Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session P44: Electrons, Phonons, Electron Phonon Scattering, and Phononics IIIFocus

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Sponsoring Units: DCOMP DMP Chair: Olivier Delaire, Duke University Room: 704 
Wednesday, March 4, 2020 2:30PM  3:06PM 
P44.00001: Theoretical Spectroscopy of 2D Materials: Excitonphonon coupling in resonant Raman and luminescence spectroscopy Invited Speaker: Ludger Wirtz 2D materials are known to exhibit very pronounced excitonic effects due to the confinement of electrons and holes in a layer and due to the weak dielectric screening of the electronhole interaction. In spectroscopy involving vibrational degrees of freedom, the excitonphonon coupling must therefore be included in order to obtain a qualitative understanding of the spectra and in order to obtain quantitative results. We present our methods for the calculation of excitonphonon coupling via a finite displacement [1] and via a diagrammatic approach [2,3], both using manybody perturbation theory. 
Wednesday, March 4, 2020 3:06PM  3:18PM 
P44.00002: Strong phononassisted Auger recombination in PbSe Xie Zhang, Jimmy Shen, Chris Van de Walle It is commonly believed that direct Auger recombination dominates in narrowgap semiconductors. With the example of the prototypical narrowgap semiconductor PbSe, we demonstrate that this is not necessarily the case. By explicitly calculating both the direct and phononassisted indirect Auger recombination coefficients of PbSe from first principles, we show that a number of puzzling anomalies in the Auger recombination in PbSe can be well understood. These insights shed new light on the impact of electronphonon coupling on Auger recombination in IVVI semiconductors, which is critical for improved device design. 
Wednesday, March 4, 2020 3:18PM  3:30PM 
P44.00003: Fully Anharmonic, NonPerturbative FirstPrinciples Theory of ElectronicVibrational Coupling in Solids Marios Zacharias, Matthias Scheffler, Christian Carbogno The coupling between nuclear vibrations and the electronic structure plays a pivotal role for many material properties, including optical absorption and electronic transport. In this regard, however, todays stateoftheart methodologies rely on two approximations [1]: the harmonic (phonon) approximation for the nuclear motion and the linear response description of the electronic structure with respect to harmonic displacements. In this work, we overcome both these approximations by performing fully anharmonic ab initio molecular dynamics (aiMD) calculations and by accounting for the nonperturbative, selfconsistent response of the wavefunctions along the aiMD trajectory. By this means, we obtain fully anharmonic, vibronically renormalized spectral functions, from which macroscopic material properties like temperaturedependent band gaps and electronic transport coeffiecients are obtained. We validate our approach using silicon as an example, for which the traditional electronphonon coupling formalism is recovered. Using cubic SrTiO_{3 }as example, we further demonstrate that anharmonic electronicvibrational coupling effects not captured in traditional formalisms play a decisive role in complex materials like perovskites. 
Wednesday, March 4, 2020 3:30PM  3:42PM 
P44.00004: Phononmediated Optical and Electronic Transport Properties of BAs Kyle Bushick, Kelsey Mengle, Sieun Chae, Zihao Deng, Emmanouil Kioupakis While boron arsenide (BAs) has attracted attention for its ultrahigh thermal conductivity, open questions remain about its use in semiconducting devices. To address this problem, we apply density functional and many body perturbation theory to understand its electronic and optical properties and guide device applications. Since BAs has an indirect band gap of approximately 2 eV and a direct gap of 4.1 eV, absorption of visible light is exclusively mediated by phonons. We therefore calculate the indirect and direct optical absorption spectra to assess its potential in photovoltaics and examine how excitonic effects alter the absorption. Our results are in excellent agreement with experimental data. We also calculate the effect of strain on the band alignment and the electron and hole mobilities, and show that biaxial tensile strain increases the mobilities of both carriers by over 50%. Finally, we determine the band offsets of BAs heterostructures with nearly latticematched ZnSnN_{2} and InGaN to guide heterostructure design. Our work elucidated the functional properties of BAs for technologically relevant device applications. 
Wednesday, March 4, 2020 3:42PM  3:54PM 
P44.00005: Electron and Phonon Hydrodynamics in Antimony Alexandre Jaoui, Benoit Fauque, Kamran Behnia We present a study of the electrical and thermal resistivities in millimetric samples of the semimetal Sb down to subKelvin temperatures. By applying a large magnetic field, we can clearly separate the electronic and phononic thermal resistivities. The WiedemannFranz (WF) law is recovered at low temperature (T ≈ 4K) in all samples. Yet, a sizedependent departure from the WF law is observed at higher temperatures. We show that this deviation is due to a mismatch between the prefactors of the thermal and electrical T^{2}resistivities. By analogy with the case of normalstate liquid ^{3}He, where fermionfermion collisions generate a thermal conductivity inversely proportional to temperature [1], we can associate the larger T^{2}thermal resistivity to the momentumconserving electronic collisions [2]. In this scenario, the sizedependence of the ratio of the thermal to the electrical T^{2}prefactors in Sb points to a hydrodynamic flow of electrons [3]. Furthermore, in the 1K7K range, we also report on a large increase of the thermal diffusivity of phonons, in the same samples, i.e. a signature that the flow of phonons is partially hydrodynamic. 
Wednesday, March 4, 2020 3:54PM  4:06PM 
P44.00006: Unraveling a New Heat Transport Regime at The Nanoscale. Giuseppe Barbalinardo, Zekun Chen, Shunda Chen, Davide Donadio The understanding of heat transport in nanoscale semiconductors is of fundamental importance because of its huge technological impact in electronics and renewable energy harvesting and conversion. A particularly interesting question is related to the understanding of how thermal properties of dielectrics, like silicon, change when their size of the order of a few hundred nanometers, which is the characteristic size of state of the art electronic circuits. In this size range heat transport is in a regime in between ballistic and diffusive, which is inaccurately described by either approximation. 
Wednesday, March 4, 2020 4:06PM  4:18PM 
P44.00007: Electron wind force in an atomistic nonequilibrium molecular dynamics simulation Davide Mandelli, Franco Pellegrini, Giuseppe E. Santoro, Erio Tosatti We demonstrate electronic friction on metals by addressing the reverse process – the force imparted on the atoms by the flow of an electronic current in the metal. By well established Ehrenfest dynamics we implement a quantumclassical simulation of the motion of a chain of classical atoms forming a contactless monatomic ring, where electrons flow by nearest neighbor hopping between atomiclike orbitals in an electric field generated by a linearly growing external magnetic field threading the ring. In the real time dynamics, electronphonon scattering events take the form of LandauZener processes, each of them imparting momentum to the classical atoms, that can vibrate and also dissipate to a bath. The realistic features of electron conduction and resistivity are recovered in this model, plus a microscopic description of the currentinduced momentum transfer to the atoms which vibrate and to the whole chain which drifts. When one additional adatom is added to the chain, the quantum mechanics of the wind force and the resulting electromigration drift are controlled by the weak hybridization of the adatom orbital to the chain atomic orbitals where the current flows, suggestive of a rather general mechanism. 
Wednesday, March 4, 2020 4:18PM  4:30PM 
P44.00008: Thicknessdependent electronphonon coupling and transport in metals from first principles Sushant Kumar, Ravishankar Sundararaman Recent strides in thinfilm deposition technology have made the next generation of ultrathin plasmonic devices and ultrascaled metallic interconnects plausible. It has led to a renewed interest in the study of properties of metals and semiconductors in the low dimensional limit. Here, starting with a single layer, we systematically investigate the effect of thickness on the electronic properties and electronphonon (eph) coupling of ultrathin metallic films. Such ab initio studies have typically been challenging because of the inability of the stateoftheart DFT codes to correctly predict the phonon dispersion curves of finite thickness materials, especially in the q→0 limit. We propose a new computationallyinexpensive correction scheme for the phonon force matrix based on rotational invariance of the elasticity tensor. This scheme facilitates accurate computation of phonon bandstructures and eph coupling for arbitrary film geometries. Using this capability, we determine the variation of electronic transport properties as a function of layer number, fully accounting for effects such as phonon confinement. These ab initio calculations give us an insight into the effect of surface and interfacial strain on the eph scattering in thin metallic films. 
Wednesday, March 4, 2020 4:30PM  4:42PM 
P44.00009: Phonon interactions in rock salt and fluorite structures Lyuwen Fu, Mark Alan Mathis, Enda Xiao, Chris Marianetti Space group irreducible derivatives of the Born Oppenheimer potential are computed from density functional theory for materials with the rock salt and fluorite structures, including PbTe and ThO2. We utilize our recently developed group theoretical approach which allows the extraction of the irreducible derivatives from finite displacement calculations on the smallest possible supercells using the smallest possible number of calculations. Quadratic (i.e. phonons), cubic, and quartic irreducible derivatives have been computed. The fidelity of the irreducible derivatives is tested via comparison to strain derivatives of the phonons. Both dynamics and thermodynamics are evaluated using our irreducible Taylor series, yielding observables such as phonon linewidths, thermal conductivity, thermal expansion, mean square displacements, etc. Results on PbTe and ThO2 are compared to experiment. 
Wednesday, March 4, 2020 4:42PM  4:54PM 
P44.00010: Predictive calculations of electron and heat transport in ultrawidebandgap semiconductors Emmanouil Kioupakis, Kelsey Mengle Ultrawidebandgap semiconductors are used for energyefficient power electronics, but there is a pressing need to imrpove on current materials. Beta gallium oxide (βGa_{2}O_{3}) outperforms materials such as Si, SiC, and GaN due to its wide band gap and corresponding large breakdown field. However, three drawbacks of βGa_{2}O_{3} are its lack of ptype doping, its low thermal conductivity, and its inferior electron mobility. Using firstprinciples calculations we calculate the electronphonon coupling of βGa_{2}O_{3} to understand the origin of the limits to the electron mobility and thermal conductivity. We also explore rutile germanium dioxide (rGeO_{2}) as a semiconductor for powerelectronic applications. The calculated band gap and optical absorption spectrum is in good agreement with optical measurements. We also determine the phononlimited carrier mobilities and lattice thermal conductivity as a function of temperature and crystal orientation. We find that the electronic and thermal transport properties of rGeO_{2} outperform current materials, while it can also be ambipolarly doped. Our results highlight the potential of rGeO_{2} for power electronics. 
Wednesday, March 4, 2020 4:54PM  5:06PM 
P44.00011: Optical absorption in gallium oxide Hartwin Peelaers, Chris Van de Walle Transparent conducting oxides (TCOs) are a technologically important class of materials used in optoelectronic devices, as TCOs balance two conflicting properties: transparency and conductivity. The requirement of transparency is typically tied to the band gap of the material being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers, required for conductivity, can also lead to optical absorption. This absorption can occur through direct absorption to higherlying conduction band states, or by an indirect process, for example mediated by phonons or charged impurities. 
Wednesday, March 4, 2020 5:06PM  5:18PM 
P44.00012: Low and high field transport in 2dimensional electron gas in β(Al_{x}Ga_{1x})_{2}O_{3}/Ga_{2}O_{3} heterostructures Avinash Kumar, Uttam Singisetti βGa_{2}O_{3} is an emerging widebandgap semiconductor for potential application in power and RF electronics. The 2dimensional electron gas (2DEG) in β(Al_{x}Ga_{1x})_{2}O_{3}/Ga_{2}O_{3} heterostructures show the promise for high speed transistors. We will present both the low and high field 2DEG transport properties in the AlGa_{2}O_{3}/Ga_{2}O_{3} heterostructure. A selfconsistent PoissonSchrodinger simulation of heterostructure is used to obtain the subband energies and wavefunctions. Intrasubband, intersubband, 2D3D, 3D2D and 3D3D scattering rates are calculated for all the different scattering mechanisms. The electronic structure, assuming confinement in a particular direction, and the phonon dispersion is calculated based on first principle methods under DFT and DFPT framework. Phonon confinement is not considered for the sake of simplicity. The different scattering mechanisms that are included in the calculation are phonon (polar and nonpolar), remote impurity, alloy and interfaceroughness. We include the full dynamic screening in polar optical phonon scattering. We will use Full Band Monte Carlo to calculate the velocityfield profile in different crystal directions. We will also report the low field mobility by solving the Boltzmann transport equation using Rode’s iterative method. 
Wednesday, March 4, 2020 5:18PM  5:30PM 
P44.00013: Phonon RIXS calculations using Green's function Momentum Average technique Krzysztof Bieniasz, Mona Berciu, Steven Johnston Spectroscopic experiments are the most extensively used probes for characterizing the physical properties of condensed matter. However, their understanding often requires a theoretical calculation of the spectra in question, usually starting from some simplified model of the underlying system. One of the problems that are key to the understanding of many properties of materials is the physics of phonons and their interaction with the conduction electrons. These are the issues lying at the base of conventional superconductivity and it is hoped that their better understanding in high temperature superconductors might help elucidate some of the mysteries still surrounding those materials. The RIXS spectroscopy is a novel, emerging technique, that only recently started to approach the resolving power necessary to observe the phononic features. Thus, theoretical predictions concerning phonon RIXS are now becoming timely in this rapidly progressing field of research. Our aim is to use the Momentum Average Green's function technique and apply it to the problem of phonon RIXS calculations, thus going beyond the current dichotomy of exact diagonalization vs. perturbative expansion methods that are the two dominating methodologies in RIXS theory. 
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