Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S36: Ultrafast Dynamics of Coupled Electrons and Atomic Motions: Experiments and Precise SimulationsInvited

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Sponsoring Units: DCOMP Chair: Andre Schleife, University of Illinois at UrbanaChampaign Room: 601/603 
Thursday, March 5, 2020 11:15AM  11:51AM 
S36.00001: Terahertz frequency switching in topological Weyl semimetals Invited Speaker: Aaron Lindenberg I will describe femtosecondresolution crystallographic measurements probing dynamic switching responses driven by terahertz light pulses in topological Weyl semimetals, focusing on the 2D transition metal dichalcogenide WTe_{2}. We describe a new mechanism for driving large amplitude interlayer shear oscillations with ~1% strain amplitudes, leading to a topologically distinct metastable phase. We further show that such shear strain serves as an ultrafast, energyefficient means to induce more robust, wellseparated Weyl points or to annihilate all Weyl points of opposite chirality. I will also discuss new efforts investigating other means for manipulating the topological phase diagram and interlayer stacking of this material through the application of pure electric fields and through doping/intercalation as probed by both optical and transport measurements. This work defines new possibilities for ultrafast manipulation of the topological properties of solids and for the development of new types of topological optoelectronic devices. 
Thursday, March 5, 2020 11:51AM  12:27PM 
S36.00002: Observables of realtime lattice dynamics in timedependent density functional theory Invited Speaker: Hannes Huebener I will discuss realtime simulations of lattice vibrations in solids and their signatures in spectroscopy. In particular signatures of electronphonon 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 nonequilibrium state, such signatures become relevant also in timeresolved measurements. I will discuss how realtime timedependent density functional calculations can be used to understand spectral features of electronphonon coupling of driven electronic structure in solids. 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. 
Thursday, March 5, 2020 12:27PM  1:03PM 
S36.00003: Toward Precise Ab Initio Simulations of the Ultrafast Dynamics of Electrons and Phonons Invited Speaker: Marco Bernardi Recent progress in combining density functional theory and related ab initio methods with kinetic equations are enabling spectacular advances in computing carrier dynamics in materials from first principles. We will first review this framework and early ab initio calculations of electron scattering rates and carrier thermalization, and then discuss recent developments. The talk will focus on a numerical approach to evolve in time the coupled Boltzmann transport equations (BTEs) of electrons and phonons, using ab initio electronphonon and phononphonon interactions together with a parallel algorithm to explicitly time step the BTEs. Our approach can simulate the electron and phonon dynamics up to hundreds of picoseconds (with a femtosecond time resolution), and the accuracy of the interactions used in the calculations can be validated by computing transport properties. We show example calculations on graphene and semiconductors, for which we compute carrier cooling rates, moderesolved phonon dynamics, transient absorption, and timeresolved structural snapshots and diffuse Xray scattering. We will show how this approach can be extended to include excitons, by discussing calculations of excitonphonon interactions and realtime exciton dynamics. We will outline code development efforts, open problems and future directions. 
Thursday, March 5, 2020 1:03PM  1:39PM 
S36.00004: Recent advances in light induced superconductivity Invited Speaker: Andrea Cavalleri I will discuss how coherent electromagnetic radiation at infrared and TeraHertz frequencies can be 
Thursday, March 5, 2020 1:39PM  2:15PM 
S36.00005: Timedependent potential energy surfaces from the exact factorization: A predictive firstprinciples approach to ultrafast nonadiabatic 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 BornOppenheimer surface. To go beyond the adiabatic approximation is notoriously difficult as one has to start from the full Hamiltonian of interacting electrons and nuclei. We deduce an exact factorization [1] of the full electronnuclear wave function into a purely nuclear part and a manyelectron 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 nonadiabatic phenomena. The successful prediction of ultrafast laserinduced isomerization processes [2], the description of decoherence [3], calculations of the molecular Berry phase without invoking the BornOppenheimer approximation [4] and accurate predictions of vibrational spectroscopy [5], especially dichroism, will demonstrate the power of this new approach. To tackle nonadiabatic phenomena in solids, such as laserinduced phase transitions, the equations of motion of the exact factorization are “densityfunctionalized” [6], leading to a coupled set of KohnSham equations for electrons and phonons [7]. 
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