Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L20: Electrons, Phonons, Electron-Phonon Scattering, and Phononics IVFocus Live
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Sponsoring Units: DCOMP DMP Chair: Ivana Savic, Univ Coll Cork |
Wednesday, March 17, 2021 8:00AM - 8:36AM Live |
L20.00001: Theoretical Spectroscopy of 2D Materials: Exciton-phonon coupling in resonant Raman and in luminescence spectroscopy Invited Speaker: Ludger Wirtz 2D and layered materials are known to exhibit very pronounced excitonic effects due to the confinement of electrons and holes in a layer and due to the weak dielectric screening of the electron-hole interaction. A theoretical description of optical spectra that include vibrations must therefore take into account exciton-phonon coupling in order to obtain meaningful results. We present our methods for the calculation of exciton-phonon coupling via a finite displacement [1] and via a diagrammatic approach [2,3], both using many-body perturbation theory. |
Wednesday, March 17, 2021 8:36AM - 8:48AM Live |
L20.00002: Anharmonic Peierls model in the low electronic density limit Alberto Nocera, John Sous, Adrian Feiguin, Mona Berciu The Peierls model of electron-phonon coupling is usually derived assuming small displacements of the atomic orbitals from their equilibrium configuration. In this case, the nearest-neighbor electronic hopping is modified at first order by a term which is linear in the phononic displacements. What happens if we relax this assumption? The inclusion of anharmonic terms is indeed expected to be important for large phononic displacements and large electron-phonon couplings. |
Wednesday, March 17, 2021 8:48AM - 9:00AM Live |
L20.00003: Optical absorption in gallium oxide Hartwin Peelaers, Chris Van de Walle Transparent conducting oxides (TCOs) are a technologically important class of materials used in optoelectronic devices, as TCOs balance two conflicting properties: transparency and conductivity. The requirement of transparency is typically tied to the band gap of the material being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers, required for conductivity, can also lead to optical absorption. This absorption can occur through direct absorption to higher-lying conduction band states, or by an indirect process, for example mediated by phonons or charged impurities. |
Wednesday, March 17, 2021 9:00AM - 9:12AM Live |
L20.00004: Nonequilibrium phonon distribution in current-driven nanostructures Guanxiong Chen, Sergei Urazhdin Electric energy of current flowing in materials or devices is dissipated mostly as Joule heat. However, in nanoscale systems, the generated phonons can escape before they thermalize, which can result in the breakdown of the Joule heating approximation. |
Wednesday, March 17, 2021 9:12AM - 9:24AM Live |
L20.00005: Phonon thermodynamics of rare-earth-iron Laves phases below and above the Curie temperature Sofia Gomez, Bethuel Khamala, Jorge Munoz The cubic rare-earth-iron Laves phases (C15 structure) have been studied extensively because of the useful magnetic and magneto-mechanical properties of its members, particularly giant magnetostriction, but less attention has been paid to interactions between the magnetism and the phonons. We investigated several thermal properties of YFe2, ErFe2 and TbFe2 in the quasiharmonic approximation via both spin-polarized and non-spin polarized density functional theory calculations and obtained phonon dispersions and phonon density of states (DOS) curves below and above the Curie temperature. Comparison to measurements of the Fe-specific phonon DOS curves of the materials obtained via nuclear resonant inelastic x-ray scattering suggests that the quasiharmonic model does not completely encapsulate the physics of these systems. |
Wednesday, March 17, 2021 9:24AM - 9:36AM Live |
L20.00006: Anomalous softening of LA phonons in Clathrate Ba8Ga16Ge30 Susmita Roy, Dan Parshall, Eric Toberer, Mogens Christensen, Dmitry Reznik Phonon-Glass Electron Crystal (PGEC) materials such as clathrates have both glass-like low phonon thermal conductivity and crystal-like high electrical conductivity [1]. This property is of interest due to potential applications in thermoelectric devices. Ba8Ga16Ge30, one of the clathrate compounds, has guest atom Ba trapped inside the Ga/Ge cage structure. There are theoretical [2] as well as experimental investigations on the avoided crossings of acoustic phonons and the flat guest atom modes, which is presumed to be the reason behind its low lattice thermal conductivity [1]. We measured a complete phonon spectrum in several hundred Brillouin zones using inelastic time-of-flight neutron scattering to better understand the origin of low thermal conductivity of Ba8Ga16Ge30. Analysis of the data using the Phonon Explorer software revealed that slope of the longitudinal acoustic branch along the [h 0 0] direction is anomalously reduced and close to that of the transverse acoustic mode, which will decrease the sound velocity and, as a consequence, reduce the thermal conductivity. |
Wednesday, March 17, 2021 9:36AM - 9:48AM Live |
L20.00007: Revisiting the chiral charge-density wave in TiSe2 Darshana Wickramaratne, Richard D Schaller, Gary P Wiederrecht, Goran Karapetrov, Igor Mazin Titanium diselenide (TiSe2) is described as an excitonic insulator that exhibits a chiral charge density wave (CDW) phase. This is based in part on pump-probe measurements that have observed non-thermal melting of the CDW phase and chiral optical transitions in the CDW phase. The microscopic origin of this finite chirality remains an open question since the CDW structure is centrosymmetric and non-thermal melting of the CDW phase alone does not provide sufficient evidence for the condensation of excitons. |
Wednesday, March 17, 2021 9:48AM - 10:00AM Live |
L20.00008: Electronic noise of warm electrons in semiconductors from first-principles Alexander Choi, Peishi Cheng, Austin Minnich The ab-initio theory of low-field electronic transport properties such as carrier mobility in semiconductors is well-established. However, an equivalent treatment of electronic fluctuations about a non-equilibrium steady state, which are readily probed experimentally, remains less explored. Here, we report a first-principles theory of electronic noise for warm electrons in semiconductors. In contrast with typical numerical methods used for electronic noise, no adjustable parameters are required in the present formalism, with the electronic band structure and scattering rates calculated from first-principles. We demonstrate the utility of our approach by applying it to GaAs and show that spectral features in AC transport properties and noise originate from the disparate time scales of momentum and energy relaxation, despite the dominance of optical phonon scattering. Our formalism enables a parameter-free approach to probe the microscopic transport processes that give rise to electronic noise in semiconductors. |
Wednesday, March 17, 2021 10:00AM - 10:12AM Live |
L20.00009: Ab-initio electronic noise of warm electrons in n-type silicon Benjamin Hatanpaa, Alexander Choi, Peishi Cheng, Austin Minnich Ab-initio calculations of low-field mobility are now routine for semiconductors like silicon, but the transport and fluctuational properties at higher fields as well as their spectral features remain unexplored. Here, we report an ab-initio study of AC transport and fluctuational properties of warm electrons in n-type silicon, using a recently developed numerical approach (arXiv:2009.11395). No adjustable parameters are required in this approach, as the scattering rates and band structure are calculated from first-principles. Transverse and longitudinal noise temperatures are reported, and the temperature dependence of intervalley scattering investigated. |
Wednesday, March 17, 2021 10:12AM - 10:24AM Live |
L20.00010: Electron-phonon interactions in transition metal oxides in the framework of DFT+U Jinsoo Park, Jin-Jian Zhou, Iurii Timrov, Andrea Floris, Matteo Cococcioni, Nicola Marzari, Marco Bernardi First-principles approaches for computing electron-phonon (e-ph) interactions enable quantitative studies in a wide range of solids. However, e-ph interactions in many transition-metal oxides (TMOs) remain challenging to treat due to the dominant on-site Coulomb repulsion from open-shell d electrons. Here we develop calculations of e-ph interactions within the framework of Hubbard-corrected density functional perturbation theory (so-called DFPT+U), which can describe the linear response of TMOs and provide an improved treatment of electron self-interactions. Employing a Hubbard U parameter computed ab initio, we demonstrate fully first-principles calculations of the e-ph coupling and the resulting electron spectral functions in various TMOs. While standard DFT e-ph calculations lead to unphysically divergent e-ph coupling, DFT+U restores the correct physics, giving well-behaved e-ph matrix elements that properly include the Frohlich interaction. Our results highlight the key role of the Hubbard U term on e-ph interactions. They further provide a broadly applicable method for predicting e-ph interactions and transport properties in TMOs with localized open-shell d electrons. |
Wednesday, March 17, 2021 10:24AM - 10:36AM Live |
L20.00011: Ultrafast control of material optical properties via the infrared-resonant Raman effect Guru Khalsa, Nicole Benedek, Jeffrey Moses The Raman effect - the inelastic scattering of light by lattice vibrations - provides an important tool for conversion of light from one color to another in optical physics, and is ubiquitous in materials characterization because of its fundamental connection to crystal symmetry. The Raman effect is dominated by changes to the electronic susceptibility in the UV and visible frequency ranges. However, in the mid- and far-IR, nonlinear contributions to the lattice polarization provide additional Raman pathways, which have been little explored. |
Wednesday, March 17, 2021 10:36AM - 10:48AM Live |
L20.00012: Recent technical developments in the EPW code Hyungjun Lee, Samuel Poncé, Elena R Margine, Feliciano Giustino EPW is an open-source Fortran code to study electron-phonon interactions based on density-functional perturbation theory and maximally localized Wannier functions. Since its first release in 2016 as one of the core modules of the Quantum ESPRESSO suite, EPW has been widely used to investigate a variety of phonon-mediated quantum processes in real materials. |
Wednesday, March 17, 2021 10:48AM - 11:00AM Live |
L20.00013: Finite temperature electronic properties of diamond-like carbon materials Arpan Kundu, Marco Govoni, Michele Ceriotti, Francois Gygi, Giulia Galli Accurate calculations of electron-phonon coupling are essential to predict the finite temperature (T) properties of materials and molecules with light atoms. We present an approach to compute electron-phonon coupling where the electronic structure is treated from first principles, e.g. at the DFT level of theory, and nuclear quantum effects are incorporated using either path-integral molecular dynamics or molecular dynamics with a quantum thermostat [1]. In particular, we carried out simulations for diamond, diamondoids, and amorphous carbon by coupling the first-principle molecular dynamics code Qbox (http://qboxcode.org) with i-PI (http://ipi-code.org), a path integral simulation package. We illustrate the role of anharmonicity and disorder in determining electron-phonon coupling, and we compare the zero-temperature limit of our simulations with the results recently reported at T=0 [2]. |
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