Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L20: Firstprinciples Modeling of Excitedstate Phenomena in Materials VIII: Excited State Dynamics From First PrinciplesFocus

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Sponsoring Units: DCOMP DMP Chair: Sivan RefaelyAbramson, Lawrence Berkeley National Laboratory Room: BCEC 157A 
Wednesday, March 6, 2019 11:15AM  11:51AM 
L20.00001: Getting to the core of valence excitations from first principles Invited Speaker: David Prendergast Excitedstate processes in molecules, materials, and at interfaces can reveal intricate details of energy transfer between electronic and nuclear degrees of freedom: photoinduced chemistry, charge transfer, nonthermal melting, etc. Accurate theoretical methods can hypothesize on the ultrafast evolution of these excited states and advanced pumpprobe characterization can reveal spectral signatures of the same, however, combining prediction and interpretation of ultrafast phenomena works best when we can simulate both pump and probe to directly connect experiment and theory. To this end, we focus on simulating corelevel spectroscopy of ground and excited valence states from first principles. Valence excited states can be modeled in different ways, depending on the context, but here we focus on two approaches based on density functional theory (DFT): constrainedDFT and realtime timedependent (TD) DFT. Coreexcited states are generally modeled with DeltaSCF or linearresponse TDDFT, and we outline our approach to combine valence and core excited states for direct interpretation of ultrafast Xray and XUV probes. An abundance of local physical and chemical detail on valence excitations is evident through probing coreexcitations, which we explore, atom by atom, in a predictive fashion. 
Wednesday, March 6, 2019 11:51AM  12:03PM 
L20.00002: Ab Initio Simulation of Carrier Dynamics in Electric Fields in Bulk Semiconductors William McCorkindale, JinJian Zhou, Marco Bernardi Carrier dynamics in the presence of electric fields governs the performance of semiconductor devices. The carrier drift velocity as a function of electric field (socalled velocityfield curve) has been a key material property since the early days of semiconductor physics. Yet, velocityfield curves cannot be computed from first principles, and their simulation still relies on decadeold semiempirical Monte Carlo approaches. We present a first principles method for solving the Boltzmann Transport Equation (BTE) in the presence of electric fields, which allows us to accurately compute velocityfield curves. The approach extends an efficient scheme we recently developed to solve the BTE with ab initio electronphonon scattering. Including the electric field is nontrivial – it makes the equations computationally ‘stiff’, a challenge we solve by developing an implicit numerical method to timestep the BTE. We demonstrate the stability of the algorithm, and simulate the relaxation of carriers to steadystate distributions in an electric field. Using GaAs as a case study, we obtain the first fully ab initio velocityfield curves, which correctly exhibit the Gunn effect (negative differential resistance at moderate Efields) and the drift velocity saturation at high field. 
Wednesday, March 6, 2019 12:03PM  12:15PM 
L20.00003: Ab Initio Radiative Lifetimes in Gallium Nitride Vatsal Jhalani, HsiaoYi Chen, Maurizia Palummo, Marco Bernardi Wurtzite GaN is the primary semiconductor for efficient solid state lighting. The radiative recombination and excited carrier dynamics are challenging to measure in GaN due to the ultrafast (fs – ps) timescales at play and the presence of defects and interfaces in devices. Here, we present ab initio calculations of the radiative lifetime as a function of temperature in bulk GaN. We compute the exciton energies and wavefunctions using a combination density functional theory and GWBethe Salpeter equation (BSE) method. We derive an equation for the temperature dependent radiative lifetime for an anisotropic bulk crystal within the ab initio BSE framework. The radiative lifetimes in GaN obtained with this approach are in excellent agreement with experiment. We discuss the importance of including spinorbit coupling, which, though weak in GaN, is essential to obtaining accurate radiative rates. Combined with our previous calculations of excited carrier relaxation in GaN [1], we can obtain from first principles key device parameters such as the hot carrier cooling time and the carrier diffusion lengths, with important technological implications. 
Wednesday, March 6, 2019 12:15PM  12:27PM 
L20.00004: FirstPrinciples Study of Hot Electron Dynamics on Silicon Quantum DotMolecule System Jian Cheng Wong, Lesheng Li, Yosuke Kanai Controlling hot carriers in nanomaterials is an active area of research, central to various optoelectronic applications. In our previous work^{1}, a shortlived hot electron transfer from a hydrogenterminated Si(111) surface to the cyanidin molecule was observed during hot electron relaxation within the conduction manifold of the silicon, followed by interfacial electron transfer of picosecond timescale. We expand this study onto a silicon quantum dot attached to a cyanidin molecule to investigate the extent to which these observations change. Hot electron dynamics are investigated using firstprinciples simulation based on fewest switches surface hopping method combined with firstprinciples molecular dynamics and GW calculation. 
Wednesday, March 6, 2019 12:27PM  12:39PM 
L20.00005: Rashba effect on spinresolved carrier dynamics excited by circularlypolarized light from firstprinciples Sushant Kumar, Feng Wu, Adela Habib, Yuan Ping, Ravishankar Sundararaman Recent optical pump/Xray probe experiments have shown that circularly polarized light can be used to manipulate spin states in materials exhibiting strong Rashba splitting like organometallic halide perovskites (OMHPs) and transition metal dichalcogenides (TMDs). The ability to exploit the dynamics of spin makes these materials lucrative candidates for spinoptoelectronic applications [1]. While considerable experimental work has been performed on probing the spin dynamics in TMDs and OMHPs, efforts to model this ultrafast spindependent dynamics of charge carriers have so far been limited primarily to effective model Hamiltonians. A key challenge is accounting for electronphonon scattering effects on the spin dynamics explicitly, due to the requirement of ultrafine Brillouinzone sampling arising from the disparate energy scales of electrons and phonons. We present a completely parameterfree ab initio approach to study spin and carrier dynamics, using Wannier functions to facilitate efficient calculation of electronphonon scattering. Using this approach, we present calculations of spinresolved photoexcited carrier distributions and phononassisted spin relaxation times for TMDs and ferroelectric oxides. 
Wednesday, March 6, 2019 12:39PM  12:51PM 
L20.00006: Spindependent electron transfer dynamics: key role of bandstructure Daniel SanchezPortal, Moritz Müller We investigate theoretically the charge transfer dynamics of coreexcited Ar on Co(001) and Fe(110). For these systems, recent coreholeclock measurements [1] of the lifetime of the photoexcited 4s level of Ar* have shown a clear dependence on the spin of the excited electron. Minority electrons decay significantly faster than majority electrons. We investigate such processes using Green's functions techniques on top of density functional theory calculations, explicitly including the effect of the coupling to the semiinfinite substrate [2, 3]. Our results agree with the observed behavior and allow analyzing in detail the origin of this phenomenon. The key ingredient is the spindepence of the bandstructure of the substrate and, in particular, the different positions of the Ar* 4s level within the projected band gap appearing around Gamma in the dispersive bands of Co(001) and Fe(110) for both spin channels. A simple model incorporating this effect successfully describes most of the observed phenomenology. 
Wednesday, March 6, 2019 12:51PM  1:03PM 
L20.00007: Ultrafast Excited State Dynamics of Coupled Carriers and Phonons from Ab Initio Calculations Xiao Tong, JinJian Zhou, Marco Bernardi The outofequilibrium dynamics and equilibration processes of excited states in materials are crucial to understanding timeresolved spectroscopies. Several ultrafast phenomena occurring on a subpicosecond time scale are governed by the coupled dynamics of electrons and atomic vibrations (phonons), including transient structural distortions and excited electron thermalization due to the competing electronphonon (eph) and phononphonon (phph) interactions. Here, we show ab initio calculations of the coupled ultrafast dynamics of carriers and phonons. We develop an efficient algorithm to timestep the coupled electron and phonon Boltzmann transport equations, which include both the eph and phph scattering on the same footing. Our approach allows us to study the time evolution of the excited electron populations as well as the outofequilibrium phonons together with the transient structural distortions they induce. We apply our method to silicon and graphene, and compare the results with timeresolved spectroscopies probing both the structural and the electron dynamics, such as freeelectron laser and timeresolved electron diffraction experiments. 
Wednesday, March 6, 2019 1:03PM  1:15PM 
L20.00008: Electrondefect interactions and low temperature carrier mobility from first principles ITe Lu, JinJian Zhou, Marco Bernardi Electrondefect (ed) interactions control charge and spin transport at low temperature and induce wellknown conductance fluctuation and weak localization effects. While established ab initio methods and code exist for treating electronphonon (eph) interactions, ab initio ed calculations are still in their infancy, mainly due to the formidable computational cost of computing ed matrix elements and selfenergies from calculations on supercells containing the defect. In this talk, we formulate an efficient approach for computing ed matrix elements using mainly unit cell quantities, and demonstrate its numerical implementation in the PERTURBO code. Using this approach, we can compute and systematically converge the ed scattering rates for neutral defects such as vacancy and interstitial atom in silicon, and obtain the corresponding defectlimited low temperature mobility. The results deviate in important ways from broadly used empirical approaches to treat ed interactions, highlighting the shortcomings of these simple models. Efforts on interpolating ed matrix elements, computing higherorder ed processes, and releasing the ed routines in PERTURBO will be discussed. 
Wednesday, March 6, 2019 1:15PM  1:27PM 
L20.00009: Simulating Electron Beam – Materials Interactions with RealTime Electron Dynamics Jacek Jakowski, David Lingerfelt, Panchapakesan Ganesh, Bobby G Sumpter Timedependent electron dynamics is used to model interaction of electron beam with materials for beam energy and positions relevant to scanning transmission electron microscopy. We investigate the realtime and linear response framework for simulating the response of small molecular systems including benzene and pyrene. The position dependence of selection rules for electron beam induced electronic excitaitons is discussed Higher order terms in the multipolar expansion of the electrostatic potential are shown to contribute significantly to the energy loss probability except in the high impact parameter regime where the electric fields emanated by the electron beam are essentially homogeneous over the volume of the material. Results of this study have implications for the prediction of electron energy loss spectra from first principles, and can lead to a more complete understanding of mechanisms underlying both directed nanoscaled materials manipulations and accidental radiation damage sustained by materials subjected to electron beams with energies below the material’s knockon threshold. 
Wednesday, March 6, 2019 1:27PM  1:39PM 
L20.00010: Coherent excitonvibrational dynamics and energy transfer in conjugated organics Tammie Nelson, Dianelys OndarseAlvarez, Nicolas Oldani, Beatriz Hernandez, Laura AlfonsoHernandez, Johan Galindo, Valeria D Kleiman, Sebastian FernandezAlberti, Adrian E Roitberg, Sergei Tretiak Excited state dynamics simulations reveal a ubiquitous pattern in the evolution of photoexcitations for a broad range of molecular systems. Symmetries of the wavefunctions define a specific form of the nonadiabatic coupling that drives quantum transitions between excited states, leading to a collective asymmetric vibrational excitation coupled to the electronic system. This promotes periodic oscillatory evolution of the wavefunctions, preserving specific phase and amplitude relations across the ensemble of trajectories. The simple model proposed here explains the appearance of coherent excitonvibrational dynamics due to nonadiabatic transitions. We demonstrate universality of these phenomena by inspecting photoinduced dynamics in several common cases for organic conjugated materials. These include a linear oligomer, nanohoop, treelike dendrimer, and molecular dimer. In all these molecules, ultrafast dynamics and exciton transport is directly simulated using our atomistic nonadiabatic excitedstate molecular dynamics (NEXMD) package. Coherent dynamics observed in these systems persists on the timescale of 100s of fs at room temperature and in the presence of a bath, which agrees with experimental spectroscopic reports on various materials. 
Wednesday, March 6, 2019 1:39PM  1:51PM 
L20.00011: Modification of excitation and charge transfer in cavity quantumelectrodynamical chemistry Christian Schäfer, Michael Ruggenthaler, Heiko Appel, Angel Rubio Energy transfer in terms of excitation or charge is one of the most 
Wednesday, March 6, 2019 1:51PM  2:03PM 
L20.00012: Nonequilibrium dynamics of spin and charge correlation in strongly correlated systems from pumpprobe spectroscopy ChenYen Lai, JianXin Zhu The ultrafast pumpprobe techniques are used to probe the elementary excitations in materials. We study the nonequilibrium process of the onedimensional extended Hubbard model under transient laser pump pulse. In nonpolarized cases, the competition between charge and spin results in bondorderwave between spindensitywave and chargedensitywave phases in equilibrium. We focus on the regime near the phase boundaries and how those spin and charge correlations affected by different pump pulse frequency and strength. Furthermore, in the spinpolarized systems, the Bethe strings state emerges and the nonequilibrium dynamics of the fractional excitation are investigated. The effects from photoinduced charge carriers near the phase boundaries are investigated by time evolving black decimation. These effects should be measurable by timeresolved angleresolved photoemission spectroscopy. (LAUR1830001) 
Wednesday, March 6, 2019 2:03PM  2:15PM 
L20.00013: Ultrafast decay of lowsymmetry photoinduced atomic forces. Shane O'Mahony, Jose QueralesFlores, Ivana Savic, Éamonn Murray, Felipe MurphyArmando, Stephen B Fahy Generation and control of atomic forces in optically excited systems is crucial to understanding photocatalysis, renewable energy and laser annealing. E_{g}symmetry coherent phonons are excited in groupV semimetals by ultrafast (<100 fs) optical pulses when the radiation is polarised perpendicular to the 3fold symmetry axis of the crystal. The phonon driving force is consistent with an initially unbalanced occupation of electronic states in symmetryequivalent regions of the Brillouin zone, which decays to fullysymmetric occupation of the zone on fs timescales. Measured temperaturedependence of the force decay time in Bi and Sb [1] suggests relaxation of the excited electronic distribution by electronphonon (elph) scattering. 
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