Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P29: Electrons, Phonons, Electron Phonon Scattering and Phononics IIFocus

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Sponsoring Units: DCOMP DMP Chair: Vidvuds Ozolins, Yale Univ Room: LACC 406A 
Wednesday, March 7, 2018 2:30PM  3:06PM 
P29.00001: Why do halfHeusler materials often have large thermoelectric power factor? Invited Speaker: Jiawei Zhou , Hangtian Zhu , TeHuan Liu , Qichen Song , Ran He , Jun Mao , Zihang Liu , Wuyang Ren , Bolin Liao , David Singh , Zhifeng Ren , Gang Chen Advancements for thermoelectric materials benefit from understandings of the underlying charge transport mechanism. One example is given by the “band engineering” approach, which seeks to enhance the power factor by tuning the electron density of states. Despite tremendous work that shows enhanced electrical properties via this strategy, the large power factors often possessed by the halfHeusler system – with a highest value ever reported for semiconductors at room temperature (>100 μW/cmK^{2} in Tidoped NbFeSb) – remains unclear. Using first principles electron transport calculation, we reveal that such high power factor results from a distinct transport regime where the electron scattering by acoustic phonons can be strongly suppressed, making halfHeusler a unique system that contrasts traditional viewpoints that acoustic phonons often limit the charge transport. The electronphonon interaction is rationalized via chemical bonding concepts, through which we find that the weak coupling strength is protected as a result of the crystal symmetry of halfHeusler phases. Large room temperature power factors well above 100 μW/cmK^{2} are predicted for several compositions. We believe the results will stimulate future work into discovering new thermoelectric materials with exceptional power factors. 
Wednesday, March 7, 2018 3:06PM  3:18PM 
P29.00002: Acoustic Deformation Potentials of ntype PbTe from First Principles Ronan Murphy , Felipe MurphyArmando , Stephen Fahy , Ivana Savic There are conflicting reports in the literature regarding the strength of intravalley acoustic phonon scattering in PbTe [1,2], and the role it plays in PbTe's excellent thermoelectric properties. Efforts to calculate electronphonon scattering in PbTe are further complicated due to the tendency of the standard density functional theory to produce an inverted band gap in materials with heavy elements when spin orbit coupling is taken into account [3]. In this work, we calculate the acoustic deformation potentials [4] in ntype PbTe from first principles using both the local density approximation and hybrid functionals. We find that the choice of the exchange correlation functional does not substantially affect the calculated deformation potential values once the physically correct representation of the conduction and valence band states near the band gap has been obtained. Our calculations show that the acoustic deformation potentials of ntype PbTe are much weaker than previously thought [1]. 
Wednesday, March 7, 2018 3:18PM  3:30PM 
P29.00003: Electronphonon scattering and temperature dependence of electronic transport properties of PbTe from first principles Jiang Cao , Jose Daniel QueralesFlores , Ronan Murphy , Stephen Fahy , Ivana Savic In PbTe, one of the most efficient thermoelectric materials, the temperature dependence of the mobility suggests the major role of acoustic phonon scattering [1]. However, recent studies showed that longitudinal optical (LO) phonon scattering is dominant in PbTe [2]. Here we build accurate models of electronic and phonon bands, and electronphonon scattering from first principles. By solving the Boltzmann equation in the momentum relaxation time approximation, we calculate mobility and thermoelectric transport properties. Our approach allows us to obtain a very fine kspace resolution while significantly reducing computational cost, and study the impact of individual phonons on electronic transport. We find that polar LO scattering dominates in PbTe, while acoustic scattering is relatively weak. However, at high temperatures and doping concentrations, the nonpolar LO contribution becomes appreciable, thus changing the temperature dependence of the mobility to that characteristic for acoustic phonons. Our calculated mobility and thermoelectric properties of PbTe are in very good agreement with experiments. 
Wednesday, March 7, 2018 3:30PM  3:42PM 
P29.00004: Dislocation strain as the mechanism of phonon scattering at grain boundaries Jeff Snyder , Riley Hanus For 50 years, we have commonly been using Casimir’s theory that describes the scattering of heatcarrying lattice vibrations (phonons) on the sample boundaries to also describe the reduction of thermal conductivity due to grain boundaries. In the frequencyindependent Casimir model, phonons simply cannot travel across the boundaries, which is not the case in grain boundaries. This and a growing body of experimental and computational evidence shows that the modification of the Casimir model is necessary for grain boundaries. In this talk I will discuss our analysis of phonon scattering that controls the thermal conductivity of many common thermoelectric materials. We find that the grain boundary dislocation strain model can substitute for the Casimir model. More importantly, the two models can be distinguished at low temperature in finegrained materials such that experimental evidence supports the grain boundary dislocation strain model. In this way, we suggest that grain boundaries themselves are best conceptualized as a collection of dislocations. Since strain and grain boundary structures can vary, we should be able to engineer grain boundaries or grain complexions (including extrinsic atoms) to disrupt phonon transport without harming electron transport. 
Wednesday, March 7, 2018 3:42PM  3:54PM 
P29.00005: Firstprinciples study of intrinsic mobilities in monolayer TMDs with twodimensional Fröhlich interaction Jinlong Ma , Wu Li In previous firstprinciples calculations of twodimensional (2D) polar materials, the Fröhlich interaction has not been considered due to the lack of effective treatment to isolate the layers. By introducing the truncated Coulomb interactions into Wannier function interpolation, we successfully capture the converged longrange coupling of longitudinal optical (LO) phonons and calculate the intrinsic mobilities of 2Hmonolayer TMDs. For MoS_{2} and WS_{2}, the decrease caused by polar effect is small as the dominant scattering is from longitudinal acoustic (LA) phonons, while for MoSe_{2}, MoTe_{2} and WSe_{2}, the decrease is large due to the comparable contribution from LO phonons. The WS_{2} has the largest mobilities, while the MoTe_{2} has the smallest mobilities which are much smaller than previous estimations. The mode dependent analysis shows the mean free paths are smaller than 20 nm at room temperature. 
Wednesday, March 7, 2018 3:54PM  4:06PM 
P29.00006: Ab Initio Calculations of Nonequilibrium Coupled Carrier and Phonon Dynamics Xiao Tong , JinJian Zhou , Marco Bernardi The dynamics and equilibration of coupled excited electrons and phonons is important for understanding novel electronics and timeresolved spectroscopy. Nonequilibrium distributions of electrons and phonons coupled through electronphonon and phononphonon interactions can be described within the framework of the Boltzmann transport equation (BTE). Here, we compute from first principles the scattering and coupled ultrafast dynamics of nonequilibrium electrons and phonons by simultaneously timestepping the electron and phonon BTEs using a fourthorder Runge Kutta scheme. Numerical and parallelization schemes are discussed in detail. Our approach is applied to graphene and silicon, where analysis of the timedependent electron and moderesolved phonon distributions provides unprecedented insight into the equilibration process of excited electrons. 
Wednesday, March 7, 2018 4:06PM  4:18PM 
P29.00007: Intrinsic phononlimited charge carrier mobilities in thermoelectric SnSe Wu Li , jinlong ma Within past few years, tin selenide (SnSe) has attracted intense interest due to its remarkable thermoelectric potential for both n and ptype crystals. In this work, the intrinsic phononlimited electron/hole mobilities of SnSe are investigated using Boltzmann transport equation based on firstprinciples calculated electronphonon interactions. We find that the electrons have much larger mobilities than the holes. The modespecific analysis shows that the highest longitudinal optical phonons, rather than previously assumed acoustic phonons, dominate the scattering processes and consequently the mobilities in SnSe. At room temperature, the largest mean free paths of electrons and holes in SnSe are about 18 nm and 10 nm, respectively. 
Wednesday, March 7, 2018 4:18PM  4:30PM 
P29.00008: Electronphonon coupling from the GW method: Linearresponse perturbation theory Zhenglu Li , Gabriel Antonius , Meng Wu , Felipe da Jornada , Steven Louie We have developed a new theoretical and computational method to calculate electronphonon coupling from first principles, based on the GW method that includes quasiparticle selfenergy effects. This GW perturbation theory (GWPT) applies monochromatic (fixed wavevector) phonon perturbations to the electron selfenergy, using linear response theory, to calculate the electronphonon matrix elements including manyelectron effects. GWPT shares a similar spirit as densityfunctional perturbation theory but goes beyond the independentparticle picture. The GWPT method naturally scales linearly with the number of phonon modes, therefore largely reduces the computation costs, and increases the accuracy compared with frozenphonon technique. We will present the general formalism, the implementation of GWPT, and apply the method to several physical systems. 
Wednesday, March 7, 2018 4:30PM  4:42PM 
P29.00009: Effect of plasmonLO phonon coupling on the mobility of GaN Nakib Protik , David Broido Wurtzite gallium nitride (wGaN) is a wide bandgap, polar semiconductor. Its wide use in electronic devices makes understanding of carrier transport critically important. Traditionally, the phononlimited electron mobility calculation is carried out assuming an equilibrium phonon gas, although the electronphonon interaction invariably leads to nonequilibrium distributions of both the electron and the phonon systems. Using a recently developed method [1] which treats the transport of the coupled plasmonLO phonon modes in conjunction with the electron transport, we calculate the carrier mobility of wGaN for various doping levels and at different temperatures. Density functional theory is combined with analytic models to obtain the electronphonon scattering rates. The mobility is calculated by performing an iterative solution of the coupled electron and plasmonLO phonon Boltzmann transport equations. 
Wednesday, March 7, 2018 4:42PM  4:54PM 
P29.00010: Electronic Transport Coefficients from First Principles Christian Carbogno , Zhenkun Yuan , Matthias Scheffler The KuboGreenwood formalism has been employed successfully for the firstprinciple assessment of electronic transport coefficients at elevated temperatures and/or for disordered systems [1]. However, its application to crystalline systems closer to room temperature is still computationally prohibitive, since extremely large supercells are required to reach the bulk limit. In this work, we discuss the physical origin of this limitation and possible strategies to overcome it. In particular, we investigate to which extent the asymptotically exact extrapolation procedure [2] that we recently developed for the assessment of vibrational transport can be extended to electronic transport. For this purpose, we perform ab initio calculations both for direct (GaAs) and indirect semiconductors (Si) and critically discuss the obtained results with respect to existing experimental and theoretical data [3]. 
Wednesday, March 7, 2018 4:54PM  5:06PM 
P29.00011: Lattice Dynamics and ElectronPhonon Coupling of the LightActinides PaU Alloy Omar De La Pena Seaman , Rolf Heid , KlausPeter Bohnen We have studied the vibrational and electronphonon (eph) coupling properties of the Pa_{1x}U_{x} actinide alloy. This system have been analyzed within the framework of density functional perturbation theory, using a mixedbasis pseudopotential method and the virtual crystal approximation (VCA) for modeling the alloy. The energetics of the system is analyzed as the groundstate crystal structure is changed form bct (Pa) to orthorhombic (U). In particular, the fullphonon dispersion as well as the Eliashberg spectral function (α^{2}F(ω)) and the electronphonon coupling (λ) parameter have been calculated for specific Ucontent (x), and their evolution analyzed as x changes from Pa(x=0) to U(x=1). Our results are discussed on the light of available experimental data. 
Wednesday, March 7, 2018 5:06PM  5:18PM 
P29.00012: Influence of SpinOrbit Coupling on Electron Mobility of WO_{3} Wennie Wang , Youngho Kang , Karthik Krishnaswamy , Chris Van de Walle High doping concentrations can be achieved in tungsten trioxide (WO_{3}), which has an ABO_{3} perovskite structure with the Asite atom absent. Understanding the transport properties of these carriers is critical in many of display applications for which WO_{3} is used. With stateoftheart firstprinciples calculations, we investigate the role of electronphonon scattering in electron transport. We use density functional theory based on hybrid functionals to calculate electronic structure and vibrational spectra. A comparison of using analytical models versus ab initio methods is given, and the microscopic role of spinorbit splitting in the scattering mechanism is examined. 
Wednesday, March 7, 2018 5:18PM  5:30PM 
P29.00013: Phononlimited Hole Mobility in Naphthalene Crystal from ab initio Band Theory NienEn Lee , JinJian Zhou , Luis Agapito , Marco Bernardi We compute from first principles the electronphonon scattering and the phononlimited hole mobility of naphthalene crystal in the framework of ab initio band theory. Our calculations combine GW electronic bandstructures, ab initio electronphonon scattering, and the Boltzmann transport equation. The calculated hole mobility is in very good agreement with experiment between 100−300 K, and we can predict its temperature dependence with high accuracy. We show that intermolecular phonons control the mobility due to their large scattering phase space near the band edge, but contrary to common notions, intramolecular phonons possess the strongest coupling to holes. Our work provides a quantitative framework for computing charge transport in organic crystals, and is a first step toward reconciling bandlike transport with carrier hopping in organic semiconductors. 
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