes were produced by Plasma Ion Implantation (PII) in an RF ICP plasma chamber using methane feedstock gas. The electroluminescence spectrum of the devices was fitted with a set of Gaussian peaks corresponding to known emission centers including disordered silicon (broad white background), buried porous silicon and hydrogenated carbon-rich silicon. Some of the emission peaks exhibit a peak intensity at a drive current density of several A/cm$^2$, followed by a drop in emission intensity at higher drive current densities. In this presentation we discuss a possible model for this observed drop in electroluminescent intensity. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L28.00007: Sub-250nm room-temperature optical gain from AlGaN/AlN multiple quantum dot structures Emanuele Francesco Pecora, Wei Zhang, Lin Zhou, David J. Smith, Jian Yin, Roberto Paiella, Luca Dal Negro, Theodore D. Moustakas There are many pressing but yet unrealized applications for optoelectronic materials and devices that can function well into the deep-UV. Group-III nitrides, in particular AlGaN, are particularly suited to cover UV spectral ranges. An intense research effort is targeting the investigation and demonstration of deep-UV lasing from these materials. We developed AlGaN/AlN MQWs by Molecular Beam Epitaxy under a novel growth mode that introduces band structure potential fluctuations and high-density of nanocluster-like features within the AlGaN wells. A characterization of this material will be presented. The Variable-Stripe Length technique, a well-established methodology for measuring optical gain coefficient, is applied for a detailed quantification of the gain properties and polarization. We demonstrate optical gain in AlGaN nanostructures down to 230 nm at room temperature with a maximum net modal gain value of 118 $\pm$ 9 cm-1 at the highest excitation fluence of 15 $\mu$J/cm2. The optical gain threshold was measured to be 5 $\pm$ 1 $\mu$J/cm2 from which we estimate the density of optically excited carriers at the threshold to be 1.4 x 10$^{17}$ cm-3, which is two orders of magnitude lower than what currently achieved by quantum well structures. Moreover, we demonstrate that gain is TE-polarized. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L28.00008: Room temperature low threshold stimulated emission of electron beam-pumped AlGaN-based deep UV laser structures emitting below 250 nm A. Nikiforov, W. Zhang, J. Woodward, J. Yin, E. Pecora, L. Zhou, L. Dal Negro, R. Paiella, D. Smith, T. Moustakas, A. Moldawer The development of semiconductor lasers, operating in the deep UV, will find a number of applications such as identification of biological and chemical agents, non-line-off -sight free space communications and point of site medical diagnostics. In this paper we report the growth of QW laser structures in the configuration 6H-SiC / AlN / AlGaN - AlN MQWs /AlN by PAMBE. A novel growth mode was developed in which arriving active nitrogen species and aluminum atoms dissolve in the excess liquid Ga covering the surface of the growing film and incorporate into the AlGaN film from the liquid phase. This liquid phase epitaxy (LPE) growth was found to introduce band structure potential fluctuations and high-density of nanocluster-like features within the AlGaN wells. The structure and microstructure of these devices were investigated by AFM, XRD and TEM and their emission properties were investigated by electron beam pumping at room temperature. The investigated laser structures were found to emit in the 235-250 nm range and stimulated emission was observed at a threshold power of 20-40 KW / cm$^{2}$. This low threshold value is attributed to nanoclusters-like features in the wells. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L28.00009: Electronic transport through InGaN heterojunctions Mathieu C\'esar, Donping Liu, Hong Guo InGaN nanowires have recently sparked great interest for their high tunability and potential in applications like solid-state lighting (LEDs) and concentrated photovoltaics. Determination of device characteristics from first principles modeling is of great importance. In order to treat quantum transport properties of nanoelectronic devices with atomistic disorder, a non-equilibrium vertex correction (NVC) theory was recently developed and implemented into the Keldysh non-equilibrium Green's function (NEGF) -based density functional theory (DFT). NEGF-DFT-NVC enables the representation of disordered structures such as the InGaN heterojunction under non-equilibrium conditions. Electronic and transport properties of a InGaN heterojunction are investigated using this accurate ab initio method. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L28.00010: Effects of Internal Fields on the Optical Emission in Nanostructured III-N LEDs Krishna Yalavarthi, Sasi Sundaresan, Ky Merrill, Shaikh Ahmed Nanostructured optical emitters can accommodate a broader range of lattice mismatch, be used in full-solar-spectrum light emitting diodes, and provide higher temperature stability of the threshold current and the luminescence. However, strong quantum confinement and certain symmetry-lowering mechanisms (caused by various internal fields) lead to pronounced optical polarization anisotropy and strong suppression of interband transitions in these structures. The objective of this work is to study the competing effects of various internal fields on the electronic structure and optical properties of nanostructured III-N LEDs. A multiscale approach has been employed where: 1) the NEMO 3-D tool is used to calculate the atomistic strain distribution and one-particle electronic states within a sp3s*d5 tight-binding framework, and 2) the outputs from NEMO 3-D are then coupled to the Synopsys TCAD tool to determine the terminal electrical and optical properties of the device. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L28.00011: Accurate first-principles calculation of the rare earth crystal field Fei Zhou, Vidvuds Ozolins Rare earth (RE) doped wide band-gap semiconductors play an important role in solid state lighting. Many aspects of the performance of these materials are characterized and determined by the $f$-electron crystal field (CF). However, CF effects are usually rather small for $f$ electron: the CF splitting is at the order of 0.1 eV, compared to several eV for d-electrons. Therefore accurate theoretical description of RE crystal field is challenging. We present a first-principles method of CF calculation based on an improved LDA+U method. By careful cancellation of errors, the method can reach relatively high accuracy for the CF parameters. As a demonstration we calculate the experimentally well-characterized RE:LaF$_3$ system, which has low point-group symmetry and a large number of CF parameters, representing a stringent test of theory. The predicted CF excitation energies of Ce:LaF3 agree within about 10 meV with experiment, and within several meV if the errors in the free-ion parameters are excluded. Work is underway to apply the method to other materials for solid-state lighting and laser applications. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L28.00012: The role of extended defects in Nitride Semiconductors on the performance of optoelectronic devices T. Moustakas The question is why nitride semiconductors, which are plagued by high concentration of extended defects, produce efficient minority carrier optoelectronic devices. The equilibrium phase of these materials is the wurtzite and the metastable phase is the zinc blende. However, since the enthalpy of formation of these two structures differ by only a few meV, the conversion from one to the other can occur easily by the creation of stacking faults on the close packed planes. Thus, stacking faults are one of the most abundant defects in these materials even when they are grown homoepitaxially. Since the basal plane stacking fault is the equivalent of a monolayer of a cubic domain and since the energy band gaps of the wurtzite and their cubic counterparts differ by about 0. 2 eV, one expects that stacking faults introduce band structure potential fluctuations. Such fluctuations are beneficial to the performance of lasers and LEDs since they lead to exciton localization and efficient radiative recombination. Other types of abundant defects in heteroepitaxially grown materials are threading dislocations. The insensitivity of the performance of LEDs and lasers to such defects is due to the strong ionicity of these materials as well as to the deep band structure potential fluctuations in the InGaN and AlGaN alloys. Due to the strong ionicity the surface states at free surfaces and dangling bonds in edge dislocations have moved towards the band edges and act as traps rather non-radiative recombination centers. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L28.00013: Surface scattering from ceramic phosphors Alan Lenef, John Kelso, Christopher Peters Scattering from phosphor converters and epitaxial surfaces is critical for solid state lighting device performance. Volume and surface scattering in solid state lighting devices can play a critical role in efficiency/efficacy, color points, and color angular consistency. Surface scattering in particular has not been well characterized in solid state lighting devices and can be complex to model. Because large angle scattering is important in lighting applications, surface scattering models generally require vector electromagnetic theory to avoid ambiguities often associated with scalar theory at these angles. Furthermore, surface features are often on the order of a few wavelengths, bringing ray tracing approaches into question. In this work, experimental angular scattering measurements are made on ceramic phosphor components where surface scattering dominates. The surface ceramic grain structure is responsible for the scattering. The results are compared to approximate statistical vector theory predictions that use the height autocorrelation functions as input. The autocorrelation measurements were derived from atomic-force microscopy topography measurements. Resulting predictions are in fairly good agreement with measurements. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L28.00014: Light emission from electrically stressed ZnO nanorods Luca Lucera, Lhacene Adnane, Kadir Cil, Venkata Manthina, Alexander Agrios, Helena Silva, Ali Gokirmak Zinc oxide (ZnO) nanorods were grown on various substrates by a chemical growth process based on a ZnO seed solution, and starting from Zinc acetate (ZnAc) material. The nanorods were grown on insulating silicon (low doped) and oxidized silicon substrates, and also over patterned conducting (highly-doped) nanocrystalline silicon microwires. When high voltage is applied directly to the ZnO film using tungsten needles ($\sim $ 50-60 V across $\sim $ 5-10 $\mu $m), high intensity blue and white light emission is observed, both in air and under high vacuum (10$^{-4}$ - 10$^{-5}$ Torr). Blue light appears as broad bright flashes covering a large area whereas white light is more localized and appears to come from individual nanostructures. The results suggest a combination of electroluminescence and photoluminescence processes that take place after an electrical breakdown (possibly across individual ZnO nanorods) that is observed as an exponential increase in current. Percolative conduction and light paths are also observed during the measurements. Measurements of the ZnO films of rods on conducting silicon substrate give more repeatable results, likely due to the higher probability of conducting paths between the two probes. The electrical stress results in significant self-heating and modification of the ZnO nanostructures and the contacts.\\[4pt] [1] Greene L. E. et al. Solution-Grown Zinc oxide nanowires. Innorganic Chemistry. Vol 45. 7535-7543. (2006) [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L28.00015: A First-Principles Analysis of the Crystal Structure, Band Gap Energy, Polarization, and Piezoelectric Properties of ZnO-BeO Solid Solutions Liang Dong, S. Pamir Alpay The electrical properties, the spontaneous polarization, and the piezoelectric response of ZnO can be tailored by alloying ZnO with BeO for various optoelectronics applications. We present here the results of a study that employs density functional theory to analyze the crystal structure, the band structure, elastic constants, spontaneous polarization, and piezoelectric properties of Zn$_{1-x}$Be$_{x}$O solid solutions. Our findings indicate that Zn$_{1-x}$Be$_{x}$O alloys may have a different crystal structure than the end components ZnO and BeO that crystallize in the prototypical wurtzite structure (P6$_{3}$mc). It is shown that orthorhombic lattices with Pmn2$_{1}$, Pna2$_{1}$, or P2$_{1}$ structures may have lower formation energies than the wurtzite lattice at a given Be composition. The band gap energies of Zn$_{1-x}$Be$_{x}$O in the wurtzite and the orthorhombic structures are nearly identical and the bowing of the band gap energy increases with increasing Be concentration. The spontaneous polarization of Zn$_{1-x}$Be$_{x}$O in the orthorhombic lattice is markedly larger compared to the wurtzite structure while the piezoelectric polarization in the wurtzite and orthorhombic structures varies linearly with the Be concentration. [Preview Abstract] |
Session L29: Focus Session: Semiconductor Qubits - Single and Multi-Qubit Demonstrations
Sponsoring Units: GQIChair: Amir Yacoby, Harvard University
Room: 259A
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L29.00001: Coherent control and detection of spin qubits in semiconductor with magnetic field engineering Invited Speaker: Yasuhiro Tokura Electrical control and detection of the spin qubits in semiconductor quantum dots (QDs) are among the major rapidly progressing fields for possible implementation of scalable quantum information processing. Coherent control of one-[1-3] and two-[4,5] spin qubits by electrical means had been demonstrated with various approaches. We have used an engineered magnetic field structure realized with proximal micro-magnets to transduce the spin and charge degrees of freedom and to selectively address one of the two spins [3]. We have demonstrated an all-electrical two-qubit gate consisting of single-spin rotations and interdot spin exchange in double QDs. A partially entangled output state is obtained by the application of the two-qubit gate to an initial, uncorrelated state. Our calculations taking into account of the nuclear spin fluctuation show the degree of entanglement. Non-uniform magnetic field also enables spin selective photon-assisted tunneling in double QDs, which then constitutes non-demolition spin read-out system in combination with a near-by charge detector [6]. \\[4pt] In collaboration with R. Brunner, Inst. of Phys., Montanuniversitaet Leoben, 8700, Austria, M. Pioro-Ladri\`{e}re, D\'{e}p. de Phys., Universit\'{e} de Sherbrooke, Sherbrooke, Qu\'{e}bec, J1K-2R1, Canada, T. Kubo, Y. -S. Shin, T. Obata, and S. Tarucha, ICORP-JST and Dep. of Appl. Phys., Univ. of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.\\[4pt] [1] F. H. Koppens, et al., Nature 442, 766 (2006).\\[0pt] [2] K. C. Nowack, et al., Science 318, 1430 (2007).\\[0pt] [3] M. Pioro-Ladri\`{e}re, et al., Nature Physics 4, 776 (2008).\\[0pt] [4] J. R. Petta, et al., Science 309, 2180 (2005).\\[0pt] [5] R. Brunner, et al., Phys. Rev. Lett. 107, 146801 (2011).\\[0pt] [6] Y. -S. Shin, et al., Phys. Rev. Lett. 104, 046802 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L29.00002: Coherent Singlet-Triplet Oscillations in a Silicon-based Double Quantum Dot Brett Maune, Matthew Borselli, Biqin Huang, Thaddeus Ladd, Peter Deelman, Kevin Holabird, Andrey Kiselev, Ivan Alvarado-Rodriguez, Richard Ross, Adele Schmitz, Marko Sokolich, Christopher Watson, Mark Gyure, Andrew Hunter We have performed coherent spin manipulation of a singlet-triplet qubit in a Si/SiGe double quantum-dot device fabricated in an undoped heterostructure. A charge stability diagram showed that the (0,0) charge state was reached and Pauli spin blockade was detected at the (1,1)-(0,2) anticrossing. A singlet-triplet splitting of $\sim $140 $\mu $eV in the (0,2) charge state provided a read-out window sufficiently wide for singlet-triplet discrimination. We used the S/T$_{-}$ spin funnel, Rabi oscillation, and $T_{2}$* pulsing experiments to measure (1,1) exchange energies spanning $\sim $0.6-700 neV over a large detuning range and measured a $T_{2}$* of 360 ns, consistent with theoretical expectations for our device. Sponsored by the United States Department of Defense. Approved for Public Release, Distribution Unlimited. The views expressed are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L29.00003: Tunable singlet-triplet splitting in a few-electron Si/SiGe quantum dot Zhan Shi, Christie Simmons, Jonathan Prance, John Gamble, Mark Friesen, Donald Savage, Max Lagally, Susan Coppersmith, Mark Eriksson The singlet-triplet energy splitting in a double quantum dot is an important parameter for singlet-triplet qubits, because it determines the energy gap for both initialization and readout. This splitting can also be used to perform gate operations in a newly proposed hybrid qubit [1]. We describe measurements in which we tune the singlet-triplet energy splitting by changing gate voltages on a Si/SiGe double quantum dot [2]. We argue that the energy is changed largely by lateral translation of the dot, which changes the local atomic structure that the electrons experience in the quantum dot, leading to variations in the valley-orbit coupling. We present calculations indicating the experimental results are consistent with the first excited state of the dot having non-zero valley-orbit coupling. [1] Z. Shi, et al., e-print: http://lanl.arxiv.org/abs/1110.6622. [2] Z. Shi, et al., e-print: http://lanl.arxiv.org/abs/1109.0511. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L29.00004: Electric dipole spin resonance measurement of spin-orbit anisotropy in InSb nanowire quantum dots Sergey Frolov, Stevan Nadj-Perge, Vlad Pribiag, Johan van den Berg, Sebastien Plissard, Erik Bakkers, Leo Kouwenhoven Indium antimonide nanowires are considered a leading platform for the generation of Majorana fermion bound states, and as hosts of spin-orbit quantum bits based on single electrons. Both efforts are motivated by the strong spin-orbit interaction in the bulk InSb. Here we present measurements of the strength and orientation of the effective spin-orbit magnetic field in InSb nanowire double quantum dots. Spin-orbit interaction induces avoided level crossings between triplet (1,1) and singlet (0,2) double dot states. These avoided crossings are observed in the spectrum of the electric dipole spin resonance performed on strongly-coupled double dots. We find that the spin-orbit field is oriented perpendicular to the nanowire axis, and parallel to the substrate plane. This orientation is consistent with Rashba spin-orbit interaction and is favorable for Majorana experiments. The strength of spin-orbit interaction is characterized by the spin-orbit length which we estimate to be 200 nm. This translates into a temperature scale of 3K for the observation of Majorana states. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L29.00005: Coherent manipulation of spin-orbit quantum bits in InSb nanowires Stevan Nadj-Perge, Johan van den Berg, Sergey Frolov, Vlad Pribiag, Sebastien Plissard, Erik Bakkers, Leo Kouwenhoven Semiconductor nanowires with strong spin-orbit coupling are becoming an attractive platform for spin-based quantum computation. Here we demonstrate coherent transitions between spin-orbit doublet states of individual electrons in indium antimonide (InSb) nanowire quantum dots induced by gigahertz-frequency electric fields. The spin-orbit doublet states form a qubit which initialization and detection relies on Pauli blockade in the double quantum dot (1,1) configuration. The maximum Rabi frequency exceeds 100 MHz and more than 10 periods of coherent oscillations are observed. We estimate fidelities of single qubit rotations and analyze qubit decoherence times by performing spin echo sequence. The two qubits in a double quantum dot are individually addressable due to different g-factors. The coupling between the qubits can be mediated by exchange interaction. The fidelities of single qubit rotations are sufficiently high to permit the implementation of two-qubit quantum gates such as controlled-not (C-NOT) or controlled-phase (C-Phase). [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L29.00006: Spin Relaxation and Manipulation in Spin-orbit Qubits Massoud Borhani, Xuedong Hu We derive a generalized form of the Electric Dipole Spin Resonance (EDSR) Hamiltonian in the presence of the spin-orbit interaction for single spins in an elliptic quantum dot (QD) subject to an arbitrary (in both direction and magnitude) applied magnetic field. We predict a nonlinear behavior of the Rabi frequency as a function of the magnetic field for sufficiently large Zeeman energies, and present a microscopic expression for the anisotropic electron g-tensor. Similarly, an EDSR Hamiltonian is devised for two spins confined in a double quantum dot (DQD). Finally, we calculate two-electron-spin relaxation rates due to phonon emission, for both in-plane and perpendicular magnetic fields. Our results have immediate applications to current EDSR experiments on nanowire QDs, g-factor optimization of confined carriers, and spin decay measurements in DQD spin-orbit qubits. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L29.00007: Spin coherence in Multi-electron GaAs double dots Andrew P. Higginbotham, Ferdinand Kuemmeth, Christian Barthel, Charles M. Marcus, Micah P. Hanson, Arthur C. Gossard Experimental investigation of spin manipulation and readout in multi-electron double quantum dots is reported. For occupations of order 10 electrons, spin blockade in both transport and pulsed-gate measurements is identified. Exchange rotations, dynamical decoupling, S/T+ beamsplitter experiments, and single-shot readout can all be implemented similar to the single-electron case. These demonstrations are relevant for the practical operation and scaling of GaAs spin qubits. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L29.00008: Fast Exchange Oscillations in Quantum Dot Spin Qubits Shannon Harvey, Michael Shulman, Oliver Dial, Hendrik Bluhm, Vladimir Umansky, Amir Yacoby The exchange splitting, J, is an important tool in semiconductor spin qubits, as it can drive both single qubit rotations and two qubit entangling operations. However, qubits operating under exchange can be dephased by electrical noise, as J is a function of the local electrostatic environment. We investigate the exchange interaction in a singlet-triplet qubit created in double quantum dot by measuring exchange oscillations ranging in frequency over three orders of magnitude. In particular, we resolve exchange oscillations at frequencies up to 30GHz, where both electrons occupy the same quantum dot. Here, J saturates to approximately the singlet-triplet splitting, and no longer depends on the electrostatic environment. In this regime exchange oscillations are insensitive to electrical noise. Since semiconductor spin qubits are usually limited by dephasing, these potentially dephasing-free exchange oscillations offer a new regime for studying quantum control in spin qubits. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L29.00009: Coherent electrical manipulation of a quantum dot qubit Matthew Pooley, Raj Patel, Antoine Boyer de la Giroday, Ian Farrer, Christine Nicoll, David Ritchie, Anthony Bennett, Mark Stevenson, Martin Ward, Niklas Skold, Andrew Shields We demonstrate the initialization and coherent manipulation of a spin-based qubit in a InAs quantum dot (QD), implementing a spin-flip gate with a fidelity of 97\%. We will discuss new measurements on the effect of fluctuating nuclear magnetic field which have implications on state initialisation, operation fidelity, and qubit storage time. An exciton spin-state is initialised by absorption of an incident polarised photon. The exciton state of QDs has two spin-eigenstates separated by fine-structure splitting ($s$), which can be manipulated using applied electric field. Superposition spin-states precess around the Bloch-sphere with an angular frequency $|s|/\hbar$. An electric field is used to vary the rate and axis of the spin-state precession, and thus control the time-evolution of the stored qubit. Nuclear magnetic field fluctuations induce additional random variations in $s$, the effects of which we investigate to understand the ideal initialisation and storage regime. Subsequent radiative decay maps the spin of the exciton onto the polarisation of the emitted photon. These results demonstrate a photon-spin interface, which has potential applications in scalable optical quantum computing schemes. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L29.00010: Fabrication and Characterization of Si MOS-Based Triple Quantum Dot Devices Hong Pan, Hongwen Jiang, Rusko Ruskov, Charles Tahan We have fabricated electrostatically defined, few electron triple quantum dot (TQD) devices in a silicon metal-oxide-semiconductor structure. The devices show good electrical stability and gate controllability. We have obtained charge stability diagrams for identifying the charging states of the TQD. In this talk, we will describe the technical challenges in the TQD fabrication and present the experimental results of tuning three dots into a resonance. We will also discuss the prospect of these devices to encode a spin qubit that uses exchange interaction alon and possible ways to perform coherent manipulations within the tunable range of inter-dot tunneling of these devices. The work is supported by ARO. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L29.00011: Pairwise control of exchange interaction between individual spins in a triple quantum dot G. Granger, Sergei Studenikin, G. Aers, A. Kam, P. Zawadzki, L. Gaudreau, R. Wasilewski, M. Pioro-Ladriere, A. Sachrajda The original spin qubit proposal [1] suggested a linear array of spins for quantum computations and the exchange interaction for 2 qubit operations. An essential component of the proposal was the ability to control pairwise the exchange interaction between neighbouring pairs of spins. In this work we experimentally demonstrate such a pairwise control of the exchange interaction between three spins localized in a triple quantum dot (TQD) device. The TQD potential was formed using electrostatic lateral split-gate technology on a GaAs/GaAlAs heterostructure with a high-mobility two-dimensional electron gas [2]. We employ fast pulsing technique based on the Landau-Zener-Stuckelberg (LZS) approach for creating and manipulating coherent superpositions of three spin quantum states [3]. We show that we are able to maintain coherence when increasing the exchange coupling of one spin with another while simultaneously decreasing its coupling with the third.\\[4pt] [1] D. Loss, and D.P. DiVincenzo, Phys. Rev. A57, 120-126 (1998).\\[0pt] [2] L. Gaudreau , et al., Appl. Phys. Lett. v.95, 193101 (2009). \\[0pt] [3] J.R. Petta, H. Lu, and A.C. Gossard, Science v.327, 669-672 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L29.00012: Entanglement of Two Singlet-Triplet Qubits Michael Shulman, Oliver Dial, Shannon Harvey, Hendrik Bluhm, Vladimir Umansky, Amir Yacoby Semiconductor spin qubits are promising candidates for quantum computation because of their potential for scalability. However, their weak interaction with the environment, which leads to long coherence times, makes two-qubit operations challenging. We perform the first two-qubit operation between singlet-triplet qubits. The two qubit operation relies on the capacitive coupling between two adjacent qubits to generate a CPHASE gate. In order to combat low frequency noise we use a dynamically decoupled gate that maintains the two-qubit coupling while decoupling each qubit from its fluctuating environment. Using state tomography we measure the two-qubit density matrix and show that the operation produces the expected state. We extract a concurrence of 0.44 and a Bell state fidelity of 0.72, each providing definitive proof of entanglement. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L29.00013: Two-axis Control and Readout of an Exchange-Only Spin Qubit in a GaAs Triple Quantum Dot J. Medford, J.M. Taylor, B.D. Armstrong, D.P. DiVincenzo, C.M. Marcus, H. Lu, A.C. Gossard The initialization, full control, and readout of a GaAs triple quantum dot exchange qubit is demonstrated. Appropriate depletion gate design has enabled control and single-shot readout along multiple Bloch sphere axes in a three electron device. Rotations around both non-orthogonal control axes are projected along three separate Bloch sphere axes, and quality factors $(Q \equiv \omega / T_2^*)$ of 500 are observed. Finally, we analyze decoherence and dynamical decoupling schemes unique to this system. [Preview Abstract] |
Session L30: Focus Session: Topologically Protected Qubits I
Sponsoring Units: GQIChair: Sumanta Tewari, Clemson University
Room: 259B
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L30.00001: Non-Abelian Braiding of Lattice Bosons Eliot Kapit, Paul Ginsparg, Erich Mueller We report on a numerical experiment in which we use time-dependent potentials to braid non-abelian quasiparticles. We consider lattice bosons in a uniform magnetic field within the fractional quantum Hall regime, where $\nu$, the ratio of particles to flux quanta, is near 1/2, 1 or 3/2. We introduce time-dependent potentials which move quasiparticle excitations around one another, explicitly simulating a braiding operation which could implement part of a gate in a quantum computation. We find that different braids do not commute for $\nu$ near $1$ and $3/2$, with Berry matrices respectively consistent with Ising and Fibonacci anyons. Near $\nu=1/2$, the braids commute. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L30.00002: Parafermion braid statistics in quasi-one-dimensional networks David Clarke, Jason Alicea, Kirill Shtengel One dimensional systems with Majorana zero modes at phase boundaries may be thought of as physical realizations of a discrete quantum wire model first put forth by Kitaev [1]. Proposed methods for braiding such Majorana fermions in one-dimensional wire networks [2] have greatly expanded the set of plausible avenues toward topological quantum computation. Recently, a generalization of the Kitaev model to parafermion modes has been developed.[3] Here, we describe the transport of such parafermion modes along the chain by the adiabatic transformation of the Hamiltonian, analogous to the transport of Majorana fermion modes. We determine the (braid) transformations of the ground state sector allowed by the adiabatic exchange of the parafermion modes in wire networks. We show that, as with Majorana fermions, none of the parafermion braid sets are universal for quantum computation. Certain parafermion chain models, unlike Majorana fermion systems, become universal with the addition of measurement operations. We discuss possible physical realizations of the parafermion models. \\[4pt] [1] J Alicea et al., Nature Physics 7, 412-417 (2011) \\[0pt] [2] A. Kitaev, arXiv:cond-mat/0010440v2 \\[0pt] [3] P. Fendley, unpublished [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L30.00003: Ettingshausen effect due to Majorana modes Chang-Yu Hou, Kirill Shtengel, Gil Refael, Paul Goldbart Due to the presence of Majorana fermions (zero mode) at the vortex core of topological superconductor, each vortex carries an extra entropy $s_0= k_B ln[2]/2$ that is independent of temperature. Utilizing this special property of Majorana fermions, one can show that the edge states appearing at the edge of a topological superconductor can be cooled (heated) due to the motion of the vortices. We will also discuss possible experimental setups to observe this cooling thermoelectric mechanism tied with the extra entropy carried by the vortex. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L30.00004: Quantum information transfer between topological and spin qubit systems Martin Leijnse, Karsten Flensberg In this talk I will introduce a method to coherently transfer quantum information, and to create entanglement, between topological qubits and conventional spin qubits. The transfer method uses gated control to transfer an electron (spin qubit) between a quantum dot and edge Majorana modes in adjacent topological superconductors. Because of the spin polarization of the Majorana modes, the electron transfer translates spin superposition states into superposition states of the Majorana system, and vice versa. Furthermore, I will discuss how a topological superconductor can be used to facilitate long-distance quantum information transfer and entanglement between spatially separated spin qubits. \\ References: \\ M. Leijnse, K. Flensberg, PRB 84, 140501(R) (2011) \\ M. Leijnse, K. Flensberg, PRL, in print, arXiv:1107.5703 \\ [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L30.00005: Braiding anyons and communication between topological and non-topological systems Haitan Xu, Jacob Taylor Quasi-particles with non-Abelian statistics are intriguing in both fundamental and applied physics. Here we propose a ``proof of principle'' experimental setup for braiding anyons and observing non-Abelian statistics using nearest-neighbor spin interactions inspired by the Kitaev honeycomb model. We also show an explicit method for teleportation between the topological and non-topological systems. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L30.00006: Electrical manipulation of Majorana Fermions in an interdigitated superconductor-ferromagnet device Shu-Ping Lee, Jason Alicea, Gil Refael We show that a topological phase supporting Majorana fermions can form in a 2DEG adjacent to an interdigitated superconductor-ferromagnet structure. An advantage of this setup is that the 2DEG can inherit the required Zeeman splitting and superconductivity from a single interface, allowing one to utilize a wide class of 2DEG's including the surface states of bulk InAs. We demonstrate that the interdigitated device supports a robust topological phase when the finger spacing $\lambda$ is smaller than the Fermi wavelength $\lambda_F$. In this regime the electrons effectively see a ``smeared" Zeeman splitting and pairing field despite the interdigitation. The topological phase survives even in the opposite limit $\lambda<\lambda_F$, though with a reduced bulk gap. We also describe how to electrically generate a vortex in this setup to trap a Majorana mode, which can be detected through edge tunneling spectroscopy. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L30.00007: Interacting topological phases in multiband nanowires Roman Lutchyn, Matthew P.A. Fisher We show that semiconductor nanowires coupled to an s-wave superconductor provide a playground to study effects of interactions between different topological superconducting phases supporting Majorana zero-energy modes. We consider quasi-one dimensional system where the topological phases emerge from different transverse subbands in the nanowire. In a certain parameter space, we show that there is a multi-critical point in the phase diagram where the low-energy theory is equivalent to the one describing two coupled Majorana chains. We study effect of interactions as well as symmetry-breaking perturbations on the topological phase diagram in the vicinity of this multi-critical point. Our results shed light on the stability of the topological phase around the multi-critical point and have important implications for the experiments on Majorana nanowires. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L30.00008: Magnetic Control of Majorana Edge Modes in Topological Insulator-Ferromagnet-Superconductor Heterostructures Xiaoting Zhou, Chen Fang, Wei-Feng Tsai, Jiangping Hu A surface of a strong 3D topological insulator (TI) doped with ferromagnetic atoms can be spin-polarized and similar to a 2D quantum anomalous Hall state. If an s-wave superconductivity can be induced by proximity effect on such a surface, a 2D topological superconducting phase is obtained. If we consider a TI-ferromagnet(FM)-superconductor(SC) heterostructure, a 2D time-reversal symmetry breaking topological superconducting (TSC) phases with Majorana edge mode(s) will be realized. We demonstrate that the existence of the edge modes critically depend on the combination of the directions and magnitudes of spin polarization on all surfaces, and that a model describing the states on only one surface is insufficient. We find that the number, the positions and the chirality of these edge modes corresponding to various TSC phases can be engineered by controlling the ferromagnetism on different surfaces. Our results are obtained by self-consistently solving for the edge modes in a 3D lattice model for topological insulator in contact with an s-wave BCS superconductor. We also offer an analysis to illustrate the underlying physics, using an effective 2D theory for the surface states. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L30.00009: Self-consistent Study of Majorana Fermions on a Topological Insulator Surface $\pi$ Junction Mahmoud Lababidi, Erhai Zhao It has been proposed that a Josephson $\pi$ junction that resides on the 2D surface of a 3D topological insulator (TI) is host to the Majorana fermion. We present a microscopic study of the $\pi$ junction TI surface through self-consistent calculations with the Bogoliubov-de Gennes equation. We calculate the order parameter, the singlet correlation function along with the energy spectrum, local density of states, and the spectral function. We also show the evolution of the energy dispersion of Majorana fermion as a function of the TI chemical potential. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L30.00010: Unpaired Majorana fermions in a layered topological superconductor Babak Seradjeh, Eytan Grosfeld We study the conditions for the existence of unpaired Majorana modes at the ends of vortex lines or the side edges of a layered topological superconductor. We show that the problem is mapped to that of a general Majorana chain and extend Kitaev's condition for the existence of its nontrivial phase by providing an additional condition when a supercurrent flows in the chain. Unpaired Majorana bound states may exist in a vortex line that threads the layers if the spin-orbit coupling has certain in-layer components but, interestingly, only if a nonzero supercurrent is maintained along the vortex. We discuss the exchange statistics of vortices in the presence of unpaired Majorana modes and comment on their experimental detection. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L30.00011: Quantum point contact as a probe of a topological superconductor Michael Wimmer, Anton Akhmerov, Jan Dahlhaus, Carlo Beenakker We calculate the conductance of a ballistic point contact to a superconducting wire, produced by the s-wave proximity effect in a semiconductor with spin-orbit coupling in a parallel magnetic field. The conductance $G$ as a function of contact width or Fermi energy shows plateaus at half-integer multiples of $4e^2/h$ if the superconductor is in a topologically nontrivial phase, supporting Majorana fermions. In contrast, the plateaus are at the usual integer multiples in the topologically trivial phase without Majorana fermions. Disorder destroys all plateaus except the first, which remains precisely quantized in the case of a topological superconductor, consistent with previous results for a tunnel contact. The advantage of a ballistic contact over a tunnel contact as a probe of the topological phase is the strongly reduced sensitivity to finite voltage or temperature. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L30.00012: Genetic braid optimization for topological quantum computation R.B. McDonald, H.G. Katzgraber In topologically-protected quantum computation quantum gates can be carried out by adiabatically braiding quasiparticles in two space dimensions, reminiscent of entangled world lines. Bonesteel {\em et al.}~[Phys.~Rev.~Lett.~{\bf 95}, 140503 (2005)] showed recently how to find braids that yield a universal set of quantum gates. Mathematically, the problem of executing a gate becomes that of finding a product of the matrices in that set that approximates the gate, up to an error. To date efficient methods to compute these gates only strive to optimize for accuracy. We explore the possibility of using evolutionary (genetic) algorithms to efficiently find optimal braids while allowing the user to optimize for the relative utilities of accuracy and length. Furthermore, when optimizing for error only, the method can efficiently produce braids of error $\sim 10^{-6}$ outperforming brute force approaches. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L30.00013: Quantum memory on topological spin glass Jeongwan Haah, Sergey Bravyi We show that any topologically ordered local stabilizer model of spins in three dimensional lattices that lacks string logical operators can be used as a reliable quantum memory against thermal noise. It is shown that any local process creating a topologically charged particle separated from other particles by a distance $R$ must cross an energy barrier of height $c \log R$. This property makes the model glassy. We devise an efficient decoding algorithm that should be used at the final read-out, and prove a lower bound on the memory time until which the fidelity between the outcome of the decoder and the initial state is close to 1. The memory time increases as $L^{c \beta}$ where $L$ is the system size and $\beta$ the inverse temperature, as long as $L < L^\star \sim e^\beta$. Hence, the optimal memory time scales as $e^{c\beta^2}$. Our bound applies when the system interacts with thermal bath via a Markovian master equation. We give an example of a strictly local stabilizer code that satisfies all of our assumptions. We numerically verify for this example that our bound is tight up to constants. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L30.00014: 3D local qupit quantum code without string logical operator Isaac Kim Recently Haah introduced a new class of local quantum error correcting code embedded on a cubic lattice without any string logical operator. We present new codes with similar properties by relaxing the condition on the local particle dimension. The resulting code is well-defined when the local Hilbert space dimension is prime. These codes can be divided into two different classes: the local stabilizer generators are either symmetric or antisymmetric with respect to the inversion operation. We lower bound the number of encoded qudits by computing the commutation relation between the logical operators confined on a plane. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L30.00015: Topological Decoding through Artificial Confinement Guillaume Duclos-Cianci, David Poulin 2D topological stabilizer codes have attracted a lot of attention in recent years for two main reasons. First, they provide exactly solvable models which exhibit topological order and anyonic excitations. Second, they naturally lead to quantum stabilizer error-correcting codes having macroscopic minimum distance. Although these codes are robust at zero temperature, quasi-particles appear and freely diffuse in the system at any finite temperature. If this diffusion is unchecked, errors will occur. Consequently, active error-correction is needed. We want to propose a cellular automaton that would perform this correction. It would ``manually'' confine the quasi-particles by simulating articifial attraction between them and moving them accordingly. We obtained encouraging preliminary results for error-correction and hope to generalize them to fault-tolerance. [Preview Abstract] |
Tuesday, February 28, 2012 5:30PM - 5:42PM |
L30.00016: Andreev Bound states in One Dimensional Topological Superconductor with Broken Spatial Inversion Symmetry Xiong-Jun Liu, Kai Sun, Sankar Das Sarma We study the Andreev bound states (ABSs) at the Josephson junction of one dimensional topological superconductors (SC) when the spatial inversion symmetry (SIS) is broken. While in the absence of inversion symmetry, we show a hidden symmetry for the Bogoliubov de Gennes equations in the case of SC gap much smaller than Fermi energy in addition to the particle-hole symmetry, due to which the ABSs are predicted to carry irrational charge, with the charge value solely depending on the SIS breaking term, regardless of the details of the superconductor order parameter and whether the disorder scattering is present or not. We demonstrate that in the tunneling transport spectroscopy the irrationally charged ABSs are measured by the resonant differential tunneling conductance. [Preview Abstract] |
Session L31: Focus Session: Topological Insulators: Synthesis and Transport - Magnetism
Sponsoring Units: DMPChair: Nuh Gedik, Massachusetts Institute of Technology
Room: 260
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L31.00001: Thin films of magnetically doped topological insulators Invited Speaker: Cui-Zu Chang The interplay between the Dirac surface state and ferromagnetic order in topological insulators can lead to a number of very exotic quantum phenomena. To observe the quantum phenomena such as quantized anomalous Hall (QAH) effect in magnetically doped topological insulators, the materials are required to be in the form of thin film with tunable chemical potential and carrier-independent ferromagnetism. In this talk, I will report our recent progress in molecular beam epitaxy growth, chemical potential tuning and electronic properties of the magnetically doped topological insulator thin films. By Cr doping, we have realized both n-type and p-type conductivity in (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$ thin films. Remarkably their ferromagnetism was found independent of the type and concentration of carriers. Moreover, the anomalous Hall effect is significantly enhanced at low carrier concentration regime, with the anomalous Hall angle reaching an unusually large value of 0.2 and the zero field Hall resistance reaching one quarter of the quantum resistance (h/e2). These findings pave the way to ultimately observing the QAH effect and other quantum effects in magnetic topological insulators. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L31.00002: Proximity induced ferromagnetism at the interface between a topological insulator (TI) Bi$_{2}$Se$_{3}$ and a ferromagnetic insulator (FI) Peng Wei, Ferhat Katmis, Badih Assaf, Don Heiman, Pablo Jarillo-Herrero, Jagadeesh Moodera The ferromagnetic phase of the surface states of a TI is predicted to carry many exotic properties, for example quantum anomalous Hall effect, magnetic monopole, and magneto-electric effects etc. In our study, we explore this novel phase utilizing the proximity induced exchange splitting to introduce ferromagnetism close to the surface of the Bi$_{2}$Se$_{3}$ film. High quality Bi$_{2}$Se$_{3}$ thin films were grown using molecular-beam-epitaxy, and in-situ deposited the ferromagnetic insulator (FI) EuS over this film. Magnetization measurements demonstrated a magnetic moment of more than 7$\mu _{B}$ per Eu$^{2+}$ ion (bulk value), and reaching up to 11$\mu _{B}$ per Eu$^{2+}$ ion for 1nm thick EuS film, showing the unambiguous existence of excess ferromagnetism. The transport studies of these TI/FI bilayers unveiled a clear switching behavior of the magnetoresistance in the Bi$_{2}$Se$_{3}$ film. There was significant temperature dependence seen in both MR and the coercivity. Due to the near range nature of the exchange interactions, these extra magnetic moments and the MR results are attributed to come from the induced ferromagnetism at the Bi$_{2}$Se$_{3}$ surface. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L31.00003: Possible transport evidence for a surface state gap in a magnetically doped topological insulator D.M. Zhang, A. Richardella, D.W. Rench, A. Kandala, T.C. Flanagan, P. Schiffer, N. Samarth, S.-Y. Xu, H. Beidenkopf, A. Yazdani, M.Z. Hasan, A.L. Yeats, B.B. Buckley, P. Klimov, D.D. Awschalom We report magnetoresistance measurements in thin films of a magnetically doped topological insulator Bi$_{2-x}$Mn$_x$Se$_3$ synthesized by molecular beam epitaxy. We observe a crossover from positive magnetoresistance to negative magnetoresistance at low temperature ($T \la 15$ K), accompanied by onset of ferromagnetic signatures (hysteresis and anisotropic magnetoresistance). The observations are consistent with the prediction of a transition of diffusive quantum transport from the symplectic to the unitary class due to a magnetically induced surface state gap. This interpretation is supported by the observation of strongly suppressed surface states at the Dirac point in angle-resolved photoemission spectroscopy. We use the magneto-optical Kerr effect, anomalous Hall effect, SQUID magnetometry, electron microscopy and scanning tunneling microscopy to clarify the source of the ferromagnetism in these samples. Supported by DARPA, ONR and NSF-MRSEC. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L31.00004: Magnetoconductance crossover and non-linear Hall effect in MBE-grown Cr-doped Bi$_{2}$Te$_{2}$Se Badih A. Assaf, Peng Wei, Ferhat Katmis, Jagadeesh S. Moodera, Don Heiman Recent predictions have shown that the magnetoconductance (MC) of a topological insulator should show a crossover from negative MC to positive MC when time-reversal symmetry is broken [1]. Observations of this crossover have been reported in 3 QL thick Cr-Bi$_{2}$Se$_{3 }$[2] in the quasi-2D regime where surface hybridization is not negligible. We report magneto-transport results on 15 QL thick Cr-doped and undoped Bi$_{2}$Te$_{2}$Se films grown on Si (111) substrates by MBE. The undoped film exhibits weak anti-localization at low temperatures. In the Cr-doped film, a crossover from negative to positive MC is observed versus temperature at T=12 K. SQUID measurements show that the sample is ferromagnetic in this temperature range. In addition, a non-linear Hall voltage is observed despite the large bulk carrier concentration. This paves the way for further experiments on (Bi,Cr)$_{2}$Te$_{2}$Se in our search for the quantum anomalous Hall effect. [1] H.Z. Lu et al. Phys. Rev. Lett. \textbf{107}, 076801(2011). [2] M.Liu et al. arXiv 1103.3353(2011). [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L31.00005: Ferromagnetism in chromium doped topological insulator thin films and nanoplate crystals Zhiyi Chen, Lukas Zhao, Inna Korzhovska, Haiming Deng, Limin Huang, Simone Raoux, Jean Jordan-Sweet, Stephen O'Brien, Lia Krusin-Elbaum The surface states of topological insulators are protected by time-reversal symmetry. Introducing magnetic impurities should break this symmetry and open a gap in the otherwise gapless surface states. Recent first-principle calculations predict that when topological insulators are doped with transition metal elements, such as Cr or Fe, a \emph{magnetically ordered} insulating state will form -- a state that in thin (quasi-2D) samples may support a quantized Hall conductance. Here we report on electrical and magnetic characterization of thin Cr doped topological insulators: Sb$_2$Te$_3$ nanoplate crystals and $\sim 50$ nm thin films of Bi$_2$Te$_3$. Electrical contacts to samples were lithographically defined, with \textit{rf} sputtered films grown on pre-patterned substrates. Low-temperature in-plane resistivity, Hall, and magnetization measurements were performed in up to 5 T magnetic fields. For 5 at\% Cr content, a distinct ferromagnetic hysteretic response is observed at temperatures below 10 K. Hysteretic loops, also observed in Hall resistivity, indicate low-$T$ coercive fields of the order of 0.5 T. Correlation of transport and magnetic measurements indicating anomalous Hall effect, and strong dependence on dopant concentration and sample thickness will be presented. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L31.00006: Ferromagnetism in vanadium doped thin films of a topological insulator Bi$_2$Te$_3$ Lukas Zhao, Zhiyi Chen, Inna Korzhovska, Haiming Deng, Simone Raoux, Jean Jordan-Sweet, Myriam Sarachik, Lia Krusin-Elbaum Recent first-principle calculations predict a new class of ferromagnetic systems that are distinctly different from the conventional dilute magnetic semiconductors. A novel ferromagnetic topological insulator (ferro-TI) state can be obtained when topological insulator are doped with certain transition metal elements. In the the quasi-2D limit these ferro-TIs are expected to support a quantized anomalous Hall effect. Here we report on electrical and magnetic characterization of vanadium doped thin ($\sim 50$ nm) films of a topological insulator Bi$_2$Te$_3$. Films were grown by \textit{rf} sputtering on S$_3$N$_4$/Si substrates with lithographically pre-patterned contact pads. Low-temperature in-plane and Hall resistivity measurements were performed in magnetic fields up to 5 T fields. We find that below 100 K, V-doped films display \textit{negative linear magnetoresistance}, which at lower temperatures becomes hysteretic. Hall resistivity is also hysteretic, suggesting an unusual ferromagnetic ordering below 10 K. Moreover, V-doping turns the \textit{p}-type conduction in as-grown films into \textit{n}-type. The doping and thickness dependence of these effects will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L31.00007: Controllable ferromagnetism of iron doped topological insulator Shan Qiao, Zhen Liu, Fuhao Ji, Bin Li, Fuchun Xi, K. Kuroda, Mao Ye, K. Miyamoto, A. Kimura The higher than room temperature ferromagnetism was found in iron doped Bi2Se3. Samples generated by different processes have different magnetic characters. The Curie temperature is independent on iron concentration which against all discovered dilute magnetic systems. EXAFS observations show that the local structure of iron in samples with paramagnetic character is complex. On the contrary, that with ferromagnetic character is very simple that the iron atoms make up small single atom, dimer or trimer structures and these structures randomly distributed in Bi2Se3 crystal. The ferromagnetism can be enhanced or suppressed by the shift of Fermi edge by co-doping of Mg and Fe to Bi2Se3 crystal. The less than 3 atoms small structure cannot have room temperature ferromagnetism, so we believe that the higher than room temperature controllable ferromagnetism is intrinsic character of iron doped topological insulator. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L31.00008: In-plane anisotropy of Fe atoms on Bi$_2$Se$_3$(111) T. Wehling, A. Lichtenstein, J. Honolka, S. Stepanow, K. Kern, V. Sessi, N. Brookes, J. Mi, B.B. Iversen, P. Hofmann, A.A. Khajetoorians, J. Wiebe, T. Schlenk, R. Wiesendanger Topological insulators exhibit a linearly dispersing gapless topological surface state where both the spin and momentum degrees of freedom are locked. The topological nature of this state results in interesting effects such as suppression of back-scattering. Recently, the robustness of these surface states against magnetic order has come under heavy investigation. Here, we explore the magnetic properties of single Fe adatoms on the Bi2Se3 surface, in the coverage range $< 1$\% , with combined non-local x-ray magnetic circular dichroism techniques and local low temperature scanning tunneling spectroscopy. We reveal that the adatoms heavily relax into the surface and exhibit a magnetic easy axis within the surface-plane, contrary to recent reports. Furthermore, we demonstrate, using $ab-initio$ approaches, how the easy axis can reorient from out-of-plane to in-plane when considering the interplay of Coulomb interactions, spin-orbit coupling, and dynamic hybridization effects. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L31.00009: Topological insulators with magnetic impurities in the bulk Gilad Rosenberg, Marcel Franz We show that a three dimensional topological insulator with magnetic impurities could have a regime where the surface is magnetically ordered but the bulk is not. This is in contrast to conventional materials where bulk ordered phases are typically more robust than surface ordered phases. This difference originates from the topologically protected gapless surface states characteristic of topological insulators. We study the problem using a mean field approach, using two concrete models that give the same qualitative result, with some interesting differences. This work could help explain recent experimental results showing the emergence of a spectral gap in the surface state of Bi$_2$Se$_3$ doped with Mn or Fe atoms, but with no measurable bulk magnetism. \\[4pt] [1] Massive Dirac Fermion on the Surface of a Magnetically Doped Topological Insulator, Y. L.~Chen {\em et al.}, Science { \bf 329}, 659-662 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L31.00010: Interface-driven magnetism in topological insulators Bi2Te3 and Bi2Se3 Kyungwha Park Topological insulators (TIs) draw great attention due to their unique quantum properties and applications. TIs possess metallic surface states within band gaps induced by spin-orbit coupling (SOC), and they allow only an odd number of Dirac cones in dispersion of the surface states at a given surface. Novel physical phenomena and applications proposed including TIs, critically rely on stability and topological nature of the surface states when TIs are interfaced with other types of materials. Despite many studies on TIs, it still remains unclear how the surface states of TIs behave in contact with other materials. In this talk, we present our study of an effect of interfaces on the surface states of TIs Bi2Te3 and Bi2Se3, using density-functional theory including SOC self-consistently. We simulate interfaces using adsorption layers on TI films in an asymmetric fashion. We discuss unexpected results on stability and topological nature of the surface states, as well as changes in their spin structure and energy gaps at zero momentum. Our findings reveal importance of interfaces and a possibility of engineering new hybrid TI structures using adsorption layers. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L31.00011: Magnetic structure of magnetic semiconductor NdBiPt studied by elastic neutron diffraction Roger Mueller, Alexandre Desilets-Benoit, Andrea Bianchi, Luc Lapointe, Zahra Yamani, Michel Kenzelmann We report a study of the magnetic structure of the antiferromagnetic (AFM) half-heusler NdBiPt via neutron scattering. NdBiPt exibits an AFM transition at $T_{\mathrm N} = 2.2 \mathrm{K}$ with an ordered moment of 3.6$\mu_{\mathrm B}$ per Nd atom, as determined from magnetization measurement. The insulating state of NdBiPt has been proposed to be a new class of a three dimensional topological insulator, which possesses surfaces states at step edges on facets, with properties that are analogous to those observed in quantum Hall states. However, this would require an orientation of the magnetic moments in the ordered state along the crystallographic direction $[001]$. Triple-axis neutron axis diffraction on a single crystal was used to study the magnetic order. We found that in the ordered state consists of a conventional two sublattice antiferromagnet with the spins oriented along $[001]$ with very little fluctuations present close to T$_{\mathrm N}$. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L31.00012: Iron impurities on Sb (111) surface and their effects on topological surface state Jinhee Han, Hyungjun Lee, Hyoung Joon Choi We study iron impurities on Sb (111) surface and their effects on topological surface state by using an \textit{ab-initio }pseudopotential density-functional method. We implemented the spin-orbit interaction into the SIESTA in a form of additional fully non-local projectors. To calculate electronic structure of topological surface states, we consider a slab of Sb using a supercell containing 20 atomic layers with experimental bulk Sb lattice parameters. We determine atomic positions of Fe impurities on Sb (111) surface by minimizing the total energy, and calculate surface band structures near the Fermi level. To find effects of the impurity on the surface states of Sb (111) surface, we simulate ARPES spectra as a function of impurity density on the surface. From the results, we find that Fe impurity states are present near Fermi level and they strongly interact with the surface states. This work was supported by the NRF of Korea (Grant Nos. 2009-0081204 and 2011-0018306) and KISTI Supercomputing Center (Project No. KSC-2011-C2-04). [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L31.00013: Impurities in Bi2Se3 topological insulator: ab initio calculation Tome M. Schmidt, Roberto H. Miwa, Adalberto Fazzio Topological insulators are materials that have a bulk as an ordinary insulator but have protected conducting states on their surface. The surface states with an odd number of Dirac cones are robust against time-reversal invariant perturbation. However the interaction of magnetic impurities with the Dirac fermions can break time reversal symmetry open unp a surface band gap. In this work we investigate the magnetic anisotropy and spin-texture of Co impurities embedded at the interlayer vdW spacings and onto the topmost Se network of the topological insulator Bi$_{2}$Se$_{3}$. The interaction of the magnetic impurity with the surface spin texture break time reversal symmetry, opening up a surface band gap. For Co atom adsorbed onto the surface, the net magnetic moment is aligned perpendicular to the surface plane, with anisotropy energy of 6$\sim $meV. On the other hand for the Co impurity at the vdW interlayers, the net magnetic moment is aligned in-plane. While pristine Bi$_{2}$Se$_{3}$ presents helical spin-texture in the massless surface Dirac cone and states resonant within the valence band, the presence of Co impurity reduces the planar spin helicity of now massive Dirac fermions. On the other hand O impurities do not break the protected surface Dirac cones, but they move the position of the Dirac crossing upwards. Also we observe that there is an energy barrier for the O$_{2}$ molecule when it approximates to the Bi$_{2}$Se$_{3}$ surface, but atomic O inside the Bi$_{2}$Se$_{3}$ is more stable than the formation of O$_{2}$ molecules. [Preview Abstract] |
Session L32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Mechanisms of Ferroelectricity
Sponsoring Units: DMP DCOMPChair: Ronald Cohen, Carnegie Institution of Washington
Room: 261
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L32.00001: General Theory for the Ferroelectric Polarization Induced by Spin-Spiral Order Hongjun Xiang, Erjun Kan, Yuemei Zhang, M.-H. Whangbo, Xingao Gong Multiferroics display magnetic, polar and elastic order parameters simultaneously and hence present fascinating fundamental physics and potentially promising applications. The multiferroic phenomenon has been explained by several different models. However, none of them can correctly describe the ferroelectric polarization of triangular-lattice antiferromagnets induced by helical spin-spiral order. To resolve this problem, we develop a general theory for the ferroelectric polarization induced by spin-spiral order on the basis of symmetry considerations and then evaluate the coefficients needed to specify the general theory on the basis of density functional calculations. Our theory not only explains the ferroelectricity of triangular-lattice antiferromagnets driven by helical spin-spiral order, but also incorporates all known models of magnetic-order-driven ferroelectricity as special cases. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L32.00002: Modeling functional piezoelectricity in perovskite superlattices with competing instabilities Charles Swartz, Xifan Wu Multi-component Perovskite Superlattices (SLs) of the form ABO$_{3}$, provide a very promising avenue for the design of materials with multifunctional properties. Furthermore the interfaces of such multi-component SLs are home to competing anti-ferrodistortive and ferroelectric instabilities which can produce unexpected functionalities. However, at present first principles calculations exceeding more than 10 units cells, are particularly costly as they scale with the valence electrons as $N^{3}$. We present a first-principles modeling technique that allows us to accurately model the piezoelectric strains of paraelectric/ferroelectric SLs, BaTiO$_{3}$/CaTiO$_{3}$ and PbTiO$_{3}$/SrTiO$_{3}$, under a fixed displacement field. The model is based on a maximally localized wannier center layer polarization technique, as well as a truncated cluster expansion, that makes use of the fact that such PE/FE SLs have been shown to have highly localized ionic and electronic interface effects. The prediction of the piezoelectricity for a SL of an arbitrary stacking sequence will be demonstrated. We also use our model to conduct a systemic study of the interface effects on piezoelectric response in the above SLs paying special attention to a strong non-linear effect observed in Bulk SrTiO$_{3}$. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L32.00003: Competing (anti-)ferrodistortive and ferroelectric instabilities in SrTiO$_{3}$ and layered La$_{2}$Ti$_{2}$O$_{7}$ Ulrich Aschauer, Nicola Spaldin We present first-principles calculations on the competition between antiferrodistortive (AFD) and ferroelectic instabilities in oxides. High temperature cubic SrTiO$_{3}$ is well known to undergo an AFD transformation at around 105 K. Further reducing the temperature shows a softening of the ferroelectric polar phonons, however the material remains incipient-ferroelectric with an overall paraelectric behavior. This behavior is believed to be linked to a suppression of the polar instability by the AFD one, the mechanism still being debated. Our calculations show that freezing in the AFD indeed reduces the polar instability. At the theoretical equilibrium angle, the material however still retains a polar instability with a double-well depth of $\sim$0.2 meV per SrTiO$_{3}$ unit, inline with the material being incipient-ferroelectric. A change in polar eigenvectors with increasing AFD rotation together with a decomposition of forces into long- and short-range components allows us to propose an underlying mechanism. We will further discuss a similar suppression mechanism observed in layered La$_{2}$Ti$_{2}$O$_{7}$, where conventional ferroelectricity is suppressed by ferrodistortive modes, these modes however still leading to improper ferroelectricity due to the layered structure. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L32.00004: First Principles Study of Piezoelectricty in Improper Ferroelectrics Kevin Garrity, Karin Rabe Piezoelectric materials are key components of many important technologies, and discovering materials with improved piezoelectric responses is a major goal of materials science. In particular, finding new mechanisms for piezoelectricity which allow for high piezoelectric coefficients, especially in lead-free materials, could have great technological impact. Recently, there has been a renewed interest in improper ferroelectrics, which are materials where a non-zero polarization is induced indirectly by the coupling of the polar distortion to non-polar unstable modes, frequently oxygen octahedral rotations. This mechanism for creating a polarization may also offer the possibility of increased coupling to strain, leading to high piezoelectric coefficients. Here, we use first principles density functional theory to investigate the mechanism of the piezoelectric response of Ca$_3$Ti$_2$O$_7$, an improper ferroelectric which we find to have large piezoelectric coefficients. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L32.00005: Engineering polar perovskites from centric polyhedra building blocks James Rondinelli, Craig Fennie Increasing demands for electric field-tunable electromagnetic (EM) materials has renewed interests in ferroelectricity and its coupling to EM properties in perovskite oxides. Using density functional computations combined with group theoretical methods, we detail the crystal-chemistry criteria that enable the rational design of new perovskite oxides displaying octahedral rotation-induced ferroelectricity---electric polarizations ($P$) without second-order Jahn-Teller cations from $B$O$_6$ building blocks. We show that interleaving two bulk perovskites to form an ordered and layered arrangement of $A$-site cations [chemical composition $(A,A^\prime)B_2$O$_6$] produces a new trilinear free energy term coupling $B$O$_6$ rotations to $P$. This symmetry rule combined with an energetic condition, describing the lattice dynamical properties of the bulk materials, enables the routine design of synthetic ferroelectric perovskites. We illustrate these guidelines and achieve sizeable electric polarizations in layered gallate and aluminate perovskites, i.e.\ bulk materials with no tendencies to ferroelectricity. Finally, we argue that this strategy could be exploited for the design of ferroelectricity in a variety of crystal classes exhibiting flexible cation--anion polyhedral frameworks. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L32.00006: Crossover between hybrid improper ferroelectricity and proper ferroelectricity in layered perovskites Andrew Mulder, Nicole Benedek, Craig Fennie Recent progress in designing materials with unconventional mechanisms of ferroelectricity has shown that nominally non-polar octahedral rotations can induce an electrical polarization in certain layered perovskites, e.g., the n=2 Ruddlesden-Popper Ca$_3$Mn$_2$O$_7$, and certain A-site ordered AA'B$_2$O$_6$ double perovskites. In these (what have recently been termed) hybrid improper ferroelectrics, two unstable octahedral rotations of different symmetries couple trilinearly with the polarization. A key question that is still unclear is what the consequences of this coupling are in determining whether these materials exhibit conventional proper ferroelectricity, as in SrBi$_2$Ta$_2$O$_9$, or something resembling improper ferroelectricity. In this talk we discuss this question of proper vs improper in A$_3$B$_2$O$_7$ layered perovskites. First we develop simple criteria for realizing this novel form of ferroelectricity based solely on the properties of the ABO$_3$ parent perovskites. Then we explore how composition and epitaxial strain lead to different ferroelectric behaviors arising from the same rotation-polarization coupling. Finally we show how strain can tune a single material between rotation-driven hybrid improper ferroelectricity and conventional proper ferroelectricity. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L32.00007: Competition and cooperation between octahedral rotations and ferroelectricity in simple and layered ABX$_3$ perovskites Invited Speaker: Nicole Benedek Researchers have been studying octahedral rotations in perovskites for over half a century. It is now known that such lattice distortions are generally driven by the coordination preferences of the A-site cation. The role of octahedral rotations in modulating the magnetic, electronic and orbital properties of perovskites has been elucidated and a set of empirical rules for rationalizing the structures of known materials and for predicting the structures of as yet unsynthesized materials has been established. Despite this progress, there remains a long-standing problem concerning octahedral rotations and ferroelectricity: there are very few ferroelectric ABX$_3$ perovskites with octahedral rotations. This has lead to the widespread assumption that octahedral rotations compete with and suppress ferroelectricity. In this talk, I will describe our recent work on the interaction between ferroelectricity and octahedral rotations in simple and layered perovskites. Using a combination of Density Functional Theory and simple crystal chemical models, we have shown that in contrast to the common assumption, ferroelectricity and octahedral rotations do not always compete. In particular, I will discuss the manner in which rotations can actually induce ferroelectricity in ABX$_3$ perovskites and present strategies for designing new functional materials based on this mechanism. In a related direction, I will also discuss the role of layering in inducing hybrid improper ferroelectricity in some Ruddlesden-Popper phases and double perovskites. Our approach provides a chemically intuitive picture -- one that combines first-principles lattice dynamics with a local description of bonding -- to explain why particular materials adopt particular structures. Such knowledge is at the foundation of the current materials-by-design effort. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L32.00008: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L32.00009: A Unified Formalism toward Polarization, Magnetization and the $\theta$-term in a Periodic Insulator Kuang-Ting Chen, Patrick Lee The traditional perturbative calculation expands in powers of the gauge potential. This procedure proves problematic when a uniform electric or magnetic field is present. Here we provide a perturbative expansion of the electronic Green's functions directly in powers of the fields. On the other hand, the first order correction to the free energy to an insulator with periodic boundary conditions in the presence of the electric field is actually a Berry's phase. To express the Berry's phase in terms of the Green's functions, one is required to extend the Green's function to one extra dimension. With the trick, one can then calculate the effective action to arbitrary order of the electric and magnetic field. One new result we have obtained is that the $\theta$-term is given by the combination of the Green's functions in the extended momentum space, similar to the Wess-Zumino-Witten term, even without time reversal symmetry. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L32.00010: Observation and Theory of Intrinsic Ferromagnetism in Ferroelectric Materials J. Lashley, K. Gofryk, D.J. Safarik, J.L. Smith, I.E. Dzyaloshinskii Quantized waves obeying Bose-Einstein statistics will contribute a $T^{3/2}$ term to the specific heat if the dispersion relation goes as q2. We measure the magnetic and electric field dependence of the specific heat on the ferroelectric material tri-glycine sulphate (TGS) over the temperature range 0.05 K $<$ T $<$ 350 K. We detect a $T^{3/2}$ term in the specific heat in the low-temperature limit, which is taken to be the dielectric analog to magnetic spin wave. Near the Curie temperature ($T_{C}$ = 320 K), the shape of the specific-heat anomaly is thermally broadened. However, the anomaly changes to the characteristic sharp lambda-shape expected for a continuous transition with the application of either a magnetic field or electric field, giving the expected entropy change at $T_{C}$ of $R$ln2. These results are explained on the basis that the frequencies of optical dipole oscillations are split by the magnetic field, and the resulting gas of excitations are paramagnetic. Consequently they contribute to the specific heat near $T_{C}$, which increases with magnetic field. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L32.00011: Dramatic effect of the near-electrode layer configurations on the phase transition characteristics of ferroelectric-paraelectric superstructures Burc Misirlioglu, Arkadi Levanyuk Ferroelectric-paraelectric superstructures have attracted great interest. A few experimental reports have emphasized the effect of individual layer thickness on the transition temperature of these systems. Here, we theoretically show that the phase transition characteristics of these systems are very sensitive to the structural configuration near the electrodes. The phase transition is homogeneous in the system only if the layers contacting the electrodes are paraelectric layers of 1/2 thickness of other individual layers. If the ferroelectric layer contacts the electrode (and the other.electrode is contacted by a paraelectric layer) the phase transition temperature is higher than in the previous structure and the spatial distribution of the polarization at the phase transition is inhomogeneous being maximal near the electrode and disappearing far from it. A striking result is that the transition temperature in one unit bilayer is the same as a system consisting of any number of bilayers. Moreover, the profile of the polarization is unchanged upon addition of new bilayer units to the system. We also discuss general features of the domain structures below the phase transition temperature and transition anomalies. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L32.00012: The phase diagram of Sr$_{1-x}$Eu$_{x}$TiO$_{3}$: Crossover from displacive to order-disorder dynamics Zurab Guguchia, Hugo Keller, Alexander Shengelaya, J\"urgen K\"ohler, Annette Bussmann-Holder The phase diagram of Sr$_{1-x}$Eu$_{x}$TiO$_{3}$ is determined experimentally by EPR and resistivity measurements and analyzed theoretically in terms of the self-consistent phonon approximation (SPA) as a function of x (0.0 $\le $ x $\le $ 1.0). It is observed that the oxygen octahedral tilting instability temperature T$_{S}$ increases nonlinearly with x. The theoretical analysis demonstrates that a crossover from displacive to order-disorder dynamics takes place for x$\ge $0.25, signaled by a change in the local double-well potential and the soft mode temperature dependence. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L32.00013: Electrocaloric effect in ferroelectric alloys from atomistic simulations Sergey Lisenkov, Inna Ponomareva Caloric effects, such as magnetocaloric and electrocaloric effects, have attracted a lot of attention recently in the context of increasing interest in energy conversion and renewable energy materials and devices. Here we develop and use accurate first-principles-based simulations to study electrocaloric effect (ECE) from an atomistic point of view. In particular, we develop a computational technique that allows both direct and indirect simulations of ECE within the {\it same} atomistic framework. We then use such a tool to provide first systematic comparison between ECE estimates obtained from direct and indirect approach which will allow us to bridge the macroscopic and atomistic description of ECE. The results of our direct atomistic simulations are then used to explore the intrinsic features of ECE in ferroelectrics with multiple transitions. [Preview Abstract] |
Session L33: Invited Session: Frontiers of Granular Physics
Sponsoring Units: GSNP DFDChair: Eric Weeks, Emory University
Room: 106
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L33.00001: Packing Nonspherical Particles: All Shapes Are Not Created Equal Invited Speaker: Salvatore Torquato Over the past decade there has been increasing interest in the effects of particle shape on the characteristics of dense particle packings, since deviations from sphericity can lead to more realistic models of granular media, nanostructured materials, and tissue architecture. It is clear the that the broken rotational symmetry of a nonspherical particle is a crucial aspect in determining its resulting packing characteristics, but given the infinite variety of possible shapes (ellipsoids, superballs, regular and irregular polyhedra, etc.) it is desirable to formulate packing organizing principles based the particle shape. Such principles are beginning to be elucidated; see Refs. 1 and 2 and references therein. Depending upon whether the particle has central symmetry, inequivalent principle axes, and smooth or flat surfaces, we can describe the nature of its densest packing (which is typically periodic) as well as its disordered jammed states (which may or may not be isostatic). Changing the shape of a particle can dramatically alter its packing attributes. This tunability capability via particle shape could be used to tailor many-particle systems (e.g., colloids and granular media) to have designed crystal, liquid and glassy states. \\[4pt] [1] S. Torquato and F. H. Stillinger, ``Jammed Hard-Particle Packings: From Kepler to Bernal and Beyond," {\it Rev. Modern Phys.} {\bf 82}, 2633 (2010). \\[0pt] [2] Y. Jiao and S. Torquato, Communication: ``A Packing of Truncated Tetrahedra That Nearly Fills All of Space and its Melting Properties," {\it J. Chem. Phys.} {\bf 135}, 151101 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:42PM |
L33.00002: Network Analysis of Granular Flows Invited Speaker: Michelle Girvan The flow of granular materials is important to many natural and industrial processes, yet connecting microscale materials properties of grains to bulk flow behavior has remained a challenging task. Our work leverages tools from complex network theory to study granular flow at multiple scales. By characterizing the statistical properties of time-evolving contact networks using metrics like average path length, giant component size, and modularity, we are able to identify how macroscale system features like the loss of reversibility are connected to micro- and meso- scale rearrangements in the contact network. In addition, we employ a network-based approach to explore the role of rotations in facilitating cooperative rearrangements. For both the reversibility and rotation studies, we apply network analysis to time-dependent contact network data obtained from both experiments and simulations and show that this approach can provide new insights on how bulk system properties are connected to particle-scale motion. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L33.00003: Swimming in a granular frictional fluid Invited Speaker: Daniel Goldman X-ray imaging reveals that the sandfish lizard swims within granular media (sand) using axial body undulations to propel itself without the use of limbs. To model the locomotion of the sandfish, we previously developed an empirical resistive force theory (RFT), a numerical sandfish model coupled to an experimentally validated Discrete Element Method (DEM) model of the granular medium, and a physical robot model. The models reveal that only grains close to the swimmer are fluidized, and that the thrust and drag forces are dominated by frictional interactions among grains and the intruder. In this talk I will use these models to discuss principles of swimming within these granular ``frictional fluids". The empirical drag force laws are measured as the steady-state forces on a small cylinder oriented at different angles relative to the displacement direction. Unlike in Newtonian fluids, resistive forces are independent of speed. Drag forces resemble those in viscous fluids while the ratio of thrust to drag forces is always larger in the granular media than in viscous fluids. Using the force laws as inputs, the RFT overestimates swimming speed by approximately 20\%. The simulation reveals that this is related to the non-instantaneous increase in force during reversals of body segments. Despite the inaccuracy of the steady-state assumption, we use the force laws and a recently developed geometric mechanics theory to predict optimal gaits for a model system that has been well-studied in Newtonian fluids, the three-link swimmer. The combination of the geometric theory and the force laws allows us to generate a kinematic relationship between the swimmer's shape and position velocities and to construct connection vector field and constraint curvature function visualizations of the system dynamics. From these we predict optimal gaits for forward, lateral and rotational motion. Experiment and simulation are in accord with the theoretical prediction, and demonstrate that swimming in sand can be viewed as movement in a localized frictional fluid. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:54PM |
L33.00004: Geometrically Cohesive Granular Materials Invited Speaker: Scott Franklin Geometrically cohesive granular materials (GCGM) are collections of particles whose individual shape leads to entanglements that resist extension forces, resulting in a non-zero Young's modulus. Examples include long, thin (anisometric) rods, arcs of varying length, and U-shaped staples. I will report on experimental and computational work that investigates the peculiar rigidity of GCGM. These include canonical stress-strain and vibration-induced melting experiments on U-shaped staples that have revealed a non-monotonic dependence of collective rigidity on particle shape. For concave particles, rigidity appears proportional to an ``entanglement number'' --- the number of neighbors that pass through the area partially bounded by the particle. Computational and analytic work on arcs and staples confirm the non-monotonic behavior of the entanglement number, and simulations that match the experimental conditions are underway to confirm entanglement as the basic mechanism of GCGM's rigidity. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:30PM |
L33.00005: Strain-stiffening in random packings of entangled granular chains Invited Speaker: Eric Brown Random packings of granular chains are presented as a model polymer system to investigate the consequences of entanglements in the absence of Brownian motion. The packings are compressed uniaxially and the structure is characterized by x-ray tomography. For short chain lengths, these packings yield when the shear stress exceeds the scale of the confining pressure, similar to packings of spherical particles. In contrast, packings of chains which are long enough to bend into closed loops exhibit strain-stiffening, in which the effective stiffness of the material increases with strain, similar to many polymer materials. The latter packings can sustain stresses orders-of-magnitude greater than the confining pressure, and do not yield until the chain links break. These strain-stiffening packings are found to contain system-spanning clusters of entangled chains. [Preview Abstract] |
Session L34: Focus Session: Impact of Ultrafast Lasers IV: Applications I
Sponsoring Units: DCPChair: Nancy Levinger and Amber Kummel, Colorado State University
Room: 107A
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L34.00001: Adding a dimension to the infrared spectra of interfaces: 2D SFG spectroscopy via mid-IR pulse shaping Invited Speaker: Martin Zanni Sum-frequency generation spectroscopy provides an infrared spectrum of interfaces and thus has widespread use in the materials and chemical sciences. In this presentation, I will present our recent work in developing a 2D pulse sequence to generate 2D SFG spectra of interfaces, in analogy to 2D infrared spectra used to measure bulk species. To develop this spectroscopy, we have utilized many of the tricks-of-the-trade developed in the 2D IR and 2D Vis communities in the last decade, including mid-IR pulse shaping. With mid-IR pulse shaping, the 2D pulse sequence is manipulated by computer programming in the desired frequency resolution, rotating frame, and signal pathway. We believe that 2D SFG will become an important tool in the interfacial sciences in an analogous way that 2D IR is now being used in many disciplines. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L34.00002: Ultrafast Broadband Infrared Source for Nonlinear Infrared Spectroscopy Krupa Ramasesha, Aritra Mandal, Luigi De Marco, Andrei Tokmakoff Existing optical parametric amplifiers can generate 200cm-1 to 400 cm-1 of bandwidth at mid-infrared frequencies, which limit the use of these pulses for studying vibrational dynamics. We have developed a new broadband infrared source that can generate pulses with 1000 cm-1 of bandwidth, spanning most of the mid-infrared region of the spectrum. These pulses can allow us to simultaneously probe higher frequency vibrations like the OH stretch and the fingerprint region at lower frequencies. By focusing the first, second and third harmonics of a 25 fs 800 nm pulse in air to create a plasma, we have been able to generate infrared pulses with a broad spectrum ranging from 3 microns to 9 microns and a sub-100 fs pulse duration. This source can thus allow us to study dynamics that involve distinct vibrational transitions on ultrafast timescales. This presentation will discuss our efforts in characterizing the broadband infrared source and the progress we are making towards setting up a pump-probe 2D IR experiment using these broadband infrared pulses. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L34.00003: Development of a Tunable Ultra-Broadband Mid IR Pulsed Source for Nonlinear Spectroscopy Mark Cheng, Anthony Reynolds, Heather Widgren, Munira Khalil We generate ultra- broadband mid-IR pulses tunable from 2.5 -- 8 $\mu $m by focusing 800 nm/400 nm pulses into various gas media. The input 800 nm light is doubled to 400 nm in a type I BBO crystal. The two orthogonally polarized $\omega $/2$\omega $ pulses encounter a birefringent calcite crystal for time delay compensation and are subsequently focused in various gas media (air, argon, neon and nitrogen) contained within a 1.2 m gas cell using a 1 m focal length silver mirror. The tunability of the broadband mid-IR pulses arises from different gases, pressure of gases and the amount of incident 800 nm/400 nm light focused into the gas cell at a given pressure. We measure IR energies as high as 0.5 $\mu $J/pulse for an input 800 nm energy of 3 mJ/pulse in 900 Torr of Argon. The mid IR pulses exhibit $\sim $2{\%} long term stability. The ultrabroadband IR pulses have a spectral bandwidth of $\sim $2000 cm$^{-1}$ corresponding to a sub-cycle pulse centered at 4.5 $\mu $m. We will present our preliminary efforts on using the ultrabroadband IR pulses in nonlinear experiments. The broad spectral content of this novel source affords the possibility of probing multiple vibrations in a coherent manner. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 4:06PM |
L34.00004: Transient IR Probes of Spinning Molecules in an Optical Centrifuge Invited Speaker: Amy Mullin A high-power optical centrifuge based on ultrafast laser pulses is used to drive molecules into very high rotational states. The optical centrifuge consists of reversed-chirped laser pulses that generate a linearly polarized electric field that angularly accelerates over the time of the pulse. Molecules trapped in the optical field are spun into high rotor states through sequential Raman transitions. The properties of the spinning molecules and their spatial distribution are interrogated with high-resolution state-resolved IR transient absorption probing. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L34.00005: Ultrafast Nonlinear Opto-Electronic Spectroscopy Shawn M. Perdue, Joonhee Lee, Alejandro Rodriguez Perez, V. Ara Apkarian Using a single color of light and rf beam tagging, parametric nonlinear optical mixing processes can be read not just in photons, but also in electrons. The scheme allows ultrafast time-resolved opto-electronics, which we demonstrate at a tunneling junction. Acousto-optic modulators are used to frequency tag ultrashort laser pulse trains, whereby carrier beats generated in nonlinear mixing processes are down shifted by sampling at the repetition rate of the laser (80 MHz). The method down converts optical carrier frequencies (PHz) to baseband (kHz), where direct current readout is possible with single electron sensitivity. Various parametric processes can be identified through their one-to-one map in baseband. Illustrative implementations that will be presented include the determination of the temporal profile of field-emitted ultrashort electron packets and characterization of the highly nonlinear dynamics involved in light-induced tunneling emission. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L34.00006: Coherence-Modulated Third Harmonic Generation for Second Hyper-Raman Spectroscopy of Molecules at an Interface Kevin Dillman, Jesse Wilson, Randy Bartels, Nancy Levinger We have developed a method of probing the low-frequency (sub-1000 cm$^{-1}$) vibrational modes of molecules at interfaces using third-harmonic generation (THG). The THG process is enhanced at an interface due to the differences in the third-order nonlinear susceptibilities of the materials. We have used this method to collect low-frequency second hyper-Raman spectra from BGO, BaF$_{2}$ and CdWO$_{4}$ crystals. In addition, we have observed coherent second hyper-Raman scattering arising from CCl$_{4}$ molecules at the liquid-glass interface. We are presently extending these techniques to observe resonant second hyper-Raman scattering from dye molecules adsorbed on gold nanoparticles in order to gain surface enhancement effects. We aim to use this method to characterize the environment at interfaces of reverse micelle systems. The development of this method is significant because we can sensitively probe the low-frequency vibrational modes of only those molecules at an interface. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L34.00007: New generation ultrafast fiber-lasers with automated pulse compression for biomedical imaging Bai Nie, Dmitry Pestov, Ilyas Saytashev, Sergey Arkhipov, Andy Chong, Hui Liu, Frank Wise, Marcos Dnatus A double-clad Yb-doped all-normal-dispersion fiber laser is demonstrated to produce 22 nJ pulses at 42.5 MHz repetition rate. With a 3 nm intracavity spectral filter, self-similar evolution is formed in the gain segment. The formation of self-similar evolution allows the achievement of both short pulse duration and high pulse energy form a fiber laser oscillator. These pulses are characterized and compressed via multiphoton intrapulse interference phase scan to as short as 42 fs and 10 nJ/pulse. Adaptive compression underlies the achievement of 250-kW peak power, which enables efficient second and third harmonic generation with spectra spanning 30 nm and 20 nm, respectively. Using the newly developed fiber laser, multiphoton imaging on live tissues is demonstrated. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L34.00008: Ionization and Coulomb explosion of small hydrocarbons exposed to short intense laser pulses Sergiy Bubin, Kalman Varga We have performed first principles numerical simulation of high field ionization in small hydrocarbons followed by a Coulomb explosion. The process was driven by 790 nm 27 fs laser pulses of high intensity (of the order of 10$^{15}$ W/cm$^2$), for which case there exists recent experimental data [S. Roither et al., Phys. Rev. Lett. 106, 163001 (2011)]. We have analyzed the spectra of ejected protons and investigated the ionization-fragmentation mechanism that takes place when molecules are subjected to short intense laser pulses. The results of our simulations support the all-at-once (concerted) fragmentation scenario proposed by Roither et al. At the same time we also observed some quantitative differences between the theoretical and experimental spectra. [Preview Abstract] |
Session L35: Focus Session: DFT IV: Ground-State DFT: New Directions
Sponsoring Units: DCPChair: Sam Trickey, University of Florida
Room: 107B
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L35.00001: Progress in fractional perspectives of density functional theory Invited Speaker: Weitao Yang Density functional theory of electronic structure is widely and successfully applied in simulations throughout engineering and sciences. However, there are major failures for many predicted properties. These errors can be characterized and understood through the perspective of fractional charges and fractional spins introduced recently. The fractional perspectives offer a possible pathway forward. I will report progress in excited states, spin state splitting, open-shell singlet states, fukui functions and band gaps. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L35.00002: Generalized-gradient approximations with non-vanishing exchange-correlation magnetic torque Giovanni Scalmani, Stefano Pittalis, Michael J. Frisch, Giovanni Vignale The description of systems of interacting electrons in the presence of magnetic fields, within spin-density functional theory (SDFT), requires the non-collinear magnetization density vector $\mathbf{m}(\mathbf{r})$ to be used as basic variable, along with the particle density $n(\mathbf{r})$. Futhermore, for a meaningful description of spin-dynamics, the magnetization density and its conjugate exchange-correlation (xc) field $\mathbf{B_{xc}}(\mathbf{r})$ must not be constrained to be locally parallel at every point in space. It is well known that the local density approximation (LDA) cannot, by construction, provide such a non-collinear configuration of the two vector fields. Here we show how popular generalized gradient approximations (GGAs), developed assuming collinear spin-density, can be used to describe non-collinear magnetization states, including the presence of non-vanishing local torque between $\mathbf{m}(\mathbf{r})$ and $\mathbf{B_{xc}}(\mathbf{r})$. Unlike previous attempts to extend the use of collinear GGAs to the domain of non-collinear magnetization densities, the approach we introduce is invariant with respect of spin-rotations, globally satisfies the \emph{zero-torque theorem}, reduces to the proper collinear limit and is numerically stable. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L35.00003: New gradient functional for SDFT derived from spin spirals in the uniform electron gas F.G. Eich, E.K.U. Gross The development of functionals in SDFT depending on gradients of the spin magnetization is a long standing challenge. We present a new functional based on the spin-spiral state of the uniform electron gas. Comparing the principal idea of the new functional to the LSDA and GGAs, we highlight the intrinsic way non-collinearity is built into the proposed approximation. As key feature the functional yields exchange-correlation magnetic fields that are non-collinear w.r.t.~the spin magnetization, while obeying the zero-torque theorem by construction. This means that an adiabatic application of the functional within TD-SDFT accounts for the local torque exerted by the exchange-correlation field and retains the numerical simplicity of explicit density functionals. An implementation of the functional based on the RPA-treatment of spin spirals in the electron gas is shown. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L35.00004: Tuning the correlation energy in second-order M{\o}ller-Plesset perturbation theory with an effective electron-electron interaction potential Tim Kowalczyk, Troy Van Voorhis For practical electronic structure calculations on large molecules, wave function theories (WFT) remain less popular than density functional theory (DFT) despite the systematic improvability of WFT. The greater computational cost of WFT is largely to blame and is two-fold in nature: WFT generally presents unfavorable scaling with system size relative to DFT, {\em and} WFT results converge more slowly than those of DFT with respect to basis set size. We propose that each of these issues can be partially mitigated by the introduction of an effective electron-electron interaction potential, in lieu of the true $1/r$ potential, for evaluating the correlation energy in WFT. We discuss the design and optimization of such an effective interaction, in the context of second-order M{\o}ller-Plesset perturbation theory (MP2), for two distinct purposes: to accelerate basis set convergence of MP2 correlation energies, and to re-scale the MP2 correlation to improve agreement with experiment and/or higher-level methods. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L35.00005: Toward Improved Semilocal and Nonlocal Density Functionals for Atoms, Molecules, and Solids Invited Speaker: John P. Perdew Semilocal density functionals construct the exchange-correlation energy density at a point from the electron density and orbitals in the neighborhood of that point. They can be constructed nonempirically, and work best for sp-bonded systems near equilibrium. They increase in sophistication from the local spin density approximation to the generalized gradient approximation to the meta-GGA. For a molecule like CO on a transition metal surface, it appears that only a meta-GGA can give a good simultaneous description of the lattice constant and surface energy of the metal, on the one hand, and the adsorption energy of the molecule on the other [1]. I will discuss two remaining deficiencies of the revised TPSS meta-GGA [2]: its artificial order-of-limits problem, and its need for more information about non-bonded interaction. When electrons are shared over stretched bonds, full nonlocality is needed, and typically empirical parameters are also needed. This suggests that we don't yet know enough about the full nonlocality of the density functional for the exchange-correlation energy. \\[4pt] [1] J. Sun, M. Marsman, A. Ruzsinszky, G. Kresse, and J.P. Perdew, Phys. Rev. B 83, 121410 (2011). \\[0pt] [2] J.P. Perdew, A. Ruzsinszky, G.I. Csonka, L.A. Constantin, and J. Sun, Phys. Rev. Lett. 103, 026403 (2009). [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L35.00006: Energy expressions for model exchange potentials: Beyond the Levy--Perdew virial relation Alex P. Gaiduk, Viktor N. Staroverov The common way to assign energies to Kohn--Sham exchange potentials is by using the Levy--Perdew virial relation. However, for model potentials that are not functional derivatives, this approach leads to energy expressions that lack translational invariance. We point out that there is a more general procedure for constructing density functionals from model potentials, of which the Levy--Perdew relation is just a special case. Using this generalization we propose a method for converting model potentials into density functionals that ensures translational invariance of the energy. To illustrate our approach we construct a competitively accurate exchange functional from the model potential of van Leeuwen and Baerends. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L35.00007: Laplacian-based models for the exchange energy Antonio C. Cancio, Christopher E. Wagner, Shaun Wood Recent Quantum Monte Carlo data for the exchange-correlation energy density of pseudopotential systems strongly suggest the value of using the Laplacian of the density as a variable for constructing first order corrections to the local density approximation of density functional theory. We report on an exchange functional built upon these observations and extended to the all-electron case. THe model keeps the typical properties of constraint-based generalized gradient approximations (GGA) and also has a finite-valued potential at the nucleus, unlike the GGA. Problems with oscillatory behavior in the potential due to higher order derivatives are controlled by a curvature minimization constraint. The results are tested against exact potentials for the He and Ne atom. A combination of gradient and Laplacian as suggested by a gradient expansion of the exchange hole gives the best overall results. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L35.00008: A non-empirical improvement of PBE and its hybrid PBE0 Alberto Vela, Jorge M. del Campo, Jos\'e L. G\'azquez, S.B. Trickey We present a non-empirical re-parameterization of the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) exchange-correlation functional and of the related PBE hybrid (PBE0) obtained by imposing the constraint that, for the hydrogen atom, the exchange energy cancels the Coulomb repulsion energy. The new parameterization is validated with well-known test sets. The results for the re-parameterized PBE GGA, called PBEmol, show a substantial improvement over the original PBE in predicted heats of formation, while retaining the quality of the original PBE functional for description of all the other properties considered. The results for the hybrids indicate that, although the PBE0 functional provides a rather good description of those properties, the predictions of the re-parameterized functional are, except in the case of the ionization potentials, modestly better. Also, the results are better than B3LYP, except for the case of the ionization potentials and the harmonic frequencies. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L35.00009: Effect of the orbital-overlap dependence on Meta Generalized Gradient Approximation Jianwei Sun, John Perdew, Bing Xiao, Adrienn Ruzsinszky The dimensionless inhomogeneity parameter, $\alpha $, characterizing the extent of orbital overlap, is disentangled from the other dimensionless inhomogeneity parameter, s, the reduced density gradient, in terms of constructing a meta generalized gradient approximation (MGGA) for the exchange functional. We show that the formation of the intershell region inside an atom is associated with increase of$\alpha $, which suggests MGGA should expect a monotonically decreasing $\alpha $ dependence for a wide range of density. This leads to a simple nonempirical MGGA exchange functional, which interpolates between the sigle-orbital regime for confinement systems, where $\alpha $=0, and the slowly varying density regime, where $\alpha \approx \mbox{1}$, and then extrapolates to $\alpha \to \infty $. The new MGGA exchange functional, combined with the variant of the Perdew-Burke-Erzerhof (PBE) GGA correlation as used in the revised Tao-Perdew-Staroverov-Scuseria (revTPSS) MGGA [1], performs equally well for atoms, molecules, surfaces, and solids, with an implication of a tight Lieb-Oxford bound. \\[4pt] [1] J.P. Perdew, A. Ruzsinszky, G.I. Csonka, L.A. Constantin, and J. Sun, Phys. Rev. Lett. 103, 026403 (2009). [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L35.00010: The derivative discontinuity in density functional theory from an electrostatic description of the exchange and correlation potential Xavier Andrade, Alan Aspuru-Guzik We present an approach that we have recently proposed [{\it Phys. Rev. Lett.} {\bf 107}, 183002 (2011)] to approximate the exchange and correlation (XC) term in density functional theory. In our approach the XC potential is considered as an electrostatic potential, generated by a fictitious XC density, which is in turn a functional of the electronic density. In this picture, the exact asymptotic limit for low density regions, wrongly predicted by many XC functionals, can be imposed as a local condition. Based on this XC density representation we develop a correction scheme that fixes the asymptotic behavior of any approximated XC potential for finite systems. The procedure is simple, computationally inexpensive, and does not depend on adjusted parameters. Additionally, from the correction procedure it is possible to extract an approximation to the derivative discontinuity of XC energy. This value can be used to directly obtain the gap of the system as a ground-state property. Results are presented for the application of the method to atoms and small molecules. The correction results in a significant improvement in the value of the ionization energy and the gap, with errors that are comparable to the results given by orbital dependent functionals. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L35.00011: Multi-Scale Approach to Simulations of Kelvin Probe Force Microscopy Ali Sadeghi, Alireza Ghasemi, Stefan Goedecker, Alexis Baratoff, Thilo Glatzel, Ernst Meyer The distance dependence and atomic-scale contrast recently observed in nominal contact potential difference(CPD) signals simultaneously recorded during non-contact atomic force microscopy on surfaces of insulating and semiconducting samples have stimulated theoretical attempts to explain how the applied bias voltage affects electrostatic forces acting on the atomic scale. We attack the problem in two steps. First, the electrostatics of the macroscopic tip-cantilever-sample system is treated by a finite-difference method on an adjustable nonuniform mesh. It has the advantages of getting a systematically increasable accuracy as well as the ability of considering the cantilever which turns out to be important for insulating samples. The resulting electric field near the tip apex is then inserted into a series of wavelet-based density functional theory calculations. Results are obtained for a reactive disordered neutral silicon nano-scale tip interacting with a NaCl(001) sample. Bias-dependent forces and resulting atomic displacements are computed to an accuracy of 1 pN. Theoretical expressions for Kelvin signals and local contact potential difference (LCPD) are obtained combining both contributions to the electrostatic force and evaluated for several tip oscillation amplitudes. [Preview Abstract] |
Session L36: Focus Session: New Energy IV
Sponsoring Units: DCPChair: Bruce Garrett, Pacific Northwest Research Laboratory and Anders Nilsson, SLAC
Room: 107C
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L36.00001: Molecular photovoltaics: Tricks from theory, from fission to gaps to traps Invited Speaker: Mark Ratner |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L36.00002: Study of excitonic energy transport in thin-film J-aggregates Stephanie Valleau, Semion Saikin, Man-Hong Yung, Alan Aspuru-Guzik The concentration and transfer of light through materials is one of the main current scientific goals in the ongoing quest for a new clean energy source. Molecular structures with optimal exciton transfer properties find widespread applications, ranging from solar cells and photonic devices to photographic and lithographic systems. J-aggregates of organic dye molecules are a good example of such structures where a strong interaction between molecular electronic transitions results in a partial exciton delocalization and a large exciton diffusion length. In this presentation I will discuss theoretical aspects of exciton dynamics in two dimensional films of J-aggregates with an emphasis on macroscopic transport properties that could be used for device modeling. As specific illustrations I will use results of our recent studies of exciton dynamics in aggregates of cyanine dyes. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L36.00003: Control of exciton delocalization pathways using ZnTPP functionalization Robert Bartynski, Sylvie Rangan, Senia Coh, Jonathan Rochford, Elena Galoppini Zn(II)-Tetraphenylporphyrin (ZnTPP) derivatives are attractive candidates for photoinduced electron-transfer mediators in dye sensitized solar cells. We have investigated the influence on solar cells efficiency, of the energy alignment and of the molecular adsorption geometry at the ZnTPP/metal-oxides interface. In this work, using x-ray, UV and inverse photoemission spectroscopies in conjunction with density functional theory (DFT) calculations, we have determined the energy alignment of molecular levels with respect to the substrate band edges for several ZnTPP derivatives adsorbed on ZnO(11-20) and TiO$_{2}$(110) surfaces. The ZnTPP derivatives were functionalized with COOH anchoring groups, to allow upright or flat adsorption on the surfaces. While the energy alignment is found similar for all of these systems, large differences in devices efficiencies are observed. We have thus explored the adsorption geometry of the same ZnTPPs at the surface of ZnO and TiO$_{2}$ using UV-visible absorption and NEXAFS spectroscopies and scanning tunnel microscopy. It is found that for ZnTPPs, upright adsorption opens deleterious exciton delocalization pathways, due to dipole/dipole interactions competing with electron transfer to the substrate. Choosing the adsorption geometry is thus critical for the electronic pathway control. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 4:06PM |
L36.00004: Coupled polaronic and ion transport in nanocrystalline metal oxide electrodes Invited Speaker: Kevin Rosso We report new computational methods and fundamental understanding in the dynamics of coupled charge and ion transport in nanoscale metal oxides. The methods attack the multi-scale problem of simulating the collective diffusivities of ions and charge compensating e-/h+ carriers in single crystal particles, across particle-particle grain boundaries, and through networks of grains for select systems. Methods include embedded quantum mechanical clusters at the DFT and MP2 levels of theory for atomic-scale polaronic and ion transport kinetics, classical DFT-based free energy calculations for grain-scale conductivity in the framework of the Poisson-Nernst-Planck formalism, and phase field simulation of charged particle diffusivity for conductivity at the grain network scale. This combination of approaches is one of a kind in terms of its multi-scale range, scaling, and computational efficiency. We are presently focused on coupled electron and Li+ ion transport in polymorphs of TiO2, and also in mixed valence spinel oxides, for electrode conductivity optimization and improving energy storage materials performance for Li+ batteries. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L36.00005: Electrochemical properties of Pt-Co-Zr thin films on high surface area NSTF supports Charles Hays, Poyan Bahrami, Michael Errico, James Kulleck, Daniel Konopka, Adam Kisor, Stuart Cooley, Harold Greer Nanostructured thin film supports (NSTF) are a promising fuel cell (FC) technology demonstrated by 3M [1]. We have examined the electrochemical performance of Pt-Co-Zr films deposited onto NSTF supports by dc-magnetron sputtering. In this presentation, we will present results of microstructural, composition, and electrochemical properties, for NSTF supported (Pt$_{3}$Co)$_{100-x}$Zr$_{x}$ thin films, with 10 $<$ x $<$ 40 (At. {\%}). Electrochemical measurements show that the films are electrochemically stable, and active for the oxygen-reduction-reaction (ORR), with ORR kinetic current densities at 0.9 V (vs. NHE), up to 57X greater than those of Pt(111) films measured in the same cell. The composition dependence of the ORR, and relevant physical properties will be discussed. \\[4pt][1] M. K. Debe, A. J. Steinbach, G. D. Vernstrom et al, J. Electrochemical Soc. 158, B910 (2011). \\[4pt] Acknowledgements: The research in this presentation was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We acknowledge support from the Department of Energy (DE-PS36-08GO98101). We thank our collaborators Dr. M. Debe, Dr. R. Atanasoski, and G. D. Vernstrom of 3M Corp. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L36.00006: Enhanced Surfactant Adsorption on Activated Carbon through Manipulation of Surface Oxygen Groups John Collins, Deyang Qu, Michelle Foster Passive energy storage is a necessary component for balancing the lifecycle budget with new forms of green energy. The work presented describes how surface oxygen groups (SOG) on granulated activated carbon have been manipulated using Nitric Acid in a controlled, stepwise fashion. The structure and surface functionality of the activated carbon samples were characterized using DRIFTS, Raman Spectroscopy and Porosimetry. Total surface area was found to increase proportionally with the removal of heteroatom material, exposing previously insulated active sites responsible for SOG attachment. Broad oxide peaks were deconvoluted and analyzed, allowing for absolute identification of evolving functionality at each oxidation stage. SOGs were maximized on the third oxidation cycle with the presence of conjugated aromatic, phenol, lactone, and carboxylic acid groups. FSN Zonyl nonionic was applied to all oxidized samples at various concentrations. Total adsorbed surfactant was quantified for each concentration / oxidation scheme using attenuated total reflection. The relative quantity and polarity of chemisorbed surfactant were qualitatively assessed for each equilibrium concentration. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L36.00007: Coarse-grained potential models for structural prediction of carbon dioxide in confined environments Tarun Sanghi, Narayana Aluru Geological storage of carbon dioxide is a promising option to reduce the level of carbon dioxide in the atmosphere and mitigate its effect on climate change. In geological storage, high-pressure carbon dioxide is injected into the underground porous rock formations, where it gets trapped inside the tiny nanometer size pores of the rocks. Thus, a good understanding of the microstructure of carbon dioxide inside nanoscale confinements is of great practical importance in developing an efficient technology for carbon dioxide storage. In this work, we discuss the systematic development of coarse-grained single-site (CGSS) potential models to study the structure of carbon dioxide in confined environments. These single-site potentials allow computationally efficient simulations, which are several orders of magnitude faster than all-atom MD (AAMD) simulations. The potential models are used in our earlier proposed multi-scale quasi-continuum theory, called EQT, and in coarse-grained MD (CGMD) to predict the equilibrium structure of carbon dioxide confined inside graphite slit pores. The results obtained from both EQT and CGMD are found be in good agreement with those obtained from computationally expensive AAMD simulations. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 5:18PM |
L36.00008: Multiscale Simulation of Electrochemical{\_} Phenomena: Fuel Cells and Batteries Invited Speaker: Ryan Jorn Results will be presented from multiscale simulations of two important systems from renewable energy technology, fuel cell proton membranes and electrochemical cells. In the first case, the solvation and transport of hydrated protons in proton exchange membranes (PEMs) such as Nafion$^{TM}$ will be described using a novel multi-state reactive molecular dynamics (MD) approach. The multi-state MD methodology allows for the treatment of explicit (Grotthuss) proton shuttling and charge defect delocalization which, in turn, can strongly influence the properties of the hydrated protons in various aqueous and complex environments. The role of PEM hydration level and morphology on these properties will be further described. A new multiscale computational methodology for describing the mesoscopic features of the proton transport will also be described, which can be coupled to the results from the molecular-scale simulations. On the second topic, a computationally efficient method will be presented for the treatment of electrostatic interactions between polarizable metallic electrodes held at a constant potential and separated by an electrolyte. The method combines a fluctuating uniform electrode charge with explicit image charges to account for the polarization of the electrode by the electrolyte, and a constant uniform charge added to the fluctuating uniform electrode charge to account for the constant potential condition. The method is used to calculate electron transport rates using electron transfer theory; these rates are incorporated in a multiscale approach to model oxidation/reduction reactions in an electrochemical cell efficiently. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L36.00009: Collective Behavior of Water on Platinum David Limmer, Adam Willard, David Chandler We present the results of molecular dynamics simulations of a interface between water and a platinum electrode. Using importance sampling techniques we probe a variety of collective phenomenon that emerge at the interface. We consider platinum electrodes with two different geometries and discuss how different behaviors result from a competition between geometrical frustration and favorable local interactions. [Preview Abstract] |
Tuesday, February 28, 2012 5:30PM - 5:42PM |
L36.00010: Superstructured tungsten oxide photoanodes Robert Coridan, Kevin Arpin, Paul Braun, Nathan Lewis Tungsten oxide is a robust and stable semiconductor for photoanodic applications in aqueous solutions. Typical deposition techniques like electroplating or sputtering produce granular films which increase interfacial recombination of minority carriers. This has a deleterious effect on the photovoltaic performance of these materials. Using a variety of templating methods, we explore multiscale structuring strategies for increasing the surface area of the photoanode while maintaining significant light absorption. We describe photoelectrochemical and reflectivity measurements on structured and templated tungsten oxide photoanodes and consider how these results guide future photosynthetic electrochemical device design. [Preview Abstract] |
Session L37: Informal Education and Public Outreach - Policy Questions
Sponsoring Units: FEdChair: Gary White, SPS/AIP
Room: 108
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L37.00001: Selectively Bringing Down the Curtain on OPERA Superluminal Neutrino Papers Robert Garisto Picture this: instead of watching from afar the daily flow of superluminal proposals trying to explain OPERA's preliminary data, you are the PRL Editor charged with deciding what to do with each paper. How do you address the validity of a manuscript whose starting point involves circumventing Einstein? I'll discuss the general issues of controversial claims and data-driven floods of theory papers through the unusual lens of the OPERA-motivated papers. Along our ten minute journey, I'll show some interesting correspondence. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L37.00002: Superluminal Neutrinos: The Good Kind of Science Controversy Michael Lucibella After OPERA released results indicating neutrinos were traveling faster than the speed of light, a number of scientists said that the team shouldn't have prematurely engaged in ``science by press release.'' However, controversial claims like this, if handled properly, can actually be a boon to science. Getting the public interested in cutting edge physics is notoriously difficult, but the public is always interested in hearing about a controversy. The press conference and following debate becomes a great ``teaching moment'' which offers the public a unique opportunity to get an inside and in-depth look at how science works. Scientists willing to publicly engage in this kind of civil controversy are important because when other scientific controversies arise, ones with major public policy implications, the public starts out with a better understanding of how science works, and scientists have better practice managing questions from the public. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L37.00003: Controversial Science and the Media James Riordon The possibility that the OPERA collaboration has detected superluminal neutrinos was among the most controversial topics in physics news in decades, and one of the most widely covered stories in all of science in 2011. Word of the research initially reached journalists and the public prior to publication in peer-reviewed journals. Understandably, many physicists are concerned that the significance of controversial science may be exaggerated or distorted when news organizations report on science at such an early stage. I will offer an overview of the ways the story was promoted by the media relations personnel, and outline the rationales that motivate media relations efforts along with the associated benefits and drawbacks that can result. Finally, I will examine the accuracy and completeness of the superluminal neutrino news stories that ultimately were made available to the general public. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L37.00004: Controversial Physics: Perfect Public Outreach Opportunity Rebecca Thompson The goal of public outreach is to excite and engage the public in physics. What can be more exciting than controversy? When OPERA announced their discover of superluminal neutrinos, controversy within the physics community quickly followed. This result could overturn a century of established physics. From a public outreach perspective there was no better way to bring people usually unaware of current research into the discussion of this result. If handled well this could be used as a gateway to interest in other physics research. The public drive to learn more about this particular result can be harnessed to create interest in other cutting edge physics research and drive the public to continue their informal physics learning. If the results of OPERA and eventually proven incorrect as many physicists believe they will, that will not erase the public's new-found interest in physics but hopefully continue to fuel it. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L37.00005: Informal Physics Education: Outreach from a National Laboratory Jose Sanchez, Patricia Dixon, Roxanne Hughes This presentation highlights strategies for K-20 teaching and learning about materials research in informal settings. The National High Magnetic Field Laboratory's Center for Integrating Research {\&} Learning is in a unique position to conduct programs that reach K-20 students and teachers. As part of a national laboratory the Center provides the infrastructure around which informal education programs are implemented, including the nationally-recognized programming as well as facilitating scientists' educational outreach in the community. Research Experiences for Undergraduates, focuses on encouraging women and other underrepresented groups to pursue STEM careers reaching approximately 200 students many of whom have pursued careers in research as well as academia. The Research Experiences for Teachers program has provided internships for over 150 teachers; the Center also reaches over 10,000 students each year through school and community outreach. Success of informal education programs relies heavily on establishing strong mentoring relationships between scientists and K-20 students and teachers. The Center's success at maintaining diverse programming that transforms how materials education is presented beyond the traditional classroom is the focus for this presentation. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L37.00006: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L37.00007: Houston, we have a problem: What the Voting Public Believes about Science Funding Jodi Lieberman In an election year with a tight Federal budget outlook, the views of the voting public are critical to forecasting what may be in store for the National Science Foundation, Department of Energy Office of Science and National Institute for Standards and Technology. It is these people who weigh in with their legislators to identify what is important and what should be cut. The results of some recent polling commissioned by the APS and other scientific societies reveal that the ge neral public may not be as supportive of Federal science funding as has been previously believed. Jodi Lieberman will present the results of that polling data and how it is likely to impact the next Federal science budget. [Preview Abstract] |
Session L39: Focus Session: Superconducting Qubits: 3D Cavities
Sponsoring Units: GQIChair: Lev Bishop, Joint Quantum Institute and CMTC
Room: 109B
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L39.00001: How coherent are Josephson junctions? Invited Speaker: Hanhee Paik Superconducting quantum circuits based on Josephson junctions are a promising technology to realize a electronically controlled, solid-state based large-scale quantum information processor but their future prospects rely on the intrinsic coherence of Josephson junctions and the engineering of the isolated environment for the quantum circuits. We introduce a new architecture for superconducting quantum circuits employing a single-Josephson junction in a three dimensional waveguide cavity where we carefully engineer the environment of the qubit to effectively reduce the coupling of the qubit to the environment while maintaining sufficient coupling to the control signal. With this architecture we demonstrate that Josephson junction qubits are at least an order of magnitude more coherent with $T^{Ramsey}_{2}$ $\sim$ 10 to 20 $\mu$s without the use of spin echo than previously reported and highly stable, enabling us to observe the physics in a Josephson junction with a unprecedented level of precision. These results suggest that the overall quality of Josephson junctions will allow error rates of a few 10$^{-4}$, approaching the error correction threshold. We will also discuss how to scale this architecture and perform two-qubit gates. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L39.00002: Bottom-up construction of artificial molecules for superconducting quantum processors Stefano Poletto, Chad Rigetti, Jay M. Gambetta, Seth Merkel, Jerry M. Chow, Antonio D. Corcoles, John A. Smolin, Jim R. Rozen, George A. Keefe, Mary B. Rothwell, Mark B. Ketchen, Matthias Steffen Recent experiments on transmon qubits capacitively coupled to superconducting 3-dimensional cavities have shown coherence times much longer than transmons coupled to more traditional planar resonators. For the implementation of a quantum processor this approach has clear advantages over traditional techniques but it poses the challenge of scalability. We are currently implementing multi-qubits experiments based on a bottom-up scaling approach. First, transmon qubits are fabricated on individual chips and are independently characterized. Second, an artificial molecule is assembled by selecting a particular set of previously characterized single-transmon chips. We present recent data on a two-qubit artificial molecule constructed in this way. The two qubits are chosen to generate a strong Z-Z interaction by matching the 0-1 transition energy of one qubit with the 1-2 transition of the other. Single qubit manipulations and state tomography cannot be done with ``traditional'' single tone microwave pulses but instead specifically shaped pulses have to be simultaneously applied on both qubits. Coherence times, coupling strength, and optimal pulses for decoupling the two qubits and perform state tomography are presented [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L39.00003: Quasiparticle tunneling in a single-junction transmon qubit Diego Rist\'e, Josephine van Leeuwen, Leonardo DiCarlo The recent increase in transmon qubit quality factor into the million range [1] makes non-equilibrium quasiparticle tunneling a potentially limiting mechanism for qubit coherence. We investigate the dynamics of quasiparticle tunneling in a single-junction transmon qubit with relaxation time $T_1=85~\mathrm{\mu s}$ ($Q=2.6$ million). The qubit operates at moderate ratio of Josephson to charging energy, $E_J/E_C\sim30$, where charge parity in the qubit islands is encoded in the qubit transition frequency. Using Ramsey-type and stimulated echo experiments, we investigate quasiparticle tunneling across the qubit junction on time scales short and long compared to $T_1$. We observe that the quasiparticle tunneling time for the single-junction qubit is at least as long as $T_1$, but shorter than the $1~\mathrm{ms}$ repetition rate. This result is consistent with recent theory and qualitatively different from the two-junction transmon. The dephasing time $T_2^{\ast}=10~\mathrm{\mu s}$ is limited by slow background charge fluctuations and extended to $T_2=95~\mathrm{\mu s}$ using dynamical decoupling. \\[4pt] [1] Paik et al. arXiv:1105.4652v4 [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L39.00004: Varying Cavity Quality Factor in situ for a Transmon in Circuit QED Andrei Petrenko, Adam Sears, Gerhard Kirchmair, Hanhee Paik, Luyan Sun, Gianluigi Catelani, Leonid Glazman, Robert Schoelkopf Superconducting transmon qubits have recently been studied within 3D cavities. In addition to increasing the coherence times of the qubits this has enabled a simple scheme for varying the quality factor Q (or decay rate $\kappa$) of a cavity in situ. This decay rate plays an important role in our understanding of a number of effects in circuit quantum electrodynamics, many of which have direct bearing on qubit decoherence processes. Here we study how adjusting the cavity Q affects the coherence times of a single qubit within the 3D architecture. We demonstrate that varying the coupling enables us to not only examine the limitations of qubit $T_1$ due to the Purcell Effect, but also probe new decoherence mechanisms such as the dephasing due to photon shot noise. By understanding and minimizing these effects, we obtain record coherences times $T_2$ and $T_2^{Echo}$ of $\sim 27 \mu s$ and $\sim 47 \mu s$ respectively. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L39.00005: Dephasing Due to Shot Noise in the Strong Dispersive Limit of Circuit QED Adam Sears, Andrei Petrenko, Gerhard Kirchmair, Hanhee Paik, Luyan Sun, Gianluigi Catelani, Leonid Glazman, Robert Schoelkopf The design parameters of superconducting qubits inside resonant cavities have evolved over time to minimize decoherence, allow fast pulses and enable high fidelity readout. The two are often coupled so strongly that the dispersive shift of the qubit due to a single photon in the cavity (or AC Stark shift) is much larger than a linewidth. In this strong dispersive regime, the passage of any photons can lead to an unintended and complete measurement of the qubit state. We study photon shot noise dephasing in this limit for a transmon and derive a simple relation between the dephasing rate and the product $\bar{n}\kappa$, where $\bar{n}$ is the average cavity occupancy and $\kappa$ is the cavity decay rate. We find good experimental agreement for a large range of $\kappa$, varied \textit{in situ} using a simple mechanism, and note several ways this can influence qubit experiments. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L39.00006: Effective model of nonlinear circuit quantum electrodynamics Simon Nigg, Michel Devoret, Steven Girvin Superconducting electronic circuits containing nonlinear elements such as Josephson junctions are of interest for quantum information processing. The low-energy spectrum of such circuits can now be measured to a precision of better than one part per million. A precise knowledge of their Hamiltonian that goes beyond current models is thus desirable. In this talk I will show how to quantize a superconducting, weakly nonlinear circuit from the knowledge of its classical linear admittance matrix. This approach represents a change of paradigm in circuit quantum electrodynamics and may potentially become a useful alternative to the standard models based on the language of atomic physics and quantum optics. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L39.00007: Designing a three mode circuit QED experiment Brian Vlastakis, Gerhard Kirchmair, Hanhee Paik, Simon Nigg, Luigi Frunzio, Steven Girvin, Michel Devoret, Robert Schoelkopf Circuit QED employs the coupling of nonlinear elements to resonant modes of an electronic circuit. We demonstrate that all resonant modes will attain some degree of nonlinearity from even a single nonlinear element. This can result in individually addressable transitions for each mode and allow direct control of each quantum state. Furthermore, we show that the transition frequency of any one mode will depend on the state of all other modes. These state dependent shifts can be used to directly readout the quantum state of one mode probing another. We illustrate this behavior by coupling two three-dimensional resonators to a superconducting transmon qubit and present a method to determine the Hamiltonian for this system using a nonlinear circuit QED model. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L39.00008: Experiments on a three mode circuit QED system Gerhard Kirchmair, Brian Vlastakis, Hanhee Paik, Simon Nigg, Luigi Frunzio, Steven Girvin, Michel Devoret, Robert Schoelkopf Current research in superconducting circuit QED is working towards combining an increasing number of cavities and qubits to investigate larger scale quantum systems. Here we will discuss measurements on a system consisting of two three-dimensional microwave resonators coupled to a single transmon qubit. We demonstrate that each mode of the system has sufficient anharmonicity to coherently manipulate the state of the lowest two energy levels. This allows us to measure the coherence of a single excitation in a mode and detect the frequency shift due to excitations of the other modes. These effects are important to consider when using a resonator as a quantum memory to decouple the quantum state from the rest of the system. Furthermore we show that we can use the state dependent shifts to detect the quantum state of one mode with another. The full characterization of the system allows us to determine the Hamiltonian and compare it to the theoretical predictions obtained with a nonlinear circuit QED model. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L39.00009: Material and Geometric Effects in 3D Transmon Qubits Matthew Ware, M.P. Defeo, J.D. Strand, B. Xiao, B.L.T. Plourde, Stefano Poletto, Chad Rigetti Optimization of coherent behavior is a key ingredient for any scalable architecture using qubits. Recent breakthroughs in novel qubit designs have resulted in significant improvements in coherence by coupling superconducting qubits to 3-dimensional microwave cavities. We are investigating material and geometric factors affecting the coherence of these 3D transmon qubits. Various loss mechanisms limiting the qubit coherence will be discussed. The role played by device geometry and size in determining the effective qubit-cavity coupling will also be explored. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L39.00010: 3D Microwave Cavity for Qubit Measurement Sergey Novikov, V. Zaretskey, B. Suri, Z. Kim, B. S. Palmer, F. C. Wellstood We have investigated the loss mechanisms of the $\textrm{TE}_{101}$ mode (resonant frequency $\textrm{f}_0 = 8\textrm{ GHz}$) of a superconducting Al microwave cavity. The internal quality factor $Q_{int}$ of the cavity has been measured for a range of temperatures from 23 mK to 360 mK in a low photon number regime and from 360 mK to $\textrm{T}_c \sim$ 1.1 K in a high photon number regime, both with and without a sapphire chip in the cavity. With sapphire present, $Q_{int} \sim 10^6$ was strongly reduced by an applied magnetic field. Without sapphire, $Q_{int} \sim 4\times10^6$ was only weakly dependent on the applied field. The frequency stability of the cavity and the use of the cavity for qubit readout (following recent experiments by H. Paik \textit{et. al.}\footnote{arXiv:1105.4652v4}) will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L39.00011: Characterization of superconducting accelerator cavity at millikelvin temperature for use in quantum computation Bing Li, Yang Ge, Alex Romanenko, Lance Cooley, David Schuster Quantum computation using 2D superconducting circuits has been advancing rapidly but has been limited to coherence times of a few microseconds.~ On the other hand, 3d superconducting cavities for accelerators routinely achieve quality factors exceeding ten billion, corresponding to coherence times exceeding one second. ~~With the recent demonstration of coupling a superconducting qubit to a 3d resonator [1], it should be possible to take advantage of this advanced accelerator technology. ~However, accelerator cavities are typically used above 2 K and at 1 W input powers. ~The ultimate residual resistance of accelerator cavities is not yet well understood and it is unclear if they will maintain their exceptional properties at millikelvin temperatures and ultra-low powers.~ We present measurements of a 3.9 GHz accelerator cavity from 2K-20mK and at powers less than one attowatt.~~ [1] Paik, et. al. arXiv:1105.4652 [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L39.00012: Improving the Coherence Time of Microwave Cavities Matthew Reagor, Hanhee Paik, Luyan Sun, Eric Holland, Luigi Frunzio, Robert Schoelkopf A superconducting cavity resonator is able to store quantum states of light, protect qubits from decoherence and place bounds on material losses. The resonator's utility in all three goals is inherently tied to its quality factor. We report recent progress in improving the quality factors of aluminum waveguide cavities in the quantum regime. We will also report on the use of these cavities to measure the dielectric properties of low-loss substrates and the surface impedance of bulk superconductors and thin films. [Preview Abstract] |
Session L40: Focus Session: Systems Biology and Biochemical Networks II
Sponsoring Units: DBIO GSNPChair: Yuhai Tu, IBM Watson Research Center
Room: 156A
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L40.00001: Microbial interaction networks in soil and in silico Invited Speaker: Kalin Vetsigian Soil harbors a huge number of microbial species interacting through secretion of antibiotics and other chemicals. What patterns of species interactions allow for this astonishing biodiversity to be sustained, and how do these interactions evolve? I used a combined experimental-theoretical approach to tackle these questions. Focusing on bacteria from the genus Steptomyces, known for their diverse secondary metabolism, I isolated 64 natural strains from several individual grains of soil and systematically measured all pairwise interactions among them. Quantitative measurements on such scale were enabled by a novel experimental platform based on robotic handling, a custom scanner array and automatic image analysis. This unique platform allowed the simultaneous capturing of $\sim $15,000 time-lapse movies of growing colonies of each isolate on media conditioned by each of the other isolates. The data revealed a rich network of strong negative (inhibitory) and positive (stimulating) interactions. Analysis of this network and the phylogeny of the isolates, together with mathematical modeling of microbial communities, revealed that: 1) The network of interactions has three special properties: ``balance'', ``bi- modality'' and ``reciprocity''; 2) The interaction network is fast evolving; 3) Mathematical modeling explains how rapid evolution can give rise to the three special properties through an interplay between ecology and evolution. These properties are not a result of stable co-existence, but rather of continuous evolutionary turnover of strains with different production and resistance capabilities. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L40.00002: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L40.00003: Regulatory schemes to achieve optimal flux partitioning in bacterial metabolism Lei-Han Tang, Zhu Yang, Sheng Hui, Pan-Jun Kim, Xue-Fei Li, Terence Hwa The flux balance analysis (FBA) offers a way to compute the optimal performance of a given metabolic network when the maximum incoming flux of nutrient molecules and other essential ingredients for biosynthesis are specified. Here we report a theoretical and computational analysis of the network structure and regulatory interactions in an E. coli cell. An automated scheme is devised to simplify the network topology and to enumerate the independent flux degrees of freedom. The network organization revealed by the scheme enables a detailed interpretation of the three layers of metabolic regulation known in the literature: i) independent transcriptional regulation of biosynthesis and salvage pathways to render the network tree-like under a given nutrient condition; ii) allosteric end-product inhibition of enzyme activity at entry points of synthesis pathways for metabolic flux partitioning according to consumption; iii) homeostasis of currency and carrier compounds to maintain sufficient supply of global commodities. Using the amino-acid synthesis pathways as an example, we show that the FBA result can be reproduced with suitable implementation of the three classes of regulatory interactions with literature evidence. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L40.00004: A Realtime Active Feedback Control System For Coupled Nonlinear Chemical Oscillators Nathan Tompkins, Seth Fraden We study the manipulation and control of oscillatory networks. As a model system we use an emulsion of Belousov-Zhabotinsky (BZ) oscillators packed on a hexagonal lattice. Each drop is observed and perturbed by a Programmable Illumination Microscope (PIM). The PIM allows us to track individual BZ oscillators, calculate the phase and order parameters of every drop, and selectively perturb specific drops with photo illumination, all in realtime. To date we have determined the native attractor patterns for drops in 1D arrays and 2D hexagonal packing as a function of coupling strength as well as determined methods to move the system from one attractor basin to another. Current work involves implementing these attractor control methods with our experimental system and future work will likely include implementing a model neural network for use with photo controllable BZ emulsions. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L40.00005: Stochastic gene expression with bursting and positive feedback Thierry Platini, Hodjat Pendar, Rahul Kulkarni Stochasticity (or noise) in the process of gene expression can play a critical role in cellular circuits that control switching between probabilistic cell-fate decisions in diverse organisms. Such circuits often include positive feedback loops as critical elements. In some cases (e.g. HIV-1 viral infections), switching between different cell fates occurs even in the absence of bistability in the underlying deterministic model. To characterize the role of noise in such systems, we analyze a simple gene expression circuit that includes contributions from both transcriptional and translational bursting and positive feedback effects. Using a combination of analytical approaches and stochastic simulations, we explore how the underlying parameters control the corresponding mean and variance in protein distributions. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L40.00006: mRNA Noise Reveals that Activators Induce a Biphasic Response in the Promoter Kinetics of Highly Regulated Genes Katie Quinn, Tsz-Leung To, Narendra Maheshri A dominant source of fluctuations in gene expression is thought to be the process of transcription. The statistics of these fluctuations arise from the kinetics of transcription. Multiple studies suggest the bulk of fluctuations can be understood by a simple process where genes are inactive for exponentially distributed times punctuated by geometric bursts of mRNA. Yet it's largely unknown how cis and trans factors affect the two lumped kinetic parameters, burst size and burst frequency, that describe this process. Importantly, how these parameters are regulated in a single gene can qualitatively affect the dynamical behavior of the network it is embedded within. Here, we ask whether transcriptional activators increase gene expression by increasing the burst size or burst frequency. We do so by deducing these parameters from steady-state mRNA distributions measured in individual yeast cells using single molecule mRNA FISH. We find that for both a synthetic and natural promoter, activators appear to first increase burst size, then burst frequency. We suggest this biphasic response may be common to all highly regulated genes and was previously unappreciated because of measurement techniques. Furthermore, its origins appear to relate to cis events at the promoter, and may arise from combinations of basal and activator-dependent bursts. Our measurements shed new light on transcriptional mechanisms and should assist in building synthetic promoters with tunable statistics. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L40.00007: Determining the stability of genetic switches: Explicitly accounting for mRNA noise Michael Assaf, Elijah Roberts, Zan Luthey-Schulten Cells use genetic switches to shift between alternate gene expression states, e.g. to adapt to new environments or to follow a developmental pathway. Here, we study the dynamics of switching in a generic-feedback on/off switch. Unlike protein-only models, we explicitly account for stochastic fluctuations of mRNA, which have a dramatic impact on the genetic switch dynamics. Employing a semi-classical theory to treat the underlying chemical master equations, we obtain accurate results for the quasi-stationary distributions of mRNA and protein copy numbers and for the mean switching time, starting from either state. Our analytical results agree well with extensive Monte Carlo simulations. Importantly, one can use the approach to study the effect of varying biological parameters, and of extrinsic noise, on the switch stability. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L40.00008: Quantitative Model of microRNA-mRNA interaction Javad Noorbakhsh, Alex Lang, Pankaj Mehta MicroRNAs are short RNA sequences that regulate gene expression and protein translation by binding to mRNA. Experimental data reveals the existence of a threshold linear output of protein based on the expression level of microRNA. To understand this behavior, we propose a mathematical model of the chemical kinetics of the interaction between mRNA and microRNA. Using this model we have been able to quantify the threshold linear behavior. Furthermore, we have studied the effect of internal noise, showing the existence of an intermediary regime where the expression level of mRNA and microRNA has the same order of magnitude. In this crossover regime the mRNA translation becomes sensitive to small changes in the level of microRNA, resulting in large fluctuations in protein levels. Our work shows that chemical kinetics parameters can be quantified by studying protein fluctuations. In the future, studying protein levels and their fluctuations can provide a powerful tool to study the competing endogenous RNA hypothesis (ceRNA), in which mRNA crosstalk occurs due to competition over a limited pool of microRNAs. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L40.00009: On the control of gene expression in small RNA post-transcriptional regulation pathway: role of conserved weak targets Daniel Jost, Andrzej Nowojewski, Erel Levine Small RNA molecules play critical regulatory roles in organisms across all kingdoms of life. Many small RNA families achieve target-specificity via base-pairing of a very short (6-8 nucleotides) ``seed'' region with the targeted mRNA, and consequently many genes carry a matching seed in their sequence. Evidence in bacteria and animals suggest that a single small RNA may regulate the gene expression of many different targets, although most of them very weakly. On the other hand, in all cases we are aware of where the functionality of a small RNA has been carefully studied, only a small number of target genes were identified as being phenotypically relevant. Here, we present a Langevin formalism which describes the dynamics of the different interacting entities (small RNA and targets), including the stochasticity of the underlying biochemical reactions and the effect of transport. Using analytical or numerical computations, we study the influence of (many) weak targets on the mean and noise properties of (few) principal targets. In particular, we argue that the role of these weak targets is to confer robustness to the regulation of the principal targets without significantly affecting their temporal responses to changing environments. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L40.00010: Approaches to Chemical and Biochemical Information and Signal Processing Vladimir Privman We outline models and approaches for error control required to prevent buildup of noise when ``gates'' and other ``network elements'' based on (bio)chemical reaction processes are utilized to realize stable, scalable networks for information and signal processing. We also survey challenges and possible future research. \\[4pt] [1] Control of Noise in Chemical and Biochemical Information Processing, V. Privman, Israel J. Chem. 51, 118-131 (2010).\\[0pt] [2] Biochemical Filter with Sigmoidal Response: Increasing the Complexity of Biomolecular Logic, V. Privman, J. Halamek, M. A. Arugula, D. Melnikov, V. Bocharova and E. Katz, J. Phys. Chem. B 114, 14103-14109 (2010).\\[0pt] [3] Towards Biosensing Strategies Based on Biochemical Logic Systems, E. Katz, V. Privman and J. Wang, in: Proc. Conf. ICQNM 2010 (IEEE Comp. Soc. Conf. Publ. Serv., Los Alamitos, California, 2010), pages 1-9. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L40.00011: Organizing biochemical reactions: Lessons from cyanobacteria Niall Mangan, Michael Brenner Cyanobacteria are model organisms for photosynthesis and are of interest for bio-fuel production and carbon dioxide sequestration. I present a mathematical model of the carbon concentrating mechanism (CCM) in cyanobacteria. The CCM is a combination of transporters and enzymes distinctively organized in the cell, which increase the internal concentration of carbon dioxide, and improve the efficiency of converting carbon dioxide to sugar. I find that the internal carbon concentration can be completely described by solutions in two parameter regimes of the model. These solutions correspond to varying transporter and enzymatic activity, which can be directly connected to experimental measurements. I also find a dependence of the carbon concentration on the spatial organization of the reactions within the cell. Understanding the CCM in cyanobacteria gives us insight into design principles for the cellular organization of biological reactions. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L40.00012: ABSTRACT WITHDRAWN |
Session L41: Pattern Formation, Nonlinear Dynamics, Computational Fluid Dynamics
Sponsoring Units: DFDChair: Daphne Klotsa, University of Michigan
Room: 156B
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L41.00001: A Thermodynamic Model for Behavioral Intelligence Alexander Wissner-Gross, Cameron Freer Recent advances in cosmology and computer science have hinted at a potentially deep connection between intelligence and thermodynamics. Here we attempt to elucidate that connection by showing that a generalization of entropic forces can induce archetypically intelligent behaviors in a variety of classical mechanical systems. These results suggest a simple, but general, thermodynamic model for behavioral intelligence. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L41.00002: Data collapse and critical dynamics in neuronal avalanche data Thomas Butler, Nir Friedman, Karin Dahmen, John Beggs, Lee DeVille, Shinya Ito The tasks of information processing, computation, and response to stimuli require neural computation to be remarkably flexible and diverse. To optimally satisfy the demands of neural computation, neuronal networks have been hypothesized to operate near a non-equilibrium critical point. In spite of their importance for neural dynamics, experimental evidence for critical dynamics has been primarily limited to power law statistics that can also emerge from non-critical mechanisms. By tracking the firing of large numbers of synaptically connected cortical neurons and comparing the resulting data to the predictions of critical phenomena, we show that cortical tissues in vitro can function near criticality. Among the most striking predictions of critical dynamics is that the mean temporal profiles of avalanches of widely varying durations are quantitatively described by a single universal scaling function (data collapse). We show for the first time that this prediction is confirmed in neuronal networks. We also show that the data have three additional features predicted by critical phenomena: approximate power law distributions of avalanche sizes and durations, samples in subcritical and supercritical phases, and scaling laws between anomalous exponents. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L41.00003: Self Organized Criticality as a new paradigm of sleep regulation Plamen Ch. Ivanov, Ronny P. Bartsch Humans and animals often exhibit brief awakenings from sleep (arousals), which are traditionally viewed as random disruptions of sleep caused by external stimuli or pathologic perturbations. However, our recent findings show that arousals exhibit complex temporal organization and scale-invariant behavior, characterized by a power-law probability distribution for their durations, while sleep stage durations exhibit exponential behavior. The co-existence of both scale-invariant and exponential processes generated by a single regulatory mechanism has not been observed in physiological systems until now. Such co-existence resembles the dynamical features of non-equilibrium systems exhibiting self-organized criticality (SOC). Our empirical analysis and modeling approaches based on modern concepts from statistical physics indicate that arousals are an integral part of sleep regulation and may be necessary to maintain and regulate healthy sleep by releasing accumulated excitations in the regulatory neuronal networks, following a SOC-type temporal organization. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L41.00004: Frost nucleation, growth and propagation on a hydrophobic surface Jos\'e Guadarrama-Cetina, Anne Mongruel, Wenceslao Gonz\'alez-Vi\~nas, Daniel Beysens We report experimental results on the condensation of water vapor on a substrate (-9~$^{\circ}$C) at supercooled conditions. The resulting breath figure grows until the liquid to solid phase transition takes place. The frost seeds start to grow by deposition at the expense of neighboring supercooled water drops that evaporate. Sometimes the propagation (due to the growth of the ice) is faster than the evaporation of the drops, hence they transit to the solid state via a percolation mechanism. In this work [1], we analyze the growth of supercooled condensed drops (first stage), the growth of the ice crystals and the evolution of the supercooled water drops (intermediate and late stages). We also consider the liquid - solid front propagation (growth of the frost figure).\\[4pt] [1] J. Guadarrama-Cetina, A. Mongruel, W. Gonz\'alez-Vi\~nas, D. Beysens. In preparation [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L41.00005: Pattern formation in oscillatory fluid flows Daphne Klotsa, Michael Swift Rigid spherical particles in oscillating fluid flows form interesting structures as a result of fluid mediated interactions. Here we show that two spheres under horizontal vibration align themselves at right angles to the oscillation and sit with a gap between them, which scales in a non-classical way with the boundary layer thickness. The details of this behavior have been investigated through experiments and simulations. We then look at a collection of spherical particles, which form chains perpendicular to the direction of oscillation. Comparing experiments and simulations we study the stages of evolution from a dispersed initial configuration to an ordered chain structure. We investigate the details of the interactions and find that the nonlinear hydrodynamic effect of steady streaming is the driving force. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L41.00006: Dynamic effects induced by renormalization in anisotropic pattern forming systems Matteo Nicoli, Adrian Keller, Stefan Facsko, Rodolfo Cuerno The dynamics of patterns in large two-dimensional domains remains a challenge in nonequilibrium phenomena. Often it is addressed through mild extensions of one-dimensional equations. We show that full two-dimensional generalizations of the latter can lead to unexpected dynamic behavior. As an example we consider the anisotropic Kuramoto-Sivashinsky equation, which is a generic model of anisotropic pattern forming systems and has been derived in different instances of thin film dynamics. A rotation of a ripple pattern by 90$^{\circ}$ occurs in the system evolution when nonlinearities are strongly suppressed along one direction. This effect originates in nonlinear parameter renormalization at different rates in the two system dimensions, showing a dynamic interplay between scale invariance and wavelength selection. Potential experimental realizations of this phenomenon are identified. A. Keller, M. Nicoli, S. Facsko, and R. Cuerno, Phys. Rev. E 84, 015202(R) (2011). [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L41.00007: Self-Sustained Front Propagation in Disordered Flow Severine Atis, Harold Auradou, Dominique Salin, Laurent Talon We generate propagative fronts resulting from a balance between molecular diffusion and non-linear chemical reaction. These fronts behave as solitary waves with a constant velocity and a stationary concentration profile. The interaction between this self-sustained system and a disordered flow leads to complex structures formation. We have performed experiments of the front propagation over a wide range of stochastic flow rates, in porous media. We have determined the structure and the velocity distribution measured along the front. The concentration profile displays salient spatial features such as scaling laws and pattern formation. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L41.00008: Sidebranching in the Dendritic Crystal Growth of Ammonium Chloride Andrew Dougherty We report measurements of the dendritic crystal growth of NH$_4$Cl from supersaturated aqueous solution at small supersaturations. Sidebranch growth in this regime is challenging to model well, and the origin of the sidebranches is not fully understood. The early detection of sidebranches requires measurements of small deviations from the smooth steady state shape, but that shape is not well known at the intermediate distances relevant for sidebranch measurements. One model is that sidebranches result from the selective amplification of microscopic noise. We compare measurements of the sidebranch envelope with predictions of the noise-induced sidebranching model of Gonz\'alez-Cinca, Ram\'irez-Piscina, Casademunt, and Hern\'andez-Machado [Phys Rev. E, 63, 051602 (2001)]. We find that the measured amplitude is somewhat larger than predicted, and the shape of the sidebranch envelope is also different. A second model is that sidebranches result from small oscillations of the tip. We have observed no such oscillations, but very small ones can not be ruled out. No measurement of the tip region can be completely free of contamination from early sidebranches, so it can be challenging to distinguish between an oscillating tip and a smooth tip with sidebranches starting nearby. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L41.00009: Burning invariant manifolds in spatially disordered advection-reaction-diffusion Dylan Bargteil, Tom Solomon, John Mahoney, Kevin Mitchell We introduce burning invariant manifolds (BIMs) which act as barriers to front propagation, similar to the role played by invariant manifolds as barriers to passive transport in two-dimensional flows. We present experimental studies of BIMs in a spatially disordered, time-independent flow. We generate the flow with a magnetohydrodynamic technique that uses a DC current and a disordered pattern of permanent magnets. The velocity field is determined from this flow using particle tracking velocimetry, and reaction fronts are produced using the Ferroin-catalyzed Belousov-Zhabotinsky (BZ) chemical reaction. We use the experimental velocity field and a three-dimensional set of ODEs to predict from theory the location and orientation of BIMs. These predicted BIMs are found to match up well with the propagation barriers observed experimentally in the same flow using the BZ reaction. We explore the nature of BIMs as one-sided barriers, in contrast to invariant manifolds that act as barriers for passive transport in all directions. We also explore the role of projection singularities in the theory and how these singularities affect front behavior. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L41.00010: Chaotic advection of immiscible fluids Benjamin Vollmayr-Lee, Daniel Beller, Sohei Yasuda We consider a system of two immiscible fluids advected by a chaotic flow field. A nonequilibrium steady state arises from the competition between the coarsening of the immiscible fluids and the domain bursting caused by the chaotic flow. It has been established that the average domain size in this steady state scales as a inverse power of the Lyapunov exponent. We examine the issue of local structure and look for correlations between the local domain size and the finite-time Lyapunov exponent (FTLE) field. For a variety of chaotic flows, we consistently find the domains to be smallest in regions where the FTLE field is maximal. This raises the possibility of making universal predictions of steady-state characteristics based on Lyapunov analysis of the flow field. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L41.00011: Hamiltonian traffic dynamics in microfluidic-loop networks Raphael Jeanneret, Denis Bartolo Recent microfluidic experiments revealed that large particles advected in a fluidic loop display long-range hydrodynamic interactions. However, the consequences of such couplings on the traffic dynamics in more complex networks remain poorly understood. In this letter, we focus on the transport of a finite number of particles in one-dimensional loop networks. By combining numerical, theoretical, and experimental efforts, we evidence that this collective process offers a unique example of Hamiltonian dynamics for hydrodynamically interacting particles. In addition, we show that the asymptotic trajectories are necessarily reciprocal despite the microscopic traffic rules explicitly break the time reversal symmetry. We exploit these two remarkable properties to account for the salient features of the effective three-particle interaction induced by the exploration of fluidic loops. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L41.00012: Intensity statistics of branched flow Jakob Metzger, Ragnar Fleischmann, Theo Geisel Branched flow is a universal phenomenon of particle and wave flows which are subjected to weak, correlated disorder. It has been observed on length scales ranging from a few micrometres, affecting the transport properties of semiconductor devices [1], up to several thousand kilometres, influencing sound propagation through the ocean [2]. It is also responsible for the appearance of large and hazardous freak waves and tsunamis [3]. While the statistics of the number of such branches has recently been calculated [4], the influence on the statistics of the intensity of the waves remains an open question [5]. Here, we show how the classical ray intensity impacts on the wave intensity statistics, and illuminate the role played by the decoherence of the wavefunction.\\ {[}1{]} Topinka et al., Nature 410, 183 (2001), Jura et al., Nat. Phys. 3, 841 (2007)\\ {[}2{]} Wolfson \& Tomsovich, J. Acous. Soc. Am. 109, 2693 (2001)\\ {[}3{]} Berry, Proc. R. Soc. A 463, 3055 (2007); Heller et al., J. Geophys. Res. 113, C09023 (2008)\\ {[}4{]} Metzger, Fleischmann and Geisel, Phys. Rev. Lett. 105, 020601 (2010)\\ {[}5{]} H\"ohmann et al., Phys. Rev. Lett. 104, 093901 (2010), Arecchi et al., Phys. Rev. Lett. 106, 153901 (2011), Ying et al., Nonlinearity 24, R67 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L41.00013: Flow past a circular cylinder with momentum injection: Optimal control cylinder design Subhash Reddy, Prasad Patnaik The primary aim of this work is to suppress vortex shedding behind a circular cylinder by placing two small rotating control cylinders very close to it and hence injecting momentum into the boundary layer. The position and circulation strengths of the control cylinders are the important aspects of our study. Solving the complete Navier-Stokes (NS) equations can be time consuming while identifying the position and circulation strength of the control cylinders. Instead, reduced order models (ROM) can be used to save the computational expenditure associated with solving the complete NS model. Physics-based approaches to reduced order modeling include many of the techniques for modeling and simplification commonly used in fluid dynamics analysis such as potential flow analysis, vortex methods etc. Each of these are approximations to the full NS equations and each can serve as effective ROMs under appropriate conditions. In the present study, we try to achieve potential flow behavior by optimum positioning of the control cylinders and hence potential flow analysis is carried out with different analytical methods like F\"oppl vortex model and conformal mapping techniques. For these optimum values, the analytical solution obtained is compared with the numerical viscous flow simulations. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L41.00014: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L41.00015: Simulating Tablet Dissolution in Complex Hydrodynamic Environment with Lattice-Boltzmann Method Arpon Raksit, Ning Sun, Vadim Pozin, Dilip Gersappe Using the Lattice-Boltzmann method, we developed a 3D mesoscopic model to study the drug-dissolution process in a complex hydrodynamic environment involving spatially varying velocity and shear forces. The results showed turbulent flow in region above tablet, which was also obtained by visualization experiments. The dissolution profiles obtained by incorporating detailed kinetics showed good agreement with case studies from literature. The influence of the paddle speed and the size of the system were studied, and a multicomponent approach was also incorporated. Our results show how that the hydrodynamic environment would affect the dissolution process by changing the local concentration of components near the tablet and by the particle erosion under high fluid velocity. The code was also successfully parallelized so that the simulation of comparatively large system is now possible. [Preview Abstract] |
Session L42: Focus Session: Physics of Cancer II -- Physical Aspects
Sponsoring Units: DBIOChair: Clare Yu, University of California, Irvine
Room: 156C
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L42.00001: A Condensed Matter Physicist Looks at Cancer and the Tumor Microenvironment Invited Speaker: Clare Yu We will discuss what physics can bring to cancer biology, and give an example by taking a closer look at the tumor microenvironment. Cancer cells do not act alone. They get their cues from the their environment which consists of the extracellular matrix and the cells (fibroblasts) that form it. These cues can be both chemical and mechanical in nature. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L42.00002: Senescent cells in growing tumors Stefano Zapperi, Caterina A.M. La Porta, James P. Sethna Tumors are defined by their intense proliferation, but sometimes cancer cells turn senescent and stop replicating. In the stochastic cancer model in which all cells are tumorigenic, senescence is seen as the result of random mutatations, suggesting that it could represent a barrier to tumor growth. In the hierarchical cancer model a subset of the cells, the cancer stem cells, divide indefinitely while other cells eventually turn senescent. Here we formulate cancer growth in mathematical terms and obtain distinct predictions for the evolution of senescence in the two models. We perform experiments in human melanoma cells which confirm the predictions of the hierarchical model and show that senescence is a reversible process controlled by survivin. We conclude that enhancing senescence is unlikely to provide a useful therapeutic strategy to fight cancer, unless the cancer stem cells are specifically targeted. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L42.00003: Clustering of brain tumor cells: a first step for understanding tumor recurrence Evgeniy Khain, M.O. Nowicki, E.A. Chiocca, S.E. Lawler, C.M. Schneider-Mizell, L.M. Sander Glioblastoma tumors are highly invasive; therefore the overall prognosis of patients remains poor, despite major improvements in treatment techniques. Cancer cells detach from the inner tumor core and actively migrate away [1]; eventually these invasive cells might form clusters, which can develop to recurrent tumors. In vitro experiments in collagen gel [1] followed the clustering dynamics of different glioma cell lines. Based on the experimental data, we formulated a stochastic model for cell dynamics, which identified two mechanisms of clustering. First, there is a critical value of the strength of adhesion; above the threshold, large clusters grow from a homogeneous suspension of cells; below it, the system remains homogeneous, similarly to the ordinary phase separation. Second, when cells form a cluster, there is evidence that their proliferation rate increases. We confirmed the theoretical predictions in a separate cell migration experiment on a substrate and found that both mechanisms are crucial for cluster formation and growth [2]. In addition to their medical importance, these phenomena present exciting examples of pattern formation and collective cell behavior in intrinsically non-equilibrium systems [3]. \\[4pt] [1] A. M. Stein et al, Biophys. J., 92, 356 (2007). \\[0pt] [2] E. Khain et al, EPL 88, 28006 (2009). \\[0pt] [3] E. Khain et al, Phys. Rev. E. 83, 031920 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L42.00004: Targeting tumor acidity Yana K. Reshetnyak, Donald M. Engelman, Oleg A. Andreev One of the main features of solid tumors is extracellular acidity, which correlates with tumor aggressiveness and metastatic potential. We introduced novel approach in targeting of acidic tumors, and translocation of cell-impermeable cargo molecules across cellular membrane. Our approach is based on main principle of insertion and folding of a polypeptide in lipid bilayer of membrane. We have identified family of pH Low Insertion Peptides (pHLIPs), which are capable spontaneous insertion and folding in membrane at mild acidic conditions. The affinity of peptides of pHLIP family to membrane at low pH is several times higher than at neutral pH. The process of peptides folding occurs within milliseconds. The energy released in a result of folding (about 2 kcal/mol) could be used to move polar cargo across a membrane, which is a novel concept in drug delivery. pHLIP peptides could be considered as a pH-sensitive single peptide molecular transporters and conjugated with imaging probes for fluorescence, MR, PET and SPECT imaging, they represent a novel in vivo marker of acidity. The work is supported by NIH grants CA133890 and GM073857 to OAA, DME, YRK. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L42.00005: Stress Modulus of Cancer Cells Keith Bonin, Martin Guthold, Xinyi Guo, Justin Sigley Our main goal is to study the different physical and mechanical properties of cells as they advance through different stages of neoplastic transformation from normal to the metastatic state. Since recent reports indicate there is significant ambiguity about how these properties change for different cancer cells, we plan to measure these properties for a single line of cells, and to determine whether the changes vary for different cellular components: i.e. whether the change in physical properties is due to a change in the cytoskeleton, the cell membrane, the cytoplasm, or a combination of these elements. Here we expect to present data on the stress modulus of cancer cells at different stages: normal, mortal cancerous, immortal cancerous, and tumorigenic. The cells are Weinberg cell line Human Mammary Epithelial (HME) cells. Atomic force microscope (AFM) probes with different diameters are used to push on the cell membrane to measure the local, regional and global cell stress modulus. Preliminary results on normal HME cells suggests a stress modulus of 1.5 $\pm$ 0.8 kPa when pushing with 7 $\mu$m spherical probes. We anticipate reporting an improved value for the modulus as well as results for some of the Weinberg cancer cells. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L42.00006: Cancer-stroma evolutionary dynamics in stress-gradient microenvironment Amy Wu, Guillaume Lambert, Robert Austin, James Sturm, Zayar Khin, Ariosto Silva In order to study the evolution of drug resistance in cancer, it is important to mimic the tumor microenvironment, in which cells are exposed to not uniform concentrations but rather gradients of drugs, nutrients, and other factors Compared to traditional in-vitro methods, microfluidic structure enables better control of the temporal and spatial profile of gradients. Here we demonstrate a microfluidic Doxorubicin gradient environment with heterogeneous landscape, and culture multiple myeloma (8226-S, expressing RFP) and bone marrow stroma (HS-5, expressing GFP) cell lines together. The myeloma cells are not directly motile, but they are able to migrate via the adhesion to motile stroma cells. The indirect motility mechanism of the myeloma cells is crucial for the adaptation to stress environment. Finally, we will report the co-culture dynamics under the stress of doxorubicin gradients, observing for cellular migrations and growth [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L42.00007: Dynamic density functional theory of solid tumor growth: Preliminary models John Lowengrub Cancer is a complex system whose dynamics and growth result from nonlinear processes coupled across wide ranges of spatio-temporal scales. The current mathematical modeling literature addresses issues at various scales but the development of theoretical methodologies capable of bridging gaps across scales needs further study. We present a new theoretical framework based on Dynamic Density Functional Theory (DDFT) extended, for the first time, to the dynamics of living tissues by accounting for cell density correlations, different cell types, phenotypes and cell birth/death processes, in order to provide a biophysically consistent description of processes across the scales. We present an application of this approach to tumor growth. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L42.00008: The physics of Eukaryotic cell crawling through elastic media Elnaz Alipour Baum-Snow, Charles Wolgemuth Understanding the motion of cells through deformable media, such as the extra-cellular matrix (ECM), is important for understanding many biological processes, such as cancer metastasis, wound healing, and organismal development. Crawling eukaryotic cells exert dipole-distributed traction stresses on the external environment. These stress distributions pull backwards at the front of the cell and forward at the rear of the cell. Recent experiments have shown the magnitude of the deformations induced in a collagen matrix by migrating cancer cells. We propose a model to understand cell movements through the ECM, by considering a dipole-crawler moving through an isotropic, linear elastic medium. This model captures the major features of the deformations that are induced by motile cancer cells in collagen. In addition, the model suggests that the deformations that are induced in the matrix can provide a mechanism by which distal cells can interact with one another through matrix-mediated interactions. We, therefore, study the forces, torques, and trajectories of two cells migrating through the ECM. Our analysis suggests a mechanism that may be relevant for the collective migration of cells during cancer metastasis and other processes where numerous cells move through the ECM. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L42.00009: Monte Carlo Simulations for Radiobiology Nicole Ackerman, Magdalena Bazalova, Kevin Chang, Edward Graves The relationship between tumor response and radiation is currently modeled as dose, quantified on the mm or cm scale through measurement or simulation. This does not take into account modern knowledge of cancer, including tissue heterogeneities and repair mechanisms. We perform Monte Carlo simulations utilizing Geant4 to model radiation treatment on a cellular scale. Biological measurements are correlated to simulated results, primarily the energy deposit in nuclear volumes. One application is modeling dose enhancement through the use of high-Z materials, such gold nanoparticles. The model matches in vitro data and predicts dose enhancement ratios for a variety of in vivo scenarios. This model shows promise for both treatment design and furthering our understanding of radiobiology. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L42.00010: A two-phase mixture model of avascular tumor growth Deniz Ozturk, M. Burcin Unlu, Sirin Yonucu, Ugur Cetiner Interactions with biological environment surrounding a growing tumor have major influence on tumor invasion. By recognizing that mechanical behavior of tumor cells could be described by biophysical laws, the research on physical oncology aims to investigate the inner workings of cancer invasion. In this study, we introduce a mathematical model of avascular tumor growth using the continuum theory of mixtures. Mechanical behavior of the tumor and physical interactions between the tumor and host tissue are represented by biophysically founded relationships. In this model, a solid tumor is embedded in inviscid interstitial fluid. The tumor has viscous mechanical properties. Interstitial fluid exhibits properties of flow through porous medium. Associated with the mixture saturation constraint, we introduce a Lagrange multiplier which represents hydrostatic pressure of the interstitial fluid. We solved the equations using Finite Element Method in two-dimensions. As a result, we have introduced a two-phase mixture model of avascular tumor growth that provided a flexible mathematical framework to include cells' response to mechanical aspects of the tumor microenvironment. The model could be extended to capture tumor-ECM interactions which would have profound influence on tumor invasion. [Preview Abstract] |
Session L43: Invited Session: Precursors to the Folding and Aggregation of Biological Molecules
Sponsoring Units: DBIO DPOLYChair: Peter G. Vekilov, University of Houston and Vassiliy Lubchenko, University of Houston
Room: 157AB
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L43.00001: Postranslational modifications significantly alter the binding-folding pathways of proteins associating with DNA Invited Speaker: Garegin Papoian Many important regulators of gene activity are natively disordered, but fully or partially order when they bind to their targets on DNA. Interestingly, the ensembles of disordered states for such free proteins are not structurally featureless, but can qualitatively differ from protein to protein. In particular, in random coil like states the chains are swollen, making relatively few contacts, while in molten globule like states a significant collapse occurs, with ensuing high density of intra-protein interactions. Furthermore, since many DNA binding proteins are positively charged polyelectrolytes, the electrostatic self-repulsion also influences the degree of collapse of the chain and its conformational preferences in the free state and upon binding to DNA. In our work, we have found that the nature of the natively disordered ensemble significantly affects the way the protein folds upon binding to DNA. In particular, we showed that posttranslational modifications of amino acid residues, such as lysine acetylation, can alter the degree of collapse and conformational preferences for a free protein, and also profoundly impact the binding affinity and pathways for the protein DNA association. These trends will be discussed in the context of DNA interacting with various histone tails and the p53 protein. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:42PM |
L43.00002: Mesoscopic Aggregation in Protein Solutions Invited Speaker: Vassiliy Lubchenko Long-lived mesoscopic clusters of a dense protein liquid are observed in concentrated solutions of numerous proteins. These clusters are a necessary kinetic intermediate for the formation of solid aggregates of native and misfolded protein molecules. We propose a novel physicochemical mechanism, by which the clusters consist of an off-equilibrium mixture of single protein molecules and long-lived protein-containing complexes. The puzzling mesoscopic size of the clusters is determined by the lifetime of the complexes and their diffusivity. We have predicted and observed a number of interesting kinetic and thermodynamic behaviors that are associated with the mesoscopic clusters. These behaviors include: (a) Ostwald-like ripening of the clusters (b) a crossover to long-range density fluctuations at high concentrations; (c) a universal, diffusion-like scaling of the autocorrelation function of light scattered off the protein solution; (d) non-trivial dependencies of the cluster size and volume fraction on the protein concentration in the solution. Our analysis of the anisotropic Coulomb interactions suggests for mesoscopic clusters to form in lysozyme solutions, protein molecules must undergo conformational changes. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L43.00003: Protein misfolding and aggregation Invited Speaker: Dave Thirumalai |
Tuesday, February 28, 2012 4:18PM - 4:54PM |
L43.00004: The impact of conformational transformations on the pathways and kinetics of protein self-assembly into extended matrices Invited Speaker: Jim De Yoreo The concept of a folding funnel with kinetic traps is used to describe folding of individual proteins. Using in situ AFM to investigate both collagen fibril and S-layer membrane assembly on mica, we show this concept is equally valid during self-assembly of proteins into extended matrices. Moreover, considering the conformational changes required to achieve the ordered structure is critical to understanding the pathway to the final state and the kinetics of assembly. In the collagen system, both the pathway and the final conformational state can be finely tuned by varying the ionic strength. This alters the relative strengths of the collagen-collagen and collagen-mica binding free energy, which we quantify using dynamic force spectroscopy. Moreover, when conditions result in the ordered D-band structure of collagen, the emergence of order catalyzes the further transformation leading to non-linear attachment kinetics. In the S-layer system, there is a kinetic trap associated with conformational differences between a long-lived transient state and the final stable state. Both ordered tetrameric states emerge from clusters of an amorphous precursor phase, however, they then track along two different pathways. One leads directly to the final low-energy state and the other to the kinetic trap. Over time, the trapped state transforms into the stable state. By analyzing the time and temperature dependencies of formation and transformation we find the energy barriers to formation of either state to be nearly identical, but once the high-energy state forms, the barrier to transformation to the low-energy state is large. Thus the transient state exhibits the characteristics of a kinetic trap in a folding funnel. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:30PM |
L43.00005: The role of proteins and peptides in shaping the structure and microstructure of biogenic and biomimetic crystals Invited Speaker: Boaz Pokroy In the course of biomineralization organisms form crystals which demonstrate superior characteristics as compared to their non-biogenic counterparts. One of the main reasons for this is that these crystals are actually hybrid nano-composites. In fact, each biogenic single crystal is not only encapsulated by an intercrystalline organic phase but in addition there are intracrystalline proteins within each individual single crystal. These proteins are the key players in the formation of biominerals and are vital in the precursor phase of their formation. I will show that both in biogenic and biomimetic crystals, these proteins have a major effect on their atomic structure and microstructure. This effect is in the form of lattice distortions which relax upon mild annealing. The distortions can be detected by means of high-resolution synchrotron diffraction. The activation energy of the relaxation of the lattice distortions is rather low and is comparable to the energy needed for protein unfolding. Furthermore, it will be shown that these intracrystalline proteins can stabilize metastable phases of calcium carbonate and even stabilize a previously unidentified twin law in calcite. [Preview Abstract] |
Session L44: Focus Session: Nano to Mesoscale Structure in Ordered Soft Matter: Liquid Crystal Phases
Sponsoring Units: DPOLYChair: Marina Ruths, University of Massachusetts Lowell
Room: 157C
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L44.00001: DILLON MEDAL BREAK
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Tuesday, February 28, 2012 3:06PM - 3:42PM |
L44.00002: Nano to Meso-scale Structure in Liquid Crystals: the Cybotactic Nematic Phase of Bent-core Mesogens Invited Speaker: Oriano Francescangeli The extent of molecular order and the resulting broken symmetry determine the properties and mesophase type of liquid crystals (LCs). Thermotropic bent-core mesogens (BCMs) represent a new class of LCs exhibiting substantially different physical properties than traditional linear (calamitic) materials. In recent years BCMs have become the focus of intense experimental and theoretical investigation, with several exciting new developments. These include chiral mesophases composed of achiral BCMs, giant flexoelectricity, biaxial nematic ($N)$ order, a ferroelectric response in the $N$ phase, and a large flow birefringence. A key issue that is currently widely debated concerns the actual nature of the $N$ phase of BCMs which gives rise to some of the above mentioned effects and is unambiguously identified by a peculiar low-angle X-ray diffraction pattern (the ``four-spot pattern''). The consensus emerging is that this $N$ phase of BCMs constitutes a new type of mesophase, namely, a cybotactic nematic ($N_{cyb})$ phase unrelated to pretransition cybotaxis, in agreement with experimental [1-3] and theoretical findings [4]. This $N_{cyb}$ phase is composed of nanometer-size clusters of BCMs exhibiting a relatively high degree of internal order---orientational as well as translational order (strata) imposed by close packing the BCM nonlinear shape. This peculiar supramolecular structure of the $N_{cyb}$ mesophase of BCMs---evanescent, biaxial clusters of tilted and stratified nonlinear mesogens percolating the nematic fluid---accounts for their unusual properties, e.g., biaxial order [4], ferroelectric response [1], and extraordinary field-induced effects [5]. In this talk I will give an overview of the most recent developments and the current state of research on this subject. \\[4pt] [1] O. Francescangeli \textit{et al}., Adv. Funct. Mater. \textbf{19},2592 (2009). \\[0pt] [2] O. Francescangeli and E.T. Samulski, Soft Matter \textbf{6}, 2413 (2010) \\[0pt] [3] O. Francescangeli \textit{et al}., Soft Matter \textbf{7}, 895 (2011). \\[0pt] [4] A.G. Vanakaras and D.J.Photinos, J. Chem. Phys. \textbf{128}, 154512 (2008). \\[0pt] [5] O. Francescangeli \textit{et al}., Phys. Rev. Lett. \textbf{107}, 207801 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L44.00003: Nanoscale Positional Order Correlations: Swarms, Cybotactic Groups, Clusters, and Pretransitional Fluctuations in Liquid Crystals Satyendra Kumar, Dena Agra-Kooijman, Bharat Acharya Short-range molecular associations in organic liquids were first described as ``cybotactic'' groups [1] followed by the development of the swarm theory [2] to explain the structure, strong light scattering, and flow behavior of the nematic (N) liquid crystal phase. However, these ideas became inconsequential with the advent of the Oseen-Frank's continuum theory [3]. In 1970, de Vries reinvoked \textit{cybotactic} groups for the N phase of bis-(4'-n-octyloxybenzal)-2-chloro-l,4-phenylenediamine. These were eventually understood to be SmC pretransitional fluctuations, i.e., small correlated regions of the lower symmetry phase near the transition. Thermotropic biaxial mesophases have resurrected the faith in \textit{cybotacticity} in the guise of a new word - ``clusters''. Previous x-ray studies of normal organic fluids, and calamitic, lyotropic, and bent-core mesogens show that these clusters fall into three groups depending on the relative contributions of normal liquid structure and pretransitional fluctuations. A comparison with other organic and inorganic fluids will also be made.\\[4pt] [1] G.W. Stewart, Phys. Rev. \textbf{35}, 726 (1930).\\[0pt] [2] L.S. Ornestein and W. Kast, Trans. Farad. Soc. \textbf{29}, 931 (1933).\\[0pt] [3] FC Frank, Discuss. Faraday Soc. \textbf{25}, 19 (1958); W. Oseen, Ark. Mat., Astron. Fys. \textbf{19}, 1 (1925). [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L44.00004: Randomized Grain Boundary Liquid Crystal Phase D. Chen, H. Wang, M. Li, M. Glaser, J. Maclennan, N. Clark The formation of macroscopic, chiral domains, in the B4 and dark conglomerate phases, for example, is a feature of bent-core liquid crystals resulting from the interplay of chirality, molecular bend and molecular tilt. We report a new, chiral phase observed in a hockey stick-like liquid crystal molecule. This phase appears below a smectic A phase and cools to a crystal phase. TEM images of the free surface of the chiral phase show hundreds of randomly oriented smectic blocks several hundred nanometers in size, similar to those seen in the twist grain boundary (TGB) phase. However, in contrast to the TGB phase, these blocks are randomly oriented. The characteristic defects in this phase are revealed by freeze-fracture TEM images. We will show how these defects mediate the randomized orientation and discuss the intrinsic mechanism driving the formation of this phase. This work is supported by NSF MRSEC Grant DMR0820579 and NSF Grant DMR0606528. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L44.00005: Resonant X-ray Scattering Studies of Smectic and Columnar Bent-Core Liquid Crystal Phases Ronald Pindak, Philippe Barois, Virginie Ponsinet, Cesar Folcia, Josu Ortega, LiDong Pan, Shun Wang, Suntao Wang, Cheng-Cher Huang Resonant X-ray scattering provides a direct probe of orientational structures in liquid crystals with periodicities that range from molecular dimensions (0.1 nm) to dimensions that can be observed with visible light (1.0 micron). We have recently applied this technique to study the orientational ordering of bent-core molecules in the smectic B$_{2}$ phase and the columnar B$_{1}$ phase. Using resonant scattering ``forbidden'' reflections due to glide or screw symmetry elements can be measured and an analysis of their polarization state enabled us to identify a chiral anticlinic antiferroelecrtic B$_{2}$ phase (Smectic C$_{A}$P$_{A}$) coexisting with an achiral synclinic antiferroelectric B$_{2}$ phase (Smectic C$_{S}$P$_{A}$) [1]. We were also able to determine the structure of a columnar B$_{1}$ phase and study the transition mechanism between the B$_{1}$ and B$_{2}$ phases [2]. \\[4pt] [1] V. Ponsinet, et al., Phys. Rev. E 84, 011706 (2011).\\[0pt] [2] C. Folcia, et al., Phys. Rev. E 84, 010701R (2011). [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L44.00006: Polyhedral Liquid Crystalline Vesicles Mark Bowick Vesicles with internal liquid-crystalline order can assume a variety of shapes depending on the ratio of the Frank moduli to the bending rigidity. Using both analytic and numerical tools one can we show that the possible low free energy morphologies include nano-fibers, faceted tetrahedral vesicles, ellipsoidal vesicles and cylindrical vesicles. The tetrahedral vesicle is a particularly fascinating example of a faceted liquid-crystalline membrane. Faceted liquid vesicles may lead to the design of supra-molecular structures with tetrahedral symmetry and new classes of nano-carriers. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L44.00007: Packing Cheerios: Simulation studies of torus-shaped hard particles Andrew Konya, Robin Selinger We perform simulation studies of hard torus-shaped particles under compression or in granular flow. A major challenge in performing simulations of non-spherical hard particles is determination of possible overlap between particle pairs. To simplify this calculation, we model a torus particle as an assembly of overlapping hard spheres arranged in a ring, and implement GPU acceleration to create an efficient Monte Carlo algorithm. For particles shaped approximately like Cheerios, with major radius $R$=1 and minor radius $r$=0.6, the hexagonal columnar crystal structure has packing fraction of about 2/3, but---as easy to observe in your cereal box---the system is easily trapped in a glassy disordered state. Preliminary simulation studies of the disordered state formed via rapid compression show short-range orientational correlations in which neighboring particles are either parallel or at right angles. We also examine structures that form when particles rain down onto a flat surface. Results are compared with the known liquid crystal phases of oblate ellipsoids and experiments on discotic colloidal phases. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L44.00008: Effect of CNTs and Induced Chirality on Smectic- Smectic Liquid Crystal Phase Transitions P. Kalakonda, G.S. Iannacchione, R. Basu, C. Rosenblatt, R.P. Lemieux High-resolution calorimetry results are presented of carbon nanotubes (CNTs) and the liquid crystal (LC) 9OO4 nano-colloidal dispersions as a function of temperature, scan rate, and CNT concentration ($0$, $0.025$, $0.05$, $0.20$ wt/$\%$). The CNT used have an enantiomeric excess that has been shown to induce chirality into this LC. The pure LC exhibits the phase sequence $I$-$N$-Sm$A$-Sm$C$-Sm$B$-$Cr$ on cooling with the expected heat capacity $C_p$ signatures, except for the Sm$A$-Sm$C$ transition, manifesting a double-$C_p$ peak $\sim 2$~K apart at low effective scan rates ($< 0.5$~K~min$^{-1}$). The introduction of CNTs results in the $I$-$N$, $N$-Sm$A$, and Sm$A$-Sm$C$ double $C_p$ features shifting to higher temperatures by $\sim 1$~K and remain sharp. However, the Sm$C$-Sm$B$ and Sm$B$-$Cr$ transitions shift to lower temperatures by $\sim 3-4$~K and broaden dramatically with increasing CNT content. We interpret these observations as a consequence of the $\pi$-$\pi$ interactions between the phenyl rings of 9OO4 and the graphene surfaces that induces bulk chirality, and the pinning of the director parallel to the CNT long-axis far from the surface. The balance of these two mechanisms may stabilize phases that lack any in-smectic-plane ordering. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L44.00009: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L44.00010: Mesophase behavior and effect of polydispersity in assemblies of polyhedral particles Fernando Escobedo, Umang Agarwal Mesophase behavior of polyhedral particles is uniquely linked to the inherent interactions embedded in their geometrical shape. A two-parameter model based on particle shape anisotropy and order of symmetry has been proposed for predicting phase behavior of polyhedral particles. The focus of the current work is to explore the phase behavior of a distinct class of polyhedral shapes, which emerge at different growth stages of PbSe nanocrystal formation. The~body of knowledge~that is emerging from these studies~may prove useful in designing optimal self-assembly strategies for many desired nanostructures; e.g., as~in our ongoing efforts in understanding the nanocrystal~superlattice~formation for solar cell applications. Moreover the effect of particle size polydispersity is explored by simulating the assembly of two representative shapes exhibiting totally different mesophase behavior. It is found that while mesophases are quite resilient to particle size anisotropy, the ordered structures are a complex function of the polydispersity and geometric attributes. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L44.00011: Submolecular organization of de Vries smectics with tuned frustration between SmA- and SmC- promoting elements Dena Mae Agra-Kooijman, S Dey, Q. Song, D. Nonnenmacher, F. Giesselmann, R. Lemieux, Satyendra Kumar Structure-property relationships and the SmA-SmC (AC) transition were investigated with x-ray diffraction in de Vries smectics compounds with tuned frustration between SmA and SmC promoting elements in the molecules. These are isometric analogues of a compound with a 2-phenylpyrimidine core that combines a trisiloxane-terminated alkoxy side-chain with a chloro-terminated alkoxy side-chain. The results reveal the local molecular structure in which, both siloxane and hydrocarbon segments are segregated and oriented parallel to the director in the SmA phase. But the siloxane segments oriented at an angle ($\sim $14$^{\circ})$ different from the remaining hydrocarbon part of the molecule. This provides the first direct evidence of a kinked molecular conformation and nano-segregation of the molecule in the SmC phase. The two parts of the molecule possess different orientational order, siloxane part being more disordered, in both phases. The rate of change of the tilt angle with temperature appears to be different in the three compounds investigated. [Preview Abstract] |
Session L45: Friction and Brushes
Sponsoring Units: DPOLYChair: Mark Stoykovich, University of Colorado
Room: 159
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L45.00001: DILLON MEDAL BREAK
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Tuesday, February 28, 2012 3:06PM - 3:18PM |
L45.00002: Velcro$^{\mbox{{\textregistered}}}$ as a Mesoscopic Model System for Stick-Slip Motion Lisa Mariani, Cara Esposito, Piotr Habdas, Paul Angiolillo The Amontons-Coulomb (AC) laws of friction, established during the 18$^{th}$ Century, serve to explain many of the phenomenological observations of friction in the macroscopic world. The AC laws for friction do not adequately explain certain systems, which undergo stick-slip motion, however. The hook-and-loop system (Velcro), in particular, exhibits easily observed stick-slip motion. Velcro evinces clear evidence of stick-slip dynamics that is independent of sliding velocity in accordance with Coulomb but, the maximum static friction force $F_s^{\max } $ and kinetic friction $F_k $ are keenly dependent on ``area of contact'' (hook number) in contrast to accepted law, but consistent with recent studies of frictional dynamics in nanoscopic systems. Both the $F_s^{\max } $ and $F_k $ as a function of area follow power law dependences with an exponent of approximately 2/3. Moreover, the fluctuations of the kinetic friction $F_k $ also follow a power law dependence with an exponent of approximately 1/2 in accordance with random walk theory. On the other hand, the $F_s^{\max } $ and $F_k $ both follow a linear dependence with applied load in accordance with the classical theory of AC. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L45.00003: The Use of Microscale Geometry to Tailor Stimulus-Responsive Surface Friction Lin Han, Jie Yin, Lifeng Wang, Khek-Khiang Chia, Robert Cohen, Michael Rubner, Mary Boyce, Christine Ortiz The capability to tailor stimulus-responsive surface friction, including sensitivity profile, range, temporal response and deformation mechanisms, holds great potential for an array of engineering and biomedical applications. In this study, the pH-dependent friction of layer-by-layer assemblies of poly(allylamine hydrochloride) and poly(acrylic acid) (PAH/PAA) were quantified for structures of a continuous planar film and anisotropic microtube forests via lateral force microscopy. By comparing experiments to microstructure-specific finite element modeling, a mechanistic change from surface adhesion-dominated friction ($\mu $=0.11) to viscoelasticity-governed shear (=0.017) was predicted upon ionic crosslink density reduction of PAH/PAA from pH 5.5 to 2.0 for the film (6.5$\times $ decrease). The responsiveness of $\mu $ was further tuned by the tube forest geometry to be 3.5$\times $. At pH 5.5, $\mu $ (=0.094) was lower than the film due to discrete tube bending/buckling and smaller tip-sample interface stress. At pH 2.0, $\mu $ (=0.027) was higher because of inter-tube contact and weaker substrate effect. This study provides an excellent platform to quantitatively access and design dynamic substrates with tailorable stimulus-responsive surface friction. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L45.00004: Viscous friction of hydrogen-bonded matter Aykut Erbas, Dominik Horinek, Roland R. Netz Amontons' law successfully describes friction between macroscopic solid bodies for a wide range of velocities and normal forces. For the diffusion and forced sliding of adhering or entangled macromolecules, proteins and biological complexes, temperature effects are invariably important and a similarly successful friction law at biological length and velocity scales is missing. Hydrogen bonds are key to the specific binding of bio-matter. Here we show that friction between hydrogen-bonded matter obeys in the biologically relevant low-velocity viscous regime a simple equations: the friction force is proportional to the number of hydrogen bonds, the sliding velocity, and a friction coefficient $\gamma_{\rm HB}$. This law is deduced from atomistic molecular dynamics simulations for short peptide chains that are laterally pulled over hydroxylated substrates in the presence of water and holds for widely different peptides, surface polarities and applied normal forces. The value of $\gamma_{\rm HB}$ is extrapolated from simulations at sliding velocities in the range from $v=10^{-2}$ m/s to 100 m/s by mapping on a simple stochastic model and turns out to be of the order of $\gamma_{\rm HB} \simeq 10^{-8}$ kg/s. 3 hydrogen bonds act collectively. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L45.00005: Molecular mechanisms of friction at soft polymer interfaces Frederic Restagno, Celine Cohen, Christophe Poulard, Liliane Leger Polymer molecules strongly anchored to a solid substrate and interdigitated into bulk crosslinked elastomer have been shown recently to efficiently promote adhesion and friction between substrate and elastomer. Concerning friction, the regime of low surface coverage in surface anchored chains has been fully and quantitatively accounted for by the pull off mechanisms, where individual chains are dynamically extracted from the elastomer. Then, the stretching energy of these chains dominates the friction losses. We focus here on the dense surface coverage regime. We present systematic experiments performed on the polydimethylsiloxane (PDMS) - silica system, and determine molecular weight and sliding velocity dependences of the friction stress. We show that the friction is dominated by the shear thinning of the grafted layer confined between the elastomer and the substrate, and responding to the shear solicitation like a melt, but with very long relaxation times. We also show that the friction stress appears highly sensitive to the molecular organization inside the surface anchored polymer layer, comparing end grafted and strongly adsorbed layers having otherwise the same molecular characteristics (molecular weight of the chains, and thickness of the surface anchored layer). [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L45.00006: Frictional properties of the end-grafted polymer layer in presence of salt solution Maryam Raftari, Zhenyu Zhang, Graham J. Leggett, Mark Geoghegan We have studied the frictional behaviour of grafted poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) films using friction force microscopy (FFM). The films were prepared on native oxide-terminated silicon substrates using the technique of atom transfer radical polymerization (ATRP). These brushes had constant grafting density (1.18 nm2), and of a thickness of $\sim$66 nm, as measured by ellipsometry. We show that single asperity contact mechanics (Johnson-Kendall-Roberts (JKR) and Derjaguin-M\"uller-Toporov (DMT) models) as well as a linear (Amontons) relation between applied load and frictional load all apply to these systems depending on the concentration of salt and the nature of the FFM probe. Measurements were made using gold-coating and polymer functionalized silicon nitride triangular probes. Polymer functionalized probe included growth the PDMAEMA with same method on tips. The frictional behaviour are investigated between PDMAEMA and gold coated and PDMAEMA tips immersed in different concentrations of KCl, KBr and KI. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L45.00007: Nanofluidity of Polymer Melt: Two velocity hydrodynamics Sergei Obukhov, Albert Johner According to the traditional macroscopic theory, the flow of the entangled polymer melt is associated with the relaxation processes in the network of entanglements. Each entanglement is formed by polymer chains, and these chains are moving randomly along their reputation tubes. Because of this random motion of polymers each entanglement has a finite lifetime. The lifetime is equal to the time needed for a polymer to leave its original tube. We should point out, that the motion of individual polymer chain in a tube is assumed to be completely random (diffusive). It means, that according to traditional theory, in average there is no systematic displacement of polymers with respect to the network of entanglements. The local polymer flow velocity in this theory is just the instant local velocity of individual entanglements forming this network. But the drift of individual polymers provides additional mechanism of a flow of a polymer melt, which becomes dominant at small scale. We suggest two velocity hydrodynamic equations which describe combined contribution of these two mechanisms. For illustration of this method we solve the problem of mobility of a small particle in a polymer melt. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L45.00008: Poly(ethylene oxide) Chains Are Not ``Hydrophilic'' When They Exist As Polymer Brush Chains Hoyoung Lee, Dae Hwan Kim, Kevin N. Witte, Kimberly Ohn, Je Choi, Kyungil Kim, Mati Meron, Binhua Lin, Bulent Akgun, Sushil Satija, You-Yeon Won By using a combined experimental and theoretical approach, a model poly(ethylene oxide) (PEO) brush system, prepared by spreading a poly(ethylene oxide)-poly($n$-butyl acrylate) (PEO-PnBA) amphiphilic diblock copolymer onto an air-water interface, was investigated. The polymer segment density profiles of the PEO brush in the direction normal to the air-water interface under various grafting density conditions were determined from combined X-ray and neutron reflectivity data. In order to achieve a theoretically sound analysis of the reflectivity data, we developed a new data analysis method that uses the self-consistent field theoretical modeling as a tool for predicting expected reflectivity results for comparison with the experimental data. Using this new data analysis method, we discovered that the effective Flory-Huggins interaction parameter of the PEO brush chains is significantly greater than that corresponding to the theta condition, suggesting that contrary to what is more commonly observed for PEO in normal situations, the PEO chains are actually not ``hydrophilic'' when they exist as polymer brush chains, because of the many body interactions forced to be effective in the brush situation. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L45.00009: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L45.00010: Tunable Surface Properties from Bioinspired Polymers Wendy van Zoelen, Adrianne M. Rosales, Ronald N. Zuckermann, Rachel A. Segalman Tunability of surface properties is of importance for a variety of coating applications, including antifouling coatings. We have investigated the surface properties of polypeptoids, a class of non-natural biomimetic polymers based on an N-substituted glycine backbone, that combine many of the advantageous properties of bulk polymers with those of synthetically produced proteins, including controllable chain shape, sequence, and self-assembled structure. We demonstrate the influence of the amount and sequence of hydrophobic monomers in a predominantly hydrophilic peptoid chain on surface properties. Especially the surface reconstruction behavior of block copolymers of these amphiphilic polypeptoids with polystyrene upon contact with water will be addressed. It has been found that surface reconstruction of peptoid chains that contain a sequence of only three fluorinated monomers and up to forty-two hydrophilic monomers occurs within seconds, whereas reorganization of surfaces containing five fluorinated monomers was an order of magnitude slower. Surfaces with higher fluorine content also showed lower settlement of spores of the green algae Ulva. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L45.00011: Concentration Fluctuations of a Semidilute Polymer Solution in Good Solvent Near a Repulsive Surface Catherine Yeh, Philip Pincus, Alexandra Zidovska The concentration profile of a semidilute polymer solution in good solvent near a repulsive surface has been previously calculated.\footnote{J.~F.~Joanny, L.~Leibler, P.-G.~de Gennes, J. Polym. Sci. 17, 1073 (1979)} In this work we consider fluctuation corrections to the mean field concentration profile in the presence of a repulsive surface using the Cahn-Hilliard square-gradient approach extended to polymer interfaces. Our results predict that at strongly repulsive surfaces, a polymer in good solvent exhibits concentration fluctuations associated with the surface in addition to fluctuations of the bulk polymer solution. We compare our predictions with current experiments which have measured fluctuations in the concentration of interphase chromatin (DNA with its associated proteins) inside the nucleus of mammalian cells \textit{in vivo} using ultrafast high space resolution spinning disc confocal microscopy. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L45.00012: Kinetics of Polymer Interfacial Reaction Shuo Zhang, Jeffrey Koberstein Germanium crystals modified with high quality azide functional monolayers are used to directly monitor in situ the kinetics of interfacial ``click'' reactions with complementary alkyne end-functional poly(n-butyl acrylate) (PnBA) and polystyrene (PS) by attenuated total reflectance infrared spectroscopy (ATR-IR). In the presence of copper (I), the azide-modified Ge substrates react quantitatively with PnBA and PS via a 1,3-dipolar cycloaddition reaction. Time-resolved ATR-IR measurements show two regimes of kinetic behavior, as predicted by theory. In the first regime the rate is rapid and is controlled by diffusion of the polymer through the solvent, scaling with the square root of time. The rate slows considerably in the second regime, limited by penetration of the reacting polymer through the covalently bound polymer brush layer, scaling with the natural logarithm of time. The influence of polymer size and solvent quality are reported. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L45.00013: Influence of Substrate Surface Energy on the Morphological Evolution of PS/PMMA Blend Thin Films Yifu Ding, Dae-Up Ahn, Zhen Wang, Ian Campell, Mark Stoykovich In this study, the morphological evolutions of PS/PMMA blend thin films are systematically characterized during thermal annealing on both preferential and non-preferential surfaces. The evolution of phases on preferential surfaces was dictated by the preferential wetting of the components, and the coarsening process of the PMMA domains. PS droplets with high spatial correlation were formed in a moderately asymmetric PS/PMMA blend because of PS dewetting via a controlled nucleation mechanism. In contrast, a more asymmetric blend evolved into PS droplets via a random nucleation process, such that the PS droplets did not exhibit spatial correlation and had a broad size distribution. The morphological evolution of the blends on relatively non-preferential surfaces was also dictated by domain coarsening, but proceeds without the formation of PMMA wetting layer. As a result, a diverse set of non-equilibrium, micro- and nanoscale morphologies were observed. [Preview Abstract] |
Session L46: Dillon Medal Symposium
Sponsoring Units: DPOLYChair: Frank Bates, University of Minnesota
Room: 160ABC
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L46.00001: John H. Dillon Medal Lecture: Self-assembly of rod-coil block polymers Invited Speaker: Rachel Segalman While the self-assembly of coil-like block copolymers into morphologies with well-defined nanometer-sized periodicities is now classic, the translation of this strategy to functional polymers with non-Gaussian chain shapes is significantly more complicated. Control and predictive understanding of nanoscale structure in rod-coil block copolymer systems are of particular importance in both conjugated polymers (for optoelectronic devices) and biomimetic polymers. Our work is targeted at gaining a fundamental understanding of the thermodynamics and kinetics of self-assembling functional rod-coil block copolymer systems, and then applying this to conjugated polymers to understand structure-property relationships in plastic electronics. We demonstrate that phase space for weakly segregated model systems is described by the Flory-Huggins block interaction parameter (\textit{$\chi $N}), the Maier-Saup\'{e} parameter (\textit{$\mu $N}) (which parameterizes rod-rod interactions and also scales with temperature), the coil volume fraction (\textit{$\varphi $}$_{coil})$, and a geometrical asymmetry term (\textit{$\nu $}) to account for aspect ratio differences in the rod and coil. Further, we also find that careful molecular design to moderate molecular interactions is essential in creating controllable systems. In particular, both sidechain substitution and sequence control can be used to control melting temperatures and liquid crystalline interactions in order to create a processing window in which self-assembly can occur. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L46.00002: Wrinkles and Folds as Photonic Structures in Polymer Photovoltaics Yueh-Lin Loo, Jong Bok Kim, Pilnam Kim, Howard Stone, Nicolas Pegard, Jason Fleischer, Soong Ju Oh, Cherie Kagan We exploit the elastic instabilities of polymer surfaces under compressive mechanical stress to generate wrinkles and deep folds with prescribed dimensions and at pre-specified coverage over large areas. These wrinkles and deep folds act as photonic structures; they increase light coupling into and trapping within polymer photovoltaics. Devices on these surfaces show a 79{\%} increase in the external quantum efficiency (EQE) in the visible compared to analogous devices on flat surfaces. ~More significantly, we observe an exponential increase in near-infrared light absorption in these devices. In both experiments and numerical simulations, we find that these structures extend the useful range of energy conversion by $>$200 nm, corresponding to a 600{\%} increase in the EQE in the near-infrared where light is otherwise minimally absorbed.~ While we demonstrate this concept with polymer photovoltaics, the controlled introduction of compressive stress provides a straightforward and economical route to large-scale patterning of photonic structures for flexible opto-electronics. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L46.00003: Molecular Exchange Dynamics in Block Copolymer Micelles Frank Bates, Jie Lu, SooHyung Choi, Timothy Lodge Poly(styrene-$b$-ethylene propylene) (PS-PEP) diblock copolymers were mixed with squalane (C$_{30}$H$_{62})$ at 1{\%} by weight resulting in the formation of spherical micelles. The structure and dynamics of molecular exchange were characterized by synchrotron small-angle x-ray scattering (SAXS) and time resolved small-angle neutron scattering (TR-SANS), respectively, between 100 \r{ }C and 160 \r{ }C. TR-SANS measurements were performed with solutions initially containing deuterium labeled micelle cores and normal cores dispersed in a contrast matched squalane. Monitoring the reduction in scattering intensity as a function of time at various temperatures revealed molecular exchange dynamics highly sensitive to the core molecular weight and molecular weight distribution. Time-temperature superposition of data acquired at different temperatures produced a single master curve for all the mixtures. Experiments conducted with isotopically labeled micelle cores, each formed from two different but relatively mondisperse PS blocks, confirmed a simple dynamical model based on first order kinetics and core Rouse single chain relaxation. These findings demonstrate a dramatic transition to nonergodicity with increasing micelle core molecular weight and confirm the origins of the logarithmic exchange kinetics in such systems. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L46.00004: Direct Imaging of Nanoscale Ionic Clusters in a Polymer Electrolyte Membrane Nitash Balsara, Kenneth Downing, Sergey Yakovlev One of the factors hindering development of technologies such as fuel cells that rely on polyelectrolyte membranes (PEMs) is the lack of quantitative morphological characterization. While it has been recognized that clustering of ionic groups, can impede proton transport rates, model-free methods to quantify clustering in PEMs have not been developed. We present the first electron micrographs of sulfonic acid clusters in a polymer electrolyte membranes (PEM). The clusters are spherical with an average diameter of 1.4 nm and a standard deviation of 0.25 nm. Obtaining images of densely packed clusters of this size in a soft material is non-trivial due to their overlap in projection. Imaging of the sulfur-rich clusters by dark field microscopy was facilitated by the spontaneous formation of thin, cluster-containing layers on the top and bottom surfaces of free-standing films. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L46.00005: Mechanisms underlying conductivity of lamellar block copolymer electrolytes Venkat Ganesan, Victor Pryamitsyn Recent experiments have reported intriguing trends for the molecular weight (MW) dependence of the conductivity of block copolymer lamellae which are opposite that exhibited by homopolymer matrices. Using coarse-grained simulations of the transport of penetrant ions, we probe the possible mechanisms underlying such behavior. Our results indicate that the MW dependence of the conductivity of homopolymeric and block copolymeric matrices owe their origins to different mechanisms. On the one hand, the solubility of penetrants in block copolymer matrices themselves exhibit a MW dependence which arises from the MW dependence of the thickness of the conducting phase relative to the interfacial zones. Moreover, distinct mechanisms are shown to be responsible for the mobilities of ions in homopolymer and block copolymers. In the former, the mobility effects associated with the free ends of the polymers play an important role. In contrast, in block copolymer lamellae, the interfacial zone between the blocks presents a zone of hindered mobility for ions and manifests as a molecular weight dependence of the ionic mobility. Together, the preceding mechanisms are shown to provide a plausible explanation for the experimentally observed trends for the conductivity of block copolymer matrices. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L46.00006: Physical Aspects of Photodynamic Corneal Collagen Crosslinking Julia Kornfield Healthy vision depends on the stability of the shape of the cornea, which provides most of the~lens power of the optical system of the eye. ~Diseases in which the cornea progressively undergoes irregular deformation over time (e.g., keratoconus) can be treated clinically by inducing additional protein-protein crosslinks using a photosensitizing drug and a tailored dose of light. Unfortunately, the treatment moving through clinical trials is toxic to cells in and on the cornea. A path to a safer treatment is offered by the nanostructure of the corneal stroma---reminiscent of a HEX phase in block copolymers with 30nm~diameter collagen cylinders~spaced 60nm center-to-center in a hydrogel matrix of~proteoglycans and water. ~We show that using a photosensitizing drug~that sequesters itself in~the collagen fibrils can minimize the toxicity of therapeutic protein-protein cross-linking. Photorheology and transport measurements are used to quantify the parameters of a simple physical model that is useful for optimizing clinical protocols. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L46.00007: Directed Assembly of Block Copolymer Cylinders: Fundamental Physical Limits to Cylinder Spacing Vindhya Mishra, Glenn H. Fredrickson, Edward J. Kramer Understanding the fundamental 2D physics of disordering and defect generation in block copolymer films is important in setting the limits for directed assembly based block copolymer lithography. Our experiments on monolayer films of cylindrical morphology block copolymer show that the monolayer structure disorders at a lower temperature compared to the bulk order-disorder transition temperature by thermal generation of a critical density of dislocations (point defects in the monolayer). We demonstrate experimentally and theoretically how this process sets lower limits on the monolayer cylinder spacing and thus pattern spacing that can be achieved by directed assembly of a given block copolymer using graphoepitaxy. Self-consistent field theoretic simulations are used to predict the compressional elastic constant $B$ of the cylinder monolayer and cylinder spacing $a$ as a function of \textit{$\chi $N} and $f$, the minor block volume fraction. In turn these are used to estimate the formation energies $E_{d}$ ($\sim $\textit{Ba}$^{3})$ of dislocations in cylinder monolayers of various block copolymers. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L46.00008: Measurement of Diffusion in Entangled Rod-Coil Triblock Copolymers B.D. Olsen, M. Wang Although rod-coil block copolymers have attracted increasing attention for functional nanomaterials, their dynamics relevant to self-assembly and processing have not been widely investigated. Because the rod and coil blocks have different reptation behavior and persistence lengths, the mechanism by which block copolymers will diffuse is unclear. In order to understand the effect of the rigid block on reptation, tracer diffusion of a coil-rod-coil block copolymer through an entangled coil polymer matrix was experimentally measured. A monodisperse, high molecular weight coil-rod-coil triblock was synthesized using artificial protein engineering to prepare the helical rod and bioconjugaiton of poly(ethylene glycol) coils to produce the final triblock. Diffusion measurements were performed using Forced Rayleigh scattering (FRS), at varying ratios of the rod length to entanglement length, where genetic engineering is used to control the protein rod length and the polymer matrix concentration controls the entanglement length. As compared to PEO homopolymer tracers, the coil-rod-coil triblocks show markedly slower diffusion, suggesting that the mismatch between rod and coil reptation mechanisms results in hindered diffusion of these molecules in the entangled state. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L46.00009: Taking a Hard Look at Soft X-ray Scattering Alexander Hexemer, Cheng Wang For many years neutron and X-ray scattering has been a trusted tool for polymer scientist. In the hard X-ray regime contrast for scattering has been limited to the difference in electron density between different components. At the Advanced Light Source we have been developing a techniques that combines the advantages of traditional scattering with chemical sensitive spectroscopy. Using soft X-rays at selected photon energies we can tune the contrast between different polymers based on there chemical makeup. We have applied this to a variety of polymer systems. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L46.00010: Neither crystalline nor amorphous: measuring disorder in polymers and assessing its effect on charge transport Alberto Salleo, Jonathan Rivnay, Rodrigo Noriega, Michael Toney Conjugated polymers displaying high mobility are semicrystalline. Thin films of these materials are comprised of ordered regions (crystallites) and disordered regions. Because of the inherent anisotropy of polymers, the crystallites exhibit varying degrees of disorder in different directions. I will show a quantitative measurement of disorder as applied to these materials, which allows us to quantify a paracrystalline parameter g. This parameter can be used to rank polymers. I will show how g is related to the electronic structure of the polymer and with the presence of electronic traps in particular. By studying the dependence of g on molecular weight we can get to a definition of polymer behavior in an electronic transport sense. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L46.00011: Shear Alignment of Perpendicular Lamellae in Block Copolymer Thin Films Richard Register, Saswati Pujari, Paul Chaikin Thin supported block copolymer films, containing a single layer of cylindrical microdomains lying parallel to the substrate, can be effectively aligned by applying a shear stress to the molten, ordered film. Such films have been used effectively as contact masks for pattern transfer via reactive ion etching, permitting the fabrication of in-plane nanowire arrays, where the nanowires are aligned over macroscopic (cm) distances. Such a nanowire array could also be formed from a film which contains lamellae whose interdomain interfaces lie perpendicular to the substrate; such a template film would in principle allow for the formation of nanostructures of high aspect ratio, provided that the lamellae can be aligned along a single in-plane direction while retaining their perpendicular orientation. We have generated such films of perpendicular lamellae in a polystyrene-poly(methylmethacrylate) diblock, PS-PMMA, by neutralizing the substrate with a random terpolymer brush. Shearing the film, using a moving polydimethylsiloxane (PDMS) pad in contact with the film surface, can indeed produce alignment over cm-scale distances; however, the orientational order is poorer and the defect density higher than in typical cylinder-forming systems, and a significantly higher stress is required. After peeling off the PDMS pad, both PS and PMMA blocks are exposed at the surface in thinner films, but for films thicker than one domain spacing, the lower-energy PS block tends to cap the film surface, overlaying aligned perpendicular lamellae. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L46.00012: Role of nanoscale morphology on the nano and macro-scale performance of polythiophene based polymer solar cells Peter Green, Jojo Amonoo, Emmanouil Glynos, Chelsea Chen Maximization of the short circuit current, J$_{SC}$, the open circuit voltage, V$_{OC}$, and the fill factor (FF) to achieve highest power conversion efficiencies (PCEs) in donor/acceptor, polymer/polymer, solar cells is dependent on optimization of variables associated with the active material's chemical and morphological structure. Control of the nanoscale structure of polythiophene (P3HT)/phenyl-C61-butyric acid methyl ether (PC$_{61}$BM) active materials was achieved through use of a novel low temperature processing strategy. With the use of energy filtered transmission electron microscopy (EF-TEM), electron and X-ray diffraction, together phase contrast, deflection and photocurrent measurements at the nanoscale, we were able to tailor nanoscale morphologies to achieve increases in the J$_{SC}$ by a factor of 1.2 and the PCE by 30{\%}, beyond that using conventional heat treatments for processing. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L46.00013: Semiconducting block copolymers as nano-structuring agents for high-efficiency and annealing-free bulk hetero-junction organic solar cells Georges Hadziioannou, S\'ebastien-Jun Mougnier, Cedric Renaud, Cyril Brochon, Guillaume Fleury, Dargie Deribew, Eric Cloutet, Laurence Vignau Three main requirements for the industrial development of the polymer solar cells have to be addressed in order to obtain a competitive technology: a fabrication process compatible with common polymer printing technologies, an enhanced life time stability and an improved power conversion efficiency (\textit{PCE}). The active layer nano-structure in bulk heterojunction organic solar cells plays a key role on the properties of charge transfer, transport and consequently on the \textit{PCE}. Ideally a bi-continuous network of donor-acceptor domains with a length scale comparable to the exciton diffusion length is required. To obtain an optimized nano-structured active layer, an annealing process (thermal and/or solvent) is commonly performed leading to an increase of the photovoltaic performance. Currently, the implementation of common printing technologies for the fabrication of polymer solar cells on a mechanically flexible polymer substrate is impeded by this annealing step. In order to overcome the limitations above a novel approach based on an annealing-free fabrication process will be presented making use of a block copolymer as a nano-structuring agent for the polymer/fullerene derivative blend. [Preview Abstract] |
Session L48: Polymers and DNA Coated Colloid Particles
Sponsoring Units: DPOLY DBIOChair: Alexei Tkachenko, Brookhaven National Laboratory
Room: 161
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L48.00001: DILLON MEDAL BREAK
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Tuesday, February 28, 2012 3:06PM - 3:18PM |
L48.00002: Structure and Assembly of Polymeric Dots Formed by Conjugated Polymers Naresh Osti, Dilru Ratnaweera, Thusitha Etempawala, Umesh Shrestha, Dvora Perahia Rigid conjugated polymers assume extended conformation in dilute solution and often behave as colloidal suspensions. When forced into a collapsed configuration, they form highly fluorescing particles, or poly-dots, which have demonstrated potential as intracellular imaging markers, as well as building blocks for light harvesting devices. The current work investigates the structure and stability of poly-dots of \textit{di-alkyl} \textit{para polyphenyleneethynylene} (PPE) conjugated polymers in solution and follows their assembly at interfaces. Small angle neutron scattering measurements of the poly-dots in water have shown that at low concentrations, stable unimolecular spherical poly-dots are formed with a polydispersity that corresponds to that of the polymer itself. With increasing concentration, yet, below the critical micelle concentration of these rod-like polymers in good solvents, the size and density of the NPs increases, however the spherical symmetry is retained. The nature and length of the substituents affect the internal density and the degree of swelling of the poly dots. Atomic Force Microscopy results show that these PPE poly-dots assemble into arrays with different symmetries, depending on molecular parameters and assembly conditions. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L48.00003: Molecular dynamics study of polymer-silica nanoparticle hybrids: Building blocks for directed assembly Sabina Maskey, Flint Pierce, Gary Grest, Dvora Perahia Molecular dynamic simulations have been used to study the conformation and interactions of a polymer nanoparticle hybrid that consists of \textit{para} dialkyl phenyleneethynylenes (PPEs) grafted to a silica nanoparticle, with the goal of deriving the factors that control their assembly. PPEs are electro-optically active polymers whose conformation determines their degree of conjugation and their assembly mode which in turn affects the electro-optical properties of the nanoparticle-polymer complexes. When confined to a nanoparticle surface, the PPE chains are fully extended in good solvents but cluster as the quality of the solvents is decreased. Tuning the degree of clustering by tuning the solvent-polymer interaction is expected to direct the assembly of the particles. Results for the conformation of grafted PPE molecules on a single nanoparticle and the forces between two nanoparticles as a function of solvent quality will be presented. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L48.00004: Structural and Dynamical Characteristics of Polyelectrolyte Dendrimer Solutions Revealed by Neutron Scattering and Atomistic Simulation Bin Wu, Yun Liu, Lionel Porcar, Peter Falus, Changwoo Do, Michalea Zamponi, Kunlun Hong, Gregory Smith, Wei-Ren Chen Solutions of polyelectrolyte dendrimers were investigated using small angle neutron scattering (SANS) and dynamical measurements including quasi-elastic neutron scattering, neutron spin-echo and high resolution NMR. The goal of the experiments was to understand the structural and dynamical responsiveness polymer toward the variation of molecular charge. Experimental spatial correlation functions and temporal correlation functions such as intermediate scattering functions and the dynamic structure factor were evaluated quantitatively. Complementary atomistic simulations were developed for providing the microscopic interpretation of the scattering measurements and for investigating the material characteristics that are not accessible experimentally. Based on this synergistic approach, we attempt to provide a detailed understanding of the microscopic mechanisms underlying the observed electrostatic responsive properties in these very important classes of materials. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L48.00005: A theoretical study of the phase behavior of spherical colloids decorated with adhesive domains Homin Shin, Kenneth Schweizer We propose a nonlinear self-consistent phonon theory for the self-assembly of spherical particles with patterned adhesive surfaces, such as Janus colloids. The approach is first tested against the known crystallization behavior of hard spheres, and also homogeneous particles that interact via short range attractions. Janus colloid pair interactions are described by an anisotropic extension of the Baxter adhesive sphere potential where particles attract only if their hydrophobic domains are in contact. Given various crystalline symmetries, the effective harmonic potential experienced by a colloid confined to its Wigner-Seitz cell is self-consistently computed. The characteristic vibrational displacements or localization lengths are determined by the lattice symmetry as well as the strength and surface pattern of adhesive interactions. The crystal free energy is then computed, and thermodynamic stability evaluated, including pressure-driven solid-solid transitions, and the fluid-solid coexistence boundary based on the Baxter solution of the Percus-Yevick integral equation for adhesive hard sphere liquids. A primary goal is to evaluate the influence of patch size, attraction strength, and geometric patterning on the formation of heterogeneous crystalline phases. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L48.00006: In situ electrochemical small-angle neutron scattering (eSANS) for quantitative structure and redox properties of polymer-coated nanoparticles Vivek Prabhu, Vytas Reipa, Peter Bonnesen, Adam Rondinone, Eric Formo Rapid growth in nanomaterial applications (energy, cosmetics and healthcare products) highlights limitations of available physicochemical characterization methods. An in situ electrochemical small-angle neutron scattering (eSANS) methodology was devised that enables direct measurements of nano and colloid material dispersion structure while undergoing reduction-oxidation (redox) reactions. By combining the electrochemical signal with contrast variant SANS, the structure of the polymer-nanoparticle complexes can be examined under electrochemical conditions. Specially-synthesized poly(ethyleneglycol)-stabilized zinc oxide nanoparticles were examined by eSANS showing an irreversible change in nanoparticle-complex structure during the potential cycle. We will report on the kinetics of the nanoparticle transformation as measured at BL-6 EQSANS, Spallation Neutron Source, Oak Ridge National Laboratory. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L48.00007: Topological Interaction by Entanglement of DNA Lang Feng, Ruojie Sha, Nadrian Seeman, Paul Chaikin We find and study a new type of interaction between colloids, Topological Interaction by Entanglement of DNA (TIED), due to concatenation of loops formed by palindromic DNA. Consider a particle coated with palindromic DNA of sequence ``P1.'' Below the DNA hybridization temperature ($T_m$), loops of the self-complementary DNA form on the particle surface. Direct hybridization with similar particle covered with a different sequence P2 do not occur. However when particles are held together at $T > T_m$, then cooled to $T < T_m$, some of the loops entangle and link, similar to a Olympic Gel. We quantitatively observe and measure this topological interaction between colloids in a $\sim 5^{\circ} C$ temperature window, $\sim 6^{\circ} C$ lower than direct binding of complementary DNA with similar strength and introduce the concept of entanglement binding free energy. To prove our interaction to be topological, we unknot the purely entangled binding sites between colloids by adding Topoisomerase I which unconcatenates our loops. This research suggests novel history dependent ways of binding particles and serves as a new design tool in colloidal self-assembly. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L48.00008: Measuring in situ primary and competitive hybridization events on microspheres Valeria Milam, James Hardin Understanding hybridization events at surfaces is crucial for optimizing nucleic acid detection platforms as well as DNA-mediated colloidal assembly. We used flow cytometry to measure time-dependent primary and competitive hybridization events of perfectly matched and mismatched targets on microsphere surfaces. In addition to more conventional sample preparation involving multiple wash and resuspension steps prior to measurement, we sampled the reaction volume directly for in situ measurements to minimize potential dissociation events between weaker partner strands during wash steps. Similar to prior reports for oligonucletide solutions, the nearly identical rates for primary hybridization events on microsphere surfaces were independent of target sequence and reached an equilibrium value within 30 min. The extent of in situ primary hybridization events for immobilized probes, however, deviated from solution model predictions. In situ competitive hybridization events were at least 100-fold slower than primary hybridization events and did not appear to reach equilibrium. The kinetics of competitive hybridization events on microspheres are consistent with predicted effects stemming from toehold effects or base length differences between primary and secondary targets. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L48.00009: You can always get what you want Zorana Zeravcic, Sahand Hormoz, Jesse Collins, Vinothan Manoharan, Michael Brenner Colloidal particles coated with DNA strands can self assemble into complex structures. Which structures are formed and with what yield depends on the specifics of the design rules. We numerically study the directed self assembly of DNA coated colloidal particles. By testing different design rules for self-assembly with short-range interactions and studying the stability of equilibrium structures, we uncover the principles for always getting a desired assembled structure. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L48.00010: Rigid and Soft Minima of DNA-colloid Clusters Jesse Collins, Zorana Zeravcic, Vinothan Manoharan, Michael Brenner What are the limits of self-assembly via short-ranged, isotropic potentials? We investigate how carefully tuning the interaction matrix--breaking the permutation symmetry--of a small number of particles leads to unique ground states and novel energy landscape features, including soft local and global minima. We coat microspheres with highly specific and thermodynamically optimized DNA-sequences, and observe a few of these at a time organize in wells and droplets. DIC and other imaging techniques reveal 3D cluster structure, and fluorescence reveals the identity of each bead in the cluster. Although our experiments equilibrate timely over only a small temperature window, they elucidate how discrete sphere packing geometry governs both the statistical equilibrium and theoretical ``zero T'' cluster probabilities. Finally, I'll describe how relaxing some constraints can shift equilibria and flatten maxima on the energy landscapes of these specific spheres. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L48.00011: Uniform colloidal clusters from random aggregation of bidisperse spheres Nicholas B. Schade, Miranda C. Holmes-Cerfon, Elizabeth R. Chen, Dina Aronzon, Jesse W. Collins, Jonathan A. Fan, Vinothan N. Manoharan Using a combination of experiment and simulation, we investigate the structures that form when colloidal spheres cluster around smaller spheres. We use either oppositely charged particles or particles coated with complementary DNA sequences to form the clusters, and we observe them under optical microscopy. We find that random sphere parking serves as a useful model for cluster self-assembly in these systems and that the sphere diameter ratio controls the distribution of cluster sizes. In particular, near a critical diameter ratio, random parking produces tetrahedral clusters in theoretically unlimited yield. Experimentally we observed 90\% tetramer yield near this geometrical singularity. We investigate how this method can be used to assemble tetrahedral plasmonic resonators from metallo-dielectric nanospheres in order to create a bulk, isotropic, optical metamaterial. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L48.00012: The Internal Structure of Nanoparticle Dimers Linked by DNA Fernando Vargas Lara, Ching-Jung Cheng, Oleg Gang, Francis W. Starr The self-assembly of inorganic units controlled by the interactions of biological molecules, like DNA, has received attention for the possibility to specify higher-order structure, with potential biological, optical and electronic applications. In biology, self-assembly of complex materials (eg. bone, spider silk) frequently occurs in a stepwise, hierarchical fashion. Here, we consider a first step towards a hierarchical approach for synthetic nanostructures of nanoparticles (NPs) linked by DNA. The most basic unit in this multiscale approach is a dimer of NPs linked by DNA. We use a coarse-grained molecular model to explain experimental measurements of the separation of two DNA-coated NPs connected by linking single-stranded DNA (ssDNA). We show that the dimer separation is primarily controlled by the number of DNA links between NPs. If these links are not constrained to lie along the axis between NPs, the separation is limited by off-axis connections that force the NPs to be closer. We also determine how the number of connections alters the effective persistence length of the ssDNA that connects the dimer. We discuss how these dimers might be used for subsequent assembly at larger scales. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L48.00013: Specific interactions in complex mixtures: effects on the thermodynamic stability of multicomponent protein solutions William Jacobs, Daan Frenkel Multicomponent protein solutions, such as the cytosol, comprise complex networks of specific interactions in a crowded environment of molecules with nonspecific interactions, with dissociation constants spanning many orders of magnitude. We investigate the phase behavior of a multicomponent lattice model with both specific and non-specific interactions. We use bit strings to encode the binding strength between interacting patches on particles at neighboring lattice sites. The boundary of the well-mixed dilute phase is calculated for a statistical ensemble of mixtures using semi-grand Monte Carlo simulations and multicanonical histogram reweighting techniques. We examine the sensitivity of this phase boundary to the distribution of component interactions and demonstrate that the phase behavior is extremely sensitive to the high-end tail of the distribution of interaction strengths. [Preview Abstract] |
Session L49: Focus Session: Fluctuation-Induced Forces in Soft Matter and Polymeric Systems - Charge, Shape and Nucleation
Sponsoring Units: DPOLYChair: Zvonimir Dogic, Brandeis University
Room: 162A
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L49.00001: DILLON MEDAL BREAK
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Tuesday, February 28, 2012 3:06PM - 3:18PM |
L49.00002: Ion-induced interactions between charged macroions and dielectric inhomogeneities Jos Zwanikken, Monica Olvera de la Cruz We present a theoretical study of the interactions between macroions in ionic solutions, that are induced by the surrounding electrolyte. In a first scenario, we consider responsive nanoparticles with positively and negatively charged surface groups, and predict that thermal fluctuations of the net surface charge are responsible for an effective attraction, as a chemical analogue of the atomic London-forces. In a second scenario, we consider nanoparticles in solvents with a low dielectric permittivity, where the Coulomb interactions between ions easily exceed the thermal energy. We predict an ionic condensation in the confinement between two nearby nanoparticles, where the suppression of fluctuations leads to a locally dense state of ions, and a consequent strong effective attraction between the nanoparticles. The induced potential between the nanoparticles is also related to the ion-nanoparticle interactions, by e.g. Van der Waals, surface charge, or image charge attractions. Recently developed theoretical methods are discussed, based on earlier work [1,2]. \\[4pt] [1] J. W. Zwanikken, and M. Olvera de la Cruz, Phys. Rev. E 82, 050401(R) (2010). \\[0pt] [2] J. W. Zwanikken, P. K. Jha, and M. Olvera de la Cruz, J. Chem. Phys. 135, 064106 (2011); [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L49.00003: Dielectric many-body effects in arrays of charged cylindrical macromolecules Daniel W Sinkovits, Kipton Barros, Jure Dobnikar, Matej Kandu\v{c}, Ali Naji, Rudolf Podgornik, Erik Luijten Nonuniform dielectric constants are a ubiquitous aspect of condensed-matter systems, but nevertheless widely ignored in simulations. Analytical work suggests that the polarization effects resulting from these inhomogeneities can produce many-body interactions that qualitatively alter the behavior of systems driven by electrostatic interactions, but such work relies on approximations. Recently, we have developed an algorithm that computes the fluctuating polarization charge at the interface between dielectric materials during a molecular dynamics simulation, without approximation. Here, we apply this approach to investigate arrays of charged cylindrical macromolecules in the presence of explicit counterions. We study the dielectric many-body effects as a function of separation, dielectric constant variation, and counterion valency. Our findings have implications for the aggregation of polyelectrolytes such as F-actin or DNA. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L49.00004: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 3:42PM - 4:18PM |
L49.00005: Influence of probe shape on polymer configurations and entropic forces Invited Speaker: Yacov Kantor The free energy of long polymers is frequently dominated by entropy with the interaction energy playing a minor role. In the absence of an energy scale, the corresponding forces are then governed by the thermal energy scale $k_{B}T$ and by the length scales associated with the experimental set-up. Recent advances in single molecule manipulation techniques have brought the accuracy of position and force determination into the range where the measurement of relatively small deformations becomes possible. In these situations the detailed shape of probes to which the molecule is attached must be taken into account. The behavior of a polymer of size $R_0$ attached to the rounded tip of a probe (sphere, paraboloid, spherocylinder) with radius of curvature $R$, differs qualitatively for large and small values of the ratio $s=R_0/R$. The scaled compliance (inverse force constant) $S/R_0^2$, is anisotropic and quite large in the direction parallel to the surface when $s\sim 1$ [1]. When a cone with a polymer attached to its sharp tip approaches a plate, then for cone-plate separation $h\ll R_0$ the polymer-mediated force between them [2] is given by $F=Ak_{B}T/h$. The coefficient $A$ can be related to geometry-dependent correlation exponents of long polymers. We computed $A$ for phantom polymers, and for self-avoiding polymers by $\epsilon$-expansion, as well as by numerical simulations in 3 dimensions. \\[4pt][1] R. Bubis, Y. Kantor and M. Kardar, Europhys. Lett. \textbf{88}, 48001, 2009. \\[0pt][2] M. F. Maghrebi, Y. Kantor and M. Kardar, Europhys. Lett., \textit{in press}, 2011 (arXiv:1109.5658). [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L49.00006: Topological obstacles and the forces on them in systems of strongly interacting directed polymers David Zeb Rocklin, Paul M. Goldbart, Shina Tan Owing to their extended structure and inpenetrability, polymers are strongly influenced by topological obstacles. To shed light on this issue we consider a model system of noncrossing directed polymers in two dimensions. As first exploited by de Gennes [1], the configurations of this system can be mapped onto the worldlines of noninteracting fermions---an analogy that enables the application to the polymer system of techniques initially developed for one-dimensional quantum systems. Via this approach, we discuss how an obstacle that forces a fixed number of polymers to pass to one side of a single topological constraint is associated with a large fluctuation of the quantum system. In addition, via the use of techniques from quantum hydrodynamics, we find that such a constraint on a system of noncrossing polymers generates an effective, long-ranged repulsion between the polymers. This repulsion causes a void to appear in the polymer fluid and generates a super-Hookean force opposing the constraint. [1] P.-G. de Gennes, J. Chem Phys. 48, 2257-2259 (1968). [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L49.00007: Critical exponents in the presence of a tip Mohammad F. Maghrebi, Yacov Kantor, Mehran Kardar The behavior of Landau-Ginzburg model of a critical system, appropriate to describe SAW polymers, in the presence of a conical boundary is studied within mean field and by epsilon expansion in d=4-epsilon dimensions. New exponents emerge for correlation functions near the boundaries, in this case the tip of the cone. In the limit of a sharp cone we find a new exponent which we interpret for a SAW polymer by using its fractal dimension. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L49.00008: Trapping fluctuations in confined carbon dioxide near the supercritical ridge Manfred Heuberger, Erich Schurtenberger We investigate the properties of a confined supercritical carbon dioxide film. Density fluctuations are theoretically predicted near the critical point and along the supercritical extension of the liquid-gas coexistence line (ridge). We have confined carbon dioxide between two atomically smooth surfaces in a new generation of surface forces apparatus. The nanometers to micrometer films are found to be laterally divided into nano domains of two distinct refractive indices. The two-refractive index structure is apparent, yet different on behavior, above or below the supercritical ridge. This non-equilibrium phase separated film exhibits long-range attractive forces (up to 500 nm), which results from an interfacial energy between the different domains. High refractive nano domains can be trapped and stretched to lengths of several micrometers, until we observe coalescence of the domains. Our observations suggest that density fluctuations can be trapped and pinned between surfaces in the form of nano strings of significantly different molecular order. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L49.00009: Homogeneous crystal nucleation from the melts of flexible $n$-alkanes by molecular simulation Peng Yi, Gregory C. Rutledge Homogeneous crystal nucleation from a dense melt is particularly interesting for chain molecules due to their anisotropy and conformational flexibility. We report our molecular simulation study of the homogeneous crystal nucleation from $n$-alkane melts. For short $n$-alkane (C20) (J.Chem.Phys. 135, 024903), the nucleation trajectory was sampled using brute force molecular dynamics (MD) simulations at about 20{\%} supercooling and the nucleation free energy was sampled using the Monte Carlo (MC) umbrella sampling method for temperatures ranging from 10{\%} to 20{\%} supercooling. In the MD simulation, we identified the induction period unambiguously and calculated the nucleation rate through a mean-first-passagetime analysis. A typical critical nucleus consists of a bundle of stretched segments organized into a cylindrical shape. The remaining CH2 groups form a disordered interfacial layer. By fitting the free energy curve sampled by MC to the cylindrical nucleus model, the crystal-melt interfacial free energies are calculated. We have found good agreement between the melting temperature and the interfacial free energies obtained in our simulation and those from experiment. For a long $n$-alkane above the entanglement length (C150), MD simulation of nucleation was performed at super-cooling as small as 15{\%}. Chain folding was observed during the nucleation stage, and thickening of crystallites was observed during the subsequent crystal growth. The resulting crystal-amorphous interface is characterized in terms of loops, bridges and tails. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L49.00010: ABSTRACT WITHDRAWN |
Session L50: Focus Session: Organic Electronics and Photonics - Morphology
Sponsoring Units: DMP DPOLYChair: Mark Dadmun, University of Tennessee
Room: 162B
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L50.00001: DILLON MEDAL BREAK
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Tuesday, February 28, 2012 3:06PM - 3:18PM |
L50.00002: Real-time observation of optoelectronic properties during poly(3-alkylthiophene) crystallization Victor Ho, Bryan Boudouris, Leslie Jimison, Michael Toney, Alberto Salleo, Rachel A. Segalman Poly(3-alkylthiophenes) (P3ATs) are commonly used semiconducting polymers in organic electronic applications where the nanoscale morphology of the active layer largely dictates device performance. However, during solution processing, a large driving force for crystallization results in thin film morphologies that are kinetically trapped and cannot be easily controlled. We show that rational side chain design, particularly the use of branched alkyl chains, can reduce the melting transition increasing thermal control relative toP3HT. Importantly, the lower crystallization transition temperature provides the opportunity to monitor crystallization in real-time using grazing-incidence x-ray diffraction and UV-vis absorption spectroscopy, and the relative degree of crystallinity is observed to increase gradually over the course of crystallization. In contrast to the gradual increase in crystallinity and optical properties, the field-effect mobility monitored during crystallization exhibits a sharp increase of approximately two orders of magnitude. We propose that the difference in time scales may be due to the formation of a percolated network between electrodes, and that increases in the degree of crystallinity beyond this point are not probed by the transistor geometry. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L50.00003: In Situ Annealing Study of Organic Photovoltaic Morphology via Non-invasive Polarized Neutron Reflectivity Brett Guralnick, Michael Mackay, Brian Kirby, Charles Majkrzak Polarized neutron reflectivity, a non-invasive technique, allows the unambiguous density distribution within a thin film to be determined. By utilizing this technique with organic photovoltaics is it possible to study the same device pre- and post-annealing. We studied a bulk herterojunction cell consisting of an organic semiconductor (P3HT) and a nanoparticle electron acceptor (PCBM). We found a shift in the location of PCBM within the organic film which migrates toward the anode and cathode following annealing. However, while some PCBM can reach the substrate interface it never fully blooms to the air interface and pure P3HT resides at the surface for the thick (200 nm) films used in this study. These results differ from previous research in that the same device was characterized allowing a true study on the effect of annealing to be performed. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L50.00004: Solvent Studies for Solution Processing of Polymer-Fullerene Bulk Heterojunctions Margaret J. Sobkowicz, Ronald L. Jones, R. Joseph Kline, Dean M. DeLongchamp A major advantage of polymer solar cells over higher-efficiency photovoltaic alternatives is the low cost of printing technologies. Solution-based film preparation relies on the self-assembly of the polymer and fullerene phases as the layer dries from the casting ink into a bulk heterojunction morphology. The nanoscale morphology must facilitate optimal charge separation and transport; thus solution processing parameters heavily influence the device performance. Current technology uses chlorinated aromatic solvents and small processing windows with stringent requirements on ink properties. In this study, the stability of various ink formulations is investigated to develop more reliable, sustainable alternatives. Model solutions of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are prepared using a range of solvents and solvent mixtures. P3HT solution crystallization behavior is investigated using differential scanning calorimetry, UV-visible absorbance measurements and neutron and light scattering. The behavior of the solutions under shear is investigated in order to predict the success of disparate printing and coating techniques. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L50.00005: Morphology in Hybrid Oxide/Polymer Core-Shell Nanowire Photovoltaics Jie Jiang, Sohrab Ismail-Beigi We use first principles theory (DFT) to gain insight into the Poly(3-hexylthiophene) (P3HT)-ZnO nanowire binding morphologywhere the P3HT is bound to ZnO via SiO$_3$ linkers [1]: understanding the experimentally realized morphology is the first step in trying to predict and enhance electronic energy alignments conducive to photovoltaic operation. For large diameter ZnO nanowires, the morphology is that of P3HT bound to the ($10\overline{1}0$) surface of ZnO. The existence of a lattice mismatch of the ZnO surface and the polymer backbone creates a competition between linker binding and strain energy. We solve this realization of the classic Frenkel-Kontorova model based on ab initio parameters to obtain the lowest energy binding morphology on the surface. For small ZnO nanwire diameters, curvature effects become important: the P3HT polymer must bend around the nanowire, and the reduction of linker spacing due to curvature enlarges the lattice mismatch for helical wrappings. We use DFT to estimate these curvature energies, and we predict a morphology change from helical wrapping to linear alignment with the nanowire axis at a diameter close to 25 nm. [1] S. Zhang, Adv. Mater. (in press, 2011). [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L50.00006: An Improved Forcefield for Molecular Modeling Oof Crystalline Poly(3-Hexyl Thiophene) Ram Bhatta, Yeneneh Yimer, David Perry, Mesfin Tsige A fundamental understanding of molecular structure and dynamics of poly(3-hexyl thiophene) (P3HT), one of the most promising organic solar cell materials, is crucial for improving the efficiency of organic solar cells containing P3HT as the donor. Molecular dynamics (MD) simulations can produce the correct structures and dynamics of P3HT provided that robust forcefields are employed for this system. The forcefields that are currently used for MD simulations of P3HT are mostly taken from the analogous thiophene molecule, bi-thiophene. However, such forcefields may lack to produce the correct morphology and stacking properties of P3HT. We present the results of MD simulations using an improved forcefield for P3HT. In the improved forcefield the torsional and partial atomic charge parameters for both the alkyl side chains and backbone were derived from ab initio calculations. Our results from MD simulations are compared with available experimental and theoretical data and the range of accessible structures is explored. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L50.00007: Device simulation of morphologies that are consistent with small angle scattering and reflectometry Daniel Olds, Phillip Duxbury Through the use of a dynamic Monte Carlo simulation, we are able to evaluate the efficiency of bulk heterojunction morphologies of P3HT/PCBM based photovoltaic devices that are consistent with neutron reflectometry and SANS data. We have developed a method to efficiently generate simulated small angle scattering data from hypothetical nanoscale systems such as polymer-fullerene bulk heterojunctions found in organic photovoltaic devices. We will show how this method can be used to accurately calculate the scattering information of well known systems such as a polydisperse collection of hard and soft spheres. We will then demonstrate this method on the calculated device morphologies, and show how the simulated scattering can grant insight into the validity of assumptions based on traditional fitting methods. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L50.00008: Three dimensional device simulations to model conducting probe AFM measurements of organic semiconductors with fibrous morphologies Kanokkorn Pimcharoen, Danial Olds, Jiebing Sun, Pengpeng Zhang, Phillip Duxbury Organic semiconductors offer a promising material for many optoelectronic devices, with device performance depending significantly on the nanoscale morphology. Atomic force microscopy (AFM) is one of the major instruments for investigating the dependence of current-voltage response on nanoscale structures. We are developing computational methods for fundamental study of charge transport probed by these measurements, using continuum device models and Kinetic Monte Carlo simulations. The simulations are performed on complex three dimensional model morphologies that are consistent with the topology observed in AFM measurements. The calculated IV response of these models is compared with CP-AFM measurements. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L50.00009: Self-Limiting Crystal Microstructure in Poly(3-hexylthiophene) Chad Snyder, Ryan Nieuwendaal, Dean DeLongchamp, Jessica Henry Polymeric semiconductors, such as poly(3-hexylthiophene-2,5-diyl) (P3HT), hold great potential for a variety of technologies. These solution processable materials are promising as active layers for low cost or large area flexible electronic or optoelectronic devices that can be prepared through high-throughput deposition processes, such as inkjet or roll-to-roll printing. Of the myriad of materials currently under examination, P3HT is one of the most widely studied materials because of its electronic properties and commercial availability. However, most, if not all, commercially available P3HT is produced with some non-negligible level of regiodefects that has a predefined impact on its crystalline fraction. We examine the effect of regiodefects in P3HT on the semicrystalline microstructure, i.e., lamellar thickness and distribution, and overall crystallinity, and discuss the impact of these effects on potential device performance and bulk heterojunction morphology. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L50.00010: X-ray and neutron reflectivity and electronic properties of PCBM-poly(bromo)styrene blends and bilayers with poly(3-hexylthiophene) Stuart Kirschner, Nathaniel Smith, Kevin Wepasnick, Howard Katz, Brian Kirby, Julie Borchers, Daniel Reich We used neutron reflectivity to complement x-ray reflectivity characterization of PCBM-based layers formed on poly(3-hexylthiophene) (P3HT). Single-layer analyses were used to provide reliable scattering length density values for bilayer fitting. Atomic force microscopy analyses showed trends similar to the reflectivity experiments when observing upper surfaces. Styrene polymers added to PCBM in small concentrations (ca. 10 percent) led to processing advantages while retaining substantial electron mobility, about 0.001 cm$^{2}$/V s. The further introduction of a relatively heavy bromo atom substituent on the styrene rings greatly increased the film smoothness, as revealed by increases of the oscillation amplitudes in the reflectivity. In addition, the bromine heavy atom increased x-ray reflectivity scattering length density of the upper layer. Finally, we confirm that P3HT is capable of extracting PCBM from a subsequently deposited overlying layer, consistent with predictions based on published phase diagrams of the P3HT-PCBM system. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L50.00011: Controlled phase separation in conjugated polymer blends using quadruple hydrogen bonding interactions Yen-Hao Lin, Rafael Verduzco All-polymer photovoltaic blends have significant promise for low-cost, solution processed photovoltaics, but large-scale phase separation can lead to non-optimal active layer structures for charge-separation and transport. We propose a novel strategy for controlling phase separation in conjugated polymer blends using quadrupole hydrogen bonding interactions. We investigate a series of end-modified conjugated and coil-like polymers in blends, including poly(3-hexyl thiophene) (P3HT), poly(styrene), poly(ethylene glycol), poly(9,9-dioctyl fluorene). Polymers are end-terminated with the multiple hydrogen bonding group 2-ureido-4(1H)-pyrimidinone (UPy) using isocyanate chemistry. Atomic force microscopy and grazing incidence x-ray scattering show phase separation is suppressed and, in some cases, P3HT crystallite orientation is improved in films for blends of UPy functionalized polymers. These results show the quadruple hydrogen bonding groups can prevent large-scale phase separation and direct the orientation of polymer chains. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L50.00012: Synthesis and Characterization of All-Conjugated Block Copolymers Prepared via Click Chemistry Rafael Verduzco, Kendall Smith All-conjugated block copolymers with both hole-conducting and electron-conducting polymer blocks can be used to address fundamental questions regarding the structure, optoelectronic properties, and photovoltaic performance of organic photovoltaic blends, but synthetic challenges have precluded comprehensive studies on such systems. Here, we present a novel synthetic approach for preparing all-conjugated block copolymers and detailed studies of their nanoscale structure and optical properties. Our synthetic approach is based on copper-catalyzed azide-alkyne ``click'' chemistry and enables us to prepare block copolymers with a poly(3-alkylthiophene) block covalently linked to a conjugated polymer prepared by Suzuki polycondensation polymerization, including poly(9,9-dioctyl fluorene), poly(9,9-dioctyl fluorene-alt-benzothiadiazole) and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2',2''-diyl) (PFOTBT). A combination of x-ray diffraction, grazing-incidence x-ray scattering, atomic force microscopy, and fluorescence quenching measurements give insight into their microstructure and potential for use in high-performance all-polymer photovoltaics. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L50.00013: Undulation instability in drop-cast poly(3-hexylthiophene) film originated from self-assembly Min Sang Park, Avishek Aiyar, Jung Ok Park, Elsa Reichmanis, Mohan Srinivasarao In this study, we characterize the undulated structures which appear at the edge of drop-cast regio-regular poly(3-hexylthiophene) (rr-P3HT, head-to-tail $>$ 95{\%}) film using optical microscopy and atomic force microscopy. We propose that these periodic structures originate from the undulations of the layered structure of liquid crystal-air interface. Evidence of rr-P3HT solution forming liquid crystalline phases at higher concentrations was obtained by the observation of distinct birefringence and characteristic textures under crossed polarizers using an optical microscope. Synchrotron x-ray diffraction pattern provides additional structural information at the undulated area compared with those at the area without undulated pattern. Based on these experimental results, we propose rr-P3HT solution can form a lyotropic liquid crystal at specific concentrations. This work was partially supported by NSF funding (DMR-0706235). [Preview Abstract] |
Session L51: Self Assembly: Mostly Colloids, Lipids and Surfactants
Sponsoring Units: DCMP DFDChair: Arjun G Yodh, University of Pennsylvania
Room: Boston Convention Center 154
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L51.00001: Basic parameters affecting nanoparticle self-assembly: An experimental approach Chakra Joshi, Yevgen Kryukov, Jacques Amar, Terry Bigioni Understanding the basic parameters that govern the nanoparticle self-assembly process is important for high-quality monolayer formation and technological advances. A complete theory that explains nanoparticle self assembly, in the bulk and at the liquid-air interface, is lacking. In this paper, dodecanethiolated gold nanoparticles were used as a model system for studying the forces that govern self assembly. These nanoparticles are known to make compact and highly-ordered monolayers at the liquid-air interface via a mechanism that is analogous to epitaxial growth of atomic layers. Epitaxial theory was used as a starting point to study the nanoparticle self-assembly at the liquid-air interface. Experimental measurements were successfully interpreted using an epitaxy-based analysis, including flux of nanoparticles onto the liquid air-interface, decay rate of the island density, and the dependence of critical nucleus size on nanoparticle diameter. Furthermore, anomalous diffusion was observed as was a remarkable ordering of islands at the liquid-air interface. This ordering was determined to be due to a long-range repulsive force between islands. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L51.00002: Self-assembly of gold nanoparticles at water/vapor interface Gary S. Grest, J. Matthew D. Lane The self-assembly of coated gold nanoparticles at the water/vapor interfaces are studied using explicit-atom molecular dynamic simulations. While it is often assumed that uniformly coating spherical nanoparticles with short organic ligands lead to symmetric nanoparticles, we find that the high curvature of small nanoparticle and the relatively short dimensions of the coatings can produce highly asymmetric coatings. At an interface this asymmetry of the ligands tends to orient the nanoparticles with the surface to minimize free energy. First esults for individual gold nanoparticles of diameter 2-8 nm coated with alkanethiol ligands of various lengths and different end group will be presented. Results for the self-assembly of the multiple nanoparticles at the water/vapor interfaces will then be presented for the diameter 2 and 4 nm nanoparticles which show how these asymmetric and oriented coatings affect the interactions between nanoparticles and the structure of the resulting aggregate. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L51.00003: Electric Field Driven Self-Assembly of Colloidal Rods Jaime Juarez, Kundan Chaudhary, Qian Chen, Steve Granick, Jennifer Lewis The ability to assemble anisotropic colloidal building blocks into ordered configurations is of both scientific and technological importance. We are studying how electric field-induced interactions guide the self-assembly of these blocks into well aligned microstructures. Specifically, we present observations of the assembly of colloidal silica rods (L/D $\sim $ 4) within planar electrode cells as a function of different electric field parameters. Results from video microscopy and image analysis demonstrate that aligned microstructures form due to the competition between equilibrium interactions of induced dipoles and non-equilibrium processes (i.e., electro-osmosis). Under the appropriate electric field conditions ($\sim $ kHZ AC fields), aligned colloidal rod fluids form over large areas on the electrode surface. The superposition of a DC electric field to this aligned colloidal rod fluid initiates their condensation into a vertically oriented crystalline phase. Ongoing work is now focused on exploring how temporal changes to electric fields influence colloidal rod dynamics and, hence, the assembly kinetics of aligned colloidal monolayers. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L51.00004: Electron-induced Three Dimensional Self-assembly and Disassembly of Molecules on a Gold Surface Qing Li, Chengbo Han, Miguel Fuentes-Cabrera, Humberto Terrones, Bobby Sumpter, Jerry Bernholc, Jieyu Yi, Zheng Gai, Art Baddorf, Petro Maksymovych, Minghu Pan The immensely successful methodology of molecular self-assembly on surfaces has produced thousands of new applications and paved ways to new research areas, such as molecular electronics and the dip-pen nanolithography. Here we demonstrate a seminal example of non-thermal control over molecular self-assembly, where hot-electrons transform a largely disordered layer of hydrocarbon molecules, into a highly ordered, densely packed and three-dimensional monolayer on a gold surface. Subsequently, hot-electron/hot-hole injection can heal the defects within the self-assembled layer, and even entirely and reversibly disassemble it. From a theoretical analysis we have identified that electron-induced processes allow the formation of a very strongly-bonded molecule, and yet it is inaccessible by thermally-activated reactions due to a large number of competing processes. This work thus demonstrates the feasibility of accessing and controlling non-thermal reaction pathways that may lead to unique and controllable order-disorder transitions in supported molecular layers. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L51.00005: Self assembly of anisotropic colloidal particles Daniel Florea, Hans Wyss Colloidal particles have been successfully used as ''model atoms'', as their behavior can be more directly studied than that of atoms or molecules by direct imaging in a confocal microscope. Most studies have focussed on spherical particles with isotropic interactions. However, a range of interesting materials such as many supramolecular polymers or biopolymers exhibit highly directional interactions. To capture their behavior in colloidal model systems, particles with anisotropic interactions are clearly required. Here we use a colloidal system of nonspherical colloids, where highly directional interactions can be induced via depletion. By biaxially stretching spherical PMMA particles we create oblate spheroidal particles. We induce attractive interactions between these particles by adding a non-adsorbing polymer to the background liquid. The resulting depletion interaction is stronger along the minor axis of the oblate spheroids. We study the phase behavior of these materials as a function of the ellipsoid aspect ratio, the strength of the depletion interactions, and the particle concentration. The resulting morphologies are qualitatively different from those observed with spherical particles. This can be exploited for creating new materials with tailored structures. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L51.00006: Adsorption of Core-Shell Nanoparticles at Liquid-Liquid Interfaces Emanuela Del Gado, Lucio Isa, Esther Amstad, Konrad Schwenke, Patrick Ilg, Martin Kroeger, Erik Reimhult The use of nanoparticles as building blocks for the self-assembly of functional materials has been rapidly increasing in recent years. In particular, two-dimensional materials can be effectively self-assembled at liquid interfaces thanks to particle localization and mobility at the interface in combination with tailoring of specific interactions. Many recent advances have been made in the understanding of the adsorption and assembly at liquid interfaces of small hydrophobic nanoparticles, stabilized by short-chain rigid dispersants, but the corresponding studies on core-shell nanoparticles sterically stabilized by extended hydrophilic polymer brushes are presently missing. Such particles offer significant advantages in terms of fabrication of functional, responsive and bio-compatible materials. We present here a combination of experimental and numerical data together with an intuitive and simple model aimed at elucidating the mechanisms governing the adsorption of iron oxide nanparticles (5-10nm) stabilized by low molecular weight poly(ethylene glycol) (1.5-10 kDa). We show that the adsorption dynamics and the structure of the final assembly depend on the free energy of the particles at the interface and discuss the thermodynamics of the adsorpt [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L51.00007: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L51.00008: Encapsulation by Janus Oblate Spheroids Wei Li, Ya Liu, Genevieve Brett, James Gunton The micro/nano encapsulation technology has acquired considerable attention in the fields of drug delivery, biomaterial engineering, and material science. Based on recent advances in chemical particle synthesis, we propose a preliminary model of encapsulation system inducted by self-assembly of Janus oblate ellipsoids, the particles with oblate ellipsoidal cores and two semi-surfaces coded with dissimilar chemical properties. Using Monte Carlo simulation, we investigate the encapsulation system with spherical particles as encapsulated guests in different densities. We study the anisotropic effect brought by encapsulating agent's geometric shape and chemical composition on encapsulation morphology and efficiency. In the relative high encapsulation efficiency we observe from the simulation, we believe this method of encapsulation is of potential value in practical use. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L51.00009: Particle Deposition in Drying Drops of Colloidal Suspensions Containing Different Surfactants Tim Still, Peter J. Yunker, A.G. Yodh When a drop of water containing small solid particles dries, most of the solid material is deposited in a ring-shape stain after evaporation (the so-called coffee ring), driven by initial contact line pinning and a subsequent outward-flow. The fluid dynamics and, hence, the deposition mechanism in such suspensions can be dramatically changed when surfactants are introduced into the system. In a colloidal model-system, the ionic sodium dodecyl sulfate (SDS) produces a concentration-driven Marangoni flow counteracting the outward-flow of the coffee ring effect. SDS locally concentrates at the air/water interface next to the contact line, leading to a reduced local surface tension. Thus, a circulating flow (`Marangoni eddy') is introduced that prevents particles from deposition. This flow is visualized by the movements of the dragged particles using video microscopy. Other surfactants can influence this highly non-equilibrium systems in completely other ways. E.g., the non-ionic Polaxamer block-copolymer surfactants lead to a uniform particle deposition, which we explain by hydrophilization of the colloidal particles. Controlling the solid deposition in drying drops is of major importance for many technical applications. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L51.00010: Formation of Lipid-Based Nanodiscs and Their Dependence of Temperature and Chemical Composition Ying Liu, Yongkun Yang, Mu-Ping Nieh Phospholipid mixtures composed of \textit{1,2-dipalmitoyl-sn-glycero-3-phosphocholine }(DPPC), \textit{1,2-dihexanoyl-sn-glycero-3-phosphocholine }(DHPC) and \textit{1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (sodium salt)}(DPPG) and\textit{ 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (ammonium salt)} (PEGylated DSPE) and cholestrol were found to form nanodiscs (bicelles) in both non-ionic and phosphate buffer solutions . The structure of the aggregates is resolved using dynamic light scattering, transmission electron microscopy and small angle neutron scattering. The effects of temperature and chemical composition (e.g., PEGylated DSPE and cholesterol) on the structural variation and polydispersity will be discussed in this presentation. These nanodiscs have the potential of serving as a model delivery carrier for hydrophobic molecules for their biological compatibility and capability of incorporating with targeting molecules. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L51.00011: How to control GUV shape transformations Kejia Chen, Adam Szmelter, Sung Chul Bae, Steve Granick Using a microfluidic platform, we expose giant unilamellar vesicles (GUVs) to programmed time-varying profiles of osmotic pressure. In response to these conditions that intentionally do not approach equilibrium, water flows in and out, and the excess area changes in response. Shape transformations are observed that were not previously reported, nor predicted theoretically. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L51.00012: Aqueous Gemini Surfactant Self-Assembly into Complex Lyotropic Phases Mahesh Mahanthappa, Gregory Sorenson In spite of the potentially wide-ranging applications of aqueous bicontinuous lyotropic liquid crystals (LLCs), the discovery of amphiphiles that reliably form these non-constant mean curvature morphologies over large phase windows remains largely serendipitous. Recent work has established that cationic gemini surfactants exhibit a pronounced tendency to form bicontinuous cubic (e.g. gyroid) phases as compared to their parent single-tail amphiphiles. The universality of this phenomenon in other surfactant systems remains untested. In this paper, we will report the aqueous LLC phase behavior of a new class of anionic gemini surfactants derived from long chain carboxylic acids. Our studies show that these new surfactants favor the formation of non-constant mean curvature gyroid and primitive (``Plumber's Nightmare'') structures over amphiphile concentration windows up to 20 wt{\%} wide. Based on these observations, we will discuss insights gained into the delicate force balance governing the self-assembly of these surfactants into aqueous bicontinuous LLCs. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L51.00013: Shear-Driven Circulation Patterns in Lipid Membrane Vesicles Francis Woodhouse, Aurelia Honerkamp-Smith, Raymond Goldstein Recent experiments [C. V{\'e}zy, G. Massiera, and A. Viallat, Soft Matter 3, 844 (2007)] have shown that when a near-hemispherical lipid vesicle attached to a solid surface is subjected to a simple shear flow it exhibits a pattern of membrane circulation much like a dipole vortex. This is in marked contrast to the toroidal circulation that would occur in the related problem of a drop of immiscible fluid attached to a surface and subjected to shear. This profound difference in flow patterns arises from the lateral incompressibility of the membrane, which restricts the observable flows to those in which the velocity field in the membrane is two-dimensionally divergence free. We theoretically study these circulation patterns within the simplest model of membrane fluid dynamics. A systematic expansion of the flow field is developed for differing bulk fluid viscosities incorporating a non-zero membrane shear viscosity and curvature effects. It is shown how such studies can allow measurements of the membrane viscosity from flow field data. New experimental results utilising this method are discussed. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L51.00014: Self standing nanoparticulate networks by self assembly surfactant H$_1$ mesophase Guruswamy Kumaraswamy, Kamendra Sharma, Sayam Sen Gupta We show that nanoparticles (size $>$ 10 nm) that are dispersed in nonionic surfactant/water system, assemble into networks on cooling into the H$_1$ phase, independent of particle surface chemistry. Coating the particles with a crosslinkable polymer, and covalent coupling of the coated particle assemblies in the H1 phase allows us to form free standing particulate networks that are stable after surfactant removal. Thus, dynamic templating of surfactant H$_1$ domains is a facile technique that involves near ambient temperatures, and a benign water wash for template removal. The network mesh size can be varied from the sub-micron to tens of microns by controlling the cooling rate. Particle networks can be flow-oriented prior to crosslinking, and interpenetrating networks can also be formed. We will show examples of macroporous nanoparticulate networks formed using nanoparticles of inorganic oxides, polymer latices, as well as bionanoparticles such as proteins. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L51.00015: SAXS on ice crystals reveals fractal structure on nanometer length scales Jesse Hopkins, Ryan Badeau, Matthew Warkentin, Robert Thorne We have used small angle x-ray scattering (SAXS) to probe ice formation in supercooled aqueous solutions and water. The SAXS shows that the ice formed in supercooled aqueous solutions and water has power law behavior that is invariant across a wide range of solute type, concentration, and temperature. We interpret this power law as scatter from fractal structures in the ice. The consistency of this power law across four different solutes and in pure water, and at temperatures between 150 K and 220 K suggests an underlying similarity between macroscopically/visually different forms of ice on length scales of ~10-100 nm. Time dependent SAXS curves reveal two scattering regimes, one occurring at early times and one dominating at later times, which we interpret within the context of fractal scatterers. Finally, we use scaling collapses on the data to extract information about the time and temperature dependence of the ice growth. We interpret this within the established framework of the ice nucleation and growth community. [Preview Abstract] |
Tuesday, February 28, 2012 5:30PM - 5:42PM |
L51.00016: Molecular Dynamics simulations of flow-structure interactions in fluids containing cylindrical micelles and micelle-nanoparticle complexes Radhakrishna Sureshkumar, Ashish Sangwai, Abhinanden Sambasivam Coarse-grained (CG) force fields, benchmarked against fully atomistic ones, are used in Molecular Dynamics simulations to predict shape transitions and binary interactions in cationic surfactant micelles as well as to understand the molecular mechanisms of self-assembly of micelles with noble metal nanoparticles germane to plasmonics. Non-equilibrium MD simulations are conducted to probe the effect of flow shear on cylindrical micelle dynamics and estimate properties such as tumbling frequency, relaxation time and scission energy. Simulations are also performed to understand flow-mediated alignment and merger of two cylindrical micelles which is hypothesized to be the mechanism underlying the formation of shear-induced structures in micellar fluids. [Preview Abstract] |
Session L52: Focus Session: Extreme Mechanics - Origami, Creasing, and Folding
Sponsoring Units: GSNP DFDChair: Ross Hatton, Carnegie Mellon University
Room: 153C
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L52.00001: Extreme Folding Invited Speaker: Erik Demaine Our understanding of the mathematics and algorithms behind paper folding, and geometric folding in general, has increased dramatically over the past several years. These developments have found a surprisingly broad range of applications. In the art of origami, it has helped spur the technical origami revolution. In engineering and science, it has helped solve problems in areas such as manufacturing, robotics, graphics, and protein folding. On the recreational side, it has led to new kinds of folding puzzles and magic. I will give an overview of the mathematics and algorithms of folding, with a focus on new mathematics and sculpture. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L52.00002: Geometry in the mechanics of origami Marcelo A. Dias, Christian D. Santangelo We present a mechanical model for curved fold origami in which the bending energies of developable regions are balanced with a phenomenological energy for the crease. The latter energy comes into play as a source of geometric frustration, allowing us to study shape formation by prescribing crease patterns. For a single fold annular configuration, we show how geometry forces a symmetry breaking of the ground state by increasing the width of the ribbon. We extend our model to study multiple fold structures, where we derive geometrical constraints that can be written as recursive relations to build the surface from valley to mountain, and so on. We also suggest a mechanical model for single vertex folds, mapping this problem to an elastica on the sphere. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L52.00003: Photo-Origami -- Using Light to Bend, Fold, and Buckle Jennie Ryu, Matteo D'Amato, Kevin Long, Xiaodong Cui, H. Jerry Qi, Martin Dunn We describe photo-origami, a method to program spatially- and temporally-variable mechanical, chemical, and optical fields into a polymer that enable controllable, sequenced, macroscopic bending and folding to create three-dimensional structures. We combine mechanical and optical stimuli to locally rearrange the polymer's network topology which allows us to program a residual stress state into the film; upon release of mechanical constraints, we realize a wide variety of desired shapes. We demonstrate, through a combination of theory, simulation-based design, synthesis, and experiment, the operative phenomena and capabilities of photo-origami. We demonstrate architectures that rely on bending, folding, instabilities, and post-buckling behavior to achieve their three-dimensional form, starting from a flat sheet. We also describe a theory that couples the hereditary nature of photophysics, chemistry, and large-deformation mechanics and enables simulations of the fabricated structures that are in good agreement with the experiments. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L52.00004: Pleated and Creased Structures Levi Dudte, Zhiyan Wei, L. Mahadevan The strategic placement of curved folds on a paper annulus produces saddle-shaped origami. These exotic geometries resulting from simple design processes motivate our development of a computational tool to simulate the stretching, bending and folding of thin sheets of material. We seek to understand the shape of the curved origami figure by applying the computational tool to simulate a thin annulus with single or multiple folds. We aim to quantify the static geometry of this simplified model in order to delineate methods for actuation and control of similar developable structures with curved folds. Miura-ori pattern is a periodic pleated structure defined in terms of 2 angles and 2 lengths. The unit cell embodies the basic element in all non-trivial pleated structures - the mountain or valley folds, wherein four folds come together at a single vertex. The ability of this structure to pack and unpack with a few degrees of freedom leads to their use in deployable structures such as solar sails and maps, just as this feature is useful in insect wings, plant leaves and flowers. We probe the qualitative and quantitative aspects of the mechanical behavior of these structures with a view to optimizing material performance. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L52.00005: Hierarchical Stress Focusing in Elastic Ridge Lee Walsh, Benny Davidovitch A crumpled or confined elastic sheet contains many stress-focusing structures and singularities, primarily ridges and vertices, which may contain much of the strain. We seek to determine the degree and quality of stress focusing within the geometry of a single ridge. Previous work on the ridge assumes the asymptotic limit of infinitely sharp vertices. However, in a physically realistic sheet any vertex or intersection of ridges will naturally have a finite radius of curvature greater than the sheet's thickness. We simulate these more physically realistic boundary conditions in a ridge using Surface Evolver. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L52.00006: Stress focusing and collapse of a thin film under constant pressure Eugenio Hamm, Nicolas Cabezas Thin elastic sheets and shells are prone to focus stress when forced, due to their near inextensibility. Singular structures such as ridges, vertices, and folds arising from wrinkles, are characteristic of the deformation of such systems. Usually the forcing is exerted at the boundaries or at specific points of the surface, in displacement controlled experiments. On the other hand, much of the phenomenology of stress focusing can be found at micro and nanoscales, in physics and biology, making it universal. We will consider the post-buckling regime of a thin elastic sheet that is subjected to a constant normal distributed force. Specifically, we will present experiments made on thin elastoplastic sheets that collapse under atmospheric pressure. For instance, in vacuum-sealing technology, when a flat plastic bag is forced to wrap a solid volume, a series of self-contacts and folds develop. The unfolded bag shows a pattern of scars whose structure is determined by the geometry of the volume and by the exact way it stuck to its surface, by friction. Inspired by this everyday example we study the geometry of folds that result from collapsing a hermetic bag on regular rigid bodies. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L52.00007: Creasy modeling of a compressed elastic surface Tuomas Tallinen, L. Mahadevan Compression of an elastic layer attached to a rigid substrate leads to nucleation and growth of creases. We explore crease formation by a numerical model allowing control of compressive strain, anisotropy and bulk modulus. We address questions on arrangement and geometry of creases and model also the stabilizing effect of surface tension at small scales. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L52.00008: Creasing instability of elastomers under uniaxial compression Dayong Chen, Ryan Hayward Soft polymers placed under compressive stress can undergo an elastic creasing instability in which sharp folds spontaneously form on the free surfaces. This process may play an important role in contexts as diverse as brain morphogenesis, failure of tires, and electrical breakdown of soft polymer actuators, but our understanding of this instability is still quite limited. We describe a simple experimental system to study creasing of thin elastomer films under uniaxial compression. The equilibrium depths, spacings and shapes of creases are characterized and found to show excellent agreements with numerical results. Further, we use this system to explore the important roles played by surface energy and adhesion in the onset and hysteretic behavior of creases. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L52.00009: Sulcus formation in a compressed elastic half space John Biggins, L. Mahadevan When a block of rubber, biological tissue or other soft material is subject to substantial compression, its surfaces undergo a folding instability. Rather than having a smooth profile, these folds contain cusps and hence have been called creases or sulcii rather than wrinkles. The stability of a compressed surface was first investigated by Biot (1965), assuming the strains associated with the instability were small. However, the compression threshold predicted with this approach is substantially too high. I will introduce a family of analytic area preserving maps that contain cusps (and hence points of infinite strain) that save energy before the linear stability threshold even at vanishing amplitude. This establishes that there is a region before the linear stability threshold is reached where the system is unstable to infinitesimal perturbations, but that this instability is quintessentially non-linear and cannot be found with linear strain elasticity. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L52.00010: Compression induced folding of a sheet: An integrable system Haim Diamant, Thomas A. Witten The apparently intractable shape of a fold in a compressed elastic film lying on a fluid substrate is found to have an exact solution. Such systems buckle at a nonzero wave vector set by the bending stiffness of the film and the weight of the substrate fluid. Our solution describes the entire progression from a weakly displaced sinusoidal buckling to a single large fold that contacts itself. The pressure decrease is exactly quadratic in the lateral displacement. We demonstrate a subtle connection to the sine-Gordon problem, which reveals a new symmetry in the folding phenomenon. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L52.00011: Wrinkles or creases in a bi-layer structure Lihua Jin, Anesia Burns, Ryan Hayward, Zhigang Suo Wrinkles and creases are different modes of instability. In this work, we try to answer for a bi-layer structure with different modulus ratios and thickness ratios of the film and substrate whether wrinkles or creases form first when the bi-layer is under uniform compression. The onset of wrinkles corresponds to a bifurcation point, and we use the linear perturbation method to analyze the critical strain for the onset of wrinkles. Since the initiation of creases is autonomous, we directly apply the critical condition for crease initiation in a half space calculated by the finite element method in the literature to the situation of a bi-layer structure with finite thickness. By comparing the critical strains for the formation of wrinkles and creases under different modulus and thickness ratios, a phase diagram of the formation of wrinkles or creases is obtain. Although the critical strains for both wrinkle and crease initiation depend on the state of strain, remarkably the phase diagram is independent of the state of strain. As a result, creases tend to set in for more compliant and thicker films, while wrinkles tend to set in for stiffer and thinner films. Especially, when the modulus ratio of the film and substrate is smaller than 1.67, creases always form earlier than wrinkles, no matter what the thickness ratio is. We further verify the result experimentally by compressing a bi-layer of polymers with different modulus and thickness ratios. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L52.00012: Wrinkles and Folds in Ultra-Thin Polymer Films Yuri Ebata, Andrew B. Croll, Alfred J. Crosby Wrinkles and folds are observed in many biological systems during morphogenesis processes. However, the mechanics of how these wrinkles and folds form are not completely understood. Studying the mechanics of wrinkles and folds will not only provide us with fundamental insights of nonlinear deformation processes but also allow for the fabrication of unique patterned surfaces that can be controlled reversibly. In this study, we examine wrinkles and folds of polystyrene films of thickness ranging from 5 nm to 180 nm attached to uniaxially-strained polydimethylsiloxane substrates. The strain is released incrementally to apply increasing compressive strain to the attached film. The wavelength and the amplitude of local out-of-plane deformation are measured as global compression is increased to distinguish between different buckling modes. The transition from wrinkling to folding is observed by tracking the statistics of amplitude distribution sampled across a large lateral area, and a critical strain map is constructed to observe how film thickness affect the resulting buckling modes. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L52.00013: Relaxation mechanisms in the unfolding of thin sheets Benjamn Thiria, Mokhtar Adda-Bedia When a thin sheet is crumpled, creases form in which plastic deformations are localized. Here we study experimentally the relaxation process of a single fold in a thin sheet subjected to an external strain. The unfolding process is described by a quick opening at first, and then a progressive slow relaxation of the crease. In the latter regime, the necessary force needed to open the folded sheet at a given displacement is found to decrease logarithmically in time, allowing its description through an Arrhenius activation process. We accurately determine the parameters of this law and show its general character by performing experiments on both Mylar and paper sheets. [Preview Abstract] |
Session L53: Frontiers of Statistical Physics
Sponsoring Units: GSNP DCMPChair: Susan Coppersmith, Univ. of Wisconsin, Madison
Room: 153B
Tuesday, February 28, 2012 2:30PM - 3:06PM |
L53.00001: Lars Onsager Prize Lecture: A Random Walk Through Theoretical Physics Invited Speaker: Ian Affleck A historical account will be given of my efforts to apply conformal field theory techniques to experimentally relevant models of condensed matter. This began with a so far unsuccessful attempt to find the exact critical exponents for the localization transition in the integer quantum Hall effect, using techniques developed by field/string theorists. It was followed by a program to classify critical behavior of Heisenberg antiferromagnetic spin chains of arbitrary spin magnitude. It eventually led to a general theory of the low energy behavior of quantum impurity models including exact solutions for non-Fermi liquid critical points. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:42PM |
L53.00002: Dannie Heineman Prize for Mathematical Physics Lecture: Understanding Nonequilibrium via Rare Fluctuations Invited Speaker: Giovanni Jona-Lasinio Irreversible processes are a hot subject in statistical mechanics. During the last decade through the effort of several people, including the recipient of the prize and his collaborators, a progress in understanding stationary nonequilibrium states has been achieved. The key has been the study of rare fluctuations. The talk will review some basic ideas, results and perspectives. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L53.00003: Experimental confirmation of Landauer's principle John Bechhoefer, Yonggun Jun Landauer's principle, formulated in 1961, postulates that irreversible logical or computational operation such as memory erasure must dissipate heat, no matter how slowly they are performed. For example, to ``reset to one'' a memory that can be in state 0 or 1 requires at least kT ln2 of work, which is dissipated as heat. In 1982, Bennett pointed out a link to Maxwell's Demon: Were Landauer's principle to fail, it would be possible to repeatedly extract work from a heat bath. We report the first confirmation of Landauer's principle in an experimental system, where a virtual double-well potential is created via a feedback loop. We observe the position of a charged, fluorescent, colloidal particle in water and calculate and then apply a force = -grad U(x,t) via an electric field. In a first experiment, the probability of ``erasure'' (resetting to one) is unity, and at long cycle times, we observe that the work is compatible with kT ln2. In a second, the probability of erasure is zero; the system may end up in two states; and, at long cycle times, the measured work tends to zero. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L53.00004: The Basis of the Second Law of Thermodynamics in Quantum Field Thoery David Snoke, Gangqiang Liu, Steven Girvin We derive the quantum Boltzmann equation for a closed system with a two-particle collision process on the basis of quantum field theory. In the thermodynamic limit, the system evolves deterministically and irreversibly towards equilibrium, on the time scale of the scattering time of the particles. This irreversibility is related to the loss of information which comes from the vanishing off-diagonal phase coherence in the system. By calculating the time evolution of the off-diagonal elements of the generalized density matrix, we show that these terms decay rapidly due to the interaction. In the case of Bose-Einstein condensates, all phase coherence is not lost. We deduce the onset of phase coherence in a Bose-Einstein condensate, which gives rise to macroscopic wavelike behaviors of Bose systems. We also derive the H-theorem by combining our results with standard definitions of entropy. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L53.00005: The Stochastic Geometry of non-Gaussian Fields Thomas Beuman, Ari Turner, Vincenzo Vitelli Gaussian random fields pervade various areas of physics and have distinctive and well understood stochastic properties. Here we study the stochastic geometry of a random surface, whose height is given by a nonlinear function of a Gaussian field. We find that, as a result of the non-Gaussianity, the density of maxima and minima no longer match and calculate the relative imbalance between the two. We perform similar calculations for the density of umbilical points, which are topological defects of the lines of curvature. Our results apply to the analysis of speckle patterns generated by nonlinear random waves and more generally to detect and quantify non-Gaussianities present in any scalar field that can be represented as a smooth two-dimensional surface. [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L53.00006: Differential geometry of the space of Ising models Benjamin Machta, Ricky Chachra, Mark Transtrum, James Sethna We use information geometry to understand the emergence of simple effective theories, using an Ising model perturbed with terms coupling non-nearest-neighbor spins as an example. The Fisher information is a natural metric of distinguishability for a parameterized space of probability distributions, applicable to models in statistical physics. Near critical points both the metric components (four-point susceptibilities) and the scalar curvature diverge with corresponding critical exponents. However, connections to Renormalization Group (RG) ideas have remained elusive. Here, rather than looking at RG flows of parameters, we consider the reparameterization-invariant flow of the manifold itself. To do this we numerically calculate the metric in the original parameters, taking care to use only information available after coarse-graining. We show that under coarse-graining the metric contracts very anisotropically, leading to a ``sloppy'' spectrum with the metric's Eigenvalues spanning many orders of magnitude. Our results give a qualitative explanation for the success of simple models: most directions in parameter space become fundamentally indistinguishable after coarse-graining. [Preview Abstract] |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L53.00007: Growth Inside a Corner: Limiting Interface Shape Jason Olejarz, Paul Krapivsky, Sidney Redner, Kirone Mallick We investigate a simple model for crystal growth in which elemental cubes are stochastically deposited onto the inside of a three-dimensional corner. The interface of this crystal evolves into a deterministic limiting shape in the long-time limit. We incorporate known results from the corresponding two-dimensional system and use geometrical symmetries of the three-dimensional problem to conjecture an equation of motion for the interface profile which we solve analytically. The agreement between the result of the calculation and simulations of the growth process is excellent. We also present a generalization of our equation of interface motion to arbitrary spatial dimension. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L53.00008: The magnetisation distribution of the Ising model for $d\ge 5$ Per Lundow, Anders Rosengren The magnetisation distribution of the Ising model for $d>4$ is excellently fitted by a generalised binomial distribution. We have computed exactly an ansatz expression which can be fitted to the distribution near $T_c$. Though the ansatz is long and complicated it only has three parameters, besides the number of vertices, which then provides us with details about the distribution. This method also provided us with an estimate of $T_c$ for $d=6$. For extremely dense regular graphs, such as a complete bipartite graph, we can show what the parameter values are and that these values give asymptotically correct behaviour of eg the susceptibility and the free energy. Also a possible approach for $d\le 4$ will be briefly discussed as well as boundary effects for $d=5$. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L53.00009: A Topological Phase Transition in Models of River Networks Jacob Oppenheim, Marcelo Magnasco The classical Scheidegger model of river network formation and evolution is investigated on non-Euclidean geometries, which model the effects of regions of convergent and divergent flows - as seen around lakes and drainage off mountains, respectively. These new models may be differentiated by the number of basins formed. Using the divergence as an order parameter, we see a phase transition in the number of distinct basins at the point of a flat landscape. This is a surprising property of the statistics of river networks and suggests significantly different properties for riverine networks in uneven topography and vascular networks of arteries versus those of veins among others. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L53.00010: Finite size scaling of the dynamical free-energy in the interfacial regime of a kinetically constrained model Vivien Lecomte, Thierry Bodineau, Cristina Toninelli Glassy phenomena have proven difficult to understand: they present a variety of features --~slow dynamics, ageing, dynamical heterogeneity, frustration~-- which make their study arduous from a theoretical point of view. Kinetically Constrained Models (KCMs) are a simple class of lattice gas whose dynamics present features similar to those of glassy phenomena, with the advantage that no disorder is present in the model --~making them easier to study. A dynamical approach has been recently proposed: it consists in determining the large deviation function associated to the probability distribution function of time-integrated observables quantifying the ``activity'' of histories followed by the system. We determine the finite size corrections to the large deviation function of the activity in a KCM (the Fredrickson-Anderson model in one dimension), in the regime of dynamical phase coexistence. Numerical results agree with an effective model where the boundary between active and inactive regions is described by a Brownian interface. We show that the scalings of this physical picture are reflected in the finite size scaling of the dynamical free energy of the model. We expect the same picture to hold in other kinetically constrained models where the particle numberis not conserved. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L53.00011: Effective Temperature Dynamics of Radiation Induced Amorphization Ido Regev, Xiangdong Ding, Turab Lookman Materials exposed to radiation suffer structural changes over time. Typically, after exposure to radiation, a crystal will gradually lose its periodic structure and become amorphous. A theory of radiation amorphization should provide a description of the structural evolution. We study radiation amorphization in a simple molecular dynamics model and show that one can describe the amorphous steady-state using a structural effective temperature (different from the thermal bath temperature). We derive a theory that predicts the value of the steady-state effective-temperature as a function of the thermal bath temperature for a constant intensity of radiation. The theory agrees well with simulations results. [Preview Abstract] |
Session L54: Superconductivity: Mostly Devices and Applications
Sponsoring Units: DCMPChair: John Clarke, University of California, Berkeley
Room: 152
Tuesday, February 28, 2012 2:30PM - 2:42PM |
L54.00001: Proximity Induced Superconductivity in Ferromagnetic Nanowires: Magnetoresistance Oscillations and Steps in I-V Characteristics Meenakshi Singh, Mingliang Tian, Jian Wang, He Lin, Moses Chan Ferromagnetic order requires electronic spins to be aligned parallel whereas singlet superconducting order requires spins to be aligned antiparallel. This spin incompatibility limits the superconducting proximity effect to $\sim $1 nm in bulk ferromagnets. In ferromagnetic nanowires contacted with superconducting W electrodes however, the proximity effect is seen to extend to $\sim $ 400 nm [Wang et al., Nat. Phys. 6, 389 (2010)]. The mechanism behind the long range proximity in these systems is not certain. We have studied single crystalline Co nanowires contacted with normal electrodes with a single superconducting W strip patterned on the nanowire. The long range proximity effect is found to persist in this geometry. Robust magnetoresistance oscillations were found when an external field parallel to the axis of the wire was applied. In addition, regularly spaced peaks were seen in the dI/dV vs. V characteristics of the sample. The origin of the oscillations and the peaks is not understood. [Preview Abstract] |
Tuesday, February 28, 2012 2:42PM - 2:54PM |
L54.00002: Imaging point-like defects in two dimensional superconductors: theory and experiment Hilary Noad, Julie Bert, Katja Nowack, Beena Kalisky, Ilya Sochnikov, Thomas Lippman, John Kirtley, Kathryn Moler Point defects in two-dimensional superconductors produce significant local variations in the superconducting properties. A model [1] of point defects in a weak, two-dimensional superconductor, based on London's equations, shows that defects appear as haloes of decreased diamagnetic susceptibility as seen by the imaging kernel of a scanning SQUID susceptometer. We report theoretical limits on the defect strength and superconducting Pearl length $\Lambda$ required for defects to be visible. We compare these models to our experimental data showing similar haloes in the superconducting state of several types of superconducting films as a function of magnetic field, gate voltage, temperature, and height. Our ability to image these defects offers new possibilities for studying the interplay between materials properties and superconducting phenomena in thin film systems.\\[4pt] [1] V.G. Kogan and J.R. Kirtley, Phys. Rev. B \textbf{83}, 214521 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 2:54PM - 3:06PM |
L54.00003: Half-quantum vortices in superconducting networks Ilya Sochnikov, Kathryn Moler, Victor Vakaryuk We study numerically the magnetic field and temperature dependence of vortex occupation in superconducting wire networks assuming that the underlying material allows for the presence of half-quantum vortices. We focus on thermodynamic stability of half-quantum vortices, which are believed to be present in large size networks even when a stability of a single isolated half-quantum vortex is not expected. The spatial arrangements of half- and full vortices in networks are studied as well. [Preview Abstract] |
Tuesday, February 28, 2012 3:06PM - 3:18PM |
L54.00004: DNA templating for \textit{in situ} growth and measurement of 1D superconducting wires Liuqi Yu, Jingjiao Guan, Peng Xiong Controllable and reproducible fabrication of sub-10 nm metal wires is of broad scientific and technological interest. One notable example is nanowires of disordered superconductors; such a small width is generally required to place them in the 1D regime where questions remain on the nature of superconductivity and superconducting fluctuations. Molecular templating based on macromolecules, such as carbon nanotubes and DNA strands, offers a relatively straightforward and reliable method of producing nm-wide wires. Here, we report the fabrication of nanowire templates using a unique procedure of DNA stretching.\footnote{J.J. Guan \textit{et al}, Adv. Mater. 19, 1212 (2007).} DNA wires are assembled onto Si/SiO$_{2}$/Si$_{3}$N$_{4}$ substrates across trenches with an overhang, fabricated via e-beam lithography and combination of dry and wet etching. The location and orientation of the DNA wires can be well controlled. The template will be used as the substrate for \textit{in situ} growth and electrical measurement of 1D superconductor nanowires in a custom dilution refrigerator. The experiments are expected to enable a close examination of the emergence and evolution of superconductivity in true 1D limit as the wire cross-section is varied in situ on one and the same sample. [Preview Abstract] |
Tuesday, February 28, 2012 3:18PM - 3:30PM |
L54.00005: The role of dissipation in quantum phase slip production in superconducting nanowires Dima Mozyrsky We study dynamics of quantum phase slips (QPS) in ultrathin superconducting wires. At sufficiently low temperatures momentum conservation is known to pose a constraint on QPS creation, which can be lifted due to inhomogeneous external potential or due to production of quasiparticles. We argue that while the former mechanism is weak in nanowires with diameters significantly exceeding the Fermi wavelength (which is the case for most up-to-date QPS experiments), the dissipation caused by quasiparticles is likely to be the major source for the QPS generation. We argue that dissipation can be enhanced in superconductors with broken time reversal symmetry, and therefore such systems are likely to exhibit phase slips at ultralow temperatures. Our studies may clarify recent experiments (Nature Physics 5, 503 (2009)) showing the evidence of QPS at sufficiently high values of bias-current. [Preview Abstract] |
Tuesday, February 28, 2012 3:30PM - 3:42PM |
L54.00006: Quantum oscillations in superconducting mesoscopic rings of Sr$_2$RuO$_4$ Xinxin Cai, Yiqun Ying, Neal Staley, Yan Xin, David Fobes, Tijiang Liu, Zhiqiang Mao, Ying Liu Sr$_2$RuO$_4$ has attracted considerable amount of attention recently because of its potential application in topological quantum computing as a $p_x\pm ip_y$ superconductor. Cantilever magnetometry measurements carried out on doubly connected Sr$_2$RuO$_4$ samples of a micron size were found to exhibit jumps in magnetization suggesting the presence of both integer and half-integer flux quanta. To search for quantum oscillations with periods corresponding to integer and half-integer flux, we prepared micron size rings of Sr$_2$RuO$_4$ from thin flakes of Sr$_2$RuO$_4$ obtained by mechanical exfoliation and carried out transport measurements at low temperatures. Photolithography technique was applied to make contact probes on the flake and focused ion beam (FIB) was used in the fabrication. Disorder introduced by the FIB process employing high-energy beams of Ga ions was characterized by transmission electron microscopy. We succeeded in the preparation of superconducting rings of roughly 1 $\mu$m in diameter with 200$\sim$400nm line width. Strong oscillations of ring resistance with a conventional period of full flux quantum have been observed. In some samples a different period of oscillations was also found at high field range. The nature of these oscillations will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 3:42PM - 3:54PM |
L54.00007: Superconductivity of a Sn film controlled by an array of Co nanowires Zhiyuan Wei, Zuxin Ye, Daya Rathnayaka, Igor Lyuksyutov, Wenhao Wu, Donald Naugle Superconducting properties of a hybrid structure composed of ferromagnetic Co nanowire arrays and a superconducting Sn film have been investigated. Ordered Co nanowires arrays with 60 nm, 150 nm and 200 nm diameter were electroplated into the pores of self organized anodic aluminum oxide (AAO) membranes. Hysteretic dependence of the Sn film superconducting properties on applied magnetic field and critical current enhancement at moderate fields has been observed. This behavior strongly depends on the ratio of the Sn film thickness to the Co nanowire diameter. [Preview Abstract] |
Tuesday, February 28, 2012 3:54PM - 4:06PM |
L54.00008: Measurements on Superconducting Nanorings Smaller than the Coherence Length Stephen Snyder, Michael J. Erickson, Joseph Kinney, Yeonbae Lee, J.J. Nelson, Allen Goldman The Little-Parks experiment on superconducting cylinders is an important demonstration of fluxoid quantization in superconductors. The transition temperature oscillations in magnetic field have a period of $h/2e$ for the micro cylinders in their studies, which was further evidence for Cooper paring at the time {[}W. A. Little, R. D. Parks, \emph{PRL} \textbf{9}, 9 (1964){]}. However recent theoretical works have suggested that in superconducting loops smaller than the coherence length this period changes from $h/2e$ to $h/e$, for details see {[}F. Loder, et al. \emph{PRB} \textbf{78}, 174526 (2008){]} and references therein. The destructive regime has also been observed experimentally in cylinders whose diameter is small compared to the coherence length {[}Y. Liu, et al. \emph{Science} \textbf{294}, 2332 (2001){]}. We present experimental work in an effort to achieve this limit in Al nanorings prepared by electron beam lithography. These measurements achieve a regime hitherto unexplored in nanorings with interesting consequences. [Preview Abstract] |
Tuesday, February 28, 2012 4:06PM - 4:18PM |
L54.00009: Flux noise in SQUIDs: calculations of geometrical dependence Sean O'Kelley, Keenan Pepper, Steven Anton, Jeffrey Birenbaum, John Clarke Low frequency (1/$f$) magnetic flux noise in SQUIDs is understood to arise from the random reversal of electron spins localized at the surface of the superconducting film. Analytical results$^1$ that assume independent electron spins predict that the spectral density at 1~Hz scales with the washer geometry approximately as $R/W$ in the limit $R/W\gg1$. Here, $R$ is the outer radius and $W$ is the linewidth. We present numerical calculations that reproduce the analytical result in the appropriate limit and extend these results to arbitrary values of $R/W$. In addition, a logarithmic dependence on $W$, evident when $R/W$ is fixed and $W$ is varied, is reproduced and discussed. The contribution of spins at the edge of the film is also computed. We compare the predicted geometrical scaling to our recent measurements of several SQUIDs with varying geometries. Our calculation that is valid for all values of $R/W$ enables us to investigate a possible breakdown of the independent spin model in our experimental data. $^1$R. C. Bialczak \textit{et al}., Phys. Rev. Lett. \textbf{99}, 187006 (2007). [Preview Abstract] |
Tuesday, February 28, 2012 4:18PM - 4:30PM |
L54.00010: Geometry and temperature dependence of low frequency flux noise in dc SQUIDs Jeffrey Birenbaum, S.M. Anton, S.R. O'Kelley, V. Bolkhovsky, D.A. Braje, G. Fitch, M. Neeley, W.D. Oliver, F.C. Wellstood, John Clarke Measurements of low frequency magnetic flux noise in dc SQUIDs demonstrate a spectral density $S_\Phi(f)=A^2/f^\alpha$ in which the magnitude $A$ scales only weakly with the washer geometry and typically $0.5<\alpha<1$. An analytical model assuming non-interacting spins localized to the surface of the SQUID loop predicts that $A^2\propto R/W$ for $R/W\gg1$. Here, $R$ and $W$ are the outer radius and linewidth, respectively. Our numerical calculations extend the analytical results to arbitrary washer geometries. We compare both models to flux noise measurements on several dc SQUIDs fabricated simultaneously on a single chip using a Nb trilayer process. Multiple SQUIDs were measured within a single cool-down. The SQUID geometries were divided into two categories: fixed $W$=500~nm with $3 |
Tuesday, February 28, 2012 4:30PM - 4:42PM |
L54.00011: Investigation of 1/f flux noise in superconducting circuits Steven Sendelbach, Antonio Puglielli, Chang-Beom Eom, Jacob Podkaminer, Kwang Hwan Cho, Robert McDermott Low-frequency 1/f flux noise is a dominant source of dephasing in the Josephson phase and flux qubits. Recent work has revealed the presence of a high density of unpaired spins at the surfaces of superconducting thin films; it is now believed that these spins are the source of the noise, although the microscopic noise mechanism is not understood. Here we describe experiments on SQUIDs and Josephson phase qubits designed to shed light on the underlying noise mechanism, and we describe efforts to develop novel materials with reduced levels of noise. [Preview Abstract] |
Tuesday, February 28, 2012 4:42PM - 4:54PM |
L54.00012: Critical Exponents and Cluster Analysis on STM Studies of a Cuprate Superconductor Benjamin Phillabaum, Erica Carlson, Karin Dahmen In this presentation we will show results from a new kind of analysis for surface probes allowing us to gather some information about the bulk behavior of the sample that is studied. This presentation will introduce the analysis in as general a way as practical and use a particular example to concretely demonstrate how it can be applied. The particular example is the case of a local nematic order observed in a Scanning Tunneling Microscopy (STM) experiment on a BSCCO sample from which we determine which model best describes the behavior of nematic orientation using critical exponents derived from the properties of clusters and correlations of these orientations. These results would highlight key physical contributions to the nematic's orientation and guide further studies into the underlying physics. We fully expect similar impact on the study of other systems for which this analysis can be performed. [Preview Abstract] |
Tuesday, February 28, 2012 4:54PM - 5:06PM |
L54.00013: Extracting the Bosonic Spectra of Pb Using Superconducting-Tip STS and Comparing it with the Cuprates F.C. Niestemski, S. Johnston, A.W. Contryman, C.D. Camp, T.P. Devereaux, H.C. Manoharan In high-temperature superconductors the meaning of the common feature labeled ``peak-dip-hump'' is still a point of great debate. In terms of scanning tunneling spectroscopy (STS) this refers to the shape of satellite features that occur outside the coherence peaks in the $dI/dV$ spectra. There are many conflicting interpretations and labeling schemes for this feature in both the hole- and electron-doped cuprates. The path to resolving this confusion is to study a well-understood BCS superconductor to better observe the way that the STM measures bosonic information. Utilizing the ultra-low electronic noise of our home-built low-temperature STM, and utilizing a superconducting tip for increased spectral resolution, we recreate the original McMillan and Rowell S-I-S junction\footnote{W. L. McMillan and J. M. Rowell Phys. Rev. Lett., \textbf{14}, 108-112 (1965)} with the STM equivalent (S-Vacuum-S). This method provides very high energy resolution for both the filled and empty electronic states in both the superconducting and normal state. We compare this data to first-principle Eliashberg calculations and relate this data to ``peak-dip-hump'' in the high T$_c$ case. [Preview Abstract] |
Tuesday, February 28, 2012 5:06PM - 5:18PM |
L54.00014: Comparison between Simmons's Equations and Quantum Tunneling Experimental Results in A Thin Film Lianxi Ma The theoretical predictions of J.G. Simmons's equations are compared with quantum tunneling experimental results and a discrepancy is found at bias voltage $V_{b}=U$/$e$, where U is the barrier's potential height and e is the electron charge. Specifically, he divided the bias voltage into 2 regions: $V_{b} \quad < \quad U$/$e$ and $V_{b} \quad > \quad U$/$e$, and the I -- V characteristics are different in these two regions. The derived equations show a kink on differential conductance d$I$/d$V$ vs. $V_{b}$ at $V_{b}=U$/$e$, because starts at this bias the thickness of the insulation film decreases with $V_{b}$ in addition to the lowering of the barrier's average height. Therefore, the differential conductance decreases more rapidly in the region $V_{b} \quad > \quad U$/$e$ than in the region $V_{b} \quad < \quad U$/$e$. However, in tunneling experiment in which Pt is used as conductor and solid neon as insulator, we have not observed such kink even the bias was increased to 4 volts. Our speculation is either 1) there should not be a kink on conductance at $V_{b}=U$/$e$ so Simmons's equations need to be modified; 2) the kink should exist but bias voltage is not high enough to observe it in the experiments. [Preview Abstract] |
Tuesday, February 28, 2012 5:18PM - 5:30PM |
L54.00015: Size effect on supercurrent screening in large array superconducting anti-dot thin films Hsiang-Hsi Kung, Ting-Hui Chen, Chia-Tso Hsieh, Chi-Chih Ho, Ken-Hui Lin, Wen-Tau Juan, Wei-Li Lee We fabricated well-ordered and large array niobium (Nb) anti-dot thin films using a monolayer of polymer/nanosphere hybrid as a template. The hole diameter and center-to-center distance can be tuned independently. By applying a perpendicular magnetic field, we observed pronounced oscillations with field in both magnetization and resistivity due to the criteria of the flux quantization, which is reminiscent of the well-known Little-Parks experiment in a thin-walled superconducting cylinder. By varying the hole diameter, the detailed size effect on supercurrent screening can be explored and will be discussed. [Preview Abstract] |
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