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 M20: Electrons, Phonons, ElectronPhonon Scattering, and Phononics VFocus Live

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Sponsoring Units: DCOMP DMP Chair: David Parker, Oak Ridge National Lab 
Wednesday, March 17, 2021 11:30AM  12:06PM Live 
M20.00001: Gauge invariance of heat and charge transport coefficients Invited Speaker: Stefano Baroni Transport coefficients have been recently shown to be largely independent of the microscopic representation of the current density of the conserved quantity being transported (charge/mass/energy) [1]. This remarkable gauge invariance has been leveraged to lay down a rigorous densityfunctional theory of heat transport [1], as well as a general approach to it in solids, that nicely bridges the BoltzmannPeierls kinetic model, which applies to crystals, and the AllenFeldman one, which applies to glasses [2]. In the case of charge transport, a combination of gauge invariance and Thouless’ quantisation of particle transport [3] allows one to express the electrical conductivity of a stoichiometric ionic conductor in terms of integervalued, scalar, and timeindependent atomic oxidation numbers, instead of realvalued, tensor, and timedependent Born charges [4]. The departure of non stoichiometric systems from this picture, due to the existence of localised electron pairs, can be fathomed in terms of topological effects on charge transport [5]. In this talk I will review these concepts and report on some key applications of them to liquids and glasses. 
Wednesday, March 17, 2021 12:06PM  12:18PM Live 
M20.00002: Remote freecarrier screening to boost the mobility of Fröhlichlimited 2D semiconductors Thibault Sohier, Marco Gibertini, Matthieu Verstraete Van der Waals heterostructures provide a versatile tool to manipulate the properties of 2D materials and optimize their performance. Here we use ab initio calculations and semianalytical models to find strategies that boost the mobility of a currentcarrying 2D semiconductor by including a metallic layer in the heterostructure. Freecarrier screening from the metallic "screener" remotely suppresses electronphonon interactions in the currentcarrying layer, thus enhancing mobility. This concept is most effective in 2D semiconductors whose scattering is dominated by electronphonon interactions which are sensitive to screening, in particular the common case of Fröhlich coupling. To obtain the screened interaction we develop a novel approach to combine the electrostatic response of any heterostructure, based on the response of the individual layers computed within densityfunctional perturbation theory. We use GaSe as a prototype, and place it in an heterostructure with doped graphene as the "screener" layer and BN as a separator. Remote screening leads to an enhancement by a factor 3 in GaSe, with a mobility that is almost constant, around 500 to 600 cm^{2}/Vs, over a wide range of carrier densities from from 10^{11} to 10^{13} cm^{2}. 
Wednesday, March 17, 2021 12:18PM  12:30PM Live 
M20.00003: Superconductivity in La_{2}Ni_{2}In Jannis Maiwald, Igor Mazin, Alexander V Gurevich, Meigan Aronson In this talk we will report^{1} on the physical properties of single crystaline La_{2}Ni_{2}In. From measurements of the electrical resistivity and specific heat as wel as Density Functional Theory calculations, we find that La_{2}Ni_{2}In is a weakly correlated metal, with almost completely quenched Ni magnetism, leaving only a weak Stoner enhancement of the density of states. Superconductivity was discovered at temperatures below 0.9 K. A detailed analysis of the field and temperature dependencies of the electrical resistivity, magnetic susceptibility, and specific heat at the lowest temperatures reveals that La_{2}Ni_{2}In is a dirty typeII superconductor with likely swave gap symmetry. Nanoclusters of ferromagnetic inclusions significantly affect the subgap states resulting in a nonexponential temperature dependence of the specific heat C(T) at T«T_{c}. 
Wednesday, March 17, 2021 12:30PM  12:42PM Live 
M20.00004: Thermodynamics of the Invar Transition: Phonons vs. Magnetism Stefan Haegeli Lohaus, Pedro Guzman, Camille Bernal, Guoyin Shen, Esen Ercan Alp, Brent Fultz Phonons, electrons, spins and their interactions determine the thermodynamic behavior of materials. These interactions are at the core of the invar effect, where the volume of a material is unchanged with increasing temperatures or pressures. The invar transition is characterized by a magnetovolume transition with competing thermodynamic effects: a change in magnetism from a ferromagnetic high spin state to a low spin state with temperature or pressure is accompanied by a decrease in volume, counteracting the usual thermal expansion. We revisit the classic FeNi invar system (Fe64Ni36) to explore the underlying interactions between phonons and magnetism. By measuring the phonon spectrum through nuclear inelastic Xray scattering and the magnetic evolution though synchrotron Moessbauer experiments at several pressures and temperatures, we quantified the changes in vibrational and magnetic entropies through the invar transition. These two pieces of entropy have opposing effects on the free energy, explaining the underlying thermodynamic drive of the invar effect and the resulting unchanging volume that gives invar its name. 
Wednesday, March 17, 2021 12:42PM  12:54PM Live 
M20.00005: Energy dissipation mechanisms during the formation of charged defect centers Shuaishuai Yuan, Kirk H Bevan The nonequilibrium dynamics associated with the formation of charged defect centers are essential to the operational properties of various technologies including batteries, oxide electronics, photocatalysis, and sensors. However, atomistic insights into the formation dynamics of charged defect centers in advanced materials are challenging because their nonequilibrium distortions can stretch over 10,000 atoms or more. In this work, we theoretically study the lattice relaxation process associated with charged defect formation in a model halide. From this investigation, we discern the atomic dynamics, energy dissipation, phonon modes, and phononphonon process that contributes to the short time scale interaction between charged defect centers and their surrounding lattice at the initial formation stage. Through this work, general findings can be arrived at to inform the study of the adiabatic formation dynamics of charged defect centers in a wide range of systems and applications. 
Wednesday, March 17, 2021 12:54PM  1:06PM Live 
M20.00006: High field transport and hot electron noise in GaAs from first principles Peishi Cheng, Alexander Choi, Austin Minnich Hot carrier transport and fluctuations is a topic of fundamental and practical interest. Although abinitio calculations of lowfield mobility are now routine, an equivalent framework for calculating spectral noise power has only recently been developed and is limited to weak electric fields. Here, we report a firstprinciples approach to compute the spectral noise power of a hot electron gas with no adjustable parameters. We apply the formalism to GaAs with fields exceeding kV/cm. Our work will provide a microscopic perspective of the electronphonon interactions that lead to current noise in a hot electron gas. 
Wednesday, March 17, 2021 1:06PM  1:18PM Live 
M20.00007: Phonon RIXS theory for a mobile electron Krzysztof Bieniasz, Steven S. Johnston, Mona Berciu Electronphonon coupling lies at the heart of conventional superconductivity and, it is hoped, its better understanding in high temperature superconductors might help elucidate some of the mysteries still surrounding those materials. RIXS is a novel, emerging spectroscopic technique, that only recently started approaching the resolving power needed for phononic features. Thus, theoretical predictions concerning phonon RIXS are becoming timely in this rapidly developing field. We apply the Momentum Average (MA) Green's function technique to the problem of phonon RIXS calculations, in order to improve on the widely utilized treatment derived from a localised electron theory, while avoiding the finite size issues inherent in exact diagonalization. We show that the role of electron mobility is important, especially for a strongly screened core hole, resulting in significant departures from the predictions of the localised theory. This points to MA as a promising and flexible alternative to existing approaches to theoretical treatment of RIXS. 
Wednesday, March 17, 2021 1:18PM  1:30PM Live 
M20.00008: Phonon frequencies from path integral quantum correlators in strongly anharmonic systems Tommaso Morresi, Rodolphe Vuilleumier, Michele Casula The recent discovery of hightemperature highpressure superconductivity in H3S and LaH10 reveals the fundamental role played by hydrogen in making these materials superconducting. The solid state of pristine hydrogen has been the object of intense research, in quest of a metallic phase which could superconduct at room temperature. 
Wednesday, March 17, 2021 1:30PM  1:42PM Live 
M20.00009: FDPML study of phonon scattering and localization in 2D nanoparticulates. Ongira Chowdhury, Joseph Feser We have developed a frequencydomain perfectly matched layer (FDPML) method capable of studying modebymode phonon transport on extraordinarily large domain sizes (tens of billions of atoms using parallel computing) with atomic resolution. In this talk, we will show progress towards understanding scattering from composites with randomly distributed embedded nanoparticles over a wide range of volume fractions. We show that the mean free paths obtained by the FDPML for composites with loading up to ~15%vol can successfully be captured by a simple model based on the scattering crosssection from an isolated particle for wavelengths in the Mie regime. We also explore trends of localized modes in embedded nanoparticulate composites using both Landauer and modal decomposition approaches. 
Wednesday, March 17, 2021 1:42PM  1:54PM Live 
M20.00010: Simulation method for thermoelectric transport in complex bandstructure bipolar materials Patrizio Graziosi, Neophytos Neophytou We present a computational method to compute the transport coefficients in bipolar complex bandstructure materials. We couple the Boltzmann transport method within the energy and momentum dependent relaxation time approximation to DFT bands for halfHeusler materials, considering both electronphonon and ionized dopant scattering. The full energy and momentum dependence of the relaxation times is essential in capturing the correct transport features. We then compare the conventional combination of unipolar transport coefficients with the simultaneous consideration of the full bipolar effects in the calculation of the transport coefficients. A large difference between the two treatments exists for narrow bandgap materials. 
Wednesday, March 17, 2021 1:54PM  2:06PM Live 
M20.00011: Oxidation states, Thouless' pumps, and nontrivial transport in nonstoichiometric electrolytes Paolo Pegolo, Federico Grasselli, Stefano Baroni Thouless' quantization of adiabatic particle transport permits to associate an integer topological charge with each atom of an electronically gapped material. Under suitable conditions, they provide a rigorous definition of oxidation states and atoms can be identified as integercharge carriers in electrolytes [1]. In these conditions, charge transport is convective, i.e. it cannot occur without substantial ionic flow. 
Wednesday, March 17, 2021 2:06PM  2:18PM Live 
M20.00012: Ultrafast Photoinduced Structural Phase Transition in Strained Ca_{2}RuO_{4} Anita LNU, Oleg Gorobtsov, Hari Nair, Nathaniel Schreiber, Jacob P Ruf, Kyle Shen, Darrell Schlom, Andrej Singer Ultrafast timeresolved xray diffraction at recently developed Xray freeelectron lasers enables investigating the dynamics of photoinduced structural phase transitions. Here, we have studied the metalinsulator transition (MIT) of a strained Ca_{2}RuO_{4} thin film. We excited the strained film with optical pulses and interrogated the structural dynamics with ultrafast timeresolved xray diffraction at Japanese XFEL SACLA. Upon photoexcitation of the predominantly lowtemperature phase, we observed a one picosecond rise in normalized scattering intensity corresponding to the hightemperature phase. Additionally, the Bragg peak associated with the hightemperature phase shifts in the reciprocal space, suggesting a unit cell volume expansion within one ps. Our results reveal a photoinduced volume modifying phase transition with a transition time rivaling much longer timescales observed in bulk systems. The ultrafast nature of phase transition suggests that the structural transition is nonthermal, potentially enabling the tuning of quantum material properties in ways not possible by other methods. 
Wednesday, March 17, 2021 2:18PM  2:30PM On Demand 
M20.00013: Orbital hybridization and Negative Gruneisen parameters of bccbased intermetallic FeTi at high pressure Bethuel Khamala, Jorge Munoz FeTi is a brittle intermetallic material that crystallizes in the bccbased CsCl structure and is stable until it melts at 1600 K. We investigated its electronic band structure and phonon dispersion relations using DFT in which the majority of the phonon modes decrease in energy or remain unchanged with decreasing volume. This behavior is usually observed in invar materials, but unlike them, FeTi is nonmagnetic and there is negligible change in the Fermi surface with pressure. The behavior occurs more generally in materials that show negative thermal expansion, but unlike most of those materials, the crystal structure of FeTi is not particularly open and it is stable at high pressure. In this talk, we show experimental measurements of the phonon densityofstates curves performed via nuclearresonant inelastic xray scattering in a diamondanvil cell (DAC) at pressures up to 55 GPa and xray diffraction also in a DAC at pressures up to 25 GPa that are consistent with the volume range of the calculated negative Gruneisen parameters, as well as an analysis of the calculated force constants, frozenphonons, charge densities, and band structures that preliminarily point towards orbital hybridization as the origin of the observed negative Gruneisen parameters. 
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