Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D43: Electrons, Phonons, Electron Phonon Scattering, and Phononics IFocus
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Sponsoring Units: DCOMP DMP Chair: Ivana Savic, Univ Coll Cork Room: 702 |
Monday, March 2, 2020 2:30PM - 3:06PM |
D43.00001: Gauge invariance of heat and charge transport coefficients Invited Speaker: Stefano Baroni Transport coefficients in extended systems have been recently demonstrated to be largely independent of the microscopic representation of the current density of the conserved quantity being transported (charge/mass/energy) [1]. This remarkable gauge invariance of transport coefficients has been leveraged to lay down a rigorous density-functional theory of heat transport [1], as well as a general approach to it, valid in the low-temperature regime, that nicely bridges the Boltzmann-Peierls kinetic model, that applies to crystals, and the Allen-Feldman one, that applies to glasses [2]. In the case of charge transport, a combination of gauge invariance and Thouless’ quantisation of particle transport [3] allows one to express the electrical conductivity of an insulating fluid in terms of integer-valued, scalar, and time-independent atomic oxidation numbers, instead of real-valued, tensor and time-dependent Born charges [3]. In this talk I will review these concepts and report on some key applications of them to liquids and glasses. |
Monday, March 2, 2020 3:06PM - 3:18PM |
D43.00002: Microscopic derivation of coarse-grained, energy-conserving generalized Langevin dynamics Sergei Izvekov Properly simulating non-equilibrium phenomena such as thermal transport in condensed matter systems requires the conservation of system’s internal energy. This precludes application of the coarse-grained (CG) generalized Langevin equation (GLE) dynamics. Attempts to address this issue have been pursued phenomenologically for dissipative particle dynamics (DPD, a Markovian variant of the CG GLE dynamics) by introducing an energy conserving extension of DPD (DPD-E). We present here a rigorous microscopic derivation of energy conserving variants of the CG GLE dynamics by extending the CG equations of motion to include the GLE for certain internal energy observables of the microscopic system. The derivation is performed using the Mori-Zwanzig projection operator method with the recently introduced interpretation of the Zwanzig projection operator.1 Our extension of the GLE dynamics to quasi-equilibrium conditions (necessary to observe heat transport), are based on the generalized canonical ensemble approach and transport equation method. We derive closed microscopic expressions for conductive heat transfer coefficients. After employing the Markov approximation, we compare the equations of motion to the published DPD-E equations. |
Monday, March 2, 2020 3:18PM - 3:30PM |
D43.00003: Cavity Control of Nonlinear Phononics Dominik Juraschek, Tomas Neuman, Johannes Flick, Prineha Narang Nonlinear interactions between phonon modes govern the behavior of vibrationally excited solids and molecules. We demonstrate here theoretically that optical cavities can be used to control the redistribution of energy from a highly-excited coherent infrared-active phonon state into the other vibrational degrees of freedom of the system. The hybridization of the infrared-active phonon mode and the fundamental mode of the cavity induces a polaritonic splitting that we use to tune the nonlinear interactions with other vibrational modes in and out of resonance. We show that not only can the efficiency of the redistribution of energy be enhanced or decreased, but also the underlying scattering mechanisms may be changed. |
Monday, March 2, 2020 3:30PM - 3:42PM |
D43.00004: A hydrodynamic description of the vibrational modes of ordered and disordered systems. matteo baggioli, alessio zaccone In ordinary crystals, Debye theory represents a very successful framework to characterize the dynamics of the vibrational excitations – the phonons, and to predict macroscopic quantities such as specific heat. Disordered systems and in general systems with dissipation provide a challenge to this description, which is manifested in several anomalous properties, e.g. the Boson Peak (BP) excess in the VDOS and the linear in temperature specific heat. Given the universality of these anomalies and their recent observation also in ordered single crystals with no disorder, a more general derivation (not necessarily relying on the presence of disorder) is needed. In this talk we provide such description using a hydrodynamic- effective field theory approach. We prove that a universal BP emerges naturally from the competition between propagation-elasticity and dissipation/diffusive damping. We also show a dependence of the BP in function of the material density which is in perfect agreement with the experimental data. Additionally, we show that the existence of hydrodynamic quasi-localized diffusive mode – the diffusons – can explain the linear in T specific heat in glasses and be therefore an alternative to the well-known TTLS theory. |
Monday, March 2, 2020 3:42PM - 3:54PM |
D43.00005: Angular momentum radiation from current-carrying molecular junctions Zuquan Zhang, Jing-Tao Lü, Jian-Sheng Wang We study the radiation of angular momentum (AM) from current-carrying molecular junctions using the nonequilibrium Green’s function method. The dependence of the the radiation on the molecular geometry is taken into account with a real-space tight-binding model coupled to the electromagnetic field. We study the relation between rotational symmetry of the molecule and the light radiation. We analyze the resonant effects when the applied bias crosses over the energy levels of the molecule. Our study maybe useful for generating radiation with AM from molecular junctions by simply applying an electrical bias. |
Monday, March 2, 2020 3:54PM - 4:06PM |
D43.00006: Ultrafast dynamics of hot carriers in bulk semiconductors and in accumulation layer: energy relaxation and screening effects. Jelena Sjakste The rapid development of the computational methods based on density functional theory, on the one hand, and of the time- energy- and momentum- resolved spectroscopy, on the other hand, allows today an unprecedently detailed insight into the processes governing hot electron relaxation dynamics, and into the role of the electron-phonon coupling. Recently, 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. In this work, we will present our new results, both experimental and theoretical, on hot electron relaxation in silicon. Numerous additional experiments were performed with respect to previous work, and a new interpretation of the measured relaxation times is provided [1]. Moreover, we will present our recent results on the hot electron relaxation in InSe. InSe is a quasi-2D material which was shown recently to have potential interest for optoelectronics [2]. In this work, we will discuss our new results on the relaxation and cooling dynamics in doped InSe. |
Monday, March 2, 2020 4:06PM - 4:18PM |
D43.00007: Finite temperature full multiple scattering calculation of ultrafast x-ray absorption spectroscopy Tun Sheng Tan, John Rehr, Joshua Kas Recent advances in ultrafast time-resolved (TR) x-ray absorption spectroscopy (XAS) have allowed us to probe the interaction between electronic temperature (Te) and lattice temperature (Ti) on the femtosecond time-scale. We present a finite temperature multiple scattering Green's function calculations of XAS, which accounts for the finite Te of the system, to describe these extreme conditions. The effect of temperature on lattice can be included at different levels of approximation such as using correlated Debye (CD) model or dynamical matrix. We apply the method to TR-XANES of an Fe-MgO layered hetero-structure [1] and a copper system [2]. We are able to qualitatively reproduce the experimental results with FEFF using CD model. We also obtain estimates of Te based on a shift in the edge and Ti based on the size of the Debye-Waller factor. |
Monday, March 2, 2020 4:18PM - 4:30PM |
D43.00008: Variational Treatment Beyond the Mean-Field Approximation for the Half-Filled SSH model Stepan Fomichev, Mona Berciu The Su-Schrieffer-Heeger (SSH) model is a common approach to quantifying the effect of electron-phonon coupling on many-body behavior. Developed initially for polyacetylene chains, it assumes that the leading order effect of lattice motion is the modulation of the electronic tight-binding hopping amplitudes. While the SSH model successfully predicts Peierls distortion, the lattice is often treated semiclassically, as there is no known analytical solution for quantum phonons. Motivated by the successes of the momentum average approximation applied to a single Holstein polaron, and extensions to boson-modulated hopping and multiple phonon branches, we generalize the technique to a half-filled (one carrier per unit cell) one-dimensional SSH model. We begin by studying the SSH model in the Born-Oppenheimer approximation, and find that an expansion of the hopping amplitudes to second order in the atomic positions is needed for internal consistency. Using the Peierls distortion ansatz, we find that the acoustic and optical phonon branches emerge even at this mean-field level. We then systematically expand the variational space around this improved mean-field solution to study the nature of the ground state in the non-adiabatic limit. |
Monday, March 2, 2020 4:30PM - 4:42PM |
D43.00009: Nanoscale Friction Control in Transition Metal Dichalcogenides Antonio Cammarata, Tomas Polcar One of the main difficulties in understanding and predicting frictional response is the intrinsic complexity of highly non-equilibrium processes in any tribological contact. |
Monday, March 2, 2020 4:42PM - 4:54PM |
D43.00010: Electrical tuning of vibrational modes in transition metal dichalcogenides Florian Belviso, Tomas Polcar, Antonio Cammarata Understanding frictional phenomena at nanoscale in layered materials is an important stage in order to control layer sliding, occurring in nanoelectromechanical devices. |
Monday, March 2, 2020 4:54PM - 5:06PM |
D43.00011: Effect of the presence of small molecule of contaminants on nanofriction in layered MX2 Transition Metal Dichalcogenides: An ab initio investigation Jamil Missaoui, Tomas Polcar, Antonio Cammarata The control of friction at the nanoscale plays a central role in nanoengineered devices such as nano-electromechanical systems, the latter more and more based on multi-layered Transition Metal Dichalcogenides (TMDs). In the present work, we study the intrinsic friction in MX2 (M = transition metal, X = chalcogen anion) TMDs in the presence of small molecules of contaminants from the atmosphere. |
Monday, March 2, 2020 5:06PM - 5:18PM |
D43.00012: The Order of Longitudinal and Transverse Optical Phonon Modes in Monoclinic Oxides Rafal Korlacki, Mathias Schubert, Alyssa Mock, Sean Knight, Vanya Darakchieva In monoclinic crystals, as required by symmetry, the infrared-active phonon modes have their transition dipoles oriented parallel (Au modes) and perpendicular (Bu modes) to the symmetry axis. In the absence of additional symmetry constrains, the Bu modes are distributed within the monoclinic plane, the pairs of the transverse optical (TO) and longitudinal optical (LO) modes (TO-LO pairs) do not need to be parallel to each other, and this leads to the apparent violation of the TO-LO rule. The analysis of the limiting frequencies within the Born-Huang approach reveals that TO-LO pairs either belong to inner or outer phonon modes. Inner modes are nested within the outer modes. The directional limiting frequencies are bound to within outer mode frequency regions not occupied by inner mode pairs. Hence, an unusual phonon mode order can occur where both lower-frequency as well as upper-frequency limits for the directional modes can be both transverse and/or longitudinal modes. This behavior will be demonstrated in the results of first principles calculations of phonon modes for three different monoclinic crystals: β-Ga2O3 (space group 12), CdWO4 (space group 13), and Y2SiO5 (space group 15), and compared with experimental results based on infrared spectroscopic ellipsometry. |
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