Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B1: Recent Advances in Density Functional Theory II
Sponsoring Units: DCP DCOMPChair: Viktor Staroverov, University of Western Ontario (Canada)
Room: 103/105
Monday, March 3, 2014 11:15AM - 11:27AM |
B1.00001: Correlation Energy of the Homogeneous Electron Gas from Adiabatic Connection Fluctuation-Dissipation Theory including Exact Exchange kernel Nicola Colonna, Stefano de Gironcoli We have developed an expression for the electronic correlation energy via the Adiabatic Connection Fluctuation-Dissipation Theorem (ACFDT) going beyond the Random-Phase Approximation (RPA) by including exact exchange contribution to the kernel (RPAx). Our derivation is valid and efficient for general systems. It is based on an eigenvalue decomposition of the time dependent response function of the Many Body system in the limit of vanishing coupling constant, evaluated by Density Functional Perturbation Theory. We tested the accuracy of this approximation on the homogeneous electron gas. Within RPAx, the correlation energy of the homogeneous electron gas improves significantly with respect to the RPA results up to densities of the order of $r_s \approx 10$. However, beyond this value, the RPAx response function becomes pathological and the approximation breaks down. We have also evaluated the dependence of the correlation energy on the spin magnetization of the system. Both RPA an RPAx are in excellent agreement with accurate Quantum Monte Carlo results. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B1.00002: Molecular dissociation within the adiabatic connection fluctuation dissipation framework Maria Hellgren, Nicola Colonna, Stefano de Gironcoli The adiabatic connection fluctuation dissipation (ACFD) framework provides an exact expression for the correlation energy in terms of the dynamical density-density response function where the latter can be approximated using time-dependent density functional theory or many body perturbation theory. The first level of approximation is the so-called random phase approximation (RPA) which already incorporates many desirable features known to be difficult to capture with standard correlation functionals. For example, it contains the weak van der Waals forces, and the problem of large static correlation errors which appear in the dissociation limit of molecules are completely absent within the RPA. However, many properties are in quantitative error with experiment and certain features of strong electron correlation are missing. We will here show how the inclusion of exchange effects in the response function yields correlation energies, van der Waals coefficients and molecular dissociation energies in excellent agreement with experimental values. Some attention will be given to how an open-shell atom should be described in the dissociation limit. This further allows one to analyze a given approximate ACFD functional in terms of a so-called fractional charge analysis. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B1.00003: Coupled-Cluster and Linear-Response Time-Dependent Density-Functional Theory Perspectives on Particle-Particle Random-Phase Approximation Degao Peng, Helen van Aggelen, Stephan Steinmann, Yang Yang, Weitao Yang The particle-particle random-phase approximation (pp-RPA) recently attracts extensive interests in quantum chemistry recently. Pp-RPA is a versatile model to calculate ground-state correlation energies, and double ionization potential/double electron affinity. We inspect particle-particle random-phase approximation in different perspectives to further understand its theoretical fundamentals. Viewed as summation of all ladder diagrams, the pp-RPA correlation energy is proved to be analytically equivalent to the ladder coupled-cluster doubles (ladder-CCD) theory. With this equivalence, we can make use of various well-established coupled-cluster techniques to study pp-RPA. Furthermore, we establish linear-response time-dependent density-functional theory with pairing fields (TDDFT-PF), where pp-RPA can be interpreted as the mean-field approximation to a general theory. TDDFT-PF is closely related to the density-functional theory of superconductors, but is applied to normal systems to capture exact N plus/minus 2 excitations. In the linear-response regime, both the adiabatic and non-adiabatic TDDFT-PF equations are established. This sets the fundamentals for further density-functional developments aiming for pp-RPA. These theoretical perspectives will be very helpful for future study. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B1.00004: Exchange-correlation energies from pairing matrix fluctuations and the particle-particle Random Phase Approximation Invited Speaker: Helen van Aggelen Despite their unmatched success for many applications, commonly used local, semi-local and hybrid density functionals still face challenges when it comes to describing long-range interactions, static correlation and electron delocalization. Density functionals of both the occupied and virtual orbitals are able to address these problems. The particle-hole Random Phase Approximation (ph-RPA), for instance, has recently known a revival as a density functional approximation, justified by the adiabatic-connection-fluctuation-dissipation (ACFD) theorem. We formulate an adiabatic connection for the correlation energy in terms of pairing matrix fluctuations, similar in form to the ACFD theorem. With numerical examples of the particle-particle Random Phase Approximation (pp-RPA), the lowest-order approximation to the pairing matrix fluctuation, we illustrate the potential of density functional approximations based on this adiabatic connection. The pp-RPA is size-extensive, self-interaction free, fully anti-symmetric, describes the strong static correlation limit in $\mathrm{H_2}$ and eliminates delocalization errors in $\mathrm{H_2^+}$ and other single-bond systems. It gives good non-bonded interaction energies -- competitive with the ph-RPA -- with the correct $R^{-6}$ asymptotic decay as a function of the separation $R$, much better atomization energies than the ph-RPA, and reaction energies of similar quality. The adiabatic connection in terms of pairing matrix fluctuations thus paves the way for promising new density functional approximations. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B1.00005: Double, Rydberg and Charge Transfer Excitations from Pairing Matrix Fluctuation and Particle-Particle Random Phase Approximation Yang Yang, Helen van Aggelen, Weitao Yang Double, Rydberg and charge transfer (CT) excitations have been great challenges for time-dependent density functional theory (TDDFT). Starting from an $(N\pm2)$-electron single-determinant reference, we investigate excitations for the $N$-electron system through the pairing matrix fluctuation, which contains information on two-electron addition/removal processes. We adopt the particle-particle random phase approximation (pp-RPA) and the particle-particle Tamm-Dancoff approximation (pp-TDA) to approximate the pairing matrix fluctuation and then determine excitation energies by the differences of two-electron addition/removal energies. This approach captures all types of interesting excitations: single and double excitations are described accurately, Rydberg excitations are in good agreement with experimental data and CT excitations display correct 1/R dependence. Furthermore, the pp-RPA and the pp-TDA have a computational cost similar to TDDFT and consequently are promising for practical calculations. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B1.00006: Dynamical second-order Bethe-Salpeter equation kernel: a method for electronic excitation beyond the adiabatic approximation Du Zhang, Stephan Steinmann, Weitao Yang We present a dynamical second-order kernel for the Bethe-Salpeter equation to calculate electronic excitation energies. The derivation takes explicitly the functional derivative of the exact second-order self energy with respect to the one-particle Green's function. It includes naturally a frequency dependence, going beyond the adiabatic approximation. Perturbative calculations under the Tamm-Dancoff approximation, using the configuration interaction singles (CIS) eigenvectors, reveal an appreciable improvement over CIS, time-dependent Hartree-Fock and adiabatic time-dependent density functional theory results. The perturbative results also compare well with equation-of-motion coupled-cluster and experimental results. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B1.00007: On the Edge of Koopmans' Theorem Paul Grabowski, Kieron Burke It is well known that the density of an atom falls off exponentially with increasing distance to the nucleus, with a falloff length inversely proportional to the square root of the ionization energy. It is less well known what happens when the ionization energy goes to zero, which is the case if the nuclear charge is artificially reduced to the critical value. At this critical value, there is a normalizable state at the bottom of the continuum, but the density only falls off as an exponential of the square root of the radius. We calculate this state for the two-electron atom using the pseudospectral method finding the critical value of the nuclear charge to 12 digits. This is a single-center system with strong correlation, and so is a difficult test case for DFT methods. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:39PM |
B1.00008: DFT calculations with the exact functional Invited Speaker: Kieron Burke I will discuss several works in which we calculate the exact exchange-correlation functional of density functional theory, mostly using the density-matrix renormalization group method invented by Steve White, our collaborator. We demonstrate that a Mott-Hubard insulator is a band metal [1]. We also perform Kohn-Sham DFT calculations with the exact functional and prove that a simple algoritm always converges [2]. But we find convergence becomes harder as correlations get stronger. An example from transport through molecular wires may also be discussed [3].\\[4pt] [1] Lucas O. Wagner, E. M. Stoudenmire, Kieron Burke, Steven R. White, Phys. Rev. Lett. 111, 093003 (2013).\\[0pt] [2] E.M. Stoudenmire, Lucas O. Wagner, Steven R. White, Kieron Burke, Phys. Rev. Lett. 109, 056402 (2012).\\[0pt] [3] J.P. Bergfield, Z.-F. Liu, Kieron Burke, C.A. Stafford, Phys. Rev. Lett. 108, 066801 (2012) [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B1.00009: Uniform semiclassical approximations for many-particle systems Raphael Ribeiro, Kieron Burke Semiclassical analysis is used to construct uniform asymptotic approximations to the quantum one-body and kinetic energy densities of a system of noninteracting particles in a 1D potential well in the limit of infinite particle number. The approximations encode the appropriate limiting behavior of the electron density at the bulk, edge and classically-forbidden regions. High accuracy is obtained even when far from the limits assumed in the derivations. The field of density functional theory [1] is impacted in at least two ways. First, the semiclassical kinetic energy density is orbital-free, thereby providing a rare analytical development that is not based on the gradient expansion to the kinetic energy functional. Second, several results are obtained on the global and local asymptotic behavior of the quantum density and kinetic energy density everywhere in configuration space which might be established as new guiding principles for the development of approximate functionals in Kohn-Sham DFT. \\[4pt] [1] A.~Cangi P.~Elliott, D.~Lee and K.~Burke. Semiclassical origins of density functionals. {\em Phys. Rev. Lett.}, 100(25):256406 [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B1.00010: Benchmarking Density Functional Theory with Density Matrix Renormalization Group and Lessons For Higher Dimensions Thomas E. Baker, Lucas O. Wagner, E. Miles Stoudenmire, Steven R. White, Kieron Burke Kohn-Sham Density Functional Theory (DFT) is a mathematically exact method that requires approximation to the exchange correlation energy which may exclude features seen in experiment or provide inadequate estimates. Meanwhile, we may use Density Matrix Renormalization Group (DMRG), a numerical method which can accurately treat strongly correlated electrons in one dimension, to find exact DFT quantities such as the Kohn-Sham potential [1]. We use DMRG in one dimension as a benchmark to test new functionals. Further, recommendations for calculations in two and three dimensional systems are discussed as well as computational proof of principles [2]. [1] E.M. Stoudenmire, et.~al., {\it Phys.~Rev.~Lett.} {\bf 109}, 056402 (2012) [2] Lucas O. Wagner, et.~al., {\it Phys.~Rev.~Lett.} {\bf 111}, 093003 (2013) [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B1.00011: Ensemble density-functional theory for excited states: exact results versus approximations Zenghui Yang, Aurora Pribram-Jones, Kieron Burke, Richard Needs, Carsten Ullrich The ensemble density-functional theory is an exact excited-state theory, but it is not used much in practice due to unsatisfactory approximated ensemble functionals. Unlike in ground-state density-functional theory, few exact conditions are known for the ensemble; because of this, the development of approximate functionals has been slow. We present a method for inverting the ensemble density to obtain the corresponding ensemble Kohn-Sham potential, and we illustrate it for highly accurate quantum Monte-Carlo densities of the helium atom. The resulting exact ensemble Kohn-Sham potential of helium shows prominent features that do not exist in known approximate ensemble functionals. In particular, the first excitation energy calculated from the exact ensemble is demonstrated to be invariant with respect to the mixing parameter of the ensemble. No known approximation has this exact property. [Preview Abstract] |
Session B2: Focus Session: Quantum Control of Molecular, Nano, and Plasmonic Materials I
Sponsoring Units: DCPChair: Yossi Paltiel, Hebrew University
Room: 102
Monday, March 3, 2014 11:15AM - 11:51AM |
B2.00001: Real-time observation of ultrafast Rabi oscillations between excitons and plasmons in J-aggregate/metal hybrid nanostructures Invited Speaker: Christoph Lienau Surface plasmon polaritons (SPPs), optical excitations at the interface between a metal and a dielectric, carry significant potential for guiding and manipulating light on the nanoscale. Their weak optical nonlinearities, however, hinder active device fabrication, $e.g.$, for all-optical switching or information processing. Recently, strong optical dipole coupling between SPPs and nonlinear quantum emitters with normal mode splittings of up to 700 meV has been demonstrated [1,2]. The predicted ultrafast energy transfer between quantum emitters and SPP fields could be a crucial microscopic mechanism for switching light by light on the nanoscale. Here, we present the first real-time observation of ultrafast Rabi oscillations in a J-aggregate/metal nanostructure, evidencing coherent energy transfer between excitonic quantum emitters and SPP fields. We demonstrate coherent manipulation of the coupling energy by controlling the exciton density on a 10-fs timescale, a step forward towards coherent, all-optical ultrafast plasmonic circuits and devices. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B2.00002: Nonlinear optics of hybrid nano-materials under strong coupling conditions Maxim Sukharev Modern optics fueled with both tremendous advances in nano-fabrication and laser physics is currently experiencing significant growth. We are presently witnessing a unique situation - the research centered at interaction of matter with electromagnetic radiation is fully diving into nanoscale, where one considers purely quantum systems optically driven by nano-materials. The possibilities are vast ranging from fundamental ideas on single atom/molecule optical manipulation, through control of light far below the diffraction limit, to optical engineering and photonic circuitry. Despite progress, the research in optics of quantum media coupled to nano- materials is not complete. Many recent works consider just several quantum emitters driven by near-fields altered by plasmonic materials with a few very promising attempts to include collective effects, which as I will show in this talk play a pivotal role in quantum optics of nano-materials. I will discuss general concepts of nano- plasmonics (one of the most promising sub-fields of nano-optics) with several examples ranging from linear spectroscopy to nonlinear transient absorption. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B2.00003: Ultrafast Coherent Nanoscopy and Control of Plasmonic Nanostructures Dmitri Voronine, Charles Ballmann, Alexei Sokolov Space-time-resolved nonlinear optical spectroscopy with nanoscale spatial and femtosecond temporal resolution may provide structural and dynamical information of various ultrafast processes such as energy and electron transfer, protein folding, etc. Theoretical analysis of tip-enhanced coherent anti-Stokes Raman scattering (TECARS) using a new plasmonic nanostructure is presented. Two tips are used to provide near-field enhancement and control of the nanoantenna response. TECARS signals from different hot spots are obtained by laser pulse shaping and tip manipulation. Various applications of time-resolved surface-enhanced coherent Raman spectroscopy and strategies for manipulating the spectra are discussed. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B2.00004: Coherent Field Emission by Superfocused Plasmons Julia Majors, Alejandro Rodriguez Perez, Joonhee Lee, V. Ara Apkarian Field emission takes place upon focusing propagating surface plasmon polaritons (SPP) at the apex of a sharp metal tip. The effect is demonstrated with remotely launched SPPs on a silver probe tip. We couple femtosecond laser pulses through a grating inscribed on the taper of a smooth, silver wire 30um from the apex. Field-emitted current is directly correlated with the radiation of the super-focused SPP at the apex. Both current and radiation at the apex are measured as a function of incident polarization on the grating. In the absence of incident light at the apex, the local field of the ``naked'' surface plasmon modulates the tunneling barrier that drives the field emission. We give a detailed analysis of the governing dynamics in the presence and absence of an applied extractor field, and clearly distinguish contributions to tunneling current from Fermi electrons, athermal electron-hole pair distribution created by Landau damping of the plasmon, and the thermalized electrons. Independent of the distribution of the electrons in the metal half-space the emission acquires coherence by the time-dependent field of the plasmon in the vacuum half-space. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B2.00005: Manipulating Energy Flow at the Nanoscale by Coupling Plasmons of Metal Nanostructures to Resonant Molecules Invited Speaker: Gary Wiederrecht Collective hybrid excitations resulting from the coupling of metal nanostructures with organic molecules present unique opportunities for manipulating light-matter interactions at the nanoscale. In this talk, I discuss recent studies that are examples of the breadth of phenomena that are possible. First, the interactions of coupled plasmonic nanostructures with azobenzene-based polymers are described, in which the spatial features of the plasmonic near-field can be used to manipulate molecular motion. The directional molecular transport that results is shown to be useful for imaging the spatial and polarization features of the optical near-field. The modeling of this effect is described. Second, the coupling of excitonic molecular aggregates to metal nanostructures produces coherent coupling that provides added structure to the optical extinction spectra of metal nanoparticles, thereby by providing a photonic handle with which to manipulate energy flow on an ultrafast timescale. Monitoring the rate of energy flow as a function of photon energy reveals important information about the energy dissipation channels and the structural interactions between molecule and metal. Third, the strongly enhanced optical nonlinearity resulting from coupled plasmonic nanorods is described. The closely spaced nanorod material exhibits nonlocality of the optical response that has an unusually strong nonlinear dependence on incident light intensity. Electromagnetic modeling confirms the nonlocal response of the plasmonic metamaterial. The broader impact of collective hybrid excitations on nanophotonics applications is described. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B2.00006: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 1:15PM - 1:27PM |
B2.00007: Coherent polarization dynamics in three-photon photoemission from Cu(111) surface Cong Wang, Xuefeng Cui, Sean Garrett-Roe, Hrvoje Petek We investigate the surface states on Cu(111) by multi-photon photoemission using tunable ultra-short (\textless 15 fs) laser pulses. The angle-resolved photoemission spectra in the pre-resonant region for the two-photon excitation from the Shockley surface (SS) state to the n$=$1 image potential state are consistent with the well-known band structures. An anomaly is observed, however, for excitation at 610 nm (2.04 eV), where the tripling of the SS band occurs, the SS appearing as three parallel bands with the same effective mass. Excitation with the doubled frequency does not support this phenomenon. Instead we attribute this to parallel excitation pathways where the multi-photon photoemission occurs in response to the external and internal fields. The external field at 2.04 eV can drive the multiphoton absorption from SS, but also the coherent local field associated with the transition from top of the d-band to the sp-band at its crossing with the Fermi level can drive the photoemission. We examine the coherent nonlinear polarization dynamics by performing interferometric two-pulse correlation measurements. Fourier transform with respect to delay axis provides 2D linear and nonlinear spectra of the coherent polarization leading to the observed phenomena. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B2.00008: Resonant secondary light emission from plasmonic Au nanostructures at high electron temperatures created by pulsed laser excitation David Cahill, Jingyu Huang, Wei Wang, Catherine Murphy We study continuous-wave (cw) and pulsed laser excitation of aqueous suspensions of Au nanorods (AuNRs) as a model system for secondary light emission from plasmonic nanostructures. Resonant secondary emission contributes significantly to the background commonly observed in surface-enhanced Raman scattering (SERS) and to the light emission generated by pulsed laser excitation of metallic nanostructures that is often attributed to two-photon luminescence (TPL). The intensity of anti-Stokes emission collected using cw laser excitation at 785 nm is described by a 300 K thermal distribution of excitations. Excitation by sub-picosecond laser pulses at 785 nm broadens and increases the intensity of the anti-Stokes emission in a manner that is consistent with electronic Raman scattering by a high temperature distribution of electronic excitations predicted by a two-temperature model. Broadening of the pulse duration using an etalon reduces the intensity of anti-Stokes emission in quantitative agreement with the model. Experiments using a pair of sub-picosecond optical pulses separated by a variable delay show that the time-scale of resonant secondary emission is comparable to the time-scale for equilibration of electrons and phonons. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B2.00009: Cooperative amplification of energy transfer in plasmonic systems T.V. Shahbazyan, V.N. Pustovit, A.M. Urbas We study cooperative effects in energy transfer (ET) from an ensemble of donors to an acceptor near a plasmonic nanostructure. We demonstrate that in cooperative regime ET takes place from plasmonic superradiant and subradiant states rather than from individual donors leading to a significant increase of ET efficiency. The cooperative amplification of ET relies on the large coupling of superradiant states to external fields and on the slow decay rate of subradiant states. We show that superradiant and subradiant ET mechanisms are efficient in different energy domains and therefore can be utilized independently. We present numerical results demonstrating the amplification effect for a layer of donors and an acceptor on a spherical plasmonic nanoparticle. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B2.00010: Proximity Resonance and Localized Surface Plasmons Bo Liu, Eric Heller The collective excitation of conduction electrons in subwavelength nanostructures is known as Localized Surface Plasmon(LSP)[1]. Such plasmon modes has been intensively studied using noble nanoparticles . More recently, the possibility of building terahertz metamaterials supporting such LSP modes has been explored in graphene microribbons and microdisks. Unlike Surface Plasmon Polaritons(SPPs) at metal-insulator interface, LSP can be directly excited by light illumination and holds promise for applications in ultrasensitive biosensing, nano-optical tweezers and improved photovoltaic devices. In this paper, we consider the interaction of two LSPs in the weak coupling regime and show how an effect similar to the proximity resonance in the quantum scattering theory\cite{a2} gives rise to an asymmetric(quadrupole) mode with increased damping rate. The existence of this asymmetric mode relies on a small phase retardation between the two LSPs. This phase retardation, though small, is key to both increased damping rate for the asymmetric mode and reduced damping rate for the symmetric mode. When this small phase retardation is removed by changing the polarization of the exciting light,we show that the asymmetric mode can not be excited and the symmetric mode shows increased damping. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B2.00011: Two-photon activation of photoactive ligands bound to gold surfaces Brenden A. Magill, Xi Guo, Erich M. See, Roberto L. Reyes, Richey M. Davis, Webster L. Santos, Hans D. Robinson Photoactive crosslinkers are useful tools for optically driven assembly of nano-particles. We report on the use of ultra-short laser pulses to affect localized photoreactions in o-nitrobenzyl-based photoactive ligands bound to a gold surface with thiol groups. The reaction is activated through a combination of thermal activation and two-photon absorption, while at higher power densities, ligands can be ablated from the surface through breaking of the gold-thiol bond. We will present data on the interplay of these three effects as a function of laser power and exposure time, and demonstrate assembly of nanoparticles onto optically patterned surfaces. Finally, we will discuss how this effect could be used to create well-defined nanoparticle assemblies where great binding-site selectivity can be obtained through the combination of high electromagnetic intensity enhancements at plasmon hotspots and the nonlinear scaling of photoactivation efficiency in two-photon absorption processes. We acknowledge financial support from the National Science Foundation and the Institute for Critical Technology and Applied Science. [Preview Abstract] |
Session B3: Undergraduate Research - Society of Physics Students II
Sponsoring Units: SPSChair: Daniel Golombek, Society of Physics Students- American Institute of Physics
Room: 107
Monday, March 3, 2014 11:15AM - 11:27AM |
B3.00001: Gold and Gallium Nanoparticle Growth on Silicon (100) Rees Madsen, Hunter L. Brown, Sadie Ames, J. Leland Rasmussen, Samuel Tobler Nanoparticles are used for various applications in today's research. Some researcher's interests involve using the nanoparticles to grow silicon nanowires on a silicon substrate. Before growing nanowires can be accomplished a study must be made of the formation of nanoparticles. Most often the metal used to make the nanoparticles is gold. In this study both gold and gallium were used to make the nanoparticles, by thermal evaporation. The gold and gallium nanoparticles were grown on silicon (100). Between one to three monolayers of material was added to the substrate, with the particle sizes ranging from 0.5 microns to 3 microns in diameter. Densities of nanoparticles varied based on the time of growth and on the intensity of the source. The variable sizes were seen with sample temperatures between 700 C and 900 C measured using a disappearing filament optical pyrometer. The growth process occurred at pressures below 3e-7 Torr. This presentation will summarize the growth process and show the similarities and differences between the two metals. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B3.00002: Optimization of Thick, Large Area YBCO Film Growth Through Response Surface Methods J. Porzio, C.H. Mahoney, M.C. Sullivan We present our work on the optimization of thick, large area YB$_2$C$_3$O$_{7-\delta}$ (YBCO) film growth through response surface methods. Thick, large area films have commercial uses and have recently been used in dramatic demonstrations of levitation and suspension. Our films are grown via pulsed laser deposition and we have optimized growth parameters via response surface methods. Response surface methods is a statistical tool to optimize selected quantities with respect to a set of variables. We optimized our YBCO films' critical temperatures, thicknesses, and structures with respect to three PLD growth parameters: deposition temperature, laser energy, and deposition pressure. We will present an overview of YBCO growth via pulsed laser deposition, the statistical theory behind response surface methods, and the application of response surface methods to pulsed laser deposition growth of YBCO. Results from the experiment will be presented in a discussion of the optimized film quality. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B3.00003: Synthesis and characterization of the physical properties of \textit{RE}$_{3}$TiSb$_{5}$ (\textit{RE} $=$ La, Ce) Tracy Stevenson, Chantal Douglas, Daniel Jackson, Derrick VanGennep, James Hamlin \textit{RE}$_{3}$TiSb$_{5}$ (\textit{RE}$=$ La, Ce) is synthesized to investigate the physical properties and test for superconductivity. Ce$_{3}$TiSb$_{5}$ was synthesized using flux growth in both Sn and Zn fluxes while La$_{3}$TiSb$_{5}$ was synthesized using Sn flux. Magnetic, resistive, and heat capacity measurements all indicate that neither of these compounds are superconducting. Based on our results La$_{3}$TiSb$_{5}$ is non-magnetic and Ce$_{3}$TiSb$_{5}$ has an antiferromagnetic phase transition near 5 K. Neither compound exhibits typical metallic transport behavior. This is in contradiction to previous results. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B3.00004: Optimization of growth conditions for (La$_{1-y}$Pr$_{y})_{1-x}$Ca$_{x}$MnO$_{3}$ thin films on annealed oxide substrates Brian Schaefer, Daniel Grant, Amlan Biswas Consistent growth of flat, epitaxial thin films is essential for uncovering the unique transport characteristics of rare-earth manganite systems. We have developed pulsed laser deposition growth conditions for (La$_{1-y}$Pr$_{y})_{1-x}$Ca$_{x}$MnO$_{3}$ (LPCMO, $y =$ 0.4, 0.5, 0.6) thin films on annealed NdGaO$_{3}$ (NGO) and SrTiO$_{3}$ (STO) substrates. The extra annealing step for NGO and STO produces atomically flat substrates with well-defined terraces of unit cell step height. Films grown on these annealed substrates demonstrate better lattice matching compared to films grown on as-received substrates. Consequently, annealing substrates before film growth leads to higher quality thin films with a more controllable thickness. We demonstrate that these optimized growth parameters yield LPCMO thin films that are also atomically flat, as confirmed by atomic force microscopy. We are using these thin films to restrict phase growth to reduced dimensions and to study the origin of thermodynamic phase competition due to first order transitions in manganites. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B3.00005: Synthesis and characterization of hydrothermally grown ZnO nanomaterials for biomedical applications Austin Shearin, Anagh Bhaumik, Adam Wanekaya, Robert Delong, Kartik Ghosh Nanomaterials have been of recent importance in the biomedical field due to their use in drug delivery applications, magnetic resonance imaging, and cell separation. Intrinsically nanomaterials of ZnO are having low cytotoxicity and genotoxicity which is suitable for several biomedical applications. The aim of this work has been to synthesize high quality ZnO nanostructures using hydrothermal process with varied growth parameters. X-ray diffraction studies on the high quality synthesized materials confirmed the hexagonal crystal structure as well as the nano-crystallite size of ZnO. Raman spectroscopy has been done on the nanostructured ZnO to understand the different phonon modes present in the molecule. Scanning electron microscopy was used to observe shape and size of the synthesized nanomaterials. Future work to be done is to study interaction kinetics between ZnO nanostructures with biomolecules such as ATP, RNA and protein. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B3.00006: Colossal piezoresistance in phase separated manganites Maria Viitaniemi, In Hae Kwak, Amlan Biswas Hole-doped manganese oxides (manganites) exhibit piezoresistance, which is defined as a change in electrical resistance as a function of strain. At low temperatures and for particular chemical compositions, manganites can also exist in a thermodynamic phase separated state. In this phase separated state, piezoresistance can increase dramatically, a phenomenon called colossal piezoresistance (CPR). By modifying an existing low temperature probe and measurement setup, we measured the resistance of thin films of the manganite (La$_{\mathrm{1-y}}$Pr$_{\mathrm{y}})_{\mathrm{1-x}}$Ca$_{\mathrm{x}}$MnO$_{\mathrm{3}}$ (LPCMO). We used a three-point beam bending method to control the compressive or tensile strain on these thin films. The resulting change in resistance and thermal hysteresis reveals phase separation and CPR in LPCMO. We are performing similar tests on La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ (LSMO) thin films. LSMO is of interest to us because it may be possible to induce phase separation in this material at room temperature and above. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B3.00007: Low-cost Flexible Memristor Fabrication Inna Kirilyuk, Nadine Gergel-Hackett Memristors are two-terminal electronic devices that exhibit unique electrical characteristics, including nonvolatile electrical switching between resistive states. These unique electrical characteristics may enable the use of memristors as logic and/or memory components in novel computer architectures. Flexible memristors have been shown to not only exhibit the electrical characteristics unique to the devices, but are also operable after 4,000 flexes, require voltages less than 10 V, show on:off ratios \textgreater 10,000:1, are nonvolatile for up to 14 days, and are fabricated at room-temperature with sol-gel solution processing. To increase accessibility to the technology and decrease production costs, we are developing methods of flexible memristor fabrication that are low-cost compared to current conventional fabrication. This low-cost fabrication includes exploring alternative materials and processes for device contacts, synthesizing and storing titanium dioxide sol-gel using standard wet chemistry tools without the use of a glove box, and using a low-cost spinner to spin sol-gel onto devices. The fabrication is performed entirely in an undergraduate lab setting by undergraduate students. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B3.00008: Electrical and structural Characterization of Ba(Sc$^{3+}$, Sb$^{5+})_{\mathrm{y}}$Ti$_{0.90}$O$_{3}$ with y $=$ 0.05 and 0.10 Julio Cantu, Jerry Contreras, Daniel Potrepka, Frank Crowne, Arthur Tauber, Steven Tidrow Ba(Sc$^{3+}$, Sb$^{5+})_{\mathrm{y}}$Ti$_{0.90}$O$_{3}$, with y $=$ 0.05 and 0.10, is investigated through temperature dependent electrical and structural characterization. The material is electrically characterized from 10 Hz to 2MHz for dielectric constant, tunability, dissipation factor and figure of merit over the temperature range of -50 $^{\circ}$C to 125 $^{\circ}$C. In addition, lattice parameters and structural changes of the material are reported as a function of temperature as obtained using x-ray diffraction and Rietvield refinement. Properties of these electric-field tunable materials are discussed in terms of ferroelectrics, non-relaxor versus relaxor behavior, and a ferrorelectric dipole-like glass state. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B3.00009: Electrical and Structural Characterization of Ba(Y, Ta)$_{\mathrm{x}}$ Ti$_{\mathrm{1-2x}}$O$_{3}$ With x $=$ 0.025 and 0.05 Jerry Contreras, Steven C. Tidrow, Daniel Potrepka, Frank Crowne, Arthur Tauber Ba(Y, Ta)$_{\mathrm{x}}$ Ti$_{\mathrm{1-2x}}$O$_{3}$, with x $=$ 0.025 and 0.05, is investigated through temperature dependent electrical and structural characterization. The material is electrically characterized from 10Hz to 2 MHz for dielectric constant, tunability, dissipation factor and figure of merit over the temperature range -50 $^{\circ}$C to 125 $^{\circ}$C. In addition, lattice parameters and structural changes of the material are reported as a function of temperature as obtained using x-ray diffraction and Rietveld refinement. Properties of these electric-field tunable materials are discussed in terms of ferroelectrics, non-relaxor versus relaxor behavior, and a ferroelectric dipole-like glass state. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B3.00010: Electrical and Structural Characterization of Ba(In,Sb)$_{y}$Ti$_{1-2y}$O$_{3}$ With y $=$ 0.05 and 0.10 Oscar Guerrero, Jerry Contreras, Daniel Potrepka, Frank Crowne, Aurthur Tauber, Steven Tidrow Ba(In,Sb)$_{0.05}$Ti$_{0.90}$O$_{3}$ and Ba(In,Sb)$_{0.10}$ Ti$_{0.80}$O$_{3}$ are investigated through temperature dependent electrical and structural characterization. The material is electrical characterized from 10 Hz to 2 MHz for dielectric constant, tunability, dissipation factor and figure of merit over the temperature range -50 $^{\circ}$C to 125 $^{\circ}$C. In addition, lattice parameters and structural changes of the material are reported as a function of temperature as obtained using x-ray diffraction and Rietveld refinement. Properties of these electric-field tunable materials are discussed in terms of ferroelectrics, non-relaxor versus relaxor behavior, and a ferroelectric dipole-like glass state. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B3.00011: Interfacial Layer Optimization in Organic Photovoltaics Joshua Litofsky, Evan Lafalce, Xiaomei Jiang Organic photovoltaic devices (OPVs) based on benchmark $\pi -$conjugated polymer polythiophene and electron acceptor PCBM are made up of a sandwich-like structure of multifunctional layers. Interfacial layers (IL) facilitate charge transport between the charge generation layer and the electrodes and enhance charge extraction. Optimizing the IL thus provides one mean of maximizing the efficiency of OPVs. Various electron transport layers such as ZnO and LiF were used, and hole transport layers included PEDOT:PSS and V$_{\mathrm{2}}$O$_{\mathrm{5}}$. Two different device architectures were explored: conventional structure with ITO serving as an anode and inverted structure when ITO acts as a cathode. Using various deposition techniques, we worked to optimize IL thickness and film formation methods. By analyzing device shunt and series resistances using a standard diode equation, we were able to identify the optimal parameters for device performance. The combination of thin IL with electrodes of appropriate work function yielded much better results compared to the control device with no IL. We can use these results and techniques to further optimize future OPV devices based on other novel material systems. This work was supported by the NSF REU grant {\#} DMR-1263066: REU Site in Applied Physics at USF. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B3.00012: Estimating Ultra-High Energy Cosmic Ray Data as seen from the JEM-EUSO Fluorescence Detector for the planned space based JEM-EUSO detector Jeremy Fenn, Lawrence Wiencke Ultra-high energy cosmic rays (UHECRs) are subatomic particles with energies above 10$^{18}$ eV. UHECRs are of interest because they are the highest energy particles known to exist. Their source(s), compositions, and the acceleration mechanisms to produce them with energies beyond 10$^{20}$ eV remain unknown. The Pierre Auger Observatory, located in Argentina, is the world's largest UHECR observatory. It is one of the few a hybrid detectors in the world that combines surface (SD) and fluorescence (FD) detectors. The hybrid detection system is advantageous as it provides a more accurate reconstruction of the incoming cosmic ray's energy and trajectory as it travels through the atmosphere. However, even with the advantage of a hybrid detector, the Pierre Auger has limitations being a ground based observatory. The next generation in UHECR detection is the planned JEM-EUSO mission. The JEM-EUSO mission will consist of a fluorescence detector telescope attached to the International Space Station (ISS). The JEM-EUSO detector is expected to receive an exposure level to UHECRs many times that of the Pierre Auger Observatory by viewing a much larger volume of the atmosphere. In this presentation, I will discuss how data from specific UHECRs collected by the Pierre Auger Observatory is analyzed and altered to estimate what their signatures would look like from space at the planned JEM-EUSO detector. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B3.00013: Aiming lasers into the sky from the Pierre Auger Cosmic Ray Observatory at astrophysical objects of interest Steven Hackenburg, Lawrence Weincke Ultra High Energy Cosmic Rays at are the highest energy particles known to exist, they are also some of the rarest with a flux less than 1 per century per square km. To study cosmic rays the worlds largest cosmic ray observatory, the Pierre Auger Observatory was built and completed in 2008. The data collected from the observatory hints at a correlation between cosmic ray arrival directories on earth and certain active galaxies. The Central Laser Facility (CLF) is located in the middle of the observatory, which recently had upgrades added on June 2013. The CLF has a UV pulsed laser. If the laser is fired into the atmosphere, a laser track is created that is similar to the signatures left behind by cosmic ray events in the atmosphere. Hence, the laser is used to calibrate the detectors at the observatory and create benchmark data. An example of benchmark data is artificial sky maps, consisting of reconstructed laser tracks, pointed toward Galaxies of interest. This presentation will describe the technique of using the lasers as a benchmarking tool to create artificial sky maps. Improvements to the timing and pointing accuracy of the method will also be discussed. [Preview Abstract] |
Session B4: Focus Session: Antiferromagnets on a Triangular Lattice
Sponsoring Units: GMAGChair: Oleg Starykh, University of Utah
Room: 112/110
Monday, March 3, 2014 11:15AM - 11:51AM |
B4.00001: Molecular Quantum Magnetism in LiZn$_2$Mo$_3$O$_8$ Invited Speaker: Martin Mourigal Considerable theoretical and experimental efforts are devoted to understanding frustrated two-dimensional antiferromagnets, searching for quantum spin-liquid states hosting deconfined fractional spin excitations. To make quantitative comparisons with theory, the sensitivity to defects and site mixing inherent to magnetic transition metal oxides is a significant challenge. Spin degrees of freedom delocalized on stable organic molecules or inorganic clusters offer an interesting alternative. The layered insulating material LiZn$_2$Mo$_3$O$_8$ is such a compound. It comprises spin-1/2 Mo$_3$O$_{13}$ molecules organized on the triangular lattice [1]. Its thermo-magnetic properties suggest it hosts collective magnetic phenomena with hints of a possible valence-bond condensation and absence of long-range spin order. Inelastic neutron scattering from powder specimen of LiZn$_2$Mo$_3$O$_8$ reveal the presence of gapless collective magnetic excitations at low-energy that are surprisingly broad in momentum space and involve at most a third of the spins. The corresponding structure factor is consistent with the presence of valence-bonds involving nearest-neighbor and next-nearest-neighbor spins [2]. No magnetic signal is apparent at higher energies, suggesting that the remaining spins contribute as a broad continuum rather than as a well defined resonance. LiZn$_2$Mo$_3$O$_8$ thus offers an example of molecular based spin-liquid material with collective excitations consistent with a disordered or dynamic ground-state.\\[4pt] [1] J. P. Sheckelton, J. R. Neilson, D. G. Soltan, and T. M. McQueen, Nature Mater. {\bf 11}, 493496 (2012).\\[0pt] [2] M. Mourigal, W. T. Fuhrman, J. P. Sheckelton, A. Wartelle, J. A. Rodriguez-Rivera, D. L. Abernathy, T. M. McQueen, and C. L. Broholm, arXiv:1309.1165 [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B4.00002: Local-scale magnetic studies of the condensed valence-bond state in LiZn$_{2}$Mo$_{3}$O$_{8}$ John Sheckelton, Francesca Foronda, LiDong Pan, Camilla Moir, Ross McDonald, Tom Lancaster, Peter Baker, N. Peter Armitage, Takashi Imai, Stephen Blundell, Tyrel McQueen The reduced molybdenum oxide LiZn$_{2}$Mo$_{3}$O$_{8}$ is an insulating material composed of layers of Mo$_{3}$O$_{13}$ clusters, with the clusters arranged on a triangular lattice and non-magnetic Li/Zn inter-layers. A formal electron count results in each molybdenum cluster acting as S$=$1/2 magnetic unit. Superexchange between clusters is mediated through Mo-O-Mo oxo bridges that lead to a frustrated magnetic state. Local-scale magnetic measurements indicate the existence of a gapless spin excitation spectrum that persists down to the lowest temperatures measured. In addition, these local probe measurements indicate local magnetic behavior that corresponds to bulk measurements. The data presented are consistent with expected magnetic responses of a condensed valence-bond state. Structural and measured magnetic properties and ongoing research will be discussed. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B4.00003: Dynamical structure factor of the triangular-lattice antiferromagnet Alexander Chernyshev, Martin Mourigal, Wesley Fuhrman, Michael Zhitomirsky We have elucidated the role of magnon interaction and spontaneous decays in the spin dynamics of the triangular-lattice Heisenberg antiferromagnet by calculating its dynamical structure factor within the spin-wave theory. Explicit theoretical results for neutron-scattering intensity will be shown for spins $S = 1/2$ and $S = 3/2$. The dynamical structure factor exhibits unconventional features such as quasiparticle peaks broadened by decays, non-Lorentzian lineshapes, and significant spectral weight redistribution to the two-magnon continuum. This rich excitation spectrum illustrates the complexity of the triangular-lattice antiferromagnet and provides distinctive qualitative and quantitative fingerprints for experimental observation of decay-induced magnon dynamics. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B4.00004: Possible Structural Distortions in NiGa$_{2}$ S$_{4}$ indicated by T-dependent Raman Modes Michael Valentine, Satoru Nakatsuji, Tomoya Higo, Collin Broholm, Natalia Drichko NiGa$_{2}$S$_{4}$ contains two dimensional sheets of spin-1 Ni$^{2+}$ ions arranged in a triangular lattice where ferromagnetic nearest neighbor interactions and antiferromagnetic third nearest neighbor interactions lead to magnetic frustration which suppresses three dimensional magnetic ordering above 1.5K [1]. We studied structural distortions in NiGa$_{2}$S$_{4}$ by Raman spectroscopy on single crystals in the energy range of 150 cm$^{-1}$ to 500 cm$^{1}$. For temperatures below 300K the 446 cm$^{-1}$ A$_{\mathrm{1g}}$ mode splits and additional E$_{\mathrm{g}}$ modes are observed between 250 cm$^{-1}$ and 450 cm$^{-1}$. These high energy features are associated with sulfur vibrations but are not predicted to occur on the basis of the point group symmetry inferred from x-ray diffraction. We discuss possible lattice distortions due to magneto-elastic coupling and their potential effects on low temperature frustrated magnetism. \\[4pt] [1] C. Stock et al., Phys. Rev. Lett. 105, 037402 (2010) [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B4.00005: Ba NMR studies of the triangular lattice antiferromagnets Georgios Koutroulakis, Tong Zhou, Cristian Batista, Yoshitomo Kamiya, Joe Thompson, Stuart Brown, Haidong Zhou Ba$_3$MSb$_2$O$_9$, with $M$=Co, Ni are triangular lattice magnetic systems with near-neighbor antiferromagnetic exchange. Previous studies have shown that Ba$_3$CoSb$_2$O$_9$ has a stabilized up-up-down spin configuration with in-plane field and the resultant one-third magnetization plateau has been observed. On the other hand, for the $M$=Ni system with 6H-B structure there has been no evidence of a magnetic ordered phase and thus it is being seen as a candidate spin-liquid material. Existing NMR data show a very broad Ba line comprised of signals from three different Ba sites, and the relaxation rate show a very weak temperature dependece, which is similar to the Co compound in the high symmetry phase. Here we report on Ba nuclear magnetic resonance (NMR) spectroscopy and spin-lattice relaxation measurements for both compounds. For the Co system, we will report data revealing the magnetization process up to 30T and present a detailed picture of the phase diagram. For the Ni compound, we are reporting the temperature evolution of the spectra and the temperature dependence of the relaxation rate for both Ba and Sb. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B4.00006: Successive magnetic phase transitions and multiferroicity in quasi-two-dimensional triangular lattice Heisenberg antiferromagnets Ba$_{3}$CoNb$_{2}$O$_{9}$ and Ba$_{3}$MnNb$_{2}$O$_{9}$ M. Lee, J. Hwang, E.S. Choi, J. Ma, C.R. Dela Cruz, M. Zhu, X. Ke, Z.L Dun, H.D. Zhou We have measured magnetic, dielectric and thermodynamic properties of quasi-two-dimensional triangular lattice antiferromagnet (TLAF), Ba$_{3}$CoNb$_{2}$O$_{9}$ (S$=$ 1/2) and Ba$_{3}$MnNb$_{2}$O$_{9}$ (S$=$ 3/2). At zero magnetic field, Ba$_{3}$CoNb$_{2}$O$_{9}$ undergoes a two-step transition at 1.36 K and 1.10 K and enters a 120 degree ordered state. By applying magnetic fields, a series of magnetic phases with fractional saturation magnetization (1/3, 1/2, 2/3 (or $\sqrt 3 $/3Ms) are observed. The collinear spin phase with 1/3 Ms becomes more robust at lower temperatures due to quantum fluctuations. For Ba$_{3}$MnNb$_{2}$O$_{9}$, the 120 degree ordered state is stabilized below 3.10 K at zero field. Under the magnetic field, successive magnetic phase transitions are observed with fractional magnetization 1/3 and 1/2 Ms. The 1/3 Ms phase becomes more stable at higher temperatures due to thermal fluctuations. The ferroelectricity emerges in all spin states in both compounds regardless of the spin chirality. Therefore, Ba$_{3}$CoNb$_{2}$O$_{9}$ and Ba$_{3}$MnNb$_{2}$O$_{9}$ are unique TLAFs exhibiting not only a series of magnetic phase transitions but also multiferroicity. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B4.00007: Calorimetric determination of magnetic phase diagram of Ba$_{3}$CoSb$_{2}$O$_{9}$ Nathanael Fortune, Scott Hannahs, Ju-Hyun Park, Haidong Zhou, Christopher Aoyama, Yasumasa Takano We report heat capacity measurements as a function of temperature, magnetic field, and field angle for the spin 1/2 triangular antiferromagnet Ba$_{3}$CoSb$_{2}$O$_{9}$, for fields up to the saturation field of 32.5 T and temperatures down to 0.1 K. The system behaves as a frustrated Heisenberg antiferromagnet with weak xy anisotropy. For magnetic fields applied along the a axis, 3 main magnetic phases are expected [1], each corresponding to a different spin arrangement: a low field ``Y'' phase, an intermediate ``up - up - down'' phase, and a high field 'V' phase. We find instead 5 distinct magnetic phase transitions with increasing field, suggesting that the ``Y' and ``V'' phases split into alternating and non-alternating co-planar subphases. \\[4pt] [1] A.V. Chubukov \& D.I. Golosov, J.Phys.Cond.Mat 3, 69 (1991). [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B4.00008: The Angular Dependent Magnetic Phase Diagram of Cs$_{2}$CuCl$_{4}$ Scott T. Hannahs, Nathanael Fortune, Yasumasa Takano, Toshio Ono, Hidekazu Tanaka We present a determination of the phase diagram of the $S = {1\over2}$ quasi-2D triangular Heisenberg quantum antiferromagnet Cs$_{2}$CuCl$_{4}$ at temperatures to down 100mK and fields up to the saturation field of 9 tesla. We have determined the low temperature phase boundaries as a function of angle to the magnetic field using a unique low temperature rotatable calorimeter. Measurements at several angles intermediate to the in-plane and perpendicular directions elucidate the evolution of the complex phase diagram between these two principal axis. For fields directed along the a-axis (perpendicular to the plane of the triangular lattice), we observe a series of magnetic phases. New, unexpected phases arise at intermediate angles as we rotate the magnetic field away from the a-axis, into the bc plane. Variation of phase boundaries with field angle within the bc plane reflect interactions due to the Dzyaloshinskii-Moriya mechanism. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B4.00009: Exact Diagonalization Study of the Heisenberg Anisotropic Triangular Model Using Twisted Boundary Conditions Mischa Thesberg, Erik S. Sorensen The anistropic triangular model, which is believed to describe materials such as Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$, is dominated by incommensurate spiral physics and is thus extremely resistant to numerical analysis on small system sizes. In this talk we will discuss new exact diagonalization work using twisted boundary conditions to study the phase diagram of this model. With these boundary conditions we are able to extract the inter- and intra-chain ordering $q$-vectors for the $\frac{J'}{J} < 1$ region, as well as identify a phase transitions at $\frac{J'}{J} \sim 0.50/0.88$ (depending on finite system geometry). We also explore the nature of the two phases. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B4.00010: Spin-Current Order in Anisotropic Triangular Antiferromagnets O.A. Starykh, A.V. Chubukov We analyze instabilities of the collinear up-up-down state of a two-dimensional quantum spin-$S$ spatially anisotropic triangular lattice antiferromagnet in a magnetic field. We find, within the large-$S$ approximation, that near the end point of the plateau, the collinear state becomes unstable due to the condensation of two-magnon bound pairs rather than single magnons. The two-magnon instability leads to a novel two-dimensional vector chiral phase with alternating spin currents but no magnetic order in the direction transverse to the field. This phase breaks a discrete $Z_2$ symmetry but preserves a continuous $U(1)$ one of rotations about the field axis. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B4.00011: Phase Diagram of Spin-1/2 Triangular-Lattice Antiferromagnets with Exchange Anisotropy and a Magnetic Field Daisuke Yamamoto, Giacomo Marmorini, Ippei Danshita A triangular-lattice spin system is a fundamental model of geometric frustration. Recent experimental developments in magnetic materials synthesis and in frustrated optical lattices of ultracold atoms have renewed interest in studying magnetic properties of ideal two-dimensional frustrated systems over wide range of external field and anisotropy. We study the spin structures of $S=$1/2 antiferromagnets on a triangular lattice using a large-size cluster mean-field method combined with a scaling scheme. We determine the ground-state phase diagram of the spin model in the plane of magnetic field and XXZ anisotropy, and compare it with the classical counterpart in order to discuss the quantum effects. We find that a nontrivial continuous degeneracy existing in the classical model is broken up into two first-order phase transitions between which a non-classical phase emerges as a result of the selection by quantum fluctuations. We also use the dilute Bose gas expansion in the vicinity of the saturation field and interpret one of the first-order transitions as the 0-$\pi $ transition of the relative phase between two magnon Bose-Einstein condensates. We suggest that the quantum phase transitions can be observed in current or near-future experiments. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B4.00012: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 2:03PM - 2:15PM |
B4.00013: Topological defects in a spin-nematic phase on the triangular lattice Hiroaki Ueda, Nic Shannon Topological defects play an important role in the theory of nematic phases in liquid crystals. However, relatively little is known about their role in quantum spin nematics[1,2,3]. Here we consider the topological defects which could arise in such a state. The model we consider is the spin-1 bilinear biquadratic model on the triangular lattice, tuned to an SU(3) point[4,5,6]. We classify defects by homotopy theory, and explore how they evolve into the neighboring anti-ferroquadrupolar spin-nematic phase. \\[4pt] [1] B. A. Ivanov, R. S. Khymyn, and A. K. Kolezhuk, Phys. Rev. Lett. 100, 047203 (2008).\\[0pt] [2] T. Grover and T. Senthil, Phys. Rev. Lett. 107, 077203 (2011). \\[0pt] [3] C. Xu and A. W. W. Ludwig, Phys. Rev. Lett, 108, 047202 (2012). \\[0pt] [4] A. Lauchil, F. Mila and K. Penc, Phys. Rev. Lett. 97, 087205 (2006).\\[0pt] [5] H. Tsunetsugu and M. Arikawa, J. Phys. Soc. Jpn. 75, 083701 (2006).\\[0pt] [6] A. Smerald and N. Shannon, arXiv:1307.5131. (accepted for publication in PRB) [Preview Abstract] |
Session B6: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Electric Control of Magnetism in Oxides
Sponsoring Units: DMP GMAGChair: Will Echtenkamp, University of Nebraska-Lincoln
Room: 108
Monday, March 3, 2014 11:15AM - 11:51AM |
B6.00001: Full Electric Field Control of Exchange Bias Invited Speaker: Stephen Wu Exchange bias is the shift of a magnetic hysteresis curve due to interfacial magnetic coupling between a ferromagnet (FM) and an antiferromagnet (AFM). This ubiquitous effect has long been used in the electronics industry to bias the magnetization of FM layers in magnetic devices. Its continued understanding is of critical importance to advance the development of future high-density magnetic storage media and other novel magnetic devices. However, due to the technological limitations of manipulating and observing an atomically thin interface, exchange bias is not well understood. In this talk we present a multiferroic field effect device with BiFeO$_{3}$ (BFO) (antiferromagnetic-ferroelectric) as the gate dielectric and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) (ferromagnetic) as the conducting channel, which exhibits the direct, bipolar electric control of exchange bias. Here the magnetic states at the AFM/FM interface can be directly manipulated with electric fields and the results can be observed as a change in exchange bias polarity and magnitude. Control of exchange bias at this level has significant implications because it represents a form of electric field control of magnetism and may potentially offer a route toward the eventual full electric field control of magnetization. In this device, exchange bias is reversibly switched between two stable states with opposite exchange bias polarities upon ferroelectric poling of the BFO. No field cooling, temperature cycling, or additional applied magnetic or electric field beyond BFO poling is needed for this bipolar modulation effect. Detailed temperature dependent measurements and a model will be presented which will attribute this effect to the coupled antiferromagnetic-ferroelectric order in BFO along with the modulation of interfacial exchange interactions due to ionic displacement of Fe$^{3+}$ in BFO relative to Mn$^{3+/4+}$ in LSMO. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B6.00002: Voltage controlled exchange bias in an all-thin-film Cr$_{2}$O$_{3}$ based heterostructure Will Echtenkamp, Christian Binek Spintronics utilizes the electron's spin degree of freedom for an advanced generation of electronic devices with novel functionalities. Controlling magnetism by electrical means has been identified as a key challenge in the field of spintronics, and electric control of exchange bias is one of the most promising routes to address this challenge. Previously, robust isothermal electric control of exchange bias has been achieved near room temperature utilizing a bulk single crystal of Cr$_{2}$O$_{3}$ [1,2]. In this study electric control of exchange bias in an all-thin-film system is demonstrated with significant implications for device realization. In particular, voltage controlled switching of exchange bias in a Cr$_{2}$O$_{3}$ based magnetoelectric magnetic tunnel junction enables nonvolatile memory storage with virtually dissipationless writing at, or above, room temperature. Additionally, unique physical properties which arise due to the Cr$_{2}$O$_{3}$ thin film geometry are highlighted.\\[4pt] [1] Xi He, et. al, Nat. Mater. 9, 579-585 (2010)\\[0pt] [2] W. Echtenkamp, Ch. Binek, Phys. Rev. Lett. 111, 187204 (2013) [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B6.00003: Full control of magnetism in a manganite bilayer by ferroelectric polarization Shuai Dong, Elbio Dagotto An oxide heterostructure made of manganite bilayers and ferroelectric perovskites is predicted to lead to the full control of magnetism when switching the ferroelectric polarizations. By using asymmetric polar interfaces in the superlattices, more electrons occupy the Mn layer at the $n$-type interface side than at the $p$-type side. This charge disproportionation can be enhanced or suppressed by the ferroelectric polarization. Quantum model and density functional theory calculations reach the same conclusion: a ferromagnetic-ferrimagnetic phase transition with maximal change $>90\%$ of the total magnetization can be achieved by switching the polarization's direction. This function is robust and provides full control of the magnetization's magnitude, not only its direction, via electrical methods. Reference: S. Dong and E. Dagotto, Phys. Rev. B 88, 140404(R) (2013). [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B6.00004: \textit{In Situ} Electrical Biasing Studies of Magnetoelectric Coupling in La$_{1-x}$Sr$_{x}$MnO$_{3}$-PbZr$_{x}$Ti$_{1-x}$O$_{3}$ Thin Film Oxide Heterostructures Steven Spurgeon, Ian McDonald, Esther Huang, Rama Vasudevan, Samuel Lofland, Brian Kirby, Nagarajan Valanoor, Mitra Taheri Thin film La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) -- PbZr$_{x}$Ti$_{1-x}$O$_{3}$ (PZT) magnetoelectric heterostructures possess desirable properties for a range of spintronic applications, but a poor understanding of interfacial coupling dynamics has made them difficult to implement. Here we present a series of magnetization studies utilizing direct \textit{in situ} electrical biasing and switching of PZT polarization. We show that a piezoelectric strain effect gives rise to significant changes in the bulk saturation magnetization of LSMO. We complement these measurements with novel \textit{in situ} polarized neutron reflectometry measurements that reveal the spatial extent of induced magnetization. We then correlate these magnetic measurements with local structural and chemical probes to elucidate a structural basis for the observed magnetic properties. From these results we suggest ways to tune coupling for a particular application and we also propose ways to extend these techniques to other composites. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B6.00005: Using neutron scattering to explore new magnetoelectric phenomena in both thin films and skyrmion lattices Invited Speaker: Jonathan White Neutron scattering continues to be an invaluable tool for exploring the microscopic magnetic properties of magnetoelectric (ME) and multiferroic materials. Here I will present studies where neutron scattering techniques less commonly used for studying MEs have provided pivotal insight into new ME coupling phenomena. Firstly, we have used polarized neutron reflectometry (PNR) in a study of multiferroic and strained orthorhombic (o-) LuMnO$_{3}$ thin films [1]. Unlike bulk o-LuMnO$_{3}$ which is a commensurate antiferromagnet, the films display drastically different properties and are simultaneously incommensurately antiferromagnetic and ferromagnetic at low temperature. The pivotal PNR experiments allowed us to measure the spatial distribution of the ferromagnetic magnetization in the films, and show that the ferromagnetism is most pronounced close to the film-substrate interface which is highly strained due to the lattice mismatch. We could further show the ferromagnetism and antiferromagnetism in the film to be directly coupled, and so demonstrate the promising functional properties of these films. Secondly, we have used small-angle neutron scattering (SANS) to study the topologically protected magnetic spin vortices, or skyrmions, in the chiral-lattice ME insulator Cu$_{2}$OSeO$_{3}$. Until 2012, skyrmions had been observed only in (semi)conducting B20 compounds where it is known that they can be manipulated by conduction electrons. From our SANS experiments on Cu2OSeO3 [2], we show that applied electric \textit{fields} can control the skyrmion lattice orientation in insulators, and in an essentially lossless manner that is dependent on both the size and sign of the electric field. These results provide the first evidence for a the electric field control of topologically protected magnetism in bulk magnetoelectrics. \\[4pt] [1] J.S. White \textit{et al.}, Phys. Rev. Lett. \textbf{111}, 037201 (2013).\\[0pt] [2] J.S. White \textit{et al.}, J. Phys.: Condens. Matter \textbf{24}, 432201 (2012). [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B6.00006: Submicronic Spatial Mapping of Magnetoelectric Coupling in an Electronically Inhomogeneous System Gervasi Herranz, Ondrej Vlasin, Nico Dix, Florencio Sanchez Electric-field control of data stored in magnetic units prefigures a promising alternative to nowadays conventional electronics. A development of such technology demands a complete understanding of the dynamics and magnetoelectric response at small scales. Yet, present experimental approaches are hampered by the extreme difficulty of having simultaneous access to magnetism and ferroelectricity. Here we present an innovative approach that exploits optics to achieve a magnetoelectric coupling mapping with unprecedented resolution. More specifically, we used the effects that ferroelectricity and magnetism exert on light polarization, by electro-optic and magneto-optic effects, respectively. The analysis was performed at room temperature in a Pt(10 nm)/BaTiO$_{\mathrm{3}}$ (120 nm)/La$_{\mathrm{2/3}}$Sr$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$ (15 nm) trilayer. We uncovered a stunningly large coupling by which the magnetization was modulated by up to above 50{\%}. The magnetoelectric coupling was, however, distributed non uniformly with micron-scale inhomogeneities. The origin of such a large effect is discussed in terms of electric-field modulation of competing electronic phases in La$_{\mathrm{2/3}}$Sr$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$. Additionally, our work emphasizes the potential of intrinsically electronically inhomogeneous systems for large magnetoelectric responses. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B6.00007: Solving magnetoelectric coupling at La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ interfaces Jinling Zhou, Vu Thanh Tra, Robbyn Trappen, Disheng Chen, Matthew Marcus, Catherine Jenkins, Charles Frye, Evan Wolfe, Srinivas Polisetty, Ying-Hao Chu, Mikel Holcomb La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3 heterostructures exhibit interfacial magnetoelectric (ME) coupling. ME coupling is the coupling between the magnetic order and the electric order in a system, the understanding of which would lead to innovative device designs. In our talk, we will discuss our x-ray absorption spectroscopy and x-ray magnetic circular dichroism experimental results and disclose how solving polar catastrophe at the interface of this system unravels the coupling mechanism. Potentially, our results may lead to design of ME materials capable of stronger coupling. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B6.00008: Magnetic coercive field changes due to electric field generated anisotropy in (La$_{1-y}$Pr$_{y}$)$_{1-x}$Ca$_{x}$MnO$_{3}$ thin films Daniel Grant, Brian Schaefer, Amlan Biswas The hole-doped manganite (La$_{1-y}$Pr$_{y}$)$_{1-x}$Ca$_{x}$MnO$_{3}$ (LPCMO) shows phase competition between ferromagnetic metallic and anti-ferromagnetic charge order insulator regions due to the interplay between magnetic, electronic, and structural interactions. Of particular interest is the possibility of utilizing the phase competition to modify the magnetic properties of LPCMO using an electric field. We will present electric field dependent magnetization data on LPCMO thin films grown on (110) NdGaO$_{3}$ substrates which show shifts in the coercive magnetic fields when an in-plane electric field is applied to the sample. The electric field effect is also influenced by the in-plane magnetic anisotropy of the thin films. The proposed dielectrophoresis model offers a qualitative scenario through which we can explain these observations. This model states that application of an electric field can cause an alignment of the ferromagnetic metallic regions to create an anisotropic low resistance path, which could affect the in-plane shape anisotropy of phase separated LPCMO thin films. We will also discuss our results on LPCMO microstructures. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B6.00009: Electric field control of spin transfer torque in multiferroic tunnel junctions Artur Useinov, Alan Kalitsov, Julian Velev, Nicholas Kioussis Based on model calculations we predict that the spin transfer torque (STT) in magnetic tunnel junctions with ferroelectric barriers can be strongly influenced by the saturated polarization of the barrier. The STT in such multiferroic tunnel junctions is calculated within the non-equilibrium Keldysh formalism generalized for non-collinear transport and implemented in the framework of a single-band tight-binding (TB) model. We calculate the bias dependence of both the in-plane (T$_{\mathrm{\parallel }})$ and out-of-plane (T$_{\mathrm{\bot }})$ components of STT as a function of the ferroelectric polarization (P) in the barrier. We find that the components of STT strongly depend on both the magnitude and the direction of the polarization. In particular switching of the polarization direction can dramatically alter the value of the STT and can even lead to a change of sign of T$_{\mathrm{\parallel }}$ and the voltage-induced part of T$_{\mathrm{\bot }}$. The effect is proportional to the magnitude of the polarization. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B6.00010: Magnetic control of electric polarization in (Cu,Ni)B$_2$O$_4$ Khanh Nguyen, Nobuyuki Abe, Mitsuru Akaki, Masashi Tokunaga, Kyota Kubo, Takahiko Sasaki, Taka-hisa Arima We report the generation and control of electric polarization using an external magnetic field in a noncentrosymmetric system nickel doped copper metaborate (Cu,Ni)B$_2$O$_4$ and discuss the origin of this effect. In this material, weak ferromagnetic moment can be rotated by applying an electric field. While this implies spin-driven ferroelectricity, the previous study via examining the structure parameters and dielectric constant showed no clear evidence for this effect, which is successfully observed in this study. Applying a magnetic field along the $[110]$ or $[1-10]$ axis induces electric polarization along the $[001]$ axis. The polarization is reversed by switching the magnetic field direction between the $[110]$ and $[1-10]$ axes. The result can be well explained in the framework of spin-dependent metal-ligand hybridization. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B6.00011: Origin of magnetoelectric response induced by respective magnetic ions $R^{3+}$/Fe$^{3+}$ in a chiral antiferromagnet $R$Fe$_{3}$(BO$_{3})_{4}$ Takashi Kurumaji, Kenya Ohgushi, Yoshinori Tokura Recent discoveries of the spin-induced ferroelectricity in frustrated magnets and the strong ME correlation in noncentrosymmetric magnets have stimulated the revived interest on the ME phenomena [1]. Rare earth iron borates $R$Fe$_{3}$(BO$_{3})_{4}$, whose structures possess a noncentrosymmetric space group ($R$32 or $P$3$_{1}$21), have recently been discovered to show multiferroicity [2]. While their magnetic and ME properties were extensively investigated, the origin of the $P$, or specifically the relationship between the electricity and the respective magnetism of iron ions (Fe) and rare-earth ions ($R)$, remains elusive. We measured the $P$ under a magnetic field and observed the linear ME effect and/or the spontaneous $P$ which are ascribed to spins of Fe and/or magnetic moments of $R$. We constructed a model for the spin-induced $P$ at the Fe/$R$ sites, with which we could reproduce the observed behavior of the magnetic field dependence of $P$. Thus, we could extract the respective contributions to $P$ from Fe and $R$ magnetic ions. \\[4pt] [1] T. Arima, J. Phys. Soc. Jpn. 80 (2011) 052001 \\[0pt] [2] A. M. Kadomtseva et al. Low Temp. Phys. 36 (2010) 511 [Preview Abstract] |
Session B7: Focus Session: Metalorganic Magnets & Films
Sponsoring Units: GMAGChair: Michael Baker, New York University
Room: 106
Monday, March 3, 2014 11:15AM - 11:27AM |
B7.00001: Metal / Metal - Free Phthalocyanine Crystalline ``Alloys": Organic Analogues to Diluted Magnetic Semiconductors M. Furis, N. Rawat, L.W. Manning, R. Headrick, S. McGill Magnetic properties of organic crystalline thin films alloys of Phthalocyanines made of a mixture of organo-soluble derivatives of (Co-Pc) and metal-free ($H_{2}$Pc) molecules are investigated. Thin films with metal to metal-free Pc ratios ranging from 1:1 to 1:10 are fabricated using a novel solution processing method that produces macroscopic grains with molecules stacked parallel to each other along the crystalline c-axis. Magnetic Circular Dichroism (MCD) measurements reveal a substantial enhancement of spin-dependent exchange interactions involving the spin polarized electrons localized on the d-orbitals of cobalt and the delocalized $\pi$-electrons, in comparison to the pure Co-Pc films. More importantly, they indicate the presence of a low temperature ferromagnetic state in these films whose characteristics depend on the mixing ratio. We hypothesize this ordered state is caused by an organic analogue of the RKKY mechanism where the delocalized $\pi$-electrons play the role of the conduction and/or valence band electrons that mediate and control the sign of exchange between the localized unpaired d-electron spins. This hypothesis is supported by luminescence results that indicate the $\pi$-electrons wavefunctions extend over at least two neighboring Co spins. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B7.00002: Exchange Mechanisms in Long Range Ordered Thin Film Organic Magnetic Semiconductors N. Rawat, R. Headrick, M. Furis, S. Mcgill, L. Kilanski, R. Waterman Magnetic exchange mechanisms in crystalline thin films of Metal Phthalocyanines (M-Pc) are explored using Magnetic Circular Dichroism (MCD) and SQUID measurements up to 10 T and 2K. Long range ordered thin films of organo-soluble derivatives of Co-Pc and Mn-Pc were fabricated using solution processing technique. In the case of Mn-Pc, our measurements show enhanced hybridization of ligand $\pi$-electronic states with the Mn d-orbitals. MCD active states beyond 1 $\mu$m have been observed for the first time, providing crucial information on the orbital arrangements of MPc's that result in competing (co-existing) long-range superexchange and indirect exchange reminiscent of RKKY. The evolution of Zeeman splitting of specific MCD-active states is very well described by enhanced effective $\pi$-electrons g-factors as large as 100, analogous to diluted magnetic semiconductors (DMS) systems. In Co-Pc MCD data indicates a weaker exchange interaction between delocalized charge carriers and d-like spin-polarized electrons, however SQUID measurements reveal magnetic ordering up to 180K. A comparison between Mn-Pc and Co-Pc and earlier results from the spin 1/2 Cu-Pc and their non-magnetic Zn-Pc counterpart, offers an interesting view on the role of long range order in magnetic interactions. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B7.00003: Charge transport through a 4f spin state in a single molecule magnet Cyrus F. Hirjibehedin, Ben Warner, Philipp Seibt, Michael Waters, Andrew J. Fisher, Joris van Slageren, Fadi El Hallak The coupling between charge and spin in nano-scale systems is of fundamental interest and also key for creating novel devices at this scale. There may be advantages in utilizing magnetism produced by f-shell states, especially in controlled local environments such as molecules. Recently, it has been shown that charge transport through a molecule can access f-shell states despite their localization. Here we show that for charge transport through DyPc$_2$ that is strongly coupled to a copper surface it is possible to directly access the 4f spin. Spatially resolved scanning tunneling spectroscopy shows a variation in the amplitude of a Fano line shape near the Fermi energy, indicative of a Kondo effect due to screening of a localized spin coupled to a metallic continuum. The spin is attributed to the 4f states on Dy rather than the delocalized spin of an electron on the Pc ligands. This work demonstrates that the coupling to the surface can define which spins are present on a molecule as well as whether the spin state can be accessed in transport. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B7.00004: Potential for spin-based information processing in a thin-film molecular semiconductor Marc Warner, Salahud Din, Igor Tupitsyn, Gavin Morley, Marshall Stoneham, Jules Gardener, Zhenlin Wu, Andrew Fisher, Sandrine Heutz, Christopher Kay, Gabriel Aeppli Organic semiconductors are studied intensively for applications in electronics and optics, and even spin-based information technology, or spintronics. Fundamental quantities in spintronics are the population relaxation time ($T_{1})$ and the phase memory time ($T_{2})$: $T_{1}$ measures the lifetime of a classical bit, in this case embodied by a spin oriented either parallel or antiparallel to an external magnetic field, and $T_{2}$ measures the corresponding lifetime of a quantum bit, encoded in the phase of the quantum state. Here we establish that these times are surprisingly long for a common, low-cost and chemically modifiable organic semiconductor, the blue pigment copper phthalocyanine, in easily processed thin-film form of the type used for device fabrication. At 5 K, a temperature reachable using inexpensive closed-cycle refrigerators, $T_{1}$ and $T_{2}$ are respectively 59 ms and 2.6 ms, and at 80 K, which is just above the boiling point of liquid nitrogen, they are respectively 10 ms and 1 ms, demonstrating that the performance of thin-film copper phthalocyanine is superior to that of single-molecule magnets over the same temperature range. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B7.00005: Tunable magnetoresistance driven by magnetically sensitive negative differential resistance in an asymmetrically coupled single molecule junction Ben Warner, Fadi El Hallak, John Sharp, Mats Persson, Andrew J. Fisher, Cyrus F. Hirjibehedin Using scanning tunneling microscopy, we study the effects of interactions between individual magnetic molecules that are separated from an underlying copper surface by a thin-insulating layer of copper nitride (Cu$_2$N). For electrical transport through a junction containing an individual iron phthalocyanine (FePc) molecule on Cu$_2$N, we observe two novel magnetoresistance behaviors that arise from negative differential resistance (NDR) that shifts by unexpectedly large amounts in a magnetic field. Because voltage is dropped asymmetrically in this double barrier junction, the FePc can become transiently charged when its states are aligned with the Fermi energy of the Cu, resulting in the observed NDR effect. Furthermore, the asymmetric coupling magnifies the observed voltage sensitivity of the magnetic field dependence of the NDR - which inherently is on the scale of the Zeeman energy - by almost two orders of magnitude. These findings represent a new basis for making magnetoresistance devices at the single molecule scale. Furthermore, the enhancement of the energy scales created by asymmetric coupling of the junction can be combined with other multi-step tunneling processes to allow for the investigation of other phenomena that normally would be difficult to observe. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B7.00006: Coupling TbPc2 single molecule magnets to antiferromagnetic FeMn layers Corneliu Nistor, Cornelius Krull, Aitor Mugarza, Christian Stamm, Svetlana Klyatskaya, Mario Ruben, Marcio Soares, Sebastian Stepanow, Pietro Gambardella Coupling of single molecule magnets to magnetically ordered (ferromagnetic or antiferromagnetic) layers is a novel research field that has potential applications in molecular-scale spintronic devices. In this study we explore the possibility to magnetically couple TbPc2 molecules to FeMn layers deposited on a Cu (100) substrate. Using X-ray magnetic circular dichroism we demonstrate that, following field cooling, the out-of-plane Tb magnetization loop is vertically shifted and, furthermore, the Tb and Fe magnetization are antiferromagnetically coupled. Additionally, it is found that the Fe magnetization loop is vertically shifted and that this vertical shift depends on the elemental composition of the FeMn layer. The hysteretic behavior of the Tb magnetization together with the horizontal shift of the Tb loop are consistent with the hypothesis that a fraction of the TbPc2 molecules are coupled to the uncompensated Fe spins through a ligand-mediated superexchange mechanism. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B7.00007: Spin transport through one-dimensional transition metal organometallic cluster systems Ivan Stich, Lucia Horvathova, Rene Derian, Lubos Mitas Using very accurate quantum Monte-Carlo methods we have studied vanadium-benzene multidecker clusters, V$_{\mathrm{n}}$Bz$_{\mathrm{n+1}}$, in the range n $=$ 1 - 3. The most important prospective applications of these and related systems are in spintronics as spin filters, which requires them to be half-metal ferromagnets, featuring a semiconducting gap for majority spin electrons and metallic behavior for minority spin electrons. We find that while magnetic structure of these systems is consistent with ferromagnetic coupling, their electronic structure is not consistent with half-metallic behavior as previously assumed, but rather this system is a ferromagnetic insulator with large and broadly similar $\uparrow $-/$\downarrow $-spin gaps implying thus a limited potential of these materials for spintronic applications unless they are further modified or functionalized. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B7.00008: Thin film organic magnet based on vanadium methyl tricyanoethylenecarboxylate Yu Lu, Megan Harberts, Chi-Yueh Kao, Howard Yu, Arthur Epstein, Ezekiel Johnston-Halperin We report a new organic magnetic thin film based on vanadium and methyl tricyanoethylene carboxylate (MeTCEC) prepared by low temperature chemical vapor deposition (CVD). The magnetic ordering temperature, Tc, is above 300K and x-ray spectroscopy reveals the composition of the thin film is V[MeTCEC]$_{\mathrm{2}}$. IR spectroscopy shows both cyano-vandium and carboxylate-vanadium bonding confirming the predicted chemical structure. The temperature dependence of the magnetization reveals spin glass behavior in the thin film below a blocking temperature of 190K. The hysteresis reveals a soft ferrimagnet with coercive field of 10 Oe at 5 K and 20 Oe at 300 K, respectively. Finally transport measurements show semiconducting behavior with an activation energy of 0.56 eV, consistent with the expected bandgap. Taken together, these properties reveal significant potential for a new class of semiconducting organic-based magnetic materials that complement the existing library of M[TCNE] compounds (where M $=$ V, Co, Fe, Mn). [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B7.00009: A study of the spin-gaps in the organic conductor Perylene2[Pt(mnt)2] with the application of high magnetic field using an inductive AC susceptibility method Laurel Winter, James Brooks, Pedro Schlottmann, Manuel Almeida, Shermane Benjamin, Claude Bourbonnais To further understand the spin-charge coupling present in the dual-chain organic conductor Per$_2$[Pt(mnt)$_2$] we utilized an inductive ac susceptibility method to study the spin-Peierls (SP) ordered state.$^2$ Besides reaffirming the coexistence of the SP-CDW below 8 K and 20 T, our measurements also showed the emergence of a second spin-gapped phase above 20 T that coincides with the previous observed field-induced insulating phase. Our results provide support for the continued coupling of the charge and spin order parameters even in high magnetic fields and hints at the possibility of further spin and charge gaps above 45 T. $^2$ L.E. Winter, J.S. Brooks, P. Schlottmann, M. Almeida, S. Benjamin, and C, Bourbonnais, $\mathit{Europhys. Lett.}$, $\mathbf{103}$ (2013) 37008. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B7.00010: Spin-dipole moment in low symmetry structures Biplab Sanyal, Sumanta Bhandary, Soumyajyoti Haldar, Olle Eriksson The spin-dipole contribution (T$_{z}$) is usually neglected in x-ray magnetic circular dichroism measurements for bulk systems, as the value is negligible compared to the spin moment. However, in the last few years, it has been demonstrated quite clearly from experiments and theory that T$_{z}$ can acquire relatively large values for systems with low dimensions, e.g., organometallic molecules like Fe porphyrine/phthalocyanine [1] or small inorganic clusters. In some cases, the large T$_{z}$ contribution can be opposite to the spin moment and hence, the effective moment (2S+7T$_{z}$) turns out to be very small [2]. With the aid of first principles density functional calculations, the role of T$_{z}$ will be demonstrated for organometallic molecules and magnetite nanoparticles. The calculated effective moments follow the same trend as experimental measurements.\\[4pt] [1] S. Bhandary {\it et al.}, Phys. Rev. Lett. {\bf 107}, 257202 (2011); S. Bhandary {\it et al.}, Phys. Rev. B {\bf 88}, 024401 (2013). \\[0pt] [2] H. Herper {\it et al.}, Phys. Rev. B {\bf 87}, 174425 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B7.00011: Novel effects at metal-organic interfaces: Magnetic interactions between high-spin molecules and non-magnetic metals triggered by interface chemistry Amilcar Bedoya, Luis Hueso The adsorption of molecules on metal surfaces presents a rich variety of physical phenomena, which move from the creation of interface dipoles to hybridization and charge-transfer via strong chemisorption. In the strong interaction regime, some metal-molecule systems could even undergo a surface rearrangement and lead to the formation of new magnetically active phases, which could be used as templates for spin-injection or magnetization switching. For this purpose, we study the interaction of novel high-spin quinoline molecules (Tb$_{3}$q$_{9})$ with non-magnetic metallic surfaces. The molecules preserve their structural, chemical and magnetic properties when deposited onto noble metal (Au) and passivated (Si02) surfaces; while the adsorption on reactive metals such as Cu induces a magnetic phase at the interface involving molecular Tb-atoms, as measured via SQUID magnetometry and X-ray magnetic circular dichroism (XMCD). Remarkably, the magnetic ordering persists up to room-temperature for the Tb$_{3}$q$_{9}$/Cu system and is linked to a chemically-triggered change in structure and stoichiometry of the interfacial species. The occurrence of a molecular-driven magnetic phase at otherwise nonmagnetic metal surfaces highlights the importance of interface chemistry to tailor new magnetic interfaces and functional hybrid structures for spintronic applications. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B7.00012: Metal free half metallic graphitic carbon nitride on BN layer Jisang Hong, Arqum Hashmi We have investigated structural, adsorptive, and magnetic properties of metal free graphitic carbon nitride ({\it g}-C$_4$N$_3$) layer on hexagonal BN layer (h-BN) using the optB88-vdW van der Waals density functional theory. The free standing {\it g}-C$_4$N$_3$ layer is known to have (2 $\times$ 2) surface reconstructed structure with 0.3 $\AA$ buckling feature. However, the surface reconstruction disappears on BN layer and the {\it g}-C$_4$N$_3$ layer becomes flat. Interestingly, the {\it g}-C$_4$N$_3$/BN hybridized system has a new lattice constant which differs from that of either BN or {\it g}-C$_4$N$_3$ and this lattice change is responsible for adsorption of {\it g}-C$_4$N$_3$ on BN layer. More surprisingly, we have observed half metallic behavior in {\it g}-C$_4$N$_3$ even on BN layer. We propose that our theoretical prediction can be verified using normal incidence of K-edge X-ray magnetic circular dichroism (XMCD) measurement and also our finding indicates that the {\it g}-C$_4$N$_3$/BN system can be utilized for novel metal free spintronics material. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B7.00013: Ab-initio study of p magnetism in CaN and CaC monolayers on Cu(001) Hadi Akbarzadeh, Maedeh Zahedifar, Zahra Nourbakhsh, S. Javad Hashemifar Ab-initio calculations are performed to study the p ferromagnetic CaC and CaN compounds in the zinc-blend (zb) and rock-salt (rs) structures and their monolayers on Cu(001). It is observed that within the generalized gradient functional both structures of the bulk compounds are half-metal, while the rs structure exhibits higher stability. It is argued the strong interatomic exchange interaction in these systems controls the splitting of the spin resolved bond points while it has no considerable effect on total bond strength. In contrast to the bulk compounds, the CaC and CaN monolayers on Cu(001) generally favor the zb structure and the anion terminated monolayers are more stable than the cation terminated ones. On the other hand, the anion terminated systems are non- or weak magnetic systems while the less stable cation terminated layers exhibit strong magnetization. In the case of CaN monolayer on Cu(001), the nudged elastic band calculations show an activation barrier of 1.18 eV per CaN unit between the higher energy ferromagnetic and the stable nonmagnetic terminations. Therefore, epitaxial growth of a Ca terminated CaN thin film on Cu (001) is likely a practical way to form a novel half-metal/metal junction. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B7.00014: Giant Rashba effect and Dirac points in deep d-orbital noble metal surface states Ryan Requist, Polina Sheverdyaeva, Paolo Moras, Carlo Carbone, Erio Tosatti The chiral spin polarization, band splitting, and topological states generated by Rashba spin-orbit interaction at crystal surfaces and interfaces have received a lot of attention recently. Most studies have focused on $sp$ states near the Fermi energy, which are relevant for transport and have long lifetimes. Far less explored, although in principle stronger, are Rashba effects within d states, including those deep below the Fermi energy. Here we report a joint ARPES/first principles study of ``giant" Rashba effects in the deep $d$ surface states of a 20-layer Ag film grown on Au(111) and a 20-layer Au film grown on Ag(111). Several surface states predicted in [1], some split by $\sim$1 eV, are clearly observed in good overall agreement with first principles calculations. We also find Dirac points at the time-reversal symmetric $M$ point within a large spin-orbit-induced bulk gap, which are visible in both Ag and Au and display all the characteristics of topological surface states, such as chiral spin polarization and robustness to perturbation. Unlike the usually symmetric dispersion at $\Gamma$ point Dirac cones, these $M$ point cones are strongly anisotropic away from the degeneracy point. [1] R. Mazzarello, A. Dal Corso, E. Tosatti, Surf. Sci. 602, 893 (2008). [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B7.00015: Ferromagnetic ordering in an insulator by itinerant electrons J.N.B. Rodrigues, Aigu L. Lin, A.H. Castro Neto, S. Adam, Andrew T.S. Wee Motivated by recent experimental work of variable range hopping of electrons between magnetic nanoparticles in oxidized graphene, we consider theoretically an ensemble of randomly oriented classical Heisenberg magnetic moments which are superparamagnetic at room temperature and have negligible magnetostatic coupling. Itinerant electrons hopping through random sites experience a Zeeman coupling with these moments. Using Monte Carlo simulations, we demonstrate that this generates an effective electron-mediated coupling between the Heisenberg moments giving rise to spontaneous magnetization of the sample. We make predictions for the temperature dependence of this magnetization and compare with experimental data. [Preview Abstract] |
Session B8: Focus Session: Spin Transfer Torque
Sponsoring Units: GMAGChair: Olle Heinonen, Argonne National Laboratory
Room: 104
Monday, March 3, 2014 11:15AM - 11:27AM |
B8.00001: Mode coupling in spin-torque oscillators: first-principle derivation Olle Heinonen, Yan Zhou, Dong Li A number of recent experimental works have shown that the dynamics of a single spin torque oscillator can exhibit complex behavior that stems from interactions between two or more modes of the oscillator. Examples are observed mode-hopping or mode coexistence$^{\mathrm{1-3}}$. There has been some initial work indicating how the theory for a single-mode (macro-spin) spin torque oscillator should be generalized to include several modes and the interactions between them. In the present work, we derive such a theory starting with the Landau-Lifshitz-Gilbert equation for magnetization dynamics. We compare our results with the single-mode theory, and show how the coupled-mode theory is a natural extension of the single-mode. Argonne National Laboratory is a US DOE Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. References: [1] S. Bonetti, V. Tiberkevich, G. Consolo et al., \textit{Phys. Rev. Lett. }\textbf{105}, 217204 (2010). [2] P. Muduli et al., \textit{Phys. Rev. Lett}. \textbf{108}, 207203 (2012). [3] R. Dumas, E. Iacocca, S. Bonetti et al., \textit{Phys. Rev. Lett.} \textbf{110}, 257202 (2013). [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B8.00002: Nanowire Spin Torque Oscillator Driven by Spin Orbit Torques Andrew Smith, Zheng Duan, Liu Yang, Brian Youngblood, Ilya Krivorotov We report microwave signal emission from a spin torque oscillator driven by spin orbit torques in a 17 um long Py(5 nm)/Pt(5 nm) ferromagnetic nanowire with an 1.8 um long active region. The emitted signal arises from excitation of the bulk and edge spin wave eigenmodes of the nanowire and detected with anisotropic magnetoresistance. This type of self-oscillatory dynamics is qualitatively different from the previously reported self-localized nonlinear bullet mode excited by spin orbit torques in extended ferromagnetic films. The eigenmode self-oscillations in the nanowire geometry are enabled by geometric confinement suppressing nonlinear magnon scattering. Our work demonstrates feasibility of spin torque oscillators with a micrometer-scale active region. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B8.00003: Magnetization reversal in orthogonal spin transfer magnetic devices Georg Wolf, Andrew D. Kent, Bartek Kardasz, Mustafa Pinarbasi Orthogonal spin transfer (OST) magnetic devices have distinct magnetization dynamics and switching characteristics compared to conventional collinearly magnetized devices. A perpendicular magnetized layer provides a large initial spin torque on the free layer magnetization and thus initiates magnetization dynamics. In order to read out the information stored in the OST device, the free layer forms a magnetic tunnel junction with an in plane magnetized reference layer, which also exerts a spin torque on the free layer. The combination of those two spin torques leads to different switching dynamics of the free layer. Quasistatic and fast pulsed measurements have been conducted to explore the state diagram and magnetization dynamics of such devices. The absolute value of the switching current I$_s$ is in general smaller for the antiparallel (AP) to parallel (P) transition, due to the angular dependence of the reference layer torque. I$_s$ also has a weak field dependence for this transition, indicating that the reference layer torque governs this transition. On the other hand, the P to AP transition shows a stronger field dependence of I$_s$ and occurs for both current polarities. Both these features denote the influence of the spin-torque generated from the perpendicular polarizer. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B8.00004: Spin wave mode coexistence on the nanoscale: A consequence of the Oersted field induced asymmetric energy landscape Invited Speaker: Randy Dumas The emerging field of magnonics relies on the systematic generation, manipulation, and detection of spin waves (SWs). Nanocontact spin torque oscillators (NC-STOs) provide an ideal platform to study spin transfer torque induced SW emission [1, 2]. In analogy to two species competing for the same food supply it has been argued that only one SW mode can survive in the steady state [3]. However, as evidenced in many experiments clear signatures of mode-hopping are often observed [1, 4]. Here, we present a third possibility, namely that under the correct experimental conditions, mode \textit{coexistence }can be realized in NC-STOs [5]. Micromagnetic simulations reveal that the SW modes are spatially separated under the NC. Mode coexistence is facilitated by the local field asymmetries induced by the spatially inhomogeneous Oersted field in the vicinity of the NC and further promoted by SW localization. Finally, both simulation and experiment reveal a weak low frequency signal exactly at the difference of the mode frequencies, consistent with inter-modulation of two coexistent modes. \\[4pt] [1] S. Bonetti, V. Tiberkevich, G. Consolo, G. Finocchio, P. Muduli, F. Mancoff, A. Slavin, and J. {\AA}kerman, Phys. Rev. Lett. \textbf{105}, 217204 (2010). \\[0pt] [2] M. Madami, S. Bonetti, G. Consolo, S. Tacchi, G. Carlotti, G. Gubbiotti, F.B. Mancoff, M.A. Yar, and J. {\AA}kerman, Nature Nanotechnology \textbf{6}, 635 (2011). \\[0pt] [3] F. M. de Aguiar, A. Azevedo, and S. M. Rezende, Phys. Rev. B \textbf{75}, 132404 (2007).\\[0pt] [4] P. K. Muduli, O. G. Heinonen, and J. {\AA}kerman, Phys. Rev. Lett. \textbf{108}, 207203 (2012).\\[0pt] [5] R.K. Dumas, E. Iacocca, S. Bonetti, S.R. Sani, S.M. Mohseni, A. Eklund, J. Persson, O. Heinonen, and Johan {\AA}kerman, Phys. Rev. Lett. \textbf{110}, 257202 (2013). [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B8.00005: Spin-Torque Ferromagnetic Resonance in PMA Thin Film Structures Luis Vilela-Le\~ao, Chi-Feng Pai, Yongxi Ou, Yun Li, Daniel Ralph, Robert Buhrman Thin film systems with strong perpendicular magnetic anisotropy (PMA) are important for many spintronic device applications. For example for magnetic memory, strong PMA can enable ultra-high density storage, in combination with enhanced non-volatility and low write energy. Recently, some interesting phenomena, including the spin Hall effect and spin orbit fields, have been reported in normal metal/ferromagnetic (NM/FM) structures with PMA. These effects, which arise from spin-orbit coupling in the structure, convert charge current into torques (a field-like torque and/or a damping-like torque) on the magnetization of the FM that can be strong enough to switch the magnetization, generate persistent magnetic oscillation, and efficiently move domain walls. Here we show that spin-torque ferromagnetic resonance (ST-FMR) can be effectively employed to characterize the anisotropy, the spin-orbit torques, and the magnetic damping of a PMA structure. We will report on ST-FMR results from several PMA systems, including W/Hf(t)/CoFeB/MgO and W/CoFeB where we have found large values for both the first order and second order terms of the magnetic anisotropy and where we have measured intrinsic and extrinsic contributions to the effective magnetic damping. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B8.00006: Field-modulated spin torque ferromagnetic resonance: Characterization of spin wave eigenmodes in magnetic tunnel junctions Yu-Jin Chen, Alexandre Goncalves, Igor Barsukov, Liu Yang, Jordan Katine, Ilya Krivorotov A common technique for measurements of magnetic parameters in nanoscale magnetic tunnel junctions (MTJs) is spin torque-driven ferromagnetic resonance (ST-FMR) based on an amplitude-modulated microwave drive [1,2]. We demonstrate a technique of broadband ST-FMR based on magnetic field modulation for measurements of spin wave properties in magnetic nanostructures [3]. Application of the field-modulated ST-FMR technique to MTJs gives reliable information on magnetization dynamics for arbitrary magnetic state of the MTJ, including the case of collinear magnetizations. This configuration is difficult to measure in conventional ST-FMR due to the weak spin torque drive. The improved signal-to-noise ratio and improved sensitivity of field-modulated ST-FMR allow us to measure the entire spectrum of low-frequency standing spin waves. We find the magnetic field dependence of the measured spin wave eigenmodes to be in good agreement with micromagnetic simulation results, which allow us to identify the observed modes as the free layer eigenmodes and to determine their spatial profiles [3]. \\ 1. A. A. Tulapurkar et al., Nature 438, 339-342 (2005)\\ 2. J. C. Sankey et al., Phys. Rev. Lett. 96, 227601 (2006)\\ 3. A. M. Goncalves et al., Appl. Phys. Lett. 103, 172406 (2013) [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B8.00007: Control of Propagating Spin Waves via Spin Transfer Torque in a Metallic Bilayer Waveguide Kyongmo An, Daniel Birt, Chi-Feng Pai, Kevin Olsson, Daniel Ralph, Robert Buhrman, Xiaoqin Li We investigate the effect of a direct current on propagating spin waves in a CoFeB/Ta bilayer structure. Using the micro-Brillouin light scattering technique, we observe that the spin wave amplitude may be attenuated or amplified depending on the direction of the current and the applied magnetic field. Our work suggests an effective approach for electrically controlling the propagation of spin waves in a magnetic waveguide and may be useful in a number of applications such as phase locked nano-oscillators and hybrid information processing devices. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B8.00008: Experimental observation of exchange-mode spin-wave via domain wall annihilation Seonghoon Woo, Tristan Delaney, Geoffrey Beach Spin waves (SWs) in magnetic nanostructures have generated great interest recently, motivated by the possibility of high-speed, low-power magnonic devices applications. A number of micromagnetic researches, therefore, have been conducted, revealing the particular behaviors of SWs in nanostructured ferromagnets. However, SWs' short attenuation length prevents them from being observed and used experimentally. Generating large-amplitude exchange-mode SWs, which is thus indispensable for real device applications, are still challenging because their very short wavelengths cannot be directly excited. Here, we present the first experimental evidence of the exchange-mode SWs. Using micromagnetics, we firstly show that the annihilation of two DWs releases their exchange energy by a mean of localized SW burst, which has broad range band and intense amplitude. Another micromagnetic result also shows that the collision-induced SWs inside a nanowire can cause the depinning of a DW with an assisting magnetic field. By taking advantage of an anisotropic magneto-resistance (AMR) effect and relative electrical measurements, we observe the generation/annihilation of DWs and the contribution of generated SWs to the DW depinning process experimentally. The additional depinning field of $\sim$ 8 Oe caused by SWs can be readily achieved, enough to propagate a standstill DW in a well-defined pinning-free nanostructure. This work shows the first experimental observation of exchange-mode SWs and highlights a new route towards SW-integrated spintronic devices. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B8.00009: Onset and annihilation of droplet solitons in Spin Torque Nano-Oscillators with perpendicular magnetized free layers Ferran Macia, Dirk Backes, Andrew Kent Nanometer scale electrical contacts to ferromagnetic thin films (STNOs) can provide sufficient current densities to excite magnetization dynamics resulting in either localized or propagating short wavelength spin waves. These oscillations can be detected through the magnetoresistance because of the change in the relative orientation between the current polarization and the free layer magnetization. We have fabricated point contacts to continuous magnetic bilayers where the polarizer magnetic film has in-plane magnetic anisotropy and with free layers with different magnetic anisotropies ranging from in-plane to perpendicular magnetic anisotropy (PMA). Our measurements on STNOs with perpendicularly magnetized free layers indicate that over a region of magnetic field and current there is an onset of an excitation with characteristics consistent with the formation of a \textit{droplet soliton}. We have systematically studied the state diagram of these excitations that shows both their onset and annihilation. We also studied the onset and annihilation of droplet solitons in arrays of STNOs. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B8.00010: Macrospin model of spin-transfer oscillators: an energy space approach Daniele Pinna, Andrew Kent, Daniel Stein A direct current applied to a nanomagnet produces a spin-transfer torque that drives the magnetization out of equilibrium. In this talk we discuss an effective theory to characterize the magnetization dynamics by focusing on its diffusive evolution over the energy landscape. The procedure allows us to model macrospin behavior with a one dimensional stochastic differential equation. We model spin-transfer oscillators (STOs) with a spin-current at an angle to the easy plane of a biaxial magnet (i.e. have a component along the magnet's hard axis). We trace the properties of stable out-of-plane precessional states and discuss their hysteretic behavior on applied current. We discuss the structure of the expected linewidth and phase noise, along with how the oscillator frequency is expected to depend on applied current. Finally, contributions due to thermal noise will be outlined and some thermally activated properties described. D. Pinna, A. D. Kent, D. L. Stein, Phys. Rev. B 88, 104405 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B8.00011: State Diagram of Orthogonal Spin-Transfer Spin-Valve Devices Li Ye, Georg Wolf, Daniele Pinna, Andrew Kent Orthogonal spin transfer (OST) devices that incorporate an out-of-plane magnetized polarizing layer with an in-plane collinear spin valve are expected to exhibit ultrafast magnetization switching as well as large amplitude precessional modes. The current and field dependence of the switching thresholds are also distinct from the collinear spin-valves because of the combined effect from in-plane reference layer (RL) and polarizing layer (PL). Here we present an experimental investigation of complete current-field state diagrams, demonstrating reversal between parallel (P) and anti-parallel (AP) states and dynamic states of the free layer in both OST pseudo spin valve and spin valve devices, where in the latter a synthetic anti-ferromagnetic layer (SAF) is used as reference layer. Switching from AP (P) to P (AP) states is observed at both positive and negative current with a different field dependence of the critical current, reflecting spin polarization asymmetry between AP and P states and different RL and PL spin torque efficiencies. High frequency noise spectra have also been acquired providing evidence of out-of-plane precessional modes, where an intermediate resistance is seen in quasistatic measurements. Modeling of this orthogonal spin transfer system is also discussed. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B8.00012: Distribution of domain wall spin torque in magnetic metals and ferromagnetic semiconductors Elizabeth Golovatski The design and implementation of many spintronic devices[1] will be dependent on good models of spin torque and domain wall motion caused by coherent carrier transport[2]. We model spin torque in N\'eel walls[3] using a piecewise linear transfer-matrix method[4], and calculate the spin torque distribution[5] throughout the system. We examine the differences in spin torque for ferromagnetic semiconductors (where the Fermi energy is much less than the spin splitting) and magnetic metals (where the Fermi energy is much greater than the spin splitting). We find that the torque distribution is more asymmetric for adiabatic torques and more symmetric for non-adiabatic torques in a magnetic metal vs. a ferromagnetic semiconductor, leading to differences in velocities for two domain walls in close proximity. [1] S. Parkin et al., Science 320, 190 (2008) [2] M. Yamanouchi et al., Nature 428, 539 (2004) [3] G. Vignale and M. Flatt\'e, Phys. Rev. Lett. 89, 098302 (2002) [4] E. Golovatski and M. Flatt\'e, Phys. Rev. B, 84, 115210 (2011) [5] J. Xiao et al., Phys. Rev. B, 73, 054428 (2006) [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B8.00013: Current-Induced Domain-Wall Motion in Perpendicularly Magnetized Magnetic Nanowires with Unflatted Surfaces Hirofumi Morise, Tsuyoshi Kondo, Shiho Nakamura We study the current-induced domain wall motion in perpendicularly magnetized magnetic nanowires with unflatted surfaces numerically. There, the axis of the anisotropy is assumed to be normal to the surfaces, that is, it is inclined continuously along the extended direction of the nanowire. The relationship between the current density and the velocities of domain walls are investigated by use of micromagnetics simulations. Comparing with the motion in a flat nanowire, the existence of an additional exchange energy due to the curvature significantly affects the motion of the domain walls. [Preview Abstract] |
Session B10: Focus Session: Mechanics of Cells and Biological Networks II
Sponsoring Units: DBIOChair: Maria Kilfoil, University of Massachusetts
Room: 201
Monday, March 3, 2014 11:15AM - 11:51AM |
B10.00001: to be determined by you Invited Speaker: Dennis Discher |
Monday, March 3, 2014 11:51AM - 12:03PM |
B10.00002: Matrix mimicry and nuclear biophysics: biostructures are maintained where stresses are high Dennis Discher You are more collagen than any other protein. We will describe various collagenous assemblies as thin and thick matrices that help elucidate how important matrix physical properties are to cells. We start with tissues analyses that reveal power law scaling of collagen levels versus of the stiffness of real tissues conform to polymer physics. Our unbiased `omics methods also reveals similar scaling for a main structural protein of the nucleus, a filamentous coiled-coil protein akin to collagen but in the same family as keratin in your hair and nails. This nuclear protein called lamin-A protects DNA from stresses that differ between tissues as stiffness does. Ultimately, the data suggests lamin-A is mechanosensitive in ways similar to collagen: degradation and turnover is repressed by stress -- which is distinct from the conventional ``stress it and break it'' of dead materials. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B10.00003: Response of microscale cell/matrix constructs to successive force application in a 3D environment Alan Liu, Christopher Chen, Daniel Reich Mechanical dilation of arteries by pulsatile blood flow is directly opposed by coordinated contraction of a band of smooth muscle tissue that envelops the vessels. This mechanical adaptation of smooth muscle cells to external loading is a critical feature of normal blood vessel function.~While most previous studies on biomechanical systems have focused on single cells or large excised tissue, we utilize a device to apply forces to engineered smooth muscle microtissues. This device consists of arrayed pairs of elastomeric micro-cantilevers capable of magnetic actuation. Tissues are formed through self-assembly following the introduction of cell-infused collagen gel to the array. With this system, we are able to dynamically stretch and relax these sub-millimeter sized tissues. The timing and magnitude of the force application can be precisely controlled and thus can be used to mimic a wide range of physiological behavior. In particular, we will discuss results that show that the interval between successive force applications mediates the both the subsequent mechanical and active dynamics of the cell/matrix composite system. Understanding this process will lead to better understanding of the interplay between cell and extracellular matrix responses to mechanical stimulus at a novel length scale. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B10.00004: High contrast single molecule tracking in the pericellular coat Jan Scrimgeour, Louis T. McLane, Jennifer E. Curtis The pericellular coat is a robust, hydrated, polymer brush-like structure that can extend several micrometers into the extracellular space around living cells. By controlling access to the cell surface, acting as a filter and storage reservoir for proteins, and actively controlling tissue-immune system interactions, the cell coat performs many important functions at scales ranging from the single cell to whole tissues. The cell coat consists of a malleable backbone - the large polysaccharide hyaluronic acid (HA) - with its structure, material properties, and ultimately its bio-functionality tuned by a diverse set of HA binding proteins. These proteins add charge, cross-links and growth factor-like ligands to the coat To probe the dynamic behavior of this soft biomaterial we have used high contrast single molecule imaging, based on highly inclined laser illumination, to observe individual fluorescently labeled HA binding proteins within the cell coat. Our work focuses on the cell coat of living chondrocyte (cartilage) cells, and in particular the effect of the large, highly charged, protein aggrecan on the properties of the coat. Through single molecule imaging we observe that aggrecan is tightly tethered to HA, and plays an important role in cell coat extension and stiffening. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B10.00005: Dynamic non-linear response of cross-linked actin networks: an energy dissipation approach Sayantan Majumdar, Margaret L. Gardel Cross-linked bio-polymer networks that primarily maintain the shape and rigidity in eukaryotic cells show striking non-linear mechanical properties. Here, we study the steady-state energy dissipation ($E_{diss}$) over a complete sinusoidal shear strain cycle for a macroscopic assembly of reconstituted network of actin filaments cross-linked with Filamin A, over wide range of strain amplitude and frequency values. For small values of the applied strain amplitudes (linear regime) $E_{diss}$ increases monotonously with the increasing frequency over the entire frequency range studied but in the non-linear regime (larger applied strain amplitudes), a clear saturation in $E_{diss}$ is observed at higher frequencies. Also, the normalized dissipated energy distribution binned over the fixed strain intervals along the shear cycle show frequency dependence in the nonlinear regime but remains frequency independent in the linear regime. Remarkably, the monotonously increasing behavior of $E_{diss}$ with frequency is also observed in the non-linear regime when a more rigid cross-linker A-Actinin is used, suggesting the importance of flexibility of cross-linkers in controlling the non-linear mechanical response in this class of materials. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 1:15PM |
B10.00006: Mechanics of composite actin networks: in vitro and cellular perspectives Invited Speaker: Arpita Upadhyaya Actin filaments and associated actin binding proteins play an essential role in governing the mechanical properties of eukaryotic cells. Even though cells have multiple actin binding proteins (ABPs) that exist simultaneously to maintain the structural and mechanical integrity of the cellular cytoskeleton, how these proteins work together to determine the properties of actin networks is not well understood. The ABP, palladin, is essential for the integrity of cell morphology and movement during development. Palladin coexists with alpha-actinin in stress fibers and focal adhesions and binds to both actin and alpha-actinin. To obtain insight into how mutually interacting actin crosslinking proteins modulate the properties of actin networks, we have characterized the micro-structure and mechanics of actin networks crosslinked with palladin and alpha-actinin. Our studies on composite networks of alpha-actinin/palladin/actin show that palladin and alpha-actinin synergistically determine network viscoelasticity. We have further examined the role of palladin in cellular force generation and mechanosensing. Traction force microscopy revealed that TAFs are sensitive to substrate stiffness as they generate larger forces on substrates of increased stiffness. Contrary to expectations, knocking down palladin increased the forces generated by cells, and also inhibited the ability to sense substrate stiffness for very stiff gels. This was accompanied by significant differences in the actin organization and adhesion dynamics of palladin knock down cells. Perturbation experiments also suggest altered myosin activity in palladin KD cells. Our results suggest that the actin crosslinkers such as palladin and myosin motors coordinate for optimal cell function and to prevent aberrant behavior as in cancer metastasis. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B10.00007: Nonlinear Elasticity: From Single Chain to Networks and Gels Andrey Dobrynin, Jan-Michael Carrillo, Fred MacKintosh Biological and polymeric networks show highly nonlinear stress-strain behavior leading to material hardening with increasing deformation. Using a combination of theoretical analysis and molecular dynamics simulations we develop a model of network deformation that describes nonlinear mechanical properties of a broad variety of biological and polymeric networks and gels by relating their macroscopic strain-hardening behavior with molecular parameters of the network strands. The starting point of our approach is a nonlinear force/elongation relation for discrete chain model with varying bending rigidity. This theory provides a universal relationship between the strain-dependent network modulus and the network deformation as a function of the first invariant and chain elongation ratio that depends on a ratio of the unperturbed chain size to chain dimension in a fully extended conformation. The model predictions for the nonlinear shear modulus and differential shear modulus for uniaxial and shear deformations are in a very good agreement with both the results of molecular dynamics simulations of networks and with experimental data for biopolymer networks of actin, collagen, fibrin, vimentin, neurofilaments, and pectin. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B10.00008: Composite fiber networks mechanics Catalin Picu, Ali Shahsavari Random fiber networks are present in many soft biological and engineering materials. In most cases, these networks are composite, in the sense that they are constructed from multiple fiber types. In this work we develop elements of a theoretical understanding of the elasticity of these structures. To this end, we consider systems made from a softer base and varying fractions of stiff fibers and investigate the effect of various system parameters on the overall behavior. The small strain elasticity depends strongly on the presence of a small concentration of stiff fibers for some types of base networks, but is essentially insensitive to these additions for other types. The way in which the stiff fibers are cross-linked to the soft fibers and to themselves is also important. These issues will be discussed within a framework general enough to make the conclusions relevant for diverse applications. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 2:15PM |
B10.00009: Elasticity on the edge of stability: what Maxwell can teach us about biology Invited Speaker: Fred MacKintosh Life makes use of filamentous proteins for many structures, both in cells and tissues. In the cell, the cytoskeleton consists of networks of protein biopolymers for mechanical stability, organization and transport within the cell. Extracellular proteins such as collagen and fibrin form similar networks. One hundred and fifty years ago, Maxwell taught us about the minimal conditions for stability of simple spring-based networks [J. C. Maxwell, Philos. Mag. 27, 27 (1864).]. Interestingly, as a function of connectivity, such networks exhibit second-order rigidity transitions. We discuss recent theoretical and experimental progress in understanding the mechanics of such networks. We focus particularly on implications of the marginal state of networks near, and below Maxwell's isostatic connectivity. We show how fields such as stress, molecular motor activity and thermal fluctuations can stabilize networks. In the process, this can help us to understand long-standing problems in collagenous tissue mechanics. [Preview Abstract] |
Session B11: Focus Session: Teaching at the Intersection of Physics and Biology
Sponsoring Units: DBIOChair: Mark Reeves, The George Washington University
Room: 203
Monday, March 3, 2014 11:15AM - 11:51AM |
B11.00001: Cultivating the Physical Biology Mindset Invited Speaker: Rob Phillips Biological experiments now regularly result in data that emphasize functional relationships between key parameters such as level of gene expression and number of transcription factors or motor velocity and applied force. This trend towards quantitative dissection of biological problems has been acknowledged explicitly in learned reports such as ``Bio2010'' and the recent NAS report ``A New Biology for the 21st Century.'' These reports repeatedly emphasize the need for a new biology characterized by what one might call ``biological numeracy'' and for overhauling biological education in a way that is consistent with this kind of biological research. In this talk, I will describe my own experience in introducing courses aimed at introducing physical biology both in the lecture hall and in the laboratory. One of the most interesting aspects of the physics-biology interface is the question of what constitutes understanding and here, I will describe my views on the role of polarizing predictions as a test of such understanding with special emphasis on examples from signaling and regulation. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B11.00002: Resources and approaches for teaching physics to pre-health and life science majors Ralf Widenhorn As science is advancing, the skill set for a physician or medical researcher today and in the future is very different than it has been in the past. As an example, the American Association of Medical Colleges revised the Medical College Admissions Test (MCAT) to reflect this dynamic environment. Because of these changes, the needs of students entering into these professions are often not met by a traditional physics course. Developing curriculum for an introductory physics course that helps to prepare life science and pre-health students can be challenging for many physics instructors who lack a strong foundation in biology or medicine. This presentation will address various approaches that physics instructors without a background in life sciences can use to successfully teach an introductory physics course for life science and pre-heath students. For these courses, an online resource may be a useful tool. Online resources already exist today, but their utility relies on active engagement and sharing of teaching material by physics instructors possessing a background in both physics and the life sciences. This talk will address ways for the biomedical physics community to contribute to this effort. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B11.00003: Teaching wave phenomena via biophysical applications Daniel Reich, Mark Robbins, Robert Leheny, Steven Wonnell Over the past several years we have developed a two-semester second-year physics course sequence for students in the biosciences, tailored in part to the needs of undergraduate biophysics majors. One semester, ``Biological Physics,'' is based on the book of that name by P. Nelson. This talk will focus largely on the other semester, ``Wave Phenomena with Biophysical Applications,'' where we provide a novel introduction to the physics of waves, primarily through the study of experimental probes used in the biosciences that depend on the interaction of electromagnetic radiation with matter. Topic covered include: Fourier analysis, sound and hearing, diffraction - culminating in an analysis of x-ray fiber diffraction and its use in the determination of the structure of DNA - geometrical and physical optics, the physics of modern light microscopy, NMR and MRI. Laboratory exercises tailored to this course will also be described. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B11.00004: Experiences Gained Creating a Biophysics Major at a Predominately Undergraduate Institution Justin Link, Steven Herbert Xavier University, a liberal arts predominately undergraduate institution (PUI) located in Cincinnati, OH, implemented a Biophysics major in the Department of Physics in spring 2012. The program is built upon foundational physics courses and is unique due to the possible selection of upper-division courses that students elect to take towards their undergraduate degree. A capstone course is offered to bring all prior knowledge in the fundamental sciences together to approach complex problems in biology. Due to the flexibility of the program, it serves students well who are interested in pursuing advanced degrees in Biophysics or Biomedical Engineering. It also offers students interested in the health professions an alternate path towards medical school which can be advantageous in the application process. This session will express some of the advantages and challenges to creating such a program at a liberal arts PUI and discuss the capstone course within the major. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B11.00005: Supporting students in building interdisciplinary connections across physics and biology Invited Speaker: Chandra Turpen Our research team\footnote{This work has been done with Benjamin Dreyfus, Benjamin Geller, Julia Svoboda Gouvea, Wolfgang Losert, Edward Redish, and Vashti Sawtelle.} has been engaged in the iterative redesign of an Introductory Physics course for Life Science (IPLS) majors to explicitly bridge biology and physics in ways that are authentic to the disciplines. Our interdisciplinary course provides students opportunities to examine how modeling decisions (e.g. knowing when and how to use different concepts, identifying implicit assumptions, making and justifying assumptions) may differ depending on canonical disciplinary aims and interests. Our focus on developing students' interdisciplinary reasoning skills requires 1) shifting course topics to focus on core ideas that span the disciplines, 2) shifting epistemological expectations, and 3) foregrounding typically tacit disciplinary assumptions. In working to build an authentic interdisciplinary course that bridges physics and biology, we pay careful attention to supporting \textit{students} in constructing these bridges. This course has been shown to have important impacts: a) students seek meaningful connections between the disciplines, b) students perceive relevance and utility of ideas from different disciplines, and c) students reconcile challenging disciplinary ideas. Although our focus has been on building interdisciplinary coherence, we have succeeded in maintaining strong student learning gains on fundamental physics concepts and allowed students to deepen their understanding of challenging concepts in thermodynamics. This presentation will describe the shifts in course content and the modern pedagogical approaches that have been integrated into the course, and provide an overview of key research results from this project. These results may aid physicists in reconsidering how they can meaningfully reach life-science students. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B11.00006: Energy as a Unifying Theme for a Models Approach to Instruction Stephen Tsui, Clarisa Guelman, Charles De Leone Biological science students who are accustomed to transmission-based modes of instruction are often challenged by the model-based problem solving that is unique to physics. To address this challenge, California State University San Marcos (CSUSM) adapted a UC Davis originated models-based curriculum for the introductory physics course for life-science majors. In this approach, the course content sequence was recast, such that energy and thermal physics is studied first, as opposed to kinematics. Throughout the sequence, unifying ideas of energy models and model-based problem solving are explicitly emphasized. We will present a brief description of this course and discuss how the models-based approach has been realized at CSUSM, along with presenting evidence of associated student outcomes from our 12-year experience with this course. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B11.00007: Biotic games and cloud experimentation as novel media for biophysics education Ingmar Riedel-Kruse, Paulo Blikstein First-hand, open-ended experimentation is key for effective formal and informal biophysics education. We developed, tested and assessed multiple new platforms that enable students and children to directly interact with and learn about microscopic biophysical processes: (1) Biotic games that enable local and online play using galvano- and photo-tactic stimulation of micro-swimmers, illustrating concepts such as biased random walks, Low Reynolds number hydrodynamics, and Brownian motion; (2) an undergraduate course where students learn optics, electronics, micro-fluidics, real time image analysis, and instrument control by building biotic games; and (3) a graduate class on the biophysics of multi-cellular systems that contains a cloud experimentation lab enabling students to execute open-ended chemotaxis experiments on slimemolds online, analyze their data, and build biophysical models. Our work aims to generate the equivalent excitement and educational impact for biophysics as robotics and video games have had for mechatronics and computer science, respectively. We also discuss how scaled-up cloud experimentation systems can support MOOCs with true lab components and life-science research in general. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B11.00008: A course on physical models of living systems Philip Nelson What is a ``Physical model'' of a biological system? Are such models valuable for students? I'll describe some mechanical systems incorporating feedback control: the governor, toggle, and relaxation oscillator. Students who understand these systems at a tactile level, seeing them in action in the classroom, gain a better understanding of control networks arising in cellular homeostasis, program switching, and the cell cycle respectively. Moreover, I've found that some students respond better to physical ideas when they are motivated by biological examples; in this light, studying physical models of living systems can actually enhance learning of physics itself. I'll give details of an undergraduate course dedicated to topics like these, which attracts students from many different majors, and describe resources I've made available for constructing such courses. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B11.00009: Initial experience with a calculus-based IPLS course at Vanderbilt M. Shane Hutson, Erin C. Rericha By implementing research results from the PER community, we have designed a new calculus-based IPLS course and began teaching two sections of this course in Fall 2013, both taught by biological physicists. This course differs from Vanderbilt's other introductory physics offerings in two major ways. First, it seeks to implement PER-based active learning strategies including just-in-time teaching, peer instruction and context-rich problems. The latter are specifically designed within biomedical contexts. Second, the course content has been chosen to closely align with the core competencies delineated in the HHMI-AAMC report \textit{Scientific Foundations for Future Physicians}. We provide students with a very explicit accounting (in the syllabus) of how this course will contribute to 5 of the 8 \textit{SFFP}-competencies and 21 of its 37 learning objectives. Throughout the course and associated labs, we make repeated, explicit and hopefully authentic connections between physics and the life sciences. The chosen text reinforces our approach through well-developed biomedical applications of physics concepts. We will report what we've seen work and not work in our first implementation of an IPLS course and detail results regarding student learning and student attitudes towards physics. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B11.00010: The Physics of Life: A Biophysics Course for Non-science Major Undergraduates Raghuveer Parthasarathy Enhancing the scientific literacy of non-scientists is an important goal, both because of the ever-increasing impact of science and technology on people's lives, and because understanding contemporary science enables enriching insights into the workings of nature. One route to improving scientific literacy is via general education undergraduate courses - i.e. courses intended for students not majoring in the sciences or engineering - which in many cases provide these students' last formal exposure to science. I describe here a course on biophysics for non-science-major undergraduates recently developed at the University of Oregon. Biophysics, I claim, is a particularly useful vehicle for addressing scientific literacy. It involves important and general scientific concepts, demonstrates connections between basic science and tangible, familiar phenomena related to health and disease, and illustrates how scientific insights proceed not in predictable paths, but rather by applying tools and perspectives from disparate fields in creative ways. In addition, it highlights the far-reaching impact of physics research. I describe the general design of this course and the specific content of a few of its modules, as well as noting aspects of enrollment and evaluation. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B11.00011: Toward Better Physics Labs for Future Biologists John Giannini, Kim Moore, Wolfgang Losert We have developed a set of laboratories and hands on activities to accompany a new two-semester interdisciplinary physics course that has been successfully developed and tested in two small test classes of students at the University of Maryland, College Park (UMD) in 2012-2013, and is currently being used on a wider scale. We have designed the laboratories to be taken accompanying a reformed course in the student's second year, with calculus, biology, and chemistry as prerequisites. This permits the laboratories to include significant content on physics relevant to cellular scales, from chemical interactions to random motion and charge screening in fluids. One major focus of the laboratories is to introduce the students to research-grade equipment and modern physics analysis tools in contexts relevant to biology, while maintaining the pedagogically valuable open-ended laboratory structure of reformed laboratories. Lab development procedures along with some preliminary student results from these two small test classes are discussed. [Preview Abstract] |
Session B12: Invited Session: Exploring the Dynamics of Evolution and Ecology of Biological Systems
Sponsoring Units: DBIO GSNPChair: Alexandre Morozov, Rutgers University
Room: 205
Monday, March 3, 2014 11:15AM - 11:51AM |
B12.00001: Exploring the Dynamics of Evolution and Ecology of Biological Systems Invited Speaker: Jin Wang We established the potential and flux landscape theory for evolution. We found explicitly the conventional Wright's gradient adaptive landscape based on the mean fitness is inadequate to describe the general evolutionary dynamics. We show the intrinsic potential as being Lyapunov function (monotonically decreasing in time) does exist and can define the adaptive landscape for general evolution dynamics for studying global stability. The driving force determining the dynamics can be decomposed into gradient of potential landscape and curl probability flux. Non-zero flux causes detailed balance breaking and measures how far the evolution from equilibrium state. The gradient of intrinsic potential and curl flux are perpendicular to each other in zero fluctuation limit resembling electric and magnetic forces on electrons. We quantified intrinsic energy, entropy and free energy of evolution and constructed non-equilibrium thermodynamics. The intrinsic non-equilibrium free energy is a Lyapunov function. Both intrinsic potential and free energy can be used to quantify the global stability and robustness of evolution. We investigated an example of three allele evolutionary dynamics with frequency dependent selection (detailed balance broken). We uncovered the underlying single, triple, and limit cycle attractor landscapes. We found quantitative criterions for stability through landscape topography. We also quantified evolution pathways and found paths do not follow potential gradient and are irreversible due to non-zero flux. We generalized the original Fisher's fundamental theorem to the general (i.e., frequency dependent selection) regime of evolution by linking the adaptive rate with not only genetic variance related to the potential but also the flux. We show there is an optimum potential where curl flux resulting from biotic interactions of individuals within a species or between species can sustain an endless evolution even if the physical environment is unchanged. We offer a theoretical basis for explaining the corresponding Red Queen hypothesis proposed by Van Valen. Our work provides a theoretical foundation for evolutionary dynamics. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B12.00002: Genetic constraints on adaptive evolution in principle and in practice Invited Speaker: Daniel Weinreich ~~Geneticists have long recognized that pairs of mutations often produce surprising effects on the organism, given their effects in isolation. Such mutational interactions are called epistasis. Importantly, epistasis among mutations influencing an organism's survival or reproductive success can constrain the temporal order in which mutations will be favored by natural selection. After exploring these theoretical considerations more fully, we will demonstrate substantial epistatic constraint on the evolution of an enzyme that confers bacterial antibiotic resistance. Such epistatically induced constraints turn out to be rather common in enzyme evolution, and we will briefly discuss recent work that seeks to explicate its mechanistic basis using methods of molecular and structural biology. Finally we observe that the epistatic interaction between two mutations itself often varies with genetic context, implying the existence of higher-order interactions. We present a computational framework for assessing magnitude of epistatic interactions of all orders, and show that non-negligible epistatic interactions of all orders are common in a diverse set of biological systems. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B12.00003: A biophysical perspective on molecular evolution Invited Speaker: Claus Wilke The field of molecular evolution investigates how genes and genomes evolve over time. It has its origin in the late 1960s, when the first DNA and protein sequences were becoming available. With rapid progress in sequencing technologies came ever increasing demand for computational tools to study molecular evolution. Today, molecular evolution is among the largest subfields of evolutionary biology, and arguably one of the most computationally advanced. A side effect of the strong emphasis on developing sophisticated methods for sequence analysis has been that the underlying biophysical objects represented by the sequences, DNA molecules, RNA molecules, and proteins, have taken a back-seat in much computational molecular-evolution work. The vast majority of algorithms for sequence analysis, for example, operate purely on strings of letters, and don't incorporate any information of the biophysical reality that these letters represent. However, DNA, RNA, and proteins are three-dimensional physical objects composed of many interacting particles. We thus expect that their genetic evolution over time is shaped to some extent by these physical properties. Here, I will discuss the extent to which biophysical properties of proteins shape genetic evolution, and how we can use these properties to improve evolutionary analyses. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:39PM |
B12.00004: Acceleration of Evolutionary Spread by Long-Range Dispersal Invited Speaker: Oskar Hallatschek The spreading of evolutionary novelties across populations is the central element of adaptation. Unless population are well-mixed (like bacteria in a shaken test tube), the spreading dynamics not only depends on fitness differences but also on the dispersal behavior of the species. Spreading at a constant speed is generally predicted when dispersal is sufficiently short-ranged. However, the case of long-range dispersal is unresolved: While it is clear that even rare long-range jumps can lead to a drastic speedup, it has been difficult to analyze the ensuing stochastic growth process. We present a simple self-consistent argument supported by simulations that accurately predicts evolutionary spread for broad distributions of long distance dispersal. In contrast to the exponential laws predicted by deterministic ``mean-field'' models, spread is either according to a super-linear power-law or a stretched exponential law, depending on the tails of the dispersal kernel. Fluctuations and the relation to supercritical long-range percolation are discussed. Due to the simplicity of our model, which lacks any complex interactions between individuals, we expect our results to be applicable to a wide range of spreading processes. Our results may be used, in particular, to estimate the spread of modern human epidemics, which are greatly accelerated by the human aviation. Based on joint work with Daniel S. Fisher, Stanford. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 2:15PM |
B12.00005: Experimental evolution and epistasis in variable environments Invited Speaker: Sander Tans Environmental changes trigger cellular responses, but also impose selective pressures on the underlying regulatory systems. To disentangle this complex interplay we follow a synthetic biology approach. By linking the output of regulatory systems to bacterial growth, quantified temporally variable selective pressures can be applied to regulatory systems. This approach allows one to explore how networks evolve in complex variable environments. Epistatic interactions that underlie evolutionary constraint have mainly been studied for constant external conditions. However, environmental changes may modulate epistasis and hence affect genetic constraints. We investigate genetic constraints in the adaptive evolution of a novel regulatory function in variable environments, using the lac repressor, LacI, as a model system. We systematically reconstructed mutational trajectories from wild type LacI to three different variants that each exhibit an inverse response to the inducing ligand IPTG, and analyzed the higher-order interactions between genetic and environmental changes. We find epistasis to depend strongly on the environment. As a result, mutational steps essential to inversion but inaccessible by positive selection in one environment, become accessible in another. We present a graphical method to analyze the observed complex higher-order interactions between multiple mutations and environmental change, and show how they can be explained by a combination of mutational effects on allostery and thermodynamic stability. This dependency of genetic constraint on the environment should fundamentally affect evolutionary dynamics and phylogenetic analysis. [Preview Abstract] |
Session B13: Focus Session: Fe-Based Superconductors-1111's,122's
Sponsoring Units: DMPChair: Athena Sefat, Oak Ridge National Laboratory
Room: 207
Monday, March 3, 2014 11:15AM - 11:51AM |
B13.00001: A Study of Hydrogen Anion Substitution in 1111-type Iron Arsenides Invited Speaker: Hideo Hosono Hydrogen is the simplest bipolar element and its valence state can be controlled from $+$1 to $-$1. We have synthesized the 1111-type iron arsenides CaFeAsH and LnFeAsO1$-$xHx (Ln $=$ lanthanide; 0 $\le $ x $\le $ 0.5) with the ZrCuSiAs type structure by a high-pressure synthesis method. The position and valence state of the substituted H were determined by neutron diffraction and density functional theory calculations. The close similarity in the structural and electrical properties of CaFeAsH and CaFeAsF indicated the formation of the hydride ion (H$-)$, which is isovalent with the fluoride ion (F$-)$, in the 1111-type iron arsenides. When some of the O2$-$ ions in LnFeAsO are replaced by H$-$, superconductivity is induced by electron doping to the FeAs-layer to maintain charge neutrality. Since the substitution limit of hydrogen in LnFeAsO (x $\approx $ 0.5) is much higher than that of fluorine (x $\approx $ 0.2), the hydrogen substitution technique provides an effective pathway for high-density electron-doping, making it possible to draw the complete electronic phase diagram of LnFeAsO. The x-T diagrams of LnFeAsO$_{1-x}$H$_x$ (Ln $=$ La, Ce, Sm, Gd) have a wide superconducting (SC) region spanning the range x $=$ 0.04 to 0.4, which is far from the parent antiferromagnetic region near x $=$ 0.0. For LaFeAsO$_{1-x}$H$_x$, another SC dome region was found in the range x $=$ $\sim$ 0.2 to $\sim$ 0.5 with a maximum Tc $=$ 36 K, in addition to a conventional SC dome located at x $\sim$ 0.08 with maximum Tc $=$ 29 K. Density functional theory calculations performed for LaFeAsO$_{1-x}$H$_x$ using virtual crystal approximation indicated that the newly observed Tc is correlated with the appearance of degeneration of the Fe 3d bands (dxy, dyz and dzx), which is caused not only by regularization of the tetrahedral shape of FeAs$_4$ due to chemical pressure effects but also by selective band occupation with doped electrons. Very recently, a new AFM phase was found around x$=$0.5, suggesting that the double dome Tc structure reflects the presence of two AFM phases at x$=$0 and 0,5. In this talk, I review the recent progress in superconductivity in 1111-type iron (oxy)arsenides and related compounds induced by hydrogen anion substitution. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B13.00002: Orbital Fluctuation Theory for LaFeAsO$_{1-x}$H$_x$: Pnictogen Height Instability and Superconductivity due to Orbital Fluctuations Youichi Yamakawa, Seiichiro Onari, Hiroshi Kontani The isostructural transition in the tetragonal ($C_4$) phase, with sizable change in the As-height, is realized in heavily H-doped LaFeAsO, Pr-doped CaFe$_2$As$_2$, and Na-doped BaFe$_2$As$_2$. Here, we study the mechanism of spin-fluctuation-driven structure transition in LaFeAsO$_{1-x}$H$_x$ by using the self-consistent vertex correction (SC-VC) method. In heavily-doped case ($x\sim0.5$), the non-nematic orbital order is caused by the VC due to $d_{xy}$-orbital spin fluctuations, and triggers the $C_4$ isostructural transition. In lightly-doped case ($x\sim0$), the orthorhombic phase is realized by the orbital-nematic order, which originates from the VC due to ($d_{xz},d_{yz}$)-orbital spin fluctuations. Both nematic and non-nematic orbital fluctuations contribute in realizing higher-$T_{\rm c}$ superconductivity. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B13.00003: Transport properties of LaFeP$_{1-x}$As$_{x}$O$_{1-y}$F$_{y}$: Evidence for two superconducting states Kwing To Lai, Akira Takemori, Shigeki Miyasaka, Setsuko Tajima Resistivity and Hall coefficient of polycrystalline LaFeP$_{1-x}$As$_{x}$O$_{1-y}$F$_{y}$ with $x$ = 0 -- 1.0 and $y$ = 0 -- 0.1 have been investigated. In the $T_{c}$-$x$ phase diagram for F-free ($y$ = 0) samples, two superconducting domes have been revealed at $x$ = 0 -- 0.3 ($T_{c}^{max}$ $\sim $12 K) and 0.6 -- 0.8 ($T_{c}^{max}$ $\sim $10 K). Hall effect measurements suggest that the electronic states in these two dome regions are different from each other. For $y$ = 0.05, double peaks of $T_{c}$ are observed at $x$ $\sim $ 0.4 and 0.8, while only one dome with $T_{c}^{max}$ $\sim$28 K is observed for $y$ = 0.1 [1]. These changes can be regarded as the fusion of two $T_{c}$ domes upon F doping. \\[4pt] [1] S. Miyasaka et al., to be published in J. Phys. Soc. Jpn. (arXiv:1310.2731). [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B13.00004: Comparison of specific heat and magneto-resistance measurements in the same SmFeAsO$_{1-x}$F$_{x}$ crystals Stanislaw Galeski, Philip Moll, Nikolai Zhigadlo, Janusz Karpinski, Bertram Batlogg We have performed resistivity and specific heat measurements in the same sub-microgram single crystals of an iron-based superconductor SmFeAsO1-xFx (T$_{c} \approx $ 50K). This allowed for the first direct comparison of Hc2 curves from thermodynamic measurements with estimates from the magneto-resistance at commonly used criteria (10, 50, 90{\%} $\varrho _{N})$. A criterion of 40-50{\%} $\varrho_{N}$ well describes H$_{c2}$(T) for both in and out of plane fields. We attribute the low field dependence of the criterion to filamentary superconductivity. The challenging heat capacity measurement on microscopic crystals (50$\mu$m in diameter, 10$\mu$m thick) was done using a commercially available gas-nanocalorimeter. The thermodynamic data was in good agreement with previous experiments performed on crystals from the same batch by other groups. H$_{c2}$ slopes of 1.6 T/K for fields parallel to the c-axis and 12.3 T/K in the ab-plane were found yielding a $\xi $ anisotropy $\gamma \approx $ 7. This demonstrates that our experimental technique is both relatively fast to set up and furthermore reliable in fields up to 6T. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B13.00005: Effect of nematic order on the spin fluctuation spectrum of LaFeAsO Qiang Zhang, Rafael M. Fernandes, Jiaqiang Yan, R.W. McCallum, Thomas A. Lograsso, Songxue Chi, David Vaknin, Robert J. McQueeney Inelastic neutron scattering measurements on the LaFeAsO antiferromagnetic (AFM) system reveal distinct temperature-dependent behavior in the low-energy spin dynamics. As expected, the dynamic susceptibility at the AFM wavevector peaks at the AFM transition temperature $T_{\mathrm{N}}$, but also displays an anomaly at the orthorhombic-to-tetragonal transition temperature $T_{\mathrm{S}}$. The spin-spin correlation length increases rapidly below $T_{\mathrm{S}}$ once long-range nematic order sets in. The sharp changes in both the dynamic susceptibility and the spin-spin correlation length at $T_{\mathrm{S}}$ evidence a strong effect of nematic order on the magnetic spectrum, in agreement with models that attribute the structural transition to an electronic nematic phase driven by spin fluctuations. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B13.00006: Temperature and composition dependence of the magnetically ordered, tetragonal C$_4$ phase in Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ via neutron scattering Jared Allred, Daniel Bugaris, Omar Chmaissem, Stephan Rosenkranz, Sevda Avci, Pascal Manuel, Dmitry Khalyavin, Aziz Daoud-Aladine, Duck Young Chung, Helmut Claus, Mercouri Kanatzidis, Ray Osborn Until recently, the relationship between orthorhombic/magnetic order and superconductivity was believed well established in the 122 iron arsenide family. This changed when recent observations in the hole doped 122's: Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($0.16 \leq x \leq 0.21$) under pressure and Ba$_{0.76}$Na$_{0.24}$Fe$_2$As$_2$ at ambient pressure exhibit evidence of a new electronic phase being stabilized near the end of the magnetic dome, beginning at higher compositions than the onset of superconductivity . Using combined high-intensity and high-resolution neutron diffraction we have expanded the Na region to multiple compositions $0.24\leq x\leq 0.28$ all stabilized at ambient pressure. The magnetic and structural properties both differ from the paramagnetic tetragonal phase and the antiferromagnetic orthorhombic phase. The complex relationship between structure, magnetism, and superconductivity in this regime give important insights into the underlying physics. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B13.00007: Phase diagrams of K- and Na - doped BaFe$_2$As$_2$ as probed by heat capacity and hydrostatic pressure S.L. Bud'ko, D.Y. Chung, M. Sturza, D. Bugaris, M.G. Kanatzidis, P.C. Canfield Many iron-arsenide based superconductors present a simple scaling of the jump in specific heat at superconducting transition temperature $T_c$ with the value of $T_c$, $\Delta C_p \propto T_c^3$ (so called BNC scaling). A comprehensive study of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ over the whole range of K - concentrations showed clear deviation from the BNC scaling for $x > 0.7$. At the same concentrations anomalous behavior was observed in NMR and thermal conductivity measurements. This observation suggests change of the superconducting state for $x > 0.7$. The pressure dependence of $T_c$ (up to $\sim 1$ GPa) is linear or close to linear for all measured K-concentrations In contrast, the data for the large portion of Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ ($0.2 \leq x \leq 0.9$) series follow the BNC scaling. In addition, the pressure dependence of $T_c$ (measured up to $\sim 1$ GPa) have clear non-linearities for Na concentration in 0.2-0.25 region, that may be consistent with an emergent, narrow, tetragonal $C4$ phase.\footnote{S. Avci et al., PRB 88, 094510 (2013).} [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B13.00008: Disorder effects in multiorbital $s_{\pm}$-wave superconductors: Implications for Zn-doped BaFe$_2$As$_2$ compounds Hua Chen, Yuan-Yen Tai, C.S. Ting, Matthias J. Graf, Jianhui Dai, Jian-Xin Zhu Recent experiments on Zn-doped 122-type iron pnictides, Ba(Fe$_{1-x-y}$Co$_y$Zn$_x$)$_2$As$_2$, are challenging our understanding of electron doping the 122s and the interplay between doping and impurity scattering. To resolve this enigma, we investigate the disorder effects of nonmagnetic Zn impurities on various properties of the system in the $s_{\pm}$-wave pairing state. The BdG is solved based on a minimal two-orbital model with an extended range of impurity concentrations. With increasing Zn concentration the density of states shows a gradual filling of the gap, revealing pair breaking effect. Both the averaged superconducting order parameter and superfluid density are dramatically suppressed towards the dirty limit, indicating the violation of the Anderson theorem and breakdown of the AG theory for impurity-averaged Green's functions. The superconductivity is fully suppressed close to the critical impurity concentration of $n_{\mathrm{imp}}\approx 10\%$, in agreement with recent experiments. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B13.00009: Study on possible correlation of superconductivity with defects and superparamagnetism in undoped AFe$_{2}$As$_{2}$ with A$=$Ca, Sr and Ba Kui Zhao, Bing Lv, Liangzi Deng, Yuyi Xue, Paul Chu Extensive studies have been carried out on the induction of bulk superconductivity in the Fe-pnictide 122 system with a T$_{\mathrm{c}}$ up to 38 K through doping and/or pressure. However, non-bulk superconductivity has also been detected unexpectedly in undoped AFe$_{2}$As$_{2}$ where A $=$ Ca, Sr, and Ba with T$_{\mathrm{c}} =$ $\sim$12K, $\sim$22K and $\sim$23K, respectively. The reason for the observation remains unknown. Recently, systematic investigation shows that highly anisotropic superconductivity with a T$_{\mathrm{c}}$ up to 49 K and superparamagnetism occur in rare-earth doped Ca122. Further examination reveals slight deviation from the 1:2:2 stoichiometry which correlates closely with the occurrence of non-bulk superconductivity and superparamagnetism in these samples. We have therefore decided to investigate systematically the stoichiometry, defects, magnetism and superconductivity in undoped AFe$_{2}$As$_{2}$ single crystals under different synthesis conditions where A $=$ Ca, Sr, and Ba. Results will be presented and discussed. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B13.00010: Anisotropy reversal of in-plane resistivity in hole doped Ba(Fe$_{1-x}$TM$_x$)$_2$As$_2$ (TM=Mn, Cr) Tatsuya Kobayashi, Takumi Yamada, Kiyohisa Tanaka, Shigeki Miyasaka, Setsuko Tajima We investigated the in-plane anisotropy of resistivity across the magneto-structural transition in hole doped Ba(Fe$_{1-x}$TM$_x$)$_2$As$_2$ (TM=Mn, Cr) with detwinned single crystals. When the Mn and Cr-doping levels were low, the resistivity along the a-axis with antiferromagnetic spin alignment was smaller than that along the b-axis with ferromagnetic spin alignment, which is similar to that of electron doped BaFe$_2$As$_2$ [1]. However when x exceeds 0.09 in Cr-doped case, we observed the opposite resistivity anisotropy like that of (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ [2]. We will discuss the origin of the anisotropic resistivity and the difference between the effect of Cr and K doping. \\[4pt] [1] J. H. Chu et al., Science 329, 824 (2010)\\[0pt] [2] E. C. Blomberg et al., Nat. Commun. 4, 1914 (2013) [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B13.00011: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 1:51PM - 2:03PM |
B13.00012: Emergence of high mobility hole-like carrier in Ba(Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As)$_{2}$ Yoichi Tanabe, Takahiro Urata, Khuong Huynh, Satoshi Heguri, Hidetoshi Oguro, Kazuo Watanabe, Katsumi Tanigaki An evolution of electronic states through impurity substitutions is one of key issues for understanding the electronic ground state of iron pnictides. In this talk, we will report the emergence of the hole-like carrier with high mobility in Ba(Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As)$_{2}$. A clear sign change of the Hall resistivity at low magnetic fields indicated that the electron-like high mobility carrier changes to the hole-like one through Mn substitution [1], although the nuclear magnetic resonance revealed that the Mn substitution does not introduce any carrier doping [2]. The evolution of p-type carrier will be discussed based on results of conductive tensor analyses. \\[4pt] [1] T. Urata et al., arxiv1307.2813.\\[0pt] [2] Y. Texier et al., EPL, 99, 17002 (2012). [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B13.00013: Iron-based superconductors under heavy compression after destruction of superconductivity Xiaojia Chen, Yong-Hui Zhou, Jian-Jun Ying, Takaki Muramatsu, Viktor Struzhkin, Ho-Kwang Mao, Zhu-An Xu, Xian-Hui Chen, Guang-Yong Xu, Gen-Da Gu Choosing electron and hole doped Ba$_{1-y}$K$_{y}$Fe$_{2-x}$A$_{x}$As$_{2}$ (A=Ni,Co) and Fe$_{1+y}$Se$_{1-x}$Te$_{x}$ -- typical iron pnictides and chalcogenides, we investigate pressure effects on the physical properties of these superconductors after their superconductivity is completely destroyed. Combining electrical resistance and magnetic susceptibility measurements at pressures above 60 GPa, we establish extended pressure - temperature phase diagrams for these compounds. Contrary to the general belief of the existence of Fermi liquid after the disappearance of superconductivity upon compression, we find an unexpected insulating state in these heavily compressed compounds. Some novel behaviors such as reentrance of superconductivity are also discovered. These results indicate that rich physics is still hidden in iron-based superconductors. [Preview Abstract] |
Session B14: Invited Session: Trends and Perspectives on Fundamental Polymer Physics
Sponsoring Units: DPOLY DFDChair: James Forrest, Univesrity of Waterloo
Room: 301-303
Monday, March 3, 2014 11:15AM - 11:51AM |
B14.00001: Role of enhanced segmental mobility in the deformation of polymer glasses Invited Speaker: Mark Ediger The mechanical properties of polymer glasses, including plastic flow, are important for many applications. In contrast to the flow of polymer melts, plastic flow is poorly understood at a fundamental level. One reason for this is that the deformation of polymer glasses typically occurs in a highly nonlinear regime, e.g., doubling the strain rate has little impact on the flow stress. Eyring proposed that stress increases the rate of molecular rearrangements in solids and this is the source of nonlinearity in many models. In this talk, experiments measuring molecular mobility during constant strain rate deformation of a polymer glass will be described for the first time. In these experiments on PMMA, the mobility initially increases in the pre-yield regime, by a factor of up to 160, as compared to the undeformed glass. After yield, the mobility remains constant even as the stress is decreasing; this non-Eyring effect is consistent with the view that the sample is being pulled higher on the potential energy landscape. For the range of strain rates investigated, mobility and strain rate are linearly correlated, consistent with the view that enhanced segmental mobility enables the flow of polymer glasses. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B14.00002: Monte Carlo Field-Theoretic Simulations for Melts of Symmetric Diblock Copolymer Invited Speaker: Mark Matsen Monte Carlo field-theoretic simulations (MC-FTS) are performed on melts of symmetric diblock copolymer for invariant polymerization indexes extending down to experimentally relevant values of $\bar{N} \sim10^4$. The simulations are performed with a fluctuating composition field, $W_-({\bf r})$, and a pressure field, $W_+({\bf r})$, that follows the saddle-point approximation. Our study focuses on the disordered-state structure function, $S(k)$, and the order-disorder transition (ODT). Although short-wavelength fluctuations cause an ultraviolet (UV) divergence in three dimensions, this is readily compensated for with the use of an effective Flory-Huggins interaction parameter, $\chi_e$. The resulting $S(k)$ matches the predictions of renormalized one-loop (ROL) calculations over the full range of $\chi_e N$ and $\bar{N}$ examined in our study, and agrees well with Fredrickson-Helfand (F-H) theory near the ODT. Consistent with the F-H theory, the ODT is discontinuous for finite $\bar{N}$ and the shift in $(\chi_e N)_{\rm ODT}$ follows the predicted $\bar{N}^{-1/3}$ scaling over our range of $\bar{N}$. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B14.00003: A direct quantitative measure of surface mobility in a glassy polymer Invited Speaker: Elie Raphael Thin polymer films are widely used in applications and have striking dynamical properties that differ from their bulk counterparts. With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and below the glass transition temperature $T_{\textrm{g}}$. Above $T_{\textrm{g}}$ the entire film flows, while below $T_{\textrm{g}}$ only the near surface region responds to the excess interfacial energy. An analytical thin film model for flow limited to the free surface region is developed and shows excellent agreement with sub-$T_{\textrm{g}}$ data. The system transitions from whole film flow to surface localized flow over a narrow temperature region near the bulk $T_{\textrm{g}}$. The experiments and model provide a measure of surface mobility in a sample geometry where confinement and substrate effects are negligible. \\[4pt] This work has been done in collaboration with Y. Chai, Department of Physics and Astronomy and Guelph-Waterloo Physics Institute, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1; T. Salez, Laboratoire de Physico-Chimie Theorique, UMR CNRS Gulliver 7083, ESPCI, Paris, France; J.D. McGraw, Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada, L8S 4M1; M. Benzaquen, Laboratoire de Physico-Chimie Theorique, URM CNRS; K. Dalnoki-Veress, Department of Physics and Astronomy, McMaster University and Laboratoire de Physico-Chimie Theorique, UMR CNRS; and J.A. Forrest, Department of Physics and Astronomy and Guelph-Waterloo Physics Institute,University of Waterloo. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:39PM |
B14.00004: On the Anomalous Diffusion of a Polymer Chain in an Unentangled Melt Invited Speaker: Jorg Baschnagel The dynamics of polymer chains in unentangled melts is commonly described by the Rouse model. However, various experimental and simulation studies show that certain dynamical phenomena in unentangled melts cannot be explained by the Rouse theory. One of the puzzling observations is the anomalous diffusion of the center-of-mass (CM) of a polymer chain for times $t < t_N$, where $t_N\propto N^2$ is the Rouse time of a polymer consisting of $N$ monomers. We explore two attempts to explain this observation: (i) an approach based on the effective interactions between the CMs in the melt and (ii) an approach based on the hydrodynamic flow and viscoelasticity of the melt. For approach (i) we find a partial success [1]: The theory accounts for the anomalous motion by yielding a negative power-law tail for the CM velocity autocorrelation function (CM VAF), $C_{\mathrm{cm}}(t) \propto - N^{-1}t^{-5/4}$. This prediction is in good agreement with molecular-dynamics (MD) simulations utilizing Langevin dynamics with a strong damping constant. On the other hand, for simulations with momentum conserving dynamics (i.e., the experimentally relevant situation) the prediction of approach (i) is qualitatively incorrect. In the latter case, the CM VAF rather scales as $C_{\mathrm{cm}}(t) \propto - N^{-1/2}t^{-3/2}$. This behavior can be rationalized by approach (ii). The predictions of approach (ii) are found to be good quantitative agreement with the MD simulations [2]. \\[4pt] [1] J. Farago, A. N. Semenov, H. Meyer, J. P. Wittmer, A. Johner and J. Baschnagel, Phys.\ Rev.\ E \textbf{85}, 051806 (2012).\\[0pt] [2] J. Farago, H. Meyer, J. Baschnagel and A. N. Semenov, Phys.\ Rev.\ E \textbf{85}, 051807 (2012). [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 2:15PM |
B14.00005: New questions in classical polymer physics Invited Speaker: Steve Granick This talk will show some of the most famous problems of polymer physics (the nature of entanglement, microrheology in the noncontinuum limit, pulled chains) can be enriched using deep imaging based on modern fluorescence imaging methods. These methods generate huge statistics regarding polymers whose internal conformational rearrangements can be large enough to image directly. Averages are obtained and also the fluctuations around them. From such investigations some aspects of classical polymer understanding are confirmed, but in other aspects we find surprises. Combining this with recent findings using super-resolution microscopy, a consistent new picture emerges. -- Work performed in collaboration with Juan Guan, Kejia Chen, Lingxiang Jiang, John King, Subhalakshmi Kumar, Changqian Yu, and Chi Hang Boyce Tsang. [Preview Abstract] |
Session B15: Bubbles, Interfaces & Porous Media
Sponsoring Units: DFDChair: C. Maldarelli, City University of New York
Room: 304
Monday, March 3, 2014 11:15AM - 11:27AM |
B15.00001: Attraction of Two Floating Spheres at a Viscous Oil-Water Interface Archit Dani, Geoff Keiser, Mohsen Yeganeh, Charles Maldarelli The aggregation rate of floating particles at a fluid/fluid interface by capillary forces has drawn significant interest. This 2D phenomenon plays a critical role in self-assembly arrangement relevant to pollination processes in biological contexts, the formation of dense particle laden interfaces for stabilizing emulsions in colloid science and in the bottom up assembly of materials in nanotechnologies. We present the first experiments on the merging of two Teflon particles at an interface between a mineral oil and an aqueous phase for a series of particle pairs, interfacial tension and oil viscosity. The separation distance as a function of time and pair aggregation time are both measured by optically following the movement of the particles. The experimental results are in excellent agreement with our theoretical formulation in which a drag correction accounts for the variation in particle depth of immersion. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B15.00002: Computational Fluid Dynamics of Acoustically Driven Bubble Systems Connor Glosser, Jie Lie, Daniel Dault, Shanker Balasubramaniam, Carlo Piermarocchi The development of modalities for precise, targeted drug delivery has become increasingly important in medical care in recent years. Assemblages of microbubbles steered by acoustic pressure fields present one potential vehicle for such delivery. Modeling the collective response of multi-bubble systems to an intense, externally applied ultrasound field requires accurately capturing acoustic interactions between bubbles and the externally applied field, and their effect on the evolution of bubble kinetics. In this work, we present a methodology for multiphysics simulation based on an efficient transient boundary integral equation (TBIE) coupled with molecular dynamics (MD) to compute trajectories of multiple acoustically interacting bubbles in an ideal fluid under pulsed acoustic excitation. For arbitrary configurations of spherical bubbles, the TBIE solver self-consistently models transient surface pressure distributions at bubble-fluid interfaces due to acoustic interactions and relative potential flows induced by bubble motion. Forces derived from the resulting pressure distributions act as driving terms in the MD update at each timestep. The resulting method efficiently and accurately captures individual bubble dynamics for clouds containing up to hundreds of bubbles. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B15.00003: Soap Film Hydrodynamics: In Color, and In Black and White Collin Pearsall, Yiran Zhang, Jana Rush, Subinuer Yilixiati, Vivek Sharma Iridescent colors of soap bubbles or films arise due to interference between light reflected from two surfactant-laden surfaces that are $\sim$ 100 nm - 10 micron apart. Sandwiched between these interfacial layers is a fluid that drains primarily under the influence of gravitational and capillary or interfacial forces, including disjoining pressure. Below 50 nm the thin films appear as black. We experimentally follow the drainage kinetics of soap films using imaging {\&} color science and UV-Visible spectroscopy. We find fascinating examples of two-dimensional hydrodynamics and unexplained, if not unprecedented, drainage kinetics. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B15.00004: Atomistic simulations of surfactant adsorption kinetics at interfaces Eugeniya Iskrenova, Soumya Patnaik Heat transfer control and enhancement is an important and challenging problem in a variety of industrial and technological applications including aircraft thermal management. The role of additives in nucleate boiling and phase change in general has long been recognized and studied experimentally and modeled theoretically but in-depth description and atomistic understanding of the multiscale processes involved are still needed for better prediction and control of the heat transfer efficiency. Surfactant additives have been experimentally observed to either enhance or inhibit the boiling heat transfer depending on the surfactant concentration and chemistry and, on a molecular level, their addition leads to dynamic surface tension and changes in interfacial and transfer properties, thus contributing to the complexity of the problem. We present our atomistic modeling study of the interfacial adsorption kinetics of aqueous surfactant (sodium dodecyl sulfate) systems at a range of concentrations at room and boiling temperatures. Classical molecular dynamics and Umbrella Sampling simulations were used to study the surfactant transport properties and estimate the adsorption and desorption rates at liquid-vacuum and liquid-solid interfaces. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B15.00005: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 12:15PM - 12:27PM |
B15.00006: Experimental Study of the Hydrodynamic Resistance of Liquid Droplets in Polycarbonate Microchannels Zeyad Almutairi, Carolyn Ren, David Johnson The presence of liquid droplets in a microchannel adds excess hydrodynamic resistance to the flow compared to single phase flow. The hydrodynamic resistance of liquid droplets is a function of fluid properties (viscosity ratios $\frac{\mu_d}{\mu_c}$, interfacial tension $\gamma$), geometrical properties of the droplet and the confining channel (droplet length $L_d$, microchannel width and height), and flow condition (Ca, Re). This work presents the results of an experimental examination of the transport properties of liquid droplets in a microchannel. Focus was given to the hydrodynamic resistance of droplets with lengths comparable or greater than the channel width ($L_d \ga W_{ch}$. Experiments were performed in surface modified polycarbonate microchannels since they will reduce measurement uncertainties associated with channel swelling in soft materials such as PDMS. For the droplets sizes that were examined results confirm the relation between the hydrodynamic resistance of liquid droplets and the Capillary number (Ca). It was also observed that droplet slip ($\beta = \frac{u_d}{u_{c,avg}}$) is less than 1 in all the experiments performed. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B15.00007: Impact and Penetration of Nanoparticle Suspension Drops into Porous Membranes Rakesh Sahu, Alexander Yarin, Behnam Pourdeyhimi The impacts and dynamic penetration of drops with suspended nanoparticles into porous membranes are studied experimentally and theoretically. This type of penetration is radically different from the wettability-driven imbibition. Two types of membranes are used in the experiments: (i) glass fiber filter membrane (wettable) and (ii) PTFE depth filter (non-wettable). The nanoparticle entrainment and deposition inside the membrane bulk is used to mostly visualize the ultimate penetration fronts of the carrier fluid by observing the cut cross-sections of the filter membranes, albeit also provides an insight into potentially new applications like circuit printing on nonwovens. The experimental results demonstrate that during the dynamic focusing responsible for water penetration into micro- and nanopores, water can penetrate into a non-wettable porous medium (PTFE). Water also penetrates by the same focusing mechanism into the wettable glass fiber membrane, where it additionally spreads on a much longer time scale due to the wettability-driven flow. A theory explaining dynamic penetration of liquid into porous medium after drop impact is proposed. It is used to explain and predict water penetration into the non-wettable media after drop impact, and the results are compared with the experimental data. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B15.00008: Oil droplet dynamics at a porous surface in the presence of crossflow: Implications for microfiltration of oil-water dispersions Tohid Darvishzadeh, Volodymyr Tarabara, Nikolai Priezjev The behavior of an oil droplet pinned at the entrance of a micropore and subject to clossflow-induced shear is investigated numerically by solving the Navier-Stokes equation. We found that in the absence of crossflow, the critical transmembrane pressure required to force the droplet into the pore is in excellent agreement with a theoretical prediction based on the Young-Laplace equation. With increasing shear rate, the critical pressure of permeation increases, and at sufficiently high shear rates the oil droplet breaks up into two segments. It was shown that droplet breakup at the pore entrance is facilitated at lower surface tension, higher oil-to-water viscosity ratio and larger droplet size but is insensitive to the value of the contact angle. An estimate for the increase in critical pressure due to crossflow and the breakup capillary number is obtained and validated for different viscosity ratios, surface tension coefficients, contact angles, and drop-to-pore size ratios. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B15.00009: Transport and Deposition of Nanoparticles in the Pore Network of a Reservoir Rock: Effects of Pore Surface Heterogeneity and Radial Diffusion Ngoc Pham, Dimitrios Papavassiliou In this study, transport behavior of nanoparticles under different pore surface conditions of consolidated Berea sandstone is numerically investigated. Micro-CT scanning technique is applied to obtain 3D grayscale images of the rock sample geometry. Quantitative characterization, which is based on image analysis is done to obtain physical properties of the pore network, such as the pore size distribution and the type of each pore (dead-end, isolated, and fully connected pore). Transport of water through the rock is simulated by employing a 3D lattice Boltzmann method. The trajectories of nanopaticles moving under convection in the simulated flow field and due to molecular diffusion are monitored in the Lagrangian framework [1]. It is assumed in the model that the particle adsorption on the pore surface, which is modeled as a pseudo-first order adsorption, is the only factor hindering particle propagation. The effect of pore surface heterogeneity to the particle breakthrough is considered, and the role of particle radial diffusion is also addressed in details.\\[4pt] [1] Voronov, R.S., VanGordon, S., Sikavitsas, V.I., and D.V. Papavassiliou, \textit{Int. J. Num. Methods in Fluids}, \textbf{67}, 501-517, 2011 [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B15.00010: Channeling and stress for fluid and suspension flows in self-affine fractures Tak S. Lo, Joel Koplik The flow of fluids and particulate suspensions in realistic models of geological fractures is investigated using lattice Boltzmann simulations. The bounding walls are self-affine fractal surfaces and combined to form a tight fracture, i.e. one in which where the particle size, the mean aperture and the surface roughness are all comparable. We consider pressure-driven flow of a viscous Newtonian liquid and model the particles as rigid non-colloidal solid spheres. Our focus is the channeling phenomena, where we compare the preferred paths for fluid flow and the suspended particles to the fracture aperture. The preferred paths are found to be somewhat similar for pure fluid and particulates, and not immediately related to the fracture aperture map. We further investigate the stress exerted on the fracture walls during flows in the irregular channel, which is useful in geological applications. Finally, we examine the spatial correlations in the stress and velocity distributions and compare to the statistics of the aperture field and identify the relationship between them. [Preview Abstract] |
Session B16: Statistical Mechanics of Social Systems
Sponsoring Units: GSNPChair: G Korniss
Room: 401
Monday, March 3, 2014 11:15AM - 11:27AM |
B16.00001: Statistical Mechanics of US Supreme Court Edward Lee, Chase Broedersz, William Bialek We build simple models for the distribution of voting patterns in a group, using the Supreme Court of the United States as an example. The least structured, or maximum entropy, model that is consistent with the observed pairwise correlations among justices' votes is equivalent to an Ising spin glass. While all correlations (perhaps surprisingly) are positive, the effective pairwise interactions in the spin glass model have both signs, recovering some of our intuition that justices on opposite sides of the ideological spectrum should have a negative influence on one another. Despite the competing interactions, a strong tendency toward unanimity emerges from the model, and this agrees quantitatively with the data. The model shows that voting patterns are organized in a relatively simple ``energy landscape,'' correctly predicts the extent to which each justice is correlated with the majority, and gives us a measure of the influence that justices exert on one another. These results suggest that simple models, grounded in statistical physics, can capture essential features of collective decision making quantitatively, even in a complex political context. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B16.00002: Finding Structure in the ArXiv Alexander Alemi, Ricky Chachra, Paul Ginsparg, James Sethna We applied machine learning techniques to the full text of the arXiv articles and report a meaningful low-dimensional representation of this big dataset. Using Google's open source implementation of the continuous skip-gram model, word2vec, the vocabulary used in scientific articles is mapped to a Euclidean vector space that preserves semantic and syntactic relationships between words. This allowed us to develop techniques for automatically characterizing articles, finding similar articles and authors, and segmenting articles into their relevant sections, among other applications. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B16.00003: A scientific impact indicator based on the latent ``citability'' of a researcher's publications Joao Moreira, Xiaohan Zeng, Luis Amaral How to quantify the impact of a scientist's body of work is currently a matter of great concern. The use of bibliometric indicators, such as the \emph{h}-index or the Journal Impact Factor, have become widespread despite their known limitations. We surmise that many of the deficiencies of existing bibliometric indicators arise from their heuristic nature. Here, we pursue a principled approach to the development of an indicator to quantify the scientific impact of individual researchers, grounded on the functional form of the distribution of the ultimate number of citations. We validate our approach using the publication records of 1,283 researchers from seven scientific disciplines. Our approach has three distinct advantages. First, it accurately captures the overall scientific impact of researchers, as measured by ultimate citation counts. Second, in contrast to prior bibliometric indicators, our proposed measure does not depend on the number of publications, offering the possibility to compare researchers at different career stages. Third, more than other measures, our index is resistant to manipulation and rewards publication quality over quantity. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B16.00004: Stochastic Dynamics of Lexicon Learning in an Uncertain and Nonuniform World Richard Blythe, Rainer Reisenauer, Kenny Smith We study the time taken by a language learner to correctly identify the meaning of all words in a lexicon under conditions where many plausible meanings can be inferred whenever a word is uttered. We show that the most basic form of cross-situational learning - whereby information from multiple episodes is combined to eliminate incorrect meanings - can perform badly when words are learned independently and meanings are drawn from a nonuniform distribution. If learners further assume that no two words share a common meaning, we find a phase transition between a maximally efficient learning regime, where the learning time is reduced to the shortest it can possibly be, and a partially efficient regime where incorrect candidate meanings for words persist at late times. We obtain exact results for the word-learning process through an equivalence to a statistical mechanical problem of enumerating loops in the space of word-meaning mappings. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B16.00005: Statistical mechanics of human resource allocation Jun-ichi Inoue, He Chen We provide a mathematical platform to investigate the network topology of agents, say, university graduates who are looking for their positions in labor markets. The basic model is described by the so-called Potts spin glass which is well-known in the research field of statistical physics. In the model, each Potts spin (a tiny magnet in atomic scale length) represents the action of each student, and it takes a discrete variable corresponding to the company he/she applies for. We construct the energy to include three distinct effects on the students' behavior, namely, collective effect, market history and international ranking of companies. In this model system, the correlations (the adjacent matrix) between students are taken into account through the pairwise spin-spin interactions. We carry out computer simulations to examine the efficiency of the model. We also show that some chiral representation of the Potts spin enables us to obtain some analytical insights into our labor markets. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B16.00006: Gender differences in collaboration patterns Xiaohan Zeng, Jordi Duch, Marta Sales-Pardo, Filippo Radicchi, Haroldo V. Ribeiro, Teresa K. Woodruff, Luis A.N. Amaral Collaboration plays an increasingly important role in research productivity and impact. However, it remains unclear whether female and male researchers in science, technology, engineering and mathematical (STEM) disciplines differ significantly from each other in their collaboration propensity. Here, we report on an empirical analysis of the complete publication records of 3,920 faculty members in six STEM disciplines at selected top U.S. research universities. We find that while female faculty have significantly fewer co-authors over their careers, this can be fully explained by their lower number of publications. Indeed, we also find that females tend to distribute their co-authoring opportunities among their co-authors more evenly than males do. Our results suggest that females have had a greater propensity to collaborate, in order to succeed in a historically men-dominated academic world. Surprisingly, we find evidence that in molecular biology there has been a gender segregation within sub-disciplines. Female faculty in molecular biology departments tend to collaborate with smaller teams and publish in journals and fields where typical team size is smaller. Our results identify gender-specific collaborative behaviors as well as disciplines with distinct patterns. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B16.00007: The Regional Structure of Technical Innovation Dion O'Neale There is strong evidence that the productivity \emph{per capita} of cities and regions increases with population. One likely explanation for this phenomenon is that densely populated regions bring together otherwise unlikely combinations of individuals and organisations with diverse, specialised capabilities, leading to increased innovation and productivity. We have used the REGPAT patent database to construct a bipartite network of geographic regions and the patent classes for which those regions display a revealed comparative advantage. By analysing this network, we can infer relationships between different types of patent classes - and hence the structure of (patentable) technology. The network also provides a novel perspective for studying the combinations of technical capabilities in different geographic regions. We investigate measures such as the diversity and ubiquity of innovations within regions and find that diversity (resp. ubiquity) is positively (resp. negatively) correlated with population. We also find evidence of a nested structure for technical innovation. That is, specialised innovations tend to occur only when other more general innovations are already present. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B16.00008: A criterion for condensation in kinetically constrained one-dimensional transport models Daniel Miedema In transport, increasing the number of transporting particles not necessarily results in an increase of the throughput. When the density of a complex system increases, the current can decrease rapidly due to jamming effects. Jammed particles can form many clusters or one big cluster: a condensate in real space. We study condensation in one-dimensional transport models with a kinetic constraint. We find the conditions under which the arrested clusters can grow to a macroscopic condensate of arrested particles. We apply our finding to the well-known Nagel-Schreckenberg traffic flow model to analytically proof the existence of a condensate in a deterministic limit of this model, and verify this result with simulations. These results provide insight into dynamic arrest and dynamic phase separation in one-dimensional traffic and transport. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B16.00009: Competing effects of social balance and influence P. Singh, S. Sreenivasan, B. Szymanski, G. Korniss The theory of social balance is one of the key drivers of social dynamics. We study a model of social interactions in which the dynamics of social balance is competing with external influence. In this model, each node in a social network is in one of the three possible states - leftist, rightist, centrist. Only a link between two unequal extremist nodes is considered unfriendly and a triangle is balanced if it contains even number of unfriendly links. Thus triangles formed by a centrist, a leftist and a rightist are unbalanced. In this model, at each time step with probability $p$, we pick a random node and convert it into a centrist while with probability $(1-p)$, a randomly picked triangle is checked for balance and if needed, it is balanced by updating the state of one of the nodes in the triangle. We find that there exists a critical value $p_c$ such that for $p |
Monday, March 3, 2014 1:03PM - 1:15PM |
B16.00010: A network approach in analysis of the matching hypothesis Tao Jia, Robert Spivey, Gyorgy Korniss, Boleslaw Szymanski The matching hypothesis in social psychology claimed that people are more likely to form a committed relationship with someone who is equally attractive. This phenomenon can be well interpreted by the principle of homophily that people are apt to get in touch with others similar to them. Yet, social experiments indicate that people in general tend to prefer more attractive individuals regardless of their own attractiveness. Here study the stochastic matching process for different underlying networks and different attractiveness distributions. We showed that the correlation of attractiveness within couples could purely due to the limited number of acquaintance each person has and such correlation decreases as the network becomes more sparse. We also analyzed the effect of the degree distribution and the attractiveness on the number of individuals that can not find their partners. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B16.00011: Scale-free networks with temporary link deactivation for disease avoidance Leah Shaw, Maxim Shkarayev, Ilker Tunc We study epidemic spread on scale-free networks in which nodes can temporarily deactivate their links to infected neighbors and reactivate when their neighbors recover. We find that the topology of the subnetwork consisting of active links is fundamentally different from the original network topology, and we predict the scaling exponent of the active degree distribution. Further, we derive an improved low dimensional system of mean-field equations for dynamics of nodes and links based on the distribution of a node's neighbors conditioned on the total degree. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B16.00012: Internal and external influence in the US stock market Stanislav Borysov, Yasser Roudi, Alexander Balatsky We analyze the multivariate distribution of the US stock returns using pairwise interaction models, inspired by Ising models in glasses and neural networks. Using the inference methods from neural networks analysis we find unique descriptors of the dynamics of stock returns in periods of crisis. Our findings suggest that the near crash dynamics is primarily governed by external factors (external fields), while internal network structure (J couplings) are not significantly affected. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B16.00013: Can Quantum Physics Find the Answer to the World Financial Crisis. Lamine Dieng We assume the global wealth of nations within the G-Global to be an American call option described as a stochastic process. We let the American call option to grow and to eventually generate profits to the nations of the G-Global. We show profits taken to be a discontinuous process, because when an investment banker or a country makes more profits continuously, then their vision will be guided by greed. When banks try to maximize profits continuously and so they operate on the edge of bankruptcy. We also assume the global wealth to be an index defined in terms of the expected global wealth of nations and normalized by their GDPs. We impose the following conditions: a). The sum of the GDPs of all nations making the G-Global is one (1), the normalizing GDP should not have an influence on the global wealth. All nations should be treated on the same footing. b). the change of the global wealth of nations to be a supermartingale. We set the drift term of the expectation decreasing process to be equal to zero. We obtain an Ordinary Differential Equation describing the dynamic of global wealth [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B16.00014: Prospect Theory for Online Financial Trading Yang-Yu Liu, Jose C. Nacher, Tomoshiro Ochiai, Mauro Martino, Yaniv Altshuler Prospect theory is widely viewed as the best available descriptive model of how people evaluate risk in experimental settings. According to prospect theory, people make decisions based on the potential value of losses and gains rather than the final outcome. People are risk-averse with respect to gains and risk-seeking with respect to losses, a phenomenon called ``loss aversion''. Despite of the fact that prospect theory has been well studied in behavioral economics at the theoretical level, there exist very few empirical research and most of them has been undertaken with micro-panel data. Here we analyze the trading activities of over 1.5 million members of an online financial trading community over 28 months, aiming to explore the large-scale empirical aspect of prospect theory. By analyzing and comparing the behaviour of ``winners'' and ``losers'', i.e., traders with positive or negative final net profit, we find clear evidence of the loss aversion phenomenon, an essence in prospect theory. This work demonstrates an unprecedented large-scale empirical evidence of prospect theory. It has immediate implication in financial trading, e.g., developing new trading strategies by minimizing the effect of loss aversion. It also provides opportunity to augment online social trading, where users are allowed to watch and follow the trading activity of others, by predicting potential winners based on their historical trading behaviour. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B16.00015: Finding hidden periodic signals in time series -- an application to stock prices Michael O'Shea Data in the form of time series appear in many areas of science. In cases where the periodicity is apparent and the only other contribution to the time series is stochastic in origin, the data can be `folded' to improve signal to noise and this has been done for light curves of variable stars with the folding resulting in a cleaner light curve signal. Stock index prices versus time are classic examples of time series. Repeating patterns have been claimed by many workers and include unusually large returns on small-cap stocks during the month of January, and small returns on the Dow Jones Industrial average (DJIA) in the months June through September compared to the rest of the year. Such observations imply that these prices have a periodic component. We investigate this for the DJIA. If such a component exists it is hidden in a large non-periodic variation and a large stochastic variation. We show how to extract this periodic component and for the first time reveal its yearly (averaged) shape. This periodic component leads directly to the `Sell in May and buy at Halloween' adage. We also drill down and show that this yearly variation emerges from approximately half of the underlying stocks making up the DJIA index. [Preview Abstract] |
Session B17: Focus Session: Packing of Anisotropic Particles
Sponsoring Units: GSNP DPOLYChair: Rob Hoy
Room: 402
Monday, March 3, 2014 11:15AM - 11:27AM |
B17.00001: Solving the Granular Inverse Packing Problem with Artificial Evolution Marc Miskin, Heinrich Jaeger If a collection of identical particles is poured into a container, it is obvious that different shapes will fill to different densities. But what is the shape that fills a container as close as possible to a pre-specified, desired density? We demonstrate a solution to this inverse-packing problem by framing it in the context of artificial evolution. By representing shapes as bonded spheres, we show how particles may be mutated, simulated, and selected to produce particularly dense or loose packing aggregates, both with and without friction. Moreover, we show how motifs emerge linking these shapes together. The result is a set of design rules that function as an effective solution to the inverse packing problem for given packing procedures and boundary conditions. Finally, we show that these results may be verified by experiments on 3d printed prototypes used to make packings in the real world. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B17.00002: Shape Alloys of Nanorods and Nanospheres from Self-Assembly Jaime Millan, Xingchen Ye, Michael Engel, Jun Chen, Benjamin Diroll, Sharon Glotzer, Chris Murray Mixtures of anisotropic nanocrystals promise a great diversity of superlattices and phase behaviors beyond those of single-component systems. However, obtaining a colloidal shape alloy in which two different shapes are thermodynamically co-assembled into a crystalline superlattice has remained a challenge. Here we present a joint experimental-computational investigation of two geometrically ubiquitous nanocrystalline building blocks---nanorods and nanospheres---that overcome their natural entropic tendency towards macroscopic phase separation and co-assemble into three intriguing phases over centimeter scales, including an AB2-type binary superlattice. Monte Carlo simulations reveal that although this shape alloy is entropically stable at high packing fraction, demixing is favored at experimental densities. Simulations with short-ranged attractive interactions demonstrate that the alloy is stabilized by interactions induced by ligand stabilizers and/or depletion effects. An asymmetry in the relative interaction strength between rods and spheres improves the robustness of the self-assembly process. Reference: Ye, Millan, Engel, Chen, Diroll, Glotzer, Murray, Nano Letters 13, 4980 (2013). [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B17.00003: What is the Real Lewis Law? Size-Topology Correlations for Anisotropic Objects Sangwoo Kim, Muyun Cai, Sascha Hilgenfeldt Ever since its empirical formulation in 1928, Lewis`s law has intrigued scientists, postulating a linear correlation between the average in-plane area and the number of neighbors in a two-dimensional tiling. Many supporting and dissenting results have been reported in systems as diverse as foams, Voronoi tilings in glass models, and nanocrystals. A strong size-topology correlation is consistently observed, but it is often pronouncedly nonlinear. Recently, a variant of the granocentric model explained numerous cases of nonlinear correlations, but cannot account for the linear version of the law. We revisit Lewis's original work by conducting more extensive experiments on cucumber epidermis tissue. The data confirms the linear law, but also shows that the individual cells have a pronounced anisotropy, not present in systems with nonlinear correlation laws. We demonstrate how the granocentric model can be modified taking into account the cell anisotropy, and how this feature is capable of reproducing the linear Lewis law, as well as other characteristic differences in size-topology statistical quantities. The model should be generally applicable to jammed, plane-filling systems and identifies domain anisotropy as an important ingredient in their statistical description. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B17.00004: Size-Topology Correlations and Crystallization in Tilings and Packings Sascha Hilgenfeldt Empirical studies have long shown complex statistics in polygonal tilings of the plane or the corresponding packings of objects. Using a 2D variant of the granocentric model, we provide an analytical explanation for correlations of domain size and neighbor number, as well as for the relation between the widths of these two distributions characterizing the tiling or packing. The results agree with data from a large variety of living and inanimate systems [1]. This strictly local approach also gives insight into order-disorder transitions: A dramatic narrowing of the neighbor distribution indicates crystallization, for which well-defined disorder thresholds can be extracted both in systems with continuous disorder and in bidisperse systems, in very good agreement with simulation results [2].\\ \\ $[1]$ M. P. Miklius and S. Hilgenfeldt, Phys. Rev. Lett. 108, 015502 (2012).\\ $[2]$ S. Hilgenfeldt, Phil. Mag. 93, 4018 (2013). [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B17.00005: Unified Theoretical Framework for Shape Entropy in Colloids Greg van Anders, N. Khalid Ahmed, Daphne Klotsa, Michael Engel, Sharon C. Glotzer Entropy has long been known to order colloidal systems ranging from dense suspensions of hard particles to dilute suspensions of spheres in the presence of smaller polymeric depletants. We present a framework for treating directional entropic forces (DEFs) in systems of colloidal shapes. By introducing an effective potential of mean force and torque we demonstrate that the microscopic origin of the entropic ordering of anisotropic shapes is the emergence of DEFs that tend to align neighboring particles. We define and compute these forces and show that, at the onset of ordering, they are on par with traditional depletion interactions, as well as and other forces contributing to assembly in nanocolloidal systems. By retaining only steric interactions, we allow the comparison of the role of shape to other forces present in experimental systems. Well-known cases involving spheres arise as the limit of ``zero shape.'' Our results apply to monodisperse systems and mixtures of hard particles of arbitrary shape and to systems of hard particles with traditional depletants. As such, we present a single theoretical framework that unifies the ordering of arbitrary shapes due to entropy alone, incorporating the well-known works of Kirkwood, Onsager, and Asakura and Oosawa. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B17.00006: Stress supporting structures from interlocking in random packings of granular materials Eric Brown, Shomeek Mukhopadhyay, Alice Nasto, Sulimon Sattari, David Brantley, Kevin Mitchell We present experimental results and a model for strong strain-stiffening in random packings of interlocking granular materials such as chains and staples. Measurements of stress vs. strain are made on the materials under uniaxial compression, along with x-ray tomography to observe interlocking. These packings are found to exhibit strain-stiffening and sustain stresses several orders-of-magnitude beyond those of unconfined granular materials as long as there are system-filling clusters of interlocked particles. To model this behavior, we use a mean-field theory approach. First, the conditions for system-filling clusters can be predicted by by using the area available for interlocking for a given particle shape and a random network model. This model correctly predicts, for example, the minimum chain length required to have system-filling clusters. In this strong regime, the packing stiffness can be calculated using the link stiffness, mean strain on each link, and the probability of tight links, which agrees with experiments within a factor of 2. This model explains the strength of these packings as coming from stretching the links between interlocked particles, and strain-stiffening as a result of increasing number of tightly interlocked particles with increasing strain. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B17.00007: Self-Assembly of Multi-Dimpled Spherical Particles N. Khalid Ahmed, Greg van Anders, Elizabeth R. Chen, Michael Engel, Sharon C. Glotzer Self-assembly of hard convex polyhedra has been extensively studied, demonstrating the formation of complex crystal structures. Recently synthesized multi-dimpled concave particles made of spheres have the potential for comparable complexity. Phase behavior and confinement have been studied for single-dimpled spherical cap and bowl shaped particles. Motivated by the synthesis of multi-dimpled spherical concave particles, we investigate the assembly of spherical particles with up to six dimples. We demonstrate that the assembly is controlled by competition between the spherical and the dimpled surface segments of the particle. Shrinking and swelling the inner spherical core of such particles can result in reconfigurable structures. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B17.00008: Binary mixtures of polyhedral nanoparticles: from phase separation to superstructures Mihir Khadilkar, Umang Agarwal, Fernando Escobedo Polyhedral nanoparticles have emerged as important model systems for both fundamental studies of entropic self-assembly as well as material design. The mixing of more than one shape provides a promising strategy towards achieving a greater variety of structures and properties. We explore this with the study of the phase behavior of binary mixtures of hard convex polyhedra having similar sizes but different shapes. Choosing representative particle shapes from those readily synthesizable, we find that the phase behavior of such mixtures is dependent on the interplay of mixing and packing entropy, which can give rise to miscible or phase-separated states. While expectedly many of the binary systems studied exhibit phase separation at high pressures due to the incompatible pure-component crystal structures, our study shows that the essential qualitative trends in miscibility and phase separation can be correlated to properties of the pure components, such as the relative values of the order-disorder transition pressure of each component. However, the relative size ratios and the presence of mesophases for the pure-component systems are also critical in aiding the formation of fully miscible blends of novel plastic crystalline superstructures. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B17.00009: Assembly precursors in fluids of hard polyhedra M. Eric Irrgang, Michael Engel, Sharon C. Glotzer The role of shape in entropy-driven self-assembly has recently been highlighted in computer simulations of hard anisotropic particles. A rich diversity of crystal and other solid-like phases has been demonstrated in particular for hard polyhedra. Moreover, a correlation has been observed between local structure in the fluid phase and structure of the solid-like phase[1]. Here we investigate the question of when the fluid first ``recognizes'' particle shape and anticipates a pending phase transition. We present equations of state for systems of hard polyhedra spanning the low-density fluid to high- density solid states, obtained numerically from equilibrium Monte Carlo simulations. We discuss trends in the behavior for different shapes, and show some general features common to all systems. [1] P. F. Damasceno, M. Engel, and S. C. Glotzer, Science 337, 453 (2012) [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B17.00010: Complexity in surfaces of densest packings for families of polyhedra Daphne Klotsa, Elizabeth R. Chen, Michael Engel, Pablo F. Damasceno, Sharon C. Glotzer Packings of hard polyhedra have been studied for centuries due to their mathematical aesthetic and more recently for their applications in fields such as nanoscience, colloidal matter, and biology. In all these fields, particle shape is important for structure and properties, especially upon crowding. In this talk, we explore packing as a function of shape. By combining simulations and analytic calculations, we study three 2-parameter families of hard polyhedra and report an extensive and systematic analysis of the densest known packings of more than 55,000 convex shapes. The three families have the symmetries of triangle groups (20-hedral, 8-hedral, 4-hedral) and interpolate between various symmetric solids (Platonic, Archimedean, Catalan). We find that maximum packing density surfaces reveal unexpected richness and complexity, containing as many as 130 different structures within a single family. Our results demonstrate the importance of thinking about shape not as a static property of an object, in the context of packings, but rather as but one point in a higher dimensional shape space whose neighbors in that space may have identical or markedly different packings. Finally, we propose a method to distinguish regions of packings and classify types of transitions between them. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B17.00011: Pessimal shapes for packing Yoav Kallus The question of which convex shapes leave the most empty space in their densest packing is the subject of Reinhardt's conjecture in two dimensions and Ulam's conjecture in three dimensions. Such conjectures about pessimal packing shapes have proven notoriously difficult to make progress on. I show that the regular heptagon is a local pessimum among all convex shapes, and that the 3D ball is a local pessimum among origin-symmetric shapes. Any shape sufficiently close in the space of shapes to these local pessima can be packed at a greater efficiency than they. In two dimensions and in dimensions above three, the ball is not a local pessimum, so the situation in 3D is unusual and intriguing. I will discuss what conditions conspire to make the 3D ball a local pessimum and whether we can prove that it is also a global pessimum. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B17.00012: Mean-field theory of random close packings of axisymmetric particles Lin Bo, Adrian Baule, Romain Mari, Louis Portal, Hernan Makse Finding the optimal random packing of non-spherical particles is an open problem with great significance in a broad range of scientific and engineering fields. So far, this search has been performed only empirically on a case-by-case basis, in particular, for shapes like dimers, spherocylinders and ellipsoids of revolution. Here, we present a mean-field formalism to estimate the packing density of axisymmetric non-spherical particles. We derive an analytic continuation from the sphere that provides a phase diagram predicting that, for the same coordination number, the density of monodisperse random packings follows the sequence of increasing packing fractions: spheres $<$ oblate ellipsoids $<$ prolate ellipsoids $<$ dimers $<$ spherocylinders. We find the maximal packing densities of $73.1\%$ for spherocylinders and $70.7\%$ for dimers, in good agreement with the largest densities found in simulations. Moreover, we find a packing density of $73.6\%$ for lens-shaped particles, representing the densest random packing of the axisymmetric objects studied so far. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B17.00013: The shape of jams to come: hidden geometric symmetries of jamming Peter Morse, Eric Corwin The mechanical vacuum of systems below jamming is surprisingly rich in structure. Using geometric quantities derived from the Voronoi tessellation we report on the discovery of a new phase transition preceding the mechanical jamming transition. This phase transition corresponds to the appearance of a new kind of symmetry hidden in the shape of the Voronoi cells. We characterize this symmetry by looking at properties related to the maximum inscribed sphere in each cell, moments of the volume distribution of cells, and the aspect ratios of cells. Each contains a very different signature of the jamming transition with various scaling laws. We offer several possible routes towards renormalization of this system and discuss whether a field theory could be made to explain the various phases. [Preview Abstract] |
Session B18: Focus Session: Liquid Crystals, Nano to Meso Scale Structure in Ordered Matter and Liquid Crystal I: Nanocomposites and Smectics
Sponsoring Units: DCMP GSNP DPOLYChair: Joseph MacLennan, University of Colorado, Boulder
Room: 403
Monday, March 3, 2014 11:15AM - 11:27AM |
B18.00001: Dendritic Patterns in Nematic Liquid Crystal Nanocomposites Sebastian Gurevich, Alejandro Rey Liquid crystal (LC) mixtures with nanoparticles (NP) are of fundamental interest in the development of advanced materials. Of particular interest is developing means to direct the assembly of the NPs. The interactions in LC-NP mixtures are still under active research, although important tendencies have been established. Little attention has been given to morphological instability patterns, and those mediated by diffusion of NPs are yet to be explored. Using the continuum model of Soule et al [1], we explore numerically the growth of nematic droplets in an isotropic liquid under conditions that lead to a variety of dendritic like morphologies controlled by a diffusive instability mediated by the NPs and the anisotropy of the nematic field. The numerical implementation of the model, which represents a mixture of calamitic nematic LC (cylinders) and NPs (hard spheres) of comparable size, is based on the adaptive mesh refinement scheme developed by Provatas et al, allowing access to realistic time and length scales. Our work lays the ground to developing new means to direct the assembly of NPs over large areas by exploiting the morphological instabilities at nematic-isotropic interfaces. The diversity of morphologies may also allow estimating the value of material parameters that are otherwise difficult to obtain experimentally. [1] Soft Matter, 2012, 8, 2860 [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B18.00002: Liquid Crystalline Orientational Control via the Electric Field of Localized Surface Plasmons Makiko Quint, Linda Hirst, Sayantani Ghosh We probe the effect of induced electric fields of localized surface plasmons (LSPs) generated by self-assembled gold nanoparticles (AuNPs) on the directional orientation of a thin film of nematic liquid crystal, 4-cyano-4$'$-pentylbipenyl (5CB). We excite the composite AuNP-LC with excitation tuned on and off resonance with the LSP absorption peak, and track the birefringence of the LC. Our results demonstrate re-orientation of the director of the LC when the LSPs are excited resonantly, and that this effect is temperature dependent. Our studies indicate LSP generated electric fields may offer an all-optical protocol to locally control LC orientation. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B18.00003: Behavior of anisotropic particles at air/nematic interface Iris B. Liu, Mohamed A. Gharbi, Randall D. Kamien, Shu Yang, Kathleen J. Stebe Colloidal particles with non-spherical shapes or deliberate patchiness can create capillary interactions that direct assembly in well-defined orientations. These effects have been considered only recently at simple fluid interfaces, and are largely unexplored at complex fluid interfaces. Anisotropic particles immersed in liquid crystals can also generate strong directed interactions. In this work, we explore the influence of particle geometry in colloidal interactions at nematic interfaces. We use particles of cylindrical shape, with controlled surface chemistry (anchoring and wetting properties) and report their behavior at an air/nematic interface. We study the interactions and self-assembly of these particles as a function of their aspect ratios. When cylinders are captured at the nematic interface, they induce deformation of the interface to satisfy wetting properties at particle surface. In addition, their presence induces distortion of the uniform director filed at the air/nematic interface to satisfy anchoring properties. Elastic and capillary interactions compete with each other and the resulting potential drives assembly of particles into novel structures. Recent progress in understanding colloidal interaction of anisotropic particles is presented. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B18.00004: True colloids of ferroelectric nanoparticles in liquid crystals: grand challenges and recent breakthroughs Yuriy Garbovskiy, Anatoliy Glushchenko Recent publications in the field of colloids of ferroelectric nanoparticle in liquid crystals revealed a high variety of physical effects. At the same time, there are still many inconsistencies between the results reported by different scientific groups. This fact reflects the complexity of such colloids. Particularly, inherent ferroelectricity of the nanoparticles or particles aggregation in some cases are among the important factors causing the variation of the reports. In this presentation we report experimental results based on the system which is free of the above mentioned challenges. We show how to prepare the true colloid of ferroelectric nanoparticles in liquid crystals; demonstrate impact of the ferroelectricity on the physical properties of the colloid; and present new electro-optical effects observed in these systems. We cover also a variety of possible applications of liquid crystals doped with ferroelectric nanoparticles. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B18.00005: Increasing Dispersion of Quantum Dots in Liquid Crystal Using Mesogenic Ligands Zachary Nuno, Andrea Rodarte, Blessing Cao, Ronald Pandolfi, Makiko Quint, Sayantani Ghosh, Jason Hein, Linda Hirst High concentrations of quantum dots (QDs) dispersed in liquid crystal materials will tend to aggregate together. One reason for this is the elastic cost of local liquid crystal alignment with the ligands on the surface of the QDs. We use mesogenic ligands with a flexible arm to allow the ligands to align with the director axis of the liquid crystal, thereby reducing aggregation and promoting QD dispersion in the host material. The dispersion of CdSe (core only) and CdSe/ZnS (core/shell) QDs with isotropic and mesogenic ligands is compared using fluorescence microscopy, x-ray scattering, and scanning confocal microscopy. The results from these techniques demonstrate that the mesogenic functionalized QDs do not aggregate into dense clusters as observed with the isotropic functionalized QDs. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B18.00006: Ordered and disordered colloidal particle monolayers at liquid crystal interfaces Wei-Shao Wei, Matthew Lohr, Mohamed Amine Gharbi, Kathleen Stebe, A.G. Yodh In this work, we investigate ordered colloidal particle monolayers at the air/liquid-crystal (LC) interface. Specifically, silica microparticles are treated with DMOAP to create homeotropic anchoring of LC mesogens at their surfaces. These particles are then spread on an air-exposed interface of the LC 5CB. Macroscopic ordered patterns of these microparticles form due to long-range interactions between particles that are mediated by elastic deformations of the underlying LC. Different confinement conditions lead to various self-assembled patterns ranging from hexagonal lattices to chain-like dipole formations. Using dark-field video microscopy, we track and analyze the dynamics of the colloidal particles in the hexagonal crystal packing, deriving mean squared displacements, phonon modes and density of states, etc., under several conditions. Further, heating of the nematic LC into its isotropic phase enables us to observe melting dynamics of this unusual quasi-2D crystal. The investigations provide insight into crystalline packings controlled by liquid-crystal mediated colloidal interactions. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B18.00007: Electromechanical memory effect in a ferroelectric nanoparticle-suspended liquid crystal Rajratan Basu A small quantity of BaTiO$_{\mathrm{3}}$ ferroelectric nanoparticles (FNP) was doped in a liquid crystal (LC), and the LC$+$FNP hybrid was found to exhibit an electromechanical memory effect in the isotropic phase. The permanent dipole moment of the FNPs causes the LC molecule to form short-range order surrounding the FNPs. This FNP-induced short-range order becomes more prominent in the isotropic phase when the global nematic order is absent. These short-range domains, being anisotropic in nature, interact with the external electric field. When the field goes off, these domains stay oriented due to the absence of the long range order in the isotropic phase, showing a hysteresis effect. The area under the hysteresis graph shows a significant pretransitional behavior on approaching the nematic phase from the isotropic phase. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B18.00008: Liquid Crystal properties of Silver (Ag) Nanowires as a Function of Flow Luz J. Martinez-Miranda, Liangbing Hu, Colin D. Preston We study the liquid crystal properties of nanowires of silver (Ag) as a function of flow of the solvent. Specifically, we are interested in finding the flow-concentration point where the electrical properties (IV curve) are: 1. Along one direction; and, 2. At a maximum along that particular direction. We are interested in the structure intermediate between the liquid crystal phase and the isotropic phase (the heterogenous phase) and how ``ordered'' this phase becomes with flow. Flow is varied in our case by having a substrate with gratings of varying depth in them. The flow due to the grating and the thickness of the film, plus the size of the nanowires will dictate the degree of order in the heterogeneous phase. This order dictates how the electrical properties orient in the resulting film. These studies can be expanded to include other semiconducting and/or metallic nanowires. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B18.00009: Memory effects of nematic liquid crystals in porous network: the role of geometry Francesca Serra, Shane Eaton, Marco Buscaglia, Roberto Cerbino, Giulio Cerullo, Roberto Osellame, Tommaso Bellini We exploit here the bistability of nematic liquid crystals (NLC) induced by their confinement into bicontinuous porous networks. In such a confined liquid crystal, the application of a strong external field induces the reconfiguring of topological defects, which then become locked as they entangle with the porous material. In this sense, the system has a memory of the applied field and it retains its orientation also when the field is removed. Computer simulations already showed that this effect depends on the geometry and the topology of the porous material. Incorporating liquid crystals in laser-microfabricated structures, made with two-photon polymerization, allows us to experimentally test this concept. We compare networks with different geometry and measure the memory of liquid crystals: we show that, as computer simulations predict, the cubic geometry yields the biggest memory effect. Both experiments and simulations also show that defects and anisotropies in the porous structure are important parameters that can substantially affect the memory. The small size of the scaffold (50-100 microns) and the large memory of the liquid crystals in cubic scaffold make this system promising for applications. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B18.00010: Nanoparticle Diffusion and Aggregation in 2D Smectic Membranes Kyle Meienberg, Greg Smith, Cheol Park, Joseph Maclennan, Matthew Glaser, Noel Clark We observe directly the diffusion and aggregation of nanoparticles embedded in thin, freely-suspended smectic films using reflected light microscopy in order to better understand the hydrodynamics of inclusions in finite, two-dimensional fluids. The Saffman-Delbr\"{u}ck (SD) model has been used previously to describe the diffusion of micron-sized objects in thin membranes in a variety of experimental systems. Nano-sized Buckyball aggregates embedded in smectic A films are observed to have much faster diffusion rates than predicted by SD theory. These experiments suggest that SD theory breaks down in the limit of nanometer-scale inclusions where molecular diffusion processes are dominate. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B18.00011: Two-Dimensional Diffusion of a Droplet near the Rigid Boundary of a Fluid Smectic Film Zhiyuan Qi, Cheol Park, Joseph Maclennan, Matthew Glaser, Noel Clark, Tatiana Kuriabova, Thomas Powers Homogeneous, freely-suspended fluid smectic A liquid crystal films in vacuum provide an ideal system for studying two-dimensional (2D) hydrodynamics in the absence of additional drag from the surrounding air. The 2D Brownian motion of a single oil droplet embedded in such a film and diffusing near a rigid wall was captured using digital video microscopy. Analysis confirms that the diffusion is anisotropic, with different mobilities for droplet motion parallel and perpendicular to the wall, in qualitative agreement with calculations based on the Oseen tensor assuming non-slip boundary conditions. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B18.00012: Dynamics of point defects in free-standing smectic films Kirsten Harth, Ralf Stannarius Ordering phenomena that break certain symmetries often involve the dynamics of defects. In liquid crystals, such processes are easily visualized in polarized light microscopy. Point defect annihilation has been investigated experimentally and numerically in thermotropic nematics, lyotropics and active nematics. Free-standing smectic films are ideal model systems to probe two-dimensional hydrodynamics. Films in the smectic C phase represent the simplest anisotropic fluid in two dimensions. Within elastic one-constant approximation and neglect of hydrodynamic effects, the description of defect dynamics is identical to that of point charges in electrodynamics. Whereas the equations for two point charges are easily solvable analytically, the case of three or more charges is equivalent to the classical three-body problem. We present new methods to create defect pairs and groups of defects of topological strength +1 in a controlled way. The annihilation process, the repulsion of sets of +1 defects in several geometrical configurations, and repulsion of +1 defects from larger positive net topological charges are analyzed. We discuss the influence of material flow and compare the experimental results to theory and simulations. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B18.00013: Coalescence of fluid droplets in freely-suspended smectic liquid crystal films Cheol Park, Zhiyuen Qi, Joseph Maclennan, Matthew Glaser, Noel Clark We have studied the coalescence dynamics of the lens-shaped fluid droplets embedded in freely suspended smectic A liquid crystal films. The early time expansion of the bridge connecting the merging domains as a function of the time from the onset of coalescence is measured using high speed video microscopy. Optical interference is used to extract the shape evolution of the coalesced droplet with the measured thickness profiles giving fundamental insights into the transport processes within the droplets before and after coalescence. We compare the early-time growth of the bridge between the droplets with 2D and 3D theoretical predictions and propose a model of the fluid flow between the droplets to explain the coalescence dynamics. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B18.00014: Brownian Motion of Topological Point Defects in Smectic C Films Kate Wachs, Cheol Park, Zhiyuen Qi, Joseph Maclennan, Noel Clark We observe the diffusive motion of topological point defects in the c-director field of freely-suspended smectic C liquid crystals films using reflected polarized light microscopy. Racemic films two layers thick were created and placed within a vacuum chamber. The diffusion of defects confined to islands of different thickness and diameter was measured as a function of air pressure. As the pressure decreases, the diffusion coefficient increases until it reaches the limit that results from pure 2D confinement. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B18.00015: Mutual Diffusion of Inclusions in Freely-Suspended Smectic Liquid Crystal Films Tatiana Kuriabova, Zhiyuen Qi, Zoom Nguyen, Cheol Park, Matthew Glaser, Joseph Maclennan, Noel Clark, Thomas Powers We study experimentally and theoretically the hydrodynamic interaction of pairs of circular inclusions in two-dimensional, fluid smectic membranes suspended in air. By analyzing their Brownian motion, we find that the radial mutual mobilities of identical inclusions are independent of their size but that the angular coupling becomes strongly size-dependent when their radius exceeds a characteristic hydrodynamic length. The observed dependence of the mutual mobilities on inclusion size is described well for arbitrary separations by a model that generalizes the Levine/MacKintosh theory of point-force response functions and uses a boundary-element approach to calculate the mobility matrix. [Preview Abstract] |
Session B19: Focus Session: Theory and Simulations of Macromolecules II - From Atomistic to Coarse Grained Models
Sponsoring Units: DPOLYChair: Christoph Junghans, Los Alamos National Laboratory
Room: 404
Monday, March 3, 2014 11:15AM - 11:27AM |
B19.00001: Hierarchical Modeling of Polymer/Solid Interfaces: From Ab-initio Calculations to Atomistic up to Coarse-grained Simulations Vagelis Harmandaris, Karen Johnston We present a hierarchical simulation approach in order to study nanocomposite systems. Our approach combines quantum calculations, atomistic and coarse-grained (CG) dynamic simulations [1-2] and allows quantitative modeling of complex hybrid systems over a very broad range of length and time scales. As an example we model the polystyrene/gold system. The proposed scheme consists of the following stages: (1) Ab-initio (Density Functional Theory) calculations of a single molecule adsorbed on solid surfaces. (2) All-atom molecular dynamics simulations of short polymer chains/solid systems. We further develop a methodology to obtain rigorous CG models from the atomistic data, for specific polymer/solid systems. (3) CG simulations of more realistic polymer/solid surfaces. Structural, conformational and dynamical properties of systems with longer polymer chains are studied. The width of the interphase region of the polymer films found to be property specific, ranging from about 1.5nm to a distance that is proportional to the square root of the chain length. References [1] K. Johnston and V. Harmandaris, \textit{J. Phys. Chem. C.}, \textbf{115}, (2011) 14707; \textit{Soft Matter}, \textbf{8}, (2012) 6320. [2] K. Johnston and V. Harmandaris, \textit{Macromolecules}, \textbf{2013,} \textit{46}, 5741$-$5750. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B19.00002: Exploring the Limits of the Iterative Boltzmann Inversion Roland Faller, Beste Bayramoglu We explore the limits of the purely structure based coarse-graining technique, the iterative Boltzmann inversion (IBI), for confined systems using the example of polystyrene solutions. First some technical considerations and challenges encountered in the course of the optimization process are presented. The choice of the initial potentials and the cross-dependency of the interactions as well as the order of optimization are discussed in detail. Furthermore, the transferability between different degrees of confinement is examined. We investigate if a CG force field developed for a confined polymer solution by IBI is sensitive to changes in the degree of localization or arrangement of polymers near the surfaces although the concentration is kept constant. The differences in the structure and dynamics of the chains are addressed. Results are compared with those of an unconfined (bulk) system at the same concentration. The chain dimensions and orientations as a function of the distance from the surfaces are also reported. We find that the arrangement of monomers and solvent molecules near the surfaces is an important factor that needs to be paid attention to when considering the application of a CG force field developed by IBI to different degrees of confinement. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B19.00003: Coarse graining of atactic polystyrene and its derivatives Anupriya Agrawal, Dvora Perahia, Gary S. Grest Capturing large length scales in polymers and soft matter while retaining atomistic properties is imperative to computational studies of dynamic systems. Here we present a new methodology developing coarse-grain model based on atomistic simulation of atactic polystyrene (PS). Similar to previous work by Fritz et al., each monomer is described by two coarse grained beads. In contrast to this earlier work where intramolecular potentials were based on Monte Carlo simulation of both isotactic and syndiotactic single PS molecule to capture stereochemistry, we obtained intramolecular interactions from a single molecular dynamics simulation of an all-atom atactic PS melts. The non-bonded interactions are obtained using the iterative Boltzmann inversion (IBI) scheme. This methodology has been extended to coarse graining of poly-(t-butyl-styrene) (PtBS). An additional coarse-grained bead is used to describe the t-butyl group. Similar to the process for PS, the intramolecular interactions are obtained from a single all atom atactic melt simulation. Starting from the non-bonded interactions for PS, we show that the IBI method for the non-bonded interactions of PtBS converges relatively fast. A generalized scheme for substituted PS is currently in development. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B19.00004: Coarse Grained Simulations of Entangled Polymer Dynamics Abelardo Ramirez-Hernandez, Marat Andreev, Jay D. Schieber, Juan J. de Pablo We use the Theoretically Informed Entangled Polymer Simulations (TIEPOS) approach for multicomponent polymeric systems to study the linear and non-linear rheological response of melts. In this many-chain model, the topological effect of non-crossability of polymers is described by effective fluctuating interactions, mediated by slip-springs, between neighboring pairs of macromolecules. We explore the effect of different implementations of slip-springs, namely, continuous movement of slip-springs along chains as oposite to discrete jumps between polymer segments, as well as the use of a grand-canonical approach where the total number of slip-springs fluctuates. We perform a comparison between simulation predictions and experimental data for a series of well-characterized linear polymeric melts. Our results are shown to be in quantitative agreement both in linear and non-linear rheology. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B19.00005: Systematic coarse-graining of the wormlike chain model for dynamic simulations Elena Koslover, Andrew Spakowitz One of the key goals of macromolecular modeling is to elucidate how macroscale physical properties arise from the microscale behavior of the polymer constituents. For many biological and industrial applications, a direct simulation approach is impractical due to to the wide range of length and time scales that must be spanned by the model, necessitating physically sound and practically relevant procedures for coarse-graining polymer systems. We present a highly general systematic coarse-graining procedure that maps any detailed polymer model onto effective elastic-chain models at intermediate and large length scales, and we specifically focus on the wormlike chain model of semiflexible polymers. Our approach defines a continuous flow of coarse-grained models starting from the wormlike chain model, proceeding through an intermediate-scale stretchable, shearable wormlike chain, and finally resolving to a Gaussian chain at the longest lengths. Using Brownian dynamic simulations of our coarse grained polymer, we show that this approach to coarse graining the wormlike chain model captures analytical predictions for stress relaxation in a semiflexible polymer. Since we can arbitrarily coarse grain the polymer in these dynamic simulations, our approach greatly accelerates simulations. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B19.00006: Multiresolution Modeling of Polymer Solutions: Wavelet-Based Coarse-Graining and Reverse-Mapping Ahmed Ismail, Carl Simon Adorf, Animesh Agarwal, Christopher R. Iacovella Unlike multiscale methods, which encompass multiple simulation techniques, multiresolution models uses one modeling technique at different length and time scales. We present a combined coarse-graining and reverse-mapping framework for modeling of semidilute polymer solutions, based on the wavelet-accelerated Monte Carlo (WAMC) method, which forms a hierarchy of resolutions to model polymers at length scales that cannot be reached via atomistic or even ``standard'' coarse-grained simulations. A universal scaling function is obtained so that potentials do not need to be recomputed as the scale of the system is changed. We show that coarse-grained polymer solutions can reproduce results obtained from the simulations of the more detailed atomistic system to a reasonable degree of accuracy. Reverse mapping proceeds similarly: using probability distributions obtained from coarse-graining the bond lengths, angles, torsions, and the non-bonded potentials, we can reconstruct a more detailed polymer consistent with both geometric constraints and energetic considerations. Using a ``convergence factor'' within a Monte Carlo-based energy optimization scheme, we can successfully reconstruct entire atomistic configurations from coarse-grained descriptions. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B19.00007: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 12:39PM - 12:51PM |
B19.00008: Coarse-graining using the relative entropy and simplex-based optimization methods in VOTCA Victor R\"uhle, Mara Jochum, Konstantin Koschke, N.R. Aluru, Kurt Kremer, S.Y. Mashayak, Christoph Junghans Coarse-grained (CG) simulations are an important tool to investigate systems on larger time and length scales. Several methods for systematic coarse-graining were developed, varying in complexity and the property of interest. Thus, the question arises which method best suits a specific class of system and desired application. The Versatile Object-oriented Toolkit for Coarse-graining Applications (VOTCA) provides a uniform platform for coarse-graining methods and allows for their direct comparison. We present recent advances of VOTCA, namely the implementation of the relative entropy method and downhill simplex optimization for coarse-graining. The methods are illustrated by coarse-graining SPC/E bulk water and a water-methanol mixture. Both CG models reproduce the pair distributions accurately. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B19.00009: Mesoscale simulation of entangled polymers: Part I. Coarse-Grained level Tunable DPD Joao Maia, Shagahyegh Khani, Mikio Yamanoi Dissipative Particle Dynamics (DPD) is a coarse-grained molecular dynamics based simulation method that has shown a very good potential in computational modeling of soft matter. However, it is associated with deficiencies in simulating the dynamics of entangled polymer systems. For instance, due to the upper limit of coarse-graining level the method could not be applicable to the whole mesoscopic range. Therefore, our group has proposed a new concept of DPD named Coarse-Grained level tunable DPD method in which the level of coarse graining can be tuned by adjusting the simulation parameters considering an energy balance in the system. The unphysical bond crossings that are artifacts of the soft potentials are prevented by applying an entanglement potential between the bonds. The performance of the method in capturing the entanglement effect is investigated by calculating the static and dynamic properties of polymers in entangled state. Linear and non-linear viscoelastic properties can also be predicted by the CG level tunable DPD method reasonably well. Moreover, this method is able to reproduce the 1.0 to 3.4 transition in power index of the zero shear viscosity with molecular weight which captures the Rouse to reptation behavior in entangled polymer systems. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B19.00010: Mesoscale simulation of entangled polymers: Part II. Lowe-Andersen thermostat Shaghayegh Khani, Mikio Yamanoi, Joao Maia Dissipative Particle Dynamics (DPD), despite its good potential in simulating soft matter, has some limitations when studying the entangled polymer systems. First limitation which arises from utilizing soft potentials in DPD is associated with unphysical bond crossings. The bond crossings can be avoided by introducing a segmental repulsive potential to the bonds. Another deficiency of DPD in simulating fluids is related to the Schmidt number. In standard DPD the momentum and mass transfer at the same rate and thus this dimensionless number takes a gas-like value ($\sim$ 1) when simulating fluids. In order to overcome this problem a Lowe-Andersen thermostat was used as an alternative method to DPD and the thermostat was found to be more successful in controlling the temperature in equilibrium state (independent from the time step) and over a wide range of shear rates. The ability of the method in capturing the entanglement effect and reproducing the static and dynamic properties of polymer melts and the scaling laws were investigated and the results were compared to the ones from standard DPD. The performance of the method in capturing the main features of the shear flow and reproducing linear and nonlinear viscoelastic properties was also evaluated. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B19.00011: Persistent Contacts Along the Primitive Path Scott Milner, Jing Cao In an entangled polymer melt or solution, the uncrossability of the chains effectively restricts a given chain to move perpendicular to its contour path such that the chain is confined in a tube like region. In the present work, we use MD simulations to investigate the dynamics of the tube, represented by the isoconfigurationally averaged primitive path of a self-entangled ring polymer in a melt. A new approach to find entanglement molecular weight is introduced based on identifying close contacts between points along the primitive path, which is in a good agreement with previous work. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B19.00012: Viscoelastic hydrodynamic interactions and anomalous CM diffusion in polymer melts Hendrik Meyer, Jean Farago, A.N. Semenov We have recently discovered that anomalous center-of-mass (CM) diffusion occurring on intermediate time scales in polymer melts can be explained by the interplay of viscoelastic and hydrodynamic interactions (VHI). The theory has been solved for unentangled melts in 3D [1] and 2D [2] and excellent agreement between theory and simulation is found. The physical mechanism considers that hydrodynamic interactions are time dependent because of increasing viscosity before the terminal relaxation time; it is generally active in melts of any topology. Surprisingly, the effects are relevant for both, momentum-conserving and Langevin dynamics [1,2] and this presentation will focus on the differences: The commonly employed Langevin thermostat significantly changes the CM motion on short and intermediate time scales, but approaching the Rouse time, the melt behavior is close to momentum-conserving simulations. On the other hand, if momentum-conserving simulations are run in too small a simulation box, the result looks as if a Langevin thermostat was used.\\[4pt] [1] PRL 107, 178301 (2011); PRE 85, 051807 (2012).\\[0pt] [2] PRL 109, 248304 (2012); Soft Matter 9, 4249 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B19.00013: Molecular simulation investigation of the nanorheology of an entangled polymer melt Mir Karim, Rajesh Khare, Tsutomu Indei, Jay Schieber Knowledge of the ``local rheology'' is important for viscoelastic systems that contain significant structural and dynamic heterogeneities, such as cellular and extra-cellular crowded environments. For homogeneous viscoelastic media, a study of probe particle motion provides information on the microstructural evolution of the medium in response to the probe particle motion. Over the last two decades, probe particle rheology has emerged as a leading experimental technique for capturing local rheology of complex fluids. In recent work [\textit{M. Karim, S. C. Kohale, T. Indei, J. D. Schieber, and R. Khare, Phys. Rev. E }\textbf{\textit{86}}\textit{, 051501 (2012)}], we showed that this approach can be used in molecular dynamics (MD) simulations to study the nanoscale viscoelastic properties of an unentangled polymer melt; an important conclusion of that work was that medium and particle inertia play a crucial role in analysis of the particle rheology simulation data. MD simulations have a natural advantage that they enable study of deformation and dynamics over a small length scale around the moving probe particle. In this work, the approach is extended to compare the motion of a nanoscale probe in melts of entangled and unentangled chains. The simulations will be used to elucidate the differences between the local responses of these media to the probe particle motion. In particular, results will be presented for the differences in the resultant velocity and stress fields as well as any possible structural asymmetry developed around the moving probe particle in the entangled and unentangled cases. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B19.00014: Estimation of Linear Viscoelasticity of Polymer Melts in Molecular Dynamics Simulations Based on Relaxation Mode Analysis Nobuyuki Iwaoka, Katsumi Hagita, Hiroshi Takano On the basis of relaxation mode analysis (RMA), we present an efficient method to estimate the linear viscoelasticity of polymer melts in a molecular dynamics (MD) simulation. Slow relaxation phenomena appeared in polymer melts cause a problem that a calculation of the stress relaxation function in MD simulations, especially in the terminal time region, requires large computational efforts. Relaxation mode analysis is a method that systematically extracts slow relaxation modes and rates of the polymer chain from the time correlation of its conformations. We show the computational cost may be drastically reduced by combining a direct calculation of the stress relaxation function based on the Green-Kubo formula with the relaxation rates spectra estimated by RMA. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B19.00015: Finding the Missing Physics: Simulating Polydisperse Polymer Melts Nichoals Rorrer, John Dorgan A Monte Carlo algorithm has been developed to model polydisperse polymer melts. For the first time, this enables the specification of a predetermined molecular weight distribution for lattice based simulations. It is demonstrated how to map an arbitrary probability distributions onto a discrete number of chains residing on an fcc lattice. The resulting algorithm is able to simulate a wide variety of behaviors for polydisperse systems including confinement effects, shear flow, and parabolic flow. The dynamic version of the algorithm accurately captures Rouse dynamics for short polymer chains, and reptation-like dynamics for longer chain lengths.$^{\mathrm{1}}$ When polydispersity is introduced, smaller Rouse times and broadened the transition between different scaling regimes are observed. Rouse times also decrease under confinement for both polydisperse and monodisperse systems and chain length dependent migration effects are observed. The steady-state version of the algorithm enables the simulation of flow and when polydisperse systems are subject to parabolic (Poiseulle) flow, a migration phenomenon based on chain length is again present. These and other phenomena highlight the importance of including polydispersity in obtaining physically realistic simulations of polymeric melts. 1. Dorgan, J.R.; Rorrer, N.A.; Maupin, C.M., \textit{Macromolecules }\textbf{2012}, \textit{45 }(21), 8833-8840. [Preview Abstract] |
Session B20: Focus Session: Microfluidics and Nanofluidics II - Colloidal Hydrodynamics and Active Particles
Sponsoring Units: DPOLY GSNP DFDChair: Steven Hudson, National Institute of Standards and Technology
Room: 405
Monday, March 3, 2014 11:15AM - 11:27AM |
B20.00001: The Role of Inertia in Particle Laden Flows Hamed Haddadi, Jeffrey Morris The microstructure and rheological properties of suspensions of neutrally buoyant hard spherical particles under finite inertia are studied using lattice-Boltzmann method (LBM). The suspensions are subjected to simple shear flow and the properties are studied as a function of inertia and volume fraction, ?. The inertia is characterized by shear flow Reynolds number, Re. The influence of inertia and the volume fraction is studied for 0.005 < Re < 2 and 0.1 < ? < 0.3. The topology of the streamlines and pair trajectories, specially off-plane configurations, are observed in more detail. The flow induced microstructure is investigated using the pair distribution function g(r). Different stress types generated by surface tractions, acceleration and velocity fluctuations are computed and their influence on the first and second normal stress differences, the particle pressure and the viscosity of the suspensions are detailed. In addition, the dynamics of the particle interactions are examined from a pair kinematics perspective. One specific example of inertia flow in a microfluidic set up is briefly discussed. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B20.00002: Hydrodynamic Fluctuations in Confined Particle-Laden Fluids Nicolas Desreumaux, Jean-Baptiste Caussin, Raphael Jeanneret, Eric Lauga, Denis Bartolo We address the collective dynamics of non-Brownian particles cruising in a confined microfluidic geometry and provide a comprehensive characterization of their spatiotemporal density fluctuations. We show that density excitations freely propagate at all scales, and in all directions even though the particles are neither affected by potential forces nor by inertia. We introduce a kinetic theory which quantitatively accounts for our experimental findings, demonstrating that the fluctuation spectrum of this nonequilibrium system is shaped by the combination of truly long-range hydrodynamic interactions and local collisions. We also demonstrate that the free propagation of density waves is a generic phenomenon which should be observed in a much broader range of hydrodynamic systems. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B20.00003: Microscopic origin of drag force: A new mathematical and physical interpretation Changho Kim, George Karniadakis We present a new microscopic interpretation of the friction force on a Brownian particle of finite mass suspended in a fluid: it originates from the deviation of the system trajectory due to the movement of the particle. We perform a systematic theoretical investigation on the observation that compared with the frozen dynamics of the fluid where the Brownian particle is fixed, the movement of the Brownian particle perturbs the trajectory of the fluid particles and correspondingly the force on the Brownian particle. We show that as the mass $M$ of the Brownian particle increases, the drag force becomes the ensemble average of the force deviation over the fluid configurations, whereas the thermal noise due to the fluctuation of the fluid becomes the force in the frozen dynamics. In addition, we obtain asymptotic expansions (with respect to $M$) of the friction force and thermal noise defined by the memory function and derive several expressions for single-particle Brownian motion near the Brownian limit. We perform a molecular dynamics simulation study on the Rayleigh model (i.e., a Brownian particle in an ideal gas), which provides a clear validation of the theory. We also observe the skewness in the force distributions and the bath-particle density on the simulation. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B20.00004: Defect Proliferation in Active Nematic Suspensions Prashant Mishra, Mark J. Bowick, Luca Giomi, M. Cristina Marchetti The rich structure of equilibrium nematic suspensions, with their characteristic disclination defects, is modified when active forces come into play. The uniform nematic state is known to be unstable to splay (extensile) or bend (contractile) deformations above a critical activity. At even higher activity the flow becomes oscillatory and eventually turbulent. Using hydrodynamics, we classify the active flow regimes as functions of activity and order parameter friction for both contractile and extensile systems. The turbulent regime is marked by a non-zero steady state density of mobile defect pairs. The defect density itself scales with an ``active Ericksen number,'' defined as the ratio of the rate at which activity is injected into the system to the relaxation rate of orientational deformations. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B20.00005: Aggregation and segregation of confined self-propelled particles Xingbo Yang, M. Lisa Manning, M. Cristina Marchetti We study the effect of confinement on a collection of self-propelled (SP) disks in two dimensions, interacting solely via soft elastic repulsion. Individual SP particles perform persistent random walks characterized by the self-propulsion speed v$_{\mathrm{0}}$ and the rotational diffusion rate. In a single component system, we observe spontaneous aggregation of particles at the walls at low packing fraction when their persistence length is smaller than the system size. Above the packing fraction where jamming occurs in passive disks, collective effects become important and a finite v$_{\mathrm{0}}$ is needed for aggregation. The pressure on the wall shows a non-monotonic dependence on packing fraction: a linear growth consistent with ideal gas behavior at small packing fraction and a decrease at large packing fraction. In a bidisperse system of disks with radii ratio 1 : 1.4 we find spontaneous species segregation. This arises from the interplay of self propulsion and the asymmetry in the elastic energy barriers seen by different-sized particles during collisions. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B20.00006: Quantitative kinetic theory of active matter Thomas Ihle, Yen-Liang Chou Models of self-driven agents similar to the Vicsek model [Phys. Rev. Lett. 75 (1995) 1226] are studied by means of kinetic theory [1,2]. In these models, particles try to align their travel directions with the average direction of their neighbours. At strong alignment a globally ordered state of collective motion forms. An Enskog-like kinetic theory is derived from the exact Chapman-Kolmogorov equation in phase space using Boltzmann's mean-field approximation of molecular chaos. The kinetic equation is solved numerically by a nonlocal Lattice-Boltzmann-like algorithm. Steep soliton-like waves are observed that lead to an abrupt jump of the global order parameter if the noise level is changed. The shape of the wave is shown to follow a novel scaling law and to quantitatively agree within 3 \% with agent-based simulations at large particle speeds. This provides a mean-field mechanism to change the second-order character of the flocking transition to first order. Diagrammatic techniques are used to investigate small particle speeds, where the mean-field assumption of Molecular Chaos is invalid and where correlation effects need to be included. [1] T. Ihle, Phys. Rev. E 83 (2011) 030901; 88 (2013) 040303. [2] Y.L. Chou et al, Phys. Rev. E 86, 021120 (2012). [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B20.00007: Mixing in suspensions of active particles Dmitri O. Pushkin, Julia M. Yeomans Microscopic active particles self-propelling in the surrounding fluid create flows that eventually lead to emergence of non-equilibrium states with long-ranged fluctuations. One of the technologically important consequences of these fluctuations is enhanced mixing of the surrounding fluid. It is also critical for understanding the ecology of a particular type of biological active systems, bacterial suspension, as the enhanced mixing strongly alters the fluxes of nutrients. We consider the theoretical foundations of fluid mixing enhancement in dilute suspensions of active force-free swimmers. We describe the impediments to fluid mixing imposed by the physical nature of fluid flows created by swimmers, and different ways of overcoming them. We show that fluid mixing in 3D suspensions of force-free (dipolar) swimmers is dominated by the effect of curvature of their trajectories, and obtain an exact analytical expression for the corresponding effective diffusion coefficient. Our results highlight limitations of alternative ``effective temperature'' approaches and may serve as a quantitative tool for designing technological applications. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B20.00008: Rotational manipulation of plasmonic nanoparticles in water by photon angular momentum Peter Johansson, Anni Lehmuskero, Robin Ogier, Tina Gschneidtner, Mikael Kall A photon carries energy, momentum and angular momentum, and can transfer each of these properties to material objects. It is well-known that optical gradient and radiation pressure forces caused by a focused laser beam enables trapping and manipulation of objects with strength dependent on the particle's optical properties. Moreover, the transfer of photon spin angular momentum, makes it possible to set objects into rotational motion by targeting them with a beam of circularly polarized light. We show that this effect can set $\sim$200 nm radii gold particles trapped in water in 2D by laser tweezers into rotation at frequencies that reach several kilohertz, much higher than any previously reported light driven rotation of a microscopic object, but still at low Reynolds numbers [1]. We also derive a theory for the fluctuations in light scattering from a rotating particle, and we argue that the high rotation frequencies observed experimentally is the combined result of favorable optical particle properties and a low local viscosity due to substantial heating of the particles surface layer. The high rotation speed suggests possible applications in nanofluidics, optical sensing, and microtooling of soft matter. \\[4pt] [1] A. Lehmuskero, {\it et al.}, Nano Lett.\ {\bf 13}, 3129 (2013). [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B20.00009: Effective Interactions in Systems of Active Particles Matthew Spellings, Michael Engel, Daphne Klotsa, Wenbo Shen, Greg van Anders, Sharon C. Glotzer Systems of linearly-driven active particles have been shown to exhibit fluid-solid coexistence in experiments and simulations. Still, the behavior of these and rotationally-driven anisotropic particles remains open for exploration. In this talk, we show that the addition of a constant driving torque for each particle to form an active system has a nonlinear effect on emergent forces between particles, which can be tuned to be attractive or repulsive. Effective interactions due to activity are naturally switchable and give us a new, orthogonal design dimension for assembly engineering in addition to more traditional design variables such as particle shape and enthalpic patchiness. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B20.00010: ABSTRACT MOVED TO S15.00014 |
Monday, March 3, 2014 1:15PM - 1:27PM |
B20.00011: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 1:27PM - 1:39PM |
B20.00012: Correlation between rotational and translational diffusion of a Janus nanoparticle in explicit solvent: A molecular dynamics simulation study Ali Kharazmi, Nikolai Priezjev Molecular dynamics simulations are used to study the diffusion of a single Janus particle immersed in a Lennard-Jones fluid. We consider a spherical particle with two hemispheres of different wettability. We analyzed the time dependence of the orientation tensor, particle displacement, and translational and rotational velocity autocorrelation functions. It was found that both translational and rotational diffusion coefficients increase with decreasing surface energy of the nonwetting hemisphere. We also observed that in contrast to homogeneous particles, the nonwetting hemisphere of the Janus particle tends to rotate in the direction of the displacement vector during the rotational relaxation time. Financial support from NSF (CBET-1033662) is gratefully acknowledged. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B20.00013: Diffusion of Interacting Particles in Discrete Geometries Kwinten Nelissen, T. Becker, Bart Cleuren, B. Partoens, C. Van den Broeck We evaluate the self-diffusion and transport diffusion of interacting particles in a discrete geometry consisting of a linear chain of cavities, with interactions within a cavity described by a free-energy function. Exact analytical expressions are obtained in the absence of correlations, showing that the self-diffusion can exceed the transport diffusion if the free-energy function is concave. The effect of correlations is elucidated by comparison with numerical results. Quantitative agreement is obtained with recent experimental data for diffusion in a nanoporous zeolitic imidazolate framework material, ZIF-8. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B20.00014: Applying Inkjet Technology to Dispense Colloidal Nanoparticle Fluids Annie O, Harjyot Mohar, Victor Hernandez, Arturo Estrada, Leonel Munoz, Sewan Fan, Laura Fatuzzo, Steven Jimenez The inkjet technology is widely employed to reliably deliver nanomaterials onto a substrate medium for further characterization and processing. To explore the feasibility of inkjet deposition for colloids, a novel drop-on-demand fluid dispenser is constructed to eject various types of liquids to produce atomized droplets. To make structured nanomaterials on a substrate using inkjet techniques, it is essential to determine the dynamical properties of the droplets as they are being formed. These would include the ejection speed, acceleration, terminal velocity and flight trajectories. For measuring these dynamic parameters, we successfully dispensed propylene glycol solution in different mixing ratios. This forms a reference fluid for establishing a baseline for our investigations. Our experimental data suggest that rapidly ejected droplets can be accurately modeled using Newton's equations and Stokes' law. In this conference, we describe our experiments consisting of an innovative inkjet dispensing apparatus in synchronization with a high-resolution camera imaging system. Furthermore, we plan to discuss our research efforts in dispensing microdroplets for relevant materials, such as chemical colloidal suspensions containing nanoparticles and polymer based fluids. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B20.00015: Structural disorder and anomalous water diffusion in random packing of spheres Andrea Gabrielli, Silvia Capuani, Marco Palombo, Vito D.P. Servedio, Giancarlo Ruocco Nowadays Nuclear Magnetic Resonance diffusion (dNMR) measurements of water molecules in heterogeneous systems have broad applications in material science, biophysics and medicine. Up to now, microstructural rearrangement in media has been experimentally investigated by studying the diffusion coefficient ($D(t))$ behavior in the tortuosity limit. However, this method is not able to describe structural disorder and transitions in complex systems. In this talk we show that, according to the continuous time random walk framework, the dNMR measurable parameter $\alpha $, quantifying the anomalous regime of $D(t)$, provides a quantitative characterization of structural disorder and structural transition in heterogeneous systems. This is demonstrated by comparing $\alpha $ measurements obtained in random packed monodisperse micro-spheres with Molecular Dynamics simulations of disordered porous media and 3D Monte Carlo simulation of particles diffusion in these kind of systems. Experimental results agree well with simulations that correlate the most used parameters and functions characterizing the disorder in porous media [1]. \\[4pt] [1] M. Palombo, A. Gabrielli, V.D.P. Servedio, G. Ruocco, S. Capuani, Scientific Reports \textbf{3}, 2631 (2013), doi:10.1038/srep02631 [Preview Abstract] |
Session B21: Focus Session: Soft Nanoparticles, Block Copolymer Micelles and Polymersomes I
Sponsoring Units: DPOLYChair: Thomas Epps III, University of Delaware
Room: 406
Monday, March 3, 2014 11:15AM - 11:51AM |
B21.00001: Responsive Hydrogels and Ion Gels by Self-Assembly of ABA and ABC Triblock Polymers Invited Speaker: Timothy Lodge Gels -- polymeric networks swollen with a substantial amount of solvent -- represent a fascinating class of soft materials, with wide-ranging applications in fields as diverse as biomedicine, pharmaceutics, personal care products, foods, sensors, actuators, flexible electronics, oil recovery, and adhesives. Physical gels are held together by non-covalent interactions, which may be as specific as hydrogen bonds, or as general as solvophobic association of insoluble blocks. Among the attractive features of physical gels are reversibility, stimuli-responsiveness, and tunability of macroscopic properties. In this talk two classes of physical gels will be highlighted. In one, the ability of ABC block terpolymers to form novel structures will be demonstrated, where blocks A and C are mutually immiscible and solvophobic, while B is solvophilic. In particular, the formation of gels by sequential association (first A, then C) leads to a remarkably sharp gelation transition, at a relatively low polymer concentration, compared to analogous gels formed from ABA systems. In the second class, gels formed by self-assembly of a variety of ABA systems in ionic liquids will be described, and in particular how gelation can be controlled through factors such as block chemistry, temperature, choice of ionic liquid, and application of light. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B21.00002: Translocation of a vesicle through a single pore Hamidreza Shojaei, Murugappan Muthukumar When a single vesicle is squeezed through a pore, it suffers from an elastic barrier arising from bending and stretching elasticity of the vesicle. We will discuss the free energy landscape for the translocation of a vesicle through a uniform cylindrical pore in terms of the initial radius of the vesicle, the diameter and length of the pore, and the elastic properties of the vesicle. With the quasi-equilibrium assumption and the Fokker-Planck formalism, we will present theoretical results for the translocation kinetics for the vesicle across a pore. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B21.00003: Towards Deflated Polymersomes Changqian Yu, Steve Granick The mechanical properties of polymersomes made from block copolymers present interesting and useful differences from the conventional vesicles made from lipids. Their limited water permeability and high bending modulus promote spectacular shape transformation upon external stimuli. Here we describe a vivid example of dynamic evolution of polymersomes from spherical towards the deflated state, driven by extreme osmotic shock. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B21.00004: Complex Morphology of Oppositely Charged Block Copolymer Micelles Misook Lee, Kyung Jee Min, Sheng Li, Kookheon Char The morphology of charged block copolymer micelle (BCM) complexes, consisting of polystyrene-\textit{block}-poly(acrylic acid) (PS-$b$-PAA) and polystyrene-\textit{block}-poly(4-vinyl pyridine) (PS-$b$-P4VP) micelles, was controlled by pH of aqueous solvent as well as solvent quality. To determine the effective pH range for the inter-corona combination of PAA and P4VP blocks in aqueous media, we studied the dissociation behavior of both coronas using Fourier Transform Infrared Spectroscopy. Lower pH region (4.0 \textless pH \textless 5.0) in aqueous medium offers stronger interactions between oppositely charged corona blocks, resulting in the formation of crystal-like complexes. Furthermore, the crystal habit of the micelle complex was found to be tunable by adjusting the relative size of the block copolymers and/or the pH of the aqueous medium. In the higher pH region (pH \textgreater 5.5), they first self-assembled into hierarchical bumpy spheres induced by the simple adsorption of small PS-$b$-PAA BCMs on the surfaces of PS-$b$-P4VP large compound micelles since the degree of ionization of P4VP blocks is relatively low. The final micelle morphology is highly dependent on the solvent quality. At low dimethylformamide (DMF) content, the internal structure of the BCM complex resembled spherical micelle. As the concentration of DMF increased, the internal PS-$b$-P4VP block copolymer structure transitioned from micelle-like to that resembled the morphology of the block copolymer in the bulk. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B21.00005: Stereoregularity Drives Precipitation in Polyelectrolyte Complex Formation Matthew Tirrell, Sarah Perry, Lorraine Leon, Matthew Kade, Dimitris Priftis, Katie Black, Kyle Hoffman, Jonathan Whitmer, Jian Qin, Juan de Pablo This study investigates the effect of stereoregularity on the formation of polypeptide-based complex formation and assembly into micelles, hydrogels and ordered phases. We demonstrate that fluid complex coacervate formation (rather than solid complex precipitation) between oppositely charged polypeptides requires at least one racemic partner in order to disrupt backbone hydrogen bonding networks and prevent the hydrophobic collapse of the polymers into compact, fibrillar secondary structures. Computer simulations bear this out and enable visualization of the molecular structure of the complexes. The ability to choose between conditions of fluid phase formation and solid phase formation is a useful tool in developing new self-assembled materials based on polyelectrolyte complex formation. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B21.00006: Rapid and Quasi-reversible Poly(vinyl acetate-b-vinyl alcohol) Spherical Micelle Fusion Induced by Poly(ethylene oxide) in Water Mahesh Mahanthappa, Milton Repollet-Pedrosa Methods for triggering morphology changes in aqueous dispersions of amphiphilic block copolymers (ABCs) are crucial for their development as responsive fluids with properties that may be manipulated ``on demand.'' Numerous groups have reported methods for switching the morphologies of ABCs by the incorporation of thermal and photochemical switches, the addition of salts, and changes in pH. We report a new ABC system in which a dispersion of spherical micelles may be rapidly and quasi-reversibly transformed into a solution of worm-like micelles, upon the addition of a water-soluble homopolymer. More specifically, we demonstrate that the addition of varying amounts of poly(ethylene oxide) homopolymer to a dilute dispersion of poly(vinyl acetate-block-vinyl alcohol) spherical micelles results in either (1) their immediate precipitation, or (2) their rapid fusion into worm-like micelles. Dilution of the latter solution of worm-like micelles with pure water induces their reversion into spherical micelles. By evaluating the effects of PEO molecular weight and solution concentration on the micellar interconversion process, we propose two possible mechanisms for this unexpected morphological transformation. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B21.00007: Comparing Fluid and Elastic Block Copolymer Shells Damith Rozairo, Andrew B. Croll Emulsions can be stabilized with the addition of an amphiphilic diblock copolymer, resulting in droplets surrounded and protected by a polymer monolayer. Such droplets show considerable promise as advanced cargo carriers in pharmaceuticals or cosmetics due to their strength and responsiveness. Diblock copolymer interfaces remain mostly fluid and may not be able to attain the mechanical performance desired by industry. To strengthen block copolymer emulsion droplets we have developed a novel method for creating thin elastic shells using polystyrene-b-poly(acrylic acid)-b-polystyrene (PS-PAA-PS). Characterization of the fluid filled elastic shells is difficult with traditional means which lead us to develop a new and general method of mechanical measurement. Specifically, we use laser scanning confocal microscopy to achieve a high resolution measure of the deformation of soft spheres under the influence of gravity. To prove the resilience of the technique we examine both a polystyrene-b-poly(ethylene oxide) (PS-PEO) stabilized emulsion and the PS-PAA-PS emulsion. The mechanical measurement allows the physics of the polymer at the interface to be examined, which will ultimately lead to the rational development of these technologies. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B21.00008: Polyoxometalate (POM) Macroion Decorated Polymersomes Benxin Jing, Erin Connor, Y. Elaine Zhu Polymersomes as one of the common self-assembled forms of amphiphilic block copolymers have been widely developed for applications from drug delivery to mirco/nanoreactors. The tunability of their materials properties, such as mechanical strength and permeability often relies on the chemistry of the selected polymer in a liquid medium. We have recently employed the emergent polyoxometalate (POM) nanoclusters as macroions to control their interaction and assembly with different polymersomes. For both neutral and cationic polymersomes decorated with highly charged anionic POM nanoclusters, the dispersion stability and mechanic strength can be significantly enhanced. AFM and TEM characterization further confirms the encapsulation of POM macroions into polymersomes to form inorganic-organic hybrid complexes, which lead to new potential applications in anticancer and antibacterial medicines and catalysts. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B21.00009: Tartaric Acid-Assisted Self-Assembly of Hybrid Block Copolymer Composites Li Yao, Ying Lin, James Watkins Enantiopure tartaric acid was used as an additive to increase the segregation strength of poly(ethylene oxide-block-tert-butyl acrylate) (PEO-b-PtBA) copolymers through strong, selective interactions with one of the polymer chain segments. Addition of tartaric acid to PEO-b-PtBA exhibiting cylindrical morphologies resulted in the formation of helical superstructures as observed by transmission electron microscopy. It was also found that this small acid additive can also enable phase-selective ultra-high loading of nanoparticles (NPs) into target domains of the block copolymer composites. The loading of tartaric acid can increase enthalpically favorable interactions between the nanoparticle ligands and the host domain and mitigate entropic penalties associated with NP incorporation into the target domain. A metal content of over 40 weight percent by mass of the resulting well ordered composites was achieved as measured by thermal gravimetric analysis in PEO-b-PtBA/tartaric acid/4-hydroxythiophenol functionalized Au NP hybrid system. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B21.00010: Self-assembled Structures of a Multifunctional, Structured Block Copolymer in Solution; A SANS Study Thusitha Etampawala, Manjula Senanayake, Naresh Osti, Lilin He, William Heller, Dvora Perahia The self-assembly of multi block copolymer in solutions is controlled by a delicate balance between inherent phase segregation due to incompatibility of the blocks and the interactions of the individual blocks with the solvent. We investigated the association of ABCBA penta-block copolymers, in solution using Small angle neutron scattering (SANS). The ABCBA penta-block comprises of centered randomly sulfonated polystyrene block to which rubbery polyisoprene is connected, terminated by blocks of polystyrene decorated with tertiary butyl group, kindly provided by Kraton LLC. The SANS studies have shown that the penta-block forms ellipsoidal core-shell structures with the sulfonated polystyrene in the core and Gaussian decaying chains of swollen polyisoprene and tertiary butyl polystyrene in the corona. The size of the micelle, the thickness of the corona and the aggregation number increased with increasing the solution concentration and temperature, while the solvent fraction in the core decreased. The dilute solutions promptly responded to thermal fluctuations. However, the temperature effects disappeared with increasing the solution concentration. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B21.00011: Multigeometry Nanoparticle Engineering via Kinetic Control through Multistep assembly Yingchao Chen, Xiaojun Wang, Ke Zhang, Fuwu Zhang, Jimmy Mays, Karen Wooley, Darrin Pochan Organization of block copolymers into complicated multicompartment (MCM) and multigeometry (MGM) nanostructures is of increasing interest. Multistep, co-assembly methods resulting in kinetic control processing was used to produce complex nanoparticles that are not obtained via other assembly methods. Vesicle-cylinder, separate vesicle and cylinder, disk-cylinder, and mixed vesicle nanoparticles were constructed by binary blends of distinct diblock copolymers. Initially, the vesicle former polyacrylic acid-polyisoprene and cylinder former polyacrylic acid-polystyrene which share the same hydrophilic domain but immiscible hydrophobic domain were blended in THF. Secondly, dimaine molecules are added to associate with the common hydrophilic PAA. Importantly, and lastly, by tuning the kinetic addition rate of selective, miscible solvent water, the unlike hydrophobic blocks are kinetically trapped into one particle and eventually nanophase separate to form multiple compartments and multigeometries. The effective bottom-up multistep assembly strategies can be applied in other binary/ternary blends, in which new vesicle-sphere, disk-disk and cylinder-cylinder MCM/MGM nanoparticles were programed. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B21.00012: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 2:03PM - 2:15PM |
B21.00013: On the Effect of TiO2 Nanoparticles on the Crystallization of PEO Jesus Eduardo Saldana, Alin Cristian Chipara, Alejandro Castillo, James Hinthorne, Elamin Ibrahim, Mircea Chipara Nanocomposite consisting of various amounts of TiO2 nanoparticles dispersed within polyethylene oxide (PEO) have been prepared by melt mixing. The thermal properties of these nanocomposites have been investigated by Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). TGA data revealed a weak increase of the thermal stability of the PEO matrix upon the loading with nanoparticles. The crystalline structure of PEO and TiO2 has been confirmed by Wide Angle X-Ray Scattering and electron microscopy. Isothermal and non isothermal DSC was used to investigate the melting/crystallization process. Additional information regarding the nanofiller has been obtained via Raman and FTIR spectroscopy. The shift of the melting and crystallization temperature due to the loading with TiO2 nanoparticles is analyzed. [Preview Abstract] |
Session B22: Focus Session: Organic Electronics and Photonics - Polymer Photovoltaics
Sponsoring Units: GERA DPOLYChair: Enrique Gomez, Pennsylvania State University
Room: 407
Monday, March 3, 2014 11:15AM - 11:27AM |
B22.00001: Impact of Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE) on Morphology and Charge Conduction in Conjugated Polymer and Bulk Heterojunction Thin Films Adrienne Stiff-Roberts, Ryan McCormick, Ayomide Atewologun An approach to improve organic photovoltaic efficiency is to increase vertical charge conduction by promoting out-of-plane $\pi $-$\pi $ stacking in conjugated polymers. Resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) features multiple growth parameters that can be varied to achieve a desired organic thin film property. In addition, RIR-MAPLE enables nanoscale domains in blended polymeric films and multi-layer polymeric films regardless of constituent solubility. Thus, RIR-MAPLE deposition is compared to solution-cast films as a possible approach to increase out-of-plane charge transport in polymers and bulk heterojunctions. Two common, solar cell polymers are investigated: P3HT and PCPDTBT. Materials characterization includes grazing-incidence, wide angle x-ray scattering (GIWAXS) for structural information and two techniques to determine hole mobility: organic field effect transistors to measure in-plane mobility and charge extraction by linearly increasing voltage to measure out-of-plane mobility. Initial indications are that the RIR-MAPLE films have a fundamentally different morphology compared to solution-cast films. In the case of P3HT, an enhancement in out-of-plane $\pi $-$\pi $ stacking was observed by GIWAXS in RIR-MAPLE films compared to solution-cast films. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B22.00002: The Role of Aromatic Structural Units of Conjugated Copolymers in Reaching High Solid-State Order and Optoelectronic Performances Chien-Lung Wang, Chain-Shu Hsu, Kuan-Yi Wu, Tien-Hsin Lee Solid-state order of conjugated polymers is determinative in converting molecular properties into useful optoelectronic performances. The rapid development in donor-acceptor conjugated copolymers not only prompted device performances of polymeric optoelectronics, but also created wide varieties of complicate aromatic structural units, whose role in the solid-state order remains under studied. The roles of two widely used axisymmetrical aromatic units- 5,6-difluorobenzo-2,1,3-thiadiazole, and dithienocyclopentacarbazole will be discussed in this presentation. 2-dimensional X-ray diffraction, electron diffraction and theoretical molecular simulation showed that ordered solid-state structures were reached in copolymers with strong interchain interaction and good backbone linearity. The enhanced interchain interaction was supported by higher melting temperature and dis-aggregation temperature in the solution. High mobility of 0.29 cm2/Vs and power conversion efficiency of 6.82\% were reached in copolymers possess ordered solid-state structure with long correlation lengths. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B22.00003: Photovoltaic Cells involving Nonconjugated Conductive Polymer, Iodine-doped cis-Polyisoprene (Natural Rubber) S. Jaju, M. Thakur Photovoltaic cells have been fabricated using titanium dioxide/doped cis-polyisoprene/carbon on ITO glass-substrates. Photocurrents and photo-voltages for different intensities of light (from a white light bulb, emission at 300-700 nm) have been measured. Use of the iodine-doped nonconjugated conductive polymer film (absorption $\sim$ 250 to 700 nm) has led to significant enhancement of photocurrent compared to previous reports which included undoped polymer in a different cell-structure. A maximum photocurrent of about 0.20 mA was observed for a light intensity of $\sim$ 5 mW/cm$^{2}$. The maximum photo-voltage as observed was about 0.70 V for the same light intensity. Natural rubber being inexpensive these cells may provide cheaper alternatives to other reported cell structures. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B22.00004: Engineering Molecular Order for Enhanced Stability of Organic Solar Cells Anne Guilbert, Jenny Nelson, Joao Cabral The microstructure of organic photovoltaics (OPV) is often unstable and very sensitive to processing parameters. It is known that fullerene crystallisation has a huge impact on device behaviour. However, the observation of different size and shape of fullerene crystals in different polymer matrices is not yet understood. We select as a model system amorphous regiorandom poly(3-hexylthiophene-2,5-diyl) (RRa-P3HT) and crystalline phenyl-C61-butyric acid methyl ester (PCBM). We study the nucleation and growth of PCBM crystals into RRa-P3HT matrix as a function of fullerene loading, solution concentration, film thickness, supercooling and surface energy (tuned by UV ozonolysis) using optical and atomic force microscopy, and DSC. We show that the shape of PCBM crystallites can be tuned from needles to spheroidal crystals by increasing undercooling. We argue that the different behaviour of polymer:PCBM blends can be rationalised in term of undercooling, viscosity and fragility of the composite. We finally evaluate the role of additives on the crystallisation of PCBM. By mapping the impact of the processing parameters in this model system, we establish a simple strategy towards controlling PCBM crystallisation at relevant lengthscales for OPV performance. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B22.00005: Influence of Amorphous PCPDTBT on the Morphology of Ternary Blend Solar Cell Based on P3HT/PCPDTBT/PCBM Yu Gu, Cheng Wang, Feng Liu, Jihua Chen, Ondrej Dyck, Gerd Duscher, Thomas Russell To push the efficiency of organic photovoltaic devices to a higher level, ternary blend solar cells were fabricated. Only a few successful ternary systems have been reported, which have higher efficiency than the binary references. One of these is based on P3HT/PCPDTBT/PCBM. We used x-ray scattering methods in combination with transmission electron microscopies to determine the morphology of thin films of this blend. Different morphologies were generated by varying the molecular weight of P3HT, blending ratio and thermal annealing time. It was found that P3HT crystallized under the confinement of PCPDTBT. The bundles of P3HT fibrils, composed of the oriented P3HT crystal blocks, formed a network; and a PCBM-rich phase with amorphous PCPDTBT and P3HT filled in the space between the P3HT fibrils. Such a multi-length-scale morphology produced a parallel device structure. The extended absorption and the photosensitization of PCPDTBT at the interface were attributed to the improved device performance relative to the binary references. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B22.00006: Characterization of solution structure and its importance in thin film ordering of conjugated block copolymers for organic semiconductor devices Michael Brady, Sung-Yu Ku, Justin Cochran, Cheng Wang, Craig Hawker, Edward Kramer, Michael Chabinyc Fully conjugated diblock copolymers (CBCPs) form intriguing materials alternatives to polymer-small molecule blends for their control of mesoscopic order in low-cost organic semiconductor devices. In both bulk heterojunction (BHJ) photovoltaics, consisting of an interpenetrating network with high donor-acceptor interfacial area, and ambipolar transistors, the transport of charge carriers through continuous p- and n-type paths in thin films is a controlling factor in device performance. AFM, GIWAXS, NEXAFS spectroscopy, and RSoXS are used to probe the structure of films of CBCPs with a p-type P3HT block and an n-type DPP block. Thermal annealing in the P3HT melt after casting creates ordered domains with $\sim$ 50 nm in-plane lamellar spacings, as confirmed with GISAXS and RSoXS. GIWAXS diffraction from the (h00) alkyl-stacking and (010) pi-stacking planes shows primarily edge-on orientation for crystals of both P3HT and DPP blocks. In addition, temperature-dependent solution SAXS and UV-Vis spectroscopy are used to probe the size and conformation of casting solution aggregates. Fibrillar DPP aggregates direct the crystallization of P3HT-$b$-DPP following film casting and enable the formation of wormlike domains after annealing and thus ideal morphologies for transport in organic devices. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B22.00007: Influence of copolymer additives on the morphology and performance of bulk heterojunction organic photovoltaics Anton Li, Jojo Amonoo, Bingyuan Huang, Peter Goldberg, Anne McNeil, Peter Green Device performance of polymer:fullerene photovoltaics is intimately connected to their complex bulk heterojunction morphologies. We incorporated varying amounts a fully-conjugated random copolymer, poly((3-hexylthiophene)-$r$-(3-(hexyl(oxylmethyl)thiophene))), into a blend of poly(3-hexylthiophene) and indene-C$_{\mathrm{60}}$ bisadduct, achieving up to a 20{\%} increase in power conversion efficiency. We attribute part of the improved device performance to a decrease in bimolecular recombination, as measured by photo-induced charge extraction by linearly increasing voltage. Superior carrier transport and collection are in turn correlated to the altered structure of the active layer, both internally and near the electrode interface, as revealed by energy-filtered transmission electron microscopy and atomic force microscopy. These findings illustrate the potential for conjugated copolymers to be versatile tools for tailoring the morphology and energetics of organic semiconductor blends for photovoltaic applications. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B22.00008: Correlated molecular orientation in all-polymer solar cells and its role in free charge generation Brian A. Collins, Marcel Schubert, Steffen Roland, Riccardo Di Pietro, Robbert Steyrleuthner, Koen Vandewal, Alberto Salleo, Wolfram Schindler, Konstantinos Fostiropoulos, Zhihua Chen, Antonio Facchetti, Harald Ade, Dean Delongchamp, Dieter Neher New polymers with high electron mobilities have spurred research in organic solar cells using polymer rather than fullerene acceptors due to their potential of increased diversity, stability and scalability. However, all-polymer solar cells have thus far struggled to keep up with their polymer-fullerene counterparts. We examine this issue by investigating the effect of additives on morphology correlated to optoelectronic properties within poly(3-hexylthiphene), P(NDI2OD-T2) blend devices. Resonant X-ray microscopy and scattering along with energy-filtered electron microscopy monitor the evolution of domain size and purity, while X-ray diffraction and a novel polarized X-ray scattering technique reveal a reorientation of the molecules with respect to the donor-acceptor interface, correlating well with device photocurrent. In addition, low efficiency in separating directly excited charge transfer states ties the low device efficiency to geminate recombination. Thus, anisotropic polymer electronic orbitals may necessitate correlated donor-acceptor molecular orientation for efficient charge generation and represent an extra hurtle compared with the isotropic fullerene in realizing efficient devices. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B22.00009: Designing high efficiency organic photovoltaics by controlling the ordering at the donor-acceptor interface Aditya Mohite, Wanyi Nie, Gautam Gupta, Brian Crone, Chenyu Kuo, Hsinhan Tsai, Darryl Smith, Paul Ruden, Feilong Liu, Hsing-lin Wang, Sergei Tretiak The overall power conversion efficiency in an organic solar cell depends on the balance between the rates of exciton dissociation, recombination and separation at the donor acceptor interface. Inability to design, control and engineer these interfaces remains a key bottleneck in their widespread use for the next generation organic electronic devices. Here, we show that we can control the ordering at the P3HT/C60 interface in bilayer device geometry by inserting a monolayer of oligothiophenes, which leads to a complete suppression in the exciplex (or charge transfer state) recombination. We observe that the photocurrent increases by 500{\%}, which in turn results in an increase in the overall power conversion efficiency by an order of magnitude. Moreover, we find that the oligothiophene with an odd number of rings (ter and penta oligothiophene) exhibit a much higher increase in the photocurrent in comparison to the oligothiophene with an even number of rings (tetra oligothiphene). STM measurements reveal that the oligothiophene with odd and even number of rings differ in their ordering respectively, that has a big effect on the overall device performance. We also find that this ordering is highly dependent on the side functional groups in the oligothiophenes. The mechanism of photocurrent generation will be discussed and a simple transport model will be used to explain the change in the charge transfer and recombination rates and predict current-voltage curves. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B22.00010: Origins of Reduced Nongeminate Recombination in P3HT:PCBM Organic Solar Cells Michael Heiber, Julien Gorenflot, Vladimir Dyakonov, Carsten Deibel Understanding the nongeminate recombination processes that are the dominant loss mechanisms in organic solar cells is critical to improving device performance. In P3HT:PCBM blends, nongeminate recombination has been found to be significantly slower than expected from Langevin theory and also exhibits super-second order kinetics. Several theories for this behavior have been proposed, but a complete model has not yet been reached. To shed light on this problem, we have used a combination of transient absorption spectroscopy experiments and kinetic Monte Carlo simulations. By modeling the temperature dependence of the polaron transients measured in both neat P3HT films and annealed P3HT:PCBM blend films, we demonstrate the effects of phase separation, carrier trapping, and charge transfer states on the magnitude of the recombination rate. Furthermore, we show that while neat P3HT films exhibit second order recombination and mobility behavior indicating a Gaussian density of states (DOS), P3HT:PCBM blends are complicated by super-second order recombination that is indicative of an exponential DOS and mobility measurements that are consistent with a Gaussian DOS. To unify these observations, we show that a separate distribution of charge transfer states must be included. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B22.00011: Interpreting impedance spectra of organic photovoltaic cells - Extracting charge transit and recombination rate constants Tyler Mullenbach, Yunlong Zou, Russell Holmes Impedance spectroscopy has been widely used to extract the electron-hole recombination rate constant in organic photovoltaic cells (OPVs). This technique is typically performed on OPVs held at open-circuit. Under these conditions, the analysis is simplified with recombination as the only pathway for the decay of excess charge carriers; transit provides no net change in charge density. In this work we generalize the application and interpretation of impedance spectroscopy for bulk heterojunction OPVs at any operating voltage. This, in conjunction with reverse bias external quantum efficiency measurements, permits the extraction of both recombination and transit rate constants, offering a more complete picture of charge carrier dynamics in the device. Using this approach, both rate constants are determined for OPVs with varying electron donor-acceptor pairings and compositions. It is found that neither rate constant individually is sufficient to characterize the efficiency of charge collection in an OPV. A large recombination rate constant is not, on its own, detrimental if it coincides with a large transit rate constant. The technique presented here permits a detailed understanding of how OPV architecture and processing conditions impact the transient behavior of charge carriers, elucidating the origin of optimum device configurations. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B22.00012: Impedance Spectroscopy and Electroabsorption Studies of PCPDTBT-PCBM Bulk-Heterojunction Solar Cells Christopher Green, Zane Cohick, Marian Tzolov Bulk heterojunction polymer solar cells, consisting of PCPDTBT and PCBM, offer the potential for high light absorption and effective charge collection in sub-micron thick films. An understanding of the behavior of photo-generated charge transfer complexes in the polymer/small molecule blend is needed for further increases in the efficiency of such devices. In this study, we present experimental results on impedance spectroscopy indicating the formation of trapped electrical charges upon illumination. The population of the related energy states depends on the applied voltage. These results are supported by electroabsorption spectroscopy of the solar cells and of the films of the constituting materials. They indicate an additional electroabsorption feature not present in the individual films of the PCPDTBT and PCBM. The voltage dependence of the electroabsorption signal resulted in estimation of the built-in voltage in our solar cells of 1.1 eV. Our comparison of the photocurrent spectrum with the optical absorption spectrum of the solar cells indicate more efficient energy conversion for the light absorbed in the PCPDTBT compared to the light absorbed in PCBM. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B22.00013: Large Perturbation Transient Photovoltage for the Study of Lifetime and Order of Recombination in Organic Photovoltaics Lindsay Elliott, James Basham, Kurt Pernstich, Pragya Shrestha, Lee Richter, Dean DeLongchamp, David Gundlach Organic photovoltaics remain a topic of tremendous research interest and continue to show promise for use in applications requiring flexible and portable energy harvesting solutions. Measurements methods to accurately quantify charge generation and recombination over a large dynamic range are crucial to understanding and improving device operation and performance. We report here on the combination of two measurements: large perturbation transient photovoltage (LTPV) and impedance spectroscopy. These techniques are employed to compare the populations of mobile and trapped charge carriers over a range of open circuit voltages. Charge carrier lifetime and order of recombination are extracted from the combined techniques for the relatively large change in charge carrier density over which the measurements are valid. To demonstrate the utility of our approach we apply these combined measurements to the well-studied polymer-fullerene pair, P3HT:PC61BM. Together, these optoelectronic techniques provide better understanding of the involvement of free-free versus free-trapped recombination and the difference between total population of generated charge and mobile charge carriers. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B22.00014: Device models for bilayer organic solar cells using interface rate equations Non Thongprong, Phillip Duxbury Although the generalized Shockley diode equation is often used to fit the electrical response of organic photovoltaic devices, however developing device models to relate these parameters to atomistic processes is more difficult yet essential to fundamental understanding. A useful device model for organic heterojunctions was developed by Giebink et al. [1], where the heterointerface is treated using a rate equation approach and the electric field in the donor and acceptor regions is assumed to be constant. We have developed models and computational tools combining the bilayer interface model of Giebink et al. with methods to include non-uniform electric fields in the donor and acceptor regions of the material [2]. Injection barriers and trap effects in the donor and acceptor regions are also incorporated in our computational tools. Here the model and computational methods will be briefly outlined and results for the effects of low mobility in the donor or acceptor regions will be summarized. In these models, the series resistance in the generalized Shockley equation is interpreted as a sum of total bulk resistivity of materials and barriers at each layer's contact, while the parallel resistance mainly stems from dissociation efficiency of charge transfer states at the interface of doner and acceptor. \\[4pt] [1] N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, S. R. Forrest, Phys. Rev. B (2010, APS).\\[0pt] [2] L. J. A. Koster, E. C. P. Smits, V. D. Mihailetchi, P. W. M. Blom, Phys. Rev. B (2005, APS). [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B22.00015: Characterization of phosphorescent organic light-emitting diodes using current noise cross-correlated spectroscopy Thaddee Kamdem Djidjou, Sergey Li, Andrey Rogachev Carrier injection and transport mechanism in small-molecule phosphorescent organic light-emitting diodes (PhOLED) have been investigated using current noise spectroscopy. The PhOLED devices studied consist of multilayers having the structure ITO / NPB / NPB:Irphq / Balq / Bpen:CsCO$_{3}$/ Al. We found that in high bias regime, the noise spectral density can be described by two terms, 1/ f$^{1.3}$ and 1/f$^{2.8}$. The first term disappears below 2.5 V, as does the luminance; this suggests that this term is related to bimolecular recombination in the devices. The second term is more pronounced al low frequencies and its magnitude is linearly proportional to the current in the device. This term, which exists in all bias range, is likely related to the presence of traps with a distributed time constant. For applied voltages greater than 2.4 V, the frequency-independent noise is dominated by the shot noise. The Fano factor is one in the range 2.4 - 2.5 V, and decreases to a constant value of 0.4 at higher biases. This indicates the presence of a barrier for carrier injection into the device. Our overall results confirm the utility of noise measurements for OLED characterization. [Preview Abstract] |
Session B24: Focus Session: Advances in Scanned Probe Microscopy I: Novel Approaches and Ultrasensitive Detection
Sponsoring Units: GIMSRoom: 504
Monday, March 3, 2014 11:15AM - 11:51AM |
B24.00001: Imaging quantum transport using scanning gate microscopy Invited Speaker: Benoit Hackens Quantum transport in nanodevices is usually probed thanks to measurements of the electrical resistance or conductance, which lack the spatial resolution necessary to probe electron behaviour inside the devices. In this talk, we will show that scanning gate microscopy (SGM) yields real-space images of quantum transport phenomena inside archetypal mesoscopic devices such as quantum point contacts and quantum rings. We will first discuss the SGM technique, which is based on mapping the electrical conductance of a device as an electrically-biased sharp metallic tip scans in its vicinity. With SGM, we demonstrated low temperature imaging of the electron probability density and interferences in embedded mesoscopic quantum rings [B. Hackens et al., Nat. Phys. 2, 826 (2006)]. At high magnetic field, thanks to the SGM conductance maps, one can decrypt complex transport phenomena such as tunneling between quantum Hall edge state, either direct or through localized states [B. Hackens et al., Nat. Comm. 1, 39 (2010)]. Moreover, the technique also allows to perform local spectroscopy of electron transport through selected localized states [F. Martins et al., New J. of Phys. 15, 013049 (2013); F. Martins et al., Sci. Rep. 3, 1416 (2013)]. Overall, these examples show that scanning gate microscopy is a powerful tool for imaging charge carrier behavior inside devices fabricated from a variety of materials, and opens the way towards a more intimate manipulation of charge and quasiparticle transport. This work was performed in collaboration with F. Martins, S. Faniel, B. Brun, M. Pala, X. Wallart, L. Desplanque, B. Rosenow, T. Ouisse, H. Sellier, S. Huant and V. Bayot. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B24.00002: Design and Characterization of a millikelvin dual-tip Josephson STM A. Roychowdhury, M. Dreyer, J.R. Anderson, C.J. Lobb, F.C. Wellstood We describe the design and characterization of a dual-tip Josephson STM that operates at millikelvin temperatures. We report an effective noise temperature for the STM on the order of 200 mK.\footnote{A. Roychowdhury et. al., arXiv:1311.1855 (2013)} In addition to the expected phase diffusive super current in the ultra-small Nb-Nb junction formed by one tip and the sample,\footnote{M. Ivanchenko and L.A Zil'berman, Sov. Phys. JETP, 28, 1272 (1969)} our high resolution spectroscopy at mK temperatures reveals resonant coupling between the STM junction and the electromagnetic environment it is embedded in, as predicted by P(E) theory.\footnote{G. Ingold and H. Grabert, Phys. Rev. B., 50, 395 (1994)} We have for the first time, observed Shapiro-like steps in this limit by measuring the response of the P(E) supercurrent to microwave radiation as a function of amplitude. Fits to theory\footnote{G. Falci, V. Bubunja and G. Schon, Z. Phys. B., 85, 451 (1991)} indicate that the coupling of an ultra-small Josephson junction to its environment/circuit may be used to a) directly measure dissipation channels associated with circuit resonances and b) calibrate the frequency dependent microwave attenuation in cryogenic circuits as seen by the junction. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B24.00003: A cryogenic quantum gas scanning magnetic microscope Matthew Naides, Richard Turner, Ruby Lai, Jack DiSciacca, Benjamin Lev Atom chip trapping of quantum gases will enable single-shot, large area imaging of transport through strongly correlated and topologically non-trivial materials via detection of magnetic flux at the $10^{-7}$ flux quantum level and below. By harnessing the extreme sensitivity of atomic clocks and Bose-Einstein condensates to external perturbations, the cryogenic atom chip technology we have recently demonstrated [1] will provide a magnetic flux detection capability that surpasses other techniques, while allowing sample temperatures spanning $<$10 K to room temperature. We report on experimental progress toward developing this novel quantum gas scanning magnetic microscope [1] and describe our recent proposal [2] to image topologically protected transport through a non-ideal topological insulator in a relatively model-independent fashion. \\[4pt] [1] M. Naides, R. Turner, R. Lai, J. DiSciacca, and B. L. Lev, Trapping ultracold gases near cryogenic materials with rapid reconfigurability, arXiv:1311.2065 (2013). \\[0pt] [2] B. Dellabetta, T. L. Hughes, M. J. Gilbert, and B. L. Lev, Imaging topologically protected transport with quantum degenerate gases, Physical Review B 85, 205442 (2012). [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B24.00004: First results for custom-built low-temperature (4.2 K) scanning tunneling microscope/molecular beam epitaxy and pulsed laser epitaxy system designed for spin-polarized measurements Andrew Foley, Khan Alam, Wenzhi Lin, Kangkang Wang, Abhijit Chinchore, Joseph Corbett, Alan Savage, Tianjiao Chen, Meng Shi, Jeongihm Pak, Arthur Smith A custom low-temperature (4.2 K) scanning tunneling microscope system has been developed which is combined directly with a custom molecular beam epitaxy facility (and also including pulsed laser epitaxy) for the purpose of studying surface nanomagnetism of complex spintronic materials down to the atomic scale. For purposes of carrying out spin-polarized STM measurements, the microscope is built into a split-coil, 4.5 Tesla superconducting magnet system where the magnetic field can be applied normal to the sample surface; since, as a result, the microscope does not include eddy current damping, vibration isolation is achieved using a unique combination of two stages of pneumatic isolators along with an acoustical noise shield, in addition to the use of a highly stable as well as modular `Pan'-style STM design with a high Q factor.[1] First 4.2 K results reveal, with clear atomic resolution, various reconstructions on wurtzite GaN c-plane surfaces grown by MBE, including the c(6x12) on N-polar GaN(000\underline {1}). Details of the system design and functionality will be presented. [1] Kangkang Wang, Wenzhi Lin, Abhijit V. Chinchore, Yinghao Liu, and Arthur R. Smith, Review of Scientific Instruments \textbf{82}, 053703 (2011). [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B24.00005: Theory and Experiment of Scanning Thermoelectric Microscopy with Atomic Resolution Eui-Sup Lee, Sanghee Cho, Ho-Ki Lyeo, Yong-Hyun Kim Heat, a measure of entropy, is largely perceived to be diffusive and transported incoherently by charge carriers and lattice vibrations in a material, which is hard to be spatially localized. Heat transport is therefore considered a challenging means of the local imaging of a material and its electronic states. However, Cho \textit{et al.} [1] reported a series of striking wavefunction images of epitaxial graphene by measuring thermoelectric voltages with a heat-based scanning probe microscopy. Here we present how the thermoelectric signal is related to the atomic-scale wavefunctions and what the role of the temperature is at such a length scale. An exact expression of local thermoelectric voltage is deduced, and a computer-based thermoelectric imaging simulation method with first-principles wavefunction calculations is developed and performed on pristine and defective graphene. From this analysis, we find that coherent electron and heat transport through a point-like contact produces an atomic Seebeck effect. We will also discuss the connection between Seebeck coefficient and thermal properties of a material, such as electronic heat capacity and quantum of thermal conductance, by introducing the statistically defined Fermi temperature [2]. [1] S. Cho, S. D. Kang, W. Kim, E.-S. Lee, S.-J. Woo, K.-J. Kong, I. Kim, H.-D. Kim, T. Zhang, J. A. Stroscio, Y.-H. Kim, and H.-K. Lyeo, arXiv:1305.2845, Nature Mater.\textbf{ 12}, 913 (2013). [2] E.-S. Lee, S. Cho, H.-K. Lyeo, and Y.-H. Kim, arXiv:1307.3742, \textit{submitted} (2013). [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B24.00006: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 12:51PM - 1:03PM |
B24.00007: Chemically Sensitive Imaging of MgP with STM Arthur Yu, Shaowei Li, Greg Czap, Wilson Ho Since its invention, the STM has been limited by its lack of sensitivity to chemical structures in molecules. Recent advances in scanning probe microscopy techniques, such as non-contact AFM and scanning tunneling hydrogen microscopy have enabled imaging of the internal structure and bonding of aromatic molecules such as pentacene and PTCDA. Here, we present a novel method of using the STM to image magnesium porphyrin molecules adsorbed on Au(110) with chemical sensitivity. In our previous study, we have shown that hydrogen molecules weakly adsorb on Au(110), exhibiting both vibrational and rotational IETS spectra. Exploiting the sensitivity of the vibrational and rotational mode energies to the local chemical environment, we perform dI/dV and d$^{2}$I/dV$^2$ imaging at different bias voltages, highlighting the various parts of the MgP molecule. In particular, we are able to image the positions of the nitrogen atoms in MgP. d$^{2}$I/dV$^{2}$ spectral mapping reveals that the origin of the chemical sensitivity comes from an energy shift of the rotational peak as the tip is scanned across the molecule, indicating a changing potential landscape for the H$_{2}$. Similar d$^{2}$I/dV$^{2}$ imaging with a CO terminated tip reveals no chemical sensitivity to nitrogen. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B24.00008: Electrostatic actuation of commercial AFM cantilevers Christian Long, Rachel J. Cannara We present an atomic force microscope (AFM) cantilever holder for electrostatic actuation of AFM cantilevers. The cantilever holder contains an electrode that is positioned behind the AFM cantilever, making this implementation of electrostatic actuation compatible with a wide variety of samples and commercially available AFM tips. Local electrostatic actuation of the cantilever eliminates the excitation of spurious mechanical resonances associated with the cantilever holder, making it an attractive alternative to piezoelectric actuation. Avoiding spurious mechanical resonances is most important for situations in which the cantilever's quality factor is low compared to the quality factors of the spurious mechanical resonances. These spurious resonances typically have quality factors of less than 50, so they are not an issue for applications such as routine tapping mode imaging in air. However, for applications where the quality factor of the cantilever is low, as is the case in contact resonance spectroscopy or in fluid environments, electrostatic actuation can be highly advantageous compared to piezoelectric actuation. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B24.00009: Combining Amplitude and frequency Modulation in Atomic Force Microscopy David Haviland, Daniel Forchheimer, Daniel Platz, Erik Tolh{\'e}n Dynamic AFM is usually sorted in to one of two general categories: Frequency Modulation (FM-AFM) or Amplitude Modulation (AM-AFM). These names refer to the way in which feedback is preformed while scanning over the surface. In either category the tip-surface interaction is viewed as a {\em passive} modulation of the response to a single drive frequency, modulating either the amplitude or phase of the response. Often an extra feedback (phase locked loop) is used to measure the response phase as a shift of resonance frequency. An alternative approach to measurement is {\em active} modulation of the drive force on the cantilever, while monitoring how this active modulation is modified by the tip-surface interaction. Such active modulation can be frequency or amplitude modulation, or both. The method is realized by driving the cantilever with a frequency comb and measuring the response at all frequencies in the comb in a phase coherent way. In comparison with single drive methods the frequency comb method can acquire much more information in the same amount of time. We will demonstrate how this technique allows one to go beyond simple linear approximations, allowing for rapid and accurate reconstruction of the non-liner tip-surface interaction in AFM. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B24.00010: 3D mapping and energy measurement of trap states in inter-layer dielectric films by Dynamic Tunneling Force Microscopy Ruiyao Wang, Sean King, Clayton Williams A novel atomic scale scanning probe microscopy method--Dynamic Tunneling Force Microscopy / Spectroscopy (DTFM/S) [1] has been employed to image the 3D distribution and energy level of individual trap states in inter-layer dielectric (ILD) and other insulating films. DTFM images of several films of different compositions, each around 5 nm thick, show a similar trap state areal density of order of 5x10$^{\mathrm{11}}$/cm$^{\mathrm{2}}$.The energy and depth of each state within tunneling range can be determined by performing the DTFM/S measurements as a function of applied voltage and probe tip height above the surface and a physical tunneling model. Nanometer scale conductance (c-AFM) and trap state imaging (DTFM) have been performed in the same location on one of the films, revealing the correlation between the observed trap states and the conductance of the film. The imaging and energy level measurement results will be presented and discussed. [1] J.P. Johnson, N. Zheng and C.C. Williams, Nanotech. 20, 055701 (2009) [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B24.00011: High-sensitivity SQUIDs with dispersive readout for scanning microscopy J.M. Mol, F. Foroughi, J. Arps, E. Kammerloher, P. Bethke, G.W. Gibson, Jr., Y.K.K. Fung, B. Klopfer, K. Nowack, P.A. Kratz, M.E. Huber, K.A. Moler, J.R. Kirtley, H. Bluhm In a scanning SQUID microscope, the high magnetic flux sensitivity is utilized to image magnetic properties of sample surfaces. As an alternative to the widely used DC SQUIDs, we present Nb SQUIDs for scanning with dispersive microwave readout, featuring significantly higher bandwidth and sensitivity. An on-chip shunt capacitor in parallel with the junction and flux pickup loops forms an LC resonator whose resonance depends on the flux in the SQUID. The readout utilizes a phase-sensitive detection of the reflected drive signal at the SQUID's resonance frequency. Highest sensitivities are achieved by making use of the inherent nonlinearity of the device at high excitation powers. We present a study of the characteristics and noise measurements of our sensors at 4 K. Extrapolations from our results to 300 mK indicate that flux sensitivities as low as 50 n$\Phi_{0}$Hz$^{-1/2}$ could be possible. Using high-resolution lithography, our sensors promise sub-micron spatial resolution. Integrated into a scanning microscope, they will provide a powerful tool for the study of weak magnetic effects and quantum coherent phenomena. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B24.00012: Real-Space Imaging of Molecular Structure by Single-Molecule Inelastic Tunneling Probe Zhumin Han, Chi-lun Chiang, Chen Xu, Wilson Ho The scanning tunneling microscope is one of the most powerful tools to perform real space imaging of the electronic, magnetic, optical, and vibrational signatures of a single molecule. However, the spatial distributions of these signatures do not always relate directly to the geometric structures of the molecules. In this study, a CO molecule is transferred from the surface to a STM tip. The energy and intensity of the hindered translational mode of the CO vary when the tip is scanned across an adsorbed molecule (such as cobalt phthalocyanine). By monitoring these variations in space, we are able to resolve the geometric structure of the molecule and even subtle intramolecular and intermolecular interactions. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B24.00013: Real-space imaging of interfacial water with submolecular resolution Ying Jiang Water/solid interfaces are vital to our daily lives and also a central theme across an incredibly wide range of scientific disciplines. Resolving the internal structure, i.e. the O-H directionality, of water molecules adsorbed on solid surfaces has been one of the key issues of water science yet remains challenging. Using a low-temperature scanning tunneling microscope (STM), we report the submolecular-resolution imaging of individual water monomers and tetramers on NaCl(001) films supported by a Au(111) substrate at 5 K. The frontier molecular orbitals of adsorbed water were directly visualized, which allowed discriminating the orientation of the monomers and the H-bond directionality of the tetramers in real space. Comparison with ab initio density functional theory calculations reveals that the ability to access the orbital structures of water stems from the electronic decoupling effect provided by the NaCl films and the precisely tunable tip-water coupling. [Preview Abstract] |
Session B25: Focus Session: OPV Device Physics and Charge Transport
Sponsoring Units: GERAChair: Dana Olson, National Renewable Energy Laboratory
Room: 503
Monday, March 3, 2014 11:15AM - 11:51AM |
B25.00001: Is there any Exciton (bottleneck) in an Excitonic Solar Cell: Revisiting the Prospects of Single-Semiconductor OPV Invited Speaker: Muhammad Alam The discovery dye sensitized and bulk heterojunction (BHJ) solar cells in early 1990s introduced a new class of PV technology that rely on (i) distributed photogeneration of excitons, (ii) dissociation of excitons into free carriers by the heterojunction between two organic semiconductors (OSC), and (iii) collection of free carriers through electron and hole transport layers. The success of the approach is undisputed: the highest efficiency OPV cells have all relied on variants of BHJ approach. Yet, three concerns related to the use of a pair of OSCs, namely, low Voc, process sensitivity, and reliability, suggest that the technology may never achieve efficiency-variability-reliability metrics comparable to inorganic solar cells. This encourages a reconsideration of the prospects of Single semiconductor OPV (SS-OPV), a system presumably doomed by the exciton bottleneck. In this talk, we use an inverted SS-OPV to demonstrate how the historical SS-OPV experiments may have been misinterpreted. No one disputes the signature of excitons in polymer under narrowband excitation, but our experiments show that exciton dissociation need not be a bottleneck for OPV under broadband solar illumination. We demonstrate that an alternate collection-limited theory consistently interprets the classical and new experiments, resolves puzzles such as efficiency loss with increasing light intensity, and voltage-dependent reverse photo-current, etc. The theory and experiments suggest a new ``perovskite-like'' strategy to efficiency-variability-reliability of organic solar cells.\\[4pt] In collaboration with Biswajit Ray, Purdue University. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B25.00002: Power conversion efficiency enhancement in OPV devices using spin 1/2 molecular additives Tek Basel, Valy Vardeny, Luping Yu We investigated the power conversion efficiency of bulk heterojunction OPV cells based on the low bandgap polymer PTB7, blend with C61-PCBM. We also employed the technique of photo-induced absorption, PA; electrical and magneto-PA (MPA) techniques to understand the details of the photocurrent generation process in this blend. We found that spin 1/2 molecular additives, such as Galvinoxyl (Gxl) radicals dramatically enhance the cell efficiency; we obtained 20{\%} increase in photocurrent upon Gxl doping with 2{\%} weight. We explain our finding by the ability of the spin 1/2 radicals to interfere with the known major loss mechanism in the cell due to recombination of charge transfer exciton at the D-A interface via triplet excitons in the polymer donors. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B25.00003: Effect of Copolymer Chain Architecture on Active Layer Morphology and Device Performance Jojo Amonoo, Anton Li, Matthew Sykes, Bingyuan Huang, Edmund Palermo, Anne McNeil, Max Shtein, Peter Green The optimum morphological structure that determines the device performance of bulk heterojunction thin film polymer solar cells is greatly influenced by the extent of phase separation between the polymer and fullerene components, which ultimately defines the length scales and purity of the donor- and acceptor-rich phases. Block copolymer thin films have been widely studied for their ability to microphase separate into well-defined nanostructures. Nickel-catalyzed chain-growth copolymerizations of thiophene and selenophene derivatives afforded well-defined $\pi $-conjugated copolymers of poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS) to achieve diblock, random and gradient copolymer chain architectures. This allowed us to study the effect of copolymer sequence and nanoscale morphology of P3HT-P3HS copolymer/[6,6]-phenyl-C61-butyric acid methyl ester (PC$_{\mathrm{61}}$BM) on device performance. With the use of energy-filtered transmission electron microscopy and conductive and photoconductive atomic force microscopy we found that copolymer sequence strongly influences the phase separation capabilities of the copolymer-fullerene blend in bulk heterojunction organic photovoltaic devices. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B25.00004: Computational design of co-polymer electron donors for bulk heterojunction photovoltaic solar cells Yongwoo Shin, Jiakai Liu, Xi Lin In this work, our recently developed adapted Su-Schrieffer-Heeger Hamiltonian is used to systematically explore the optoelectronic properties of thousands of pi-conjugated structures. New physical insights on the structure-property relationship are extracted and transformed into practical guiding rules in the donor materials design. For the power-efficient copolymer structures, we find that the energy variation of frontier orbitals can be controlled either independently or collectively, depending on their specific donor or acceptor structures. In particular, we find that having five-membered conjugated carbon rings in the acceptor units is essential to break the electron-hole charge conjugation symmetry, so that the LUMO levels of the copolymer can be reduced dramatically while holding the HOMO energy levels in the donor units constant. On the other hand, by incorporating heteroatoms into the donors units, we can vary the HOMO levels of the copolymers independently. Effects of introducing various side groups (-R, -O, -CO, -COO, and thiophene) on the primitive donor and acceptor structures are investigated and their results are discussed in details. Finally, unexpected localized states are found, for the first time, in our calculations for a few special co-polymer structures. These localized states, with electrons localized on one end of the copolymer chain and holes on the other end, contain large dipole moments and therefore may be treated as a new design dimension when these copolymers are placed in polar and non-polar solvent environments. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B25.00005: Measuring and Modeling Exciton Dynamics in Multichromophore Macromolecules Daniel Weingarten, Nan Hu, Michael LaCount, Andrew Ferguson, Daniel Dessau, David Walba, Jao VanDeLagemaat, Mark Lusk, Garry Rumbles, Sean Shaheen Attaining specific control over the dynamics of exciton movement in organic photovoltaics (OPV) has, thus far, been a largely unachieved goal of OPV design. Such an understanding of exciton transfer dynamics would allow for the design of macromolecules whose energetics, bandgaps, and conformational properties allow for control of exciton flow toward specific reaction site chromophores, potentially enabling non-linear improvements in energy harvesting. To better understand exciton movement we synthesized and characterized a multi-chromophoric macromolecule and measured the dynamics of exciton transfer across coupled chromophores. Our model system is a hexabenzocoronene molecule attached to six oligothiophene. We developed a kinetic model and by fitting it to the decay rates of excited states measured via time-correlated single photon counting, we were able to extract rates for exciton transfer between chromophores. Since this macromolecule exhibits liquid crystalline aggregation behavior, observing the dependence of exciton transfer rates on solution concentration yields an improved understanding of exciton movement within a single molecule as well as the dependence of that transfer process on local material structure. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B25.00006: Design principle for efficient charge separation at the donor-acceptor interface for high performance organic solar cell device Wanyi Nie, Gautam Gupta, Brian Crone, Hsing-lin Wang, Aditya Mohite The performance of donor (D) /acceptor (A) structure based organic electronic devices, such as solar cell, light emitting devices etc., relays on the charge transfer process at the interface dramatically. In organic solar cell, the photo-induced electron-hole pair is tightly bonded and will form a charge transfer (CT) state at the D/A interface after dissociation. There is a large chance for them to recombine through CT state and thus is a major loss that limit the overall performance. Here, we report three different strategies that allow us to completely suppress the exciplex (or charge transfer state) recombination between any D/A system. We observe that the photocurrent increases by 300{\%} and the power conversion efficiency increases by 4-5 times simply by inserting a spacer layer in the form of an a) insulator b) Oliogomer or using a c) heavy atom at the donor-acceptor interface in a P3HT/C60 bilayer device. By using those different functional mono layers, we successfully suppressed the exciplex recombination in evidence of increased photocurrent and open circuit voltage. Moreover, these strategies are applicable universally to any donor-acceptor interface. And we demonstrated such strategies in a bulk-heterojunction device which improved the power conversion efficiency from 3.5{\%} up to 4.6{\%}. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:27PM |
B25.00007: Understanding and Mitigating Recombination Loss in Organic Solar Cells Invited Speaker: David Ginger We study recombination losses in organic photovoltaics occurring at both the donor/acceptor interface within a bulk heterojunction, as well as at the electrode contacts. In the bulk, we discuss how the interplay of energetics and morphology, particularly the size and connectivity of domains, can alter the ratio of free carriers to triplets. Turning to the active layer/electrode interface, we also show how surface fields generated at interfaces can be used to modulate recombination rates, and probe the role of structural heterogeneity in recombination at interfaces. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B25.00008: Small Molecule Acceptors for Organic Photovoltaics David P. Ostrowski, Unsal Koldemir, Alan Sellinger, Sean E. Shaheen Organic photovoltiacs (OPVs) have demonstrated solar power conversion efficiencies in the regime of 10-12{\%} from several classes of materials, including conjugated polymers and small-molecules. Of note, in each of the classes, the electron-accepting molecule is based on C$_{\mathrm{60}}$. While C$_{\mathrm{60\thinspace }}$is a very effective electron-acceptor and transporter, it has low optical absorption strength in the solar spectrum and it is difficult to tune its optoelectronic properties. Here we present results on small molecule acceptors based on a core unit of benzothiodiazole (BT) whose optoelectronic properties are readily tunable. A library of these small molecule acceptors has been synthesized with a variety of absorbance bands in order for OPV technologies to harness a greater amount of the solar spectrum. Through utilization of a range of solvents, co-solvent mixtures and orthogonal solvents, devices are fabricated with contrasting bulk heterojunction (BHJ) morphologies or bi-layer architectures. Device performance is compared over a range of active layer morphologies, with particular emphasis on the effectiveness of photocurrent generation when light is absorbed in the acceptor molecule with subsequent charge (hole) transfer to the donor (channel 2 photocurrent generation). [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B25.00009: The influence of Morphology on Charge Transport Properties in P3HT Alex Dixon, Nikos Kopidakis, Sean Shaheen The field of Organic Photovoltaic (OPV) has been growing quickly, yet there are still several questions about the underlying physics that remain poorly understood. One question is the nature of the relationship between active layer microstructure and charge transport. To investigate this, we fabricated devices using a range of molecular weights (from 13kDa to 331kDa) of poly 3-hexothyophene (P3HT). Varying the molecular weight of P3HT causes the films to exhibit changes in microstructure, with low molecular weights forming a paraffinic-like structure and higher molecular weights forming a semi-crystalline structure. Using the Charge Extraction by Linearly Increasing Voltage (CELIV) technique, we determined the hole mobility and recombination factor for theses devices. We found that the mobility in the devices peaked at 47kDa and the recombination rate decreased with increasing molecular weight. We hypothesize that the decrease in recombination is due to spacial separation of charge carrier in the semi-crystalline regions, with the holes populating the crystalline regions and the electrons populating the amorphous areas. This improves mobility for mid rage molecular weights but defects cause it to dip at higher molecular weights. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B25.00010: Ionically Gated Hybrid Tandem Organic Photovoltaics Alexander Cook, Jonathan Yuen, Joeseph Michelli, Anvar Zakhiov In our work, `Electrochemically gated organic photovoltaics with tunable carbon nanotube electrodes', recently published in Applied Physics Letters, and our previous APS presentation, we demonstrated a hybrid device comprised of an organic photovoltaic (OPV) monolithically attached to a supercapacitor via a common transparent carbon nanotube (CNT) electrode. This structure may also be viewed as an electrochemically gated OPV in which the gate voltage gradually shifts a resistor-like device into a high efficiency photovoltaic. We have extended this concept to an electrochemically-gated, parallel tandem, organic photovoltaic device, which features two photoactive layers in addition to the supercapacitive cell. This device can be produced entirely in ambient conditions via spin-coating and lamination, and avoids many processing difficulties associated with tandem architectures. Additionally, this architecture allows us to perform experiments to better understand the undelaying phenomena in this system such as whether the electrochemical charging in the OPV device extends to the semiconducting photoactive layers or is constrained entirely to the carbon nanotube electrodes. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B25.00011: Study of free carrier recombination in carbon nanotubes photovoltaic materials Meng-Yin Wu, Thomas McDonough, Matthew Shea, Yumin Ye, Martin Zanni, Michael Arnold Semiconducting single-walled carbon nanotubes are promising photoabsorbers for future solar cells and photodetectors. Previously, we have put nanotubes in contact with electronegative accepting semiconductors such as C60-fullerenes, which spontaneously drive photoexcited electron transfer from the nanotubes to the C60. One important part of making efficient CNT/C60 photovoltaics is that the free carrier lifetime has to be longer than the charge collection time. Here, we analyze CNT/C60 planar heterojunction devices for simplicity to study the free carrier recombination lifetime. We use three measurements: photocurrent-voltage characteristics, photovoltage decay following transient optical excitation, and simple charge extraction to determine how charge density and lifetime are interrelated and how they vary under different illumination intensities. These dependencies also allow us to uncover recombination mechanisms. The results showed the free carrier lifetime decays from 130 us to 280 ns as the free carrier density increases from 3.4E13 cm$^{\mathrm{-3}}$ to 1.3E17 cm$^{\mathrm{-3}}$, with a power law dependence. These measurements of free carrier lifetime and concentration will help us in future investigations and modeling of the C60-nanotube heterointerface. [Preview Abstract] |
Session B26: Materials at High Pressure: Phase Transitions
Sponsoring Units: DCOMP DMP GSCCMChair: Li Zhang, Carnegie Institution
Room: 502
Monday, March 3, 2014 11:15AM - 11:27AM |
B26.00001: Pre-compression for Dynamic Gas Gun Loading Christopher Seagle Equation of state properties for materials off the principal Hugoniot and isentrope are currently poorly constrained. The ability to directly probe regions of phase space between the Hugoniot and isentrope under dynamic loading will greatly improve our ability to constrain equation of state properties under a variety of conditions and study otherwise inaccessible phase transitions. Large diameter ($>$4 mm) samples are typically required for dynamic loading on gas guns. We are developing the ability to pre-compress large diameter samples to 1+ GPa. Compressible materials (such as liquids and gases) possess a significantly stiffer pre-compressed Hugoniot, allowing access to lower temperature states on shock compression. Challenges and initial static and dynamic testing results of the pre-compression hardware will be discussed. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B26.00002: High-pressure phases of alumina Matthew Lyle, Chris Pickard, Richard Needs Alumina (Al2O3) has been widely used as a pressure standard in static diamond anvil cell experiments and is a major chemical component of the Earth. So a detailed knowledge of its high-pressure stability is of great importance in both materials science and deep Earth science. A phase transition is known to occur at roughly 80-100 GPa between corundum and the Rh2O3 (II) structure. A second phase transition to the CaIrO3 structure occurs at even higher pressures. Here we present a computational structure search to reveal three additional structures which are competitive at these pressures but hitherto unknown to be stable in high-pressure alumina. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B26.00003: Design and Fabrication of RF/CRF Aerogel Flier-Plates with Graded Density For Laser-Driven Quasi-Isentropic Compression Experiments Yang Shen, Bin Zhou, Ai Du, Xiuguang Huang, William Halperin Resorcinol Formaldehyde (RF)/Carbonized Resorcinol Formaldehyde (CRF) aerogel flier-plates with graded density were designed and fabricated via simple and effective approaches, for increasing the peak pressure and shaping the compression profile in Laser-driven quasi-isentropic compression (ICE) experiments. Sol-gel technique and flexible micro-mould were involved in launching density gradients into aerogel. Resorcinol (R)-formaldehyde (F)-water system catalyzed by sodium carbonate (C) was employed to provide organic RF aerogel, various of sol with different recipes were cast into the mould layer by layer; A carbon dioxide (CO$_{2})$ supercritical fluid drying (SCFD) process and a four-step pyrolysis process were applied to convert RF hydrogel into RF aerogel, RF aerogel into CRF aerogel, respectively. After a four-step pyrolysis process, RF aerogel was converted to CRF aerogel. The strategies were demonstrated to be simple and effective in launching density gradients into RF/CRF aerogel flyer-plates. It was found in the Laser-Driven Quasi-Isentropic compression experiments of Al that the rise time of the ramp compression wave was about 50{\%} longer for the graded density RF aerogel case. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B26.00004: Laser Driven, Extreme Compression Science Invited Speaker: Jon Eggert Extreme-compression science is blessed by a number of new techniques and facilities that are shattering previous experimental limitations: static pressures above 600 GPa, equation of state (EOS) experiments on pulsed-power machines, picosecond-resolved x-ray diffraction on free-electron lasers, and many new experiments on high-energy lasers. Our goals, using high-energy lasers, have been to push the limits of high pressure accessible to measurement and to bridge the gap between static- and dynamic-compression experiments by exploring off-Hugoniot states. I will review laser techniques for both shock- and ramp-compression experiments, and discuss a variety of diagnostics. I will present recent results including: impedance-matching Hugoniot experiments, absolute-Hugoniot implosive-shock radiography, coupled radiometry and velocimetry, ramp-compression EOS, and in-situ x-ray diffraction and absorption spectroscopy into the TPa regime. As the National Ignition Facility (NIF) transitions to a laser user facility for basic and applied science, we are transferring many of these techniques. The unprecedented quality and variety of diagnostics available, coupled with exquisite pulse-shaping predictability and control make the NIF a premier facility for extreme-compression experiments. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B26.00005: Quantum Monte Carlo simulations of a single iron impurity in MgO Kevin Driver, Shuai Zhang, Burkhard Militzer, R.E. Cohen Ferropericlase [(Mg,Fe)O] is the second most abundant mineral in Earth's lower mantle. A high-spin to low-spin transition in Fe2+ that occurs in the middle of the lower mantle has been observed in diamond anvil experiments and confirmed within density functional theory (DFT). The spin transition has significant influence on the physical properties and behavior of the lower mantle. However, details on the mechanism of spin transition are still being understood in both experiment and DFT [1]. Here, we aim to benchmark the high-spin to low-spin transition of a single iron atom impurity in MgO using quantum Monte Carlo (QMC). High-spin and low-spin equations of state are initially computed using density functional theory within the LDA+U approximation, which provide trial Slater-Jastrow wave functions for QMC. Equations of state are then computed with variational and diffusion Monte Carlo in 8- and 64-atom cells using the QMCPACK code.[2]. QMC results are in general agreement with experiment and DFT studies.\\[4pt] [1] J.-F. Lin, S. Speziale, Z. Mao, and H. Marquardt, Rev. Geophys., 51, 244 (2013).\\[0pt] [2] http://qmcpack.cmscc.org/ [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B26.00006: Quantum Monte Carlo Calculation for the Equation of State of MgSiO$_{3}$ perovskite at high pressures Yangzheng Lin, R.E. Cohen, Kevin P. Driver, Burkhard Militzer, Luke Shulenburger, Jeongnim Kim Magnesium silicate (MgSiO$_{3})$ is among the most abundant minerals in the Earth's mantle. Its phase behavior under high pressure has important implications for the physical properties of deep Earth and the core-mantle boundary. A number of experiments and density functional theory calculations have studied perovskite and its transition to the post-perovskite phase. Here, we present our initial work on the equation of state of perovskite at pressures up to 200 GPa using quantum Monte Carlo (QMC), a benchmark ab initio method. Our QMC calculations optimize electron correlation by using a Slater-Jastrow type wave function~with a single determinant comprised of single-particle orbitals extracted from fully converged DFT calculations.~The equation of state obtained from QMC calculations agrees with experimental data. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B26.00007: Thermally induced velocity variations of ferropericlase in Earth's lower mantle Renata Wentzcovitch Understanding the origin of lateral velocity heterogeneities in the mantle is crucial to understand the constitution of and internal processes at work in the Earth. The spin crossover in iron in ferropericlase (Fp) and the unusual and well documented elastic anomalies introduce unfamiliar effects on seismic velocities. In this work we investigate by first principles calculations potential velocity anomalies caused by lateral temperature variations in the presence of a spin crossover in Fp under mantle conditions. Anti-correlation between shear velocity (V$_{\mathrm{S}})$ and bulk sound velocity (V$_{\mathrm{\phi }})$ in the mantle has long been viewed as an indicator of compositional or mineralogical heterogeneity. This view is not entirely justified in the presence of spin crossover in ferropericlase. We also identify new effects that exist in the presence of a spin crossover. Signatures of these effects appear to exist in the lower mantle. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:39PM |
B26.00008: Design principles and high-pressure syntheses of novel superhard materials Invited Speaker: Yongjun Tian The development of novel high-performance superhard materials, guided by reliable design theories, is highly anticipated for continuous progresses in processing techniques. In the past decade, we have established the hardness model for polar covalent single crystals, and revealed an extra hardening mechanism for polycrystalline materials, which shows a hardness$-$microstructural size correlation and provides further hardening at deep nanoscale due to quantum confinement effect. Therefore, nanostructuring diamond and cBN is still an effective way to enhance hardness. Based on our model, we estimate the hardness upper limits for diamond and cBN with nanograined and nanotwinned microstructures, respectively. Transformed from graphite-like carbon and BN precursors at high pressure and high temperature (HPHT), nanograined diamond and cBN with the smallest grain size of $\sim$15 nm can be synthesized, showing enhanced hardness but reduced thermal stability. Starting from onion-like BN and carbon, we successfully synthesized nanotwinned cBN and diamond with average twin thickness of 5 nm or below at HPHT. The simultaneous enhancements in hardness, fracture toughness, and thermal stability were confirmed in our nanotwinned cBN and diamond. Our approach offers a general pathway to nano-structure superhard materials for excellent stability and ultrahardness, as well as exceptional tradeoff between hardness and toughness, through the creation of nanotwinned microstructure. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B26.00009: Structures of xenon oxides at high pressures Nicholas Worth, Chris Pickard, Richard Needs, Agnes Dewaele, Paul Loubeyre, Mohamed Mezouar For many years, it was believed that noble gases such as xenon were entirely inert. It was only in 1962 that Bartlett first synthesized a compound of xenon. Since then, a number of other xenon compounds, including oxides, have been synthesized. Xenon oxides are unstable under ambient conditions but have been predicted to stabilize under high pressure. Here we present the results of a combined theoretical and experimental study of xenon oxides at pressures of 80-100 GPa. We have synthesized new xenon oxides at these pressures and they have been characterized with X-ray diffraction and Raman spectroscopy. Calculations were performed with a density-functional theory framework. We have used the \textit{ab-initio} random structure searching (AIRSS) method together with a data-mining technique to determine the stable compounds in the xenon-oxygen system in this pressure range. We have calculated structural and optical properties of these phases, and a good match between theoretical and experimental results has been obtained. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B26.00010: The $\varepsilon $-$\eta $' transition in solid oxygen at high pressure Sabri Elatresh, Vahid Askarpour, Stanimir Bonev Despite extensive theoretical and experimental studies, the stability of solid oxygen at high pressure remains an open question. The exact structure of $\varepsilon $-O was only recently defined experimentally as consisting of O$_{8}$ units. More recent measurements have indicated that a new phase, $\eta $'-O, may be stable above $\varepsilon $-O in the pressure range from 44 to 90 GPa and at temperatures near 1000 K. However, the supporting experimental evidence is not conclusive. In this work, we study the phase diagram of solid oxygen up to 120 GPa and 1200 K from first principles. Full free energy calculations with hybrid exchange functionals have been performed to establish the mechanical and thermodynamic stability of $\eta $'-O. This structure has unusual dynamical properties, which will be discussed. Analysis of the low-temperature stability of $\varepsilon $-O to resolves inconstancies between previous experimental and theoretical results will be presented as well. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B26.00011: Pressure-induced structural transformations in nanomaterials: a linear-scaling DFT investigation Niccolo Corsini, Peter Haynes, Carla Molteni, Nicholas Hine Semiconductor nanomaterials display a number of peculiar and tunable properties that distinguish them from their bulk counterparts. Of particular interest is their response to applied pressure, as they transform from one crystalline or amorphous structure to another. Accurate simulations are important for understanding finite size effects in the atomistic mechanisms of phase transformations (difficult to observe clearly in macroscopic experiments), for the opportunity to uncover novel metastable phases stabilized in finite systems, and for potentially innovative applications of nanomaterials. First-principles methods are essential to accurately describe the bond breaking/making in phase transformations and the realistic description of surfaces (often covered by complex surfactants). However the computational cost limits both the length- and time-scales attainable. We have combined an O(N) density functional theory code for large systems and an electronic-enthalpy method[1] to apply pressure to finite systems to model with quantum mechanical precision processes induced by pressure in nanomaterials under realistic conditions. The focus is on Si, Ge and CdS nanocrystals that are currently favoured for technological applications. [1] Corsini et al, J. Chem. Phys. 139,084117 (2013) [Preview Abstract] |
Session B27: Focus Session: Petascale Science and Beyond: Applications and Opportunities for Materials Science I
Sponsoring Units: DCOMPChair: Jack Wells, Oak Ridge National Laboratory
Room: 501
Monday, March 3, 2014 11:15AM - 11:51AM |
B27.00001: Condensed Matter Applications of Quantum Monte Carlo at the Petascale Invited Speaker: David Ceperley Applications of the Quantum Monte Carlo method have a number of advantages allowing them to be useful for high performance computation. The method scales well in particle number, can treat complex systems with weak or strong correlation including disordered systems, and large thermal and zero point effects of the nuclei. The methods are adaptable to a variety of computer architectures and have multiple parallelization strategies. Most errors are under control so that increases in computer resources allow a systematic increase in accuracy. We will discuss a number of recent applications of Quantum Monte Carlo including dense hydrogen and transition metal systems and suggest future directions. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B27.00002: Peta-scale QMC simulations on DOE leadership computing facilities Jeongnim Kim Continuum quantum Monte Carlo (QMC) has proved to be an invaluable tool for predicting the properties of matter from fundamental principles. Even with numerous innovations in methods, algorithms and codes,~\footnote{Kim et al., J. Phys.: Conf. Ser., {\bf 402} 012008 (2012).} QMC simulations of realistic problems of 1000s and more electrons are demanding, requiring millions of core hours to achieve the target chemical accuracy. The multiple forms of parallelism afforded by QMC algorithms and high compute-to-communication ratio make them ideal candidates for acceleration in the multi/many-core paradigm. We have ported and tuned QMCPACK to recently deployed DOE doca-petaflop systems, Titan (Cray XK7 CPU/GPGPU) and Mira (IBM Blue Gene/Q). The efficiency gains through improved algorithms and architecture-specific tuning and, most importantly, the vast increase in computing powers have opened up opportunities to apply QMC at unprecedent scales, accuracy and time-to-solution. We present large-scale QMC simulations to study energetics of layered materials where vdW interactions play critical roles. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B27.00003: Quantum Monte Carlo simulations on Blue Gene/Q using QMCPACK: Performance and Applications Anouar Benali, Luke Shulenburger, Nichols A. Romero, Jeongnim Kim Quantum Monte Carlo (QMC) is the most accurate many-body method for computing ground-state properties in condensed-phase systems. QMC uses a stochastic sampling method to solve the many-body Schr\"{o}dinger equation. The advent of petascale supercomputing facilities and massively concurrent QMC algorithms has allowed us to study materials at unprecedented levels of accuracy. We will present the implementation and optimization of the QMCPACK [1-2] simulation package on the IBM Blue Gene/Q as well as results for a number systems including: van der Waals-dominated materials, transition metals and biological molecules. \\[4pt] [1] K. Esler, J. Kim, L. Shulenburger, and D. Ceperley, Computing in Science and Engineering 14, 40 (2012).\\[0pt] [2] J. Kim et al., Journal of Physics: Conference Series 402, 012008 (2012). [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B27.00004: Diffusion Monte Carlo characterization of a methane molecule in a (H$_{2}$O)$_{20}$ dodecahedral cage Kenneth Jordan, Michael Deible The diffusion Monte Carlo method is used to investigate the interaction of a water molecule with a dodecahedral (H$_{2}$O)$_{20}$ cage as found in the methane hydrate crystal. The DMC value of the interaction energy between the methane molecule and the cage are compared with the results of MP2 and symmetry-adapted perturbation theory (SAPT) calculations [1]. In addition, the net interaction energy is decomposed into two- and three-, and $n \ge$ four-body contributions. The two- and three-body contributions are further analyzed in terms of SAPT calculations [1,2]. \\[4pt] [1] A. J. Misquitta, R. Podeszwa, B. Jeziorski, and K. Szalewicz, J. Chem. Phys. \textbf{123}, 214103 (2005); A. Hesselmann, G. Jansen, and M. Sch\"{u}tz, J. Chem. Phys., \textbf{122}, 014103 (2005).\\[0pt] [2] R. Podeszwa and K. Szalewicz, J. Chem. Phys., \textbf{126}, 194101 (2007). [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B27.00005: Quantum Monte Carlo for Materials Design Tim Mueller, Lucas Wagner, Jeffrey Grossman The accurate calculation of formation energies is critical to evaluating the stability and chemical reactivity of newly designed materials. Comprehensive databases of formation energies can be used to screen materials for stability before they have been synthesized, but the reliability of such databases depends on the accuracy of the data they contain. Quantum Monte Carlo (QMC) is a highly accurate method that can calculate the formation energies a wide variety of chemical substances, including molecules and metals. The wide applicability of QMC calculations is made possible in part by the fact that the cost of a QMC calculation scales roughly linearly with system size when calculating energies per atom. The accuracy of QMC comes at significant computational cost, but it scales nearly linearly with the number of processors up to a large numbers of processing cores, making it well-suited for large, highly parallel computers. As the initial step towards developing a database of accurate formation energies calculated using QMC, we demonstrate how automated QMC calculations can be used to accurately calculate the formation energies of a variety of different materials. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B27.00006: Detecting phase-transitions in electronic lattice-models with DCA$^+$ Peter Staar, Thomas Maier, Thomas Schulthess The DCA$^+$ algortihm was recently introduced to extend the dynamic cluster approximation (DCA) by introducing a self-energy with continuous momentum dependence. This removes artificial long-range correlations and thereby reduces the fermion sign problem as well as cluster shape dependencies. Here, we extend the DCA$^+$ algorithm to the calculation of two-particle quantities by introducing irreducible vertex functions with continuous momentum dependence compatible with the DCA$^+$ self-energy. This enables the study of phase transitions within the DCA$^+$ framework in a much more controlled fashion than with the DCA. We validate the new method using a calculation of the superconducting transition temperature $T_c$ in the attractive Hubbard model by reproducing previous high-precision finite size quantum Monte Carlo results. We then calculate $T_c$ in the doped repulsive Hubbard model, for which previous DCA calculations could only access the weak-coupling ($U=4t$) regime for large clusters. We show that the new algorithm provides access to much larger clusters and thus asymptotic converged results for $T_c$ for both the weak ($U=4t$) and intermediate ($U=7t$) coupling regimes, and thereby enables the accurate determination of the exact infinite cluster size result. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B27.00007: Implementation of continuous-time QMC impurity solver for Dynamical Mean Field Theory Mancheon Han, Choong-Ki Lee, Hyoung Joon Choi Dynamical mean field theory maps an interacting lattice problem to an interacting impurity problem in non-interacting bath. Continuoustime Quantum Monte Carlo (CT-QMC) method is numerically exact way to obtain a solution for such an impurity problem. We developed hybridization-expansion CT-QMC (CT-HYB) impurity solver and tested its validity by studying infinite neighbor Bethe lattice which has semicircular density of states and only local Hubbard interaction. This work is supported by the NRF of Korea (Grant No. 2011-0018306). Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2013-C3-008). [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B27.00008: Quantum Chemistry on Petascale Machines: Done! What's Next? Edoardo Apra We will illustrate some recent application of parallel code NWChem on Petascale hardware related to material science topics. For example, we will report of the use of embedded cluster approach to model the ground state and excited state properties of crystalline compounds. The methods analyzed in the talk will range from Density-Functional based to wave-function based (e.g. Coupled Cluster). In the second half of the talk we will describe on-going software and algorithmic developments geared towards exploiting the aggregate resources available in upcoming 100 petaflops architectures. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B27.00009: Including short length scale correlations in quantum chemistry methods K. Bhaskaran-Nair, K. Kowalski, J. Moreno, W. Shelton, M. Jarrell Many aspects of computational chemistry and computational material science require accuracies that can only be obtained by a small class of highly accurate computational methods that appropriately account for instantaneous interactions between electrons in molecules or in materials. To aid in addressing the sign problem associated with DMFT based methods we use accurate quantum chemistry methods to treat short length scale correlations within DMFT type formulations and its cluster extensions. The local Green function is obtained from truncated variants of Configuration Interaction and Coupled Cluster methods, which efficiently describe the electron correlation effects. This work is supported by the National Science Foundation award NSF EPS-1003897 with additional support from the Louisiana Board of Regents. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B27.00010: Some lessons learned on the simulation of atomic-scale stochastic processes in carbon systems Vincent Meunier, Colin Daniels, Zachary Bullard The behaviors of many materials are rooted in stochastic processes due to spatial and temporal fluctuations in their nano- and micro- structures. This talk will be the opportunity to present preliminary results on attempts to shed light on the role played by disorder on the dynamical appearance of atomic-scale defects and how these build their way up to mesoscopic length scales and over macroscopic time scales. I will present a simple algorithm that allows translating atomic level properties into scales relevant to devices and materials systems. The algorithm enables the random introduction of elementary mutations in low-dimensional systems and leads to the investigation of the emergence of structures with new functionality and to novel nanostructures resulting from the coalescence of elementary building blocks. The mutations are introduced by local modifications to the connectivity table and are accepted based on a Metropolis algorithm. Externally imposed constraints can be introduced as needed, depending on the actual conditions to be simulated. In addition, the fast prototyping of the effect of mutations on electronic properties is made possible by the ability to enact mutations as perturbation potentials using Dyson equation to update Green functions as mutations are accepted. Results applied to the coalescence, annealing, and phase separation in a number of carbon nanostructures will be shown and compared to experiments when available. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B27.00011: Parallel in time simulations using high level quantum chemistry methods and complex empirical potentials Eric Bylaska, Jonathan Weare, John Weare Algorithms that support parallel decomposition in the time dimension are presented and applied to conventional molecular dynamics (MD) models and {\em ab initio} molecular dynamics (AIMD) models of realistic complexity. The algorithms support convenient parallel implementation to achieve significant improvement in the simulation of high level (e.g. MP2) and excited state dynamics. Given a forward time integrator propagating the system from time $t_i$ (trajectory position and velocity $\mathbf{x}_i =(\mathbf{r}_i,\mathbf{v}_i)$) to $t_{i+1}$ as $\mathbf{x}_{i+1} = \mathbf{f}_i(\mathbf{x}_i)$, the dynamics problem is transformed into a root finding problem, $\mathbf{F} = \mathbf{[x_{i}-f(x_{(i-1})]_{i}}=\mathbf{0}$, for the trajectory variables. The fixed point problem is unconditionally convergent and is solved iteratively using a variety of optimization techniques, including quasi-Newton and preconditioned quasi-Newton methods. The algorithm is parallelized by assigning a processor to each time-step entry in the columns of $\mathbf{F(\mathbf{X})}$. Less accurate but more efficient dynamical models based on simplified interactions or coarsening time-steps provide preconditioners for the root finding problem and lead to an algorithm similar to the parareal algorithm. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B27.00012: Computational Science at the Argonne Leadership Computing Facility Nichols Romero The goal of the Argonne Leadership Computing Facility (ALCF) is to extend the frontiers of science by solving problems that require innovative approaches and the largest-scale computing systems. ALCF's most powerful computer -- Mira, an IBM Blue Gene/Q system -- has nearly one million cores. How does one program such systems? What software tools are available? Which scientific and engineering applications are able to utilize such levels of parallelism? This talk will address these questions and describe a sampling of projects that are using ALCF systems in their research, including ones in nanoscience, materials science, and chemistry. Finally, the ways to gain access to ALCF resources will be presented. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B27.00013: Computational Science with the Titan Supercomputer: Early Outcomes and Lessons Learned Jack Wells Modeling and simulation with petascale computing has supercharged the process of innovation and understanding, dramatically accelerating time-to-insight and time-to-discovery. This presentation will focus on early outcomes from the Titan supercomputer at the Oak Ridge National Laboratory. ~Titan has over 18,000 hybrid compute nodes consisting of both CPUs and GPUs. In this presentation, I will discuss the lessons we have learned in deploying Titan and preparing applications to move from conventional CPU architectures to a hybrid machine. I will present early results of materials applications running on Titan and the implications for the research community as we prepare for exascale supercomputer in the next decade. Lastly, I will provide an overview of user programs at the Oak Ridge Leadership Computing Facility with specific information how researchers may apply for allocations of computing resources. [Preview Abstract] |
Session B28: Focus Session: Superconducting Qubits: Error Correction & Validation Methods
Sponsoring Units: GQIChair: Alexandre Blais, Sherbrooke University
Room: 601
Monday, March 3, 2014 11:15AM - 11:51AM |
B28.00001: Controlling Schr\"{o}dinger cat states using qubit-photon entanglement Invited Speaker: Brian Vlastakis With a toolset of conditional qubit-photon logic, we manipulate the quantum state of a cavity resonator. We map a quantum bit to a superposition of coherent states, also known as a Schr\"{o}dinger cat state. We achieve this using a superconducting transmon qubit coupled to a microwave waveguide cavity with an ideal off-resonant coupling. This dispersive interaction is much greater than decoherence rates and higher-order nonlinearities which allows simultaneous control of over one hundred photons. We extend this protocol to create superpositions of up to four coherent states. Furthermore, we explore the conditional logic used in this procedure by performing state tomography on the joint qubit-cavity system. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B28.00002: Parity check operation in a surface code plaquette segment with superconducting qubits Jerry Chow, Jay Gambetta, Srikanth Srinivasan, Easwar Magesan, Andrew Cross, David Abraham, Nicholas Masluk, Blake Johnson, Colm Ryan, Christopher Lirakis, Matthias Steffen An essential part of the two-dimensional surface code is the ability to perform $X$ and $Z$-stabilizer parity checks of code qubits via the measurement of ancilla qubits. We benchmark a complete set of high-fidelity single- and two-qubit gates on a three-qubit sub-section of a surface code layout comprised of superconducting resonators and transmons. Combining these gates with high-fidelity individual single-shot readouts, we show a parity check operation, deterministically entangling two qubits which are non-nearest neighbors. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B28.00003: Demonstrating a four-qubit network for the surface code with superconducting qubits Srikanth Srinivasan, Easwar Magesan, Jerry Chow, Jay Gambetta, Andrew Cross, Nicholas Masluk, David Abraham, Nicholas Bronn, Christopher Lirakis, Matthias Steffen In the skew-symmetric layout of superconducting qubits and resonators for the surface-code error correction protocol, studying an inner ring structure of four qubits is a critical step towards demonstrating the core operations of a full plaquette tile. We show results for quantum devices consisting of twelve quantum degrees of freedom: four transmon qubits, coupled via four bus resonators, with four independent readout resonators. We discuss engineering challenges of such devices as well as benchmarked results for control and readout. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B28.00004: Tracking Quantum Jumps of Light with Repeated Single-Shot Parity Measurements Luyan Sun, Andrei Petrenko, Zaki Leghtas, Brian Vlastakis, Gerhard Kirchmair, Katrina Sliwa, Anirudh Narla, Michael Hatridge, Shyam Shankar, Jacob Blumoff, Luigi Frunzio, Mazyar Mirrahimi, Michel Devoret, Robert Schoelkopf Quantum error correction (QEC) is required for a practical quantum computer because of the fragile nature of quantum information. A measurement-based QEC requires the measurement of error syndromes in a quantum non-demolition way and at a rate which is faster than errors occur. In a 3D circuit quantum electrodynamics architecture, we realize a parity measurement of a microwave field with about 90{\%} fidelity by mapping its parity onto an ancilla qubit. The projective nature of the parity measurement onto a degenerate parity eigenspace, the cat states, is confirmed by Wigner tomography after a single parity measurement, showing 84{\%} fidelity to ideal cats. The parity can therefore serve as an error syndrome for a recently proposed QEC scheme [Leghtas et.al. PRL (2013)]. We then demonstrate a tracking of quantum jumps of this error syndrome by repeated parity measurements. We will also discuss a quantum filter developed to mitigate the imperfections during the parity measurement for a best estimate of the photon state parity. The demonstrated extraction of error syndromes without perturbing the encoded information is essential for QEC. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B28.00005: Quantum Non-Demolition Singleshot Parity Measurements for a Proposed Quantum Error Correction Scheme Andrei Petrenko, Luyan Sun, Zaki Leghtas, Brian Vlastakis, Gerhard Kirchmair, Katrina Sliwa, Anirudh Narla, Michael Hatridge, Shyam Shankar, Jacob Blumoff, Luigi Frunzio, Mazyar Mirrahimi, Michel Devoret, Robert Schoelkopf In order to be effective, a quantum error correction scheme(QEC) requires measurements of an error syndrome to be Quantum Non-Demolition (QND) and fast compared to the rate at which errors occur. Employing a superconducting circuit QED architecture, the parity of a superposition of coherent states in a cavity, or cat states, is the error syndrome for a recently proposed QEC scheme. We demonstrate the tracking of parity of cat states in a cavity and observe individual jumps of party in real-time with singleshot measurements that are much faster than the lifetime of the cavity. The projective nature of these measurements is evident when inspecting individual singleshot traces, yet when averaging the traces as an ensemble the average parity decays as predicted for a coherent state. We find our protocol to be 99.8\% QND per measurement, and our sensitivity to parity jumps to be very high at 96\% for an average photon number $\bar{n}=1$ in the cavity (85\% for $\bar{n} = 4$). Such levels of performance can already increase the lifetime of a quantum bit of information, and thereby present a promising step towards realizing a viable QEC scheme. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B28.00006: Deterministic entanglement of two transmon qubits by parity measurement and digital feedback Diego Rist\`e, Marcin Dukalski, Christopher Watson, Gijs de Lange, Marijn Tiggelman, Yaroslav Blanter, Konrad Lehnert, Raymond Schouten, Leonardo DiCarlo While quantum measurement typically collapses quantum superpositions into a basis state, a special type of joint measurement, detecting the parity of qubit excitations, can create entanglement. Building on recent developments in quantum nondemolition measurement and feedback control in circuit QED, we realize a continuous-time parity meter for two 3D-transmon qubits using a dispersively coupled cavity and Josephson parametric amplification. Starting from a maximal superposition, we first generate entanglement with up to $88\%$ fidelity to the closest Bell state by postselecting on the odd-parity result. The infidelity is due to measurement-induced dephasing, arising from imperfect cavity resonance matching in the odd-parity subspace and finite transmission in the even. We then incorporate the parity meter into a digital qubit feedback loop to turn the generation of entanglement from probabilistic to deterministic, achieving $66\%$ fidelity to the targeted Bell state. This combination of parity measurement and conditional qubit control is at the basis of modern error correction protocols. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B28.00007: Understanding the effects of leakage in superconducting quantum error detection circuits Joydip Ghosh, Austin Fowler, John Martinis, Michael Geller The majority of quantum error detection and correction protocols assume that the population in a qubit does not leak outside of its computational subspace. For many existing approaches, however, the physical qubits do possess more than two energy levels and consequently are prone to such leakage events. Analyzing the effects of leakage is therefore essential to devise optimal protocols for quantum gates, measurement, and error correction. In this talk, I discuss the role of leakage in a two-qubit superconducting quantum error detection circuit. We simulate the repeated ancilla-assisted measurement of a single $\sigma^{z}$ operator for a data qubit, record the outcome at the end of each measurement cycle, and explore the signature of leakage events in the obtained readout statistics. An analytic model is also developed that closely approximates the results of our numerical simulations. We find that leakage leads to destructive features in the quantum error detection scheme, making additional hardware and software protocols necessary. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B28.00008: Verification methods for surface code implementations in superconducting systems Easwar Magesan, Jay M. Gambetta, Jerry M. Chow, Srikanth J. Srinivasan, Andrew W. Cross, David W. Abraham, Nicholas A. Masluk, Matthias Steffen, Chris Lirakis The surface code is a promising error-correction protocol for realizing large-scale quantum computation in superconducting qubit systems. Multi-qubit states and processes have recently been implemented with high enough fidelities in these systems to realize small building blocks of the surface code. We will discuss various metrics and tomographic protocols that can be used to characterize the performance of these building blocks and present recent experimental results that demonstrate high fidelity implementations in superconducting systems. Moving forward, these techniques will be useful for characterizing more complex surface code implementations, as well as more general error-correction strategies. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B28.00009: Reducing the impact of intrinsic dissipation in a superconducting circuit by quantum error detection Youpeng Zhong, Zongli Wang, Haohua Wang, John M. Martinis, Andrew N. Cleland, Alexander N. Korotkov A fundamental challenge for quantum information processing is reducing the impact of environmentally-induced errors. Quantum error detection and rejection (QEDR) provides one approach to handling such errors, in which errors are rejected when they are detected. Here we demonstrate a QEDR protocol based on the idea of quantum un-collapsing, using this protocol to suppress energy relaxation due to the environment in a three-qubit superconducting circuit. We encode quantum information in a target qubit, and use the other two qubits to detect and reject errors caused by energy relaxation. This protocol improves the storage time of a quantum state by a factor of roughly three, at the cost of a reduced probability of success. This constitutes the first demonstration of the extension of the effective lifetime of a quantum state using a quantum protocol. Using a similar protocol and a four-qubit superconducting circuit, we further demonstrate the protection of Bell-state entanglement against energy relaxation. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B28.00010: Tomography of microwave states based on parametric interactions Manuel Castellanos-Beltran, Michael Defeo, Adam Sirois, Leonardo Ranzani, Florent Lecocq, Raymond Simmonds, John Teufel, Jose Aumentado Due to recent innovations, Josephson junction-based superconducting circuits have emerged as a platform for performing quantum optics experiments in the microwave regime. These circuits have given us the ability to manipulate the quantum states of microwave light fields in ways that have only been possible in theory. One crucial step in our efforts to build a flexible platform for implementing these experiments in superconducting circuits is the development of a protocol for efficient measurement of the state of light in our systems. In recent years, several approaches have been developed in order to accomplish this, either by performing tomography of states within the cavity or on the itinerant photons escaping from it. In this talk, I will discuss our progress toward the goal of efficient measurement of the quantum state of light, in particular, as part of our goal to implement an ``on-chip'' optical table which utilizes parametric interactions for state preparation and measurement. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B28.00011: Characterizing measurement and feedback processes in superconducting qubit systems N. Ofek, Y. Liu, M. Hatridge, S. Shankar, M.H. Devoret, R.J. Schoelkopf New strategies for quantum control have been enabled by integrating nearly quantum-limited amplifiers with long-lived superconducting qubits. We now record high fidelity~single shot measurements that are also QND. We rely on these properties of our measurement to apply an active feedback on a quantum system. Understanding the degree to which they are QND is desirable. For example, if measurements are perfectly QND yet have finite fidelity, repeated measurements can improve the overall fidelity. In this talk, we present a formalism to quantify a number of important independent measurement parameters including fidelity and the QND degree. We then apply this formalism to characterize and optimize a feedback experiment. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B28.00012: Implementing fast sideband-modulated ``wah-wah'' pulses for driving transmon qubits with tight frequency separation V. Vesterinen, O.-P. Saira, A. Bruno, D.J. Egger, F.K. Wilhelm, L. DiCarlo Packing multiple transmon qubits in a narrow frequency band is challenging due to the limited transmon anharmonicity: control drives targeting one qubit may drive the leakage transition of another. This cross-driving effect grows with decreasing gate time, potentially imposing a quantum speed limit. The widely used DRAG (derivative removal by adiabatic gate) technique only suppresses leakage in the targeted transmon. Adding a modulation tone to a Gaussian pulse envelope in one quadrature, and complementing with DRAG in the other, has been predicted [1] to reduce both intrinsic and cross-driving leakage. We have experimentally verified the performance of this new pulse-shaping method, termed ``wah-wah,'' with two transmons in a 2D circuit QED architecture. We optimize the modulation frequency and amplitude, and characterize the gate fidelity using randomized benchmarking (RB) and quantum process tomography. Pulses on the two qubits are characterized separately and simultaneously by interleaving the RB sequences. Wah-wah pulses show decoherence-limited fidelity at gate speeds where DRAG pulses add significant error.\\[4pt] [1] R. Schutjens \emph{et al.} arXiv:1306.2279 [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B28.00013: Improving Fidelity in Superconducting Xmon Qubits: Decreasing 1/f Flux Noise Peter O'Malley, Rami Barends, Ben Chiaro, Yu Chen, Evan Jeffrey, Julian Kelley, Anthony Megrant, Josh Mutus, Charles Neill, Pedram Roushan, Daniel Sank, James Wenner, Theodore White, Andrew Cleland, John Martinis Two qubit CZ gate fidelity in our superconducting Xmon qubits is currently 99.4$\%$. To achieve 99.9$\%$ fidelity, experiments indicate that we need to reduce 1/f flux noise. We present measurements of 1/f flux noise on the Xmon from sub-Hz to MHz frequencies. At low frequencies we measure an f$^{-1.0}$ frequency dependence, which is in agreement with previous phase qubit measurements but significantly different from the f$^{-0.7}$ dependence seen in SQUIDs. We also see a dependence on geometry that agrees with a theory of magnetic defects; this points toward a qubit design that will minimize dephasing. [Preview Abstract] |
Session B29: Graphene Functionalization and Adsorbates
Sponsoring Units: DCMPChair: Dan Dougherty, North Carolina State University
Room: 603
Monday, March 3, 2014 11:15AM - 11:27AM |
B29.00001: Graphene functionalization with nitrogen and oxygen: controlled modification of the electronic properties Peter Brommer, Alexander Marsden, Neil Wilson, Gavin Bell, David Quigley For many applications it is essential to modify the electronic properties of graphene in a controlled fashion. This can be achieved via oxygen and nitrogen functionalization in ultra-high vacuum, leading to a system in which electronic and structural properties can be systematically studied. Here we present insights from DFT calculations on functionalized graphene systems, such as the low-energy configurations and simulated transmission electron microscopy (TEM) images, binding energies and effective band structures (EBS) of the N and O decorated graphene sheets. We directly compare our results with experiments on CVD grown graphene. Angle-resolved photoemission spectroscopy (ARPES - performed at the Antares beamline of Synchrotron SOLEIL, France) resolves the band structure changes on functionalization, whilst the simulated TEM images provide feedback for the interpretation of low-voltage aberration-corrected TEM measurements. Combined, the computational and experimental results have important implications for the manipulation of electronic properties in graphene by controlled functionalization. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B29.00002: Density functional theory investigation of cyclohexane as a potential functionalizing agent on graphene Ceren Sibel Sayin, Daniele Toffoli, Hande \"Ust\"unel Single molecules can functionalize graphene both covalently and non-covalently for use in various applications. Their adsorption properties also become important in graphene-based catalysis. In this talk, we present density functional theory (DFT) results of the interaction of cyclohexane, cyclohexyl and cyclohexene with pristine and defected graphene. We investigate structural and electronic changes induced in graphene upon adsorption and explore the potentiality of graphene to be employed in the catalytic oxidation/hydrogenation of these molecules. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B29.00003: Ab Initio Study of Covalently Functionalized Graphene and Carbon Nanotubes Sanjiv Jha, Mahmoud Hammouri, Igor Vasiliev, Igor Magedov, Liliya Frolova, Nikolai Kalugin The electronic and structural properties of carbon nanomaterials can be affected by chemical functionalization. We apply {\it ab initio} computational methods based on density functional theory to study the properties of graphene and single-walled carbon nanotubes functionalized with benzyne. Our calculations are carried out using the SIESTA electronic structure code combined with the generalized gradient approximation for the exchange correlation functional. The calculated binding energies, densities of states, and band structures of functionalized graphene and carbon nanotubes are analyzed in comparison with the available experimental data. The surfaces of carbon nanotubes are found to be significantly more reactive toward benzyne molecules than the surface of graphene. The strength of interaction between benzyne and carbon nanotubes is affected by the curvature of the nanotube sidewall. The binding energies of benzyne molecules attached to both semiconducting zigzag and metallic armchair nanotubes increase with decreasing the nanotube diameter. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B29.00004: Graphene functionalization by single atoms doping - a theoretical study Amir Natan, Elad Segev, Mark Hersam, Tamar Seideman We present first principles results and analysis for the electronic structure of chemically modified graphene. We analyze the cases of fluorine adsorption and nitrogen substitution and show that a simple analytical model can describe the doping level as a function of dopant concentration for both cases. We show the relationship between different physical parameters and the electronic band structure of the modified material and its doping level. Finally, we discuss the possible effects of substrate and of different dopant patterns on the band structure and possible applications. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B29.00005: Transport properties of chemically functionalized graphene Georgi Diankov, Francois Amet, Yongtao Cui, Zhi-Xun Shen, David Goldhaber-Gordon We use low-temperature transport measurements and microwave impedance microscopy to investigate the properties of graphene on hBN substrates. In particular, we study the Quantum Hall-insulator transition in pristine graphene and then study its evolution as the graphene is hydrogenated, observing the effect of the interplay between inter-defect distance and magnetic length. Using real-space imaging with microwave impedance microscopy, we observe well-defined edge states and suppression of conductivity in the bulk. We correlate the results from microwave impedance imaging with transport measurements. The study elucidates mechanisms that can be used to introduce controlled amounts of defects and thus, to tune the quantum transport properties of graphene. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B29.00006: Electrical transport in indium-decorated graphene sheets U. Chandni, E.A. Henriksen, J.P. Eisenstein Heavy adatoms on graphene are expected to alter its intrinsic properties in many novel ways. Here we report magneto-transport measurements on single layer graphene sheets which have been decorated with dilute concentrations of indium adatoms. These measurements are made using a custom-built evaporator housed in an ultra-high vacuum cryostat. This apparatus allows for the annealing of the graphene sample, the controlled deposition and removal of the In adatoms, and the actual transport measurements to all be done \textit{in situ}. As expected, we find that the In adatoms donate electrons to the graphene sheet, thereby shifting the location of the Dirac peak. More interestingly, our measurements clearly reveal how the In adatoms influence the scattering environment experienced by the Dirac electrons. Beyond merely reducing the sample mobility via enhanced charged impurity scattering, we find that the In adatoms alter the ``puddle'' landscape near the Dirac point and modify the low field magneto-resistance signatures of weak localization and anti-localization. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B29.00007: Pseudo-spin Winding Number in Hydrogenated Graphene Keyan Bennaceur, Jonathan Guillemette, Pierre L. L\'evesque, Farzaneh Mahvash, Cyril Proust, Mohamed Siaj, Richard Martel, Guillaume Gervais, Thomas Szkopek The quantum Hall effect (QHE) has been previously observed in highly resistive hydrogenated graphene, with an estimated hydrogen coverage up to 0.1\% that is sufficient to impart strongly insulating behaviour in zero magnetic field [1]. The opening of an impurity induced gap in graphene upon hydrogenation is anticipated to break local sub-lattice symmetry, and it may thus alter the Berry phase of Shubnikov-de Haas (SdH) oscillations and lead to a different Landau level (LL) sequence. Here we report the observation of SdH oscillations in a magnetic field up to 55 Tesla in graphene samples hydrogenated to different degree. The low temperature electron mobility ranges from $\sim$1 $cm^2/V.s$ to $\sim$1000 $cm^2/V.s$. Analysis of SdH oscillation frequency in 1/B indicates that the LL sequence remains four-fold degenerate. We also observe the $\nu=2$ Hall plateau in all samples. We therefore conclude that the topological part of the Berry phase, meaning the pseudo-spin winding number that determines the LL sequence [2], is preserved in hydrogenated graphene. \\[4pt] [1] J. Guillemette et al, PRL 110, 176801 (2013).\\[0pt] [2] J.N Fuchs et al Eur. Phys. J. B 77, 351-362 (2010). [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B29.00008: Triangular Lattices of Transition Metals Adsorbed on Graphene Matheus P. Lima, Carlos M. Acosta, Roberto H. Miwa, Ant\^onio J.R. da Silva, Adalberto Fazzio It is possible to control the electronic properties of graphene (GR) via adsorption. For instance, the increasing of the Spin-Orbit Coupling (SOC) in GR by adsorption of heavy atoms. When these atoms are adsorbed in GR, the SOC leads to a non-trivial opening of a band gap higher than the predicted in pristine GR. Indeed, there are several works addressed to investigate the adsorption of adatoms in GR, where the geometric arrangement of the atoms plays a quite important role. In this work, using ab initio DFT calculations, we present a detailed picture of how the electronic properties of GR are connected to the atomic lattice geometry of TMs (3d , 4d, and 5d) adatoms. Particularly, we show that triangular arrangements of ``Ru'' and ``Os'' atoms on GR give rise to: i) two Dirac cones at K-point due to the crystal field, ii) broken of the spin degeneracy due to the exchange field, leading to four Dirac cones, and iii) a non-trivial band gap opens due to the SOC, resulting the Quantum Anomalous Hall phase. The Dirac cones are ruled by a suitable coupling between the TM triangular lattice, and the GR hexagonal lattice. The electronic states near the Dirac points appear in the Local Density of States as peaks or valleys at geometric center of the TM triangles (barycenters). [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B29.00009: Quantum Monte Carlo study of magnetic correlations between adatom impurities in graphene A.D. G\"u\c{c}l\"u, Nejat Bulut We study the interaction between two adatom impurity spins in graphene within the framework of the Anderson model. In particular, we calculate the inter-impurity magnetic correlations by using the quantum Monte Carlo (QMC) technique [1]. We find that, at high temperatures, the QMC results for the magnetic correlations between the impurities are in agreement with the Ruderman-Kittel-Kasuya-Yoshida (RKKY) predictions for graphene [1]. However, as the temperature is lowered, the inter-impurity magnetic correlations become strongly enhanced over the RKKY results, which points to the significance of the electronic correlations.\\[4pt] [1] R.M. Fye and J.E. Hirsch, PRB v38, 433 (1988).\\[0pt] [2] S. Saremi, PRB v76, 184430 (2007). [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B29.00010: The sonication of graphite in various solvents and surfactants to synthesize high quality graphene as well as the nitrogen doping of graphene Daniel R. Soden, Jincheng Bai, Lifeng Dong The emergence of graphene in the scientific community has been the cause of much excitement among material scientists due to its unusual physical and photovoltaic properties. However, the much sought after monolayer graphene has proven to be difficult to produce in sufficient quantities This experiment aims to correct some of these problems, concerning itself with the synthesis of high quality graphene through continuous sonication with surfactant or solvent added throughout, as well as the issue of graphene quality as a function of sonication time. This was accomplished through the creation and addition of a solvent or surfactant solution to a graphite suspension during sonication lasting for 50, 80, 110,140, 170, and 200 minutes. The resulting suspension was then filtrated to separate out the graphene. Following this, the graphene was then doped through various methods with nitrogen to alter its properties. This completely physical method of graphene synthesis and doping provides a much simpler and more environmentally safe way to achieve the highly desired few layer graphene, and will hopefully allow for greater use of the substance in industry and its implementation into new technology. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B29.00011: Theory of spin-orbit coupling in fluorinated graphene Martin Gmitra, Tobias Frank, Susanne Irmer, Denis Kochan, Jaroslav Fabian We performed first-principles calculations of the spin-orbit coupling in graphene with fluorine adatom. The chemisorption of fluorine modifies the structural symmetry of graphene by breaking the pseudospin symmetry and inducing local corrugation towards $sp^3$ hybridization. We show that there are two dominant contributions to the spin-orbit field -- Rashba term and a term due to pseudospin inversion asymmetry (PIA). In addition to the $sp^3$ induced spin-orbit coupling enhancement the spin-orbit split of the fluorine $p$ orbitals is substantially transferred to graphene. Using group theoretical arguments we propose a realistic minimal Hamiltonian that reproduce the relevant spin-orbit effects calculated from first-principles. Our realistic effective Hamiltonian should be useful for spin transport and spin relaxation investigations. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B29.00012: Attraction-repulsion transition in the interaction of adatoms and vacancies in graphene Stephan LeBohec, Justin Talbot, Eugene Mishchenko The interaction of two resonant impurities in graphene has been predicted to have a long-range character with weaker repulsion when the two adatoms reside on the same sublattice and stronger attraction when they are on the same sublattice. We reveal that a single impurity level is responsible for such attraction. This opens up a possibility of controlling the sign of the impurity interaction via the adjustment of the chemical potential. For many randomly distributed impurities (adatoms or vacancies) this may offer a way to achieve a controlled transition from aggregation to dispersion. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B29.00013: Spin-induced modification of Dirac band on Fe-intercalated graphene system Sijin Sung, Jaewoon Yang, Paengro Lee, Jingul Kim, Mintae Ryu, Heemin Park, Chancuk Hwang, Kwangsu Kim, Jaesam Kim, Jinwook Chung Intercalation of magnetic iron atoms through graphene formed on the SiC(0001) surface is found to induce significant changes in electronic properties of graphene due mainly to the Fe-induced asymmetries in charge as well as spin distribution. From our synchrotron-based photoelectron spectroscopy data together with \textit{ab initio }calculations, we observe that the Fe-induced charge asymmetry results in the formation of a quasi-free-standing bilayer graphene while the spin asymmetry drives multiple spin-split bands. We find that Fe adatoms are best intercalated upon annealing at 600$^{\circ}$C exhibiting split linear $\pi $-bands, characteristic of a bilayer graphene, but much diffused. Subsequent changes in the C 1s, Si 2p, and Fe 3p core levels are consistently described in terms of Fe-intercalation. Our calculations together with a spin-dependent tight binding model ascribe the diffused nature of the $\pi $-bands to the multiple spin-split bands originated from the spin-injected carbon atoms residing only in the lower graphene layer. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B29.00014: Real-Time Optical Observation of Water Diffusion at a Graphene-Silica Janus Interface Sunmin Ryu, DaeEung Lee, Gwanghyun Ahn Because of the dominant role of the surface of molecules and their individuality, molecules behave distinctively in a confined space, which has far-reaching implications in many physical, chemical and biological systems. Here, we demonstrate that graphene forms a unique atom-thick interstitial space that enables the study of molecular diffusion in 2-dimensions with underlying silica substrates. Raman spectroscopy visualized intercalation of water from the edge to the center underneath graphene in real time, which was dictated by the hydrophilicity of the substrates. In addition, graphene undergoes reversible deformation to conform to intercalating water clusters or islands. Atomic force microscopy confirmed that the interfacial water layer is clearly flat and only a few angstroms thick, corresponding to one bilayer unit of normal ice. This study also proves that oxygen species responsible for the ubiquitous hole doping are located below graphene. In addition to serving as a transparent confining wall, graphene and possibly other 2-dimensional materials can be used as an optical indicator sensitive to interfacial mass transport and charge transfer. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B29.00015: Pseudo-magnetic fields in rippled nitrogenated graphene Sara Rothwell, Feng Wang, Edward Conrad, Gang Liu, Leonard Feldman, Philip Cohen We demonstrate a new form of semiconducting graphene which is fabricated via controlled silicon sublimation on carbon face SiC(000$\bar{1}$), previously seeded with a submonolayer of nitrogen. Nitrogenated graphene (NG) films between 2 - 8 layers have been examined [F. Wang et al. Nano Lett. 13, 4827 (2013)]. Scanning tunneling microscopy (STM) shows that NG films have ripples and folds over the entire surface. The ripples are of variable size but typically about 2 nm wide and 2-4 nm high. They meander from 5-20 nm in length. STM images show graphene flowing continuously over all folds. Scanning tunneling spectroscopy (STS) at 50 K shows peaks corresponding to Landau levels, implying a pseudo magnetic field of about 100 T. Little variation in peak position is noted in spectra taken across a fold. Levy et al. observed similar STS spectra taken near graphene nano bubbles [N. Levy et al. Science 329, 544 (2010)]. Near the Dirac point in NG STS spectra, the peaks are weak and broad corresponding to a bandgap of less than 1.5 eV. For a similar film, angle resolved photoemission measures an offset of 0.7 eV, which is a measurement of the portion of the gap below the fermi level. We thank N. Guisinger and T. Low for their support and valuable discussions. [Preview Abstract] |
Session B30: Focus Session: Graphene Devices: Fabrication, Characterization and Modeling: Trilayer and Bilayer Graphene: Transport and Stacking
Sponsoring Units: DMPChair: Jeanie Lau, University of California, Riverside
Room: 605
Monday, March 3, 2014 11:15AM - 11:51AM |
B30.00001: Transport in bilayer and trilayer graphene: band gap engineering and band structure tuning Invited Speaker: Jun Zhu Controlling the stacking order of atomically thin 2D materials offers a powerful tool to control their properties. Linearly dispersed bands become hyperbolic in Bernal (AB) stacked bilayer graphene (BLG). Both Bernal (ABA) and rhombohedral (ABC) stacking occur in trilayer graphene (TLG), producing distinct band structures and electronic properties. A symmetry-breaking electric field perpendicular to the sample plane can further modify the band structures of BLG and TLG. In this talk, I will describe our experimental effort in these directions using dual-gated devices. Using thin HfO$_2$ film deposited by ALD as gate dielectric, we are able to apply large displacement fields D $>$ 6 V/nm and observe the opening and saturation of the field-induced band gap E$_g$ in bilayer and ABC-stacked trilayer graphene, where the conduction in the mid gap changes by more than six decades. Its field and temperature dependence highlights the crucial role played by Coulomb disorder in facilitating hopping conduction and suppressing the effect of E$_g$ in the tens of meV regime. In contrast, mid-gap conduction decreases with increasing D much more rapidly in clean h-BN dual-gated devices. Our studies also show the evolution of the band structure in ABA-stacked TLG, in particular the splitting of the Dirac-like bands in large D field and the signatures of two-band transport at high carrier densities. Comparison to theory reveals the need for more sophisticated treatment of electronic screening beyond self-consistent Hartree calculations to accurately predict the band structures of trilayer graphene and graphenic materials in general. In collaboration with: Ke Zou, Jing Li, Fan Zhang, C Clapp, and Allan MacDonald. References: Zou and Zhu, PRB 82, 081407 (2010) Zou et al, Nano Letters, 13, 369 (2013) [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B30.00002: Electric Field Control of Stacking-Order Solitons in Trilayer Graphene Matthew Yankowitz, Joel I-Jan Wang, A. Glen Birdwell, K. Watanabe, T. Taniguchi, Pablo San-Jose, Philippe Jacquod, Pablo Jarillo-Herrero, Brian J. LeRoy Trilayer graphene exhibits two low-energy stacking configurations (Bernal and rhombohedral). In graphene flakes with both stacking configurations, the area separating them consists of a localized soliton-like region of strain where one graphene layer shifts by the carbon-carbon bond distance. Under a perpendicular electric field, Bernal-stacked trilayer graphene remains metallic whereas rhombohedrally-stacked trilayer graphene develops a band gap. Consequentially, the electric field modifies the relative energy cost of each stacking configuration, permitting rare control over the crystal structure of a material via only the application of an external electric field. We demonstrate the ability to control the stacking configuration in trilayer graphene via an electric field using scanning tunneling microscopy. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B30.00003: Broken Symmetry States in Dual Gated Rhombohedral Trilayer Graphen Yongjin Lee, David Tran, Kevin Myhro, Jairo Velasco Jr., Nathaniel Gillgren, Chung Ning Lau, Yafis Barlas, Jean-Marie Poumirol, Dmitry Smirnov, Francisco Guinea We perform low temperature transport measurements of dual-gated rhombohedral-stacked trilayer graphene device. At the charge neutral point, we observe a giant interaction-induced gap, $\sim$ 41mV that is suppressed by an interlayer potential or a critical temperature Tc $\sim$ 28K, suggesting a layer antiferromagnetic ground state with broken time reversal symmetry. In the quantum Hall regime, we observe~QH plateaus at filling factors $\nu =$0, 1, 2 and 3 in a high magnetic field. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B30.00004: Mechanism of photocurrent generated at a junction between ABA- and ABC-stacked tri-layer graphene Minjung Kim, Seon-Myeong Choi, Ho Ang Yoon, Sun Keun Choi, Jung Cheol Kim, Sang Wook Lee, Young-Woo Son, Hyeonsik Cheong Tri-layer graphene has two stacking orders, ABA and ABC stacking, which have different electronic band structures. We observed photocurrent generated at the ABA and ABC stacking junction in tri-layer graphene and investigated the mechanism of the photocurrent by measuring the back-gate voltage dependence of the photocurrent. In general, there are two mechanisms of photocurrent generated in graphene photodevices without bias; a density of states (DOS) mismatch and the Seebeck coefficient difference. The dominant mechanism of photocurrent at a junction between single- and bi-layer graphene has been suggested as being due to the difference in Seebeck coefficients [X. Xu et. al., Nano lett. 10, 562 (2010).]. Here, we studied the dominant mechanism of the photocurrent in the ABA and ABC stacking junction in tri-layer graphene. If the DOS mismatch is the dominant mechanism, the direction of photocurrent is from ABC to ABA stacking in p-doped tri-layer graphene. On the other hand, if the Seebeck coefficient difference is dominant, the direction of the photocurrent is opposite. In our devices, it has been found that the DOS mismatch is dominant. In addition, we measured the photocurrent at between at a junction single- and bi-layer graphene and bi- and tri-layer graphene for comparison. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B30.00005: Quantum Hall phase diagram of ABC-trilayer graphene Invited Speaker: Yafis Barlas At low-energies, the massive Dirac electrons in ABC-stacked trilayer graphene exhibit a cubic dispersion with a Berry phase of $3 \pi$. Landau quantization of ABC-trilayer graphene leads to a quantum Hall (QH) plateau sequence $\sigma_{xy} = \pm 4(N +3/2) e^2/h$(where $N \geq 0 $ is the Landau level index). This results in a 12-fold degenerate zero-energy Landau level (LL) which supports a degenerate set of triplet ($n=0,1,2$) LL orbitals along with the spin and valley degeneracies. In this talk, I will show that interactions within the zeroth LL induce charge gaps which drive additional integer QH plateaus at intermediate filling factors $\nu$ ($-6 < \nu < 6$). The competition of remote hopping between the layers, interactions and pseudo-spin anisotropy leads to various ferromagnetically and anti-ferromagnetically pseudo-spin ordered states. Additionally, the unique LL orbital degeneracy influences the ground state at filling factors $\nu =-5,-2,1,4$. At these filling factors, a quantum phase transition from a quantum Hall liquid state to a triangular charge-density wave occurs when an electric potential difference $\Delta _{V}$ between the layers is reduced below a critical value $\Delta_{V}^{\left( c\right)}$ [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B30.00006: Electronic correlation in ABC graphene trilayers Xu Dou, Akbar Jaefari, Yafis Barlas, Bruno Uchoa At low energies, undoped ABC-stacked trilayer graphene can be described by an effective two-band model which features a cubic noninteracting energy spectrum. The divergence of the density of states at the neutral points provides a large phase space for electronic instabilities, which differentiates it from single-layer and bilayer graphene. In the large N limit, where N is the number of fermion species, we show that the self-energy is logarithmically renormalized by Coulomb interactions in leading order in 1/N. We show that the dynamical polarization bubble is also logarithmically divergent near the edge of the particle hole continuum. We investigate the renormalization of different physical observables accounting for dynamical polarization effects. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B30.00007: Magneto-optical spectra of ABA trilayer graphene: interplay between monolayer and bilayer signatures Yen-Hung Ho, Wu-Pei Su, Ming-Fa Lin We utilize a generalized tight-binding model to study the Landau level spectra of ABA trilayer graphene. The spatially resolved Landau wave functions enable the characterization of Landau levels and the calculation of magneto-optical properties. The spectra consist of monolayer-like and bilayer-like features, in which the inter-valley symmetry is lifted, especially the levels close to zero energy. Applying a bias voltage effectively increases the splitting, and furthermore, triggers the optical transitions between monolayer-like and bilayer-like Landau states. The calculated results can be further verified by optical measurements, and this numerical method can be applied to other layered materials, such as Molybdenum disulfide. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B30.00008: Chiral electron transport in CVD bilayer graphene Kyunghoon Lee, Yun Suk Eo, Cagliyan Kurdak, Zhaohui Zhong Charge carriers in bilayer graphene have a parabolic energy spectrum. Due to this band structure they are massive quasiparticles having a finite density of state at zero energy like other non-relativistic charge carriers in conventional two dimensional materials. However, they are massive Dirac fermions which have a chiral nature similar to the case of massless Dirac fermions in single layer graphene. Coupling of pseudospin and motion of charge carrier via chirality can result in dramatic consequence for transport in bipolar regime like Klein tunneling, Fabry-Perot interference, collimation of charge carrier, Veslago lens, etc. However, little attention has been paid to chiral dependent electron transport in bilayer graphene. Here we study these properties by probing phase coherent transport behavior in CVD bilayer graphene devices with sub-200nm channel length. Complex Fabry-Perot interference patterns are observed in resonant cavities defined by local gating. By applying Fourier analysis technique, we successfully analyze and identify the origin of each individual interference pattern in bipolar and monopolar regime. Our initial results also hint at the observation of cloaking of electronic states against chiral electrons in bilayer graphene. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B30.00009: Electrochemical characterization of chemical vapor deposition grown few-layer graphene Rajaram Narayan, Mehmet Karakaya, Ramakrishna Podila, Prabhakar Bandaru, Apparao Rao The intrinsic double-layer capacitance (C$_{\mathrm{dl}})$ of graphene is an important fundamental parameter that has important implications in nano-carbon based energy storage devices. We used cyclic voltammetry to measure the C$_{\mathrm{dl}}$ of few-layer graphene (FLG) samples. Considering the fact that the specific C$_{\mathrm{dl}}$ of graphitic edge planes exceeds that of basal planes by an order of magnitude, the measured specific C$_{\mathrm{dl}}$ may be used to evaluate the relative area fraction of edge planes to that of basal planes. In our case, the specific C$_{\mathrm{dl}}$ of FLG grown on Ni foils was found to be $\sim$2-4 $\mu $F/cm$^{2}$, which is typical of basal plane capacitance, and indicating predominant basal plane coverage in our CVD process. Such samples are amenable to further physical/chemical modifications to create controlled defects which are expected to further enhance C$_{\mathrm{dl}}$. Electrochemical characterization of such ideal geometry in tandem with defects engineering can provide insights into the contribution of graphitic edge planes to charge storage in high surface area carbon electrodes. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B30.00010: Large area single and bilayer graphene with controlled orientation for each layer Lola Brown, Edward Lochocki, Christopher Guti\'errez, Abhay Pasupathy, Kyle Shen, Jiwoong Park The creation and exploration of artificial graphene structures has recently become the focus of great interest. In particular, controlling the interlayer twist angles in multilayer graphene stacks allows modulation of the overall band structure. However, producing such a structure remains difficult due to the random distribution of twist angles in as-grown samples. Here we report a novel way for creating large area graphene stacks with a pre-determined twist angle. We first grow single layer graphene whose orientation is aligned over a few cm length scale on copper foil. The overall angle alignment of the graphene is confirmed using low energy electron microscopy (LEED) and transmission electron microscopy techniques. Since the graphene is well aligned over a few centimeters, we can create large area graphene stacks with known twist angle by transferring these graphene layers while controlling the orientation of each layer during transfer. We confirm that the layers are coupled by probing the resulting band structure using angle resolved photoemission spectroscopy (ARPES), and examining their interlayer optical resonance features using spatially resolved hyperspectral (DUV-Vis-NIR wavelengths). This new method is scalable, and controllable and thus paves the way to explore and exploit the novel properties of two-dimensional crystals in artificial stacks with controlled interlayer structures. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B30.00011: Stacking Faults and Topological Kink States in Bilayer Graphene Adam Tsen, Dennis Wang, Jiwoong Park, Abhay Pasupathy, Philip Kim For bilayer graphene, the lowest energy configuration consists of two mirror-symmetric stacking orders (AB and BA), which are connected by a lattice translation. In large-area bilayer systems grown by chemical vapor deposition, domains of both stacking configurations have been observed with transmission electron microscopy (TEM), and the boundaries were found to form by the continuous strain of one layer with respect to the other. Here, we perform similar TEM measurements on bilayer graphene prepared by mechanical exfoliation and observe identical stacking faults. These structures may present important ramifications for the electronic properties of such systems. In particular, they are predicted to support topologically protected, gapless kink states, and so their presence may explain the difficulty in opening a substantial transport gap in bilayer graphene even under large electric fields. We also present preliminary transport measurements on individual stacking faults resolved by TEM. [Preview Abstract] |
Session B31: Focus Session: Computational Discovery and Design of New Materials II
Sponsoring Units: DMP DCOMPChair: Lin-Lin Wang, Iowa State University
Room: 607
Monday, March 3, 2014 11:15AM - 11:51AM |
B31.00001: Associating Specific Materials with Topological Insulation Behavior Invited Speaker: Xiuwen Zhang The first-principles (a) total-energy/stability calculations combined with (b) electronic structure calculations of band inversion, spin-polarization and topological invariants (Z2) has led to the design and prediction of specific materials that are topological insulators in this study. We classify bulk materials into four types of band-inversion behaviors (TI-1, TI-2, BI-3, BI-4), based on the number of band inversions and their distributions on various time reversal invariant k points. Depending on the inversion type in bulk, the corresponding surface states have different protections e.g., protected by time reversal symmetry (in TI-1 materials), spatial symmetry (in TI-2), or not protected (in BI-3, BI-4). Subject 1 Discovery of new TI by screening materials for a Z2 metric: Such high-throughput search in the framework of Inverse Design methodology predicts a few previously undocumented materials that are TI-1 in their ground state crystal structure. We also predict dozens of materials that are TI-1 however in structures that are not ground states (e.g. perovskite structure of II-Bi-O3). Subject 2 Design Principle to increase the gap of TI-1 materials: In HgTe-like cubic topological materials, the insulating gap is zero since the spin-orbit splitting is positive and so a 4-fold half-filled p-like band is near the Fermi level. By design of hybridization of d-orbitals into the p-like bands, one can create negative spin-orbit splitting and so a finite insulating gap. Subject 3 Unconventional spin textures of TI surface states: Despite the fact that one of our predicted TI-1 KBaBi has inversion symmetry in the bulk--a fact that that would preclude bulk spin polarization--we find a Dresselhaus-like spin texture with non-helical spin texture. This originates from the local spin polarization, anchored on the atomic sites with inversion asymmetric point groups, that is compensated due to global inversion symmetry in bulk. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B31.00002: Determination of ground-state structure of perovskite super- lattices from first principles Yuanjun Zhou, Karin Rabe We propose an efficient method to find the ground state structure (GSS) of superlattices. It is based on the assumption that in the GSS of the superlattice, structures of the constituent layers will closely resemble a low-energy state of the pure compound at the relevant epitaxial strain. This method is especially suitable for high-throughput first-principles studies for the design of functional superlattices since the information about the low-energy states for a relatively small set of pure compounds, generated in a preprocessing step and stored in a database, can be used for the structure determination of a large space of constituent and layer thickness combinations. The method is demonstrated by application to the 2:2 PbTiO3/SrTiO3 superlattice. For tensile and compressive epitaxial strain, we find the GSS consistent with previous studies. For 0\% epitaxial strain, however, our method identifies two degenerate distinct ground-state structure candidates, only one of which was previously identified; further investigation confirms a complex energy landscape for this phase. Results for the Sr$M$O3/SrTiO3 series of superlattices, where $M$ = V, Cr, Co and Fe, will also be presented. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B31.00003: Ab initio search for new $p$-type transparent conductors among oxide sulfides Kanber Lam, Giancarlo Trimarchi, Arthur J. Freeman, Kenneth Poeppelmeier, Alex Zunger Optimal $p$-type, i.e., hole-conducting, transparent materials must meet the design metrics of large band gap for transparency, and light hole effective masses and large hole content for good $p$-type conductivity. The oxide sulfides could potentially satisfy these design metrics better than oxides do, owing to the stronger hybridization between the S $p$ and metal orbitals that can produce a more dispersive valence band maximum (VBM) and lighter hole masses than in oxides. LaOCuS is the prototype $p$-type transparent conductor (TC) among oxide sulfides. Here, we perform a density functional study of $\sim 30$ oxide sulfides, based on transition metals and column II and III elements, to identify compounds in this set that meet the design metrics for $p$-type TCs. We screen these materials using band gaps and hole effective masses. The analysis of the VBM wavefunctions shows that these oxide sulfides can be classified into ``band-mixed,'' with a continuous distribution of the wavefunction on both anions, and ``band-segregated,'' with the VBM mostly originating from one of the anions. The correlation between the type of VBM wavefunction and the O and S arrangement in the material (anion mixed vs. anion segregated) provides a designing criterion for new mixed-anion $p$-type TCs. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B31.00004: First-principles data-driven discovery of new low-band-gap oxides for solar energy capture and conversion Qimin Yan, Wei Chen, Anubhav Jain, Kristin Persson, Jeffery B. Neaton We develop first-principles data driven discovery approach to explore experimentally-known oxide compounds with low band gaps. Cr-based oxide compounds comprise a nice test bed for assessing high throughput discovery of light absorbers and photocatalysts. An interesting subclass with promising band gaps, this Cr oxide testbed spans a range of electronic and magnetic properties; predicting trends across such a range can challenge for standard density functional theory and many-body perturbation theory. We focus on this set and implement a broadly-applicable high-throughput workflow for calculation of band gaps, adsorption spectra, and band edges, initially using semi-local and hybrid functionals. We develop best practices for analysis of these data, and successfully identify several promising new compounds for solar energy capture and conversion applications, which we then apply more rigorous many-body perturbation theory including GW method and beyond to further study their optical and electronic properties. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B31.00005: Origin of the large $p$-type conductivity in the misfit layered La$_5$Cu$_6$O$_4$S$_7$ oxide sulfide: a first-principles study Arthur J. Freeman, Jino Im, Kanber Lam, Giancarlo Trimarchi, Kenneth R. Poeppelmier Large $p$-type, i.e., hole, conductivity has been achieved only in very few transparent conducting oxides. Oxide sulfides can potentially display higher hole conductivity than oxides, due to the favorable hybridization between metal and sulfur orbitals at the valence band maximum. The layered oxide sulfide LaCuOS has been identified and extensively investigated as a $p$-type transparent conductor, yet its layered misfit analogue La$_5$Cu$_6$O$_4$S$_7$ was found to have an intrinsically larger hole conductivity and an optical gap of $\sim 2.0$ eV. We find through first-principles density-functional calculations that the S atoms in the chains embedded in the La-O layer in La$_5$Cu$_6$O$_4$S$_7$ can form S$_2$ dimers. Absence of dimers in the S chain results in a metallic band structure. This dimerization controls the opening of an optical gap. A random distribution of S$_2$ dimers together with isolated atoms along these S chains is a possible mechanism for the concurrent opening of an optical gap and the presence of a significant hole concentration in this material. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B31.00006: Structurally unstable III-Bi-O$_3$ perovskites are predicted to be topological insulators but their stable structural forms are just band insulators: A first principles study Giancarlo Trimarchi, Arthur J. Freeman, Xiuwen Zhang, Alex Zunger Several Bi oxides in the assumed cubic $Pm\bar{3}m$ perovskite structure have recently been identified as topological insulators or semi-metals by first-principles calculations. In these perovskites, Bi is at the octahedral site and the $A$ atom at the interstitial site is a column III cation, i.e., Al, Ga, In, Sc, Y, La. We use density functional total-energy calculations and crystal structure prediction to determine the energetically stable phases for these oxides. We find that these $Pm\bar{3}m$ $A$BiO$_{3}$ perovskites are topological insulators, confirming recent results obtained by our and other groups. However, switching the position of Bi and $A$ in the $Pm\bar{3}m$ perovskite produces trivial insulators or semimetals, as opposed to topological insulators. Indeed, symmetry-lowering via concerted tilting and internal deformation of the octahedra, stabilizes these Bi oxides, irrespective of the position of Bi, producing the stable $Pnma$ perovskite structure that is not a topological insulator. This illustrates that a simultaneous application of ``first-principles thermodynamics'' with first-principles electronic structure ($Z_{2}$ evaluation) is needed to establish stable topological insulators. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B31.00007: A new class of polar mott-insulators via heterostructruring Chanul Kim, Hyowon Park, Chris Marianetti We propose simple design rules based on charge transfer and ion size to design a new class of polar Mott insulators in perovskite-based transition metal oxides. Ab Initio DFT+U calculations are then used to selectively scan phase space in double perovskites which have strong potential to be polar and Mott insulating. We begin by exploring pairs of A-type ions (A, A') and pairs of B-type ions (B, B') in $AA^{\prime}BB^{\prime}O_{6}$ which will have nominal charge transfer consistent with valencies that are conducive to a Mott insulator. Additionally, the A-type ions are chosen to have a large size mismatch and are ordered to break symmetry, creating conditions favorable to a polar distortion. We uncover a number of materials which are strong candidates to be polar Mott insulators in experiment, including BaLaVNiO$_{6}$, BaLaVCoO$_{6}$, BaLaVCuO$_{6}$, BaLaCrNiO$_{6}$, BaBiVCoO$_{6}$, BaBiVNiO$_{6}$, and PbLaVNiO$_{6}$. Furthermore, we show that the magnetic state and the band gap are sensitive to the particular ordering of the transition metals. Finally, we discuss possible applications and the potential to grow these systems in experiment. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B31.00008: Room-temperature quantum spin Hall effect in HgTe honeycomb superlattices Cristiane Morais Smith, Wouter Beugeling, Efterpi Kalesaki, Y.-M. Niquet, Christophe Delerue, Daniel Vanmaekelbergh The recent experimental realization of self-assembled honeycomb superlattices of truncated semiconducting nanocrystals has opened a new path to engineer graphene-like structures. Atomistic band-structure calculations for honeycomb lattices of PbSe and CdSe have shown a rich band structure, with Dirac cones at the $s$- as well as at the $p$-bands, in addition to a flat $p$-band. By controlling the chemical composition of the nanocrystals, lattices with strong spin-orbit coupling can be artificially designed. We show that for HgTe a huge non-trivial gap, of order of 50 meV, opens at the K-points. We calculate the edge states using both, an atomistic calculation that takes into account $10^6$ atomic orbitals per unit cell, as well as an effective 16-bands tight-binding model, and find that the quantum spin Hall effect should be observable in this material at temperatures of the order of room temperature. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B31.00009: Development of (MnO)$_{\mathrm{1-x}}$(ZnO)$_{\mathrm{x}}$ Alloys for Water Splitting Applications Paul Ndione, Emily Warren, Haowei Peng, Stephan Lany, David Ginley, Andriy Zakutayev Using high throughput combinatorial synthesis, measurement and analysis methodologies, we rapidly investigate the composition related structural, optical, and electrical properties of (MnO)$_{\mathrm{1-x}}$(ZnO)$_{\mathrm{x}}$ alloys and identify candidates materials for a more detailed study in PEC applications. The (MnO)$_{\mathrm{1-x}}$(ZnO)$_{\mathrm{x}}$ thin films are synthesized using combinatorial pulsed laser deposition with continuous orthogonal gradients in both chemical composition and substrate temperature. The solubility limit of ZnO into MnO is determined using the disappearing phase method and found to decrease with increasing temperature. For example, (MnO)$_{\mathrm{1-x}}$(ZnO)$_{\mathrm{x}}$ deposited at 300 C exhibit only the tetrahedral wurzite (WZ) structure instead of the rocksalt (RS) one at x\textgreater 0.4. Optical measurements indicate the strong reduction of the optical band gap associated with the RS to WZ transition, and are consistent with the first-principles theory prediction of E$_{\mathrm{gap}}=$2.1 eV at a x$=$0.5 alloy composition. The values of the electrical conductivity for the Ga-doped (MnO)$_{\mathrm{1-x}}$(ZnO)$_{\mathrm{x}}$ samples deposited at 300 C from a 4{\%} Ga-doped ZnO target are determined to be \textless 2 S/cm and 100 S/cm for the RS and WZ structure respectively per atom of Ga. These results suggest that Ga-doped MnO-ZnO alloys present a promising materials system for water oxidation in a PEC cell. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B31.00010: Prediction of novel, Earth abundant Cu2O based alloys for PV applications Vladan Stevanovic, Stephan Lany Tuning the opto-electronic properties of semiconductors through alloying is essential for semiconductor industry. Currently, mostly isovalent and isostructural alloys are used (e.g. Si/Ge, GaN/InN or CdTe/ZnTe), but a vast and unexplored space of novel functional materials is conceivable when considering more complex alloys by mixing aliovalent and heterostructural constituents. The real challenge lies in the quantitative property prediction for such complex alloys to guide their experimental exploration. In our work we demonstrate how an Earth abundant p-type oxide Cu2O, can be engineered through alloying into a technologically useful absorber material. We use non-local external potentials (NLEP) fitted to GW calculations for correcting the DFT electronic structure and compute absorption coefficient of different alloy compositions and configurations. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B31.00011: Identifying Solid Sorbents for CO$_{2}$ Capture Technology by \textit{ab initio} Thermodynamic Approach Yuhua Duan Since the current technologies for capturing CO$_{2}$ are still too energy intensive, to develop new materials that can capture CO$_{2}$ reversibly with acceptable energy costs are needed. By combining thermodynamic database mining with first principles density functional theory and phonon lattice dynamics calculations, a theoretical screening methodology to identify the most promising CO$_{2}$ sorbent candidates from the vast array of possible solid materials have been proposed and validated. The calculated thermodynamic properties of different classes of solid materials versus temperature and pressure changes were further used to evaluate the equilibrium properties for the CO$_{2}$ adsorption/desorption cycles. According to the requirements imposed by the pre- and post- combustion technologies and based on our calculated thermodynamic properties for the CO$_{2}$ capture reactions by the solids of interest, we were able to identify only those solid materials for which lower capture energy costs are expected at the desired pressure and temperature conditions. At a given CO$_{2}$ pressure, the turnover temperature (T$_{\mathrm{t}})$ of an individual solid capture CO$_{2}$ reaction is fixed. Such T$_{\mathrm{t}}$ may be outside the operating temperature range ($\Delta $T$_{\mathrm{o}})$ for a particularly capture technology. In order to adjust T$_{\mathrm{t}}$ to fit the practical $\Delta $T$_{\mathrm{o}}$, in this study, we demonstrate that by mixing different types of solids it's possible to shift T$_{\mathrm{t}}$ to the a range of practical operating temperature conditions. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B31.00012: First-principles design of organo-Sn polymeric dielectrics Huan Tran, Arun Kumar, Chenchen Wang, Aaron Baldwin, Rui Ma, Gregory Sotzing, Rampi Ramprasad Following on from recent computation-based suggestions that Sn-containing polymers may be promising dielectrics, one of them, poly (dimethyltin glutarate) (pDMTG), has been synthesized. The measured dielectric constant of pDMTG is $\epsilon \simeq 7.4$, significantly higher than the current standard material used for high-energy-density applications, namely, polypropylene ($\epsilon \simeq 2.2$). By performing first-principles calculations at the level of density functional theory and using the minima-hopping method to predict the stable structures (given that just the composition is provided), we propose four structural models of pDMTG. Based on these models, various physical properties of pDMTG, e.g., dielectric constant, infrared spectra and refractive index, are determined to closely agree with experimental data. The calculated band gap of pDMTG is high ($E_{\rm g} \simeq 6.1$ eV), implying that pDMTG is a promising candidate for high-energy-density materials. The strategy that has lead to the synthesis and understanding of pDMTG shows that density functional theory is a powerful method to study and design new materials. Our work is supported by the Office of Naval Research through the Multidisciplinary University Research Initiative (MURI). [Preview Abstract] |
Session B32: Invited Session: Thermalization and Prethermalization in Isolated Systems after a Quantum Quench
Sponsoring Units: DCMP DAMOPChair: Michael Kolodrubetz, Boston University
Room: 708-712
Monday, March 3, 2014 11:15AM - 11:51AM |
B32.00001: How does an isolated quantum system relax? Invited Speaker: Joerg Schmiedmayer One of the biggest challenges in probing non-equilibrium dynamics of many-body quantum systems is that there is no general approach to characterize the resulting quantum states. Interference experiments give access to the phase of the order parameter. The full distribution functions of the interference amplitude, and the full phase correlation functions allow us to study the relaxation dynamics in one-dimensional quantum systems. Starting form a coherently split 1d quantum gas, the initial coherence slowly decays. Due to the approximate conserved quantities in our nearly integrable system, this relaxation leads to a pre-thermalized state [1], which is characterized by thermal like distribution functions but exhibits an effective temperature much lower than the kinetic temperature of the initial system. A detailed study of the correlation functions reveals that these thermal-like properties emerge locally in their final form and propagate through the system in a light-cone-like evolution [2]. Furthermore we demonstrate that the pre- thermalized state is connected to a Generalized Gibbs Ensemble and show the pathways for further relaxation towards thermal equilibrium. \\[4pt] [1] M. Gring et al., Science \textbf{337, }1318 (2012); \\[0pt] [2] T. Langen et al. Nature Physics \textbf{9}, 640-643 (2013). [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B32.00002: Universal dynamics of a degenerate unitary Bose gas Invited Speaker: Eric Cornell It has long been thought that one can not study a degenerate Bose gas with fully resonant (unitary) interactions because the gas is unstable to three-body recombination. We find empirically instead that after a Bose-Einstein condensate has been tuned from a weakly interacting state to a fully unitary gas at the peak of a Feshbach resonance, it survives for a time long enough to permit the characterization momentum-population dynamics. In particular, a high momentum tail forms and comes to a quasi-steady state in perhaps 100 microseconds, while the sample continues to survive and indeed remains degenerate for considerably longer. We show that the shape- and time-dependence of the momentum distribution scale in a universal way with sample density. This work was done in collaboration with Phil Makotyn, Deborah Jin, Cathy Klauss and David Goldberger. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B32.00003: Undephasing the generalized Gibbs ensemble Invited Speaker: Stefan Kehrein Understanding the long time limit of closed quantum many-body systems prepared in some initial non-equilibrium pure state has attracted a lot of interest in recent years. One central question is whether such a system will thermalize or not. In integrable systems the long time limit leads generically to a generalized Gibbs ensemble (GGE) description [1]. For example for the one dimensional transverse field Ising model one can prove that asymptotically all local observables can be calculated in the GGE [2]. In my talk I will show how one can approximately reverse the time arrow of this dynamics using a spin echo-like local Hamiltonian. In this sense the time evolved system never forgets that it is in a pure state and remembers the initial values of local observables like the longitudinal and transverse magnetization. The time evolution can be thought of as dephasing leading to a GGE, which can be undone with this spin echo-like setup. This and related kinds of echo dynamics will be demonstrated for the transverse field Ising model and other integrable models. \\[4pt] [1] M. Rigol, V. Dunjko, V. Yurovsky, and M. Olshanii, Phys. Rev. Lett. 98, 050405 (2007). \newline [2] M. Fagotti and F. Essler, Phys. Rev. B 87, 245107 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:39PM |
B32.00004: Equilibration and coarsening in the quantum O(N) model at infinite N Invited Speaker: Anushya Chandran The quantum O(N) model in the infinite-N limit is a paradigm for symmetry breaking. In this talk, I will investigate the physics of this model out of equilibrium, specifically its response to global quenches starting in the disordered phase. In the infinite-N limit, I will show that not only does the model not lead to equilibration on account of an infinite number of conserved quantities, it also does not relax to a generalized Gibbs ensemble (GGE) consistent with these conserved quantities. Instead, an infinite number of new conservation laws emerge at late times and the system relaxes to an emergent GGE consistent with these. Nevertheless, the late-time states following quenches bear strong signatures of the equilibrium phase diagram. Notably, we find that the model exhibits coarsening to a nonequilibrium critical state only in dimensions d $>$ 2, that is, if the equilibrium phase diagram contains an ordered phase at nonzero temperatures. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 2:15PM |
B32.00005: Fluctuation-dissipation relations in isolated quantum systems after a quench Invited Speaker: Marcos Rigol In recent years, there has been increasing interest in understanding under which conditions observables in isolated quantum systems far from equilibrium relax to the predictions of traditional statistical ensembles. Despite being guided by unitary dynamics, this has been shown to occur in nonintegrable systems [1,2] and has been understood within the eigenstate thermalization hypothesis [1-4]. In integrable systems, on the other hand, observables have been found to relax to nonthermal values, which instead can be described by generalized Gibbs ensembles (GGEs) [5]. In this talk, we review some of the early results on this topic and examine whether standard fluctuation dissipation relations apply after relaxation following a quantum quench. We focus on the dynamics of trapped hard-core bosons in one-dimensional lattices with dipolar interactions, as realized in recent experiments with ultracold gases in optical lattices, whose strength is changed during the quench. We consider both nonintegrable and integrable regimes and discuss how, at integrability, the results after relaxation depend on the properties of the initial state selected [6].\\[4pt] [1] M. Rigol, V. Dunjko, and M. Olshanii, Nature 452, 854 (2008).\\[0pt] [2] M. Rigol, Phys. Rev. Lett. 103, 100403 (2009).\\[0pt] [3] J. M. Deutsch, Phys. Rev. A 43, 2046 (1991).\\[0pt] [4] M. Srednicki, Phys. Rev. E 50, 888 (1994).\\[0pt] [5] M. Rigol, V. Dunjko, V. Yurovsky, and M. Olshanii, Phys. Rev. Lett. 98, 050405 (2007).\\[0pt] [6] E. Khatami, G. Pupillo, M. Srednicki, and M. Rigol, Phys. Rev. Lett. 111, 050403 (2013). [Preview Abstract] |
Session B34: Photocatalysis, Proton Conductors and Other Energy Topics
Sponsoring Units: GERAChair: Zhigang We, Colorado School of Mines
Room: 704
Monday, March 3, 2014 11:15AM - 11:27AM |
B34.00001: ZnO/PbTiO$_{3}$ as a Novel Catalyst for CO$_{2}$ Conversion Babatunde Alawode, Alexie Kolpak Due to its role in climate change, there is a great interest in finding ways to take advantage of the vast amount of waste CO$_{2}$ we produce by its conversion to useful substances. This is currently impractical due to the high temperatures and pressures generally required for the synthesis of compounds using CO$_{2}$ as a precursor. To make direct CO$_{2}$ capture and conversion economically viable, new materials able to catalyze the conversion reactions at significantly milder conditions will be essential. In this work, we use DFT computations to design a dynamically tunable ferroelectric oxide-supported thin film catalyst that can capture CO$_{2}$ directly from the emission stream and convert it into methanol or cyclic carbonates. One promising candidate for a dynamically tunable catalyst of this type is Zn$_{\mathrm{x}}$O$_{\mathrm{y}}$/PbTiO$_{3}$. We demonstrate that switching the polarization of the ferroelectric substrate substantially changes the surface atomic and electronic properties of the heterostructure, thereby enabling tunable absorption. We investigate reaction pathways on unsupported and supported ZnO for common CO$_{2}$ reactions. Our approach may lead not only to new technologies for reducing emissions, but also to novel catalysts that could decrease energy consumption for industrial-scale synthetic processes. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B34.00002: Dynamics of Interfacial Charge Transfer in Semiconductor Crystalline Film-Assisted Photocatalyst Wei Sea Chang, Chien Nguyen Van, Ying-Hao Chu Semiconductors are by far the most intensively studied photocatalytic materials due to their favorable electronic and optical properties. Many have reported on semiconductor particles for their size quantization effect. The discrepancies about particle sizing, however, have always been questioned as to what is the optimum particle size for quantization effects to be observed. Here, we offer a novel semiconductor/metal architecture to understand the mechanism of interfacial charge transfer in a visible-light driven redox reaction. Semiconductor BiFeO$_{3}$ (BFO) film with preferred orientation were synthesized by using pulsed laser deposition. Au nanoparticles were produced by post-thermal annealing under oxygen atmosphere to distribute over the semiconductor film surface. High resolution X-ray diffraction and transmission electron microscopy results indicate that the Au (111) nanoparticles are partially embedded on the (100)-, (110)-, and (111)-oriented BFO film. A comprehensive study of the electronic properties has been performed by ultrafast pump-probe and X-ray photoelectron spectroscopy. We seek to answer how oriented-semiconductor films with Au nanoparticles distributed uniformly over the film would affect the dynamics of photocatalytic reactions. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B34.00003: Fe doping and anion defects in bismuth pyrochlore photocatalysts Cedric Mayfield, David Barker, Vaidyananthan Subramanian, Muhammad Huda To understand the change in photocatalytic properties of Bi$_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7\thinspace }}$after incorporating localized Fe 3d electrons, the electronic properties and formation energies of anion defects and cation substitutions have been systematically studied by first principles density functional theory. We have found for each type of doping, intrinsic or extrinsic, structural distortions are localized to the defect site. For the intrinsic defects, O vacancies (O$_{\mathrm{vac}})$ are relatively shallow donors compared to O interstitials (O$_{\mathrm{int}})$. For the extrinsic defects, Fe substitutions at the Bi sites (Fe$_{\mathrm{Bi}})$ are more stable than Fe substitutions at the Ti sites (Fe$_{\mathrm{Ti}})$, however they both promote the acceptor defect levels which are critical for band gap engineering. Complex doping (combined intrinsic and extrinsic doping) was also considered to examine the defect correlations at first nearest neighbor to third nearest neighbor distances. A detailed electronic structure analysis will be presented for both pristine and doped Bi$_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B34.00004: Raising Power Output in an Acoustic Energy Harvester Michael Primrose, Orest Symko A promising approach for the conversion of heat to electricity consists of coupling a thermoacoustic heat engine to a piezoelectric device. When heated, this unit resonates at high audible frequencies which are converted to electricity. Being compact and small its power output is limited. To overcome this, several piezoelectric devices can be coupled to the acoustic engine thereby generating more electrical power at the expense of increasing the load on the engine. In the prototype studied, three PZT unimorph piezoelectric devices converted the heat-generated sound at 2.5 kHz in the engine to electrical signals which were rectified and sent to a resistive load matched to the unimorphs. Within variations in device characteristics, results show a three-fold increase in power to the load, effectively raising the power density of the converter. Such approach with multiple piezoelectric elements provides increased power output within impedance limitations of the engine. The technique, based on a device that has essentially no moving parts and is simple, shows much promise for the conversion of heat to electricity in many applications. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B34.00005: Digital superlattice model for the measurement of strain and interfacial intermixing by X-ray diffraction Yifei Meng, Honggyu Kim, Jian-Min Zuo We have developed a digital superlattice model that describes the discrete lattice fluctuation in high quality GaSb/InAs type II superlattices (T2SL) grown by molecular beam epitaxy. T2SLs have attracted considerable attention as a candidate for middle-wavelength and long-wavelength infrared light detection. However, so far the performance of T2SL materials has been limited by short carrier lifetime below the theoretical predictions. Interfacial defects have suggested as possible cause. To quantify the T2SL structure, we extract interfacial strain and composition profile at atomic monolayer scale using a combination of direct 2theta-omega scan and model fitting. The digital superlattice model we developed describes the discrete fluctuation in T2SL, which enables accurate simulation of peak widths, positions and intensities. The simulation results indicate more cation intermixing compared with anion. Also strong evidence of interfacial strain is revealed in the X-ray diffraction data. The development of this technique allows a systematic study of interfacial treatments and their influences on atomic structure of T2SL. The detailed structure information is extremely helpful for optimizing the growth and refining existing energy band calculation model. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B34.00006: Gold plasmonic effects on charge transport through single molecule junctions Olgun Adak, Latha Venkataraman We study the impact of surface plasmon polaritons, the coupling of electromagnetic waves to collective electron oscillations on metal surfaces, on the conductance of single-molecule junctions. We use a scanning-tunneling microscope based break junction setup that is built into an optical microscope to form molecular junctions. Coherent 685nm light is used to illuminate the molecular junctions formed with 4,4'-bipyridine with diffraction limited focusing performance. We employ a lock-in type technique to measure currents induced by light. Furthermore, the thermal expansion due to laser heating is mimicked by mechanically modulating inter-electrode separation. For each junction studied, we measure current, and use AC techniques to determine molecular junction resonance levels and coupling strengths. We use a cross correlations analysis technique to analyze and compare the effect of light to that of the mechanical modulation. Our results show that junction transmission characteristics are not altered under illumination, within the resolution of our instrument. We argue that photo-currents measured with lock-in techniques in these kinds of structures are due to thermal effects. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B34.00007: Increased Efficiency of a Permanent Magnet Synchronous Generator through Optimization of NdFeB Magnet Arrays Helena Khazdozian, Ravi Hadimani, David Jiles The United States is currently dependent on fossil fuels for the majority of its energy needs, which has many negative consequences such as climate change. Wind turbines present a viable alternative, with the highest energy return on investment among even fossil fuel generation. Traditional commercial wind turbines use an induction generator for energy conversion. However, induction generators require a gearbox to increase the rotational speed of the drive shaft. These gearboxes increase the overall cost of the wind turbine and account for about 35 percent of reported wind turbine failures. Direct drive permanent magnet synchronous generators (PMSGs) offer an alternative to induction generators which eliminate the need for a gearbox. Yet, PMSGs can be more expensive than induction generators at large power output due to their size and weight. To increase the efficiency of PMSGs, the geometry and configuration of NdFeB permanent magnets were investigated using finite element techniques. The optimized design of the PMSG increases flux density and minimizes cogging torque with NdFeB permanent magnets of a reduced volume. These factors serve to increase the efficiency and reduce the overall cost of the PMSG. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B34.00008: Preperation and electrochemical characterization of Sm and Gd co-doped ceria/carbonate composite electrolytes for IT-SOFC applications Sibel Dikmen, Rabia Ozsakarya, Erdal Dikmen Sm and Gd co-doped ceria based composite electrolytes were prepared by mixing nanosized powders of Ce$_{\mathrm{0.8}}$Sm$_{\mathrm{0.1}}$Gd$_{\mathrm{0.1}}$O$_{\mathrm{2-\delta }}$ (SGDC) and alkaline carbonates (Na-Li)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, (Li-K)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, and (Na-K)$_{\mathrm{2}}$CO$_{\mathrm{3}}$ at a weight ratio of 4:1. Structure of the samples was characterized by powder X-ray diffraction. The microstructure and morphology were examined by SEM. Impedance spectroscopy was used to perform electrochemical characterization. The conductivities of the samples increase as the temperature increases and for the composite electrolytes SGDC(Na-Li)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, and SGDC(Li-K)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, there is a sharp increase in conductivity at around 475 and 450$^{\mathrm{o}}$C, respectively. This sudden change in the conductivity refers to superionic phase transition in the interfaces between SGDC phase and salt phase. The single cell power density reached a maximum of 1056, 826, and 565 mWcm$^{\mathrm{-2}}$ for SGDC/ (Na-Li)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, SGDC/(Li-K)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, and SGDC/ (Na-K)$_{\mathrm{2}}$CO$_{\mathrm{3}}$ as the electrolytes, respectively. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B34.00009: Electronic structure and excitations in oxygen deficient CeO$_{2-\delta}$ from DFT calculations C. Lane, T. Jarlborg, B. Barbiellini, Yung Jui Wang, R.S. Markiewicz, Zhi Liu, Zahid Hussain, A. Bansil Mixed valent cerium oxides (ceria) are technologically important materials with remarkable properties useful for applications in heterogeneous chemical and electrochemical catalysis. We investigated the equilibrium electronic structures of supercells of CeO$_{2-\delta}$ within the Density Functional Theory (DFT), wherein properties such as lattice constants, bulk moduli and magnetic moments were well reproduced by the generalized gradient approximation (GGA) without the need to introduce the Hubbard U parameter. The chemical expansion and magnetic moment were calculated for Ce$_{4}$O$_{8-N}$ as a function of $N$, which for $N=8$, removing all the oxygen atoms, the fcc non-magnetic $\alpha$-phase of Ce was recovered. In the ground state of defective ceria, the Ce-\textit{f} majority band resides near the Fermi level, but appears at 2 eV below the Fermi level in photoemission spectroscopy experiments. We have demonstrated that x-ray photoelectron spectroscopy (XPS) relaxation effects yield a renormalization of \textit{f}-levels away from the Fermi level for electron excitation spectroscopies, which is also consistent with Ce-M and O-K x-ray absorption spectroscopy. Work supported in part by the US Department of Energy. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B34.00010: Density functional theory study of defect energies and space charge distribution at a solid-oxide electrolyte surface Chu Han, Angelo Bongiorno Yttrium-doped barium zirconate (BZY) is a proton conducting electrolyte forming a class of novel materials for new generation of solid oxide fuel cells, for hydrogen separation and purification, and for electrolysis of water. Here we use density functional theory calculations to compute the energy of protons and oxygen vacancies at the surface and in the bulk of lightly Y-doped BZY materials. We found that protons are energetically more stable at the surface termination than in the bulk of BZY by about 1 eV. In contrast, doubly-positively charged oxygen vacancies are found to form iso-energetic defects at both the terminal surface layer and in the bulk of BZY, while in the sub-surface region the defect energy raises by about 1 eV with respect to the value in the bulk. The energetic behavior of protons and oxygen vacancies in the near surface region of BZY is attributed to the competition of strain and electrostatic effects. Lattice model representations of BZY surfaces are then used in combination with Monte Carlo simulations to solve the Poisson-Boltzmann equation and investigate the implication of the results above on the structure of the space charge region at the surface of BZY materials. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B34.00011: DFT calculations of XAS and XPS processes in Ceria cells B. Barbiellini, T. Jarlborg, C. Lane, Yung Jui Wang, R.S. Markiewicz, Liu Zhi, Zahid Hussain, A. Bansil Final-state effects in X-ray absorption spectroscopy (XAS) and X-ray photoemission(XPS) have been calculated around oxygen vacancies in ceria (CeO$_2$). The method considers final-state total energies calculated using the constrained density functional theory. In the XAS final state, a core electron is extracted and then added to the valence electrons. The electronic structure is carried out self-consistently under these conditions. After the system has relaxed, one considers the total energy difference between the unperturbed state and the relaxed state to determine the XAS threshold energy. In the XPS final state, one electron is transferred from the ground-state density of states to a homogeneous plane-wave single-particle state. The total energy of the final state is constructed by using an average Kohn-Sham energy corresponding to the hole energy level and the hole density $\rho_h$. The present scheme is able to capture important XAS and XPS features observed in experiments when oxygen vacancies are created in ceria. Work supported by the US DOE. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B34.00012: Proton Diffusion in Tungsten Oxide Dihydrate: Hints from ab initio Calculations Hao Lin, Fei Zhou, Chi-Ping Liu, Vidvuds Ozolins Knowledge of proton diffusion mechanisms in tungsten oxide (WO$_{3}$) and its hydrates (WO$_{3}$${\cdot}$H$_{2}$O,WO$_{3}$${\cdot}$2H$_{2}$O) is essential for designing fuel cell membranes, electrochromics, energy storage materials and gas sensors. It is generally believed that tungsten oxide dihydrate is a better proton conductor than anhydrous tungsten oxide and monohydrate, due to the existence of fast diffusion pathways through the interlayer structural water. Aiming to test this assumption, we performed density functional theory calculations and surprisingly found that the interlayer structural water in dihydrate does not contribute to proton diffusion and that proton diffusion mechanisms are similar in dihydrate and tungsten oxide. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B34.00013: Understanding cation ordering and oxygen vacancy site preference in Ba$_{3}$CaNb$_{2}$O$_{9}$ from first-principles Hepeng Ding, Anil Virkar, Feng Liu We investigate the physical mechanism underlying the formation of the B-site cation ordering and the oxygen vacancy site selection in Ba$_{3}$CaNb$_{2}$O$_{9}$ using density functional theory calculations. We found that either cation site exchange or oxygen vacancy formation induces negligible lattice strain. This implies that the ionic radius plays an insignificant role in governing these two processes. Furthermore, the electrostatic interactions are found dominant in the ordering of mixed valence species on one or more sites, the ionic bond strength is identified as the dominant force in governing both the 1:2 B-site cation ordering along the \textless 111\textgreater direction and the oxygen vacancy site preference in Ba$_{3}$CaNb$_{2}$O$_{9}$. Specifically, the cation ordering can be rationalized by the increased mixing bonding energy of the Ca-O-Nb bonds over the Ca-O-Ca and Nb-O-Nb bonds, i.e., 1/2(Ca-O-Ca $+$ Nb-O-Nb) \textless Ca-O-Nb; while oxygen vacancy prefers a site to minimize the electrostatic energy and to break the weaker B-O-B bond. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B34.00014: QENS Investigation of Proton Diffusion in Sr/Ca-doped LaPO$_{4}$ Amal al-Wahish, N. Jalarvo, Z. Bi, C. Bridges, M.P. Paranthaman, A. Huq, K. Herwig, D. Mandrus We have investigated the diffusion dynamics of protons in hydrated~La$_{0.958}$Sr$_{0.042}$LaPO$_{4}$ and La$_{0.958}$Ca$_{0.042}$LaPO$_{4}$. These materials contain networks of tetrahedra rather than octahedra and relatively little is known about the mechanisms of proton transport in such systems. The samples were characterized by X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), and neutron powder diffraction (NPD) from room temperature to 800 $^{\circ}$C. The macroscopic and microscopic dynamics were studied using electrochemical impedance spectroscopy (EIS) and quasielastic neutron scattering (QENS). The activation energy characterizing the proton diffusion was determined in the temperature range~500-700 $^{\circ}$C using both QENS and EIS. For~La$_{0.958}$Ca$_{0.042}$LaPO$_{4}$, QENS~reveals a dynamical process that was not detected by EIS. The QENS activation energy was determined to be 0.09 eV, roughly an order of magnitude lower than the $\sim$ 1 eV inferred from EIS measurements. We will present results from the EIS, NPD and QENS analysis of the samples. [Preview Abstract] |
Session B35: Fermi Gases
Sponsoring Units: DAMOPChair: Lawrence Cheuk, Massachusetts Institute of Technology
Room: 702
Monday, March 3, 2014 11:15AM - 11:27AM |
B35.00001: Observation of antiferromagnetic correlations in the Fermi-Hubbard model R.A. Hart, P.M. Duarte, T.L. Yang, X. Liu, R.G. Hulet, T.C.L. Paiva, D. Huse, R.T. Scalettar, N. Trivedi The physics of high temperature superconductors is not well understood, although it is known that the undoped parent compounds of many of them are antiferromagnetic (AF) insulators. The Fermi-Hubbard model at half filling (one atom per lattice site) is known to exhibit a phase transition to an antiferromagnetic insulator at a low temperature. We realize the Fermi-Hubbard model by loading ultracold $^{6}$Li atoms into a three-dimensional red-detuned optical lattice. We have compensated the confining potential of the lattice with blue-detuned laser beams in order to evaporatively cool the atoms. We have cooled sufficiently to observe AF correlations using spin-sensitive Bragg scattering of near-resonant light. Comparison with Quantum Monte Carlo (QMC) calculations indicates that the temperature is between 2-3 $T_{N}$, where short-range correlations begin to develop. Bragg scattering combined with QMC provides sensitive thermometry in a previously unexplored regime. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B35.00002: Phase diagram of a one-dimensional spin-full Bose-Fermi mixture at large boson densities Alberto Nocera, Roman M. Lutchyn, Adrian E. Feiguin We determine the ground state phase diagram of a one dimensional Bose-Fermi Hubbard model with spin-full fermions using the Density Matrix Renormalization Group (DMRG) method. We focus on the regime with one fermion per site, and deep into the superfluid phase. We study the effects of the boson-fermion interaction on the fermionic pairing, as a function of the interaction strength, hopping, and bosonic density. We identify the regime in which fermionic superfluidity dominates, and a phase with coexisting CDW and bosonic superfluidity. At high boson densities we find a fermionic Wigner crystal coexisting with bosonic superfluidity. We analyze the structure of the Cooper pairs and the bosonic cloud that acts as the glue. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B35.00003: Anisotropic Weyl Fermions from Quasiparticle Excitation Spectrum of a 3D Fulde-Ferrell Superfluid Yong Xu, Ruilin Chu, Chuanwei Zhang Weyl fermions, first proposed for describing massless chiral Dirac fermions in particle physics, have not been observed yet in experiments. Recently, much effort has been devoted to explore Weyl fermions around band touching points of single particle energy dispersions in certain solid state materials (named Weyl semimetals), similar as graphene for Dirac fermions. Here we show that such Weyl semimetals also exist in the quasiparticle excitation spectrum of a three-dimensional (3D) spin-orbit coupled Fulde-Ferrell (FF) superfluid. By varying Zeeman fields, the properties of Weyl fermions, such as their creation and annihilation, number and position, as well as anisotropic linear dispersions around band touching points, can be tuned. We study the manifestation of anisotropic Weyl fermions in sound speeds of FF fermionic superfluids, which are detectable in experiments. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B35.00004: $d_{xy}$-density wave in fermion-fermion cold atom mixtures Chen-Yen Lai, WenMin Huang, David Campbell, Shan-Wen Tsai Using a functional renormalization group (FRG) calculation, we predict number density wave instabilities in a doubly-degenerate Fermi-Fermi mixture on a square optical lattice. We take both inter-species and intra-species interactions into account and treat them on an equal footing. When the two species of fermions are both at half-filling, we find two out-of-phase conventional ($s$-wave) number density waves, which arise from from a sufficiently strong on-site inter-species repulsion. Moving only one species away from half-filling, we discover that an unconventional $d_{xy}$-density emerges. When both species are away from half-filling, a superconductivity instability becomes dominant. Apart from the detailed FRG calculation, we develop an intuitive minimal model to capture the physical mechanism, which emerges from the density imbalance between the two species of fermions in the vicinity of half-filling. Our study sheds light on the search for unconventional density waves in strong correlated systems. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B35.00005: Quantum Monte Carlo study of antiferromagnetic correlations in the Positive $U$ Fermi-Hubbard model Thereza Paiva, Russell Hart, Pedro Duarte, Ernie Yang, Xinxing Liu, Randy Hulet, David Huse, Richard Scalettar, Nandini Trivedi We use determinantal quantum Monte Carlo (QMC) simulations$^{1,2}$ to investigate the fermion Hubbard model as a function of filling, temperature and population imbalance. We find that (i) the structure factor is not very sensitive to population imbalance for the ranges of temperatures currently available in the experiments; (ii) at half filling for a large range of $U$ the antiferromagnetic structure factor collapses onto a universal curve. This scaling behavior along with QMC data for other scattering angles allows us to directly compare with experimental Bragg scattering data and put constraints on the experimental Neel ordering temperature. $^1$ ``Fermions in 2D Optical Lattices: Constraints on entropy for observing antiferromagnetism and superfluidity", T. Paiva, R.T. Scalettar, M. Randeria, and N. Trivedi, Phys. Rev. Lett. 104, 066406 (2010). $^2$ ``Fermions in 3D Optical Lattices: Cooling Protocol to Obtain Antiferromagnetism", T. Paiva, Yen Lee Loh, N. Trivedi, M. Randeria and R.T. Scalettar, Phys. Rev. Lett. 107, 086401 (2011) [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B35.00006: Extended $s$-wave superfluid of repulsively interacting three-component fermionic atoms in optical lattices Sei-ichiro Suga, Kensuke Inaba We investigate pairing symmetry of the superfluid state in repulsively interacting three-component (colors) fermionic atoms in optical lattices. This superfluid state appears, when two of the color-dependent three repulsions are much stronger than the other close to half filling [1]. We evaluate the effective pairing interaction by collecting random-phase-approximation-type diagrams and ladder diagrams, and solve the Eliashberg equation within weak-coupling theory in square optical lattices. We find that pairing symmetry is an extended $s$-wave, although in the phase diagram the superfluid state is adjacent to the color-density wave or paired Mott insulator at half filling. The $k$-dependence of the superfluid order parameter is caused by quantum fluctuations of the staggered color-density wave. When the difference in the three repulsions is decreased, paring symmetry changes from an extended $s$-wave to a $d$-wave. We expect $^{6}$Li, $^{171}$Yb, $^{173}$Yb atoms and their mixtures in optical lattices to be possible candidates for observing this superfluid state.\\[0pt] [1] K. Inaba and S. Suga, \textit{Phys. Rev. Lett.} \textbf{108} (2012) 255301. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B35.00007: Suppressing the loss of ultracold molecules via the continuous quantum Zeno effect Bihui Zhu, Bryce Gadway, Michael Foss-Feig, Johannes Schachenmayer, Michael Wall, Kaden Hazzard, Bo Yan, Steven Moses, Jacob Covey, Deborah Jin, Jun Ye, Murray Holland, Ana Rey We develop theoretical methods to explain the recently observed suppression of chemical reactions between two rotational states of fermionic KRb molecules confined in 1D tubes with a superimposed optical lattice along them [Yan {\it et al}., Nature 501, 521 (2013)]. The loss suppression is a consequence of both lattice confinement and the continuous quantum Zeno effect, which in this case takes place in the regime where the two-body loss is larger than other energy scales in the lattice. To quantitatively analyze the experiment, we derive a renormalized single-band model which accounts for 3D multi-band effects, and formulate from it a rate equation and mean-field theory validated by comparing with numerically exact t-DMRG. We demonstrate that the renormalized model captures the measured dependence of the loss rate on all lattice parameters, allowing us to determine the filling fraction. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B35.00008: Pair correlations in the two-dimensional Fermi gas Vudtiwat Ngampruetikorn, Jesper Levinsen, Meera M. Parish We consider the two-dimensional Fermi gas at finite temperature with attractive short-range interactions. Using the virial expansion, which provides a controlled approach at high temperatures, we determine the spectral function and contact for the normal state. Our calculated spectra are in qualitative agreement with recent photoemission measurements [M. Feld et al., Nature 480, 75 (2011)], thus suggesting that the observed pairing gap is a feature of the high-temperature gas rather than being evidence of a pseudogap regime just above the superfluid transition temperature. We further argue that the strong pair correlations result from the fact that the crossover to bosonic dimers occurs at weaker interactions than previously assumed. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B35.00009: FFLO state of two-dimensional imbalanced Fermi gases Daniel Sheehy Trapped fermionic atomic gases exhibit superfluid states, akin to superconductivity in a metal, due to the pairing of two species of atomic fermion. In recent years, there has been much experimental and theoretical interest in studying the behavior of fermionic superfluids under an imposed population imbalance that disrupts such pair formation and superfluidity and can lead to new phases, including phase separation, imbalanced normal (Fermi liquid), and the elusive Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state predicted to exhibit a spatially-modulated superfluid state to accommodate the population imbalance. I will present recent theoretical results on fermionic atomic gases confined to a quasi two-dimensional (2D) geometry, showing that the FFLO phase in may more stable in 2D than in the bulk (3D) case (similar to the case of quasi-1D imbalanced gases), providing another possible setting for observing the FFLO state. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B35.00010: Disordered Mott Insulator in heavy-light Fermi mixture in optical lattices Anzi Hu, Maciej Maska, Jim Freericks, Charles Clark Ultracold mixtures of different atomic species have great promise of realizing novel many-body phenomena beyond Hubbard model. In a mixture of femionic atoms with large mass differences, a disordered Mott insulator can be formed as the result of the repulsive interaction between two species. The disorder Mott insulator leads to an incompressible total density of the mixture while the relative density is still compressible. Based on strong-coupling expansion and Monte Carlo calculations, we show that this phase can exist for a broad parameter region for ultracold mixtures confined by a harmonic trap and a three-dimensional optical lattice. The realization of such phase can lead to new ways of quantum control in ultracold mixtures. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B35.00011: Evaporative cooling in a compensated optical lattice P.M. Duarte, R. Hart, T.L. Yang, X. Liu, R.G. Hulet We present experimental results of evaporative cooling in a three-dimensional, red-detuned optical lattice. The lattice is compensated by the addition of three blue-detuned gaussian beams which overlap each of the lattice laser beams, but are not retro-reflected~\footnote{C.~J.~M. Mathy, et al., ``Enlarging and cooling the N\'eel state in an optical lattice,'' Phys. Rev. A {\bfseries 86,} 023606 (2012).}. The intensity of the compensating beams can be used to control the difference between the chemical potential in the lattice and the threshold for evaporation. We start with a two spin component degenerate Fermi gas of $^{6}$Li atoms at a temperature $<0.05 T_{F}$ in a dimple potential, which is obtained by rotating the polarization of the lattice retro beams to prevent the formation of standing waves. The temperature of the cloud is measured by releasing it from the dimple and fitting the momentum distribution to a Thomas-Fermi profile. We perform round-trip measurements into, and out of the lattice to study the adiabaticity of the loading as well as the effect of the compensating beams. Using the compensated lattice potential, we have reached temperatures low enough to produce antiferromagnetic spin correlations, which we detect via Bragg scattering of light. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B35.00012: Energy, decay rate, and effective masses for a moving polaron in a Fermi sea: Explicit results in the weakly attractive limit Trefzger Christian, Castin Yvan We study the properties of an impurity of mass $M$ moving through a spatially homogeneous three-dimensional fully polarized Fermi gas of particles of mass $m$. In the weakly attractive limit, where the effective coupling constant $g\to0^-$ and perturbation theory can be used, both for a broad and a narrow Feshbach resonance, we obtain an explicit analytical expression for the complex energy $\Delta E(\mathbf{K})$ of the moving impurity up to order two included in $g$. This also gives access to its longitudinal and transverse effective masses $m_\parallel^*(\mathbf{K})$, $m_\perp^*(\mathbf{K})$, as functions of the impurity wave vector $\mathbf{K}$. Depending on the modulus of $\mathbf{K}$ and on the impurity-to-fermion mass ratio $M/m$ we identify four regions separated by singularities in derivatives with respect to $\mathbf{K}$ of the second-order term of $\Delta E(\mathbf{K})$, and we discuss the physical origin of these regions. Remarkably, the second-order term of $m_\parallel^*(\mathbf{K})$ presents points of non-differentiability, replaced by a logarithmic divergence for $M=m$, when $\mathbf{K}$ is on the Fermi surface of the fermions. We also discuss the third-order contribution and relevance for cold atom experiments. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B35.00013: Persistence of Bose condensate after Fermi surface destruction in a Bose-Fermi Mixture Eric Duchon, Shizhong Zhang, Soon-Yong Chang, Mohit Randeria, Nandini Trivedi We propose a single variational wave function to investigate the ground state properties of a Bose-Fermi mixture with equal boson and fermion population. We use variational and diffusion quantum Monte Carlo techniques to study this mixture as a function of increasing attraction between bosons and fermions. Sandwiched between the expected states in weak and the strong coupling limits, we find evidence for a novel state at intermediate coupling for which we make two predictions: (I) a complete destruction of the atomic Fermi surface and emergence of a molecular Fermi sea that coexists with a remnant of the Bose-Einstein condensate, and (II) evidence for fermion pairing correlations mediated by bosons. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B35.00014: Unconventional superfluid in a two-leg fermonic ladder Shun Uchino, Akiyuki Tokuno, Thierry Giamarchi We show that a novel unconventional superfluid triggered by a spin-orbit coupling is realized in a repulsively interacting fermonic ladder system. A competition between spin singlet and triplet pairings occurs due to the breaking of inversion symmetry. We show that both superfluid orders decay algebraically with the same exponent except for the special coupling constants for which a dominant superfluid is determined solely by the spin-orbit coupling. We also propose an experiment to observe such phases with cold atoms. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B35.00015: Quantum simulation of correlated-hopping models with fermions in optical lattices M. Di Liberto, C.E. Creffield, G.I. Japaridze, C. Morais Smith By using a modulated magnetic field in a Feshbach resonance for ultracold fermionic atoms in optical lattices, we show that it is possible to engineer a class of models usually referred to as correlated-hopping models. These models differ from the Hubbard model in exhibiting additional density-dependent interaction terms that affect the hopping processes. In addition to the spin-SU(2) symmetry, they also possess a charge-SU(2) symmetry, which opens the possibility of investigating the $\eta$-pairing mechanism for superconductivity introduced by Yang for the Hubbard model. We discuss the known solution of the model in 1D (where $\eta$ states have been found in the degenerate manifold of the ground state) and show that, away from the integrable point, quantum Monte Carlo simulations at half filling predict the emergence of a phase with coexisting incommensurate spin and charge order. [Preview Abstract] |
Session B36: Invited Session: Visa and Immigration Policies for 21st Century Science
Sponsoring Units: FIPChair: Amy Flatten, Director of International Affairs, American Physical Society
Room: 703
Monday, March 3, 2014 11:15AM - 11:51AM |
B36.00001: George Washington University Visa Project--Streamlining Our Visa and Immigration Systems for Scientists and Engineers Invited Speaker: Albert H. Teich Many scientists believe that current U.S. visa and immigration systems are out of sync with today's increasingly globalized science and technology. This talk will highlight specific proposals that would facilitate the recruitment of promising STEM students by U.S. universities and better enable international scientists and engineers to visit the United States for scientific conferences and research collaboration. Most of these proposals could be implemented without additional resources and without compromising U.S. security. The talk is based on the results of an 18 month study conducted at the George Washington University's Center for International Science \& Technology Policy. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B36.00002: National Academy of Sciences: Helping Scientists Navigate \& Troubleshoot Visa Issues Invited Speaker: Kathie Bailey The International Visitors Office (IVO) is a program operated by the Board on International Scientific Organizations of the National Academy of Sciences. The IVO serves as a resource on visa-related issues for scientists and students traveling to the United States for professional activities. The speaker will address visa issues for international scientists wishing to visit the United States, tips for trouble-shooting visa issues, and statistics on the current visa system. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B36.00003: Improvements to the Visa Application System: Serving the S\&T Community, Promoting The American Economy and Keeping Us Safe Invited Speaker: Mathew Gillen The speaker will address policy changes and improvements in visa processing that help scientists and students to visit and study in the United States. The speaker will also discuss challenges involved with balancing the needs of U.S. science with national security interests. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 2:15PM |
B36.00004: Panel Discussion and Audience Q\&A |
Session B37: Focus Session: Graphene Stacking Sequence, Including Twisted Bilayers
Sponsoring Units: DMPChair: Diego Mastrogiuseppe, Ohio University
Room: 705/707
Monday, March 3, 2014 11:15AM - 11:27AM |
B37.00001: Synthesis of twisted bilayer graphene and studies of its low energy Raman modes Ting Fung Chung, Rui He, Conor Delaney, Courtney Keiser, Luis A. Jauregui, Paul M. Shand, C.C. Chancey, Yanan Wang, Jiming Bao, Yong P. Chen We have synthesized bilayer graphene on copper foils with different twist angles and stacking orders using chemical vapor deposition. Raman spectroscopy has been used to study twisted bilayer graphene (tBLG) transferred on Si/SiO2 substrate, focusing on low frequency Raman modes below 200 cm$^{-1}$. The modes are found in a small range of twist angle at which the G Raman peak is under resonance conditions with corresponding laser energy. The $\sim$ 94 cm$^{-1}$ mode (ZO'$_{\mathrm{L}})$ and $\sim$ 160 cm$^{-1}$ (ZO'$_{\mathrm{H}})$ modes (measured with a 532 nm laser) are assigned to the fundamental layer breathing vibration (ZO' mode) associated with different phonon wavenumbers, indicating different phonon scattering processes. We identify that the ZO'$_{\mathrm{L}}$ mode shares the same resonance enhancement mechanism as G Raman mode arising from van Hove singularities (vHs) in the band structure of tBLG. The ZO'$_{\mathrm{H}}$ mode was previously observed, related to the superlattice induced wavevector. The dependence of ZO'$_{\mathrm{L}}$ mode frequency and line width on the twist angle can be understood by the double-resonance Raman scattering. We also observe another lower energy Raman mode at $\sim$ 52 cm$^{-1}$, whose origin is yet to be understood. We have also measured the doping dependence of Raman modes in tBLG. Our results probe the interlayer coupling and phonon dispersions in tBLG. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B37.00002: Theory of twisted graphene bilayers near commensuration Hridis Pal, Steven Carter, Markus Kindermann It has been predicted [1,2] that a gap can arise in the Dirac spectrum of commensurately twisted graphene bilayers. Hitherto that gap has not been observed experimentally since it is difficult to produce samples with a specified twist angle. This motivates us to construct a long wavelength theory of almost commensurately rotated graphene bilayers. The theory inherits its structure from the exactly commensurate bilayer that it is close to. It thus makes the physics of commensurate graphene bilayers more easily accessible experimentally. [1] E. J. Mele, Phys. Rev. B 81, 161405 (R) (2010). [2] S. Shallcross, S. Sharma, and O. A. Pankratov, Phys. Rev. Lett. 101, 056803 (2008). [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B37.00003: Twisted Bilayer Graphene with Controlled Rotation Angles Yanan Wang, Sirui Xing, Xiaoxiang Lu, Francisco Robles-Hernandez, Shin-shem Pei, Jiming Bao With unique rotation-angle dependent electronic band structure, twisted bilayer graphene (tBLG) is expected to be a promising platform for future semiconductor electronic and photonic applications. Although tBLG has been observed in the samples prepared by silicon sublimation of SiC, chemical vapor deposition (CVD), and stacking of single-layer graphene, tBLG with controlled rotation angles has not been demonstrated. In this work, we present a simple transfer method to create tBLG domains with pre-defined rotation angles in the size of a few hundred micrometers. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B37.00004: Theoretical studies of structure-property relations in graphene-based carbon nanostructures Invited Speaker: Dimitrios Maroudas This presentation focuses on establishing relations between atomic structure, electronic structure, and properties in graphene-based carbon nanostructures through first-principles density functional theory calculations and molecular-dynamics simulations. We have analyzed carbon nanostructure formation from twisted bilayer graphene, upon creation of interlayer covalent C-C bonds due to patterned hydrogenation or fluorination. For small twist angles and twist angles near 30 degrees, interlayer covalent bonding generates superlattices of diamond-like nanocrystals and of fullerene-like configurations, respectively, embedded within the graphene layers. The electronic band gaps of these superlattices can be tuned through selective chemical functionalization and creation of interlayer bonds, and range from a few meV to over 1.2 eV. The mechanical properties of these superstructures also can be precisely tuned by controlling the extent of chemical functionalization. Importantly, the shear modulus is shown to increase monotonically with the fraction of \textit{sp}$^{\mathrm{3}}$-hybridized C-C bonds. We have also studied collective interactions of multiple defects such as random distributions of vacancies in single-layer graphene (SLG). We find that a crystalline-to-amorphous structural transition occurs at vacancy concentrations of 5-10{\%} over a broad temperature range. The structure of our defect-induced amorphized graphene is in excellent agreement with experimental observations of SLG exposed to a high electron irradiation dose. Simulations of tensile tests on these irradiated graphene sheets identify trends for the ultimate tensile strength, failure strain, and toughness as a function of vacancy concentration. The vacancy-induced amorphization transition is accompanied by a brittle-to-ductile transition in the failure response of irradiated graphene sheets and even heavily damaged samples exhibit tensile strengths near 30 GPa, in significant excess of those typical of engineering materials. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B37.00005: Scaling laws of van Hove singularities in twisted bilayer graphene Jeil Jung, Ashley DaSilva, Yang Wang, Dillon Wong, Michael Crommie, Shaffique Adam, Allan H. MacDonald Van Hove singularities (vHS) appear in twisted coupled bilayer graphene at saddle points in the band structure. The lowest energy vHS can be associated with the overlap between the displaced Dirac cones of the top and bottom layers, resulting in an approximately linear increase of its position in energy with increasing twist angle. This picture, which is applicable in the perturbative regime for moderately large twist angles, sees departures in the small angle limit due to non-perturbative coupling between the layers. Using a theory for twisted bilayer graphene [1] that incorporates all the relevant interlayer coupling compatible with momentum conservation of k-vectors of the top and bottom layers we explore the scaling laws of the vHS for sufficiently small twist angles and long period moire superlattices. We analyze the localization properties of their wave functions through their local density of states (LDOS) paying particular attention to the behavior of the states corresponding to higher energy van Hove singularities. We comment on our results in light of the experimental DOS and LDOS maps obtained through scanning tunneling microscopy. [1] R. Bistritzer and A. H. MacDonald, PNAS 108 (30), 12233 (2011) [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B37.00006: First-Principles Calculations of Off-Normal LEEM Reflectivity Spectra of Few Layer Graphene John McClain, Karsten Pohl, Jian-Ming Tang We present calculations of the off-normal low-energy electron specular reflectivity spectra of few layer graphene (FLG) systems using our first-principles theoretical approach that leverages the self-consistent scattering potentials produced by density-functional theory [1]. Our Bloch wave matching approach, which replaces the traditional analysis using multiple scattering off muffin-tin potentials, admits a straightforward handling of non-normal incident beams. Our calculated off-normal spectra for free-standing FLG reveal the shifting of the characteristic thickness-dependent oscillations in reflectivity found for energies between 0 and 7 eV in normal-incidence low-energy electron microscopy (LEEM) spectra. We also find shifts in other peaks and new features for incoming beams with in-plane momentum far from $\Gamma$. We compare the spectra to features in the in-plane band structure of FLG and to available experimental LEEM and LEED data for FLG on metallic and semiconductor substrates. We discuss modeling reflection for small deviations from normal incidence, as well as the possibility of accessing novel spectra features using wide-angle scattering. [1] McClain et al., arXiv.1311.2917. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B37.00007: Electrochemical Intercalation of Few-Layer Graphenes: Method and Characterization Shu Yang Frank Zhao, Giselle A. Elbaz, Dmitri K. Efetov, Jayakanth Ravichandran, Yinsheng Guo, Natalee Raymond, Louis Brus, Xavier Roy, Philip Kim Few layer graphene (FLG) intercalate compounds form a new generation of graphene derivative systems where novel physical phenomena such as superconductivity and magnetism may emerge. Experimental realization of FLG has been limited to the harsh intercalation processes which are often not compatible with mesoscopic device fabrication techniques. We demonstrate the in-situ intercalation and transport measurements of mechanically exfoliated FLGs using alkali metals via electrochemical methods. With suitable passivation methods, we isolate the FLG's contribution to the electrochemical current, and electronically monitor the intercalation reaction in real time, via cyclic voltammetry. We correlate the intercalation signatures from cyclic voltammetry with optical and Raman characteristics of the FLGs. Finally, we characterize the intercalated few-layer graphene compounds by transport measurements down to cryogenic temperatures. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B37.00008: Exciton Effects on Doped and Gated Twisted Bilayer Graphene Ryan Soklaski, Yufeng Liang, Li Yang Turbostratic graphite and epitaxially grown few-layer graphene (FLG) are known to exhibit significant rotational defects -- a departure from familiar Bernal stacked FLG systems. The admixing of states across rotated graphene layers occur far from their respective valleys, giving rise to saddle-point van Hove singularities. We study the effects of doping and voltage gating on twisted bilayer graphene. In particular, we perform first-principle calculations, including e-e and e-h interactions, of the optical absorption spectra of doped and gated twisted bilayer graphene. Increasing the doped carrier density enhances screening in the system, reducing both the self-energy corrections and e-h interaction effects -- an effect also seen in doped single layer graphene. On the other hand, gating the system leads to a misalignment of van Hove singularities, diminishing the joint density of states and hence the exciton strength. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B37.00009: Optical properties of twisted bilayer graphene Pilkyung Moon, Young-Woo Son, Mikito Koshino A twisted stack of two graphene layers (twisted bilayer graphene) exhibits an extremely long potential period arising from the Moir\'{e} interference between the layers. We calculate the optical absorption of twisted bilayer graphene in the absence of magnetic field and demonstrate that the spectroscopic characteristics serve as a fingerprint to identify the rotation angle between two layers\footnote{P. Moon and M. Koshino, Phys.\ Rev.\ B \textbf{87}, 205404 (2013).}. We explain the peculiar optical selection rule in terms of the symmetry of the effective Hamiltonian. We also investigate the effects of charging and gating on the optical spectrum\footnote{P. Moon, M. Koshino, and Y.-W. Son, in preparation}. In addition, we investigate the absorption spectrum and the selection rule for the fractal band regime (Hofstadter butterfly) in the presence of magnetic field. We demonstrate that the absorption spectrum exhibits a self-similar recursive pattern reflecting the fractal nature of the energy spectrum, and the optical selection rule has a nested self-similar structure as well\footnote{P. Moon and M. Koshino, arXiv:1308.0713 (2013)}. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B37.00010: Incoherent interlayer conduction in twisted bilayer graphene Youngwook Kim, S.-G. Nam, H.-J. Lee, Jun Sung Kim, H. Yun, S.W. Lee, M. Son, H.C. Choi, D.S. Lee, D.C. Kim, S. Seo Coherent motion of the electrons in the Bloch states often breaks down for the interlayer conduction in layered materials where the interlayer coupling is significantly reduced by e.g. large interlayer separation. Here, we report complete suppression of coherent conduction in twisted bilayer graphene even with an atomic length scale of layer separation. The interlayer conduction were investigated using a cross junction of monolayer graphene layers. The interlayer resistivity is much higher than the c-axis resistivity of Bernal-stacked graphite and exhibits strong dependence on temperature as well as on external electric fields. These results suggest that the graphene layers are significantly decoupled by rotation, and the incoherent electron tunneling is the main interlayer conduction channel. In this regime, the interlayer conduction is determined by the overlap of the Dirac Fermi surfaces (FS) from each layer. The angle dependence of the interlayer resistivity is found to be relatively strong at low temperatures, while it becomes moderate and monotonous at high temperatures. This demonstrates the importance of phonon-mediated conduction at high temperatures, which enhances the overlap between the momentum-mismatched FS's in twisted bilayer graphene. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B37.00011: Gated Raman Spectroscopy of Twisted Bilayer Graphene Shengqiang Huang, Kanokporn Chattrakun, Matthew Yankowitz, Arvinder Sandhu, Brian LeRoy The interaction of charge carriers with lattice vibrations in graphene exhibits many intriguing physical phenomena. Raman spectroscopy is a powerful non-destructive technique to probe these interactions. In twisted bilayer graphene, the electronic band structure and phonon dispersion depend on the rotation angle between the layers. Here we present a systematic Raman spectroscopy study of twisted bilayer graphene, using a 532 nm laser, with controllable charge densities up to 2$\times$10$^{13}$cm$^{-2}$. The twist angle is first identified by the observation of a moire pattern in STM measurements. In the angle range between 5 and 8 degrees, the R$'$ peak softens and weakens with increasing charge density. Near 12 degrees, the G peak is enhanced due to the increased density of states in twisted bilayer graphene. However, the G peak area quickly decreases with increasing charge density. Lastly, we observed several unusual effects for the G peak for all angles from 2 to 10 degrees as a function of increasing charge density. We found that the G peak broadened, split and oscillated in position. All these results demonstrate that twisted bilayer graphene has rich optoelectronic properties. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B37.00012: A Scanning Tunneling Study of Twisted Bilayer Graphene Dillon Wong, Yang Wang, Jeil Jung, Ashley DaSilva, Sergio Pezzini, Hsin-zon Tsai, Han Sae Jung, Ramin Khajeh, Youngkyou Kim, Salman Kahn, Sajjad Tollabimazraehno, Haider Rasool, Juwon Lee, Kenji Watanabe, Takashi Taniguchi, Alex Zettl, Shaffique Adam, Allan MacDonald, Michael Crommie The properties of bilayer graphene strongly depend on the angle of rotation between its two layers. We investigated the local electronic structure of twisted bilayer graphene on an insulating substrate. Using scanning tunneling microscopy, we measured the energy dependence of features in the differential tunneling conductance for many different twist angles. Comparison with theoretical calculations reveal the physical origin of these features. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B37.00013: Exchange self-energy and compressibility of multilayer graphene by wavefunction rotation method Hongki Min Multilayer graphene has chiral electronic structure which strongly depends on the stacking sequence. A fundamental issue is to understand the interplay between the chiral electronic structure and electron-electron interaction, and the exchange interaction is the leading-order correction to the electron-electron interaction. The exchange energy calculation of multilayer graphene, however, requires a large amount of computational cost, because of non-local nature of the exchange interaction and because of the absence of the analytic form of the wavefunction, which should be obtained self-consistently. We overcome this problem using the wavefunction rotation method, in which the angular part of the wavefunction is obtained analytically by attaching a phase factor that is determined by the stacking sequence, thus reducing the computational cost significantly. Using this method, we calculate the exchange self-energy and compressibility of multilayer graphene, and discuss the role of intralayer and interlayer exchanges. [Preview Abstract] |
Session B38: Invited Session: Women and the Manhattan Project
Chair: Margaret Murnane, University of Colorado BoulderRoom: 709/711
Monday, March 3, 2014 11:15AM - 11:51AM |
B38.00001: The Girls of Atomic City: The Untold Story of the Women Who Helped Win World War II Invited Speaker: Denise Kiernan |
Monday, March 3, 2014 11:51AM - 12:27PM |
B38.00002: Women and the Hanford Site Invited Speaker: Michele Gerber When we study the technical and scientific history of the Manhattan Project, women's history is sometimes left out. At Hanford, a Site whose past is rich with hard science and heavy construction, it is doubly easy to leave out women's history. After all, at the World War II Hanford Engineer Works -- the earliest name for the Hanford Site -- only nine percent of the employees were women. None of them were involved in construction, and only one woman was actually involved in the physics and operations of a major facility -- Dr. Leona Woods Marshall. She was a physicist present at the startup of B-Reactor, the world's first full-scale nuclear reactor -- now a National Historic Landmark. Because her presence was so unique, a special bathroom had to be built for her in B-Reactor. At World War II Hanford, only two women were listed among the nearly 200 members of the top supervisory staff of the prime contractor, and only one regularly attended the staff meetings of the Site commander, Colonel Franklin Matthias. Overall, women comprised less than one percent of the managerial and supervisory staff of the Hanford Engineer Works, most of them were in nursing or on the Recreation Office staff. Almost all of the professional women at Hanford were nurses, and most of the other women of the Hanford Engineer Works were secretaries, clerks, food-service workers, laboratory technicians, messengers, barracks workers, and other support service employees. The one World War II recruiting film made to attract women workers to the Site, that has survived in Site archives, is entitled ``A Day in the Life of a Typical Hanford Girl.'' These historical facts are not mentioned to criticize the past -- for it is never wise to apply the standards of one era to another. The Hanford Engineer Works was a 1940s organization, and it functioned by the standards of the 1940s. Just as we cannot criticize the use of asbestos in constructing Hanford (although we may wish they hadn't used so much of it), we cannot criticize the employment realities or the social practices of those days. If we can simply understand the past, then maybe we can learn from it. This presentation will highlight the success stories of many of Hanford's women. About 4,000 women came to the gargantuan, remote desert location, most of them young and away from home for the first time. Almost all of them were coming to a place they had never heard of and undertaking a mission that could not be explained to them because it was Top Secret. Faced with decidedly unequal opportunity, they came and took the jobs that were available, because they felt a personal dedication to the war effort. They had fun at Hanford, despite living in dusty barracks and eating mess hall food, and they left their mark on Hanford and its memories in many ways. Without them, the Site could not have functioned, and the war might not have been won as soon as it was. They then became the grandmothers of Richland, Washington, who told their stories to me in the 1990s. This presentation will show the lives of these women at Hanford during the Manhattan Project, as they worked to make the best of the situation, contribute and do their jobs. Their feelings about the work 50 years later will also be discussed. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B38.00003: After the War: Stories of the women who did scientific and technical work on the Manhattan Project Invited Speaker: Ruth Howes When the Japanese surrendered on September 2, 1945, the crash research program of the Manhattan Project abruptly ended although the labs continued to produce nuclear weapons to supplement the single implosion bomb ready for deployment. The older (over 25) members of the scientific staff at Los Alamos and other Manhattan Project sites were eager to return to jobs at universities and in industry, and the younger staff members wanted to finish graduate degrees. Women were no exception to this rule. In addition, the government launched a huge propaganda effort to persuade women to leave the workforce and make jobs available for returning GIs. Doors that had been open to women scientists and technicians abruptly closed as the economy returned to a peacetime footing. Stories of the women scientists and technicians of the Manhattan Project illustrate the strategies they used to meet these challenges as well as their remarkable determination to continue their careers. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:39PM |
B38.00004: Preserving the Manhattan Project Invited Speaker: Cynthia Kelly When future generations look back on the 20$^{th}$ century, few events will rival the harnessing of nuclear energy as a turning point in world history, science and society. Yet, the Department of Energy has not always embraced its Manhattan Project origins. The presentation will focus on the progress made over the last 20 years to preserve the properties and first-hand accounts that for decades have been threatened with demolition and indifference. Since the mid-1950s, most remaining Manhattan Project properties at the Los Alamos National Laboratory had been abandoned. Among them was a cluster of wooden buildings called the ``V Site.'' This is where scientists assembled the ``Gadget,'' the world's first atomic device tested on July 16, 1945. Regardless of its significance, the ``V Site'' buildings like all the rest were slated for demolition. The Advisory Council on Historic Preservation (ACHP) toured the properties in November 1998. Most could not believe that the world's first atomic bomb was designed in such humble structures. The properties were declared to be ``monumental in their lack of monumentality.'' A Save America's Treasures grant for {\$}700,000 was awarded to restore the properties. To raise the required matching funds, I left the Federal government and soon founded the Atomic Heritage Foundation. The presentation will trace the progress made over the last decade to generate interest and support nationwide to preserve the Manhattan Project heritage. Saving both the physical properties and first-hand accounts of the men and women have been a priority. Perhaps our most significant achievement may be legislation now under consideration by Congress to create a Manhattan Project National Historical Park. Seventy years later, the Manhattan Project is finally getting the recognition it deserves. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 2:15PM |
B38.00005: Then and Now: Women Respond to the Manhattan Project -- an illustrated talk Invited Speaker: Olivia Fermi, M.A. I am very much looking forward to visiting you, the family of physicists gathering for your annual APS conference. In different ways, my grandfather Enrico Fermi is a member of both our families. In this sense we are connected and share a common legacy, which I want to explore from the angle of two women inextricably involved with and affected by the Manhattan Project. One from the past and one alive now. These two women, despite a significant temporal and cultural gap share a remarkable number of traits and values. My talk will not offer a particular thesis or finding. Rather it will be about ways of seeing, including questioning unnoticed assumptions and belief systems. My grandmother Laura Fermi, modeled this for me as a youngster. She was at Enrico's side during the Manhattan Project years, yet in the dark about his work. What was it like to live in a climate of intellectually and patriotically charged enthusiasm, with an undercurrent of unspoken dread? Laura, just like most everyone else, discovered the true nature of the effort on the day the atomic bomb was dropped on Hiroshima. After the war and after Enrico's untimely death in 1954, in response to all she had experienced, Laura re-invented herself as an author and visionary, pioneering in both the environmental and handgun control movements. Marian Naranjo lives on the Santa Clara Pueblo near Los Alamos. Her ancestors dwelled on the Pajarito Plateau which encompasses the space where Los Alamos National Labs (LANL) is today. Her people, the Pueblo People have used the area's natural resources for ceremonial and survival for uncountable generations. They say, ``We \textit{are} this place.'' What is it like to live on land one's families have safely occupied for thousands of years, with an undercurrent of dread at the prospect of toxic waste stream products from LANL? Like my grandmother did in her place and time, Marian builds community as an integral part of her environmental and social justice activism. She is regularly a presenter at the table with LANL, DOE and other organizations; and also works to empower women and youth. Laura and Marian's lives illustrate the potential for engaged response to the Manhattan Project and its legacy. Implicit in them are fundamental moral and ethical questions. What is the nature of individual responsibility? Does it differ for men and women? How does the interplay between masculine and feminine forces affect our culture, and what does it imply for our future? [Preview Abstract] |
Session B39: Invited Session: Silicon-based Quantum Information Processing
Sponsoring Units: DCMPChair: Michael Flatte, University of Iowa
Room: Mile High Ballroom 2A-3A
Monday, March 3, 2014 11:15AM - 11:51AM |
B39.00001: Building quantum states at the silicon surface using dangling bonds Invited Speaker: Steven Schofield Scanning tunnelling microscopes can be used to deterministically introduce atomic-scale defects in semiconductors [1-3], and this is considered a promising route toward the fabrication of a solid-state quantum computer. Here, we investigate the properties of deep centre defects created in the hydrogen terminated silicon (001) surface by removing individual hydrogen atoms to form dangling bonds (DBs). We demonstrate that pairs, linear chains, and two dimensional structures of individual DBs form quantum dot type states with probability density maxima between the missing H atom sites. By using the STM tip as an electrostatic gate to control which states contribute to the STM image, we suggest the origin of these surprising and previously unobserved states are first excited states of the individual DBs [1]. Our results show that quantum states can be fabricated on silicon with atomic-scale precision, and suggest a general model of quantum state fabrication using other passivated semiconductor surfaces. \\[4pt] [1] Schofield et al., Nature Commun. 4, 1649 (2013)\\[0pt] [2] Schofield et al., PRL 91, 136104 (2003).\\[0pt] [3] Koenraad et al., Nature Mater. 10, 91, (2011) [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B39.00002: A single dopant atom in silicon sees the light Invited Speaker: Sven Rogge Optical access to a single qubit is very attractive since it allows for readout with unprecedented high spectral resolution and long distance coupling. Substantial progress has been demonstrated for nitrogen-vacancy centers in diamond (Bernien, Nature, 2013). Optical access to qubits in silicon been an important goal but has to date only been achieved in the ensemble limit (Steger, Science, 2012). Here, we present the photoionization of an individual erbium dopant in silicon (Yin, Nature, 2013). A single-electron transistor is used as a single-shot charge detector to observe the resonant ionization of a single atom as a function of photon energy. This allows for optical addressing and electrical detection of individual erbium dopants with exceptionally narrow line width. The hyperfine coupling is clearly resolved which paves the way to single shot readout of the nuclear spin. This hybrid approach is a first step towards an optical interface to dopants in silicon. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B39.00003: Qubit control in phosphorus doped silicon nanowires Invited Speaker: Adam Gali Quantum confinement can turn thin silicon nanowires (SiNWs) to wide band gap material where the large surface-to-volume ratio indicates that its electronic structure may be tailored by surface termination. Here we show an example how these properties of thin SiNWs may be utilized to host quantum bits. A phosphorus (P) donor has been extensively studied in bulk silicon to realize the concept of Kane quantum computers. In most cases the quantum bit was realized as an entanglement between the donor electron spin and the nonzero nuclei spin of the donor impurity mediated by the hyperfine coupling between them. The donor ionization energies and the spin-lattice relaxation time limited the temperatures to a few kelvin in these experiments. Here, we demonstrate by means of ab initio density functional theory calculations that quantum confinement in thin SiNWs results in (i) larger excitation energies of donor impurity and (ii) a sensitive manipulation of the hyperfine coupling by external electric field. We propose that these features may allow to realize the quantum bit (qubit) experiments at elevated temperatures with a strength of electric fields applicable in current field-effect transistor technology. We also show that the strength of quantum confinement and the presence of strain induced by the surface termination may significantly affect the ground and excited states of the donors in thin SiNWs, possibly allowing an optical read-out of the electron spin [1]. Another forms of donor-related defects as potential qubits will be also discussed.\\[4pt] Work done in collaboration with Binghai Yan, Max Planck Institute for Chemical Physics of Solids, Dresden, and Riccardo Rurali, Institut de Ci\`encia de Materials de Barcelona (ICMAB-CSIC).\\[4pt] [1] Binghai, Rurali, Gali, Nano Letters, 12, 3460 (2012). [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:39PM |
B39.00004: Physics of high field magnetic white dwarf stars -- relevance to silicon quantum information applications? Invited Speaker: Ben Murdin Shallow donor impurities in silicon, once frozen out at low temperature, share many properties in common with free hydrogen atoms [1]. They have long been the subject of spectroscopic investigation, but it is only very recently [2,3] that it has been possible to investigate the time-domain dynamics of orbital excitations such as the 1s to 2p, due to the difficulty of obtaining short, intense pulses in the relevant wavelength range. These new techniques make shallow donors, and also acceptors [4], attractive for studying atomic physics effects, and for applications in quantum information. We have measured the population dynamics [2] of electrons orbiting around phosphorus impurities in commercially-available silicon, and shown that the lattice relaxation lifetime is about 200ps, only 1 order of magnitude shorter than the radiative lifetime of free hydrogen. Recently we also showed that high magnetic fields can introduce enormous changes in the electron wavefunction [1], and that easily available fields could be used for spatial control of the Rydberg orbital, and hence the overlap with adjacent atoms. A spin off benefit of the analogy with free hydrogen, is that we can use the results to better understand the spectroscopy of free hydrogen atoms on the surface of white dwarf stars where the magnetic field can be as high as one gigagauss.\\[4pt] [1] BN Murdin et al Nature Communications 4, 1469 (2013);\\[0pt] [2] NQ Vinh, et al, Proc Nat Acad Sci USA 105, 10649 (2008);\\[0pt] [3] PT Greenland, et al Nature 465, 1057 (2010);\\[0pt] [4] NQ Vinh, et al Phys Rev X 3, 011019 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 2:15PM |
B39.00005: Engineered defect spin states in silicon carbide for sensing and computation Invited Speaker: Abram Falk Crystal defects can confine isolated electronic spins and are promising candidates for solid-state quantum bits. Alongside research efforts focusing on nitrogen-vacancy centers in diamond, an alternative approach seeks to identify and control new spin systems with an expanded set of technological capabilities, a strategy that could ultimately lead to ``designer'' spins with tailored properties. We show that the 4H, 6H and 3C polytypes of silicon carbide are all hosts for optically addressable spin states, including states in all three whose long quantum coherence times persist up to room temperature\footnote{A. L. Falk, B. B. Buckley, G. Calusine, W. F. Koehl, V. V. Dobrovitski, C. A. Zorman, P. X.-L. Feng, D. D. Awschalom. Polytype control of spin qubits in SiC. \textit{Nat. Commun.}, {\bf 4}, 1819 (2013)} and states with highly spin-dependent photoluminescence. Atomic-scale sensing with SiC defects is also an exciting and developing area of research, particularly since the polar character of the Si-C bond enhances the sensitivity of defect spin transitions to electric fields. We show that SiC defects can be used for high-sensitivity electric- and strain- field measurements\footnote{ A. L. Falk, P. V. Klimov, B. B. Buckley, V. Iv\'{a}dy, W. F. Koehl, I. A. Abrikosov, \'{A}. Gali, and D. D. Awschalom. Strain and electric field sensing with defect spins in SiC. \textit{Submitted} (2013).} and controlled on the nanometer scale through electrically driven spin resonance.\footnote{P. V. Klimov, A. L. Falk, B. B. Buckley, D. D. Awschalom, Electrically driven spin resonance in SiC color centers. \textit{ArXiv}:1310.4844 (2013).} Moreover, we use double electron-electron resonance to measure magnetic dipole interactions between spin states occupying inequivalent lattice sites of the same crystal. Together with the distinct spin transition energies of such inequivalent states, these interactions provide a route to dipole-coupled networks of separately addressable spins. [Preview Abstract] |
Session B40: Invited Session: Quantum Foundations
Sponsoring Units: DCMP GQIChair: Matt Leifer, Perimeter Institute
Room: Mile High Ballroom 2B-3B
Monday, March 3, 2014 11:15AM - 11:51AM |
B40.00001: The resource theory of stabilizer computation Invited Speaker: Joseph Emerson |
Monday, March 3, 2014 11:51AM - 12:27PM |
B40.00002: Quantum thermodynamics Invited Speaker: Jonathan Oppenheim |
Monday, March 3, 2014 12:27PM - 1:03PM |
B40.00003: The ubit model in real-amplitude quantum theory Invited Speaker: William Wootters The logical structure of quantum theory is unchanged if we replace the usual complex probability amplitudes with real amplitudes, but the physics is in general quite different. Here we consider a specific model within real-amplitude quantum theory in which a hypothetical binary quantum system, the ubit, substitutes for the complex phase factors of the standard theory. In a certain limit, this model yields an effective theory that looks very much like ordinary quantum theory, but it differs in that it exhibits spontaneous decoherence of isolated systems. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:39PM |
B40.00004: Universal Uncertainty Relations Invited Speaker: Gilad Gour Uncertainty relations are a distinctive characteristic of quantum theory that imposes intrinsic limitations on the precision with which physical properties can be simultaneously determined. The modern work on uncertainty relations employs entropic measures to quantify the lack of knowledge associated with measuring non-commuting observables. However, I will show here that there is no fundamental reason for using entropies as quantifiers; in fact, any functional relation that characterizes the uncertainty of the measurement outcomes can be used to define an uncertainty relation. Starting from a simple assumption that any measure of uncertainty is non-decreasing under mere relabeling of the measurement outcomes, I will show that Schur-concave functions are the most general uncertainty quantifiers. I will then introduce a novel fine-grained uncertainty relation written in terms of a majorization relation, which generates an infinite family of distinct scalar uncertainty relations via the application of arbitrary measures of uncertainty. This infinite family of uncertainty relations includes all the known entropic uncertainty relations, but is not limited to them. In this sense, the relation is universally valid and captures the essence of the uncertainty principle in quantum theory. This talk is based on a joint work with Shmuel Friedland and Vlad Gheorghiu. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 2:15PM |
B40.00005: Bell's theorem on arbitrary causal structures Invited Speaker: Tobias Fritz Bell's theorem is a gedankenexperiment with an underlying causal structure in the form of the letter ``M.'' I will describe how such a Bell scenario is a special case of a vastly larger class of scenarios, in which the causal structure of the ``M'' is replaced by an arbitrary directed acyclic graph (or, equivalently, by a causal set). In this formalism, the apparent difference between the notions of ``choice of setting,'' ``source,'' and ``measurement'' disappears completely and all of these become special cases of the general notion of ``event.'' I will explain how this relieves Bell's theorem of the philosophical baggage associated with free will and also present several mathematical results about these more general scenarios obtained by various people. This formalism is expected to have applications in many other areas of science: it is relevant whenever a system is probed at certain points in space and time, and at each of these points there may be hidden information not observed by the probes. [Preview Abstract] |
Session B41: Focus Session: Ferroelectrics and Anti-Ferroelectrics
Sponsoring Units: DMP DCOMPChair: Antonio Cammarata, Drexel University
Room: Mile High Ballroom 3C
Monday, March 3, 2014 11:15AM - 11:27AM |
B41.00001: Structure and Properties Across a Strain-induced Ferri-to-ferroelectric Transition James Rondinelli, Gaoyang Gou We identify a first-order, isosymmetric transition between a ferrielectric (FiE) and ferroelectric (FE) state in $A$-site ordered LaScO$_{3}$/BiScO$_{3}$ and LaInO$_{3}$/BiInO$_{3}$ superlattices using density functional calculations. Such a previously unreported ferroic transition is driven by the easy switching of cation displacements without changing the overall polarization direction or crystallographic symmetry. Epitaxial strains less than 2\% are predicted to be sufficient to transverse the phase boundary, across which we capture a $\sim\!5$X increase in electric polarization. In a fashion similar to Pb-based perovskite ceramics with a morphotropic phase boundary (MPB), we predict an electromechanical response up to 102 pC/N in the vicinity of the FiE-FE phase boundary in multidomain materials. The structural origin of the unanticipated piezoelectric enhancement is explained. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B41.00002: Structural investigations and the effect of strain on lead based double perovskites Brian Abbett, Craig J. Fennie The A$_2 B B^{\prime}$O$_6$ double perovskite structure, in which the $B$ and $B^{\prime}$ ions are ordered (typically in a rocksalt configuration), provides a versatile platform to realize new properties such as multiferroicity. In particular, compounds with a lone-pair cation on the $A$-site, such as $A$=Pb$^{2+}$, and magnetic $B$=Co, Mn, and diamagnetic $B^{\prime}$= Te, Mo, W, Re, cations have been investigated experimentally, but as of yet none have been found to display ferroelectricity, although several are known to be antiferroelectric. Here we present a first-principles study of the structural and dielectric properties of this family of compounds. We resolve any conflicting reports in the literature as to the ground state structure of compounds and predict the ground state structure when no structural data is available. Additionally, we investigate the effect of epitaxial strain on the structural and magnetic properties. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B41.00003: Effect of Epitaxial Strain on the Dynamical Properties of Ferroelectric Perovskites Kevin McCash, Brajesh Mani, Chun-Min Chang, Inna Ponomareva The use of ferroelectric perovskites in device applications is in large part determined by the strain induced by their growth on lattice mismatched substrates. The epitaxial strain resulting from such growth has been shown to dramatically alter the soft-mode dynamics of ferroelectrics. Here we take advantage of first-principles-based molecular dynamics simulations to investigate the soft-mode dynamics in epitaxial PbTiO$_{3}$ films. By calculating the complex dielectric response and extracting the soft-mode frequencies we are able to trace the intrinsic dynamics as a function of temperature and strain. Our simulations show that the interplay of applied and spontaneous strain is critical to the soft-mode dynamics and provides insights into some recent experimental findings. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B41.00004: Frustrated Antiferroelectricity in a Room-Temperature Ferrimagnet: Promising Candidate Toward Multiple State Memory PanShuo Wang, HongJun Xiang Frustration refers to the presence of competing forces that cannot be simultaneously satisfied. However, geometrical frustration in ferroelectrics is highly unusual. Here we show from first-principles calculations that the M-type hexaferrite BaFe$_{12}$O$_{19}$ exhibits frustrated antiferroelectricity, and hence resolve the experimental controversy on the local structure of the trigonal bipyramidal (TBP) site. Due to the electrostatic interaction, the high-spin Fe$^{3+}$ ions at the TBP sites are displaced from the mirror-plane sites to generate local dipole moments along the c axis. Because of the dipole-dipole interactions, the ground state of BaFe$_{12}$O$_{19}$ is a$(2\times 1)$ chain-like antiferroelectric (AFE) phase. Our work indicates that the ferroelectric state is metastable and can be reached by applying an external electric field to the AFE state, and that the FE state can be made stable at room temperature by element substitution or strain engineering. Thus M-type hexaferrites not only provide platform for studying the new physics of the frustrated antiferroelectricity, but also are promising candidates for realizing multiple state memory devices based on the coexistence of the room temperature polar order and strong ferrimagnetic order. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B41.00005: Antiferroelectricity in ZrO$_2$ and related AXY compounds from first principles Sebastian E. Reyes-Lillo, Kevin F. Garrity, Karin M. Rabe The field-induced structural transition in antiferroelectrics has important technological applications in energy-storage capacitors and piezoelectric devices. Recently, antiferroelectricity was reported in zirconia (ZrO$_2$) [1], which is a widely-used material in electronic devices. In this work, we investigate the nature of antiferroelectricity in thin film ZrO$_2$ and related AXY compounds. For ZrO$_2$, we use first principles calculations to provide strong evidence that the experimentally reported field-induced ferroelectric phase is an intrinsic property of ZrO$_2$. Using a Landau type model, we propose a switching mechanism from the nonpolar tetragonal phase to the orthorhombic polar structure, and we show how to access the ferroelectric phase through epitaxial strain. Drawing on these results, we reexamine a wide variety of related AXY compositions as candidates for antiferroelectrics. Physical descriptors that promote optimal functional properties in antiferroelectrics are identified, and results will be presented. \\[4pt] [1]~J. M\"uller~\emph{et al.}, Nano. Lett. 12, 4318 (2012). [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B41.00006: Pressure-induced transitions in ferroelectric single-crystal PbZr$_{0.54}$Ti$_{0.46}$O$_{3}$ Muhtar Ahart, R.E. Cohen, Russell J. Hemley Ceramics of PbZr$_{(1-x)}$Ti$_{x}$O$_{3}$ (PZT) are widely used in many modern electromechanical transducers. Because single crystals of these materials are difficult to grow, many intrinsic physical properties have not been well understood. Recent breakthroughs in the growth of PZT single crystals have allowed us to study their fundamental physical properties. Here, we study the pressure induced phase transitions in PbZr$_{0.54}$Ti$_{0.46}$O$_{3}$ single crystal by means of combined high-pressure Raman scattering and x-ray diffraction. Our Raman results indicate that the structural transition at 3 GPa is driven by soft optical phonons, and is accompanied by the appearance of a sharp peak near 370 cm$^{-1}$ above 3 GPa. We also observe a new structural transition occurring above 27 GPa associated with a drastic change of the Raman spectrum. The pressure evolution of the diffraction patterns for PbZr$_{0.54}$Ti$_{0.46}$O$_{3}$ show obvious splitting above 27 GPa, particularly for the pseudo-cubic [111] and [220] diffraction peaks, the results indicate a lowering symmetry transition in PbZr$_{0.54}$Ti$_{0.46}$O$_{3}$. We propose that the second transition is from rhombohedral to orthorhombic induced by a pressure above 27 GPa. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B41.00007: Antiferroelectricity in lead zirconate Invited Speaker: Alexander K. Tagantsev Antiferroelectrics are essential ingredients for widely applied piezoelectric and ferroelectric materials. Despite their technological importance, the reason why materials become antiferroelectric has remained allusive since their first discovery. Experimentally, antiferroelectrics can be recognized as materials that exhibit a structural phase transition between two non-polar phases with a strong dielectric anomaly at the high temperature side of the transition. Despite a widely spread opinion that these materials can be viewed as direct analogues of antiferromagnetics, the so-called anti-polar ionic displacements at the transition do not guaranty the antiferroelectric behavior of the material while the interpretation of such behavior does not require the incorporation of the anti-polar ionic displacements in the scenario. To get insight in the true origin of antiferroelectricity, we studied the lattice dynamics of the antiferroelectric lead zirconate using inelastic and diffuse X-ray scattering techniques and the Brillouin light scattering. Based on our experimental data, we showed that the driving force for antiferroelectricity is a ferroelectric instability. Through flexoelectric coupling, it drives the system to a state, which is virtually unstable against incommensurate modulations. However, the Umklapp interaction allows the system to go directly to the commensurate lock-in phase, leaving the incommensurate phase as a ``missed'' opportunity. By this mechanism the ferroelectric softening is transformed into an antiferroelectric transition. The remaining key parts of the whole scenario are repulsive and attractive biquadratic couplings that suppress the appearance of the spontaneous polarization and induce the anti-phase octahedral rotations in the low-temperature phase. The analysis of the results reveals that the antiferroelectric state is a ``missed'' incommensurate phase, and that the paraelectric to antiferroelectric phase transition is driven by the softening of a single lattice mode via the flexoelectric coupling. These findings resolve the mystery of the origin of antiferroelectricity in lead zirconate and suggest an approach to the treatment of complex phase transitions in ferroics. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B41.00008: Formulation of predictive models for use in first principles design of non-centrosymmetric perovskite oxides Joshua Young, James Rondinelli Because many useful electronic properties such as ferroelectricity arise solely due to the lack of inversion in a material's crystal structure, predictive microscopic models describing how to deterministically remove this symmetry operation can allow for the rapid identification and design of new polar compounds. By understanding how structural distortions influence the connectivity between oxygen polyhedra in solid state oxides, we elucidate a series of geometric design rules necessary to develop polar materials. We then apply these criteria to the family of ABO$_3$ perovskite oxides by systematically investigating how distortions of the corner-connected BO$_6$ polyhedral network influence the A-site environments, resulting in a detailed description of the octahedral rotation patterns and A- and B-site cation ordering arrangements capable of producing centrosymmetric, polar, and enantiomorphic structures. Using this as a guide, we then show how such a method allows for the targeted design of new non-centrosymmetric oxides. We conclude by using these rules in combination with density functional theory calculations to predict a series of rhombohedral (A,A$^\prime$)B$_2$O$_6$ perovskites displaying electric polarizations in their ground state. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B41.00009: Origin of Ferroelectricity in a Family of Polar Oxides: The Dion-Jacobson Phases Nicole Benedek The discovery of octahedral rotation-induced ferroelectricity has expanded the opportunities for designing materials in which the polarization is coupled to (and therefore makes possible the electric field control of) other properties, e.g. magnetism, orbital order, metal-insulator transitions. Recent work has elucidated the microscopic mechanism of octahedral rotation-induced ferroelectricity in two families of layered perovskites: AA$^\prime$B$_2$O$_6$ double perovskites and Ruddlesden-Popper (RP) phases. However, there are many other families of layered perovskites - are there octahedral rotation-induced polar materials among them also? We use symmetry arguments, crystal chemical models and first-principles calculations to elucidate the microscopic origin of ferroelectricity in the Dion-Jacobson (DJ) phases. Although ``on paper'' the phenomenology of the DJ phases appears identical to that of polar double perovskites and RP phases, the crystal chemical details regarding how the polar state emerges are different. We link trends in the magnitude of the induced polarizations to changes in structure and composition and discuss possible phase transition scenarios. Our results add surprising new richness to theories of how polar structures emerge in layered perovskites. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B41.00010: First-principles studies of low tolerance factor perovskites Sung Gu Kang, Craig J. Fennie Most perovskites form in the non-polar \textit{Pnma} structure, however, materials found in the polar subgroup of this structure, e.g., space group \textit{Pna2}$_{1}$, are rare. Here we study from first principles the structural and vibrational properties of twelve materials that span a wide range of tolerance factors (MgSnO$_{\mathrm{3}}$, ZnSnO$_{\mathrm{3}}$, MgTiO$_{\mathrm{3}}$, ZnTiO$_{\mathrm{3}}$, MgGeO$_{\mathrm{3}}$, ZnGeO$_{\mathrm{3}}$, CdSnO$_{\mathrm{3}}$, CaSnO$_{\mathrm{3}}$, CdTiO$_{\mathrm{3}}$, CaTiO$_{\mathrm{3}}$, CdGeO$_{\mathrm{3}}$, and CaGeO$_{\mathrm{3}})$. We illustrate how low tolerance factor materials that have been artificially constrained to the \textit{Pnma} structure do in fact display ferroelectric instabilities. Insight is gained by further studying the energetics for each material in the ilmenite, lithium niobate, and perovskite structures over a wide pressure range. Our first-principles results are shown to correlate with physical descriptors, such as tolerance factor, ionic radii, and electronegativity. The rationalized rules from our data analysis will guide to design the new ferroelectric/functional materials. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B41.00011: Multiferroic Aurivillius Phases: the Case of $\rm Bi_{5}FeTi_{3}O_{15}$ by \textit{Ab Initio} Yael Birenbaum, Claude Ederer The Aurivillius phases form a family of naturally layered-perovskites materials with good ferroelectric properties. $\rm Bi_{5}FeTi_{3}O_{15}$ (BFTO) is perhaps the simplest known member of this family that also incorporates magnetic degrees of freedom. However, due to the low concentration of magnetic cations in similar systems, it is unclear how long-range multiferroic behaviour can be achieved. For example, room temperature ferromagnetism has been reported for $\rm Bi_{5}Co_{0.5}Fe_{0.5}Ti_{3}O_{15}$, in contrast with no magnetic order found in $\rm Bi_{5}CrTi_{3}O_{15}$. To address this question, we establish the ferroelectric and magnetic properties of BFTO, using \textit{ab initio} electronic structure calculations, comparing two commonly used exchange-correlation functionals: PBE and PBEsol. We then discuss a potential site preference for $\rm Fe^{3+}$ and its impact on the polarisation and magnetic couplings. In addition, a brief comparison with $\rm Bi_{5}MnTi_{3}O_{15}$ will be made. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B41.00012: Structure and electronic properties of Zn$_x$Sn$_{1-x}$O$_{2-x}$ Anindya Roy, Yansha Jin, Tonghu Jiang, Michael Falk Using first-principles based hybrid-exchange calculations we look at the structural and electronic properties of Zn-Sn-O system. The oxides represented by Zn$_x$Sn$_{1-x}$O$_{2-x}$ has end members ZnO and SnO$_2$. These relatively well studied, native n-type semiconductors are technologically important. Intermediate oxides corresponding to $x=$2/3 and 1/2 have been synthesized: spinel Zn$_2$SnO$_4$ and rhombohedral ZnSnO$_3$. These mixed oxides are functionally promising for their potential as ferroelectrics, transparent conducting oxides, thermoelectrics etc. Previously, {\it ab initio} calculations investigated the structures, electronic and thermodynamic properties of these mixed oxides. However, we considerably improve our understanding of band gap values and band structure of these compounds using hybrid-exchange method. We also perform band alignment calculations, estimate work function of these intermediate oxides, and compare those values to that of the end members and to the experimental results. The existence of Zn$_2$SnO$_4$ in the spinel structure allows a number of configurations which correspond to normal, partially inverted, or inverted spinel forms. We use cluster expansion method to identify energetically most stable form before calculating other properties. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B41.00013: Semiconducting ferroelectrics for photovoltaics through Zn2+ doping into KNbO3 Fenggong Wang, Ilya Grinberg, Peter Davies, Andrew Rappe Using first-principles calculations, we design and predict six low band gap ferroelectric solid solutions by partially substituting Zn$^{2+}$ for Nb$^{5+}$ into the parent KNbO$_{3}$ material, combined with charge compensations at the $A$-sites by different combinations of higher valence cations (Ba$^{2+}$, Sr$^{2+}$, Pb$^{2+}$ and La$^{3+}$, Bi$^{3+}$). In particular, our HSE06 calculations yield a low band gap of only 2.11 eV for the 75\%KNbO$_{3}-25\%$(Sr$_{1/2}$La$_{1/2}$)(Zn$_{1/2}$Nb$_{1/2}$)O$_{3}$ (KN-SLZN) solid solution, and this can be lowered further by 0.6-0.7 eV upon in-plane compressive strains, allowing for more efficient visible light photovoltaic energy harvesting. The maintaining or enhancing of the polarizations of KN-SLZN provides an efficient charge separation route by the bulk photovoltaic effect that could make the power conversion efficiency (PCE) go beyond the Shockley$-$Queisser limit. We argue that these newly designed low band gap ferroelectric solid solutions can be experimentally synthesized and are promising for photovoltaics. In addition, we demonstrate a new strategy to engineer the band gap while maintaining the polarization of the ferroelectric perovskites, which can be well applied to other systems. [Preview Abstract] |
Session B42: Focus Session: Experiments on Samarium Hexaboride
Sponsoring Units: DMPChair: Vidya Madhavan, Boston College
Room: Mile High Ballroom 4A
Monday, March 3, 2014 11:15AM - 11:27AM |
B42.00001: High Field Magnetoresistance Measurements on the Surface States of Samarium Hexaboride using Corbino Structures Steven Wolgast, Yun Suk Eo, Gang Li, Ziji Xiang, Colin Tinsman, Tomoya Asaba, Benjamin Lawson, Fan Yu, J.W. Allen, Kai Sun, Lu Li, Cagliyan Kurdak, Dae-Jeong Kim, Zachary Fisk The recent conjecture of a topologically-protected surface state in SmB$_{6}$ and the verification of robust surface conduction below 4 K have led to a large effort to understand the surface states. Extracting carrier density and charge mobility of these states via Hall measurements is complicated because current can flow on all surfaces of a topological insulator, each of which can have different transport characteristics. We study magnetotransport of SmB$_{6}$ surfaces up to 45 T using a Corbino geometry that is sensitive to individual surfaces. The Corbino allows us to measure conductivity, $\sigma_{xx}$, in both parallel and perpendicular magnetic fields. In the parallel geometry both (110) and (100) samples show a strong negative magnetoresistance. We extracted information about the carrier mobility from the ratio of the perpendicular and parallel magnetoresistance traces. The (110) surface had the highest carrier mobility of 122 cm$^{2}$/Vs with a carrier density of $2.5\times10^{13}$ cm$^{-2}$. The conduction on both polar (100) and non-polar (110) surfaces strongly indicates that the conduction must have a non-polarity-driven origin. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B42.00002: Low Field Magnetoresistance Measurements on the Surface States of Samarium Hexaboride using Corbino Structures Yun Suk Eo, Steven Wolgast, Cagliyan Kurdak, Gang Li, Ziji Xiang, Colin Tinsman, Tomoya Asaba, Benjamin Lawson, Fan Yu, Lu Li, Kai Sun, James Allen, Dae-Jeong Kim, Zachary Fisk Recently, SmB$_{6}$ attracted great attention by numerous reports suggesting it to be an ideal strong 3D topological insulator. By spin-momentum locking, the quantum correction of conductivity of a topological surface state of this material is expected to result in weak anti-localization (WAL). To study this effect, we have performed low field magnetorestance measurements on (100) and (110) Corbino samples at temperatures down to 60 mK. Many of the Corbino samples that we have studied so far have a dip in the magnetoresistance trace that resembles the WAL feature. The size and temperature dependence of this feature are in general consistent with those expected from a quantum interference correction. However, after careful investigation we found the features shrink in amplitude with slower magnetic field sweep rates. Also, the traces have a hysteretic signal of an unknown origin. The potential coupling between a magnetic oxide layer forming on the surfaces of SmB$_{6}$ and the topological surface states will be discussed. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B42.00003: Surface transport in Topological Kondo Insulator SmB6 Jing Xia, Sean Thomas, Dae-jeong Kim, Fisk Zach, Jing Xia In this talk we will discuss the existence of topological order in a 3D strongly correlated material SmB6, which has recently been proposed theoretically as a topological Kondo insulator. We will present transport evidence for a highly conductive surface state surrounding a truly insulating bulk: At low temperature We found that the Hall resistance scales with the surface, and is independent of the thickness. Using non-local transport measurements, we demonstrate that the electric conducting is mostly along the surface at low temperatures and in zero magnetic field. We demonstrate that the surface dominated conduction is destroyed by small amounts of magnetic doping but survives non-magnetic doping. At even lower temperatures, we demonstrate the weak localization effect, which is consistent with a surface state with spin momentum locking. The Kondo-effect-like resistance saturation will also be discussed. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B42.00004: Surface electronic structure of topological Kondo insulator candidate SmB6: a high-resolution ARPES study Nasser Alidoust, Madhab Neupane, Su-Yang Xu, Chang Liu, Ilya Belopolski, Guang Bian, M. Zahid Hasan, T. Kondo, S. Shin, T.-R. Chang, H.-T. Jeng, L. Balicas, T. Durakiewicz, H. Lin, A. Bansil, D.-J. Kim, Z. Fisk The Kondo insulator SmB6 has been known to exhibit low temperature transport anomaly and has recently attracted attention as a new topological insulator candidate. By combining low temperature and high resolution of the laser-based ARPES, for the first time, we probe the surface electronic structure of the anomalous conductivity regime. We observe that the bulk bands exhibit a Kondo gap of 14 meV and identify in-gap low-lying states within 4 meV of the Fermi level on the surface of this material. The low-lying states are found to form electron-like Fermi surface pockets that enclose the X and the $\Gamma$ points of the surface BZ. These states disappear as temperature is raised in correspondence with the complete disappearance of the 2D conductivity channels. While the topological nature of the in-gap metallic states cannot be ascertained, our measurements are consistent with the predicted first-principle topological Kondo insulator phase in this material. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B42.00005: Growth and Intrinsic Physical Properties of the Kondo Insulator SmB$_{6}$ William Phelan, Seyed Koohpayeh, Patrick Cottingham, Leslie Schoop, Robert Cava, Collin Broholm, Tyrel McQueen SmB$_{6}$ is a long-studied Kondo Insulator that has come back into focus recently following theoretical predictions that it may harbor topologically protected surface states. Materials containing such surface states are referred to as topological insulators, and may impact technologically important areas such as quantum computing and spintronics. We report the preparation of single crystals of SmB$_{6}$ \textit{via} the floating zone technique, and the impact of growth conditions on the physical properties, including low temperature electrical transport. These results provide insights into the nature of the anomalous low temperature state of SmB$_{6}$. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B42.00006: Terahertz transmission studies of the topological Kondo insulator candidate SmB$_6$ Nicholas J. Laurita, Christopher M. Morris, Seyed Koopayeh, Patrick Cottingham, W. Adam Phelan, Leslie Schoop, Tyrel M. McQueen, N. Peter Armitage The Kondo insulator SmB$_6$ has long been known to display anomalous transport behavior at low temperatures (T$<10$ K) and high pressures. At low temperatures, a plateau is observed in the resistivity, contrary to the divergence expected for a normal Kondo insulator. Recent theoretical calculations suggest that SmB$_6$ may be the first topological Kondo insulator, a material with a Kondo insulating bulk, but topologically protected metallic surface states.\footnote{M. Dzero \textit{et al.}, Phys. Rev. Lett. \textbf{104}, 106408 (2010)} Here, time domain terahertz spectroscopy (TDTS) is used to investigate the temperature dependent low frequency optical conductivity of single crystals of SmB$_6$. We find evidence for a substantial bulk conductivity at a frequency of a few hundred GHz, which challenges the notion of this material as having a clean gap. The evidence for topological surface states and their properties will be discussed. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B42.00007: Magnetic Excitations of the Kondo Insulator SmB$_{6}$ Wesley Fuhrman, Jonathan Leiner, Garrett Granroth, Mark Lumsden, Pavel Alekseev, Jean-Michel Mignot, Seyed Koohpayeh, Patrick Cottingham, William Phelan, Leslie Schoop, Robert Cava, Tyrel McQueen, Collin Broholm Research within the past year indicates the long-studied Kondo insulator SmB$_{6}$ may be a topological insulator, with an insulating bulk at low temperatures and topologically protected metallic surface states. Because electron-electron interactions give rise to the insulating state, there is intense interest in SmB$_{6}$. Using time-of-flight inelastic neutron scattering, we have probed magnetic excitations over a wide range of energy and momentum transfer. Consistent with previous work there is a resonant mode near 14 meV and a broad spectrum of excitations centered near 30 meV. This data set provides a comprehensive map of the Q-dependence of the excitations throughout the Brillouin zone allowing for comparison to theoretical models describing the anomalous insulting state. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B42.00008: Two dimensional Fermi surfaces in Kondo insulator SmB$_{6}$ Gang Li, Ziji Xiang, Fan Yu, Tomoya Asaba, Benjamin Lawson, Peng Cai, Colin Tinsman, Adam Berkley, Steven Wolgast, Yun Suk Eo, Dae-Jeong Kim, Cagliyan Kurdak, James Allen, Kai Sun, Xianhui Chen, Yayu Wang, Zachary Fisk, Lu Li Samarium hexaboride SmB$_{6}$ belongs to a class of strongly correlated heavy Fermion semiconductors, in which hybridization between itinerant electrons and localized orbitals lead to opening of a charge gap at low temperature. However, the resistivity of SmB$_{6}$ does not diverge but saturates below $\sim$ 2 Kelvin. Former studies suggested that this residual conductance is contributed by intragap states with various origins. Recent theoretical developments suggest that the particular symmetry of energy bands of SmB$_{6}$ may host a topologically non-trivial surface state, i.e., a topological Kondo insulator. To probe the Fermiology of the possible metallic surface state, we use highly sensitive torque magnetometry to detect the de Haas van Alphen (dHvA) effect due to Landau level quantization. Our detailed angular and temperature data suggest two-dimensional Fermi Surfaces lie in both crystalline (001) and (101) surface planes of SmB$_6$. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B42.00009: High Field Torque Magnetometry of SmB$_6$ Colin Tinsman, Gang Li, Ziji Xiang, Fan Yu, Tomoya Asaba, Benjamin Lawson, Peng Cai, Adam Berkley, Steven Wolgast, Yun Suk Eo, Dae-Jeong Kim, Cagliyan Kurdak, James Allen, Kai Sun, Xianhui Chen, Yayu Wang, Zachary Fisk, Lu Li The Kondo Insulator SmB$_6$ has been observed to have a small, residual surface conductance apparent below 5 K. Torque Magnetometry was employed to find quantum oscillations in the magnetization -- the de Haas-van Alphen effect. Using magnetic fields up to 45 T, we were able to resolve 3 different pieces of Fermi Surface, at frequencies of 35 T, 300 T, and 400 T. Angular dependence of these oscillation frequencies indicate that they are two dimensional in nature. Additionally, Landau Level indexing analysis for the 35 T pocket gives a $-1/2$ intercept in the infinite field limit, a Berry phase contribution consistent with Dirac electronic system such as graphene. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B42.00010: Scanning Tunneling Spectroscopy and Imaging of Topological Kondo Insulators Michael Yee, Yang He, Anjan Soumyanarayanan, Dae-Jeong Kim, Zachary Fisk, Jennifer E. Hoffman Topological insulators host spin-polarized surface states which robustly span the band gap and hold promise for novel applications. Recent theoretical predictions have suggested that topologically protected surface states may similarly span the hybridization gap in some strongly correlated heavy fermion materials, particularly $SmB_6$. Scanning tunneling spectroscopy (STS) is a powerful tool for studying topological materials because it is directly sensitive to the surface states of interest, and their scattering processes. Here we present the first atomic resolution spectroscopic study of the cleaved surface of $SmB_6$ [1]. Using a combination of real space imaging and filled and empty state spectroscopy, we reveal a robust hybridization gap that universally spans the Fermi level on four distinct surface morphologies despite shifts in the f band energy. Using a cotunneling model, we separate the density of states of the hybridized bands from which the predicted topological surface states must be disentangled. Our technique lays the groundwork for understanding the first strongly correlated topological insulator, and implements a generally applicable method to quantitatively understand a wider class of Kondo insulators.\\[4pt] [1] arXiv:1308.1085 [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B42.00011: Emergence of a coherent in-gap state in SmB$_{6}$ Kondo insulator revealed by scanning tunneling spectroscopy Wei Ruan, Cun Ye, Minghua Guo, Fei Chen, Xianhui Chen, Guangming Zhang, Yayu Wang SmB$_{6}$ is a Kondo insulator that exhibits transport anomalies at temperatures below 5 K, where resistivity saturates instead of diverging. It has long been ascribed to in-gap states which become coherent at low temperatures. Recently, a host of theoretical and experimental studies suggest that SmB$_{6}$ may be a topological Kondo insulator with topological protected metallic surface states. In this talk we present STM studies of the (001) surface of cleaved SmB$_{6}$ single crystal. We have observed four different kinds of surface morphologies with similar \textit{dI/dV} spectra. Variable temperature \textit{dI/dV} spectroscopy up to 60 K reveals a gap-like density of state suppression around the Fermi level, which is due to the hybridization between the itinerant Sm 5$d$ band and localized Sm 4$f$ band. At temperatures below 40 K, a sharp coherence peak emerges within the hybridization gap near the lower gap edge. We propose that the in-gap resonance state is due to a collective excitation in magnetic origin that is specific to the mixed valent Kondo insulator. Implications of these results to the electronic structure evolution and transport anomaly in SmB$_{6}$ will be discussed. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B42.00012: Valence and moment of Sm in SmB$_6$ under pressure Nicholas Butch, Jason Jeffries, Paul Syers, Johnpierre Paglione, Yuming Xiao, Paul Chow We studied the valence and magnetic moment of Sm ions in the mixed-valent compound SmB$_6$ via x-ray spectroscopy. Using diamond anvil cells, the pressure dependence of these properties was measured to at least 20 GPa. We will discuss pressure-induced magnetic order in this compound, as well as ramifications for the sought-after ambient-pressure topological state. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B42.00013: Combinational growth and physical properties of possible topological Kondo insulator SmxB1-x films Jie Yong, Richard Ruchoski, Sean Fackler, Ichiro Takeuchi, Richard Greene Kondo insulator Samarium hexboride (SmB6) has caused great interest due to its possible topological surface state and its interplay with correlated physics. Thin films of SmB6 are highly desirable for surface sensitive measurements and novel device fabrications. Since both PLD and sputtering yield highly boron deficient films, we explored the thin film growth through combinational sputtering of a stoichiometrically SmB6 target and Boron target. Thin SmxB1-x films are fabricated with x continuously varies from 0 to 1. We found that when x\textgreater 0.14, resistivity measurements show upturns around 50K and saturations below 10K, consistent with the bulk results. Resistance ratios between 300K and 4K are around 1.5, which is consistent with a much larger surface-to-bulk ratio. The films with x \textless 0.14 are more insulating at room temperature and show insulating behavior. Other details of the characterization of these films will also be presented. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B42.00014: Polarity-driven surface metallicity in SmB$_6$ Andrea Damascelli, Z.-H. Zhu, A. Nicolaou, G. Levy, N.P. Butch, P. Syers, X.F. Wang, J. Paglione, G.A. Sawatzky, I.S. Elfimov By a combined angle-resolved photoemission spectroscopy and density functional theory study, we discover that the surface metallicity is polarity driven in SmB$_6$. Two surface states, not accounted for by the bulk band structure, are reproduced by slab calculations for coexisting B$_6$ and Sm surface terminations. Our analysis reveals that a metallic surface state stems from an unusual property, generic to the (001) termination of all hexaborides: the presence of boron 2$p$ dangling bonds, on a polar surface [1]. The discovery of polarity-driven surface metallicity sheds new light on the 40-year old conundrum of the low temperature residual conductivity of SmB$_6$, and raises a fundamental question in the field of topological Kondo insulators regarding the interplay between polarity and nontrivial topological properties. \\[4pt] [1] Z.-H. Zhu et al., Phys. Rev. Lett. to appear (arXiv:1309.2945) [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B42.00015: Effect of magnetic impurity substitution in topological Kondo insulator SmB6 Xiangfeng Wang, Yasuyuki Nakajima, Yeping Jiang, Richard Greene, Johnierre Paglione A topological insulator is a material with topologically protected metallic boundaries and an insulating bulk. The strongly-correlated Kondo system SmB6 has recently been widely investigated owing to its promise of being the first realized topological Kondo insulator. Many results have confirmed the existence of metallic surface states and provided evidence of their non-trival topological nature. Here we report a study of the effect of magnetic transition metal impurity substitution in SmB6 on transport and thermodynamic properties, providing an important insight into the nature of the surface states. [Preview Abstract] |
Session B43: Symmetry Protected Topological Phases
Sponsoring Units: DCMPChair: Maxim Dzero, Kent State University
Room: Mile High Ballroom 4B
Monday, March 3, 2014 11:15AM - 11:27AM |
B43.00001: Detection of Symmetry Enriched Topological Phases Frank Pollmann, Ching-Yu Huang, Xie Chen Topologically ordered systems in the presence of symmetries can exhibit new structures which are referred to as symmetry enriched topological (SET) phases. We introduce simple methods to detect the SET order directly from a complete set of topologically degenerate ground state wave functions. In particular, we first show how to directly determine the characteristic symmetry fractionalization of the quasiparticles from the reduced density matrix of the minimally entangled states. Second, we show how a simple generalization of a string order parameter can be measured to detect SETs. The usefulness of the proposed approached is demonstrated by examining two concrete model states which exhibit SET: (i) a spin-1 model on the honeycomb lattice and (ii) the resonating valence bond state on a kagome lattice. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B43.00002: Detecting two dimensional symmetry protected topological order in a ground state wave function Michael Zaletel Symmetry protected topological states cannot be deformed to a trivial state so long as the symmetry is preserved, yet there is no local order parameter that can distinguish them from a trivial state. We demonstrate how to detect whether a two dimensional ground state has symmetry protected topological order; the measurement play a similar role as the topological entanglement entropy does for detecting anyons. For finite abelian symmetries the measurement gives a complete characterization of the 3rd cohomology class that describes the order. The proposed measurement is validated numerically for a model with $Z_2$ symmetry protected order. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B43.00003: Symmetry-Protected Topological Entanglement Iman Marvian We propose an order parameter for the Symmetry-Protected Topological (SPT) phases which are protected under an Abelian on-site symmetry. This order parameter, called the SPT entanglement, is defined as the entanglement between A and B, two distant regions of the system, given that the total charge (associated with the symmetry) in a third region C is measured and known, where C is a connected region surrounded by A and B and the boundaries of the system. In the case of 1-dimensional systems we prove that at the limit where A and B are large and far from each other compared to the correlation length, the SPT entanglement remains constant throughout a SPT phase, and furthermore, it is zero for the trivial phase while it is nonzero for all the non- trivial phases. Moreover, we show that the SPT entanglement is invariant under the low-depth local quantum circuits which respect the symmetry, suggesting that the SPT entanglement remains constant throughout a SPT phase in the higher dimensions as well. Finally, based on the concept of SPT-Ent, we propose a new interpretation of string order parameters and also an algorithm for extracting the relevant information about the SPT phase from them. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B43.00004: Symmetry protected topological phases in two dimensions: Generalized Laughlin's argument and quantum pumps Chang-Tse Hsieh, Olabode Mayodele Sule, Shinsei Ryu, Rob Leigh We generalize Laughlin's flux insertion argument in a way that it is applicable to interacting topological phases protected by unitary symmetries -- either on-site or non-on-site -- in two spatial dimensions. Large gauge invariance of the symmetry projected partition function of the one-dimensional edge theory can be used to argue the (non)conservation of the quantum number (corresponding to the projected unitary symmetry) under the large gauge transformation. If the edge does not conserve such quantum number, there is a flux-driven ``quantum pump'' between edges of the two dimensional system, which can be diagnosed as the nontrivial symmetry protected topological phase. This also gives the criteria of stability/gappability of the edge states that respect the symmetry. For non-on-site symmetry such as parity symmetry, the one dimensional edge theory is considered as the conformal field theory on an unoriented surface, such as Klein bottle, which arise naturally from a parity symmetry projection operation. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B43.00005: Diagnosing gapless edge theory of symmetry protected topological phases via twist operators Gil Cho, Shinsei Ryu A symmetry protected topological (SPT) phase is a new phase of matter which has been actively studied recently. The bulk of a SPT phase is gapped and disordered, and thus it is featureless and difficult to be distinguished from a trivially disordered phase. Remarkably there are gapless edge modes emerging at the boundary between the vacuum and the SPT phase. The gapless edge state is protected by the symmetries of the SPT phase and is a only measurable signature of the SPT phase, and thus we can learn about the SPT phase by studying only its edge modes. One can write down a conformal field theory describing the edge modes, and we consider twist operators of the theory to diagnose the stability of the conformal field theory against symmetry-respecting perturbations. When acted on a state, a twist operator changes the boundary condition for the quantum fields in the conformal field theories. It manifests in the mode expansion of the fields and changes only the behavior of the ``zero'' mode of the fields. At the edge of the SPT phase, we consider only the twist operators consistent with symmetries. Then we investigate the algebra between the twist operators and their symmetry quantum numbers in various methods to study the stability of the edge theory. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B43.00006: Detecting symmetry protected topological states by generalized correlation Yizhuang You, Alex Rasmussen, Zhen Bi, Cenke Xu The symmetry protected topological (SPT) states has attracted much research attention recently. They classify a large family of disordered gapped quantum states that have non-trivial topological twist in their wave functions. Examples include topological insulators and the Haldane spin-1 chain. To better understand the physical properties of the SPT states, we focus on their many-body wave functions. We propose a simply way to distinguish the SPT state from the trivial state by studying the behavior of a generalized static bulk correlation function. We show that for 2D SPT states, the generalized correlation function will exhibit a long-range or quasi-long-range behavior, distinct from the short-range behavior for trivial states. This quasi-long-range behavior in the bulk is closely related to the symmetry protected gapless edge modes on the boundary of SPT state. The effective theory for the gapless edge can be described by the conformal field theory (CFT), whose central charge may be extracted from the scaling behavior of the entanglement entropy, which can be given by the wave function overlap on a double torus. We demonstrate our proposal with lattice models for both the fermion and the boson SPT states. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B43.00007: Topological Response Theory of Abelian Symmetry-Protected Topological Phases in Two Dimensions Meng Cheng, Zheng-Cheng Gu Symmetry-protected topological (SPT) phases in two-dimensions can be largely described by Chern-Simons topological field theories. We propose a topological response theory to uniquely identify the SPT orders, which allows us to obtain a systematic scheme to classify bosonic SPT phases with any finite Abelian symmetry group. We also apply the theory to fermionic SPT phases with $Z_m$ symmetry and find the classification of SPT phases depends on the parity of $m$: for even $m$ there are $2m$ classes, $m$ out of which is intrinsically fermionic SPT phases and can not be realized in any bosonic system. We outline the general classification scheme for fermionic SPT phases in two dimensions. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B43.00008: Detecting topological phase transitions of insulators in the complex crystal momentum space Xugang He, Wei Ku We present an intuitive picture of topological phase transitions in insulators via topological properties of band dispersion in the {\it complex} crystal momentum space. Specifically, the dispersion, when analytically contiuned to the complex crystal momentum space, has doubly degenerate ``branch point'' where two bands can meet, and the topological property of the branch point contains clear signature of the phase transition. In addition, the residue of the branch point in the reduced Berry curvature is shown to give the change of topological invariants across the phase transitions, thus providing a convenient way to detect topological phase transition. We demonstrate the general idea using the generic Bernevig-Hughes-Zhang (BHZ) model originated in the quantum spin Hall effect on a square lattice. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B43.00009: Phase diagram of the isotropic spin-3/2 model on the z=3 Bethe lattice Stefan Depenbrock, Frank Pollmann We study an $SU(2)$ symmetric spin-3/2 model on the $z=3$ Bethe lattice using the infinite Time Evolving Block Decimation (iTEBD) method. This model is shown to exhibit a rich phase diagram. We compute the expectation values of several order parameters which allow us to identify a ferromagnetic, a ferrimagnetic, a anti-ferromagnetic as well as a dimerized phase. We calculate the entanglement spectra from which we conclude the existence of a symmetry protected topological phase that is characterized by $S=1/2$ edge spins. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B43.00010: Quantum Distance and the Classification of Topological States of Matter Jiahua Gu, Kai Sun In this talk, we provide a generic geometrical classification for topological states of matter, which is applicable for all topologically nontrivial band insulators (with or without symmetry protections), as well as certain strongly-correlated topological states (e.g. the fractional quantum Hall effect and the fractional Chern insulators). We prove that generically, quantum distance measurement contains direct information about the topology of a quantum wavefunction. Specific examples will be provided to demonstrate this principle. The experimental implications will also be discussed. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B43.00011: Integer characterization of 2D topological insulators at finite temperature Zhoushen Huang, Daniel Arovas 2D band topological insulators (TI) are characterized by the TKNN number and its variants. However, this only works for zero temperature as the TKNN number is no longer quantized for $T>0$. We show that using Uhlmann's parallel transport for density matrices, TI at finite temperature can still be characterized by an integer, which (1) reduces to the corresponding TKNN number at $T = 0$, and (2) exhibits a phase transition, i.e. drops to zero, at a critical temperature. Prototypical models such as Haldane's honeycomb lattice model and the Bernevig-Hughs-Zhang model will be discussed. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B43.00012: Topological field theory for 2+1 TRI TSC Yingfei Gu, Xiaoliang Qi Time-reversal invariant topological superconductors (TRI TSC) are gapped TRI superconductors with topologically robust gapless modes on the boundary. In the work by X. L. Qi et al, [PRB, 87, 134519(2013)], a topological field theory description was proposed for 3+1-dimensional TRI TSC, which contains an axionic coupling between superconducting phase and electromagnetic field. In my talk, I will describe a generalization of this theory to the 2+1 dimensional TRI TSC. The 2+1d topological field theory describes a topological coupling between electromagnetic field, superconducting phase fluctuation and magneto-electric polarization. I will also talk about the corresponding physical consequences. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B43.00013: Unitary engineering of two- and three-band Chern insulators Soo-Yong Lee, Jin-Hong Park, Gyungchun Go, Jung Hoon Han In this talk, we discuss how to engineer the topological number and ordering in some two- and three-band Chern insulators. First, we investigate a way to extend the unit Chern number of a two-band lattice model such as Haldane model and Bernevig-Hughes-Zhang model to the one in possession of higher Chern numbers, relying crucially on the monopole number-changing unitary transformations. The scheme is generalized to a class of three-band model Hamiltonian where the a pair of monopole charges can be introduced to manipulate the Chern numbers of each band. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B43.00014: Analytical approach to the edge state of the Kane-Mele model Hyeonjin Doh, Gun Sang Jeon, Hyoung Joon Choi We investigate the edge state of a two-dimensional topological insulator based on the Kane- Mele model. We consider the two semi-infinite honeycomb lattices with a zig-zag and an armchair boundary, respectively. We construct the effective Hamiltonians for the edge states assuming exponentially decaying wave functions. With the boundary conditions for the both types of the boundaries, we derive the self-consistent equations for the energies and the decaying factors of the edge states. The numerical solutions of the self-consistent equations exhibit intriguing spatial behaviors of the edge states with respect to the spin-orbit coupling and the sub-lattice potential. We found the bifurcation behavior of the edge state width with respect to the sub-lattice potential in zigzag boundary. The bifurcation behavior discriminates the boundary dependencies of the edge state properties. We also discuss the relation between the sample size and the interaction parameters in the phase transition from normal insulator to topological insulator. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B43.00015: Edge states and the quantized Berry phase of general massless Dirac fermions Toshikaze Kariyado, Yasuhiro Hatsugai Topological properties of massless Dirac fermion systems are investigated in terms of the quantized (Z$_2$) Berry phase. Although the Berry phase is gauge dependent and can take any value in modulo $2\pi$, it is quantized with symmetry protection. For this protection, the chiral symmetry is often employed. Here, we show that this symmetry protection is effective in much more general situation, namely, the inversion combined with the time reversal symmetry or the spatial reflection is sufficient for the quantization. Then, the topological stability of the massless Dirac fermions in two dimension is discussed in relation to the quantized Berry phase. We also demonstrate the bulk-edge correspondence of the generic massless Dirac fermions, that is, giving topological reasoning for the existence of edge states, using a model containing the massless Dirac fermion, but having no chiral symmetry [T. Kariyado and Y. Hatsugai, arXiv:1307.7926]. Further generic applications of the symmetry protection for the bulk-edge correspondence will be discussed as well. [Preview Abstract] |
Session B44: Focus Session: Defects in Semiconductors: Computational Methods
Sponsoring Units: DMP FIAPChair: Joongoo Kang, National Renewable Energy Laboratory
Room: Mile High Ballroom 4C
Monday, March 3, 2014 11:15AM - 11:27AM |
B44.00001: Free-carrier effects on electronic and optical properties of binary oxide semiconductors Andre Schleife, Claudia Roedl While there is persistent interest in oxides, e.g., for semiconductor technology or optoelectronics, it remains difficult to achieve $n$-type and $p$-type doping of one and the same material. At the same time, higher and higher conductivities are reported for both types of doping individually. Hence, it is important to understand the corresponding influence of free carriers on electronic structure and optical properties. Modern electronic-structure calculations, based on hybrid exchange-correlation functionals and the $GW$ approximation, were performed for $n$-type (ZnO, CdO, SnO$_2$) and $p$-type (MnO, NiO) binary oxides. We use these results to analyze the influence of free carriers by computing contributions that increase (Burstein-Moss shift) or reduce (electron-electron interaction and ionized-impurity scattering) the band gaps as a function of free-carrier concentration. We also compute the carrier-concentration dependence of effective electron and hole masses and compare to experimental data. For $n$-type ZnO we compute optical absorption spectra by means of a recent extension of the Bethe-Salpeter framework. This allows us to take excitonic effects as well as the influence of free carriers on the electron-hole interaction into account. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B44.00002: Bound and Unbound: Stabilizing the Anionic States of Adamantane through Functionalization Zachary Pozun, Vamsee Voora, Michael Falcetta, Kenneth Jordan Adamantane is the simplest diamondoid, which are nanostructures of carbon with a diamond-like cage structure. Although diamondoids have been reported to have negative electron affinities (EA),\footnote{N. D. Drummond, A. J. Williamson, R. J. Needs, and G. Galli. Phys. Rev. Lett., 95, 096801 (2005).} calculating an EA for an unbound anion is not straightforward because the localized anion state is strongly coupled to the continuum. In order to determine the energy and lifetime of temporary anions, we apply the stabilization method, where the exponents of the diffuse basis functions centered on the carbon atoms are scaled during equation-of-motion calculations; the energies of the anionic states are relatively insensitive to the scaling factor as compared to the continuum states.\footnote{J. S.-Y. Chao, M. F. Falcetta, and K. D. Jordan. J. Chem. Phys., 93(2), 1125-1134 (1990)} We use this method in order to identify temporary anion states and their associated energies and lifetimes. We also demonstrate that these states can be tuned in energy based substituting the adamantane cage with substituents that either withdraw or donate charge into the carbon-carbon backbone. Thus, the unique optoelectric properties of small diamondoids can be properly calculated and easily tuned. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B44.00003: A bound exciton model of acceptors in semiconductors Yong Zhang, Jianwei Wang We point out that the electronic structure of an acceptor bears a close similarity to that of an isoelectronic impurity bound exciton with a larger electronegativity (known as ``acceptor-like bound exciton'') [Hopfield et al., PRL 17, 312(1966)], and to some extent to that of a free exciton in a semiconductor. Instead of using only one quantity \textbf{\textit{acceptor binding energy}} E$_{\mathrm{A}}$ (based on Coulomb interaction) when dealing with the electronic transitions involving an acceptor, another quantity \textbf{\textit{impurity binding energy}} E$_{\mathrm{I}}$, depending on the atomic orbital difference, is usually more important in the transition processes. E$_{\mathrm{I}}$ resembles the role of the electron bound state or conduction band edge, whereas E$_{\mathrm{A}}$ resembles the hole or exciton binding energy, respectively, in the isoelectronic impurity or free exciton case. Furthermore, instead of viewing the acceptor impurity as a ``shallow impurity'' and isoelectronic impurity as a ``deep impurity'', it would be more appropriate to view for both impurity types that the bare electron bound state involves a localized potential, and the ionized impurity has a long-range Coulomb potential. A first-principles calculation of the total energy difference yields approximately E$_{\mathrm{I}}$ -- E$_{\mathrm{A}}$, but the energy needed to generate free holes is in fact E$_{\mathrm{I}}$. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B44.00004: First principle prediction of shallow defect level binding energies and deep level nonradiative recombination rates Invited Speaker: Linwang Wang Accurate calculation of defect level energies in semiconductors and their carrier capturing rate is an important issue in ab initio prediction of semiconductor properties. In this talk, I will present our result work in ab initio shallow level calculation [1] and deep level caused nonradiative recombination rate calculation [2]. In the shallow acceptor level calculation, a large system up to 64,000 atoms needs to be used to properly describe the weakly bounded hole wave functions. The single particle Hamiltonian of that system is patched from bulk potential and central potential. Furthermore, GW calculation is used to correct the one site potential of the impurity atom. The resulting binding energy agrees excellently with the experiments within 10 meV. To calculate the nonradiative decay rate, the electron-phonon coupling constants in the defect system are calculated all at once using a new variational algorithm. Multiphonon process formalism is used to calculate the nonradiative decay rate. It is found that the transition is induced by the electron and the optical phonon coupling, but the energy conservation is mostly satisfied by the acoustic phonons. The new algorithm allows fast calculation of such nonradiative decay rate for any defect levels, as well as other multiphonon processes in nanostructures. \\[4pt] [1] G. Zhang, A. Canning, N. Gronbech-Jensen, S. Derenzo, L.W. Wang, ``Shallow impurity level calculations in semiconductors using ab initio method,'' Phys. Rev. Lett 110, 166404 (2013). \\[0pt] [2] L. Shi, L.W. Wang, ``Ab initio calculations of deep level carrier nonradiative recombination rates in bulk semiconductors,'' Phys. Rev. Lett. 109, 245501 (2012). [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B44.00005: First-principles theory of radiative and nonradiative carrier capture rates at defects in semiconductors Audrius Alkauskas, Cyrus E. Dreyer, John L. Lyons, Qimin Yan, Chris G. Van de Walle We have developed a first-principles approach to calculate radiative and nonradiative carrier capture coefficients (cross sections) at defects in semiconductors. The methodology is based on the use of hybrid density functionals that provide an excellent description of both bulk and defect properties. As test cases, we applied the methodology to selected defects in GaN and ZnO. We have obtained excellent agreement with experimental results in the few cases where they are available. For deep acceptors, radiative electron capture cross sections are of the order 10$^{\mathrm{-5}}$ {\AA}$^{\mathrm{2}}$, while nonradiative hole capture cross sections are in the range 1-500 {\AA}$^{\mathrm{2}}$. Our results will (i) be very helpful for identifying the microscopic origin of defects in GaN and ZnO; (ii) provide fundamental insights into the origin of traps in electronic devices based on these materials; and (iii) help finding and controlling the centers responsible for Shockley-Read-Hall recombination in nitride optoelectronic devices. This work was supported by DOE. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B44.00006: Recombination driven vacancy motion - a mechanism of memristive switching in oxides Xiao Shen, Yevgeniy S. Puzyrev, Sokrates T. Pantelides Wide-band gap oxides with high O deficiencies are attractive memristive materials for applications. However, the details of the defect dynamics remain elusive, especially regarding what drives the defect motion to form the conducting state. While the external field is often cited as the driving force, we report an investigation of memristive switching in polycrystalline ZnO and propose a new mechanism [1]. Using results from density functional theory calculations, we show that the motion of O vacancies during switching to the conductive state is not driven by the electric field, but by recombination of carriers at these vacancies, which transfers energy to the defects and greatly enhances their diffusion. Such mechanism originates from the large structural change of O vacancies upon capturing electrons. In addition, contrary to the hypothesis that memristive switching in polycrystalline materials is facilitated by the defect motion along the grain boundary (GB), we show in our system the vacancies move perpendicular to the GB, attaching and detaching from it during the switching process. We call it recombination driven vacancy breathing.\\[4pt] [1] X. Shen, Y. S. Puzyrev, and S. T. Pantelides, MRS Commun. 3, 167 (2013). [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B44.00007: The impact of +U term on the electronic structure of Mn and Fe ions and of the gallium vacancy in GaN: GGA+U calculations Piotr Boguslawski, Oksana Volnianska, Tomasz Zakrzewski Band structure of solids is commonly calculated in the Local Density Approximation or the Generalized Gradient Approximation to the Density Functional Theory. Their known failure is the underestimation of the band gap. Within LDA or GGA, the approach of semi-empirical character that leads to correct band gaps consists in adding the +U term for particular atomic orbitals. While the impact of the +U term on bands of an ideal crystal was extensively discussed, its impact on the electronic structure of defects is less understood. Here, we systematically analysed how the +U term affects the properties of the gallium vacancy V:Ga, and of the Mn and Fe transition metal (TM) ions in GaN. The +U term was treated as a free parameter, and it was applied to p(N) and d(TM) orbitals. The results of GGA+U calculations were compared to available experimental data. U(N)=4 eV reproduces well the gap of GaN. We find that the +U terms strongly affect the electronic structure of Mn, Fe, and V:Ga. Surprisingly, however, for U=0, the energies of the gap levels induced by these centers, and of the intra-center optical transitions, agree well with experiment. In contrast, for U(N)=U(TM)=4 eV, these energies are in substantial disagreement with experimental values by about 1-2 eV. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B44.00008: Relating the defect band gap and the density functional band gap Peter Schultz, Arthur Edwards Density functional theory (DFT) is an important tool to probe the physics of materials. The Kohn-Sham (KS) gap in DFT is typically (much) smaller than the observed band gap for materials in nature, the infamous ``band gap problem.'' Accurate prediction of defect energy levels is often claimed to be a casualty---the band gap defines the energy scale for defect levels. By applying rigorous control of boundary conditions in size-converged supercell calculations, however, we compute defect levels in Si and GaAs with accuracies of $\sim$0.1 eV, across the full gap, unhampered by a band gap problem. Using GaAs as a theoretical laboratory, we show that the defect band gap---the span of computed defect levels---is insensitive to variations in the KS gap (with functional and pseudopotential), these KS gaps ranging from 0.1 to 1.1 eV. The defect gap matches the experimental 1.52 eV gap. The computed defect gaps for several other III-V, II-VI, I-VII, and other compounds also agree with the experimental gap, and show no correlation with the KS gap. Where, then, is the band gap problem? This talk presents these results, discusses why the defect gap and the KS gap are distinct, implying that current understanding of what the ``band gap problem'' means---and how to ``fix'' it---need to be rethought. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B44.00009: Convergence of density and hybrid functional defect calculations for compound semiconductors Stephan Lany, Haowei Peng, David Scanlon, Vladan Stevanovic, Julien Vidal, Graeme Watson Recent revisions of defect formation energy calculations based on band-gap corrected hybrid functionals have raised concerns about the validity of earlier results based on standard density functionals, and about the reliability of the theoretical prediction of electrical properties in semiconductor materials in general. We show here that a close agreement between the two types of functionals can be achieved by determining appropriate values for the electronic and atomic reference energies, thereby mitigating uncertainties associated with the choice of the underlying functional. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B44.00010: From Light Impurity Doping to Complete Cation Exchange in Semiconductor Nanocrystals: The Role of Coulomb Interactions Steven Erwin, Florian Ott, David Norris Cation exchange is a reversible chemical reaction used to create new materials by replacing one type of cation with another, usually from solution. We have developed an atomistic model describing cation exchange in semiconductor nanocrystals. The model uses a small set of results obtained from DFT calculations for Ag-doped CdSe. From these we constructed a kinetic Monte Carlo model to address finite temperatures and time scales beyond the reach of DFT. Our simulations span a wide range of Ag concentrations, from light doping to full cation exchange. Thus our model provides a single conceptual framework in which these two phenomena can be understood as limiting endpoints. The results of the simulations are consistent with several experimentally observed aspects of both phenomena. An unexpected finding of our simulations is that the Coulomb interaction plays a central, but changing, role as the Ag concentration varies from light doping to fully cation exchanged. For example, if the Coulomb interaction is strongly screened then cation exchange is suppressed or stopped. When only moderately screened, Coulomb effects play an unanticipated but important role for both doping and cation exchange. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B44.00011: Electronic structure and van der Waals interactions in the stability and mobility of point defects in semiconductors Wang Gao, Alexandre Tkatchenko Point defects are abundant in materials, and significantly affect the electronic, optical, and magnetic properties of solids. However, our understanding of the stability and mobility of point defects remains incomplete, despite decades of intensive work on the subject. In the framework of density-functional theory, Perdew-Burke-Ernzerhof functional underestimates formation energies by 0.7 eV due to the electron self-interaction error, while Heyd-Scuseria-Ernzerhof (HSE) functional yields formation energies in better agreement with high-level many-body methods, but often overestimates migration barriers by up to 0.4 eV. Using HSE coupled with screened long-range vdW interactions [1], we demonstrate that HSE+vdW can accurately describe both the formation energies and migration barriers of point defects. The inclusion of vdW interactions largely changes the transition state geometries, and brings migration barrier into close agreement with experimental values for six different defects. For multiatom vacancies and point defects in heavier semiconductors, vdW energy plays an increasingly larger role [2]. [1] G. X. Zhang {\em et al.}, PRL {\bf 107}, 245501 (2011); A. Tkatchenko, {\em et al.}, PRL {\bf 108}, 236402 (2012). [2] W. Gao {\em et al.}, PRL {\bf 111}, 045501 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B44.00012: spds* Tight-Binding Model for Transition Metal Dopants in SiC Victoria R. Kortan, C\"{u}neyt \c{S}ahin, Michael E. Flatt\'e SiC is a well known, wide-band-gap semiconductor with excellent chemical, thermal and mechanical stability. These traits make it an attractive material for high temperature, hostile environment, high power and high frequency device design[1]. A necessary step in the development of SiC technology is the understanding and subsequent control of point defects[2]. In addition to altering optoelectronic properties, single dopants can add effects dependent on the specific dopant species. In particular the d-states of transition metal dopants have been predicted to allow the control of the single Ni spin state with the application of strain in diamond [3] and single Fe dopants in GaAs have a core transition that can be manipulated by a STM and produce a decrease in tunneling current[4]. Here we choose a first and second nearest neighbor spds* tight-binding model to calculate the electronic trends and defect wavefunctions of transition metal dopants in 3C-SiC. Additionally we calculate the exchange interaction between pairs of dopants.\\[4pt] [1] H. Morko\c{c}, et al, J. of Appl. Phys. 76, 1363 (1994).\\[0pt] [2] S. Greulich-Weber, Phys. Stat. Sol. (a) 162, 95 (1997).\\[0pt] [3] T. Chanier, et al, EPL 99, 67006 (2012).\\[0pt] [4] J. Bocquel, et al, Phys. Rev. B 87, 075421 (2013). [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B44.00013: Simulated doping of Si from first principles using pseudo-atoms Ofer Sinai, Leeor Kronik Semiconductor doping is a process of fundamental importance to semiconductor physics and solid-state electronics, but cannot be explicitly simulated from first-principles due to the huge system size needed for most doping scenarios. We examine the efficacy of the simulation of doping in silicon by the inclusion of ``pseudo atoms'' with fractional nuclear charge, introduced via specially-constructed pseudopotentials. These provide a net charge carrier concentration matching an arbitrary chosen doping level, at no increase of the computational cost. By extending this approach to consider minute deviations from the integer charge, we demonstrate that the electron Fermi level can be set to any value within the forbidden gap, at minimal perturbation of the electronic structure. Beyond the bulk scenario, we successfully simulate the development of the space-charge region in a heavily-doped p-n junction and examine the doping-dependence of the work function of the hydrogen-passivated (semiconducting) Si(111) surface. [Preview Abstract] |
Session B45: Topological Quantum Hall Phases
Sponsoring Units: FIAPChair: Nickolas Bonesteel, Florida State University
Room: Mile High Ballroom 4D
Monday, March 3, 2014 11:15AM - 11:27AM |
B45.00001: Bulk-Edge Correspondence in 2+1-Dimensional Abelian Topological Phases Eugeniu Plamadeala, Meng Cheng, Michael Mulligan, Chetan Nayak, Jennifer Cano, Jon Yard The same bulk two-dimensional topological phase can have multiple distinct, fully-chiral edge phases. We show that this can occur in the integer quantum Hall and Abelian fractional quantum Hall states. We give a general criterion for the existence of multiple distinct chiral edge phases for the same bulk phase and discuss experimental consequences. We show that fermionic systems can have edge phases with only bosonic low-energy excitations and discuss a fermionic generalization of the relation between bulk topological spins and the central charge. The latter follows from our demonstration that every fermionic topological phase can be represented as a bosonic topological phase, together with some number of filled Landau levels. Our analysis shows that every Abelian topological phase can be decomposed into a tensor product of theories associated with prime numbers $p$ in which every quasiparticle has a topological spin that is a $p^n$-th root of unity for some $n$. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B45.00002: Distinguishing between edge phases of a bulk quantum Hall state Jennifer Cano, Meng Cheng, Michael Mulligan, Chetan Nayak, Eugeniu Plamadeala, Jon Yard The same bulk quantum Hall state can have multiple distinct, fully-chiral edge phases. This effect can occur at both integer and fractional quantum Hall states; examples include $\nu = 8$ and $12$ and the fractions $\nu = 8/7, 12/11, 8/15, 16/5$. This raises the question: given a quantum Hall device, how do we know which edge phase it is in? This is especially interesting when one edge phase has gapless fermionic excitations and the other does not. Since no bulk measurement can distinguish the states and their quasiparticles are identical, it is necessary to probe the edge directly. Here we discuss experimental probes that can distinguish between the possible edge phases. In addition, we consider interfaces between two edge phases and the localized zero energy modes that can reside at these interfaces. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B45.00003: Supersymmetry approach to delocalization transitions in a network model of the weak field quantum Hall effect and related models. Shanthanu Bhardwaj, Vagharsh Mkhitaryan, Ilya Gruzberg We consider a recently proposed network model of the integer quantum Hall (IQH) effect in a weak magnetic field. Using a supersymmetry approach, we reformulate the network model in terms of a superspin chain. A subsequent analysis of the superspin chain and the corresponding supersymmetric nonlinear sigma-model allows to establish the analytical form of the critical line of the weak-field IQH transition, which separates the Anderson insulator and the quantum Hall insulator phases. Our results also confirm the universality of the IQH transition, which is described by the same sigma model in strong and weak magnetic fields. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B45.00004: Loop Statistics in $SU(2)_k$ String-Net Models S.L. Sondhi, Vedika Khemani, Rahul Roy Topologically ordered quantum phases are often realized as condensates of highly-fluctuating, extended ``string-net'' degrees of freedom. We posit that the non-local quantum order in these phases manifests itself in universal, geometric properties of the underlying string-nets. In this work, we consider a mapping from the $SU(2)_k$ string-net models to generalized loop models and compute statistical properties of the resulting loops. In an appropriate classical limit, we find that the loop length distribution shows critical scaling with exponents that are independent of $k$. We also report loop-length scaling for the quantum $SU(2)_2$ and $SU(2)_3$ models. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B45.00005: Semiclassical theory of Hall viscosity Rudro Biswas Hall viscosity is an intriguing stress response in quantum Hall systems and is predicted to be observable via the conductivity in an inhomogeneous electric field. This has been studied extensively using a range of techniques, such as adiabatic transport, effective field theories, and Kubo formulae. All of these are, however, agnostic as to the distinction between strongly correlated quantum Hall states and non-interacting ones, where the effect arises due to the fundamental non-commuting nature of velocities and orbit positions in a magnetic field. In this talk I shall develop the semiclassical theory of quantized cyclotron orbits drifting in an applied inhomogeneous electric field and use it to provide a clear physical picture of how single particle properties in a magnetic field contribute to the Hall viscosity-dependence of the conductivity. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B45.00006: Fractionally charged bound states of an impurity in a fractional quantum Hall system Kelly Patton, Michael Geller The single-particle spectral function for an incompressible fractional quantum Hall state of the lowest Landau level (LLL) in the presence of a short-ranged attractive impurity potential is calculated via exact diagonalization. In contrast to the noninteracting case, where only a single bound state below the LLL, electron-electron interactions strongly renormalize the impurity potential, effectively giving it a finite range, which supports many quasi-bound states (long-lived resonances). Averaging the spectral weights $Z$ of the quasi-bound states and extrapolating to the thermodynamic limit, for filling factor $\nu = 1/3$ we find evidence consistent with localized fractionally charged $e/3$ quasiparticles. For $\nu = 2/5$, the results are slightly more ambiguous, due to finite size effects and possible bunching of Laughlin-quasiparticles. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B45.00007: Monte-Carlo Study of Phase Transitions out of Symmetry-Enriched Topological phases of bosons in Two-dimensions Jong Yeon Lee, Olexei Motrunich, Scott Geraedts In this work, we studied a statistical mechanics model of two species of bosons with mutual statistics $\theta=2\pi/n$ in (2+1) dimensions. This is a model for quasi-particles in a symmetry-enriched topological quantum phase of bosons with charge fractionalization, and by studying condensation of the quasi-particles we can access nearby phases. Through a reformulation, sign problem was eliminated and we could perform Monte Carlo simulation of this model. We focused on the phase transition point between the topological insulator and trivial Mott insulator to study critical properties of the transition. By measuring correlations in terms of original variables and dual variables, with finite size scaling, we could narrow down the region of criticality, and concluded it is a continuous multi-critical point. We extracted critical exponents for the topological phase transition. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B45.00008: Analyzing pseudo potentials in ``guiding-center-only'' approach Alexander Seidel, Zohar Nussinov, Jorge Dukelsky, Gerardo Ortiz A variety of short-range interactions are known whose zero energy modes successfully describe the low energy properties of various interesting phases in the fractional quantum Hall regime. The theoretical analysis of Haldane-type pseudo potentials and their generalizations is usually based on a first quantized picture, deriving nice analytical properties of their first quantized zero mode wave functions, which have polynomial form in most standard geometries. Recently, however, the second quantized -or guiding center- form of these pseudo-potentials has enjoyed much interest, e.g., in flat band solids. In such a context, the embedding of the problem into the lowest Landau level of some larger Hilbert space is artificial, and with the construction of new models in mind, it seems beneficial to understand how to systematically ``solve'' known pseudo-potential problems in a purely second quantized picture. He we discuss some general theorems that apply to the second quantized forms of pseudo-potential Hamiltonians, and allow for the derivation of many known properties of zero modes, in particular ``squeezing'', in a second quantized language, starting from a second quantized Hamiltonian. [1] Phys. Rev. B 88, 165303 (2013) [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B45.00009: Bulk/Boundary Relations in Hydrodynamics with Quantum Anomalies Gustavo Monteiro, Alexander Abanov It is well-known that Quantum Hall systems can be described as incompressible fluids. Their effective hydrodynamics description contains anomalous topological terms that reflect broken parity. These terms together with gauge symmetry generate topologically protected gapless states propagating on the boundary. We consider similar relations between quantum anomalous terms in hydrodynamics of two and one-dimensional systems. Our starting point is the relativistic hydrodynamics in 2+1 dimensions with parity odd terms [1, 2]. These terms are present due to quantum anomalies in the underlying field theory. We introduce domain walls to our system and derive the propagating mode along these domain walls. These edge modes are chiral and their effective action generalizes the one found in [3]. Furthermore, they are present even in the absence of external magnetic field and do persist at non-zero temperatures. As result, we find some relations between the anomalous transport in the bulk and at the boundary. We also discuss similar reductions in higher dimensional cases. [1] A. Nicolis and D. T. Son, arXiv:1103.2137 [2] F. M. Haehl and M. Rangamani, arXiv:1305.6968 [3] S. Dubovsky, L. Hui, and A. Nicolis, arXiv:1107.0732 [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B45.00010: Bipartite fluctuations and entanglement spectrum in quantum Hall states Alexandru Petrescu, H. Francis Song, Stephan Rachel, Zoran Ristivojevic, Christian Flindt, Nicolas Laflorencie, Israel Klich, Nicolas Regnault, Karyn Le Hur We exploit a general relation between bipartite fluctuations of particle number or spin and the real space bipartite entanglement entropy and the entanglement R\'enyi entropies for free fermion systems [Phys. Rev. B \textbf{85}, 035409 (2012)]. We apply this method to derive the real space entanglement entropy and entanglement spectrum [Phys. Rev. Lett. \textbf{101}, 010504 (2008)] of integer quantum Hall systems and Chern insulators, focusing on continuum models, edge models at quantum point contacts and the role of sine-Gordon terms, and finite-sized lattice models. Numerical efforts will be addressed for fractional quantum Hall systems. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B45.00011: Fractional quantum Hall droplet on a lattice Martin Claassen, Thomas Devereaux In analogy to the fractional quantum Hall (FQH) liquid on a disk, we study droplets of interacting electrons in a fractional Chern insulator, in a dispersionless band with non-zero Chern number $\mathcal{C}$. We describe how the quantum geometry of such a band naturally defines a basis of momentum-space Landau levels, with radially-localized wave functions that preserve lattice rotational symmetries, in direct analogy to the lowest Landau level in the continuum. This new approach permits a direct description of the interacting droplet in terms of Haldane pseudopotentials on the disk. We then provide numerical results for the formation of a FQH liquid. We deform the host lattice model via local adiabatic modifications to ideal models with flat Berry curvature and analyze the ground state wavefunction. For $\mathcal{C} >$ 1, we discuss generalizations of the FQH droplet as multicomponent FQH systems. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B45.00012: Multi-layer fractional quantum Hall states in lattice systems Layla Hormozi We study fractional quantum Hall states of interacting particles in lattice systems subject to external magnetic fields. When the number of flux quanta per lattice plaquette is close to a rational fraction, the lowest energy states can be mapped to degenerate lowest Landau levels in the continuum, where particles carry an extra degree of freedom -- a pseudospin or layer-index. We find a class of multi-layer fractional quantum Hall states that can form in these systems with different inter- and intra-layer interactions and show that topological and spectral properties of these states can be derived from different conformal field theories that are related by a condensation/orbifolding mechanism. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B45.00013: 2D electrons in a magnetic field. Linear responses to curvature and e/m fields Alexander Abanov, Andrey Gromov Two-dimensional electron gas in a quantizing magnetic field plays an important role in condensed matter physics. At the integer filling factor its linear responses to weak e/m fields are known as expansions in wave vectors and frequencies. We generalize these known results to gravitational and mixed responses considering the system in weakly curved background. Using the obtained expansions to all orders in wave vectors and frequencies we verify the exact relations between linear response functions following from the Galilean symmetry of the model [1-2] as well as phenomenological expressions derived in [3]. We present examples of the linear responses such as charge accumulation around a disclination defect (conic singularity), non-dissipative current perpendicular to the gradient of the scalar curvature, stress in the medium produced by the inhomogeneous e/m field, etc. \\[4pt] [1] C. Hoyos, D. T. Son: `` Hall viscosity from electromagnetic response'' \newline [2] B. Bradlyn, M. Goldstein, N. Read: ``Kubo formulas for viscosity: Hall viscosity, Ward identities, and the relation with conductivity'' \newline [3] A. Abanov: ``On the effective hydrodynamics of FQHE'' [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B45.00014: Topological gaps without masses in driven graphene-like systems Thomas Iadecola, Titus Neupert, Claudio Chamon We illustrate the possibility of realizing band gaps in graphene-like systems that fall outside the existing classification of gapped Dirac Hamiltonians in terms of masses. As our primary example we consider a band gap arising due to time-dependent distortions of the honeycomb lattice. By means of an exact, invertible, and transport-preserving mapping to a time-independent Hamiltonian, we show that the system exhibits Chern-insulating phases with quantized Hall conductivities $\pm e^2/h$. The chirality of the corresponding gapless edge modes is controllable by both the frequency of the driving and the manner in which sublattice symmetry is broken by the dynamical lattice modulations. We demonstrate that, while these phases are in the same topological sector as the Haldane model, they are nevertheless separated from the latter by a gap-closing transition unless an extra parameter is added to the Hamiltonian. Finally, we discuss a promising possible realization of this physics in photonic lattices. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B45.00015: Majorana fermion qubit states and non-Abelian braiding statistics in quenched inhomogeneous spin ladders Yan Chen, Yinchen He In studying Majorana fermions (MFs) in a spin ladder model, we numerically show that their qubit state can be read out by measuring fusion excitations in quenched inhomogeneous spin ladders. We construct an exactly solvable T-junction spin ladder model that can be used to implement MF braid operations. With braiding simulated numerically as non-equilibrium quench processes, we verify that the MFs in our spin ladder model obey non-Abelian braiding statistics. Our scheme provides a promising platform to study exotic properties of MFs and a broad range of applications in topological quantum computation. [Preview Abstract] |
Session B47: Superconductors/Ferromagnet Multilayer Structures
Sponsoring Units: DCMPChair: Chia-Ling Chien, Johns Hopkins University
Room: Mile High Ballroom 4F
Monday, March 3, 2014 11:15AM - 11:27AM |
B47.00001: Proximity effect in superconductor/ferromagnet hetero-structures as a function of interface properties Julio Sarmiento, Edgar J. Patino Superconductor/ferromagnet heterostructures are currently a subject of strong research due to novel phenomena resulting from the proximity effect. Among the most investigated ones are the oscillations of the critical temperature as function of the ferromagnet thickness. The oscillatory behavior of Tc is theoretically explained as to be result of the generation of the FFLO (Fulde-Ferrel-Larkin-Ovchinnikov) state of Cooper pairs under the presence of the exchange field of the ferromagnet [1,2]. With the advancement of experimental techniques for S/F bilayers growth new questions regarding the effect of the interface transparency can to be addressed. For instance the influence of the interface roughness on the proximity effect [3]. For studying this phenomenon Nb/Co and Nb/Cu/Co samples were sputtered on SiO2 substrates with different roughness. Characterization of these samples show a significant variation of Tc with the interface roughness. This results point towards a possible relationship between transparency and roughness of the interface. References [1] J. S. Jiang, et al., Phys. Rev Lett. 74 (1995) 314. [2] C. Cirillo, et al . Phys. Rev. 72, 144511 (2005) [3] E. J. Pati\~{n}o, Study of The Influence of Domain Walls in The S/F Proximity Effect. Cambridge (2005). [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B47.00002: Signatures of triplet supercurrents in hybrid S/F structures Caroline Richard, Manuel Houzet, Julia Meyer While ferromagnetism and conventional superconductivity appear as antagonist phases, the proximity effect in hybrid S/F structures offers a unique opportunity to study their interplay. In particular, spin-triplet odd-frequency superconducting correlations may be induced in a diffusive ferromagnet. We study the Josephson effect through a long ferromagnetic bilayer in the diffusive regime [1]. For a non-collinear magnetization of the bilayer, we find that the current phase relation is dominated by its second harmonic, and corresponds to the long-range coherent propagation of two triplet pairs. Here, the superharmonicity is a signature of the Josephson coupling between a singlet superconductor and an effective triplet superconductor induced at the end of the ferromagnetic bilayer attached to the other lead. Then, we further study the critical current flowing between such two effective triplet reservoirs through a conventional superconducting layer. As a result of the competition between triplet/triplet and triplet/singlet couplings and under quite general conditions, we find that the critical current exhibits a maximum in the vicinity of the superconducting transition of the central layer. [1] C. Richard, M. Houzet, and J.S. Meyer, PRL. 110, 217004 (2013) [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B47.00003: Tunneling and Transport in Clean Ferromagnet-Superconductor Heterostructures Chien-Te Wu, Oriol Valls, Klaus Halterman We study charge and spin transport in clean Ferromagnet (F)-Superconductor (S) layered structures. By combining a transfer matrix method with a numerical self-consistent solution of the Bogoliubov-de Gennes (BdG) equations, we compute the spin dependent tunneling conductance in F-F-S trilayers in a range of exchange fields and layer thicknesses. In particular, we investigate the dependence of the tunneling conductance on the angle $\alpha$ between the magnetizations in two F layers. We find a variety of non-monotonic and switching behaviors in these heterostructures. We also present results for charge and spin transport in S-F-F-S Josephson junctions. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B47.00004: Electron transport in p-wave superconductor-normal metal junctions Ahmet Keles, Anton Andreev, Boris Spivak We study low temperature electron transport in p-wave superconductor-insulator-normal metal junctions. In diffusive metals the p-wave component of the order parameter decays exponentially at distances larger than the mean free path l. At the superconductor-normal metal boundary, due to spin-orbit interaction, there is a triplet to singlet conversion of the superconducting order parameter. The singlet component survives at distances much larger than l from the boundary. It is this component that controls the low temperature resistance of the junctions. As a result, the resistance of the system strongly depends on the angle between the insulating boundary and the d-vector characterizing the spin structure of the triplet superconducting order parameter. We also analyze the spatial dependence of the electric potential in the presence of the current, and show that the electric field is suppressed in the insulating boundary as well as in the normal metal at distances of order of the coherence length away from the boundary. This is very different from the case of the normal metal-insulator-normal metal junctions, where the voltage drop takes place predominantly at the insulator. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B47.00005: Theory of proximity induced triplet superconductivity in spin-orbit-coupled systems Xin Liu, Jainendra Jain, Chao-Xing Liu We study proximity induced triplet superconductivity in a spin-orbit-coupled system, and show that the $\vec{d}$-vector of the induced triplet superconductivity undergoes precession that can be controlled by varying the Rashba and Dresselhaus spin-orbit couplings. In particular, a long range triplet-helix mode is predicted when the two spin-orbit couplings have equal strengths. We also study the Josephson junction geometry and show that a transition between 0- and $\pi$- junctions can be induced by controlling the spin-orbit coupling strength. An experimental setup is proposed to verify these effects. Conversely, the observation of these effects can serve as a direct confirmation of the triplet nature of the superconductivity. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B47.00006: Control of Spin-Triplet S/F/S Josephson junctions E.C. Gingrich, B.M. Niedzielski, A.M. Cramer, W.P. Pratt, Jr., Norman O. Birge We present recent work on S/F'/F/F''/S Josephson junctions, where F' is a hard ferromagnet, F is a synthetic antiferromagnet (SAF), and F'' is a soft ferromagnet. With the hard and soft ferromagnets magnetizations lying in plane, and the central ferromagnet possessing a non-colinear magnetization with respect to the F' and F'' layers, the Josephson junctions experience an enhancement in their critical current due to the generation of Spin-Triplet pair correlations. The direction of the F'' layer is predicted to control the state of the junction. We are fabricating such junctions with the goal of controllably switching the state of the junction between the $0$ and $\pi$ states. By integrating the junctions into a SQUID device, the state of the junctions can be measured. We will report on our progress. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B47.00007: Exploration of long-range spin-triplet correlations in superconductor/ferromagnetic hybrid systems William Martinez, W.P. Pratt, Jr., Norman O. Birge Since the prediction of long-range spin-triplet correlations (LRTCs) in superconductor/ferromagnet (S/F) systems,[1] their realization has been investigated by many groups. From F-N bilayers to intrinsic generation of spin-triplet through domain walls, there is wide interest in observing a signal at ranges beyond the tens of $nm$ observed in earlier work.[2] In this work, we examine the propagation of LRTCs extrinsically generated through noncollinear magnetization, at long $(100nm)$ length scales. We will report on our recent progress. \\ $[1]$ A.F. Volkov, F.S. Bergeret and K.B. Efetov, Phys. Rev. Lett., \textbf{90}, 117006 (2003).\\ $[2]$ T.S. Khaire, M.A. Khasawneh, W.P.Pratt, Jr, and N.O. Birge, Phys. Rev. Lett. \textbf{104}, 137002 (2010). [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B47.00008: Dimensionality crossover in ferromagnetic/superconducting films: Role of magnetic history Luis Ruiz-Valdepe\~nas, Fernando Valdes-Bango, Luis Alvarez-Prado, Jose Martin, Elena Navarro, Maria Velez, Jose Alameda, Jose Vicent Amorphous NdCo$_{5}$ films are ferromagnetic samples with a weak perpendicular magnetic anisotropy which can show small magnetic domain sizes (less than 100 nm) with labyrinthine structures. Sputtering technique is used to fabricate Nb/Al(5nm)/ NdCo$_{5}$ superconducting films on Si substrates. The temperature dependence of the upper critical field shows features which could be related to an ``imprinting'' of the domain structure of NdCo5 layers in the superconducting Nb film. This peculiar proximity effect governs the superconductivity dimensionality crossover from 1D to a regime between 1D and 2D typical of superconducting wire network. This superconducting crossover can be connected to the NdCo$_{5}$ magnetic history. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B47.00009: Angular Dependence of Superconductivity in Superconductor / Spin Valve Heterostructures Alejandro Jara, Christopher Safranski, Ilya Krivorotov, Chien-Te Wu, Oriol Valls The superconducting condensate in superconductor / ferromagnet (S/F) multilayers consists of singlet and triplet components. For a non-collinear state of magnetization of the multilayer, all three spin components $S_{z}=(0,\pm 1)$ of the triplet condensate are generally non-zero, which can result in a long range proximity effect in S/F multilayers. Indeed, the $S_z= \pm 1$ triplet components of the condensate are immune to pair breaking by the exchange field and, unlike the singlet and the $S_z =0$ triplet components, they can penetrate deep into the ferromagnetic layers. Here we report measurements demonstrating magnetic control of the triplet component amplitude in Nb/Co/Cu/Co/CoOx superconducting spin valves. We find that for all values of the layer thicknesses employed in the experiment, Tc shows non-monotonic angular dependence with a minimum near perpendicular orientation of the Co layers. This drop in Tc is evidence of the enhanced long-range triplet amplitude in the maximally non-collinear configuration of the spin valve. We will present detailed measurements of the magnitude of this effect as a function of thicknesses of both Co and Cu layers of the spin valve. We will also compare our data to theoretical predictions of the angular dependence of Tc for this system. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B47.00010: Superconducting proximity effects in YBa$_{2}$Cu$_{3}$O$_{7}$/[Co/Pt] multilayers J.E. Villegas, C. Visani, A. Verso, F. Cuellar, C. Deranlot, R. Bernard, A.F. Volkov, F.S. Bergeret We have studied the penetration of superconducting correlations into Co/Pt multilayers and single Pt thin films deposited on top of c-axis YBa$_{2}$Cu$_{3}$O$_{7}$. We used tunneling conductance measurements across an AlO$_{\mathrm{x}}$ barrier in order to track changes in the electron density of states of the Co/Pt (or Pt) films. A large number of junctions were studied, varying several parameters. We found more pronounced, longer-ranged signatures of proximity-induced superconductivity (energy-gap below T$_{\mathrm{C}})$ in Co/Pt than in single Pt films. This is interpreted by considering a series of magnetic effects specific to the Co/Pt interface, which induce inhomogeneities in the exchange field felt by the conduction electrons and explain the presence of long-range proximity effects. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B47.00011: Proximity effect in superconductor/conical magnet/ferromagnet heterostructures James Annett, Daniel Fritsch At the interface between a superconductor and a ferromagnetic metal spin-singlet Cooper pairs can penetrate into the ferromagnetic part of the heterostructure with an oscillating and decaying spin-singlet Cooper pair density. However, if the interface allows for a spin-mixing effect, equal-spin spin-triplet Cooper pairs can be generated that can penetrate much further into the ferromagnetic part of the heterostructure, known as the long-range proximity effect. Here, we present results of spin-mixing based on self-consistent solutions of the microscopic Bogoliubov-de Gennes equations incorporating a tight-binding model. In particular, we include a conical magnet into our model heterostructure to generate the spin-triplet Cooper pairs and analyze the influence of conical and ferromagnetic layer thickness on the unequal-spin and equal-spin spin-triplet pairing correlations. It will be shown that, in agreement with experimental observations [1], a minimum thickness of the conical magnet is necessary to generate a sufficient amount of equal-spin spin-triplet Cooper pairs allowing for the long-range proximity effect [2]. \newline [1] J. W. A. Robinson, J. D. S. Witt, and M. G. Blamire. Science {\bf 329}, 59 (2010). \newline [2] D. Fritsch and J. F. Annett, arXiv:1311.3278 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B47.00012: Giant thermoelectric effects in a proximity-coupled superconductor-ferromagnet device Peter Machon, Matthias Eschrig, Wolfgang Belzig The usually negligibly small thermoelectric effects in superconducting heterostructures can be boosted dramatically due to the simultaneous effect of spin-dependent scattering and spin-filtering. Build on our idea [Phys. Rev. Lett. \textbf{110}, 047002 (2013)], we propose realistic setups to measure local thermoelectric effects in superconductor heterostructures. We focus on the Seebeck-effect which is a direct measure of the local thermoelectric response and find that a thermopower $\sim100\mu V/K$ can be achieved if a third terminal allows to drain the thermal current. A measurement of the thermopower can furthermore be used to determine quantitatively the spin-dependent interface parameters that induce the spin splitting. For applications in nano-cooling we discuss the figure of merit that we found to exceed one for realistic parameters. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B47.00013: Direct Detection of Cooper Pair Momentum in Fulde-Ferrell Superconductors Wei Chen, Rui Shen, Dingyu Xing, Ming Gong Finite momentum pairing for Cooper pairs in inhomogeneous superconductors is an important conceptual extension of the celebrated Bardeen-Cooper-Schrieffer theory of superconductivity. In the past five decades, great efforts have been paid and only indirect evidences related to the possible finite momentum pairing have been reported. In this work, we propose an Andreev interferometer based on branched Y-junction to directly detect the Cooper pair momentum in the Fulde-Ferrell (FF) superconductor, which can provide the most convincing evidences for finite momentum pairing. The subgap conductance of the interferometer is a unique function of the phase difference induced by the FF superconductor, providing full information of the Cooper pair momentums. We demonstrate that the function of the device will not be affected by other uncontrollable phases during the Andreev scattering processes. The interferometer has important applications in non-centrosymmetric superconductors, where FF type pairing is generally expected under proper in-plane magnetic field. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B47.00014: Switching Josephson coupling through a pseudo-spin-valve barrier with an exchange-field effect Burm Baek, William Rippard, Samuel Benz, Stephen Russek, Paul Dresselhaus, Matthew Pufall, Horst Rogalla With respect to information technology applications, Josephson junctions can be used in circuits that perform logic operations in picoseconds, which may result in energy-efficient, high-performance cryogenic computers, provided that memory elements can be developed that can be switched between two stable states by integrated superconducting logic circuits. We show that Josephson junctions based on pseudo-spin-valve barriers could enable such memory elements. We tuned the magnetic materials and carefully analyzed the full magnetic field dependence of the critical current in order to differentiate the controlled changes in Josephson coupling from the spatial superconducting phase modulation mediated by the remanent fields. We observed clear changes in Josephson critical current that can be either in magnitude or phase, which are explained well by the direct exchange-field effect on the spin-singlet superconducting pairs. These devices are the first that demonstrate nonvolatile, size-independent switching of the Josephson coupling in magnitude or phase, and they may allow for the first scalable superconducting memory devices. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B47.00015: Anomalous Resistance Behavior in Ferromagnet/Superconductor Heterostructure Lu-Kuei Lin, Jin-Hua Huang, Shang-Fan Lee We report on anomalous resistance peaks larger than the normal-state values in ferromagnetic NiFe wires with superconductor Nb electrodes. The resistance peaks occurs just below the onset of superconducting transition. The peak resistance temperature was suppressed and the peak height diminished by magnetic field together with the reduction of onset transition temperature. It is distinct from the charge imbalance behavior which is sensitive to magnetic field. The spin accumulation is one possible mechanism but the resistance peak is three-order larger than the predicted value. Contrary to the typical interfacial resistance contribution from band structure mismatch and electron spin imbalance, which should increase as the size of superimposed area was reduced, when we reduced the size of the ferromagnet/superconductor overlapping area, the resistance peaks vanished. Possible mechanisms of the electronic transport in the ferromagnet/superconductor interface will be discussed. [Preview Abstract] |
Session B48: General Magnetism I
Sponsoring Units: GMAGRoom: Mile High Ballroom 1A
Monday, March 3, 2014 11:15AM - 11:27AM |
B48.00001: Renormalization of spin-rotation coupling Mamoru Matsuo, Jun'ichi Ieda, Sadamichi Maekawa We show the enhancement of the spin-rotation coupling due to the interband mixing[1]. The Bloch wave functions in the presence of mechanical rotation are constructed with the generalized crystal momentum which includes a gauge potential originating from the rotation. Using the Kane model, the renormalized spin-rotation coupling is explicitly derived. As a result of the renormalization, the rotational Doppler shift in electron spin resonance, the mechanical torque on an electron spin, and the spin current generation due to elastic deformation[2] will be strongly enhanced. [1] M. Matsuo, J. Ieda and M. Maekawa, Phys. Rev. B87, 115301 (2013). [2] M. Matsuo, J. Ieda, K. Harii, E. Saitoh and M. Maekawa, Phys. Rev. B87, 180402(R) (2013). [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B48.00002: Ab Initio $\alpha\rightarrow\epsilon$ Transition Barrier in Shocked Iron Michael P. Surh, Babak Sadigh An ab initio study is presented of the $\alpha\rightarrow\epsilon$-like transition in single crystal iron under uniaxial strain (Kalantar, et. al., Phys. Rev. Lett. 95 075502 (2005)). First-principles ground state DFT calculations for ordered-spin phases predict a magnetostructural transition from ferromagnetic (FM) to an antiferromagnetic (AFM) phase at a uniaxial strain similar to experiment. However, the calculated stress is higher than is inferred from the shock. DFT also predicts a significant energy barrier for the expected atomic shuffle transition from bcc to hcp. In contrast, the experimental kinetics are consistent with a barrier-free or small-barrier transformation path. It is possible that the neglect of thermal spin disorder in the DFT calculations explains these discrepancies. To study this, the spin and atomic order are artificially separated, assuming that the atomic motion is adiabatic on spin time-scales. A Heisenberg-like Hamiltonian is fit to DFT energies of different ordered magnetic phases, and magnetic free energies are calculated for fixed atomic coordinates by Monte Carlo methods. This model predicts a significantly smaller elastic constant than DFT for the FM ground state, and it reduces the transition energy barrier versus ground state values. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B48.00003: Time-reversal invariant U(1) spin liquids in three dimensions Chong Wang, Senthil Todadri We study possible quantum U(1) spin liquids in three dimensions with time-reversal symmetry. We find there are in total eight such states, distinguished by the properties of their emergent electric/magnetic charges. Various aspects of these states will be discussed, including their spin wave-functions and relevance to pyrochlore spin ices. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B48.00004: Magnetic Phase Diagram of the Classical Kitaev-Heisenberg (KH) Model Craig Price, Natalia Perkins In this work, we numerically study the low-temperature magnetic properties of the Kitaev-Heisenberg model using classical Monte-Carlo simulations. Due to the discreteness of the KH model caused by the Kitaev interaction while in zero magnetic field, the model is magnetically ordered at low temperatures for all non-zero values of the Kitaev interaction except at two special points. The ordered phase is stabilized entropically by an order-by-disorder mechanism where thermal fluctuations of classical spins select collinear magnetic states that point along a cubic direction. We computed the H-T phase diagrams of the KH model for different orientations of the magnetic field and estimated the saturation field and its directional dependence in each phase. The low-temperature magnetic phase diagram is significantly modified both by the presence of external magnetic fields and the field's orientation with respect to cubic axes. In the Kitaev limit, the external field continuously changes the classical ground-state manifold of the geometrically frustrated classical Kitaev, thus it stabilizes different magnetic states at different strengths of the magnetic field. Our results can be used to understand the physics of Li$_2$IrO$_3$ and Na$_2$IrO$_3$ in an applied magnetic field. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B48.00005: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 12:15PM - 12:27PM |
B48.00006: Unusual physical behaviors of strongly correlated rare earth dialuminides Durga Paudyal, V.K. Pecharsky, K.A. Gschneidner, Jr. We present electronic, magnetic, and magnetostructural behaviors of rare earth dialuminides calculated by first principles. Total energy calculations show that CeAl$_{2}$ and EuAl$_{2}$ adopt antiferromagnetic ground states while dialuminides formed by other magnetic lanthanides have ferromagnetic ground states. The magnetic moment of CeAl$_{2}$ indicates that the 4f orbital moment of Ce in CeAl$_{2}$ is quenched. Eu in EuAl$_{2}$ and Yb in YbAl$_{2}$ are divalent. PrAl$_{2}$ exhibits a tetragonal distortion near ferromagnetic transition. HoAl$_{2}$ shows a first order magnetostructural transformation while DyAl$_{2}$ shows a second order transformation. The dialuminides formed by Nd, Tb, and Er are simple ferromagnet without additional anomalies in the ferromagnetic state. SmAl$_{2}$ orders ferromagnetically with less than 1 $\mu_{\mathrm{B}}$ indicating the cancellation of 4f spin moment by its orbital counterpart. Due to substantially high 4f crystal field splitting TmAl$_{2}$ shows 4f spin magnetic moment lower than expected. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B48.00007: Superconducting instability of a non-magnetic metallic band in an antiferromagnetic background Fernando Reboredo In superconducting cuprates there is experimental consensus that, for low doping, holes occupy a band primarily formed by the $p_x$ and $p_y$ oxygen orbitals in the CuO2 planes. However, it is very difficult to determine whether this band is formed by $\sigma$ or a $\pi$ bonds with the $d$ orbitals of copper. In electron doped cuprates, the location of the carriers less clear. Most ab-initio methods based in a mean field approach lack of the accuracy required to determine the location of the carriers introduced by doping due to errors in exchange, correlation and self-interaction. Nevertheless, these methods have been used to support models that consider only one Cu orbital and the $\sigma$ oxygen bonds. In this talk we consider what could happen if the carriers go elsewhere and discuss attempts to determine the location of these carriers with diffusion Monte Carlo. It is shown that if the holes occupy the $\pi$ bonds or the electrons remain centered at the cation outside the planes, a non-magnetic metallic band would form. In the presence of an antiferromagnetic background this band will be coupled to the planes by the exchange interaction in second order, developing a superconducting instability similar to the one described by BCS theory. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B48.00008: Ferromagnetism and stability of three-fermion clusters in two-dimensional lattices Pavel Kornilovitch A three-body spin-$\frac{1}{2}$ fermion problem with on-site repulsion and nearest-neighbor attraction is solved on a one-dimensional chain and on a two-dimensional square lattice by discretizing the Schroedinger equation in momentum space. Energies of bound complexes (trions) and their binding conditions are obtained for total spins S = 1/2 and S = 3/2. In the S = 1/2 sector in one dimension, a narrow but finite parameter region is identified where the ground state consists of a stable fermion pair and an unbound fermion [EPL, 103, 27005 (2013)]. In the S = 1/2 sector in two dimensions, a much wider region of pair stability is found. The stability is attributed to the formation of a centrifugal barrier for the third fermion. In the S = 3/2 sector in two dimensions, trions are found to form before triplet pairs indicating ``Borromean'' coupling. In the strong-attraction limit in two dimensions, the system transitions from an S = 1/2 ground state to a ferromagnetic S = 3/2 ground state in agreement with the Nagaoka theorem for a four-site plaquette. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B48.00009: Spin-spin interaction in the bulk of topological insulators Liang Chen, Jinhua Sun, Hai-Qing Lin We apply mean-field theory and Hirsch-Fye quantum Monte Carlo method to study the spin-spin interaction in the bulk of three-dimensional topological insulators. We find that the spin-spin interaction has three different components: the longitudinal, the transverse and the transverse Dzyaloshinskii-Moriya-like terms. If the chemical potential is located in the bulk gap of topological insulators, the spin-spin interaction decays exponentially due to the Bloembergen-Rowland interaction. And the longitudinal correlation is antiferromagnetic, the transverse correlations is ferromagnetic and the transverse Dzyaloshinskii-Moriya-like correlation is suppressed if the distance between magnetic impurities is sufficient large. When the chemical potential is in the conduction or valence band, the spin-spin interaction follows power law decay, and isotropic ferromagnetic interaction dominates in short separation limit. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B48.00010: MBE Growth of Si/MnGe Quantum Dot Superlattice with Curie Temperature beyond 400 K Tianxiao Nie, Xufeng Kou, Yabin Fan, Jianshi Tang, Shengwei Lee, Murong Lang, Chia-Pu Chu, Liang He, LiTe Chang, Kang L. Wang The realization and application of spintronic devices would be boosted dramatically if room-temperature ferromagnetism could be integrated into semiconductor nanostructures, especially when compatible with the mature silicon technology. Here, a Si/MnGe superlattice with quantum dots well aligned in the vertical direction was successfully grown by molecular beam epitaxy. Magnetic measurements found that the superlattice structure exhibited a Curie temperature beyond 400 K, which is attributed to the presence of Mn-doped quantum dot nanostructures. Such unique Si/MnGe superlattice sets a new stage for strengthening ferromagnetism due to the enhanced hole-mediation by quantum confinement, which has the potential to realize the room-temperature spin filed-effect transistor devices with lower power dissipation and low variability. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B48.00011: First-principles evidence of Mn moment canting in hole-doped Ba$_{1-2x}$K$_{2x}$Mn$_{2}$As$_{2}$ James Glasbrenner, Igor Mazin The compound BaFe$_{2}$As$_{2}$ is the proptotypical example of the 122 family of high-$T_{c}$ Fe-based superconductors that crystallize in the ThCr$_{2}$Si$_{2}$ structure. Isostructural compounds can be formed by replacing Fe with another transition metal; using Mn produces the material BaMn$_{2}$As$_{2}$. Despite its lack of superconductivity, the material is interesting in its own right. Recent experimental studies have shown that hole-doping the compound by substituting K for Ba leads to metallic behavior and a spontaneous, weak, in-plane magnetization, which was attributed to the holes fully polarizing independent of the Mn moments, producing half-metallic behavior. However the observed in-plane magnetization can also be understood as a small canting of the Mn moments. Using density functional theory, we demonstrate that a Mn moment canting occurs upon hole-doping the compound. We argue that this is due to the competition between the super- and double exchange interactions, which we support using a simple tight-binding model of the superexchange-double exchange interaction and the Andersen Force Theorem. Our calculations also rule out an in-plane polarization of As holes as an explanation for the in-plane magnetization. [1]J. K. Glasbrenner and I. I. Mazin, arXiv:1311.1537 [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B48.00012: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 1:39PM - 1:51PM |
B48.00013: Magnetic force microscopy of magnetic domains in UMn$_{2}$Ge$_{2}$ Morgann Berg, Alex de Lozanne, Ryan E. Baumbach, Jeehoon Kim, Eric D. Bauer, Joe D. Thompson, Filip Ronning UMn$_{2}$Ge$_{2}$, a distant cousin to the heavy-fermion compound URu$_{2}$Si$_{2}$, is a ternary intermetallic compound with a tetragonal crystal structure of type ThCr$_{2}$Si$_{2}$ and with space group I4/mmm. Local U and Mn moments in UMn$_{2}$Ge$_{2}$ order on their respective sublattices at temperatures near 100 and 380 K, respectively. Previous high-pressure x-ray diffraction and Kerr rotation angle measurements point to structural and magnetic phase transitions that reflect the competition between U and Mn spins at low temperatures. As U moments order with a reduction in temperature, they are predicted to align the Mn moments along the c-axis, altering the anisotropy of the material and the easy axis direction. A reduction of inter-atomic distances between the U and Mn atoms is also projected to induce hybridization between uranium 5f and manganese 3d states, leading to a delocalization of magnetic moments and reduction in magnetization. We use a variable temperature atomic force microscope in magnetic force microscopy (MFM) mode to obtain some initial images of magnetic domains in UMn$_{2}$Ge$_{2}$. Room temperature MFM images display branching magnetic domains with uniaxial anisotropy. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B48.00014: Non-hysteretic colossal magnetoelectric effect in a collinear antiferromagnet Yoon Seok Oh, Sergey Artyukhin, Jun Jie Yang, Vivien Zapf, Jae Wook Kim, David Vanderbilt, Sang-Wook Cheong Electric field control of magnetization has attracted lots of attention because of its potential applications for magnetoelectric devices such as memory, sensors and oscillators, as well as a fundamental interest. Recently, large magnetoelectric responses have been reported in various magnetoelectric systems. Most of the colossal magnetoelectric effects are in response to domain wall motion associated with the phase coexistence and metastability at the 1st order phase transition. This nature leads to hysteretic behavior of the magnetoelectric response. In applications for sensors and oscillators, the hysteresis plays a role of detrimental side-effect such as low precision, drift and asymmetric oscillation. In this talk, we demonstrate non-hysteretic colossal magnetoelectric effect in a collinear antiferromagnet and discuss colossal magnetoelectric response of magnetization as well as polarization associated with the continuous spin-flop transition. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B48.00015: Influence of pressure and chemical substitution on samarium-cobalt permanent magnets Scott McCall, Jason Jeffries, Jonathon Lee, Patrick Huang The magnetic properties of a material are generally sensitive to its crystal structure, particularly its interatomic spacing. This spacing can be adjusted through application of external pressure and by chemical substitution. Measurements on the magnetic and thermodynamic properties of samarium cobalt permanent magnets are reported as a function of applied pressure and chemical substitution on the cobalt site. The effects of these two tuning parameters will be compared and discussed. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Session B49: Focus Session: Electron Correlations and Spin Orbit: Iridates
Sponsoring Units: DMPChair: James Rondinelli, Drexel
Room: Mile High Ballroom 1C
Monday, March 3, 2014 11:15AM - 11:51AM |
B49.00001: Exploring the correlated phase behavior and electronic properties of parent and doped spin-orbit Mott phases Invited Speaker: Stephen Wilson An unusual manifestation of Mott physics dependent on strong spin-orbit interactions has recently been identified in a growing number of classes of 5d transition metal oxides built from Ir$^{4+}$ ions. Instead of the naively expected increased itinerancy of these iridates due to the larger orbital extent of their 5d valence electrons, the interplay between the amplified relativistic spin-orbit interaction (intrinsic to large Z iridium cations) and their residual on-site Coulomb interaction $U$, conspires to stabilize a novel class of spin-orbit assisted Mott insulators with a proposed J$_{eff}=$1/2 ground state wavefunction. The identification of this novel spin-orbit Mott state has been the focus of recent interest due to its potential of hosting a variety of new phases driven by correlated electron phenomena (such as high temperature superconductivity or enhanced ferroic behavior) in a strongly spin-orbit coupled setting. Currently, however, there remains very little understanding of how spin-orbit Mott phases respond to carrier doping and, more specifically, how relevant $U$ remains for the charge carriers of a spin-orbit Mott phase once the bandwidth is increased. Here I will present our group's recent experimental work exploring carrier doping and the resulting electronic phase behavior in one such spin-orbit driven Mott material, Sr$_{3}$Ir$_{2}$O$_{7}$, with the ultimate goal of determining the relevance of $U$ and electron correlation effects within the doped system's ground state. Our results reveal the stabilization of an electronically phase separated ground state in B-site doped Sr$_{3}$Ir$_{2}$O$_{7}$, suggestive of an extended regime of localization of in-plane doped carriers within the spin-orbit Mott phase. This results in a percolative metal-to-insulator transition with a novel, global, antiferromagnetic order. The electronic response of B-site doping in Sr$_{3}$Ir$_{2}$O$_{7\, }$will then be compared with recent results exploring A-site doping if time permits. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B49.00002: Controlling spin-orbital and electronic structure in iridate thin films and heterostructures Jian Liu, Jiun-Haw Chu, Claudy Rayan Serrao, Di Yi, Xavi Marti, Ramamoorthy Ramesh Epitaxial thin films and heterostructures have been showed to be a versatile platform for tuning the Mott physics and inducing novel phase in complex oxides with 3$d$ transition metal elements. The strong spin-orbit coupling in 5$d$ transition metal oxides adds a new dimension to this area and attracts strong interests since many theoretical proposals have been put forward for unconventional electronic, magnetic and topological phases. While the combination of correlation and spin-orbit coupling holds huge potential for appealing quantum states and functionalities, the fundamental challenge of realizing experimental manipulation on the spin-orbitals of the $d$-electrons and modulation on the resulting electronic structure has yet to be addressed. Here we report our study on gaining possible control on these spin-orbitals by epitaxial layering. We take perovskite SrIrO$_{\mathrm{3}}$ as a model system of 5$d$ complex oxides and investigate its response to heteroepitaxial strain. Results from x-ray spectroscopy, optical spectroscopy and transport measurements demonstrate that epitaxial constrain offers a unique pathway to tuning the spin-orbit coupling, its interaction with the ligand field, and the macroscopic electronic properties. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B49.00003: Magnetoresistance of Sr$_2$IrO$_4$ Epitaxial Thin Films O.B. Korneta, J. Nichols, J. Terzic, L.E. De Long, G. Cao, S.S.A. Seo Recent studies on Sr$_2$IrO$_4$ single crystals and thin films have revealed an intriguing insulating ground state, even though there are continued debates whether to classify this material as a Mott or Slater insulator. We have recently synthesized epitaxial Sr$_2$IrO$_4$ thin films on various substrates, which allow for the deposition of films under either tensile or compressive strain. The measurements of temperature-dependent magnetoresistance (MR), $\Delta R/R = [R(H)-R(0)]/R(0)$ on these samples reveal a negative linear MR near the room temperature, which is well above the antiferromagnetic ordering temperature ($T_{N} \approx 240$~K). However, as the temperature decreases, the {MR} becomes larger with a positive parabolic response. This behavior is very robust showing no noticeable dependence on magnetic field direction, strain, or film thickness and is remarkably different from the {MR} observed on Sr$_2$IrO$_4$ single crystals. This intriguing effect can be potentially explained by the presence of multiple conducting channels within the sample. We will discuss this model as well as other possible mechanisms for this unique phenomenon. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B49.00004: Tuning Electronic Structure of Epitaxial Sr$_{2}$IrO$_{4}$ Thin Films via Strain S.S.A. Seo, J. Nichols, J. Terzic, E.G. Bittle, O.B. Korneta, L.E. De Long, J.W. Brill, G. Cao Recent research on Sr$_{2}$IrO$_{4}$ has shown that the energy scale associated with spin-orbit coupling is comparable to the crystal-field energy and the on-site Coulomb interaction. The strong competition between these fundamental interactions creates the potential for the emergence of novel electronic states. To understand the physics of Sr$_{2}$IrO$_{4}$ and to find a way of tuning its multiple competing interactions, we have investigated the transport, magnetic, and optical properties of $c$-axis oriented Sr$_{2}$IrO$_{4}$ epitaxial thin films grown on various oxide substrates. Under tensile (compressive) strain, increased (decreased) Ir-O-Ir bond-angles are expected to result in increased (decreased) electronic bandwidths. However, the films under various strains have little change in their transport properties. In optical spectroscopic measurements, we have observed that two optical absorption peaks near 0.5 eV and 1.0 eV are shifted to higher (lower) energies under tensile (compressive) strain, indicating that the electronic-correlation energy is affected by in-plane lattice-strain and interlayer-spacing. Our observations strongly suggest that not only the electronic bandwidth, but also the magnitude of the electronic correlation energy can be manipulated by lattice strain, which provides an important insight into the physics driven by the coexistence of strong spin-orbit coupling and electronic correlation. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B49.00005: Anisotropic Electronic Properties of $a$-Axis Oriented Sr$_{2}$IrO$_{4}$ Epitaxial Thin Films J. Nichols, O.B. Korneta, J. Terzic, L.E. De Long, J.W. Brill, G. Cao, S.S.A. Seo We have successfully synthesized $a$-axis oriented Sr$_{2}$IrO$_{4}$ epitaxial thin films on LaSrGaO$_{4}$ (100) substrates by pulsed laser deposition. The tetragonal structure of the substrate allows for the film to grow with compressive strain along both in-plane directions ($b$- and $c$-axes). This results in the $c$-axis of the film being in-plane. We will present the anisotropic structural, electronic, and optical properties of these $a$-axis oriented thin films along both the $b$- and $c$-axes. X-ray diffraction confirms these films are of high quality and are fully strained along the $c$-axis while the $b$-axis undergoes strain relaxation. The $c$-axis resistivity is approximately one order of magnitude larger than that of the \textit{ab}-plane. Optical absorption spectra with E$\bot c$ polarization show both Ir 5$d$ intersite transitions and charge-transfer transitions (O 2$p$ to Ir 5$d)$, while E//$c$ spectra show only the latter. The structural anisotropy created by biaxial strain in $a$-axis-oriented thin-films also changes the electronic structure and gap energy. These $a$-axis-oriented, epitaxial thin-films provide a powerful tool to investigate the highly anisotropic electronic properties of Sr$_{2}$IrO$_{4}$. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B49.00006: First Principles Prediction of Topological Phases in Thin Films of Pyrochlore Iridates Xiang Hu, Zhicheng Zhong, Mehdi Kargarian, Andreas R\"uegg, Penghao Xiao, Chandrima Mitra, Gregory A. Fiete Using density functional theory and Hartree-Fock theory, we predict topological phases in thin pyrochlore iridate films grown along the [111] direction. Including the full orbital structure of the the relevant d-orbitals and the strong but finite-spin orbit coupling strength, we find a two-dimensional time-reversal invariant topological insulator with a gap of up to ~.15eV is possible in a bilayer geometry, and a zero magnetic field quantum anomalous Hall state is possible in a trilayer geometry with a gap of up to ~0.1eV. Our results show that while the bulk pyrochlore iridates experimentally explored so far may not be promising for insulating topological phases, the thin film geometries are. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B49.00007: Topological insulating phases in oxide multilayers using perovskites and rutiles Victor Pardo, Jose L. Lado, Daniel Baldomir Ab initio calculations combined with tight-binding modelling have been performed\footnote{Phys. Rev. B 88, 155119 (2013).} in 5d-electron-based perovskite multilayers in the large spin-orbit coupling limit. The topological properties of the systems (SrTiO$_3$)$_7$/(SrIrO$_3$)$_2$ and isoelectronic (KTaO$_3$)$_7$/(KPtO$_3$)$_2$ grown along the (111) direction have been analyzed as a function of on-site Coulomb repulsion $U$, parity asymmetry and uniaxial strain. The former is found to be a topological semimetal and the latter is a topological insulator describable as the high-U limit of the other one. This high-U phase can be driven to a trivial insulating phase by a perpendicular external electric field. In the talk, we will describe how to proceed in a similar way with rutile-based multilayered structures, where a 4d/5d electron dioxide with rutile structure, sandwiched by a band insulator like TiO$_2$ or SnO$_2$ can lead to topologically non-trivial properties if band filling and strain are tuned. We discuss also the possibility of obtaining similar topological states using isoelectronic fluorides. The electronic structure and properties of free-standing thin films will be also briefly discussed. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B49.00008: Chiral orbital angular momentum perspective on surface electronic states of SrTiO3 and KTaO3 Kyeong Tae Kang, Panjin Kim, Jung Hoon Han Tight-binding models suitable for the recently observed surface electronic bands of $\rm{SrTiO_3}$ and $\rm{KTaO_3}$ are analyzed with a view to bringing out the relevance of chiral orbital angular momentum (OAM) structure in the $t_{2g}$-derived bands. With the inversion symmetry breaking at the surface, orbital chiralities of the three bands (neglecting spin splitting) are $m = +1,0,-1$. Further inclusion of spin-orbit interaction induces linear Rashba splitting on the chiral OAM bands, but not in the non-chiral, $m=0$ band structure. Our predictions can be easily verified by circular dichroism ARPES experiment. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B49.00009: Electric Field Tuning of the Rashba splitting in $d$-electron systems Shanavas Veedu, Sashi Satpathy It has been found that the Rashba spin splitting is proportional to the electric field so that one can manipulate the electron spin through electric fields leading to potential applications in spintronics devices. Theoretical models based on phenomenological and symmetry arguments have been successful in reproducing the effects, but a complete understanding for $d$ orbital systems is still lacking. Using tight-binding Hamiltonian approaches, we show that the effect can be understood by treating the electric field as a perturbation that leads to mixing of $p$ and $f$ states with $d$ orbitals which can be shown to result in effective Hamiltonians of the Rashba type. We also propose a recipe for deriving the Hamiltonian terms using Gaunt coefficients for general lattice and orbital configurations. We have tested our predictions with density functional theory based calculations for various 3$d$ and 5$d$ systems. In the case of the perovskite oxide surface of KTaO$_3$, we find that Rashba effect originates from the first few layers near the surface and can be altered by moving the 2DEG in and out of the surface using applied fields and the model agrees well with the calculations. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B49.00010: Interaction-induced quantum anomalous Hall phase in bilayers of 3d transition-metal oxide Yilin Wang, Zhong Fang, Xi Dai In the present paper, we have studied the electronic structure of 3d transition-metal oxide LaCoO$_3$ thin film grown on the [111] surface of SrTiO$_3$. By using first-principles calculation under local density approximation implemented with Gutzwiller variational method (LDA+G), we have studied the bilayer systems of LaCoO$_3$ thin films grown along the [111] direction on SrTiO$_3$. The LDA results show that two nearly flat bands locate at the top and bottom of eg bands of Co atoms, and the Fermi level crosses the lower one, which is almost half-filled. After including both the spin-orbit coupling and the rotational invariant Coulomb interaction in the LDA+G method, we found that the Coulomb interaction will enhance the effective spin-orbit coupling, and a ferromagnetic insulator phase with a gap as large as 0.15 eV will be stabilized. Further calculations indicate that such a ferromagnetic insulator phase will have non zero Chern number one leading to quantum anomalous Hall effect. Increasing Hund's rule coupling in this system will generate a low spin to high spin transition and destroy the quantum anomalous Hall phase. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 2:15PM |
B49.00011: Total Energy Calculations using DFT+DMFT: Application to the Pressure-composition Phase Diagram of Rare-earth Element Nickelates Invited Speaker: Hyowon Park Ab-initio total energy calculations have been implemented within the fully self-consistent density functional theory plus dynamical mean field theory (DFT+DMFT) method, using a Wannier orbital basis. The method is used to calculate the structural and metal-insulator transition phase diagrams of the rare-earth element nickelate $R$NiO$_3$ perovskites as a function of rare-earth ion, pressure and temperature. This phase diagram is of interest because the insulating phase arises from a remarkable site-selective Mott state, in which unusual electronic physics is strongly coupled to a breathing-mode Ni-O bond disproportionation. Conventional DFT fails to stabilize the breathing distortion and thus does not reproduce the insulating phase. DFT+U overpredicts order, in particular finding that $La$NiO$_3$ is disproportionated, in disagreement with experiment. In contrast to these theories, the DFT+DMFT method can quantitatively reproduce the metal-insulator and structural phase diagram of all $R$NiO$_3$ perovskites in the plane of pressure and rare-earth elements. The calculated temperature dependence of the energetics of the phase transformation indicates that the thermal transition is driven by phonon entropy effects. This present method can be generally applied to nano-structured or artificially structured strongly correlated materials including heterostructures and thin films, whose electronic phases are strongly coupled to their lattice degrees of freedom. [Preview Abstract] |
Session B50: Semiconductor Nanowires: Optical and Electronic Properties
Sponsoring Units: DCMPChair: Roberto Myers, The Ohio State University
Room: Mile High Ballroom 1D
Monday, March 3, 2014 11:15AM - 11:27AM |
B50.00001: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 11:27AM - 11:39AM |
B50.00002: Carrier dynamics in silicon nanowires studied using optical-pump terahertz-probe spectroscopy Alexandre Beaudoin, Bassem Salem, Thierry Baron, Pascal Gentile, Denis Morris The advance of non-contact measurements involving pulsed terahertz radiation presents great interests for characterizing electrical properties of a large ensemble of nanowires. In this work, N-doped and undoped silicon nanowires (SiNWs) grown by chemical vapour deposition (CVD) on quartz substrate were characterized using optical-pump terahertz probe (OPTP) transmission experiments. Our results show that defects and ionized impurities introduced by N-doping the CVD-grown SiNWs tend to reduce the photoexcited carrier lifetime and degrade their conductivity properties. Capture mechanisms by the surface trap states play a key role on the photocarrier dynamics in theses small diameters' ($\sim $100 nm) SiNWs and the doping level is found to alter this dynamics. We propose convincing capture and recombination scenarios that explain our OPTP measurements. Fits of our photoconductivity data curves, from 0.5 to 2 THz, using a Drude-plasmon conductivity model allow determining photocarrier mobility values of 190 and 70 cm$^{2}$/V$\cdot $s, for the undoped and N-doped NWs samples, respectively. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B50.00003: Field-effect transistors based on Si:P nanowires with axially graded doping Jorge Barreda, Timothy Keiper, Mei Zhang, Peng Xiong Phosphorus-doped Si nanowires (NWs) have been synthesized via the vapor-liquid-solid method. Local electrical transport measurements along the length of the NWs reveal a systematic reduction of the electrical conductivity in the growth direction. These results, along with structural characterizations by SEM and AFM, point to a graded doping profile along the length of the NWs [1] as the origin of the spatial variation of the electronic properties. Due to the inherent doping gradient, Cr/Au and Cr/Ag contacts on the NWs evolve systematically from ohmic contacts on the highly-doped side (where growth starts) to Schottky junctions on the lower-doped side (where growth ends). Field-effect transistors (FETs) have been fabricated from individual as-grown Si NWs. By patterning a series of electrodes along the length of a NW, both channel-limited and Schottky barrier-limited devices were obtained from a single NW. In particular, by using two electrodes located at opposite ends of a NW, FETs limited by a single Schottky junction were consistently realized. These devices, in which the Schottky junction acts as the drain terminal and the ohmic contact as the source terminal, exhibit excellent gate modulation due to the tuning of the Schottky barrier. [1] Daniel E. Perea et al., Nature Nanotechnology 4, 315-319 (2009). [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B50.00004: Quasiballistic hole transport in Ge/Si core/shell nanowires Dharmraj Kotekar Patil, Zhaoen Su, Binbin Tian, Minh Nguyen, Jinkyoung Yoo, Shadi Dayeh, Sergey Frolov We investigate low temperature hole transport in Ge/Si core/shell nanowires. We study devices with annealed (Ni) and unannealed (Ti/Al) contacts. We observe Coulomb blockade and analyze the capacitive and quantum energy scales in Ge/Si nanowire quantum dots. In devices with Ni contacts, we study diameter and shell thickness dependence on hole mobility at low temperature. We observe Fabry-Perot oscillations indicating a quasi-ballistic transport regime. We also investigate subband-resolved transport of holes as a function of magnetic field magnitude and orientation. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B50.00005: Analysis of spin-orbit coupling effects in the Pt-Si nanowires on Si(110) surface Sehoon Oh, Hyungjun Lee, Hyoung Joon Choi We study Pt-induced nanowires on Si(110) surface by using an ab-initio pseudopotential density-functional method. A thick slab of Si atoms is considered with Pt atoms added on the Si surface. Atomic structures of Pt-induced nanowires are determined by the total-energy minimization. We calculate surface band structures near the Fermi level and simulate scanning tunneling microscopy (STM) images and angle-resolved photoemission spectra (ARPES). We analyze the effects of the spin-orbit interaction on the electronic structure qualitatively as well as quantitatively. This work was supported by NRF of KOREA (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2013-C3-008). [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B50.00006: Substrate effect on the band gap of semiconducting atomic wires Adam J. Simbeck, Saroj K. Nayak The electronic structure of free-standing and supported semiconducting atomic wires is investigated using a combination of first-principles density functional theory (DFT) and many-body perturbation theory (MBPT). The band gaps predicted from DFT for SiH$_{\mathrm{2}}$ and GeH$_{\mathrm{2}}$ atomic wires are unaffected by the presence of the substrate, whereas the gaps calculated using MBPT under the \textit{GW} approximation are reduced by about 1eV when the wires are supported. The reduction in the band gap is attributed to a change in the electronic correlation energy, which can be understood as a screened Coulomb interaction. These results highlight the importance of the role played by the substrate in manipulating the electronic and optical properties of quantum confined Si and Ge systems. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B50.00007: Optical and Electrical Characterization of Quantum Dots Decorated ZnO Nanowires for Energy Conversion Richard Mu, Anthony Mayo, Haiyang Xu, Yichun Liu Significant progress has been made recently in understanding optoelectronic properties of metallic and semiconducting quantum dots and their interactions with their surrounding nano-environments. It is shown that nanostructured photovoltaic devices do have clear advantages over the bulk counterparts to address energy challenges facing humanity. They require much less mass, not exclusively limited by materials of choice, and favoring integration for multifunctionality to be able to effectively harvest solar energy by tuning the optical gap and enhancing photon absorption across section through various nanomaterials syntheses. The other challenge is to be able to purposely control and manipulate the energy transfer pathways for particular needs. As for nanostructured photovotaic devices, charge and exciton transports must be carefully evaluated. The knowledge of charge and exciton mobility, coherent and incoherent hopping due to electronic coupling, energy redistribution and partition in may be the critical steps. CdTe and Si functionalized bare ZnO nanowires, and core/shell have been fabricated with Glazing Angle Deposition technique as the model systems. A series materials characterization techniques (confocal Raman, optical, photoluminancence and electrical) have been conducted to provide valuable information about the nanostructure. Results will be presented and discussed along with their scientific implications. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B50.00008: Size and strain dependence of electronic properties in ultrathin ZnO nanowires Xihong Peng, Andrew Copple, Qun Wei One dimensional nanostructures of group II-VI semiconductors, in particular ZnO, have drawn broad research interests in recent years due to their potential applications in nano-electronics and nano-optics. In this project, electronic structures of ultrathin wurtzite ZnO nanowires were studied using first-principles Density Functional Theory (DFT) calculations. It was found that the nanowire axial lattice constants shrink compared to that of bulk ZnO. The band gap opens for small nanowires due to the quantum confinement effect. In addition, the band gap can be further tuned using uniaxial strain. The effective masses of the electron and the hole in ZnO can be manipulated not only by the size of nanowire, but also through the applied strain. The results were cross checked using different DFT methods, including GGA, DFT$+$U, and hybrid functionals. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B50.00009: Insights into Optical Quality of GaAs/AlGaAs MOCVD Nanowires Yi-Hsin Chiu, Nicholas G. Minutillo, Greg Smith, John A. Carlin, Ezekiel Johnston-Halperin, Fengyuan Yang GaAs nanowires (NWs) are promising candidate materials for future optoelectronic device applications and fundamental physics study. In order to fully realize this potential it is important to identify a growth regime that yields high quality structural, electronic and optical characteristics. Here we explore the impact of growth temperature on NW optical quality for both (100) and (111)B oriented GaAs substrates. Au catalyzed GaAs/AlGaAs core/shell NWs are grown by metalorganic chemical vapor deposition (MOCVD) within a substrate temperature window of 410 $^{\circ}$C to 470 $^{\circ}$C. We find that, contrary to expectation, the optical quality depends on substrate orientation with (100) wire quality decreasing monotonically with increasing temperature while (111)B wire quality peaks at 430 $^{\circ}$C. This result suggests that the orientation of the NW ensembles plays a critical role in precursor diffusion and subsequent point defect density. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B50.00010: Photocurrent spectroscopy of confined quantum states in single quantum well tube nanowire devices Bekele Badada, Teng Shi, Leigh Smith, Howard Jackson, Jan Yarrison-Rice, Qiang Gao, H. Hoe Tan, Chennupati Jagadish We investigate optical transitions in GaAs/AlGaAs core-multishell nanowires Quantum Well Tubes (QWT) devices using photocurrent spectroscopy (PC) at low temperature (10K). The GaAs quantum well layer was embedded inside a thick AlGaAs shell surrounding a 50 nm diameter GaAs NW. The single nanowire devices were fabricated by standard photolithography followed by deposition of Ti (20nm)/Al (500nm) metal contacts on either end the nanowire. We present results for two sets of quantum well tubes having quantum well widths of 8nm and 4nm. The QWT nanowire devices exhibit very low (sub pA) dark current and are extremely photosensitive. PC measurements of the QWT devices exhibit photocurrent peaks corresponding to excitons confined to the GaAs core as well as ground and excited states of electrons and holes confined to the quantum well tube, and also in the barrier. Comparisons of the PC spectra with PL and PLE measurements on the same devices show that the peaks associated with the PC spectra show a close correspondence. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B50.00011: Temperature-dependent photoluminescence imaging of GaAs/AlGaAs heterostructure quantum well tubes Teng Shi, Howard Jackson, Leigh Smith, Jan Yarrison-Rice, Nian Jiang, Hoe Tan, Qiang Gao, Chennupati Jagadish Two sets of GaAs/AlGaAs core-multi shell nanowire quantum well tubes (QWTs) grown by MOCVD, with QW widths of 2nm and 6nm are dispersed onto a 4mm diameter hemispherical solid immersion lens. We obtain high spatial resolution photoluminescence (PL) images of single nanowires (NWs) from 10 K up to 120 K. High spectral resolution PL spectra reveal several narrow emission lines on high energy side of the 2nm QWT at low temperatures. In the 6nm QW, such narrow emission lines are not observed. Spatially-resolved PL images show that these localized states are randomly distributed along the NW long axis. Temperature-dependent PL imaging indicates that the quantum dot emissions disappear at temperatures above 50K. The recombination lifetime for electrons and holes in the QWT for the 2nm and 6nm QWTs are 500ps and 800 ps, respectively. We observe the recombination lifetime increases slightly with increasing temperature. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B50.00012: Ab initio study of substitutional doping of III-V nanowires Bob Schoeters, Geoffrey Pourtois, Bart Partoens Using \textit{ab initio} calculations we study the impact of substitutional dopants in thin III-V nanowires. In this study we focus on 4 different technologically relevant III-V nanowires: GaAs, InSb, InP and InGaAs, doped with either C, Si, Ge, Be, Mg or Zn. We determine the energetically most favorable positions for these dopants by looking at their formation energies. These preferred locations indicate whether a dopant will segregate to the surface or the center of the nanowire. This can have a large impact on the electronic properties, since it will lead to an inhomogeneous doping distribution. We can explain the preferential positions as a competition between a chemical and a relaxation effect. However, in reality these nanowires contain several defects, such as antisites, vacancies, dangling bonds at the nanowire surface, \dots. Therefore we also investigate the impact of these defects on the localization of the dopants. Finally we study the impact of these dopants on the electronic structure. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B50.00013: Transient Rayleigh Scattering Spectroscopy measurement of Carrier Dynamics in Zincblende and Wurtzite Indium Phosphide nanowires Yuda Wang, Mohammad Montazeri, Howard Jackson, Leigh Smith, Jan Yarrison-Rice, Tim Burgess, Suriati Paiman, Hoe Tan, Qiang Gao, Chennupati Jagadish Pump-probe transient Rayleigh spectroscopy is used to study the carrier dynamics of ZB {\&} WZ InP nanowires. Utilizing a wavelength-tunable pulse laser as probe and by adjusting the pump-probe pulse time delay, the change of the Rayleigh scattering efficiency as a function of excitation energy and time delay is measured from a single nanowire. The results are analyzed using an absorption coefficient calculated by a band-to-band transition model and the index of refraction obtained by the Kramers-Kronig relation. The temperature of the electron-hole plasma (EHP) cools via the emission of longitudinal optic (LO) and acoustic phonons. The LO phonon cooling dominates at early times when the carrier temperatures are high, after which the acoustic phonon interactions begins to dominate. The carrier concentrations of the split-off band in ZB InP and C band in WZ InP appears to be 1-2 orders of magnitude smaller than that of the HH, LH or A, B valence bands. Such measurements provide a detailed picture of electron and hole densities and temperatures as a function of time after excitation. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B50.00014: Population Dynamics of Excitons in Polytype Wurzite-Zincblende InP Nanowires Hans-Peter Wagner, Masoud Kaveh, Wolfgang Langbein, Qian Gao, Chennupati Jagadish, Gerd Duscher We investigate the population dynamics of excitons in polytype wurzite/zincblende (WZ/ZB) InP nanowires using heterodyne four-wave-mixing (HFWM) in three-beam configuration at LN temperature. The photon energy of exciting 100 fs pulses was set to 1.44 eV and 1.49 eV, resonant to excitons in ZB and WZ segments, respectively. Pump pulse fluences were varied from 0.16 to 3.2 $\mu $J/cm$^{\mathrm{2}}$. At 1.44 eV pulse energy the HFWM amplitude shows a rapid initial decay on a sub-picosecond time-scale indicating a fast conversion of ZB excitons into spatially indirect excitons at the WZ/ZB interface. For longer delays the HFWM amplitude reveals a nearly mono-exponential decay time of 2 ns at lowest pump fluence which is assigned to the lifetime of indirect electron-hole pairs. With increasing pulse fluence the decay dynamics becomes multi-exponential which is attributed to state filling of higher-energy indirect excitons. At 1.49 eV pulse energy the HFWM amplitude reveals an additional rapid decay on a 10 ps time-scale which is tentatively assigned to the trapping of A-excitons at point-defects in the WZ segments. The observed dynamics is modeled by coupled rate-equations. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B50.00015: Inelastic electron and Raman scattering from the collective excitations in quantum wires Manvir Kushwaha The nanofabrication technology has taught us that an $m$-dimensional confining potential imposed upon an $n$-dimensional electron gas paves the way to a quasi-($n$-$m$)-dimensional electron gas, with $m \le n$ and $1\le n, m \le 3$. This is the road to the (semiconducting) quasi-$n$ dimensional electron gas systems we have been happily traversing on now for almost two decades. Achieving quasi-one dimensional electron gas (Q-1DEG) led us to some mixed moments in this journey: while the reduced phase space for the scattering led us believe in the route to the faster electron devices, the proximity to the 1D systems left us in the dilemma of describing it as a Fermi liquid or as a Luttinger liquid. No one had ever suspected the potential of the former, but it took quite a while for some to convince the others on the latter. A {\em realistic} Q-1DEG system at the low temperatures is best describable as a Fermi liquid rather than as a Luttinger liquid. This has motivated us to employ the Bohm-Pines' full RPA to develop a systematic methodology for the inelastic electron and light scattering from the collective (plasmon) excitations in Q-1DEG [or quantum wires]. We will discuss in detail the results published in AIP Advances {\bf 3}, 042103 (2013). [Preview Abstract] |
Session B51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties II
Sponsoring Units: DMPChair: Xiaodong Xu, University of Washington
Room: Mile High Ballroom 1E
Monday, March 3, 2014 11:15AM - 11:27AM |
B51.00001: Photoluminescence and reflectivity measurements of single-layer transition-metal dichalcogenides A.T. Hanbicki, M. Currie, D. Gunlycke, A.L. Friedman, G. Kioseoglou, B.T. Jonker Single layers of transition metal dichalcogenides, MX$_{2}$ (M = Mo, W and X = S, Se) have been the focus of intense research recently because they are direct gap semiconductors with inequivalent K-points making them prime candidates for valleytronics. We have performed various optical measurements including photoluminescence (PL) and differential reflectivity in an attempt to elucidate such issues as intervalley scattering, peak assignment and spin-orbit splitting. For all of the MX$_{2}$ materials, excitonic emission is observed with wavelengths in the visible regime with the main emission occurring at the A-exciton. Additionally, identification of the B-exciton yields spin-orbit energies ranging from 170 meV to 390 meV. We will also discuss intervalley scattering based on measurements of the change in optical polarization as a function of excitation energy. This work was supported by ONR directly, and by NRL and the NRL Nanoscience Institute. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B51.00002: Exciton Rydberg series in mono- and few-layer WS2 Alexey Chernikov, Timothy C. Berkelbach, Heather M. Hill, Albert Rigosi, Yilei Li, \"Ozgur B. Aslan, Mark S. Hybertsen, David R. Reichman, Tony F. Heinz Considered a long-awaited semiconducting analogue to graphene, the family of atomically thin transition metal dichalcogenides (TMDs) attracted intense interest in the scientific community due to their remarkable physical properties resulting from the reduced dimensionality. A fundamental manifestation of the two-dimensional nature is a strong increase in the Coulomb interaction. The resulting formation of tightly bound excitons plays a crucial role for a majority of optical and transport phenomena. In our work, we investigate the excitons in atomically thin TMDs by optical micro-spectroscopy and apply a microscopic, \textit{ab-initio} theoretical approach. We observe a full sequence of excited exciton states, i.e., the Rydberg series, in the monolayer WS$_{\mathrm{2}}$, identifying tightly bound excitons with energies exceeding 0.3 eV - almost an order of magnitude higher than in the corresponding, three-dimensional crystal. We also find significant deviations of the excitonic properties from the conventional hydrogenic physics - a direct evidence of a non-uniform dielectric environment. Finally, an excellent quantitative agreement is obtained between the experimental findings and the developed theoretical approach. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B51.00003: Excitons as massless and massive Dirac particles in monolayer transition metal dichalcogenides Hongyi Yu, Guibin Liu, Xiaodong Xu, Wang Yao In monolayer transition metal dichalcogenides, tightly bound excitons can form at +-K valleys, where optical generation of excitonic valley polarization and coherence can be realized through a polarization selection rule. Here, we show that the the electron-hole Coulomb exchange leads to the strong coupling between the valley pseudospin of bright exciton and its motion. In the light cone, the exciton dispersion exhibits a massless Dirac cone with chirality index I=2. Moderate tensile strain provides a powerful approach to tune the exciton dispersion. When the exciton binds an electron to form a negatively charged trion, the exchange interaction with the excess electron opens up a gap and the trion behaves as a massive Dirac particle. With the optical addressability at specifiable momentum and energy, excitons in monolayer transition metal dichalcogenides may provide unique opportunities to study Dirac particles. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B51.00004: Optoelectronic Control of Spin and Pseudospin in Layered WSe$_2$ Invited Speaker: Aaron Jones Coherent manipulation of spin-like quantum numbers facilitates the development of new quantum technologies. Layered transition metal dichalcogenides provide an ideal laboratory to exploit such dynamic control of spin, pseudospin, and their interplay. Here, we discuss two examples based on monolayer and bilayer WSe$_2$. Due to the inversion asymmetry in monolayer WSe$_2$, valley pseudospins, which index the degenerate extrema of the energy-momentum bands, possess circularly polarized optical selection rules. In addition to the generation of valley polarization through optical pumping of valley excitons, we demonstrate the creation of a coherent superposition between valley states in monolayer WSe$_2$ by linearly polarized excitation. On the other hand, bilayer WSe$_2$ provides an additional quantum degree of freedom, the layer pseudospin, which corresponds to layer polarization. In AB stacked bilayers, we find the real spin is locked to layer pseudospin for a given valley, which results in the suppression of spin relaxation and electrical control of spin Zeeman splitting without an applied magnetic field. Additionally, we obtain spectroscopic evidence of interlayer and intralayer trion species, an important step toward coherent optical control in van der Waals 2D heterostructures. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B51.00005: Observation of negative terahertz photoconductivity in monolayer MoS$_{2}$ under femtosecond laser excitation Chun Hung Lui, Alex J. Frenzel, Daniel V. Pilon, Yi-Hsien Lee, Jing Kong, Nuh Gedik We observed a pronounced transient decrease of terahertz conductivity in doped monolayer molybdenum disulfide (MoS$_{2})$ after pulsed laser excitation. This anomalous phenomenon arises from the strong many-body interactions in the system, where optically produced electron-hole pairs join the doped charges to form trions, bound states of two electrons and one hole, and substantially diminish the carrier conductivity by the resulting increase of effective mass. Our results reveal the ultrafast formation and decay of trions in monolayer MoS$_{2}$ and their influence on the conductivity of the material. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B51.00006: Asymmetry in optical absorption spectra of 2D metal dichalcogenides Deepika Saini, Ramakrishna Podila, Apparao Rao Two dimensional (2D) inorganic materials have recently been shown to exhibit interesting optical phenomena that are strikingly different from their bulk counterparts. For example, exfoliated dichalcogenides exhibit an increased absorption and photoluminescence due to a change in the nature of its bandgap, i.e. a transition from an indirect to a direct bandgap. We find that the bandgap transitions in MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ in the UV-VIS regime are invariably accompanied by an asymmetric Briet-Wigner-Fano lineshape. In this talk we will discuss the origin of Fano lineshape in terms of phonon scattering and changes in the electronic band structure. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B51.00007: Effects of metal contact on 2D semiconductors Xiaoping Hong, Sufei Shi, Jonghwan Kim, Yinghui Sun, Feng Wang Thin layers of transition metal dichalcogenides (TMD) such as MoS2 have recently attracted intense interest in both fundamental research and electronic and optoelectronic applications. As atomically thin semiconductors the electronic and optical properties of single layer TMDs are greatly sensitive to their metal contacts. We use optical method to characterize the effects of metal contact on single layer MoS2, which provides valuable information about how to improve the performance of MoS2 based devices. Also it sheds light on possible manipulation of excitons in atomically thin TMDs through their metallic environment. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B51.00008: Edge Nonlinear Optics on a MoS2 Atomic Monolayer Xiaobo Yin, Ziliang Ye, Daniel Chenet, Yu Ye, Kevin O'Brien, James Hone, Xiang Zhang The structural discontinuity at an interface yields significant electric dipolar contributions to the surface optical nonlinearity, making the highly surface- and molecular-specific second-harmonic spectroscopy an indispensable tool for noninvasive study of surface sciences. Not only does it measure dipolar width across interfaces, but it also probes real-time dynamics of surface, such as atomic reconstructions, charge transfer and molecular conformational transitions. Here we study experimentally the second-order nonlinear optics on the one-dimensional edges of hexagonal molybdenum disulfide (MoS2) atomic membranes. The broken inversion symmetry of the atomically thin monolayer shows strong second-harmonic generation (SHG), in stark contrast to the centrosymmetric bulk material which is immune to the second order nonlinear processes. The destructive interference and annihilation of nonlinear waves from neighboring atomic membranes not only reveals the few-atom-wide line defects that stitch different crystal grains together but also allows the rapid mapping of crystal grains and grain boundaries over large areas which typically requires a cumbersome diffraction-filtered dark-field transmission electron microscope (TEM). More interestingly, this unique optical imaging technique enables the nonlinear optical detection of the electronic edge state at the atomic edges of two-dimensional crystals where the translational symmetry is broken. The observed edge resonance of SHG clearly indicates the electronic structure variation at the atomic edges that have been long suspected to be the active sites for electrocatalytic hydrogen evolutions. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B51.00009: Dynamics of valley polarized excitations in monolayer MoSe2 using transient spin-grating spectroscopy Fahad Mahmood, Edbert Sie, Yi-Hsien Lee, Jing Kong, Nuh Gedik We report on a transient spin-grating measurement on CVD-grown monolayer MoSe2. Two cross-polarized 1.5 eV short laser pulses interfere on the sample to generate a polarization grating. This selectively induces K and K' valley excitations, the populations of which vary sinusoidally across the surface. The decay of this valley polarization grating is studied through a diffracted probe beam. We find that the decay strongly depends on the initial population and exhibits a characteristic temperature dependence. These results provide important insights into the lifetime and mechanisms for inter-valley scattering as well as possible scattering to dark exciton states in monolayer transition metal dichalcogenides. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B51.00010: Biexciton formation in monolayer MoS2 as observed by transient absorption spectroscopy Edbert J. Sie, Yi-Hsien Lee, Alex J. Frenzel, Jing Kong, Nuh Gedik We report on the observation of biexcitons and heterobiexcitons in monolayer MoS2 measured using optical pump and probe spectroscopy. The binding energies of these biexcitons were found to be as large as 35 meV and 60 meV, respectively. This renders the four-particle, or even higher-order, electronic correlations stable against thermal fluctuations at room temperature. These results could serve as a guide for first-principle calculations of high-order electronic correlations in 2D atomic crystals, and to facilitate further investigation toward device applications. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B51.00011: Probing the dark excitons in TMDC with two photon absorption spectroscopy Ziliang Ye, Kevin Obrien, Yu Ye, Mervin Zhao, Ying Wang, Jun Xiao, Hanyu Zhu, Xiaobo Yin, Yuan Wang, Xiang Zhang When the transition metal dichalcogenide (TMDC) is reduced to the monolayer, its electronic bandgap is shifted from indirect type to direct type, resulting in a great enhancement in the photoluminescence efficiency. On the other hand, excitons usually play a very important role in determining the materials' optical properties, especially in the low dimensional forms. However, the exciton contribution has yet been experimentally identified in TMDC. Here we use the two photon absorption spectroscopy technique to probe the TMDC dark exciton transition, which has the complementary selection rule to bright exciton transition. By comparing the dark-bright exciton separation with the theoretical model, we confirm the extraordinarily large binding energy of exciton in two-dimensional TMDC. The identification of exciton contribution as well as the electronic band gap size would help to design the TMDC electronics and optoelectronics in the future. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B51.00012: Valley Carrier Dynamics in Monolayer Molybdenum Disulphide from Helicity Resolved Ultrafast Pump-probe Spectroscopy Dong Sun, Qinsheng Wang, Shaofeng Ge, Xiao Li, Jun Qiu, Yanxin Ji, Ji Feng We investigate the valley related carrier dynamics in monolayer MoS$_{\mathrm{2}}$ using helicity resolved non-degenerate ultrafast pump-probe spectroscopy at the vicinity of the high-symmetry K point under the temperature down to 78 K. Monolayer MoS$_{\mathrm{2}}$ shows remarkable helicity resolved transient reflection signals, in stark contrast to bilayer and bulk MoS$_{\mathrm{2}}$ due to the enhancement of many body effect at reduced dimensionality. The ultrafast time-resolved result shows that the valley polarization is preserved for only around 1 ps before scattering process makes it undistinguishable. We suggest that the dynamical degradation of valley polarization is attributable primarily to the exciton trapping by defect states in the exfoliated MoS$_{\mathrm{2}}$ samples. Our experiment and a tight-binding model analysis also show that the perfect valley CD selectivity is fairly robust against disorder at the K point, but quickly decays from the high-symmetry point in the momentum space in the presence of disorder. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B51.00013: Topologic connection between a (quasi) 2-D hexagonal and 3-D diamond or wurtzite structure Jianwei Wang, Yong Zhang The similarity in the geometrical arrangements of C atoms between the (111) plane of diamond and the basal plane of graphite has long been recognized.[1] Individual (111) monolayers of various IV and III-V compounds have been explored theoretically as candidates for 2-D materials beyond graphene.[2] We further point out the topologic connection between the 2-D hexagonal and 3-D diamond (zinc-blende) or wurtzite structure, namely a buckled 2-D hexagonal structure (e.g., silicene) can be viewed as a partially collapsed 3-D (111) or (0001) monolayer when the monolayer spacing is increased and in the meantime allowing the structure to relax into a (meta)stable configuration. Graphene is one of the special cases the monolayer collapses entirely. Using a density functional theory, we examine this topologic evolution for IV, III-V, and II-VI compounds and calculate the electronic structures for the quasi 2D structures so derived. When large atoms are involved, the weakened $\pi $-bond of the monolayer leads to chemical instability. Capping layers can be used to stabilize the material, which then forms an ultra-thin quantum well or superlattice.[3] [1] Li et al., JAP 73, 711(1993).[2] \c{S}ahin et al., PRB 80, 155453 (2009). [3] Esaki {\&} Tsu, IBM Res. Develop. 14, 61 (1970). [Preview Abstract] |
Session B52: Pseudogap Phenomenon in Copper-oxide Superconductors
Sponsoring Units: DCMPChair: Carmen Almasan, Kent State University
Room: Mile High Ballroom 1F
Monday, March 3, 2014 11:15AM - 11:27AM |
B52.00001: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 11:27AM - 11:39AM |
B52.00002: Neutron scattering study of novel pseudogap excitations in the high-temperature superconductor HgBa$_{2}$CuO$_{4+\delta}$ Yang Tang, M.K. Chan, C. Dorow, M. Veit, Y. Ge, M. Greven, L. Mangin-Thro, P. Bourges, Y. Sidis, D.L. Abernathy Following the discovery of universal novel magnetic order in the pseudogap phase of the cuprates [B. Fauqu\'{e} et al. PRL 96, 197001 (2006); Y. Li et al., Nature 455, 372 (2008)], our inelastic neutron scattering measurements of HgBa$_{2}$CuO$_{4+\delta}$ revealed two weakly-dispersive excitation branches that appear to be associated with this ordered state [Y. Li et al., Nature 468, 283 (2010); Y. Li et al., Nature Phys. 8, 404 (2012)]. Here we report new results on the doping and momentum dependence of the excitations. The momentum-transfer dependence of the mode intensities is inconsistent with traditional magnetic or structural form factors. Polarized neutron scattering results suggest a possible dual magnetic and structural nature of the excitations [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B52.00003: Competition between the pseudogap and superconductivity and its critical point Makoto Hashimoto, Elizabeth Nowadnick, Rui-Hua He, Inna Vishik, Brian Moritz, Yu He, Kikyohisa Tanaka, Rob Moore, Donghui Lu, Yoshiyuki Yoshida, Motoyuki Ishikado, Takao Sasagawa, Kazuhiro Fujita, Shigeyuki Ishida, Shinichi Uchida, Hiroshi Eisaki, Zahid Hussain, Tom Devereaux, Zhi-Xun Shen In high-temperature cuprate superconductors, the nature of the pseudogap remains unresolved. Recently, there have been increasing evidence that the pseudogap is a distinct order from superconductivity, but it has not been completely understood how the pseudogap affects the superconductivity. To reveal the interplay between the pseudogap and superconductivity, we have performed ARPES measurement on a wide doping range of Bi2212. From the analysis of the spectral weight, we found a singular behavior at Tc at the antinode, indicating the competition between the order parameters for the pseudogap and superconductivity. This signature for the competition at Tc becomes weaker with hole doping, and disappears at $\sim$22\% hole doping. Together with spectroscopic evidence for the ground-state critical point at 19\%, our result reveals the non-trivial termination of the interplay between the pseudogap and superconductivity. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B52.00004: Effect of the pseudogap on T$_c$ in the cuprates and implications for its origin Michael Norman, Vivek Mishra, Utpal Chatterjee, Juan Carlos Campuzano One of the most intriguing aspects of cuprates is a large pseudogap coexisting with a high superconducting transition temperature. Here, we study pairing in the cuprates from electron-electron interactions by constructing the pair vertex using spectral functions derived from angle resolved photoemission data for a near optimal doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ sample that has a pronounced pseudogap. Assuming that that the pseudogap is not due to pairing, we find that the superconducting instability is strongly suppressed, in stark contrast to what is actually observed. Using an analytic approximation for the spectral functions, we can trace this suppression to the destruction of the BCS logarithmic singularity from a combination of the pseudogap and lifetime broadening. Our findings strongly support those theories of the cuprates where the pseudogap is instead due to pairing. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B52.00005: Universal bulk charge-density-wave (CDW) correlations in the cuprate superconductors Wojciech Tabis The recent observation of bulk CDW order in YBa$_{2}$Cu$_{3}$O$_{8+\delta}$(YBCO) in competition with superconductivity is a significant development [1,2]. Using Cu $L$-edge resonant X-ray scattering, we also observe bulk CDW order in HgBa$_{2}$CuO$_{4+\delta}$(Hg1201; $T_{c}=$72K). The correlations appear below $T_{CDW}\approx $200K, well below the pseudogap temperature $T^{\ast }\approx $320K associated with unusual magnetism [3], but coincident with the onset of Fermi-liquid-like charge transport [4,5]. In contrast to YBCO, we observe no decrease of the CDW amplitude below $T_{c}$, and the correlation length is short and temperature independent. CDW correlations therefore are a universal property of underdoped cuprates, enhanced by low structural symmetry and a magnetic field [1,2], but fundamentally not in significant competition with superconductivity. We also discuss the relationship between the CDW modulation wave vector and the Fermi surface area extracted from QO experiments [6]. \\[4pt] [1] G. Ghiringhelli \textit{et al.}, Science \textbf{337}, 821 (2012).\\[0pt] [2] J. Chang \textit{et al.}, Nature Phys. \textbf{8}, 871 (2012).\\[0pt] [3] Y. Li \textit{et al.}, Phys. Rev B \textbf{84}, 224508 (2011).\\[0pt] [4] N. Bari\v{s}i\'c \textit{et al.}, PNAS \textbf{110}, 12235 (2013).\\[0pt] [5] S.I. Mirzaei \textit{et al.}, PNAS \textbf{110}, 5774 (2013).\\[0pt] [6] N. Bari\v{s}i\'{c} \textit{et al.}, Nature Physics (2013), doi:10.1038/nphys2792. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B52.00006: Doping dependence of the hole distribution in the high-temperature superconductor HgBa$_{2}$CuO$_{4+\delta}$ investigated by X-ray absorption spectroscopy Guichuan Yu, Wojciech Tabis, Dennis Nordlund, Jun-Sik Lee, Tomasz Kolodziej, Michael Veit, Chelsey Dorow, Mun Chan, Neven Bari\v{s}i\'c, Martin Greven Using polarization dependent X-ray absorption spectroscopy at the O $K$ and Cu $L$ edges, the symmetry and distribution of the doping-induced states in the simple tetragonal model cuprate superconductor HgBa$_{2}$CuO$_{4+\delta}$ are studied as a function of the effective hole concentration $p$. We find that in the heavily underdoped regime, the doped holes predominantly reside in the planar O 2$p_{\mathrm{x,y}}$ orbitals, with an occupancy that extrapolates linearly to zero at $p_{0} =$ 0 and saturates at $p_{2} \approx $ 0.1. In contrast, holes in the apical O 2$p_{\mathrm{z}}$ orbitals only emerge near the boundary of the superconducting dome at $p_{1}$ $\approx $ 0.05. Similar to the planar holes, these apical O holes also exhibit saturation behavior above $p_{2}$. These observations suggest that the apical O holes might play an important role in the formation of superconductivity, and they imply the existence of an electronic instability as optimal doping is approached. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B52.00007: Charge stripe ordering in LSCO and its comparison to LBCO and YBCO Vivek Thampy, Mark Dean, Mirian Garcia-Fernandez, Niels Bech Christensen, Stuart Wilkins, John Hill Charge stripe ordering has been shown to exist in several related members of the so-called 214 family, such as La$_{\mathrm{2-x}}$Ba$_{\mathrm{x}}$CuO$_{\mathrm{4}}$~(LBCO) and La$_{\mathrm{2-x-y}}$Sr$_{\mathrm{x}}$Nd$_{\mathrm{y}}$CuO$_{\mathrm{4}}$~(LNSCO), but had not previously been unambiguously observed in La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$CuO$_{\mathrm{4}}$~(LSCO). Using resonant soft x-ray and hard x-ray diffraction, we have observed a charge modulation in La$_{\mathrm{1.875}}$Sr$_{\mathrm{0.125}}$CuO$_{\mathrm{4}}$~with a characteristic wave-vector close to that of the other 214 compounds. Recent reports suggested that this ordering occurs at the surface of LSCO. These results establish that it persists throughout the bulk of the sample. Despite the similar wave-vector, the evolution of the stripe order through the superconducting transition ($T_{C})$ in LSCO is different from LBCO and LNSCO, which show no change in the stripe ordering across $T_{C}$. In LSCO, on the other hand, the peak intensity is clearly suppressed below $T_{C}$ and the correlation length stops growing. This bears similarity to what has been seen in YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{6+y}}$ of the 123 family, and could help establish a common motif to charge ordering between these two disparate families of superconducting cuprates. Finally, we show that despite these distinctions, the order parameter of the charge modulation in the three compounds is comparable in magnitude. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B52.00008: Doping-dependent vortex-state scanning tunneling spectroscopic (STS) studies of Ca-doped YBa$_{2}$Cu$_{3}$O$_{7-\delta}$(Y-123) M.L. Teague, C.-C. Chen, N.-C. Yeh, Z.J. Feng We present STS studies of Ca-doped Y-123 as a function of magnetic field ($H)$ and hole doping level ($p)$. Our previous STS studies at $H =$ 0 have shown that the origin of the pseudogap (PG) is due to competing orders (COs), and that the presence (absence) of PG above the superconducting (SC) transition $T_{c}$ is associated with a CO energy $\Delta_{\mathrm{CO}}$ larger (smaller) than the SC gap $\Delta_{\mathrm{SC}}$. Moreover, $\Delta _{\mathrm{SC}}$ and $\Delta_{\mathrm{CO}}$ decrease with increasing $p$ for $p$ \textgreater 0.16, and $\Delta_{\mathrm{CO}}$ \textit{\textless } $\Delta _{\mathrm{SC}}$ for $p$ \textgreater 0.23. The pairing symmetry also evolves from pure $d_{x^{2}-y^{2}}$ to ($d_{x^{2}-y^{2}} + s$) for $p$ \textgreater 0.16, where the $s$-wave component increases with $p$. Here we investigate the evolution of vortex-state ($H$ \textgreater 0) STS with $p$. For $p =$ 0.21 and $H =$ 3T, STS reveal the presence of vortices with a vortex ``halo'' size $\xi $ $\sim$ 8 nm, smaller than $\xi $ $\sim$ 10 nm for $p =$ 0.16. A PG with $\Delta_{\mathrm{CO}}$ ($\sim$ 11 meV) \textless $\Delta_{\mathrm{SC}}$ ($\sim$ 17 meV) is found inside the vortex core for $p =$ 0.21, which is consistent with the value derived from Green function analysis of the STS in $H =$ 0 and is in contrasts to the finding of an intra-vortex PG $\Delta_{\mathrm{CO}}$ ($\sim$ 32 meV) \textgreater $\Delta_{\mathrm{SC}}$ ($\sim$ 23 meV) for $p =$ 0.16. Fourier transformation of the STS also shows energy-independent wave-vectors Q$_{\mathrm{CDW}}$ and Q$_{\mathrm{PDW}}$ associated with the charge- and pair-density waves, where Q$_{\mathrm{CDW}}$ decreases with $p$ and Q$_{\mathrm{PDW}}$ is $p$-independent. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B52.00009: Universal quantum oscillations in the underdoped cuprate superconductors Neven Barisic, Sven Badoux, Mun K. Chan, Chelsey Dorow, Wojciech Tabis, Baptiste Vignolle, Guichuan Yu, Jerome Beard, Xudong Zhao, Cyril Proust, Martin Greven The observations of a very small pocket (covering only $\sim$ 2{\%} of the Brillouin zone (BZ)) for underdoped Y123 and Y124 are in stark contrast to the situation at high hole concentrations, where a large Fermi surface (FS) is observed (corresponding to $\sim$ 65{\%} of the BZ. While this result can been taken as evidence for a dramatic change of the FS associated with the quintessential CuO$_{2}$ planes, it may also be attributed to the existence of a non-universal FS piece related to the CuO chains. Consequently, it has remained a pivotal open question whether the FS reconstruction has anything to do with aspects of the unidirectional structures, or if it is a universal property of the cuprates. We have settled this issue through the observation of quantum oscillations in the magnetorestivity of a simple tetragonal compound Hg1201 at p$\approx $0.09 in pulsed magnetic fields of up to 80 T. The pocket appear approximately in the same doping, temperature and magnetic field range of the phase diagram as in the case of Y123, with a very similar effective mass and comparable cyclotron frequency. (Nat. Phys., doi:10.1038/nphys2792) [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B52.00010: Indications of an $s$-wave structure of pseudogap in high-$T_{\mathrm{c}}$ cuprate superconductors Shiro Sakai The superconducting gap in high-$T_{\mathrm{c}}$ cuprates is d wave, as established by a SQUID experiment. Although no such phase-sensitive experimental result exists for a pseudogap it has also been believed to be d wave, on ground of the observations by angle-resolved photoemission spectroscopy (ARPES) However, since APRES focuses on the occupied spectra, little is actually known about the unoccupied side. Here we propose, based on a cluster dynamical mean-field calculation on the two-dimensional Hubbard model, that the pseudogap has an s-wave structure, where the gap in the nodal region is \textit{above} the Fermi level. The s-wave structure indeed explains well the recent ARPES and STM observations of electron-hole asymmetry, as well as the anomalous behavior observed in electronic Raman spectroscopy, which captures the structure of the unoccupied spectra, essential for discriminating the s-wave pseudogap from the conventional d-wave one [S. Sakai et al., Phys. Rev. Lett. 111, 107001 (2013)] The work was done in collaboration with S. Blanc, M. Civelli, Y. Gallais, M. Cazayous, M.-A. Measson, J. S. Wen, Z. J. Xu, G. D. Gu, G. Sangiovanni, Y. Motome, K. Held, A. Sacuto, A. Georges, and M. Imada [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B52.00011: Chiral Texture of the Magnetic Moments of Unit-Cell Loop Currents in the Pseudogap Phase of Cuprate Superconductors Sergey Pershoguba, Kostya Kechedzhi, Victor Yakovenko We propose a novel chiral order parameter to explain the unusual polar Kerr effect in underdoped cuprates. It is based on the loop-current model by Varma, which is characterized by the in-plane anapole moment $\mathbf{N}$ and exhibits the magnetoelectric effect. We propose a helical structure where the vector $\mathbf{N^{(n)}}$ in the layer $n$ is twisted by the angle $\pi/2$ relative to $\mathbf{N^{(n-1)}}$, thus breaking inversion symmetry. We show that coupling between magnetoelectric terms in the neighboring layers for this structure produces optical gyrotropy, which results in circular dichroism and the polar Kerr effect.\\[4pt] S. S. Pershoguba, K. Kechedzhi, and V. M. Yakovenko, Phys. Rev. Lett. {\bf 111}, 047005 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B52.00012: Nodal electronic scattering rates, T*, and the phase diagram in hole doped cuprate superconductors Daniel Dessau, T.J. Reber, Xiaoqing Zhou, Haoxiang Li, Justin Waugh, Stephen Parham, Yue Cao, Zhe Sun, Qiang Wang, Michael Hermele, Gerald Arnold, J.S. Wen, Zhijun Xu, Genda Gu, Nick Plumb, Yoshiyuki Yoshida, Hiroshi Eisaki Based upon detailed angle resolved photoemission spectroscopy measurements of a wide range of doping of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$, we present a unified phenomenology for the non-Fermi liquid normal-state interactions (scattering rates) in the nodal direction, which we show are dominated by a single parameter that smoothly varies with doping. When viewed over typical experimental temperature ranges this phenomenology has a curvature change that mimics the T* ``pseudogap'' temperature scale observed in transport experiments, including the possible quantum critical point. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B52.00013: Exploration of pseudogap scenarios in the three-orbital Hubbard model of cuprate superconductors Y.F. Kung, C.-C. Chen, E.A. Nowadnick, S. Johnston, B. Moritz, T.P. Devereaux One intriguing question in cuprate high-temperature superconductors concerns the nature of the pseudogap, whose origin remains elusive despite intense effort in both theory and experiment. Various ordered states have been proposed, yet with conflicting results from different numerical studies. Here we use determinant quantum Monte Carlo (DQMC) simulations to investigate the possibility of orbital loop currents, d-density waves, and oxygen antiferromagnetism in the three-orbital Hubbard model. We explore the dependence of their correlation functions on temperature and doping, as well as their relative magnitudes, to shed light on how these orders depend on the parameters of the model Hamiltonian. The DQMC calculations are further compared to results from exact diagonalization. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B52.00014: Quenched disorder and vestigial nematicity in the pseudo-gap regime of the cuprates Laimei Nie, Gilles Tarjus, Steven Kivelson We carry out a theoretical analysis of the Landau-Ginzburg-Wilson effective field theory of a classical incommensurate charge-density-wave (CDW) in the presence of weak quenched disorder. While the possibility a sharp phase transition and long-range CDW order are precluded in such systems, we show that any discrete symmetry breaking aspect of the charge order (nematicity in the case of the unidirectional (stripe) CDW we consider explicitly) generically survives up to a non-zero critical disorder strength. Such ``vestigial order,'' which is subject to unambiguous macroscopic detection, can serve as an avatar of what would be CDW order in the zero disorder limit. Various recent experiments in the pseudo-gap regime of the hole-doped cuprate high-temperature superconductors are interpreted in light of these results. [Preview Abstract] |
Session B53: Surfaces, Interfaces, and Thin Films: Kinetics and Dynamics
Sponsoring Units: DCMPChair: Brad Conrad, Appalachian State University
Room: Mile High Ballroom 2C
Monday, March 3, 2014 11:15AM - 11:27AM |
B53.00001: Charge transfer and adsorption-desorption kinetics in carbon nanotube and graphene gas sensing Sang-Zi Liang, Gugang Chen, Avetik Harutyunyan, Milton Cole, Jorge Sofo Detection of molecules in the gas phase by carbon nanotube and graphene has great application potentials due to the high sensitivity and surface-to-volume ratio [1, 2]. In chemiresistor, the conductance of the materials has been proposed to change as a result of charge transfer from the adsorbed molecules. Due to self-interaction errors, calculations using LDA or GGA density functionals have an innate disadvantage in dealing with charge transfer situations. A model which takes into consideration the dielectric interaction between the graphene surface and the molecule is employed to estimate the distance where charge transfer becomes favorable. Adsorption-desorption kinetics is studied with a modified Langmuir model, including sites from which the molecules do not desorb within the experimental time. Assuming a constant mobility, the model reproduces existing experimental conductance data [1, 2]. Its parameters provide information about the microscopic process during the detection and varying them allows optimization of aspects of sensor performance, including sensitivity, detection limit and response time. [1] G. Chen et al., Sci. Rep. 2, 343 (2012). [2] G. Chen et al., Appl. Phys. Lett. 101, 053119 (2012). [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B53.00002: Dynamics of H$_{2}$O Molecules Confined in Fullerenes Roxanne Tutchton, Paul Larson, Zhigang Wu Structural and dynamical properties of confined water in nanostructures are expected to be remarkably different than those of bulk water. Water confined in nanotubes and graphene sheets has been extensively investigated theoretically, yet very few computational efforts have been made to study confined water in fullerenes though, experimentally, water molecules have been successfully encapsulated inside fullerenes as small as C$_{60}$. In this work, we carry out classical molecular dynamics simulations to investigate density and H-bond distributions inside fullerenes ranging from C$_{60}$ to C$_{540}$. Our results show that as the size of the fullerene increases, concentric shells of water molecules are formed, and the water density is higher than that of bulk water while the average H-bond per molecule is slightly lower than the bulk value. We also find that these shells of H$_{2}$O are solid-like at room temperature, but they should eventually become liquid-like at high temperatures. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B53.00003: Growth and dynamics of C$_{60}$ molecules on Ag(111) Renee Diehl, Hsin Li, Guilherme Abreu, Heekeun Shin, A.K. Shukla, Julian Ledieu, Vincent Fourn\'ee, Laura Serkovic Loli, Kristin Marino, Stephanie Su, Michael Snyder C$_{60}$ monolayers on Ag(111) form various structures that depend on the surface preparation. For films deposited at 300 K, the monolayer comprises close-packed domains that are incommensurate with the substrate. These domains progressively convert into a commensurate (2$\surd $3x2$\surd $3)R30 structure as the annealing temperature is increased. The annealing process activates the formation of vacancies beneath some of the C$_{60}$ molecules, resulting in a temperature-dependent equilibrium population of about 50{\%} of the molecules sitting on vacancies and 50{\%} sitting on top of Ag atoms in the commensurate phase. In addition to molecules in these two sites, about 0.5{\%} of the molecules image very brightly, and these molecules are in dynamical equilibrium with the other two types. This talk will focus on scanning tunneling microscopy and low-energy diffraction studies of the structure and dynamics of the monolayer phases of C$_{60}$ molecules on Ag(111). [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B53.00004: Modeling the transport of chemical warfare agents and simulants in polymeric substrates for reactive decontamination Thomas Pearl, Brent Mantooth, Mark Varady, Matthew Willis Chemical warfare agent simulants are often used for environmental testing in place of highly toxic agents. This work sets the foundation for modeling decontamination of absorbing polymeric materials with the focus on determining relationships between agents and simulants. The correlations of agents to simulants must consider the three way interactions in the chemical-material-decontaminant system where transport and reaction occur in polymer materials. To this end, diffusion modeling of the subsurface transport of simulants and live chemical warfare agents was conducted for various polymer systems (e.g., paint coatings) with and without reaction pathways with applied decontamination. The models utilized 1D and 2D finite difference diffusion and reaction models to simulate absorption and reaction in the polymers, and subsequent flux of the chemicals out of the polymers. Experimental data including vapor flux measurements and dynamic contact angle measurements were used to determine model input parameters. Through modeling, an understanding of the relationship of simulant to live chemical warfare agent was established, focusing on vapor emission of agents and simulants from materials. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B53.00005: Desorption Kinetics of Water from Poly(methyl methacrylate) Patrick Howard, Brian Familo, Thorin Kane, Ross Netusil, Marie Romano, John St. Leger, Paul Jones, Carolina C. Ilie Understanding the surface behaviors of polymers has many applications in modern medicine and engineering. In this experiment, we are analyzing the thermal desorption of water from poly(methyl methacrylate), polyethylene, and other polymers. Crystalline polymer films are being prepared using a Langmuir-Blodgett method from a water subphase, as well as by a spin coating technique. The organized, crystalline Langmuir-Blodgett films are different in structure from the more disorganized spin-coated films. The thermal desorption spectra from both techniques will be compared to show the effects of the way polymer films are obtained. Arrhenius analysis on the desorption spectra yields desorption activation energies for these polymers. The purpose of this analysis is to provide insight into the dipole interactions between the water and polymer molecules. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B53.00006: Reactive decontamination of absorbing thin film polymer coatings: model development and parameter determination Mark Varady, Brent Mantooth, Thomas Pearl, Matthew Willis A continuum model of reactive decontamination in absorbing polymeric thin film substrates exposed to the chemical warfare agent O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate (known as VX) was developed to assess the performance of various decontaminants. Experiments were performed in conjunction with an inverse analysis method to obtain the necessary model parameters. The experiments involved contaminating a substrate with a fixed VX exposure, applying a decontaminant, followed by a time-resolved, liquid phase extraction of the absorbing substrate to measure the residual contaminant by chromatography. Decontamination model parameters were uniquely determined using the Levenberg-Marquardt nonlinear least squares fitting technique to best fit the experimental time evolution of extracted mass. The model was implemented numerically in both a 2D axisymmetric finite element program and a 1D finite difference code, and it was found that the more computationally efficient 1D implementation was sufficiently accurate. The resulting decontamination model provides an accurate quantification of contaminant concentration profile in the material, which is necessary to assess exposure hazards. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B53.00007: Estimating Atomic Diffusivity in Metallic Multilayered Systems Manav Vohra, Omar Knio Nanostructured multilayered systems can support self-propagating reactions due to exothermic intermixing and small atomic diffusion distances. In the Zr-Al system under stoichiometric and adiabatic conditions, the temperature increases by about 1500 K over ambient as a result of the formation reaction. This usually results in melting in the individual Al layers, and consequently to enhanced rates of intermixing. In order to characterize this phenomenon, and accordingly quantify the associated heat release rates, we rely on transient temperature measurements of homogeneous ignition, as well as measurements of the velocity of self-propagating fronts. The former enables us to infer averaged intermixing rates in a temperature range falling below the melting point of Al, whereas the latter yield estimates at high temperatures. Implementation of the formalism leads to correlations of the atomic diffusivity that exhibit two Arrhenius branches, with a jump across the melting temperature of Al. The resulting composite Arrhenius relation can be readily incorporated into reduced reaction models,\footnote{M. Salloum and O. M. Knio. \textit{Combustion and Flame} 157.2 (2010): 288-295.} and thus exploited to predict transient, multidimensional reaction phenomena. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B53.00008: Boron diffusion mechanism and effect of interface Ge atoms in Si/SiO$_{2}$ and SiGe/SiO$_{2}$ interfaces Geun-Myeong Kim, Young Jun Oh, Chang Hwi Lee, K.J. Chang In metal-oxide-semiconductor field effect transistors (MOSFETs) it is known that implanted B dopants easily segregate to the oxide during thermal annealing after ion implantation, causing threshold voltage shift and sheet resistance increase. On the other hand, SiGe alloys have been considered as a promising material for $p$-type MOSFETs due to reduced B diffusion and high hole mobility. However, there is a lack of studies for B diffusion in Si/SiO$_{2}$ and SiGe/SiO$_{2}$ interfaces. In this work, we perform first-principles density functional calculations to study the mechanism for the B diffusion in Si/SiO$_{2}$ and SiGe/SiO$_{2}$ interfaces. We investigate the diffusion pathways and migration barriers by using the climbing nudged elastic band and dimer methods. For Si/SiO$_{2}$ interface, B in Si turns into an interstitial B and tends to intervene between the Si and bridge O atoms at the interface. The overall migration barrier is calculated to be about 2 eV, comparable to that in bulk SiO$_{2}$. In SiGe/SiO$_{2}$, interface Ge atoms enhance the stability of B-related defects in the interface region, resulting in the higher migration barrier of about 3.7 eV. Our results indicate that Si/SiO$_{2}$ interface does not hinder the B diffusion, however, the B diffusion is suppressed in the presence of interface Ge atoms. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B53.00009: X-ray Scattering from Ag (001) and Ag (111) Surfaces in Electrochemical Environments: Dynamics and Structure Robert Karl, Jr., Michael Pierce, Vladimir Komanicky, Hoydoo You, Andi Barbour, Chenhui Zhu, Alec Sandy We have investigated the Ag (001) and Ag (111) single crystal surfaces in weak electrolyte using a combination of x-ray Crystal Truncation Rod (CTR) experiments to determine structural information and X-ray Photon Correlation Spectroscopy (XPCS) to examine the nano-scale dynamics. Our structural measurements confirm earlier potential dependent measurements of H$_2$O over the Ag (111) surface [1], and also show that the Ag (001) H$_2$O interface behaves in an analogous fashion. The XPCS dynamics data reveal how the surface evolves, in real-time, relative to the point of zero charge. Both the CTR and XPCS data were collected at different applied potentials. By comparing the CTR data with the XPCS data, along with ex-situ Atomic Force Microscopy (AFM), we will investigate relationships between the dynamics of the Ag surface and the distribution of H$_2$O molecules above the crystal surface. [1] M. F. Toney et al. Nature 368, 444 (1994). [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B53.00010: Evolution of $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface of Al$_{2}$O$_{3}$(0001) generated in air Hawoong Hong, Aaron Gray, T.-C. Chiang As reported by S. Baik et al\footnote{S. Baik, D. E. Fowler, J. M. Blakely, and R. Raj, J. Am. Ceram. Soc. 68(5), 281 (1985).} $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface of Al$_{2}$O$_{3}$(0001) can be generated by annealing at a high temperature in air. We reproduced this $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface and investigated surface structures with x-ray diffraction using synchrotron radiation at Advanced Photon Source and RHEED techniques. We also annealed this $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface in a UHV chamber until the superstructure disappeared and a new $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface was generated. We will compare the results to the previous x-ray diffraction experiments\footnote{G. Renaud, B. Villette, I. Vilfan, and A. Bourret, Phys. Rev. Lett. 73, 1825 (1994).} and recent AFM/DFT investigation.\footnote{J. V. Lauritsen, M. C. R. Jensen, K. Venkataramani, B. Hinnemann, S. Helveg, B. S. Clausen, and F. Besenbacher, Phys. Rev. Lett 103, 076103 (2009). } The UHV generated $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface also appeared to preserve the $\sqrt {31} \times \sqrt {31} R9^{\circ}$ symmetries as Pd films were deposited. However, the intensity ratios between superlattice peaks went through large changes. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B53.00011: Capillary Condensation Pathways of CO2 under Templated Mesoporous Silica Confinement Bo Wang, Paul Sokol Adsorption of CO2 in porous medium has been of great current interest due to its potential for mitigating the global warming caused by greenhouse gases. In particular, the behavior of confined CO2 in mesoporous media near room temperature is particularly relevant to sequestration efforts. Realistic mesoporous systems, such as shales and coals, represent a complex fractal pore structure that complicates the interpretation of adsorption studies. We present the results of a study focused on the adsorption of CO2 in model mesoporous media with well-defined pore structures. Templated porous glasses, such as MCM-41 which has a regular network of 1D pores, provide an ideal system for fundamental studies of the adsorption process. In this study, we focus on the structure of adsorbed CO2 films which evolves in a mixture of phases and the development of nucleation occurs during the formation of high density liquid CO2 inside the confining matrix. We have used Small Angle Neutron Scattering to study the spatial distribution of material radially and transversely within the pores. The 30m SANS NG7 at NIST was used to map out the details of CO2 condensation pathway under mesoporous silica confinement. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B53.00012: Local Investigation in Dynamic Behavior of Excited Water Nanoclusters on Cu(111) Surface Yang Guo, Huiqi Gong, Li Dong, Lailai Li, Jinchuan Wang, Xinyan Shan, Xinghua Lu Dynamic behavior of water molecules on surfaces is important for surface-mediated water dissociations and reactions. Here we present investigations in dynamic behavior of excited water nanoclusters on Cu (111) surface by using a low temperature scanning tunneling microscope (STM). It is found that excess electrons in a single water nanocluster can be injected from a metallic STM tip under a positive voltage. Such injection of electrons results in both the diffusion of single H$_{\mathrm{2}}$O molecules within the nanocluster and directional diffusion of water nanoclusters on surface. The range of lateral diffusion is limited to several nanometers from the tip because of the electrical screening effect from Cu substrate for the excess electrons in the nanocluster. In addition, femto-second laser pulses are employed to excite the water nanoclusters during STM imaging with tip in the tunneling condition. Significant changes in topographic profile of H$_{\mathrm{2}}$O nanoclusters are observed under the photoexciation, as compared with that of the nanoclusters in the ground state. The results obtained in this study provide a microscopic understanding of the diffusion mechanism of excited water nanoclusters on surface. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B53.00013: Self-consistent elastic and inelastic scattering of helium atoms by the monolayer solid Xe/Pt(111) Ludwig Bruch, Flemming Hansen The inelastic scattering of a helium beam by an incommensurate monolayer solid of Xe/Pt(111) for incident energies in the range 2.5 -- 50 meV is evaluated in the one-phonon approximation. The calculations treat specific one-phonon excitations and use an extension of our wave packet propagation formulation of helium scattering to treat the elastic and inelastic scattering self-consistently. There is an inelastic (diffuse) background scattering for phonon modes with wavevectors that uniformly sample the Brillouin zone. The fraction of the beam that is scattered elastically into diffraction channels varies from almost 90\% to about 15\% over this energy range. A low energy case with trapping for more than 200 ps has been simulated. Results of the self-consistent calculations for the inelastic scattering are compared to those of previous calculations that did not include the feedback (coherent phonon annihilation) to the elastic channels. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B53.00014: Simultaneous observation of ultrafast structural dynamics and transient electrical field by picosecond electron pulses Run-Ze Li, Pengfei Zhu, Long Chen, Jie Chen, Jianming Cao, Zheng-Ming Sheng, Jie Zhang Ultrafast electron diffraction and microscopy are very promising methods to study transient structural dynamics with atomic spatial-temporal resolution. However, in these laser-pump electron-probe studies of structural dynamics, a transient electric field induced by laser excitation of the sample could deflect probing electrons, which may eventually leads to a misinterpretation of the diffraction data. Here, picosecond structural dynamics and transient surface electric field evolution, excited by femtosecond laser interaction with a metallic thin film, have been observed simultaneously in real time by ultrashort electron pulses in a transmission configuration. By tracing time dependent changes of electron diffraction and deflection angles, these two processes are found to be significantly different and distinguishable in their temporal behavior. This observation provides an effective approach to extract the otherwise obscured ultrafast structural dynamics and may help to improve the spatiotemporal resolution in ultrafast electron diffraction and microscopy studies. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B53.00015: Two-stage melting of the ice Ih (0001) surface by molecular dynamics: A molecular dynamics study Mizuki Kuga, Hiroshi Iyetomi We report a molecular dynamics simulation study of the structure and energetics of Ice Ih with a free basal (0001) surface. Especially we cast a new light on the formation of a quasi-liquid layer (QLL) at the surface prior to the melting of bulk ice. Detailed analysis of functional behavior of the total and potential energies with respect to temperature clearly separates the surface melting process into two stages. As temperature is increased, the outermost bilayer of the ideal surface first starts disordering in a progressive way with detachment of water molecules from the surface. During this first stage of surface melting, the energies show no discontinuous change as a function of temperature. Subsequently to fully developed disordering of the top layer, the surface shows stepwise melting in a narrow temperature range of around $10\,{\rm K}$. This second stage of surface melting has first-order phase-transition-like characteristics as demonstrated by structural relaxation from an initial metastable (super-heating) state to the stable state during which the surface is liquefied layer by layer. Accordingly, it turns out that each of the stable QLL states designated by the number of liquefied bilayers has its own branch of the potential energy. [Preview Abstract] |
Session B54: Focus Session: Cooperative Phenomena: Phase Transitions and Magnetocalorics
Sponsoring Units: GMAGRoom: Mile High Ballroom 1B
Monday, March 3, 2014 11:15AM - 11:27AM |
B54.00001: Use of Analysis of Variance to Analyze Universal Scaling in Magnetocaloric Materials Dustin D. Belyea, Casey W. Miller Universal scaling analysis of magnetocaloric materials is a tool used to remove the temperature and field dependence in the measured M(H,T) space, and should in theory result in a single universal curve for all materials within a single universality class. This can be used to compare materials and predict behavior beyond measurable field ranges [1,2]. However, in many cases the scaled curves show wide variations in collapsing onto the universal curve, leading to a noisy collection of scaled curves. Hypotheses as to the origin of such scaling violations include minority phases, either magnetic or structural, or both [3]. In this work we show that it is possible to measure the degree to which a material follows universal scaling using tests for homoscedasticity, namely the Brown-Forsythe test. We use experimental magnetic phase transitions as well as some computational models and then apply this test to show general trends in the changes of magnetization with the inclusion of such magnetic and structural minority phases. [1] V Franco et al, J. Phys.: Condens. Matter 20 285207 (2008) [2] V. Franco et al, J. Appl. Phys. 106, 103911 (2009) [3] V. Franco et al, J. Magn. Magn. Mater., 321(9):1115-1120 (2009) [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B54.00002: Magnetocaloric and Exchange Bias effects in Ni$_{50}$Mn$_{35.8}$Sn$_{14.2}$ alloy Vijaysankar Kalappattil, A. Biswas, T.L. Phan, S.C. Yu, N.H. Dan, M.H. Phan, H. Srikanth Heusler alloys have been a subject of intense research because of their intriguing physical properties. We have studied magnetic and magnetocaloric properties of such an alloy with composition Ni$_{50}$Mn$_{35.8}$Sn$_{14.2}$. The system undergoes two transitions upon cooling: (i) a ferromagnetic transition in austenite phase at T$_{\mathrm{C}}^{\mathrm{A}} \sim $310 K and (ii) the martensitic transition at T$_{\mathrm{MA}} \sim $ 165 K. Using Maxwell's equation, the magnetic entropy change ($\Delta $S$_{\mathrm{M}})$ is calculated from the is magnetization vs. field curves following heating, cooling, and loop protocols. A large inverse magnetocaloric effect (IMCE) is observed at T$_{\mathrm{MA}}$ in addition to the conventional magnetocaloric effect (CMCE) at T$_{\mathrm{C}}^{\mathrm{A}}$. While $\Delta $S$_{\mathrm{M}}$ is protocol dependent, we have demonstrated that a universal curve can be constructed to describe temperature (T) dependence of $\Delta $S$_{\mathrm{M}}$ corresponding to IMCE without rescaling of the T-axis and irrespective of measurement protocol. Such a universal behavior of IMCE is different from that of CMCE. The system also shows an exchange bias effect below T$_{\mathrm{MA}}$ due to phase coexistence. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B54.00003: Towards enhanced magnetocaloric effects: a combined experimental / computational methodology Karl Sandeman A simple thermodynamic analysis shows that there is scope to double the adiabatic temperature change of d-metal magnetocaloric materials in a fixed change of magnetic field [1]. If found in a a real material, such an improvement in the MCE will have two important effects: to increase the final efficiency of the cooling device and to reduce the use of rare earth materials in the magnet that is used to drive the MCE. I will give examples of a methodology for the search for new magnetocaloric materials, combining neutron scattering studies with materials modelling. We have used this approach to understand magneto-elastic coupling at a fundamental level in both Mn-based and Fe-based tricritical metamagnets [2,3]. \\[4pt] [1] K.G. Sandeman, Scr. Mater. 67 566 (2012).\\[0pt] [2] Z. Gercsi, K. Hono and K.G. Sandeman, Phys. Rev. B 83 174403 (2011) and references therein. \\[0pt] [3] Z. Gercsi et al., Phys. Rev. B 88 024417 (2013). [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B54.00004: Vector boson excitations near deconfined quantum critical points Yejin Huh, Philipp Strack, Subir Sachdev We show that the N\'eel states of two-dimensional antiferromagnets have low energy vector boson excitations in the vicinity of deconfined quantum critical points. We compute the universal damping of these excitations arising from spin-wave emission. Detection of such a vector boson will demonstrate the existence of emergent topological gauge excitations in a quantum spin system. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B54.00005: First-principles study of the magnetic and electronic properties of NiAs-type FeSe Weidong Luo, Xiaole Zhang FeSe exhibits the NiAs-type hexagonal structure under external pressure or under certain growth conditions at ambient pressure. The superconducting transition temperature of alpha-FeSe initially increases and then decreases when external pressure is applied, in which the NiAs-type FeSe is considered as the competing phase to the superconducting alpha-FeSe. We study the magnetic and electronic properties of the NiAs-type FeSe using first-principles calculations. The ferromagnetic (FM) and several antiferromagnetic (AFM) configurations of NiAs-type FeSe have been studied, and we observe dependence of the electronic structure on its magnetic configuration of NiAs-type FeSe. The results of theoretical calculations are compared to experimental observations. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B54.00006: Phase diagram of an ideal three-dimensional antiferromagnet with random magnetic anisotropy Pavel Borisov, Felio A. Perez, Trent A. Johnson, Tudor D. Stanescu, David Lederman, Michael R. Fitzsimmons, Adam A. Aczel, Tao Hong The magnetic phase diagram of three-dimensional (3D) antiferromagnets with random magnetic anisotropy (RMA) is not well understood because systems studied experimentally to date have complicated magnetic structures with competing two-dimensional and three-dimensional exchange interactions. The properties of epitaxial thin films of the 3D RMA antiferromagnet Fe$_{x}$Ni$_{1-x}$F$_{2}$ thin films grown on (110) MgF$_2$ substrates were investigated by magnetometry and neutron scattering. Fe$_{x}$Ni$_{1-x}$F$_{2}$ is an ideal 3D antiferromagnet with which to study this problem due to the single-ion anisotropy energies of the transition metal site which tend to order Ni$^{2+}$ and Fe$^{2+}$ spins perpendicular to each other. The magnetic phase diagram determined from these measurements was analyzed using mean field theory. Regions with uniaxial, oblique and easy plane anisotropy were identified. An anisotropy glass region was discovered where a Griffiths-like breakdown of long-range spin order occurs. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B54.00007: Spin Jams, frustration, and exotic entropy scaling Israel Klich, Seung-Hun Lee, Kazuki Iida When spins are regularly arranged in a triangular fashion, the spins may not satisfy simultaneously their antiferromagnetic interactions with their neighbors. This phenomenon, called frustration, usually leads to a large set of ground states and to exotic states such as spin ice and spin liquid. Here we report a novel situation: a system governed by simple Heisenberg interactions that freezes into a glass like~state induced by quantum fluctuations, in contrast to the usual mechanisms for classical spin-glass which rely on the presence of disorder. At the heart of the effect is an unusual scaling of the number of local minima, with a scaling extensive in the boundary length rather than the volume, which we describe in terms of a new tiling problem. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B54.00008: Antiferromagnetism and a Dimensional Crossover in the Model Quantum Magnet SrCu$_{2}$(BO$_{3}$)$_{2}$ Sara Haravifard, Arnab Banerjee, Jasper van Wezel, Daniel Silevitch, Antonio Moreira Dos Santos, Jonathan Lang, Edwin Kermarrec, George Srajer, Bruce Gaulin, Hanna Dabkowska, Thomas Rosenbaum SrCu$_{2}$(BO$_{3}$)$_{2}$ has corner-sharing Cu$^{2+}$ S=1/2 dimers lying on a square lattice, corresponding to the two-dimensional Shastry-Sutherland model. We conduct high resolution x-ray and neutron diffraction measurements as a function of pressure, tuning the ground state from a singlet ground state to long-range order. We report a change in crystal symmetry as a function of temperature for pressures above 4 GPa, linked to antiferromagnetism and the tilting of the dimers out of the plane. The inclusion of Dzyaloshinskii-Moriya interactions in the Shastry-Sutherland Hamiltonian helps explain the experimental results. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B54.00009: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 1:03PM - 1:15PM |
B54.00010: Violating of the Essam-Fisher and Rushbrooke formulas: quantum phase transitions and 1D Ising model Vladimir Udodov The classical Essam-Fisher and Rushbrooke relationships (1963) that connect the equilibrium critical exponents of susceptibility, specific heat and order parameter are shown to be valid only if the critical temperature $T_{C}$ \textgreater\ 0 and $T \to T_{C}$. For quantum phase transitions (PT's) and 1D Ising model with $T_{C} = $ 0$K$, these relations are proved to be of different form. This fact has been actually observed experimentally, but the reasons were not quite clear. A general formula containing the classical results as a special case is proposed. This formula is applicable to all equilibrium PT's of any space dimension. The predictions of the theory are consistent with the available experimental data and do not cast any doubts upon the scaling hypothesis. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B54.00011: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 1:27PM - 1:39PM |
B54.00012: Phase Diagram for Magnon Condensate in YIG Fuxiang Li, Wayne Saslow, Valery Pokrovsky Recently, magnons, which are quasiparticles describing the collective motion of spins, were found to undergo Bose-Einstein condensation (BEC) at room temperature in films of Yttrium Iron Garnet (YIG). Unlike other quasiparticle BEC systems, this system has a spectrum with two degenerate minima, which makes it possible for the system to have two condensates in momentum space. Recent Brillouin Light scattering studies for a microwave-pumped YIG film of thickness $d=5$ $\mu$m and field $H=1$ kOe find a low-contrast interference pattern at the characteristic wavevector $Q$ of the magnon energy minimum. In this report, we show that this modulation pattern can be quantitatively explained as due to unequal but coherent Bose-Einstein condensation of magnons into the two energy minima. Our theory predicts a transition from a high-contrast symmetric phase to a low-contrast non-symmetric phase on varying the $d$ and $H$, and a new type of collective oscillations. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B54.00013: Magnetic-field-induced quantum criticality in a spin-1 planar ferromagnet with single-ion anisotropy Maria Teresa Mercaldo, Ileana Rabuffo, Luigi DeCesare, Alvaro CaramicoD'Auria The effects of single-ion anisotropy on field-induced quantum criticality in spin-1 planar ferromagnet is explored by means of the two-time Green's function method. We work at the Tyablikov decoupling level for exchange interactions and the Anderson-Callen decoupling level for single-ion anisotropy. In our analysis a longitudinal external magnetic field is used as the non-thermal control parameter and the phase diagram and the quantum critical properties are established for suitable values of the single-ion anisotropy parameter. We find that the single-ion anisotropy has sensible effects on the structure of the phase diagram close to the quantum critical point. Indeed, for values of the uniaxial crystal-field parameter above a positive threshold a re-entrant behavior appears for the critical line, while above this value the conventional magnetic-field-induced quantum critical scenario remains unchanged. \\ M.~T. Mercaldo, I. Rabuffo, L. De Cesare, A. Caramico~D'Auria, Eur. Phys. J. B {\bf 86}, 340 (2013) [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B54.00014: Complex Magnetic behavior in CeGe$_{1.76}$ Single Crystals observed by Neutron Diffraction at Low Temperatures Wageesha Jayasekara, Wei Tian, Andreas Kreyssig, Halyna Hodovanets, Sergey L. Bud'ko, Paul C. Canfield, Robert J. McQueeney, Alan I. Goldman Magnetization measurements on CeGe$_{1.76}$ single crystals indicate two apparent magnetic transitions around 4 K and 7 K. A neutron diffraction experiment was performed at the instrument HB-1A, to study the magnetic order in detail and results will be discussed in this report. Surprisingly, several transitions have been observed: Below 7 K, incommensurate antiferromagnetic order appears with a propagation vector close to (0~0~0.25), that becomes commensurate below 5.2 K demonstrating a lock-in transition. Moreover, squaring-up behavior was also observed by the occurrence of higher harmonic Bragg peaks. An additional antiferromagnetic order was seen below 4.0 K by half-integer and integer indexed Bragg peaks. Further experiments are needed to verify the directions of the ordered magnetic moments. [Preview Abstract] |
Session B55: Invited Session: Novel Topological Phases and Surface States in Strongly Interacting Systems
Sponsoring Units: DCMPChair: Charles Kane, University of Pennsylvania
Room: Four Seasons Ballroom 1
Monday, March 3, 2014 11:15AM - 11:51AM |
B55.00001: Highly entangled quantum states of matter Invited Speaker: Xiao-Gang Wen Highly entangled quantum matter is a new class of matter that correspond patterns of intricate quantum entanglement. The phases of matter have traditionally been classified by their symmetry properties described by group theory. For decades we believe that symmetry breaking states describe all possible phases of matter. However, the discovery of topological order suggested that Landau theory does not describe all quantum phases. In topological order, the phases are not described by the patterns of symmetry, but by the patterns of long-range quantum entanglement. Recently, we have identified a new class of states, called symmetry-protected topological order, which correspond to patterns of short-range quantum entanglement with symmetry. We find that this class of quantum phases and corresponding patterns of entanglement can be described by an abstract mathematical theory - group cohomology theory. In this talk, I will review the background and the basic theory of symmetry-protected topological phases. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B55.00002: Classification and Edge States of Symmetry Protected Topological Phases Invited Speaker: Xie Chen Symmetry protected topological (SPT) phases are gapped and nonfractionalized in the bulk but can have nontrivial edge states protected by the anomalous symmetry action on the boundary. In this talk, I discuss the classification of bosonic SPT phases using group cohomology and what their edge states are like in one, two and three dimensions. In 1D, edge states of SPT chains are degenerate and carry projective representations of the symmetry. In 2D SPT systems, the edge state is either symmetry breaking or gapless which can be protected by the chiral symmetry action on the 1D boundary. For 3D SPT states, a new possibility arises on the 2D boundary besides being symmetry breaking or gapless. The edge can be both gapped and symmetric and have fractional excitations. The fractional excitations transform under symmetry in a way that is not possible in 2D and hence reflect the nontrivial SPT order in the bulk. Explicit examples are given to illustrate the possibilities in different dimensions. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B55.00003: A symmetry-respecting topologically-ordered surface phase of 3d electron topological insulators Invited Speaker: Max Metlitski A 3d electron topological insulator (ETI) is a phase of matter protected by particle-number conservation and time-reversal symmetry. It was previously believed that the surface of an ETI must be gapless unless one of these symmetries is broken. A well-known symmetry-preserving, gapless surface termination of an ETI supports an odd number of Dirac cones. In this talk, I will show that in the presence of strong interactions, an ETI surface can actually be gapped and symmetry preserving, at the cost of carrying an intrinsic two-dimensional topological order. I will argue that such a topologically ordered phase can be obtained from the surface superconductor by proliferating the flux 2hc/e vortex. The resulting topological order consists of two sectors: a Moore-Read sector, which supports non-Abelian charge e/4 anyons, and an Abelian anti-semion sector, which is electrically neutral. The time-reversal and particle number symmetries are realized in this surface phase in an ``anomalous'' way: one which is impossible in a strictly 2d system. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:39PM |
B55.00004: Classification of interacting electronic topological insulators in three dimensions Invited Speaker: Chong Wang A fundamental open problem in condensed matter physics is how the dichotomy between conventional and topological band insulators is modified in the presence of strong electron interactions. We show that there are 6 new electronic topological insulators that have no non-interacting counterpart. Combined with the previously known band-insulators, these produce a total of 8 topologically distinct phases. Two of the new topological insulators have a simple physical description as Mott insulators in which the electron spins form spin analogs of the familiar topological band-insulator. The remaining are obtained as combinations of these two ``topological paramagnets'' and the topological band insulator. We prove that these 8 phases form a complete list of all possible interacting topological insulators, and are classified by a $Z_2^3$ group-structure. Experimental signatures are also discussed for these phases. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 2:15PM |
B55.00005: Stability of Topological Superconductors to Interactions and Surface Topological Order Invited Speaker: Lukasz Fidkowski Three-dimensional topological superconductors protected by time reversal symmetry are characterized by gapless Majorana cones on their surface. Free-fermion phases with this symmetry (class DIII) are indexed by an integer $Z$, of which $\nu=1$ is realized by the B phase of superfluid $^3 He$. Previously, it was believed that the surface must be gapless unless time-reversal symmetry is broken. In this talk, we argue that a fully symmetric and gapped surface is possible in the presence of strong interactions, if a special type of topological order appears on the surface. The topological order realizes time reversal symmetry in an anomalous way, one that is impossible to achieve in purely two dimensions. For odd $\nu$, the surface topological order must be non-Abelian, and propose the simplest non-Abelian topological order that contains electronlike excitations, $SO(3)_6$, with four quasiparticles, as a candidate surface state. We also discuss Abelian theories for the surface $\nu=2,4,8$; one particular consequence of our scheme is that $\nu=16$ admits a trivially gapped time reversal symmetric surface. [Preview Abstract] |
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