Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B20: Electrons, Phonons, ElectronPhonon Scattering, and Phononics IFocus Live

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Sponsoring Units: DCOMP DMP Chair: Ivana Savic, Univ Coll Cork 
Monday, March 15, 2021 11:30AM  11:42AM Live 
B20.00001: NonPerturbative Theory of Charge Transport in Crystalline Solids Christian Carbogno, Matthias Scheffler Our understanding of charge transport in solids predominantly relies on the Boltzmann transport equation. However, its perturbative approximations for the nuclear dynamics and for its coupling to the electrons can be inaccurate in complex materials [1]. We present an alternative, nonperturbative ab initio GreenKubo approach based on a new formulation of the flux viz. polarization. It can be evaluated via ab initio molecular dynamics and only requires gaugefixed properties. At variance with Berry phase approaches [2], it is thus suitable for (semi)conductors featuring thermal electronic excitations. We demonstrate our method by calculating the electrical conductivity for 2D honeycomb lattices, for silicon, and for the perovskite SrTiO_{3}. We also compare to nonperturbative KuboGreenwood calculations and discuss why this approach has so far only been numerically applicable for materials with strong structural disorder [3]. 
Monday, March 15, 2021 11:42AM  11:54AM Live 
B20.00002: Gaussian timedependent variational principle for the finitetemperature anharmonic lattice dynamics JaeMo Lihm, CheolHwan Park The anharmonic lattice is a representative interacting bosonic manybody system. The selfconsistent harmonic approximation has been used to study the equilibrium properties of the anharmonic lattices. However, to study the dynamical properties within this method, one needs to resort to a specific selfenergy ansatz, whose validity is yet to be proven. In this presentation, we apply the timedependent variational principle, a recently emerging tool for studying the dynamical properties of interacting manybody systems, to the anharmonic lattices. Using the Gaussian variational states and the linearized equation of motion method, we theoretically prove the dynamical selfenergy ansatz of the selfconsistent harmonic approximation. The calculated dynamical and spectral properties of the lattice can be understood as that of interacting 1 and 2phonon excitations. Our work lays the groundwork for a fully variational study of dynamical properties of the anharmonic lattice and also expands the range of applicability of timedependent variational principle to firstprinciple lattice Hamiltonians. 
Monday, March 15, 2021 11:54AM  12:06PM Live 
B20.00003: Phonons of solid atomic Hydrogen with quantum Monte Carlo Kevin Ly, David M Ceperley The calculation of phonon frequencies and modes is the starting point for investigating a variety of physical properties of solids, like the specific heat, thermal expansion, and properties related to electronphonon coupling, like superconductivity. For solid Hydrogen at high pressures, the zero point motion of the protons is known to be important when determining its structures. This effect can be accounted for if the phonon frequencies are known. These calculations are routinely carried out with density functional theory (DFT). Here, we calculate the phonons of solid atomic Hydrogen with quantum Monte Carlo (QMC), a feat made possible by the calculation of forces in QMC, highly optimized wavefunctions, and careful attention to statistical error. We compare the results with those obtained within DFT. 
Monday, March 15, 2021 12:06PM  12:18PM Live 
B20.00004: Ab initio calculation of Hall mobility in semiconductors Samuel Ponce, Francesco Macheda, Elena R Margine, Nicola Marzari, Nicola Bonini, Feliciano Giustino In this talk, we will probe the accuracy limit of ab initio calculations of drift [1] and Hall [2] carrier mobilities that relies on the electronphonon coupling, within the framework of the Boltzmann transport equation [3] for 10 semiconductors. 
Monday, March 15, 2021 12:18PM  12:30PM Live 
B20.00005: Computing Nonradiative Capture Coefficients from First Principles Mark Turiansky, Audrius Alkauskas, Manuel Engel, Georg Kresse, Darshana Wickramaratne, Jimmy Shen, Cyrus Dreyer, Chris Van de Walle Semiconductor devices are susceptible to defectmediated nonradiative processes that degrade their performance. In optoelectronic devices, for example, these nonradiative transitions give rise to ShockleyReadHall recombination that limits the lightemission efficiency. The nonradiative processes occur as a result of electronphonon coupling, and a rigorous evaluation of the resulting rates is of vital importance for analysis and improvement of devices. We have developed the Nonrad code, which implements the firstprinciples formalism of Alkauskas et al. [1] for the evaluation of nonradiative capture coefficients. We will discuss several improvements to the methodology, including a treatment of electronphonon coupling within the widely used projector augmentedwave method. 
Monday, March 15, 2021 12:30PM  12:42PM Live 
B20.00006: ElectronPhonon Interactions Using Wannier Functions and the ProjectorAugmentedWave Method Manuel Engel, Carla Verdi, Martijn Marsman, Georg Kresse Electronphonon interactions play a pivotal role in simulating a wide variety of material properties. As a result, there is an ongoing effort to find more accurate and efficient algorithms to incorporate these interactions in materials simulations. To this end, we present an abinitio densityfunctionaltheory (DFT) approach for calculating electronphonon interactions within the projectoraugmentedwave (PAW) method. As the PAW method leads to a generalized eigenvalue problem, the resulting electronphonon matrix elements lack some symmetries that are usually present for traditional allelectron formulations. To allow for efficient evaluation of physical properties, we use an interpolation scheme based on Wannier functions, with the required matrix elements constructed in a supercell using finite differences. While being inherently slower than most methods operating in reciprocal space, this approach has multiple advantages. Finally, we highlight results for the phononinduced bandgap renormalization for polar and nonpolar materials obtained from the implementation of this approach in the Vienna Abinitio Simulation Package (VASP). 
Monday, March 15, 2021 12:42PM  12:54PM Live 
B20.00007: From deformation potential extraction to electronic transport simulations: an efficient and practical approach Zhen Li, Patrizio Graziosi, Neophytos Neophytou In this work, we present a firstprinciples framework to compute the thermoelectric properties of materials based on the extraction and use of deformation potentials and then the corresponding scattering rates, which is the middle ground computationally between the constant relaxation time approximation (RTA) and firstprinciples relaxation time extraction. Based on density functional theory (DFT) and density functional perturbation theory (DFPT), we compute the electronic bandstructures, phonon dispersion relations, and electronphonon matrix elements. Within the polar Wannier interpolation scheme, we consider the shortrange interactions between electrons and longwavelength phonons, and the longrange optical interactions. From the shortrange electronphonon matrix elements, we derive the acoustic deformation potential (ADP) and optical deformation potential (ODP) for longwavelength phonons. The electronic structures and deformation potentials are taken as inputs to compute the charge transport coefficients using an advanced, homedeveloped numerical simulator, which allows not only for the incorporation of electronphonon scattering, but also for other (even more) important scattering mechanisms, such as ionized impurity scattering, alloy scattering, etc. 
Monday, March 15, 2021 12:54PM  1:06PM Live 
B20.00008: Ab initio calculations of electroncharged defect interactions and low temperature mobility ITe Lu, JinJian Zhou, Jinsoo Park, Marco Bernardi Charged defects scatter electrons and control transport in materials at low temperature. Theoretical descriptions of this interaction have mainly relied on simplified effective mass models, which cannot capture the atomic details of the defectinduced perturbation potential or treat materials with complex band structures. We recently developed ab initio calculations of electrondefect (ed) interactions due to neutral defects [1,2]. Here we present a method to calculate these interactions for charged defects and impurities. We employ Wannier functions to compute the relevant ed matrix elements and develop an interpolation scheme to treat the longrange part of the electroncharged defect interaction. We demonstrate results for n and pdoped silicon, including defectlimited relaxation times and mobilities, and explain subtle aspects of low temperature transport. Our approach can overcome limitations of effective mass models as it can be applied to materials with anisotropic, multivalley, or linear band structures. It provides a powerful tool to study ionized impurity scattering and low temperature transport in complex materials. 
Monday, March 15, 2021 1:06PM  1:18PM Live 
B20.00009: Neural Network Potential for Lattice Dynamics Calculations and Thermal Conductivity Prediction Jie Gong, HyunYoung Kim, Alan McGaughey Lattice dynamics calculations can predict the phonon properties of insulating and semiconducting crystals, with force constants as the primary input. Density functional theory (DFT) allows for an ab initio method to obtain these force constants accurately. These DFT calculations, however, can be computationally expensive. This is not an issue in simple materials (high symmetry and small primitive cell), where relatively few DFT calculations are necessary. In more complex materials, however, the number of calculations required will increase significantly and tax computational resources. 
Monday, March 15, 2021 1:18PM  1:30PM Live 
B20.00010: Effect of higherorder anharmonicity on the phonon lineshapes in weaklybonded solids from first principles Navaneetha Krishnan Ravichandran Recently, it has been shown computationally that the inclusion of higherorder anharmonic phonon renormalization and higherorder fourphonon scattering is crucial to accurately represent the thermal conductivities of weaklybonded solids like sodium chloride (NaCl) [1]. Here we show, using our recentlydeveloped unified firstprinciples framework, that fourphonon scattering also critically affects the phonon lineshapes of NaCl, typically observed in inelastic neutron and Raman scattering experiments. To capture these higherorder effects accurately, our calculations include the lowestorder threephonon and higherorder fourphonon scattering processes, and a manybody selfconsistent anharmonic phonon renormalization step to address the illdefined nature of phonon quasiparticles. By performing these calculations over a broad range of temperatures, we show that fourphonon processes significantly broaden the phonon lineshapes compared to their threephonon counterparts  particularly at high temperatures, and their inclusion into the calculations is pivotal to explain the experimental data. 
Monday, March 15, 2021 1:30PM  1:42PM Live 
B20.00011: Anharmonic phonon spectra of all four phases of BaTiO_{3} Ali Hamze, Valentina Lacivita, Yan Wang, JeongJu Cho The standard firstprinciples approach for calculating lattice dynamics in crystals is based on the harmonic approximation, which expands the crystal potential to secondorder. However, higherorder (anharmonic) contributions are often important, especially in materials relevant for modern electronic, optical, thermal, and energy storage applications. In such materials, the harmonic approximation yields very poor predictions; therefore, advanced computational techniques that can incorporate anharmonic effects are essential for efficient material screening for advanced device applications. We apply two such computational methods, compressive sensing lattice dynamics (CSLD) and quantum selfconsistent ab intio lattice dynamics (QSCAILD), to all four phases (cubic, tetragonal, orthorhombic, and rhombohedral) of perovskite BaTiO_{3}, a highly anharmonic crystal with promising device applications, and calculate the phonon dispersions and static dielectric constants as functions of temperature. Finally, we compare our predictions to calculations done in the harmonic approximation, and to experiment. 
Monday, March 15, 2021 1:42PM  2:18PM Live 
B20.00012: Probing nonequilibrium structural dynamics in lowdimensional and correlated materials using ultrafast diffraction Invited Speaker: Aditya Sood Novel phenomena arise from the nonequilibrium excitation of materials, and the complex interplay between their various degrees of freedom. Understanding this at a microscopic level requires detailed knowledge about a material’s atomic structure on fast timescales. In this talk, I will discuss the application of ultrafast diffraction techniques to probe structural dynamics in correlated oxides and twodimensional (2D) materials. First, I will describe a new timeresolved ‘electricalpump’ technique based on femtosecond electron diffraction, that enables direct measurements of atomic motions in microelectronic devices. In an archetypal correlated oxide, we discover signatures of a transient metastable phase under pulsed electrical bias [1]. Next, I will discuss phonon transport in van der Waals (vdW) heterostructures, which are promising candidates for phononics applications [2]. I will describe our efforts to rationally synthesize ‘3D’ solids with tailored thermal properties using layerbylayer assembly of 2D crystals. Ultrafast Xray diffraction is demonstrated as a useful technique to probe heat transport across buried interfaces in vdW stacks [3]. Finally, using ultrafast electron diffraction, I will show how we can interrogate the coupling between charge and lattice dynamics in photoexcited 2D heterostructures, enabling direct observations of heat flow across single vdW junctions on picosecond timescales. 
Monday, March 15, 2021 2:18PM  2:30PM On Demand 
B20.00013: Evaluating Computational Shortcuts in SupercellBased Phonon Calculations of Molecular Crystals: The Instructive Case of Naphthalene Tomas Kamencek, Sandro Wieser, Hirotaka Kojima, Natalia BedoyaMartínez, Johannes P. Dürholt, Rochus Schmid, Egbert Zojer Over the past decades molecular crystals have been studied extensively  especially in terms of electronic structure  to improve their performance in organic semiconductorbased devices. Many of the properties relevant in this context are crucially affected by phonons. For example, electronphonon coupling is often found to be one of the main limiting factors for charge transport, or entropic contributions from phonons play a decisive role when it comes to reliably predicting phase stability of polymorphs close in energy. Despite their importance, measurements and ab initio simulations are often methodologically impeded in such complex systems. Thus, approximate simulation approaches are typically employed: densityfunctional based tight binding (DFTB) or classical force fields (FFs). Here, we quantitatively studied the errors one must expect when resorting to such methods for various phononrelated properties in crystalline naphthalene^{1}. Besides offtheshelf solutions using publicly available parameters for DFTB or widely used FFs such as COMPASS and GAFF, we also critically test the performance of our own parametrization of the MOFFF^{2} for naphthalene. 
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