Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session X22: Electrons, Phonons, ElectronPhonon Scattering and Phononics VIIIFocus

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Sponsoring Units: DCOMP DMP Chair: David Broido, Boston Coll Room: BCEC 157C 
Friday, March 8, 2019 8:00AM  8:36AM 
X22.00001: Computational quest for highmobility 2D materials Invited Speaker: Thibault Sohier Recently, we identified close to 2000 exfoliable 2D materials from firstprinciples calculations [1]. The next natural step is to explore their properties and look for novel or improved performance. Here, we are searching for electrostaticallydoped 2D semiconductors with superior electronic transport properties. 
Friday, March 8, 2019 8:36AM  8:48AM 
X22.00002: Lattice dynamics and thermal conductivity in antiferromagnetic semiconductor MnTe Lucas Lindsay, Sai Mu, Raphael Hermann, Stéphane Gorsse, Huaizhou Zhao, Randy Fishman We discuss electronic, magnetic, vibrational and transport properties of the antiferromagnetic semiconductor MnTe. This work derives physically justified Coulomb repulsion, magnetic structure and exchange parameters from density functional theory and the Heisenberg model. Calculated vibrational density of states, magnon and phonon dispersions compare favorably with literature values and our measurements from neutron scattering experiments. First principles PeierlsBoltzmann phonon transport calculations are also presented and show good agreement with measured values. Our work demonstrates that vibrational and magnetic degrees of freedom do not strongly couple in MnTe. 
Friday, March 8, 2019 8:48AM  9:00AM 
X22.00003: Thermal conductivity of perovskite KTaO_{3} and PbTiO_{3} from first principles Yuhao Fu, David Singh The low thermal conductivity of piezoelectric perovskites is a challenge for high power transducer applications. We report first principles calculations of the thermal conductivity of ferroelectric PbTiO_{3} and the cubic nearly ferroelectric perovskite KTaO_{3}. The calculated thermal conductivity of PbTiO_{3} is much lower than that of KTaO_{3} in accord with experiment. Analysis of the results shows that the reason for the low thermal conductivity of PbTiO_{3} is the presence of low frequency optical phonons associated with the polar modes. These are less dispersive in PbTiO_{3}, leading to a large three phonon scattering phase space. These differences between the two materials are associated with the Asite driven ferroelectricity of PbTiO_{3} in contrast to the Bsite driven near ferroelectricity of KTaO_{3}. The results are discussed in the context of modification of the thermal conductivity of electroactive materials. 
Friday, March 8, 2019 9:00AM  9:12AM 
X22.00004: Unified theory of thermal transport in crystals and disordered solids Michele Simoncelli, Francesco Mauri, Nicola Marzari The phonon Boltzmann equation formulated by Peierls [1] describes the heat conduction in perfectly ordered solids in terms of interacting phonon wavepackets. Several methods have been recently developed to solve this equation in a numerically exact way, allowing to determine the thermal conductivity of crystals [2,3] 
Friday, March 8, 2019 9:12AM  9:24AM 
X22.00005: Characterization of Lattice Thermal Transport in TwoDimensional BAs, BP, and BSb: A FirstPrinciples Study Charles Shi, Xuan Luo Recently, bulk BAs has been confirmed experimentally to have a room temperature thermal conductivity of around 1,100 W/mK. However, the monolayer hexagonal form of boron arsenide (hBAs) has seldom been studied. Here, we use a firstprinciples approach and solve the Boltzmann Transport Equation for phonons to obtain a surprisingly high thermal conductivity in this material. We determine hBAs to have a much lower Debye temperature and average phonon group velocity compared to the other monolayer boronV compounds of boron nitride (hBN) and boron phosphide (hBP), yet curiously it possesses a higher thermal conductivity. Further investigation reveals that this is due to the large phonon frequency gap caused by large mass imbalances, where there is a restricted Umklapp phononphonon scattering channel and consequently results in a higher thermal conductivity. We determine the intrinsic lattice thermal conductivity of monolayer BAs to be 362.62 W/mK at room temperature, which is considerably higher compared to the other monolayer boronV compounds of hBN (231.96 W/mK), hBP (187.11 W/mK), and hBSb (87.15 W/mK). This study opens the door for investigation into a new class of monolayer structures and the properties they possess. 
Friday, March 8, 2019 9:24AM  9:36AM 
X22.00006: High Temperature Thermal Conductivity by Machine Learning driven Atomistic Simulations Xin Qian, Ronggui Yang Understanding thermal transport is important for many high temperature applications. However, the conventional ab initio lattice dynamics method ignored the temperature dependence of force constants, thus cannot be applied to energetically unstable crystals including body centered cubic phase of group IV metals, CmCm phase of SnSe and βCu2Se, which all display soft phonon modes at static limit while they are all stable phases at high temperatures. Conventionally, this instability problem is approached by iteratively solving the high temperature harmonic force constants, but predicting thermal conductivity requires third and even forthorder anharmonic force constants. In addition, the accuracy of the force constants are significantly affected by the artificial truncation of the Taylor expansion of the potential energy surface. Using machine learning, we developed interatomic potentials for these energeticallyunstable high temperature phases to study the thermal conductivity. Using machine learningdriven molecular dynamics, the soft phonon modes are observed to renormalize to a positive frequency and the thermal conductivity is predicted using GreenKubo method including all orders of phonon anharmonicity. 
Friday, March 8, 2019 9:36AM  9:48AM 
X22.00007: Orbitally driven low thermal conductivity of monolayer gallium nitride (GaN) with planar honeycomb structure: a comparative study Zhenzhen Qin, Guangzhao Qin, Xu Zuo, Zhihua Xiong, Ming Hu Monolayer gallium nitride (ML GaN) with honeycomb structure was successfully fabricated recently in experiments, generating enormous research interest for its promising applications in nano and optoelectronics. By solving the Boltzmann transport equation (BTE) based on firstprinciples calculations, we performed a comprehensive study of the phonon transport properties of ML GaN, with detailed comparison to bulk GaN, 2D graphene, silicene and ML BN with similar honeycomb structure. We find that the thermal conductivity (κ) of ML GaN (14.93 W/mK) is more than two orders of magnitude lower than that of graphene and is even lower than that of silicene with a buckled structure. Systematic analysis is performed based on the study of the contribution from phonon branches, comparison among the phonon mode levels and phonon anharmonicity. Further deep insight is gained from the electronic structure. Resulting from the special sp orbital hybridization mediated by the Gad orbital in ML GaN, the strongly polarized Ga–N bond and its inhomogeneous distribution could lead to the intrinsic low κ of ML GaN. The unraveled orbitally driven low κ of ML GaN offers fundamental understanding of phonon transport and will shed light on further studies of phonon transport in 2D materials . 
Friday, March 8, 2019 9:48AM  10:00AM 
X22.00008: Anomalous phonons in Pb_{0.5}Sn_{0.5}Te Aashish Sapkota, John Tranquada, Genda Gu, Igor Zaliznyak, Yangmu Li, Barry L. Winn The (Pb_{0.5}Sn_{0.5})_{1x}In_{x}Te system is of current interest because of its potential connection to topological superconductivity [1,2]. Recent inelastic neutron scattering (INS) measurements on powder samples of (Pb_{0.5}Sn_{0.5})_{1x}In_{x}Te, x= 0 and 0.3, reported unexpected lowenergy peaks (in the range of 1 to 2.5 meV) in the phonon density of states, which presently lack theoretical explanation [2]. To further determine the nature of these lowenergy phonons, we recently performed an INS measurement on a Pb_{0.5}Sn_{0.5}Te (x = 0) crystal. The results reveal that dispersing longitudinal and transverse phonons are not the contributor. Instead, there exist diffusive phonons with spectral weight in the energy range of 1 to 2.5 meV and characteristics deviating from conventional behavior. This suggests that some localized nondispersive modes are the source for the anomalous lowenergy peaks in the density of states. However, a proper understandig of these diffusive excitations will require further study. [1] X.L. Qi and S.C. Zhang, Rev. Mod. Phys. 83, 1057 (2011), [2] K. Ran et al., Phys. Rev. B 97, 220502(R) (2018). 
Friday, March 8, 2019 10:00AM  10:12AM 
X22.00009: Unified ab initio thermal transport theory for insulators Navaneetha Krishnan Ravichandran, David A Broido

Friday, March 8, 2019 10:12AM  10:24AM 
X22.00010: First principles investigation of the impact of symmetry and dimensionality on thermal transport Tribhuwan Pandey, Carlos Polanco, Valentino R. Cooper, David Parker, Lucas Lindsey Symmetry and dimensionality reduction have a profound impact on lattice dynamics, phonon scattering and thermal transport. Here, we will present new insights developed from predictive first principles Boltzmann transport theory in 1DBa_{3}N and 2DInSe and compare them with their bulk counterparts. In particular, for 1D materials new symmetrybased selection rules will be introduced and their effect on thermal conductivity will be assessed. Furthermore, the challenges related to thermal transport modelling in low dimensional materials will be discussed. 
Friday, March 8, 2019 10:24AM  10:36AM 
X22.00011: Characterization of Analytical Thermal Conductivity Models Through Assesment Against Experiment and Simulation Ramya Gurunathan, Riley Hanus, Maxwell Dylla, Jeff Snyder The Klemens/Callaway equations [Phys Rev 119 (2), 507509 (1960)] model lattice thermal conductivity (κ_{L} ) versus temperature and impurity concentration. This framework highlights the dominant phonon scattering mechanisms in a system, and is used for routine interpretation of experimental trends. The model predictions are often regarded as rough estimates due to the assumption of a monatomic lattice and Debye model dispersion. In this study, the Klemens model is applied to both experimental measurements and firstprinciples simulations of κ_{L}, where imperfections are introduced to scatter phonons. We demonstrate the proper application of this model to materials with complex unit cells, and resolve discrepancies over model inputs that can yield a factor of 210 difference in the predicted κ_{L} values. Additionally, the model is demonstrated to provide robust predictions of point defect scattering strength in a wide range of materials, whose dispersion relations are known to deviate significantly from the Debye model. We demonstrate how the dispersion relation dependence of the model is, in practice, lifted, allowing for surprising agreement between analytical theory and experiment. Thus, we provide justification for using this model on systems with arbitrary dispersion relations. 
Friday, March 8, 2019 10:36AM  10:48AM 
X22.00012: Thermal conductivity of wide band gap nitrides from first principles calculations Sahil Dagli, Kelsey Mengle, Emmanouil Kioupakis Wide band gap nitrides are of interest for applications in deepUV optoelectronics and power electronics. Materials for these devices must be able to dissipate heat generated from operation, making thermal conductivity an important parameter for highpower applications. We have investigated the thermal conductivity of AlN, GaN, Al_{x}Ga_{1x}N and BN polymorphs with firstprinciples calculations, with a focus on effects from compositional and isotopic disorder. We use density functional theory to calculate the phonon frequencies and interatomic force constants. This information is used to calculate the thermal conductivity using the solution from the full Boltzmann transport equation calculation. Our results provide understanding on the benefits of using isotopically pure compositions in increasing the thermal conductivity of wide band gap nitrides. 
Friday, March 8, 2019 10:48AM  11:00AM 
X22.00013: Thermal transport through GaNAlN interfaces with mass and force variance from firstprinciples Carlos Polanco, Lucas Lindsey We present thermal conductance calculations of a GaNAlN interface using a Landauer formalism with all interatomic force constants (IFCs), including those relaxed at the interface, computed from density functional theory. A thorough convergence study of the conductance value is outlined, focusing on the effects of truncating longrange IFCs from polar materials and properly enforcing various symmetries and invariances. For example, enforcing the simple acoustic sum rule instead of using more advanced methods overestimates the conductance by 7% with a 35% overestimation of the optical phonon contributions. We compare our fully firstprinciples conductance with those using empirical mixing rules to define interfacial IFCs. Moreover, we contrast our calculations with values inferred from thermal conductivity measurements of GaNAlN superlattices. Our calculations constitute initial steps towards a fully predictive framework of interfacial thermal conductance. 
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