Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K22: Electrons, Phonons, ElectronPhonon Scattering and Phononics IIFocus

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Sponsoring Units: DCOMP DMP Chair: Boris Kozinsky, Harvard University Room: BCEC 157C 
Wednesday, March 6, 2019 8:00AM  8:36AM 
K22.00001: Predictive calculations of phononassisted quantum processes Invited Speaker: Emmanouil Kioupakis Quantum processes mediated by phonons play an important role in the characterization of materials and in the operation of electronic and optoelectronic devices. I will discuss the theory and recent results in the predictive modeling of phononmediated quantum processes in materials with atomistic calculations based on density functional and manybody perturbation theory. I will discuss the computational methodology and results for phononassisted optical absorption in metals and in indirectgap semiconductors such as silicon (Si), boron arsenide (BAs), and boron nitride (BN). I will also discuss calculations for phononassisted Auger recombination in widebandgap semiconductors, an important nonradiative mechanism that dominates carrier recombination at high freecarrier concentrations. The results shed light into the interaction of light with materials and the efficiency of light emitters. This work was performed in collaboration with Kyle Bushick, Kelsey Mengle, Guangsha Shi, Andrew McAllister, Dylan Bayerl, and Chris Van de Walle. 
Wednesday, March 6, 2019 8:36AM  8:48AM 
K22.00002: TimeResolved Second Harmonic Generation Polarimetry Study of Elemental Tellurium Honglie Ning, Omar Mehio, Eli Zoghlin, Nicholas Laurita, Stephen Wilson, David Hsieh Elemental tellurium (Te) is a chiral semiconductor that breaks inversion symmetry, which allows for strong optical second harmonic generation (SHG). We performed timeresolved SHG polarimetry measurements on Te single crystals to understand the effects of interband carrier excitations on its lattice structure. I will describe the contrasting temporal evolution of the structure and charge excitations as a function of both temperature and pump fluence. 
Wednesday, March 6, 2019 8:48AM  9:00AM 
K22.00003: Firstprinciples resonant Raman spectroscopy for 2D materials Yue Yu, Jun Jiang, Liangbo Liang, Georgios D Barmparis, Sokrates T Pantelides, Xiaoguang Zhang We implement firstprinciples method for resonant Raman spectroscopy of solids based on a secondorder Feynman diagram and the FranckCondon principle. The Raman Intensity is calculated using an expression in the form of the KramersHeisenbergDirac (KHD) formula. Sum over all phonon configurations is performed using the Fourier transform into the time domain and evaluated with the help of Feynman path integrals. The triple integration method offers a simple and fast numerical approach that accounts for all multiple phonon processes. The ground state and excited state phonon modes are calculated to provide frequencies and equilibrium atomic positions. The computed Raman intensities for MoS2/WS2 heterostructure is in agreement with experimental data. We also explore the resonance properties of the Raman spectrum, in particular its strong dependence on the energy of incident laser and on temperature. 
Wednesday, March 6, 2019 9:00AM  9:12AM 
K22.00004: Pressureinduced destabilization and periodic lattice distortion in SnX_{2} (X = S, Se) Gyanu Prasad Kafle, Christoph Heil, Hari Paudyal, Jianjun Ying, XiaoJia Chen, Viktor V. Struzhkin, Elena R Margine Using firstprinciples calculations we investigate the behavior of tin disulfide (SnS_{2})_{ }and tin diselenide (SnSe_{2}) under compression. In agreement with singlecrystal Xray diffraction, Raman, and transport measurements, we find that a commensurate √3 × √3 × 1 superlattice forms in both SnS_{2 }and SnSe_{2} at elevated pressures. We show that the pressureinduced transition to the commensurate periodic lattice distortion (PLD) phase is due to the combined effect of strong Fermi surface nesting and electronphonon coupling at a certain wave vector q = (1/3, 1/3, 0). We further compare our findings to similar PLD transitions associated with charge density wave orderings in transition metal dichalcogenides. 
Wednesday, March 6, 2019 9:12AM  9:24AM 
K22.00005: Microscopic dynamics in the Holstein model at hightemperature: Absence of diffusion Stuart Trugman, ChenYen Lai A single itinerant electron in the Holstein model is known to have a large conductivity at low temperatures as the electron is scattered by dilute thermallyexcited phonons. At high temperatures, some theories suggest the dynamics are described by simple Boltzmann diffusion since there are many thermally excited phonons. Here, we investigate the dynamics of an itinerant particle of the Holstein model at high temperature. Our results suggest that the microscopic dynamics is not diffusion in some parameter regimes. In one dimension, the particle travels in a constant direction over time, which suggests an infinite conductivity. In higher dimensions, the particle follows a closed trajectory, a type of localization, and the trajectories are fractal. 
Wednesday, March 6, 2019 9:24AM  9:36AM 
K22.00006: Generalized Fröhlich model vs accurate firstprinciples: zeropoint renormalisation in polar semiconductors and insulators. Véronique BrousseauCouture, Anna Miglio, Matteo Giantomassi, Gabriel Antonius, YangHao Chan, Michel Cote, Xavier Gonze Computing the zeropoint renormalization (ZPR) of the electronic bandgap due to electronphonon coupling from first principles (FP) is a computationaly challenging task, especially for polar materials, for which a very fine phonon wavevector sampling is required [1]. By contrast, the wellknown Fröhlich Hamiltonian gives in the perturbative regime a simple analytical formula for the polaron binding energy, based on a few parameters that can be obtained from experiments or from FP calculations. 
Wednesday, March 6, 2019 9:36AM  9:48AM 
K22.00007: Firstprinciples study of selftrapped polarons WENG HONG SIO, Carla Verdi, Samuel Ponce, Feliciano Giustino The polaron is a quasiparticles consisting of an electron or hole dressed by a cloud of phonons. Selftrapped polarons are found when the electronphonon interaction is so strong that the electron is bound by the potential of the lattice distortion that it induces. Here we investigate the mechanisms of polaron formation in widegap insulators, using Li2O2 as a prototypical example. We study selftrapping both via direct densityfunctional theory (DFT) calculations and using a perturbation theory approach. We analyze the binding energy in terms of the underlying band structure and phonon dispersions, and we compare our findings with previous calculations and with classic models of selftrapped polarons. We discuss open questions in the ab initio study of polarons and possible solutions. 
Wednesday, March 6, 2019 9:48AM  10:00AM 
K22.00008: Phonon Behaviors of Charged Nanocrystals: Effects of Free Charge Carriers Shun Wang, Butian Zhang The precise knowledge of phonons is important to understand the electronic and photonic properties of nanocrystals (NCs) and NCbased devices. It is common to introduce free charge carriers into NCs for device applications, highlighting the significance of studying the free carrier effects on phonon behaviors. In this study, we introduced the free carriers into a series of NCs (ZnO, CdSe, PbSe) and investigated the carrierdependent phonon behaviors by using Raman spectroscopy. For charged NCs, the change of phonon frequencies and the overtone to fundamental intensity ratios were explained by the screening and bandfilling effects caused by the accumulated free carriers. By using a timedependent model, the overtone to fundamental intensity ratios were further correlated with the electronphonon coupling strength of the involved excited states. The effects of free carriers on phonon behaviors, usually neglected in previous studies, should be carefully considered when analyzing the properties of charged NCs. Moreover, the relationship between the free carrier densities and Raman frequencies/relative intensities are potentially useful for nondestructive characterization of free carriers in NCs. 
Wednesday, March 6, 2019 10:00AM  10:12AM 
K22.00009: Experimental demonstration of generalized Fourier's Law for heat conduction at the nanoscale Chengyun Hua, Lucas Lindsey, Austin Minnich Heat conduction under large temperature gradients that occur over scales of mean free paths of the energy carriers in solids is a topic of intense interest. Although the failure of Fourier's law is well understood, the appropriate replacement has been the topic of debate. A concise relation that links temperature gradient to heat flux based on a rigorous mathematical interpretation is not only necessary but crucial to advance our knowledge of nanoscale heat transport. Here, we derived a generalized Fourier's law based on PeierlsBoltzmann transport equation. This generalized Fourier's law contains two parts, nonlocality of thermal conductivity, which has been previously hypothesized, and nonlocality of external effects, i.e. volumetric heat generation, which has long been ignored in literatures. We demonstrated the validity of this generalized Fourier's law through comparisons with a series of timedomain thermoreflectance (TDTR) measurements. Furthermore, we showed that misinterpreting the generalized Fourier's law in the experimental observation of nanoscale heat conduction would lead to erroneous microscopic information of phonons. To map the macroscopic observations to intrinsic phonon properties, it is crucial to appropriately take account into the microscopic heat input. 
Wednesday, March 6, 2019 10:12AM  10:24AM 
K22.00010: Computing Raman Spectra With Wavelets Using The BigDFT Code Maxime Morinière, Michel Cote, Thierry Deutsch, Luigi Genovese We report the calculations of Raman spectra using the BigDFT code. 
Wednesday, March 6, 2019 10:24AM  10:36AM 
K22.00011: Infrared reflectance, transmittance and emittance spectra from firstprinciples Giorgia Fugallo, Benoit Rousseau, Michele Lazzeri Density functional theory (DFT) is considered a predictive approach to determine anharmonic phononphonon interactions in crystals. Such a computational tool is a necessary ingredient, for example, of the first principles computational methods recently developed to determine phonon thermal transport in real systems. In this context, finding measurable quantities that can provide an independent benchmark for theoretical approaches is of paramount importance. Surprisingly, DFT has been rarely used to interpret the anharmonic features observable in infrared (IR) reflectance, transmittance and emittance spectra even though they provide a relatively direct probe to anharmonic properties in heteropolar materials and can be used to directly determine the anharmonic phonon selfenergy of the optically active mode. I will show the influence of anharmonic effects on IR spectra of MgO, which is chosen as a test material because of the availability of different kinds of radiative properties measured experimentally and I will explain the limit of validity of a perturbative (multiphonon) approach. 
Wednesday, March 6, 2019 10:36AM  10:48AM 
K22.00012: Ab Initio Charge Transport in the Polaron Regime in Transition Metal Oxides JinJian Zhou, Marco Bernardi Strong electronphonon (eph) interactions lead to the formation of polarons, quasiparticles consisting of an electron carrying a phonon cloud. Polaron formation lowers the mobility and greatly affects charge transport in transition metal oxides and other polar compounds. Here, we develop a manybody ab initio approach to compute charge transport including polaron effects. We apply our approach to the two perovskite oxides BaSnO_{3} and SrTiO_{3}, and analyze in detail how and why polaron formation affects charge transport in these materials. The calculations are connected with our recent work on SrTiO_{3}, where, using the Boltzmann Transport equation with lowestorder eph scattering, we accurately predicted the temperature dependence of the mobility in SrTiO_{3} [1], whose absolute value was however higher than experiment. We show how including polaron effects can correct the lowestorder result, improving agreement with experiment. 
Wednesday, March 6, 2019 10:48AM  11:00AM 
K22.00013: The Role of Configurational Entropy in RealTime Mass Sensing Sudeep Adhikari, Kevin Beach We present a theoretical framework for determining the mass deposited on a mechanical resonator subject to a flux of incoming particles of a single species. We consider the specific example of a vibrating nanostring and infer the history of mass deposition events from the frequency shifts in real time using a numerical optimization alogrithm that correctly compensates for the configurational entropy. Our approach is tested against simulated data and is shown to perform well. We comment on its applicability to common inverse problems in physics, such as the extraction of spectral functions from imaginarytime Green's functions, that also suffer from configurational entropy effects. 
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