Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A22: Electrons, Phonons, and Electron-Phonon Scattering IFocus
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Sponsoring Units: DCOMP Chair: David Singh, University of Missouri Room: 321 |
Monday, March 14, 2016 8:00AM - 8:36AM |
A22.00001: Ab initio phonon limited transport Invited Speaker: Matthieu Verstraete We revisit the thermoelectric (TE) transport properties of two champion materials, PbTe and SnSe, using fully first principles methods. In both cases the performance of the material is due to subtle combinations of structural effects, scattering, and phase space reduction. In PbTe anharmonic effects are completely opposite to the predicted quasiharmonic evolution of phonon frequencies and to frequently (and incorrectly) cited extrapolations of experiments. This stabilizes the material at high T, but also tends to enhance its thermal conductivity, in a non linear manner, above 600 Kelvin. This explains why PbTe is in practice limited to room temperature applications. SnSe has recently been shown to be the most efficient TE material in bulk form. This is mainly due to a strongly enhanced carrier concentration and electrical conductivity, after going through a phase transition from 600 to 800 K. We calculate the transport coefficients as well as the defect concentrations ab initio, showing excellent agreement with experiment, and elucidating the origin of the double phase transition as well as the new charge carriers. AH Romero, EKU Gross, MJ Verstraete, and O Hellman PRB 91, 214310 (2015) O. Hellman, IA Abrikosov, and SI Simak, PRB 84 180301 (2011) [Preview Abstract] |
Monday, March 14, 2016 8:36AM - 8:48AM |
A22.00002: High throughput solution of Boltzmann transport equation: phonons, thermal conductivity and beyond Jose Plata, Pinku Nath, Demet Usanmaz, Cormac Toher, Marco Fornari, Marco Buongiorno Nardelli, Stefano Curtarolo Quantatively accurate predictions of the lattice thermal conductivity have important implications for key technologies ranging from thermoelectrics to thermal barrier coatings. Of the many approaches with varying computational costs and accuracy, which have been developed in the last years, the solution of the Boltzmann transport equation (BTE) is the only approach that guarantees accurate predictions of this property. We have implemented this methodology in the AFLOW [1] high throughput materials science framework, which enables us to compute these anharmonic force constants and solve BTE to obtain the lattice thermal conductivity and related properties automatically in a single step. This technique can be combined with less expensive methodologies previously implemented in AFLOW [2] to create an efficient and fast framework to accelerate the discovery of materials with interesting thermal properties. [1] S. Curtarolo et al., Comp. Mat. Sci. 58, 218 (2012). [2] C. Toher, et. al, Phys. Rev. B 90, 174107, 2014 [Preview Abstract] |
Monday, March 14, 2016 8:48AM - 9:00AM |
A22.00003: Parallel calculations of vibrational properties in complex materials: negative thermal expansion and elastic inhomogeneity. F.D. Vila, J.J. Rehr Effects of thermal vibrations are essential to obtain a more complete understanding of the properties of complex materials. For example, they are important in the analysis and simulation of x-ray absorption spectra (XAS). In previous work\footnote{F. D. Vila \textit{et al.} Phys. Rev. B \textbf{76}, 014301 (2007).} we introduced an \textit{ab initio} approach for a variety of vibrational effects, such as crystallographic and XAS Debye-Waller factors, Debye and Einstein temperatures, and thermal expansion coefficients. This approach uses theoretical dynamical matrices from which the locally-projected vibrational densities of states are obtained using a Lanczos recursion algorithm. In this talk I present recent improvements to our implementation, which permit simulations of more complex materials with up to two orders of magnitude larger simulation cells. The method takes advantage of parallelization in calculations of the dynamical matrix with VASP. To illustrate these capabilities we discuss two problems of considerable interest: negative thermal expansion in ZrW$_2$O$_8$; and local inhomogeneities in the elastic properties of supported metal nanoparticles. Both cases highlight the importance of a local treatment of vibrational properties. [Preview Abstract] |
Monday, March 14, 2016 9:00AM - 9:12AM |
A22.00004: Lattice dynamics and electron-phonon coupling calculations using nondiagonal supercells Jonathan Lloyd-Williams, Bartomeu Monserrat Quantities derived from electron-phonon coupling matrix elements require a fine sampling of the vibrational Brillouin zone. Converged results are typically not obtainable using the direct method, in which a perturbation is frozen into the system and the total energy derivatives are calculated using a finite difference approach, because the size of simulation cell needed is prohibitively large. We show that it is possible to determine the response of a periodic system to a perturbation characterized by a wave vector with reduced fractional coordinates $(m_1/n_1,m_2/n_2,m_3/n_3)$ using a supercell containing a number of primitive cells equal to the least common multiple of $n_1$, $n_2$, and $n_3$. This is accomplished by utilizing supercell matrices containing nonzero off-diagonal elements. We present the results of electron-phonon coupling calculations using the direct method to sample the vibrational Brillouin zone with grids of unprecedented size for a range of systems, including the canonical example of diamond. We also demonstrate that the use of nondiagonal supercells reduces by over an order of magnitude the computational cost of obtaining converged vibrational densities of states and phonon dispersion curves. [Preview Abstract] |
Monday, March 14, 2016 9:12AM - 9:24AM |
A22.00005: Ab Initio Electronic Relaxation Times and Transport in Noble Metals Jamal I. Mustafa, Marco Bernardi, Jeffrey B. Neaton, Steven G. Louie Relaxation times employed to study electron transport in metals are typically assumed to be constants and obtained empirically using the Drude model. Here, we employ ab initio calculations to compute the electron-phonon relaxation times of Cu, Ag, and Au, and find that they vary significantly on the Fermi surface, spanning $\sim$15$-$45 fs. We compute room temperature resistivities in excellent agreement with experiment by combining $GW$ bandstructures, Wannier-interpolated band velocities, and ab initio relaxation times. Our calculations are compared to other approximations used for the relaxation times. Additionally, an importance sampling scheme is introduced to speed up the convergence of resistivity and transport calculations by sampling directly points on the Fermi surface. [Preview Abstract] |
Monday, March 14, 2016 9:24AM - 9:36AM |
A22.00006: First-principles calculation of LO phonon scattering in BaSnO$_3$ Karthik Krishnaswamy, Burak Himmetoglu, Anderson Janotti, Chris G. Van de Walle BaSnO$_3$ (BSO) has drawn interest owing to the recent discovery of high electron mobility, highest among the perovskite materials. In our theoretical work, we calculate the electron scattering rate due to LO phonon scattering from first-principles density functional calculations. The calculated mobility is much higher than the experimentally observed value, suggesting defect scattering as the primary limiting factor in currently grown BSO samples, and that reducing the defect density can enhance BSO’s mobility significantly. [Preview Abstract] |
Monday, March 14, 2016 9:36AM - 9:48AM |
A22.00007: Consequences of ionic and covalent bonding in Ge-Sb-Te phase change materials Saikat Mukhopadhyay, Jifeng Sun, Alaska Subedi, Theo Siegrist, David Singh Structural transformation of Ge$_{\mathrm{2}}$Sb$_{\mathrm{2}}$Te$_{\mathrm{5}}$ has attracted a great deal of research as it involves two states (crystalline and amorphous) that are stable at ambient temperature but with remarkably different physical properties, in particular, very different optical constants. The differences in physical properties in these states have been explained in terms of resonant bonding that has been generalized to the description of covalent systems with high symmetry structures such as benzene and graphite. However, given the local lattice distortions noted from both experimental and theoretical investigations, it is clear that the meaning of ``resonant bonding'' in GST is very different from that in graphite or benzene and the precise nature of bonding in this phase has not been fully established. In this talk, based on our first-principles calculations, we show that there is a strong competition between ionic and covalent bonding in the cubic phase, and establish a link between the origins of phase change memory properties and giant responses of piezoelectric materials. [Preview Abstract] |
Monday, March 14, 2016 9:48AM - 10:00AM |
A22.00008: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 10:00AM - 10:12AM |
A22.00009: Novel, discontinuous polaron transition in a two-band model Mirko M. Moeller, George A. Sawatzky, Mona Berciu The coupling of charge carriers (electrons or holes) to phonons leads to the formation of a polaron, a coherent quasi-particle consisting of the charge carrier and the cloud of phonons surrounding it and moving coherently with it. Here we present exact diagonalization and momentum average approximation results for the single polaron properties of a two-band model with phonon modulated hopping, inspired by the perovskite BaBiO$_3$. For large coupling we find that the ground state momentum changes {\em discontinuously} from $k=\pi$ to $k=0$. Such sharp transitions of the polaron's ground state properties cannot occur in the well-studied models of the Holstein or Fr\" ohlich type in which the carrier-phonon coupling modulates the on-site energies. However, they can occur in models where the carrier-phonon coupling modulates the hopping integrals such as the SSH model for which a similar yet smooth transition of the ground state momentum was recently shown to exist. We compare our findings to the SSH model and point out qualitative differences which we believe to be due to the two band nature of our model versus the single band SSH model. [Preview Abstract] |
Monday, March 14, 2016 10:12AM - 10:24AM |
A22.00010: Quasiparticle properties of the nonlinear Holstein model at finite doping and temperature. Shaozhi Li, Beth Nowadnick, Steven Johnston Models with linear electron-phonon (e-ph) interactions often predict the formation of small polarons with large lattice displacements. This directly violates the approximations made in deriving the linear model, which implies that one should consider higher order terms in the interaction. Previously we have shown that even small positive nonlinear e-ph interactions dramatically suppress charge-density-wave formation and s-wave superconductivity relative to the linear model [EPL. 109, 27007 (2015)]. In this talk, we present a determinant quantum Monte Carlo study of thesingle-particle properties of quasiparticles and phonons in a two-dimensional Holstein model that includes an additional nonlinear e-ph interaction. We show that a small positive nonlinear e-ph interaction reduces the effective coupling between electrons and phonons and hardens the effective phonon frequency. Conversely, a small negative nonlinear interaction can enhance e-ph coupling resulting in heavier quasiparticles. In addition, we find that an effective linear model fails to simultaneously capture the quantitative effects of the nonlinearity of both the electronic and phononic degrees of freedom, even though it can qualitatively reproduce properties. [Preview Abstract] |
Monday, March 14, 2016 10:24AM - 10:36AM |
A22.00011: Engtanglement of nuclear spins and phonons in ideal solids. Vadim Oganesyan, Steven Morgan, Gregory Boutis We investigate quantum many-body dynamics of nuclei in solids, in particular as they are affected by dynamical excitations of the underlying matrix, i.e. phonon modulation of dipolar couplings. Our recent work documented consequences of this coupling in calcium fluoride, where small changes in the spectrum of the free induction decay (FID) were measured, roughly consistent with theoretical estimates based on a simplified elastic model. Such theory also predicts temperature dependent enhancement of the diffusion constant of dipolar order, essentially due to growth in the phonon mean-free path. [Preview Abstract] |
Monday, March 14, 2016 10:36AM - 10:48AM |
A22.00012: Chiral Phonons and Electrical Resistivity of Ferromagnetic Metals at Low Temperatures Edgardo Solano Carrillo, Andrew J. Millis In the presence of a magnetic field (produced for example by the exchange field of a ferromagnet) phonons become chiral, with left and right circularly polarized modes in addition to the longitudinal or zero-helicity mode. The scattering of spin-split electrons by chiral phonons is investigated, with particular attention to the question of whether the scattering can account for the linear resistivity observed in metallic ferromagnets at low temperature. The theory is shown to explain the observed spin relaxation time of Ni. [Preview Abstract] |
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