Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P22: Electrons, Phonons, Electron-Phonon Scattering and Phononics IVFocus
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Sponsoring Units: DCOMP DMP Chair: David Parker, Oak Ridge National Laboratory Room: BCEC 157C |
Wednesday, March 6, 2019 2:30PM - 3:06PM |
P22.00001: Electrons modified by phonons: Adiabatic versus non-adiabatic, and the persistence of electron quasiparticles Invited Speaker: Philip B. Allen This talk will briefly review various types of renormalization of electron quasiparticles by phonons, including the question of when quasiparticle properties persist. Several new concerns will be discussed. In summary, band structure energies of order eV’s are affected by phonons of energy 100 times smaller. Semiconductor band gaps are altered by shifts of order 0.1 eV, and are temperature (T)-dependent. The component from thermal expansion is typically smaller than the renormalization from phonons. In metallic bands, an additional low T “mass renormalization”, m*/m>1 is observed. The ratio can range between 1.1 and 3.0. This is a non-adiabatic effect. At low T, metallic electrons notice the time-dependence of the vibrational displacements. The Eliashberg reformulation of conventional BCS superconductivity is closely related. The spectral functions of metallic electrons are often so broad that no recognizable quasiparticle peak is seen. Yet the quasiparticle picture functions nicely for understanding resistivity. Developments in computation have enabled good calculations of semiconductor band renormalization by phonons, which is mostly an adiabatic effect. Recently it has been realized that Froehlich polaron effects (and also piezo-polaron effects) need special treatment when computing energies of electrons close to the band gap. The (non-adiabatic) Froehlich polaron has a clear quasiparticle peak, plus phonon “satellites” in its spectral function. Surprisingly, the peak and satellites are badly misplaced in a conventional perturbative treatment. A “cumulant” version of the spectral function appears to solve the problem nicely. There are still aspects of phonon interactions with electrons that need reformulation, and remain challenges for theory and computation. |
Wednesday, March 6, 2019 3:06PM - 3:18PM |
P22.00002: The electron-phonon problem reconsidered Ilya Esterlis, Steven Kivelson, Douglas J Scalapino The phase diagram of the electron-phonon (e-p) problem, as a function of e-p coupling and temperature, is presented. We work with the Holstein model and present determinant quantum Monte Carlo results for both large and infinite ion mass. In weak-coupling the system is a Fermi-liquid (and ultimately a “low temperature” superconductor), while beyond a critical coupling strength there is a transition to a (π,π) charge-density wave state. At elevated temperatures we find a cross-over to a pseudo-gap regime, corresponding to the formation of bound pairs of electrons with large effective mass (bipolarons). We compare MC results both with predictions from Migdal theory and strong-coupling expansions. For sufficiently weak coupling the Migdal approximation works extremely well but breaks down catastrophically upon entering the pseudo-gap regime, where the physics is accurately described by an Ising lattice gas model, as expected from the strong-coupling expansion. |
Wednesday, March 6, 2019 3:18PM - 3:30PM |
P22.00003: Temperature effects on the electronic band structure of PbTe from first principles Stephen B Fahy, Jose Querales-Flores, Jiang Cao, Ivana Savic
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Wednesday, March 6, 2019 3:30PM - 3:42PM |
P22.00004: First-principles electronic lifetimes and phonon-limited mobility in Si, Diamond, GaP, GaN and SnO2 Guillaume Brunin, Henrique Miranda, Matteo Giantomassi, Gian-Marco Rignanese, Geoffroy Hautier Correctly understanding and computing the electronic transport quantities is crucial for the discovery and developments of new functional materials. Within the Boltzmann transport formalism, the computation of the electrical conductivity of a material is usually performed using the constant relaxation time approximation. In this framework, the electronic lifetime is a parameter extracted either from experimental data or from various semi-empirical models. Only recently, ab initio computations of the electronic lifetimes due to electron-phonon interactions have been reported for various semiconductors and metals, making the full ab initio computation of the phonon-limited (intrinsic) conductivity of materials possible [1-2]. |
Wednesday, March 6, 2019 3:42PM - 3:54PM |
P22.00005: Anharmonic suppression of charge-density-wave instability in bulk and monolayer NbS2 Raffaello Bianco, Ion Errea, Lorenzo Monacelli, Matteo Calandra, Francesco Mauri The superconducting transition-metal dichalcogenide 2H-NbS2, in sharp contrast to the isoelectronic compound 2H-NbSe2, at low temperature does not show any charge-density-wave (CDW) ordering co-existing with the superconductive phase. That is in strong disagreement with ab initio harmonic phonon calculations, which predict that 2H-NbS2 is dynamically unstable. |
Wednesday, March 6, 2019 3:54PM - 4:06PM |
P22.00006: Ab Initio Approach for Exciton-Phonon Interactions Hsiao-Yi Chen, Davide Sangalli, Andrea Marini, Marco Bernardi We derive a formalism to compute exciton-phonon (ex-ph) interactions in crystals within the ab initio GW-Bethe Salpeter equation (BSE) approach. Using first order perturbation theory and the Tamm-Dancoff approximation, we express the ex-ph coupling constant as a superposition of electron- and hole-phonon coupling processes. We discuss numerical calculations of such ex-ph coupling and the related scattering rates, which are challenging as they combine electron-phonon and finite-momentum BSE calculations; we carry them out by using the YAMBO code to obtain exciton dispersions and the PERTURBO code to compute electron-phonon coupling and the ex-ph scattering rates. We apply this framework to investigate ex-ph interactions in bulk hexagonal boron nitride (h-BN), for which we compute the exciton relaxation time due to ex-ph interactions and map it onto the exciton dispersions. We also employ an exciton Boltzmann transport equation to simulate the out-of-equilibrium dynamics of excitons and their equilibration with phonons in h-BN. Our work is a first step toward understanding electron-phonon interactions in composite electronic quasiparticles. It sheds light on correlated electron-phonon processes and provides new computational tools to investigate excited state dynamics. |
Wednesday, March 6, 2019 4:06PM - 4:18PM |
P22.00007: WITHDRAWN ABSTRACT
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Wednesday, March 6, 2019 4:18PM - 4:30PM |
P22.00008: Exploration of promising thermoelectric materials in half-Heusler compounds by high throughput screening Zhenzhen Feng, Yuhao Fu, Aditya Putatunda, Jordan Bavlnka, Yongsheng Zhang, David Singh High throughput (HT) method is a useful tool to screen high-performance thermoelectric materials. We first estimate the electronic transport performance using electronic fitness function (EFF), and calculate the electronic relaxation time using the deformation potential theory. Additionally, the lattice thermal conductivity is calculated by solving the linearized Boltzmann-Peierls transport equation of phonons with the ShengBTE package. Applying the three descriptors to half-Heusler compounds, the important family in thermoelectrics, we calculate 75 these compounds and select many promising thermoelectric materials. For these theoretically determined compounds, we successfully predict previously experimentally and theoretically investigated promising half-Heusler thermoelectric materials. Additionally, a few previously unreported half-Heusler compounds are predicted as promising thermoelectric materials, respectively. Our work provides the new thermoelectric candidates with perfect crystalline structure for the future investigations. |
Wednesday, March 6, 2019 4:30PM - 4:42PM |
P22.00009: First-Principles Calculations of Electron-Phonon Interaction in Solid Materials:
full-potential vs pseudopotential method Feng Gao, Guang-Lin Zhao The calculations of electron-phonon (e-ph) interactions in solid materials from first-principles remain as an important fundamental and applied problem in condensed matter physics. The pseudopotential method has been successfully used to study the valence electronic structure and related physical properties of solid materials. However, when atoms move, they will carry their true electronic potentials with them, not pseudopotentials. The derivatives of true electronic potentials with respect to atomic shifts in a solid will be quite different from the derivatives of pseudopotentials with respect to the same atomic shifts because of the smooth and non-local nature of the pseudopotentials. The essential part of the calculations of e-ph interactions is a reliable computation of the derivatives of electronic potential with respect to atomic position shifts. Consequently, determining how to calculate e-ph interactions reliably by using a pseudopotential method remains as an open question for further research. Before the problems are solved, a full-potential method for the calculation of e-ph interactions can be used to circumvent the problems. |
Wednesday, March 6, 2019 4:42PM - 4:54PM |
P22.00010: Plasmonic hot carriers in transition metal nitrides Adela Habib, Fred Florio, Ravishankar Sundararaman Transition metal nitrides (TMNs) are well established as hard protective coatings on account of their excellent mechanical properties and chemical stability. With recent advances in synthesis, they are becoming increasingly competitive for their opto-electronic properties as refractory plasmonic materials. However, their potential for plasmonic hot carrier harvesting remains largely unknown. In this talk, we show that certain TMNs have a unique wide-band plasmonic behavior that extends deep into the ultraviolet regime. From first-principles calculations, we predict plasmonic response, hot carrier generation and subsequent thermalization of all group IV, V and VI transition metal nitrides, fully accounting for direct and phonon-assisted transitions as well as electron-electron and electron-phonon scattering mechanisms. We find hot carrier lifetimes and mean free paths in these TMNs comparable to those of gold and silver. Finally, we explore mechanisms responsible for the negative real permittivity extending to high frequencies in TMNs, an exciting prospect for stable ultraviolet plasmonics. |
Wednesday, March 6, 2019 4:54PM - 5:06PM |
P22.00011: Electron-phonon coupling within Quasiparticle Self-consistent GW Savio Laricchia, Nicola Bonini, Mark van Schilfgaarde There is clear evidence that standard Density Functional Theory (DFT) understimates electron-phonon coupling interaction in many materials, including even simple sp-bonded compounds. Hybrid functionals and quasiparticle GW corrections suggest that nonlocal exchange-correlation enhances the electron-phonon interaction as a consequence of an improved description of the electronic screening. This has highlighted the need to move beyond local exchange-correlation functionals within DFT, but complete field-theoretic investigations are still missing. In this talk I will introduce the development of a field-theoretic methodology which is able to predict on an equal footing electronic quasiparticles and phonons as well as their interaction. Such an approach has been implemented within the Quasiparticle Self-consistent GW (QSGW) formalism which describes well the electronic properties for a wide range of materials, including many where standard DFT fails. |
Wednesday, March 6, 2019 5:06PM - 5:18PM |
P22.00012: Ab Initio Next-to-Leading Order Electron-Phonon Interactions: Two-Phonon Electron Scattering Processes and their Temperature and Energy Dependence Nien-En Lee, Jin-Jian Zhou, Hsiao-Yi Chen, Marco Bernardi Electron-phonon (e-ph) interactions are a current focus of first-principles calculations. The lowest-order e-ph self-energy is computed in most works, and only recently there were attempts to include higher-order e-ph interactions using the cumulant method. However, diagrammatic approaches beyond the lowest order have not been attempted for computing e-ph interactions ab initio. |
Wednesday, March 6, 2019 5:18PM - 5:30PM |
P22.00013: Simulation of time-resolved electron-phonon scattering on a Dirac cone Benjamin Nosarzewski, Arthur K Mills, MengXing Na, Fabio Boschini, Matteo Michiardi, Ryan P Day, Elia Razzoli, Alexander Sheyerman, Michael Schneider, Giorgio Levy, Sergey Zhdanovich, Alexander Kemper, David J Jones, Andrea Damascelli, Thomas Devereaux Ultrafast spectroscopy is a powerful tool for studying the dynamical properties of quantum materials driven out of equilibrium. The relaxation dynamics of the non-equilibrium state can provide insights into the properties of the elementary scattering process and many-body interactions present in equilibrium. Using the Keldysh formalism we simulate time- and angle-resolved photoemission spectroscopy on a Dirac cone to capture the quantized relaxation processes of electrons coupled to optical phonons. Including retarded electron-phonon interactions, the simulation naturally captures the appearance of a direct excitation peak at an energy set by the frequency of the pump pulse and the delayed appearance of a secondary non-thermal peak above the Fermi level due to electron-phonon scattering as observed in an experiment performed on graphite. |
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