Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V19: Predictive Modeling of Electron-Phonon Coupling in Condensed-Matter PhysicsInvited
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Sponsoring Units: DCOMP Chair: Walter Lambrecht, Case Western Reserve University Room: 278-279 |
Thursday, March 16, 2017 2:30PM - 3:06PM |
V19.00001: Electronic structure of solids, including vibrational effects : Temperature dependence and zero-point motion. Invited Speaker: Xavier Gonze Modifications of electronic eigenenergies due to vibrational effects and electron-phonon coupling are sizable in many materials with light atoms. While often neglected, they have been recently computed from first principles using different formalisms, among which the perturbation-based Allen-Heine-Cardona (AHC) approach, considering both the adiabatic and the non-adiabatic harmonic approximation. Electron-phonon coupling can be obtained from density-functional perturbation theory (DFPT) as well as from many-body perturbation theory (MBPT), e.g. the GW approximation. I will provide a brief overview of the concepts and formalisms, and present recent progresses, including : the validation of AHC theory implementation in two different software applications, with an agreed DFPT zero-point motion correction of 0.4 eV for the direct bandgap of diamond [1] ; the MBPT result for the same material, 40\% higher [2]; the breakdown of the adiabatic AHC theory for infrared-active materials, and fix of this problem in the non-adiabatic AHC theory [3,4], with results for diamond, Si, LiF, AlN, BN, and a dozen oxydes; the connection with the Frohlich Hamiltonian and polaron physics. [1] S. Poncé, G. Antonius, P. Boulanger, E. Cannuccia, A. Marini, M. Côté and X. Gonze, Computational Materials Science 83, 341 (2014). [2] G. Antonius, S. Poncé, P. Boulanger, M. Côté and X. Gonze, Phys. Rev. Lett. 112, 215501 (2014). [3] S. Poncé, Y. Gillet, J. Laflamme Janssen, A. Marini, M. Verstraete and X. Gonze, J. Chem. Phys. 143, 102813 (2015). [4] G. Antonius, S. Poncé, E. Lantagne-Hurtubise, G. Auclair, X. Gonze and M. Côté, Phys. Rev. B 92, 085137 (2015). [Preview Abstract] |
Thursday, March 16, 2017 3:06PM - 3:42PM |
V19.00002: Electronic properties with and without electron-phonon coupling Invited Speaker: Philip Allen To decent approximation, electronic properties P of solids have a temperature dependence of the type $\Delta $P(T) $= \quad \Sigma $ (dP/d$\omega _{\mathrm{i}})$[n$_{\mathrm{i}}$(T)$+$1/2], where $\omega_{\mathrm{i}}$ is the frequency of the i$^{\mathrm{th}}$ vibrational normal mode, and n$_{\mathrm{i}}$ is the Bose-Einstein equilibrium occupation of the mode. The coupling constant (dP/d$\omega_{\mathrm{i}})$ comes from electron-phonon interactions. At T$=$0, the ``1/2'' gives the zero-point electron-phonon renormalization of the property P, and at T\textgreater $\Theta_{\mathrm{D}}$, the total shift $\Delta $P becomes linear in T, extrapolating toward $\Delta $P$=$0 at T$=$0. This form of T-dependence arises from the adiabatic or Born-Oppenheimer approximation, where electrons essentially ``don't notice'' the time-dependence of thermal lattice fluctuations. In other words, the leading order theory for P is $\Delta $P(T) $= \quad \Sigma $ (d$^{\mathrm{2}}$P/du$_{\mathrm{i}}$du$_{\mathrm{j}})$\textless u$_{\mathrm{i}}$u$_{\mathrm{j}}$\textgreater , responding to the thermal average mean square lattice displacement, as if it were static. There are two situations where non-adiabatic effects alter things. (1) In metals at low T, the thermal smearing k$_{\mathrm{B}}$T of the sharp Fermi edge gets small ($\omega_{\mathrm{i\thinspace }}$\textless \textless k$_{\mathrm{B}}$T). Then non-analyticity of k-integrals requires phonon energy to be included in perturbative denominators. (2) In insulators with polar phonons, Froehlich polaron effects enter, and k-integrals diverge unless phonon energies are kept. Most non-adiabatic effects become unimportant by room temperature, but the low T consequences can be very interesting (e.g. superconductivity.) This talk will discuss the confusing history and predict some future developments in this field. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 4:18PM |
V19.00003: The role of electron-phonon coupling in carrier capture at defects Invited Speaker: Cyrus E. Dreyer Understanding capture of charge carriers by point defects is crucial to improving the performance of electronic and optoelectonic devices. For example, the process of Shockley-Read-Hall (SRH) recombination, which limits the efficiency of light emitters and photovoltaics, involves the sequential capture of an electron and hole at a defect. Capture processes can occur radiatively, through the emission of a photon, or nonradiatively, mediated by phonons. In both cases, understanding the coupling of the carrier to the vibronic structure of the defect is crucial to calculating the rate of the process. I will outline our recently-developed methodology to calculate radiative and nonradiative capture rates. Then I will demonstrate how this quantitative understanding allows us to predict defects that will result in SRH in optoelectronic devices. The material system that we explore is InGaN alloys, which are key materials for high-efficiency light emitting diodes across the visible spectrum. We find that gallium vacancies complexed with hydrogen and/or oxygen have SRH rates that will be detrimental to devices emitting at yellow and green wavelengths.\footnote{C. E. Dreyer, A. Alkauskas, J. L. Lyons, J. S. Speck, and C. G. Van de Walle, Appl. Phys. Lett. \textbf{108}, 141101 (2016).} For wider-band-gap InGaN alloys, we demonstrate that a novel mechanism involving the excited electronic states of defects is required to explain experimentally observed SRH rates.\footnote{A. Alkauskas, C. E. Dreyer, J. L. Lyons, and C. G. Van de Walle, Phys. Rev. B \textbf{93}, 201304(R) (2016).}$^,$\footnote{D. Wickramaratne, J.-X. Shen, C. E. Dreyer, M. Engel, M. Marsman, G. Kresse, S. Marcinkevicius, A. Alkauskas, and C. G. Van de Walle, Appl. Phys. Lett. \textbf{109}, 162107 (2016).} [Preview Abstract] |
Thursday, March 16, 2017 4:18PM - 4:54PM |
V19.00004: Dynamics of highly excited electrons photoinjected into GaAs: formation and decay of hot-electron ensembles. Invited Speaker: Jelena Sjakste Electron scattering by phonons is one of the major processes that determine the transport characteristics and relaxation dynamics in semiconductor-based devices. \\ Although density functional theory (DFT) -based computational methods for the calculation of the electron-phonon coupling matrix elements in metals exist since the late nineties, DFT-based calculations of the electron-phonon coupling in semiconductors started much later [1,2]. Very recently, we have developed a computational method for the calculation of the electron-phonon coupling in polar semiconductors, based on the interpolation of the electron-phonon matrix elements in Wannier representation [3]. This method allowed us to successfully interpret the dynamics of hot electron relaxation in bulk GaAs, in excellent agreement with time- and angle- resolved photoemission experiment by the group of K. Tanimura (University of Osaka, Japan). The measured, and calculated, electron-phonon scattering times turned out to be surprisingly fast, of the order of a few tens of femtoseconds. Moreover, we have demonstrated, for the relaxation of hot carriers in GaAs, the existence of two distinct relaxation regimes, one related with the momentum, and the other with energy relaxation. Both regimes are shown to be almost entirely ruled by the electron-phonon interaction [4].\\ 1. J. Sjakste, N. Vast, V. Tyuterev, Phys. Rev. Lett. 99, 236405 (2007) \\ 2. Annual Reviews of Heat Transfer, Vol. 17, Begell House Inc, Danbury, CT, USA, J. Sjakste, I. Timrov, P. Gava, N. Mingo, and N. Vast (2014). \\ 3. J. Sjakste, N. Vast, M. Calandra, F. Mauri, Phys. Rev. B 92, 054307 (2015). \\ 4. H. Tanimura, J. Kanasaki, K. Tanimura, J. Sjakste, N. Vast, M. Calandra, F. Mauri, Phys. Rev. B 93, 161203 (R) (2016). [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:30PM |
V19.00005: Predictive density matrix embedding theory of correlated systems Invited Speaker: Garnet Chan I will describe our work in density matrix embedding theory as applied to correlated quantum systems. I will focus especially on our recent work on ab-initio Hamiltonians, electron-phonon coupling, time-dependence, and spin-orbit effects. [Preview Abstract] |
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