Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V29: Electrons, Phonons, Electron Phonon Scattering and Phononics VFocus
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Sponsoring Units: DCOMP DMP Chair: Matthieu Verstraete, Univ de Liege Room: LACC 406A |
Thursday, March 8, 2018 2:30PM - 2:42PM |
V29.00001: Anomalous electron-phonon coupling in the proximity of a ferroelectric phase transition. Piotr Chudzinski, Tchavdar Todorov, Pablo Aguado-Puente, Myrta Gruening, Javier Fernández Troncoso, Jorge Kohanoff We study the problem of electron-phonon coupling in a framework of effective correlated electron-ion dynamics, hence going beyond the standard adiabatic approximation for electrons and Ehrenfest dynamics for ions' vibrations. We focus on the coupling with the transverse optical (TO) branch, a mode that plays a crucial role in the ferroelectric phase transition -- its softening drives the system to the transition. This softening results in an anomalously strong dispersion of the optical mode and brings it very close to acoustic branches. Our study is motivated by recent experimental findings in PbTe where hybridization with the LA mode and the so-called 'waterfall' of spectral weight were detected. By accounting for a strong anharmonicity present in the system close to the transition, we are able to identify a novel scattering mechanism which is driven by the presence of fluctuations of the order parameter in the critical regime. Subsequently, we consider the coupling of TO phonons with electrons as well as TO-LA phonon-phonon scattering. Since our study is dedicated to intermediate temperatures, where the phonon drag effect is the largest, we conclude with a discussion of possible implications for thermoelectric coefficients. |
Thursday, March 8, 2018 2:42PM - 2:54PM |
V29.00002: First principle calculations of phonon-limited electron mobility in GaN Tianshi Wang, Zhigang Gui, Anderson Janotti, Chaoying Ni For many advanced applications of GaN, electron mobility plays a key role in the device performance. Phonon strongly impacts electron scattering and thus electron mobility for a wide range of temperatures and carrier concentrations. However, the mechanism of electron-phonon scattering remains to be explored in GaN. Using density functional theory, we computed carrier scattering rate and transport properties for zinc blende n-type GaN with carrier concentration (n) of 1015 to 1019 cm-3 in the temperature range of 100 to 400 K. The calculated Hall mobility shows good agreement with experimental data. Furthermore, we found that the mobility remains the same for lightly doped (1015 < n < 1017 cm-3) cases, which is explained by values of the occupation numbers in scattering rate expression. We also found that electrons with mean free paths (MFPs) below 40 nm at n = 1017 cm-3 (30 nm at n = 1019 cm-3) provide dominant contribution to electron transport at 300 K, shedding light on GaN-related nanoengineering. |
Thursday, March 8, 2018 2:54PM - 3:06PM |
V29.00003: First-order resonant Raman intensities of MoS2 from first-principles Henrique Miranda, Sven Reichardt, Alejandro Molina-Sanchez, Ludger Wirtz We investigate the laser-energy dependent Raman intensities of MoS2 using two different approaches. In the first approach, we calculate the Raman intensities from finite differences of the dielectric susceptibility according to the phonon displacements. In the second, we formulate the Raman tensor in terms of time-dependent perturbation theory and calculate it using electron-photon and electron-phonon coupling matrix elements obtained from density functional theory. We demonstrate that the two approaches are formally and numerically equivalent in the adiabatic limit. The second approach allows to include the phonon dynamical effects and captures the Stokes and anti-Stokes shift with respect to the adiabatic case. This method, which is computationally more efficient, is also extended to include temperature effects with electronic lifetimes calculated from many-body perturbation theory. |
Thursday, March 8, 2018 3:06PM - 3:42PM |
V29.00004: Electronic and Phononic Thermal Conduction in Materials with Metal-Insulator Phase Transitions Invited Speaker: Junqiao Wu Metal-insulator phase transition (MIT) occurs in a wide range of materials with diverse mechanisms, such as the Mott and Peierls transitions. In contrast to existing in-depth investigations of electrical conduction across the MIT, thermal conduction of these materials has been relatively less explored owing to difficulties in accurate measurements and the high sensitivity of heat conduction to materials quality and configuration. However, exploration of electrons and phonons carried thermal conduction across the MIT is much needed because it would provide unique and deep insight into the phase transition physics and charge dynamics of the materials. In this invited talk, we will discuss our recent results in measurements and analysis of thermal conductivity of high-quality, single-crystal nanowire/nanoribbon of materials with MIT, including doped VO2 whose MIT is believed to have a coupled Mott-Peierls nature, and TaS2 and TaS3 that are typical charge density wave materials. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V29.00005: Phonon-Induced Absorption Line Shapes in Organic Semiconductors Michel Panhans, Johannes Benduhn, Karl Schellhammer, Koen Vandewal, Frank Ortmann
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Thursday, March 8, 2018 3:54PM - 4:06PM |
V29.00006: Theory of the exciton-phonon coupling Gabriel Antonius, Steven Louie The influence of electron-phonon interactions on optical absorption spectra requires a special treatment in materials with strong electron-hole interactions. We conceptualize these effects as exciton-phonon coupling [1]. Through phonon absorption and emission processes, the optically accessible excitons are scattered into other dark, finite-momentum exciton states. The exciton-phonon self energy therefore contributes to the temperature dependence of the optical transitions and their broadening. Two-bands model [2] shows how the full exciton-phonon self energy differs both qualitatively and quantitatively from commonly used approximations. We present the implementation of a first-principles calculation scheme, making use of density functional theory (DFT), density function perturbation theory (DFPT), and the Bethe-Salpeter equation (GW-BSE) formalism. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V29.00007: Thermal relaxation of electrons in semiconductors Pierre Darancet, Maria Chan, Sridhar Sadasivam Using a novel first-principles framework, we study the timescales of energy transfer and thermalization of highly energetic "hot" electrons with an atomic lattice for semiconductors [1]. In particular, we compute phonon-specific temperatures and account for both phonon-phonon and electron-phonon interactions in our semiclassical description. For polar and nonpolar semiconductors, we show that the coupled phonon and electron dynamics departs qualitatively from the two-temperature (2T) physical picture over time scales ranging from 1 to 100 ps after excitation. We demonstrate that this disagreement stems from the breakdown of the hypothesis of thermal equilibrium within the lattice subsystem, and generalize the 2T model of Allen [2] to account for slow phonon thermalization as a limiting step of electron-phonon thermalization. We discuss how our model can be used to extract more information on the electron-phonon interactions from time-resolved spectroscopy experiments. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V29.00008: Temperature-Dependent Phonons in Single-Crystal FeGe2 Yang Shen, Hillary Smith, Brent Fultz Inelastic neutron scattering (INS) was used to measured phonon dispersions in a single crystal of FeGe2 with the C16-structure at 300, 500 and 635 K. Phonon density of states (DOS) were also measured on polycrystalline FeGe2 from 325 to 1050 K, and the Fe partial DOS was obtained from polycrystalline 57FeGe2 by nuclear resonant inelastic x-ray scattering (NRIXS) at 300 K. The thermal broadening of high-energy modes is the dominant feature in the temperature dependence of the phonon spectrum. The energy shifts of the low- and high-energy parts of the spectrum were almost the same. DFT calculations performed with the quasiharmonic approximation (QHA) gave results in reasonable, but not excellent agreement with the experimental thermal energy shifts. The thermal broadening of the phonon spectrum and dispersions, especially at high energies, indicates a significant cubic anharmonicty to second order that should also induce phonon shifts. The different anharmonic contributions largely cancel, however, giving average phonon shifts in fair agreement to calculations with the QHA. |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V29.00009: Vibrational Self-Consistent Field Method Applied to the Ferroelectric Phase Transitions of Barium Titanate Joseph Prentice, Nicholas Bristowe, Richard Needs Barium titanate is the prototypical ferroelectric ceramic, but the nature of the transitions between the various ferroelectric phases and the cubic non-ferroelectric phase is still under significant debate in the literature, with several different models proposed, such as the displacive and order-disorder models. The work presented here uses a vibrational self-consistent field method to obtain the anharmonic vibrational wavefunction of the system, and uses this to obtain a fully first-principles description of the phase transitions of this material for the first time. Our results are in qualitatitive agreement with the order-disorder description of phase transitions in barium titanate, with eight equal regions of nuclear probability density at high temperature, which gives way to a lower symmetry configuration at lower temperatures. To obtain a full quantitative picture of these phase transitions, further effects must be included, and potential ways of approaching this will be discussed. |
Thursday, March 8, 2018 4:42PM - 4:54PM |
V29.00010: Anharmonicity in Equation of State Calculations: A Cross-Validation Study Raymond Clay, Luke Shulenburger, Michael Desjarlais The ability to accurately capture anharmonic effects is especially important for predicting and characterizing the behavior of materials at extreme conditions. Firstly, anharmonic contributions to the free energy can be both quantitatively and qualitatively significant for low Z materials and most materials at high enough temperatures. Secondly, since vibrational spectroscopy is one of the primary diagnostics for materials at extreme conditions, a proper inclusion of anharmonicity can sometimes be necessary to properly compare theoretical structure predictions to observed spectroscopic data. In this work, we use path integral molecular dynamics around the BCC-FCC transition in elemental lithium to benchmark and validate some of the main methods used in equation of state calculations: the vibrational self consistent field method, the 2PT model, and one variant of perturbation theory. |
Thursday, March 8, 2018 4:54PM - 5:06PM |
V29.00011: Ab-initio thermal properties of high-pressure silica Hugo Aramberri, Riccardo Rurali, Jorge Iniguez We investigate the thermal properties of silica as it undergoes a pressure-driven structural phase transition with first principles calculations. We compute the lattice thermal conductivity of SiO2 for a wide range of temperatures and pressures by solving the Boltzmann transport equation iteratively. We find that, at low temperatures, silica displays a large peak in the in-plane thermal conductivity and a highly anisotropic behavior close to the phase boundary. We further trace back the origin of this behavior by analyzing the phonon contributions to the conductivity. |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V29.00012: Phonon Anharmonicity in Cuprite Claire Saunders, Dennis Kim, Olle Hellman, Brent Fultz Abstract Body: We report large anharmonicity of optical phonons in cuprite (cuprous oxide, Cu2O). Phonon dispersions were measured at 10, 300, and 700 K by rotating a single crystal in the ARCS spectrometer at ORNL, and folding the data back into the first Brillouin zone. Ab initio calculations of harmonic phonons accounted for the dispersions at 10 K. At 300 K, however, broadenings of phonon energies made it impossible to use harmonic or quasiharmonic interpretations. The broadening was calculated from the cubic anharmonicity using the stochastic Temperature Dependent Effective Potential method (quasiharmonic phonons in an anharmonic potential), but the experimental broadening was considerably larger than calculated. Results will be compared from ongoing ab initio molecular dynamics calculations and experimental phonon linewidths with full background corrections. Cuprite is stable to a temperature of 1503 K, but the phonon spectrum at 700 K is already so broad that it may not be interpretable with phonons. Implications for vibrational entropy and chemical bonding will be discussed. |
Thursday, March 8, 2018 5:18PM - 5:30PM |
V29.00013: Temperature Driven Phase Transformation in Zeolitic Imidazolate Framework . Suchitra, Umesh V Waghmare Metal organic frameworks (MOFs) are known to exhibit a variety of crystal structures under pressure, temperature, and gas adsorption. Desolvated zeolitic imidazole frameworks show temperature dependent phase transition. On cooling to 140 K, its volume contracts by 23\%, while its symmetry remains the same. Through first-principles calculations, we attempt to understand the dynamical properties of high and low temperature phases. We identified the soft acoustic phonon modes of high temperature phase that are responsible for the phase transitions. The transition is driven by the vibrational entropy. Our calculated transition temperature is 170 K which is quite close to the experimental value of 140 K. |
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