Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B21: Electrons, Phonons, Electron Phonon Scattering and Phononics IFocus

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Sponsoring Units: DCOMP DMP Chair: David Singh, Univ of Missouri  Columbia Room: LACC 309 
Monday, March 5, 2018 11:15AM  11:51AM 
B21.00001: Force multipliers for the discovery of advanced thermoelectric materials Invited Speaker: Eric Toberer This talk will focus on the development of advanced thermoelectric materials within the Materials Genome Initiative paradigm and the prospects for widespread thermaltoelectric power conversion. These thermoelectric material discovery efforts are driven by a close coupling of theory, computation, and experimental validation. The implementation of a high throughput search of known and hypothetical compounds for thermoelectric performance (NSFDMREF) has led to the identification of new classes of thermoelectric materials. Further material development involves demonstrating materials with exceptionally strong phononpoint defect scattering crosssections and strong lattice anharmonicity. Complementary studies (NSFCAREER) focus on electronic structure selection for low Lorenz numberand tunable charge carrier scattering. These high throughput computational searches find synergy with combinatorial synthesis methods and automated measurements of sample transport properties. Together, these efforts serve as force multipliers for discovering the next generation of thermoelectric materials. 
Monday, March 5, 2018 11:51AM  12:03PM 
B21.00002: Ab initio TemperatureDependent Phonons and ElectronPhonon Coupling in SrTiO_{3} JinJian Zhou, Olle Hellman, Marco Bernardi Strong anharmonicity in perovskites and other structurally complex semiconductors and oxides leads to temperature dependent phonon frequencies and phase transitions, both of which pose a challenge to computing electronphonon scattering from first principles. Here, we show an approach to calculate temperaturedependent lattice vibrations and electronphonon coupling in such strongly anharmonic crystals. Our method combines density functional perturbation theory (DFPT), the temperature dependent effective potential (TDEP) method, and a new approach to correctly include in the lattice dynamics the longrange dipoledipole interactions. We apply this method to the canonical perovskite SrTiO_{3}, for which we compute the phonon dispersions, electronphonon coupling (including the soft modes) and electron mobility for a range of temperatures spanning the tetragonaltocubic phase transition. Our work enables ab initio calculations of electronphonon scattering and charge transport in perovskites and other anharmonic crystals with multiple phase transitions. 
Monday, March 5, 2018 12:03PM  12:15PM 
B21.00003: Electronic fitness function for screening semiconductors as thermoelectric materials Jifeng Sun, Guangzong Xing, Yuwei Li, Xiaofeng Fan, Weitao Zheng, David Singh Thermoelectric performance is a contraindicated property of matter that requires combinations of transport properties that do not occur in ordinary materials. Finding new high performance thermoelectrics requires methods for efficiently screening compounds to detect the unusual electronic structures that can decouple transport quantities, especially for power factor, the Seebeck coefficient and the conductivity. Here we present and test a simple transport function that can be efficiently calculated using standard methods and which identifies materials that decouple these transport quantities. It is large for band structures that overcome the inverse relationship between σ and S regardless of the specific characteristic that does this, i.e. complex band shapes, multiple valleys, heavylight band mixtures, band convergence, valley anisotropy, and other features. Thus this transport function provides a simple and easily used way to screen materials for potential TE performance 
Monday, March 5, 2018 12:15PM  12:27PM 
B21.00004: Abinitio thermal properties of semiconductors with higher order anharmonicities Navaneetha Krishnan Ravichandran, David Broido

Monday, March 5, 2018 12:27PM  12:39PM 
B21.00005: Renormalized lattice dynamics properties from highorder phononphonon interactions Yi Xia, Maria Chan Accurate modeling of phonon properties is important for predictive modeling of thermal transport properties. Boltzmann transport equation (BTE) combined with firstprinciples computed phonon scattering rate has in recent years been used to model lattice thermal conductivity (κ_{L}) in bulk crystals and their alloys. The essential quantities entering BTE are phonon frequencies and lifetimes, which are regularly computed in the harmonic approximation and including only threephonon process. In some cases, it may be necessary to include higherorder phononphonon interactions and temperature effects to achieve sufficient accuracy for comparison with experiments. In this talk, we will present a computational scheme to (1) efficiently correct phonon frequency shift by accounting for temperatureinduced phonon renormalization and (2) include additional phonon scattering from fourphonon processes, based on highorder interatomic force constants (IFCs) extracted from compressive lattice dynamics (CSLD). We validate our method and implementation by directly comparing results to IFCbased molecular dynamics simulations. We further demonstrate the capabilities of our scheme to accurately model lattice thermal transport properties at high temperatures through a case study of NaCl and PbTe. 
Monday, March 5, 2018 12:39PM  12:51PM 
B21.00006: Renormalized lattice dynamics and thermal transport of VO_{2} Maria Chan, Yi Xia Vanadium dioxide (VO_{2}) undergoes a firstorder metalinsulator transition (MIT) upon cooling near room temperature. The MIT is concomitant with structural changes from rutile to monoclinic, and an accurate characterization of lattice vibrations is vital for elucidating the underlying phase transition mechanism. To investigate the lattice dynamics and thermal transport properties of VO_{2} across the MIT, we demonstrate a phonon renormalization scheme based on selfconsistent phonon theory through iteratively refining vibrational free energy. Using this technique, we compute temperaturedependent phonon dispersion and lifetime, identify extremely strong anharmonicity associated with lowlying zonecenter optical mode, and point out the importance of both magnetic and vibrational entropy in driving MIT. We reveal that lattice thermal conductivity of rutile VO_{2} is nearly temperature independent as a result of the strong intrinsic anharmonicity, while that of monoclinic VO_{2} varies according to 1/T. Based on the good agreement between our predicted and experimentally measured phonon dispersion and lifetime, we will further comment on a recently identified strong violation of WiedemannFranz law in rutile VO_{2}. 
Monday, March 5, 2018 12:51PM  1:03PM 
B21.00007: Phonon Diffraction and Dimensionality Crossover in PhononInterface Scattering Riley Hanus, Anupam Garg, Jeff Snyder This theoretical work provides several mechanistic understandings of phononinterface scattering. The treatment provided here defines the interface as an array of linear defects, as opposed to a structureless planar defect which is inherent in current models. This is the basis on which prevailing models for the structure and energy of interfaces are grounded, and we extend this definition to phononinterface scattering. We apply the analytical expression derived here to phononGB strain field scattering from a symmetric tilt grain boundary. It is shown that phonon diffraction conditions arise from the periodic nature of these defect arrays as can be expected from the wavelike nature of phonons. Furthermore, for diffuse heat conduction, a dimensionality crossover is observed in the frequency (ω) dependence of the scattering rate which arrises from phase space considerations. This crossover in defect dimensionality provides a mechanistic understanding for ωdependence in phononinterface scattering, transmissivity, specularity, and the T^{2} behavior in the low temperature lattice thermal conductivity of polycrystalline and nanocrystalline materials. 
Monday, March 5, 2018 1:03PM  1:15PM 
B21.00008: Phonons at finite temperature Olle Hellman We present recent developments using the temperature dependent effective potential technique (TDEP) to model strongly nonharmonic materials. The method employs model Hamiltonians that explicitly depend on temperature. I will present applications pertaining to thermal conductivity, inelastic neutron spectra and phase stabilities. In addition, we investigate the nonadiabatic electronphonon coupling and their influence on phonon spectra, and recent additions to that deal with nuclear quantum effects and efficient stochastic sampling. 
Monday, March 5, 2018 1:15PM  1:27PM 
B21.00009: Revisiting the Lower Limit of Crystal Structure Thermal Conductivity Saikat Mukhopadhyay, David Parker, Brian Sales, Alexander Puretzky, Lucas Lindsay Phonon picture for thermal transport fails in disordered/amorphous materials as atomic vibrations are thought to carry heat by random walk among uncorrelated Einstein oscillators. Crystalline materials with low thermal conductivity (κ) are often compared with models of this ‘amorphous limit (κ_{min})’. Given the periodicity in bulk crystals, κ values are usually much higher than κ_{min}. Here we identify a moderately simple crystal, Tl_{3}VSe_{4}, with calculated κ at 300K lower than κ_{min} and unusually, lower than measured κ values: 0.16/0.30 W/mK (calculated/measured). Phonon lifetimes (calculated/measured) are so short that they may be considered “illdefined” by typical definitions. However, both measured Raman structure and temperature dependent κ show typical phononcharacteristics. We find that measured κ is wellexplained when considering two independent conduction channels from (1) typical phonons and (2) uncorrelated Einstein oscillators, suggesting that a measure of κ_{min }may be more accurate when considering both. This work draws into question the criteria that distinguish crystalline from amorphous vibrational behaviors. 
Monday, March 5, 2018 1:27PM  1:39PM 
B21.00010: Minimum thermal conductivity in the context of diffusonmediated thermal transport Matthias Agne, Riley Hanus, Jeff Snyder The concept of minimum thermal conductivity, κ_{min}, is important for materials screening and device design, particularly for thermoelectric and thermal barrier technologies. Unlike current phononbased models of κ_{min} [Phys. Rev. B 46, 6131 (1992); Surf. Coat. Technol. 163, 6774 (2003)], this study recognizes that atomic vibrations in complex materials at high temperature may be better described by diffuson quasiparticles [Philos. Mag. B 79, 17151731 (1999)]. Using the experimentally determined vibrational density of states, we derive the diffuson thermal conductivity, κ_{diff}, and show that it is defined by the average vibrational frequency, ω_{avg}. Furthermore, ω_{avg} is found to be highly correlated with the Debye temperature, allowing κ_{diff} to be estimated from readily accessible speed of sound measurements. Using κ_{diff} as an estimate of κ_{min} gives values that are 37% lower than the Cahill model and 18% lower than the Clarke model, which may reconcile some experimental findings of thermal conductivity below κ_{min}. Additionally, κ_{diff} is proposed to be a good metric for identifying materials with extraordinary physics leading to ultralow thermal conductivity, such as phonon focusing. 
Monday, March 5, 2018 1:39PM  1:51PM 
B21.00011: Electronic and vibrational properties of Ba_{3}XN (X= Bi, and Sb): Atom intercalation influence on transport anisotropy Tribhuwan Pandey, Lucas Lindsay, David Parker The electronic structure and lattice thermal conductivity (κ) of hexagonal antiperovskite Ba_{3}XN compounds are studied by employing firstprinciples density functional theory and Boltzmann transport calculations. For Ba_{3}N we find significant intrinsic anisotropy (κ_{c}/κ_{a} ~ 7.5) in thermal conductivity, with the crossplane direction having the higher conductivity. Interestingly, introducing Bi/Sb atoms (Ba_{3}SbN/Ba_{3}BiN) has little effect on the inplane thermal conductivity (κ_{a}), however κ_{c} decreases significantly, reducing the anisotropy κ_{c}/κ_{a} ~ 3.5. The anisotropy in κ for these systems originates from different bonding along the crystallographic directions, specifically from relative weak bonding between the layers in the hexagonal plane, which leads to anisotropic phonon group velocities. These results will be discussed in terms of structural bonding, group velocities and phonon life times. In particular the unusual behavior of thermal conductivity for Bi/Sb compounds will be discussed. 
Monday, March 5, 2018 1:51PM  2:03PM 
B21.00012: Thin film resistivity scaling of metals with conduction band anisotropy Miguel De Clercq, Kristof Moors, Wim Magnus, Bart Soree The resistivity of metal thin films is generally understood to increase with decreasing film thickness due to increased boundary surface and grain boundary scattering, the latter being a direct consequence of the average grain size typically reducing for thinner films. Recently, several experiments and ab initio simulations have demonstrated a dependency of resistivity scaling on the crystal orientation of the film, particularly in the case of an anisotropic Fermi surface. This anisotropy cannot be captured by the commonly used MayadasShatzkes resistivity scaling model, which adopts an isotropic effective mass approximation for the electrons. As a qualitative understanding of the impact of conduction band anisotropy is currently lacking, we have extended the MayadasShatzes approach to account for grain boundary and boundary surface scattering as well as the anisotropy of the electronic structure. Recently, we calibrated the extended model with Fermi surfaces obtained from ab initio simulations and successfully applied the model to Cu and Ru thin films, with a nearly isotropic and anisotropic Fermi surface respectively (arXiv:1711.00796). 
Monday, March 5, 2018 2:03PM  2:15PM 
B21.00013: Fecontent influence on the lattice dynamics and electronphonon coupling of Mn_{1x}Fe_{x}Si Paola Gonzalez, Omar De la Peña Seaman, Rolf Heid, KlausPeter Bohnen We have studied the lattice dynamics and electronphonon (eph) coupling properties of the Mn_{1x}Fe_{x}Si alloy as a function of Fecontent. This system have been analyzed within the framework of density functional perturbation theory, using a mixedbasis pseudopotential method and the virtual crystal approximation (VCA) for modeling the alloy. In particular, the phonon dispersion as well as the phonon linewidth (γ), and the electronphonon coupling (λ) have been calculated and analyzed with and without the inclusion of spin polarization. The observed evolution of particular phonon modes as a function of Fecontent is highly dependent of the spinpolarization, indicating the existence of a correlation between the spin ordering and the lattice dynamics. Such correlation is easily observed from the behavior of phonon linewidths and the electronphonon coupling, and is discussed in terms of the interplay of electronic and vibrational properties of the alloy. 
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