Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F8: Electrons, Phonons, and Electron Phonon Scattering IFocus

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Sponsoring Units: DCOMP Chair: Anderson Janotti, University of Delaware Room: 267 
Tuesday, March 14, 2017 11:15AM  11:51AM 
F8.00001: Anharmonic phonons and secondorder phasetransitions by the stochastic selfconsistent harmonic approximation Invited Speaker: Francesco Mauri Anharmonic effects can generally be treated within perturbation theory. Such an approach breaks down when the harmonic solution is dynamically unstable or when the anharmonic corrections of the phonon energies are larger than the harmonic frequencies themselves. This situation occurs near latticerelated secondorder phasetransitions such as chargedensitywave (CDW) or ferroelectric instabilities or in Hcontaining materials, where the large zeropoint motion of the protons results in a violation of the harmonic approximation. Interestingly, even in these cases, phonons can be observed, measured, and used to model transport properties. In order to treat such cases, we developed a stochastic implementation of the selfconsistent harmonic approximation valid to treat anharmonicity in the nonperturbative regime and to obtain, from firstprinciples, the structural, thermodynamic and vibrational properties of strongly anharmonic systems [1]. I will present applications to the ferroelectric transitions in SnTe, to the CWD transitions in NbS$_2$ and NbSe$_2$ (in bulk and monolayer) and to the hydrogenbond symmetrization transition in the superconducting hydrogen sulfide system [2], that exhibits the highest Tc reported for any superconductor so far. In all cases we are able to predict the transition temperature (pressure) and the evolution of phonons with temperature (pressure).\\ $\rm [1]$ I. Errea, M. Calandra, F. Mauri, Phys. Rev. Lett. 111, 177002 (2013) and Phys. Rev. B 89, 064302 (2014)\\ $\rm [2]$ I. Errea, M. Calandra, C. J. Pickard, J. R. Nelson, R. J. Needs, Y. Li, H. Liu, Y. Zhang, Y. Ma, F. Mauri, Nature 532, 81 (2016) [Preview Abstract] 
Tuesday, March 14, 2017 11:51AM  12:03PM 
F8.00002: Large Scale GW Calculations Including ElectronPhonon Interactions Ryan McAvoy, Marco Govoni, Giulia Galli Including the effect of electronphonon coupling in first principle electronic structure calculations is crucial for the accurate prediction of bandgaps and temperature dependent carrier lifetimes. We present results for the electronic properties of condensed and molecular systems, including electronphonon coupling, obtained by merging GW calculations [1] of eigenvalues and an efficient implementation of the FanMigdalDebyeWaller selfenergy. Our implementation does not require summation over virtual states and inversion of large dielectric matrices. Advantages of the algorithm presented here over standard techniques and its scalability will be discussed. [1] Govoni, Marco, and Giulia Galli. "Large scale GW calculations."~\textit{Journal of chemical theory and computation}~11, no. 6 (2015): 26802696. [Preview Abstract] 
Tuesday, March 14, 2017 12:03PM  12:15PM 
F8.00003: Implementation of ElectronPhonon Coupling in the KKR Formalism and its Applications to Simple Metals Carsten Eberhard Mahr, Michael Czerner, Christian Franz, Christian Heiliger Electronphonon coupling is one of the main incoherent inelastic scattering mechanisms in a wide variety of crystalline material systems at room temperature. Therefore, it is necessary to incorporate those effects in any realistic calculation of thermoelectric properties. We do so by extending our density functional theory (DFT) based KorringaKohnRostocker (KKR) Green's function formalism code. By approximating the Fr\"ohlichtype interaction with a selfenergy $\Sigma_\text{eph} = i\frac{\hbar}{2\tau}$ we are able to compute the dressed propagator $G$ by solving Dyson's equation $G = G_\text{ref} + G_\text{ref}\cdot\left(\Delta{}V+\Sigma_\text{eph}\right)\cdot{}G$, where $G_\text{ref}$ is the Green's Function of an arbitrary (though typically repulsive) reference system. The corresponding electronphonon scattering time $\tau$ is extracted from electron linewidth calculations. We demonstrate the physical validity of the beforementioned calculational scheme for nonequilibrium properties by comparing evaluated temperature dependent resistivity characteristics of transport systems consisting of copper, aluminum and other simple metals to experiment. Further, technical details of the implementation in the KKR basis set are presented. [Preview Abstract] 
Tuesday, March 14, 2017 12:15PM  12:27PM 
F8.00004: Oneshot calculation of temperaturedependent optical spectra and phononinduced bandgap renormalization Marios Zacharias, Feliciano Giustino Electronphonon interactions are of fundamental importance in the study of the optical properties of solids at finite temperatures. Here we present a new firstprinciples computational technique based on the WilliamsLax theory for performing predictive calculations of the optical spectra, including quantum zeropoint renormalization and indirect absorption [1]. The calculation of the WilliamsLax optical spectra is computationally challenging, as it involves the sampling over all possible nuclear quantum states. We develop an efficient computational strategy for performing "oneshot" finitetemperature calculations [2]. These require only a single optimal configuration of the atomic positions. We demonstrate our methodology for the case of Si, C, and GaAs, yielding absorption coefficients in good agreement with experiment. This work opens the way for systematic calculations of optical spectra at finite temperature. [1] M. Zacharias, C. E. Patrick, and F. Giustino, Phys. Rev. Lett. 115, 177401 (2015). [2] M. Zacharias and F. Giustino, Phys. Rev. B 94, 075125 (2016). [Preview Abstract] 
Tuesday, March 14, 2017 12:27PM  12:39PM 
F8.00005: Using atomic orbitals to compute and interpolate the electronphonon interaction Luis Agapito, Marco Bernardi The electronphonon interaction (EPI) controls key dynamical properties in materials, including charge and spin transport, and ultrafast carrier dynamics. Accurately computing these properties involves obtaining the EPI on very fine Brillouin zone (BZ) grids; since direct ab initio computation is prohibitively expensive, schemes to interpolate the EPI are needed. We present a new method that uses atomicorbitals (AOs) to compute and interpolate the EPI on fine BZ grids, with important advantages over the alternative Wannier function (WF) interpolation. Different from WFs, AOs are readily available from databases. Their use can be automated without the trialanderror optimization required in WFs, making AOs suitable for highthroughput study of carrier dynamics in materials. We will discuss the formalism and numerical implementation of our approach, comparing computational cost and accuracy with WFs. The impact of the completeness of the AO set (i.e., single zeta, double zeta, etc.) on the accuracy will be analyzed. Applications to complex materials for which WFs are not ideally suited will be discussed, along with processes that are naturally mapped onto AO sets, such as polaron transport in conjugated molecules and correlated oxides. [Preview Abstract] 
Tuesday, March 14, 2017 12:39PM  12:51PM 
F8.00006: Firstprinciples calculation of indirect absorption in transparent conducting oxides Hartwin Peelaers, Emmanouil Kioupakis, Chris G. Van de Walle Transparent conducting oxides (TCOs) are a technological important class of materials that combine high electrical conductivity with transparency in the visible light spectrum. The most frequently used material is Sndoped In$_2$O$_3$, also called ITO. The Sn doping provides the free carriers required for the conductivity. The transparency of the material can be attributed to the fact that direct absorption processes, either acrossthegap or by direct freecarrier absorption, require energies larger than that of visible light photons. But light absorption can still occur due to indirect freecarrier absorption, which is usually described by a phenomenological Drude model. To improve the fundamental understanding of the processes limiting the transparency in these materials, we use a fully firstprinciples methodology based on density functional theory [1]. We will discuss the importance of phonon and defectassisted absorption and compare the results for ITO with our earlier results for SnO$_2$ [1]. \\ \\~ [1] H. Peelaers, E. Kioupakis, and C.G. Van de Walle, Phys. Rev. B 92, 235201 (2015). [Preview Abstract] 
Tuesday, March 14, 2017 12:51PM  1:03PM 
F8.00007: Firstprinciples carrier transport using the Boltzmann Transport Equation in EPW Samuel Ponc\'e, Elena Roxana Margine, Feliciano Giustino Understanding transport and carrier mobilities in semiconductors is crucial in electronics, optoelectronics, and energy applications. To predict mobilities in theoretical (not yet made) materials would be a remarkable achievement. Fully predictive firstprinciples calculations of mobilities have only been made possible recently with the advance of abinitio tools to compute electronphonon interactions with high accuracy. Here, we present a comprehensive framework to study electron and hole transport in metals and insulators within the full selfconsistent linearized Boltzmann transport equation and its approximations. The theory is implemented into the free and opensource code EPW [1,2]. We study four representative semiconductors: Si, GaAs, GaN and bulk MoS$_2$ and discuss challenges associated with ultra dense sampling and polar electronphonon interactions. \newline [1] S. Ponc\'e \textit{et al.}, Comput. Phys. Commun. \textbf{209}, 116 (2016) \newline [2] http://epw.org.uk [Preview Abstract] 
Tuesday, March 14, 2017 1:03PM  1:15PM 
F8.00008: DQMC study on Holstein model with momentumdependent electronphonon coupling Weiting Chiu, Richard Scalettar We use the determinant quantum Monte Carlo (DQMC) method to study the Holstein model with a momentumdependent electronphonon coupling $\lambda(\textbf{q})$. The correlation functions and their Fourier transformed structure factors are calculated. In addition, the single particle Green's function and spectral functions are obtained. These quantities are used to infer the nature of longrange charge order as well as the renormalization of the electron and phonon propagators. Study of this model is motivated by the suggestion that electronphonon interaction with large "forward scattering" is relevant to the understanding of the physics of the thin layers of singleunitcell iron selenide (FeSe) grown on a strontium titanate (SrTiO$_{3}$) substrate, possibly giving rise to the enhanced superconducting transition temperature and replica bands seen in ARPES experiments. [Preview Abstract] 
Tuesday, March 14, 2017 1:15PM  1:27PM 
F8.00009: Electronphonon scattering rates in GaAs and AlAs under hydrostatic pressure Nandan Tandon, L. R. RamMohan, John D. Albrecht We present a firstprinciples plane wave pseudopotential study on the electronphonon (elph) scattering rates in GaAs and AlAs under hydrostatic pressure. Effect of pressure on the band gap and phonon dispersion is well studied in the past, where the direct band gap of GaAs becomes larger than the indirect band gap at X making it an indirect bandgap semiconductor over $\sim 3$GPa. Under hydrostatic pressure AlAs remains an indirect bandgap semiconductor with the bandgap decreasing as pressure increases. Phonon dispersion under hydrostatic pressure results in softening of acoustic phonons and optical frequencies shift to higher values. We will discuss the resulting effect of pressure on the elph scattering rates. The hydrostatic pressure is varied between $015$GPa. [Preview Abstract] 
Tuesday, March 14, 2017 1:27PM  1:39PM 
F8.00010: Ab initio Electron Mobility and Polar Phonon Scattering in GaAs JinJian Zhou, Marco Bernardi In polar semiconductors and oxides, the longrange nature of the electronphonon (\textit{e}ph) interaction is a bottleneck to compute charge transport from first principles. Here, we develop an efficient ab initio scheme to compute and converge the \textit{e}ph relaxation times (RTs) and electron mobility in polar materials. We apply our approach to GaAs, where using the Boltzmann equation with statedependent RTs, we compute mobilities in excellent agreement with experiment at 250$$500~K. The $e$ph RTs and the phonon contributions to intravalley and intervalley $e$ph scattering are also analyzed. Our work enables efficient ab initio computations of transport and carrier dynamics in polar materials. [Preview Abstract] 
Tuesday, March 14, 2017 1:39PM  1:51PM 
F8.00011: Firstprinciples studies of electron transport in Ga$_2$O$_3$ Youngho Kang, Karthik Krishnaswamy, Hartwin Peelaers, Chris G. Van de Walle Ga$_2$O$_3$ is a widegap semiconductor with a monoclinic crystal structure and a band gap of 4.8 eV. Its high carrier mobility and large band gap have attracted a lot of attention for use in high power electronics and transparent conductors. Despite its potential for adoption in these applications, an understanding of its carrier transport properties is still lacking. In this study we use firstprinciples calculations to analyze and compute the electron scattering rates in Ga$_2$O$_3$. Scattering due to ionized impurities and polar longitudinaloptical (LO) phonon is taken into account. We find that the electron mobility is nearly isotropic, despite the lowsymmetry monoclinic structure of Ga$_2$O$_3$. At low carrier densities ($\sim$10$^{17}$ cm$^{3}$), the mobility is limited by LO phonon scattering. Scattering by ionized impurities becomes increasingly important at higher carrier densities. This type of scattering is enhanced when compensating native point defects are present; in particular, gallium vacancies, which are triply negatively charged, can have a strong effect on mobility. These effects explain the downturn in mobility observed in experiments at high carrier densities. [Preview Abstract] 
Tuesday, March 14, 2017 1:51PM  2:03PM 
F8.00012: Phonon Calculations Using the RealSpace Multigrid Method (RMG) Jiayong Zhang, Wenchang Lu, Emil Briggs, Yongqiang Cheng, A.J. RamirezCuesta, Jerry Bernholc RMG, a DFTbased opensource package using the realspace multigrid method, has proven to work effectively on large scale systems with thousands of atoms. Our recent work has shown its practicability for high accuracy phonon calculations employing the frozen phonon method. In this method, a primary unit cell with a small lattice constant is enlarged to a supercell that is sufficiently large to obtain the force constants matrix by finite displacements of atoms in the supercell. An opensource package PhonoPy is used to determine the necessary displacements by taking symmetry into account. A python script coupling RMG and PhonoPy enables us to perform highthroughput calculations of phonon properties. We have applied this method to many systems, such as silicon, silica glass, ZIF8, etc. Results from RMG are compared to the experimental spectra measured using the VISION inelastic neutron scattering spectrometer at the Spallation Neutron Source at ORNL, as well as results from other DFT codes. The computing resources were made available through the VirtuES (Virtual Experiments in Spectroscopy) project, funded by Laboratory Directed Research and Development program (LDRD project No. 7739) [Preview Abstract] 
Tuesday, March 14, 2017 2:03PM  2:15PM 
F8.00013: Fr\"ohlich electronphonon coupling from firstprinciples in anatase TiO$_2$ Carla Verdi, Feliciano Giustino The calculation of electronphonon properties in polar materials from first principles remains largely unexplored despite their ubiquitous role in many technological applications, such as lightemitting devices and transparent electronics. In polar semiconductors and insulators the electrons can be strongly coupled to the macroscopic electric field induced by longitudinal optical (LO) phonons at long wavelength, leading to the socalled Fr\"ohlich interaction. In our work we develop a general formalism for calculating the Fr\"ohlich vertex from first principles, which can be used as a powerful tool in conjunction with \textit{ab initio} interpolation based on maximally localized Wannier functions [1]. We demonstrate our method by computing the electron lifetimes in anatase TiO$_2$ and we establish quantitatively the importance of the inclusion of the \textit{ab initio} Fr\"ohlich coupling. Finally, we address the problem of investigating the properties of polaronic quasiparticles, i.e. electrons dressed by a phonon cloud, that can easily form in polar materials and affect their charge transport properties.\\ [4pt] [1] C. Verdi and F. Giustino, Phys. Rev. Lett. \textbf{115}, 176401 (2015). [Preview Abstract] 
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