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 S18: Ultrafast Dynamics of Coupled Electrons and Atomic Motions: Experiments and Precise SimulationsInvited Live
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Sponsoring Units: DCOMP Chair: Andre Schleife, Univ. of Illinois at Urbana; Angel Rubio, Max Planck Inst Structure & Dynamics of Matter |
Thursday, March 18, 2021 11:30AM - 12:06PM Live |
S18.00001: Recent advances in light induced superconductivity Invited Speaker: Andrea Cavalleri I will discuss how coherent electromagnetic radiation at infrared and TeraHertz frequencies can be used to drive collective excitations in solids and especially experiments in which superconducting correlations can be induced at temperatures higher than the thermodynamic transition temperature. I will discuss results in cuprates, doped fullerenes and recent work in organic salts. By comparing these examples a clearer microscopic picture is emerging for driven unconventional superconductivity. |
Thursday, March 18, 2021 12:06PM - 12:42PM Live |
S18.00002: Ultrafast dynamics of coupled electrons, phonons and excitons from first principles Invited Speaker: Marco Bernardi Combining density functional theory and related methods with kinetic equations has enabled remarkable advances in computing the ultrafast dynamics of materials from first principles. After reviewing this framework, I will present a numerical approach to evolve in time the coupled Boltzmann transport equations (BTEs) of electrons and phonons, using ab initio electron-phonon and phonon-phonon interactions together with a parallel algorithm to explicitly time step the BTEs. Our approach can simulate the coupled electron and phonon dynamics up to tens of picoseconds (with a femtosecond time resolution) and its quantitative accuracy can be validated by computing transport properties. Using graphene as a case study, I will demonstrate calculations of coupled ultrafast carrier and phonon population dynamics, and simulations of time-resolved transient absorption, structural snapshots, and diffuse X-ray scattering. Extensions to include excitonic effects will also be discussed, focusing on calculations of exciton-phonon interactions and real-time exciton dynamics in two-dimensional materials. Code development efforts and future directions will be outlined. |
Thursday, March 18, 2021 12:42PM - 1:18PM Live |
S18.00003: Energy conversion pathways in graphite from attosecond X-ray spectroscopy Invited Speaker: Jens Biegert The conversion of light to fundamental excitations of matter is governed by the build-up of electronic coherences and their dephasing to excited quasiparticles due to scattering processes, which occur on atto- and femtosecond timescales. Disentangling the interplay of these mechanisms, and how they lead to a specific flow of energy inside a material, is extremely challenging since many of these effects occur on overlapping temporal scales. I will discuss the semimetal graphite which was investigated with attosecond K-shell X-ray absorption near edge structure (XANES) spectroscopy and how the combination of the measurement with theoretical modeling allows to assign the spectroscopic signatures to microscopic processes relating to the dynamic evolution of electrons, holes and phonon modes of the material. At the earliest times, already during photogeneration of carriers, we observe a competition between different carrier scattering effects and the excitation of strongly coupled optical phonons. Further, we elucidate the mechanism behind the excitation of SCOPs in graphite and their deexcitation. These measurements show the utility of our methodology even for a seemingly well-studied system like graphite for which it reveales novel insight and addresses standing questions. Since the method is generally applicable to molecules and solids, we expect it may prove valuable to address questions such as what the energy dissipation is in light-harvesting or energy storage systems, or to re-examine long-standing questions in non-equilibrium multi-body physics such as phase-transitions or superconductivity. |
Thursday, March 18, 2021 1:18PM - 1:54PM Live |
S18.00004: Time-dependent potential energy surfaces from the exact factorization: A predictive first-principles approach to ultra-fast non-adiabatic dynamics Invited Speaker: Eberhard K Gross Some of the most fascinating phenomena in physics and chemistry, such as the process of vision, as well as exciton dynamics in photovoltaic systems involve the coupled motion of electrons and nuclei beyond the adiabatic approximation, i.e. processes not captured by the dynamics on a single Born-Oppenheimer (BO) surface. To tackle the problem, we start from the exact factorization [1] of the full electron-nuclear wave function into a purely nuclear part and a many-electron wave function which parametrically depends on the nuclear configuration and which has the meaning of a conditional probability amplitude. The equations of motion of these two wave functions provide an ideal starting point to develop efficient algorithms for the study non-adiabatic phenomena. The successful prediction of ultrafast laser-induced isomerization processes [2], the description of decoherence [3], calculations of the molecular Berry phase without invoking the BO approximation [4] and accurate predictions of vibrational spectroscopy [5], especially dichroism, will demonstrate the power of this new approach. To tackle non-adiabatic phenomena in solids, such as laser-induced phase transitions, the equations of motion of the exact factorization are “densityfunctionalized” [6], leading to a coupled set of Kohn-Sham equations for electrons and phonons [7]. As a second possibility, a beyond-BO many-body Green function approach will be presented [8]. |
Thursday, March 18, 2021 1:54PM - 2:30PM Live |
S18.00005: Observables of real-time lattice dynamics in time-dependent density functional theory Invited Speaker: Hannes Huebener I will discuss real-time simulations of lattice vibrations in solids and their signatures in spectroscopy. In particular signatures of electron-phonon coupling in optical and electron spectroscopies have long been used to investigate materials. With the increasing interest in controlling and manipulating materials properties in a non-equilibrium state, such signatures become relevant also in pump-probe measurements. I will discuss how real-time time-dependent density functional calculations can be used to understand spectral features of electron-phonon coupling of driven electronic structure in solids [1]. A useful interpretative tool that emerges from such a treatment is the picture of a dressed electronic structure, which allows to discuss effects in driven systems without referring to perturbation theory [2]. |
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