Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J44: Electrons, Phonons, Electron Phonon Scattering, and Phononics IIFocus

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Sponsoring Units: DCOMP DMP Chair: David Singh, Univ of Missouri  Columbia Room: 704 
Tuesday, March 3, 2020 2:30PM  3:06PM 
J44.00001: Quantifying Uncertainty in FirstPrinciples Predictions of Phonon Properties and Thermal Conductivity Invited Speaker: Alan McGaughey We present a robust method for quantifying the uncertainty in phonon properties and thermal conductivity predicted from density functional theory calculations using the BEEFvdW exchangecorrelation (XC) functional. The procedure starts by displacing atoms in an equilibrium structure and using the energies of the perturbed structures to determine harmonic and anharmonic force constants. BEEFvdW generates an ensemble of energies for each perturbed structure as a computationally efficient postprocessing step by perturbing the XC functional and solving for the energy nonself consistently. Thus, each perturbed structure yields an ensemble of energies. This energy ensemble is then used to determine an ensemble of force constants, which is then used as input to lattice dynamics calculations and a solution of the Boltzmann transport equation. This procedure results in ensembles for the phonon frequencies, group velocities, and lifetimes, and overall heat capacity and thermal conductivity, whose spreads can be used to quantify uncertainty. Results for silicon, graphene, and graphite are presented and compared to predictions from the PBE, RPBE, and PBEsol XC functionals. 
Tuesday, March 3, 2020 3:06PM  3:18PM 
J44.00002: From Coherent State Statistics to the Frozen Phonon Model Christopher Fechisin, Eric Johnson Heller We present a formalism for employing the basis of coherent states for analytical and numerical computations involving systems at thermal equilibrium. We identify an integration measure on the complex plane which may be understood as the weight function for a thermal distribution of coherent states. When applied to phonons in a thermally occupied harmonic crystal, this method yields an intuitive and rigorously quantum mechanical justification of the frozen phonon model of thermal diffuse scattering. In this and many other applications, the semiclassical dynamics of coherent states can mediate the application of classical reasoning to otherwise intractable quantum problems, including those involving anharmonic potentials. We also discuss and refine the concept of “quasielastic” scattering. 
Tuesday, March 3, 2020 3:18PM  3:30PM 
J44.00003: Anomalous elasticity of 2D materials beyond selfconsistent approximation David Saykin, Igor Burmistrov, Valentin Kachorovskii, Igor Gornyi We study elastic properties of twodimensional crystalline materials, such as graphene. It is known that in 2D membranes strong thermal fluctuations of flexural phonons lead to dramatic change of phonon spectrum and, as a result, in anomalous materialindependent elastic properties, such as nonlinear Hooke's law under the low stress and auxetic behavior. 
Tuesday, March 3, 2020 3:30PM  3:42PM 
J44.00004: Continuity of phonon dispersion curves in layered ionic crystals Natalie A Holzwarth, Yan Li, William Kerr We investigate in detail the origin of apparent discontinuities and mode disappearances in phonon band diagrams of ionic materials having hexagonal and other anisotropic structures.^{1} The phenomenon is due to the coupling of some of the vibrational modes to long wavelength electromagnetic waves within the material as described in 1951 by Huang.^{2} Modern analyses by Giannozzi, Gonze, Baroni, and others, based on density functional theory and density functional perturbation theory, have been implemented in several first principles code packages such as ABINIT and QUANTUM ESPRESSO. These use the socalled nonanalytic correction to the dynamical matrix to correctly represent the modified longitudinal optical vibrational modes. In this work, we extend the analysis to include the transverse phononphoton modes as well. The combination of the longitudinal and transverse phononphoton mode dispersions are continuous functions of the wavevector q. These effects are demonstrated for cubic and hexagonal BN. 
Tuesday, March 3, 2020 3:42PM  3:54PM 
J44.00005: Theoretical Analysis of Vibrational Lineshapes from Molecular Dynamics Andrew Cupo, Damien Tristant, Kyle Rego, Vincent Meunier The conventional spectral method for extracting anharmonic phonon properties from molecular dynamics (MD) requires prohibitively long simulations as the fitting function relies on the infinite time approximation. To that end, we derived the spectral lineshapes for arbitrary simulation lengths, while retaining the frequency shift and lifetime as fitting parameters. The theory was illustrated for graphene, hexagonal boron nitride, and silicon at the density functional theory (DFT) level, with up to nearly a factor of nine reduction in the required simulation time to reach convergence in the vibrational properties as compared to the standard approach. Such improvement in the convergence is expected in general provided the phonon anharmonicity is sufficiently weak, resulting in welldefined renormalized phonon quasiparticles. Application of the proposed approach has the potential to be far reaching as the theory applies equally well to ab initio MD based on DFT, timedependent DFT dynamics, and parameterized forcefields and is thus expected to have important impact on topics ranging from stronglycorrelated materials with sophisticated treatment of electronelectron interactions to biological systems. 
Tuesday, March 3, 2020 3:54PM  4:06PM 
J44.00006: Influence of spinorbit coupling and Rashba interaction on the electronphonon renormalized electronic energy levels Véronique BrousseauCouture, Xavier Gonze, Michel Cote Electronphonon (ep) interaction calculations from firstprinciples are well documented in the literature. The predominance of nonadiabatic effects in the zeropoint renormalization (ZPR) of the band gap for polar materials has been recently assessed in the light of the Fröhlich interaction. 
Tuesday, March 3, 2020 4:06PM  4:18PM 
J44.00007: Anharmonicity in Zirconium Hydrides: a firstprinciples study combined with inelastic neutron scattering Jiayong Zhang, Yongqiang Cheng, Alexander Kolesnikov, Jerry Bernholc, Wenchang Lu, Anibal J. RamirezCuesta Zirconium Hydrides and Deuterides are candidates for neutron moderators and fuelrod cladding materials. We investigate anharmonic phenomena in these materials using inelastic neutron scattering (INS) and lattice dynamics calculations within the framework of density functional theory (DFT). We observe multiple sharp peaks below harmonic (free) multiphonon bands in the experimental spectra, which do not show up in the simulated INS spectra based on the harmonic approximation. We have thus carried out a detailed study of the anharmonicity by exploring the 2D potential energy surface with DFT calculations and solving the corresponding 2D singleparticle Schrodinger equation of the H/D atom in zirconium hydrides/deuterides to get the eigenfrequencies. The results describe well the experimental INS spectra, showing harmonic behavior of the fundamental modes and strong anharmonicity at higher energies. The DFT calculations were carried out with the realspace multigrid (RMG) and VASP codes, and the experimental INS spectra were measured with the VISION spectrometer at the Spallation Neutron Source, Oak Ridge National Laboratory. 
Tuesday, March 3, 2020 4:18PM  4:30PM 
J44.00008: Phononinduced topological phase transition in SnTe Jose D. QueralesFlores, Pablo AguadoPuente, Dorde Dangic, Jiang Cao, Tchavdar Todorov, Myrta Grüning, Stephen B Fahy, Ivana Savic Unlike the topologically trivial semiconductor PbTe, SnTe has an inverted band gap at the L point that gives rise to a topological crystalline insulating phase protected by mirror symmetry [1]. In this work, we calculate the temperature renormalization of the electronic band structure of SnTe. We account for the energy shift of the electronic states due to thermal expansion, and electronphonon interaction using the nonadiabatic AllenHeineCardona formalism within density functional perturbation theory [2,3]. Corrections to the electronic band structure due to electronelectron interaction are obtained using manybody perturbation theory (GW). We capture the decrease of the direct gap with temperature yielding a temperatureinduced phase transition to a topologically trivial phase at ~800 K. We find that both thermal expansion and electronphonon interaction have a considerable effect on these temperature variations. We also analyze the temperature dependence of the electronphonon selfenergy. 
Tuesday, March 3, 2020 4:30PM  4:42PM 
J44.00009: Phonon renormalization and fourphonon scattering in semiconductors and insulators Navaneetha Krishnan Ravichandran, David Broido

Tuesday, March 3, 2020 4:42PM  4:54PM 
J44.00010: Abinitio calculation of Seebeck coefficient of transitionmetal elements Hisazumi Akai, Sonju Ko Abinitio calculation of the Seebeck coefficient S of transition metal elements are performed within the framework of Kubo–Greenwood formula. The difficult points of calculating S are that, firstly, at T = 0K, the conductivity of pure metal diverges. Second, the Fermi surfaces of transition metals composed of many different states where the constant relaxation time approximation breaks down. To overcome the difficulties, we included the effects of electronphonon scattering in the calculation of S. We exploited the Korringa–Kohn–Rostoker (KKR) Green's function method combined with the Kubo–Greenwood formula. The electronphonon scattering was taken into account through abinitio phonon calculations and an alloy analogy applied to the local static phonons. The KKR coherent potential approximation (KKR–CPA) was used for the latter. The calculated Cu resistivity and the Seebeck coefficients for various transitionmetal elements at finite temperature show reasonable overall agreements with experiments. The present approach provides us with a framework applicable to a wide range of materials, including pure metals, compounds, ordered and disordered alloys. 
Tuesday, March 3, 2020 4:54PM  5:06PM 
J44.00011: Understanding the Restoring Force From a Local Bonding Perspective: A FirstPrinciples Picture of Phonons and Elasticity Ethan Ritz, Guru Bahadur Khalsa, HsinYu Ko, Robert Distasio, Nicole A Benedek Though the calculation of bulk elastic quantities and phonon frequencies using modern abinitio techniques is now routine, understanding the crystal chemical origins of these properties is still an open question. How do we relate the value of a specific bulk elastic constant or vibrational mode to chemically intuitive ideas about local structure and bonding, especially when our calculations are often done in a spatially delocalized Bloch basis? Using a basis of maximally localized Wannier functions, we partition the total electronic energy onto real space representations of occupied states, as well of the curvature of that energy with respect to mechanical deformations and phonon distortions. By understanding how the energy of each individual bond changes with these distortions, we can obtain orbitally decomposed, chemically specific understanding of bulk properties. We use this approach to explore various perovskite oxides, exploring the chemical origins of elasticity, structural phase transitions, Grüneisen parameters, and thermal expansion properties, and discuss the possibility of enhancing or controlling these properties. 
Tuesday, March 3, 2020 5:06PM  5:18PM 
J44.00012: Finite temperature electronic properties of diamond and diamondoids Arpan Kundu, Marco Govoni, Michele Ceriotti, Francois Gygi, Giulia Galli Accurate calculations of electronphonon coupling are essential to predict the finite temperature (T) properties of materials and molecules, especially those containing lightatoms. We present an approach to compute electronphonon coupling which treats the motion of ions quantum mechanically, through the use of pathintegral calculations, and the electronic states at the DFT or manybodyperturbation theory (MBPT) level. In particular, we carried out simulations for diamond and diamondoids by coupling the firstprinciple molecular dynamics code Qbox (http://qboxcode.org) with iPI (http://ipicode.org), a path integral simulation package, and we obtained singleparticle energy levels within MBPT using the WEST code (http://westcode.org). We present results for different cluster sizes and surface terminations and we compare the zerotemperature limit of our simulations with results recently reported for electronphonon coupling at T=0 [1]. 
Tuesday, March 3, 2020 5:18PM  5:30PM 
J44.00013: Electronic structure and phonons in FeTi at high pressure Bethuel O Khamala, Jorge Munoz The FeTi system is amenable to computational investigations due to its simple crystal structure and minimalist Fermi surface. A thermallydriven electronic topological transition that results in anomalous phonon softening was recently reported to occur in FeTi at elevated temperatures as new features appear in the Fermi surface and new spanning vectors increase electronic screening of particular phonon modes. We investigated the pressure dependence of the electronic structure and the phonon dispersions using density functional theory and uncovered an octahedral splitting with an energy difference that increases with pressure and a Kohn anomaly with a wavevector that decreases with pressure. The calculated Fe partial phonon density of states are in agreement with nuclearresonant inelastic xray scattering measurements and show that the phonons stiffen at different rates. 
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