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 A22: FirstPrinciples Modeling of ExcitedState Phenomena in Materials I: ElectronPhonon and PhotonPhonon InteractionsFocus Live

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Sponsoring Units: DCOMP DCP DMP Chair: Sahar Sharifzadeh, Boston University 
Monday, March 15, 2021 8:00AM  8:36AM Live 
A22.00001: Understanding electronmediated photonphonon interactions from first principles Invited Speaker: Pierre Darancet Excitations of materials using ultrafast light pulses are conducive to nonequilibrium ("hot") electron distributions that interact with the underlying atomic lattice. In the case of metals, Allen showed thirty years ago [1] that the effect of these "hot" electrons is, to a good approximation, to warm up phonons homogeneously with a single characteristic timescale controlled by electronphonon interactions. In semiconductors and lowdimensional materials, the reduced dielectric screening, and, in some cases, the higher lattice thermal conductivity weaken this hypothesis, hence calling for a more detailed physical picture. 
Monday, March 15, 2021 8:36AM  8:48AM Live 
A22.00002: Ab initio calculation of polarons: algorithms and benchmarks Chao Lian, Weng Hong Sio, Feliciano Giustino Calculations of polarons using densityfunctional theory and supercell approaches face two key challenges: (i) large computational cells are required to describe intermediate and large polarons, (ii) the formation energy and localization of the polaron wavefunction are sensitive to the exchangecorrelation functional. We developed a new approach where the polaron is expressed as a superposition of Bloch states, and the calculation of wavefunctions and energies is cast into the solution of a nonlinear system involving KohnSham energies, phonon frequencies, and elph matrix elements from densityfunctional perturbation theory [PRL 122, 246403 (2019)]. Here we report on further optimization, specifically improvements of the iterative eigensolver, parallelism, memory management, and increased modularity using EPW and Quantum ESPRESSO. We analyze the performance of the method in terms of Brillouinzone sampling and wavefunction initialization and report benchmark on prototypical polaronic systems, from ionic to covalent insulators. 
Monday, March 15, 2021 8:48AM  9:00AM Live 
A22.00003: FirstPrinciples Studies of Photoluminescence of Defects in Semiconductors Yu Jin, Marco Govoni, Giulia Galli Optically active points defects in semiconductors offer unique opportunities for quantum technology applications, and accurate predictions of their optoelectronic properties may help design efficient systems for, e.g. quantum emitters. Here we present a general strategy to compute photoluminescence (PL) spectra of point defects from first principles, as well as results for several systems, including the negatively charged nitrogenvacancy center in diamond, the neutral divacancy in silicon carbide and the carbondimer substituent in hexagonal boron nitride. Using the FranckCondon principle, we computed zerophonon lines, phonon sidebands and HuangRhys factors and we performed a detailed analysis of the electronphonon coupling of optical transitions. We discuss the results obtained using different methods to compute excited state properties, including constrained density functional theory (CDFT) and timedependent density functional theory (TDDFT). 
Monday, March 15, 2021 9:00AM  9:12AM Live 
A22.00004: Firstprinciples study of electron, phonon, magnon dispersions and heat capacities of antiferromagnetic L_{10}type MnPt Kisung Kang, David G. Cahill, Andre Schleife Electron, phonon, and magnon dispersions are important to understand magnetic phenomena such as anisotropic magnetoresistance or inelastic neutron scattering. Firstprinciples density functional theory enables simulations of these three contributions. We investigate the energy dispersions and heat capacities of antiferromagnetic L_{10}type MnPt, which is commonly utilized as a pinning layer to induce exchange bias in a ferromagnetic layer. The calculated energy dispersions lead to temperaturedependent heat capacities. The magnon dispersion from a Heisenberg model, including calculated exchange parameters and anisotropy energy, verifies the existence of a gap at the Gamma point. The T^{3} dependence of heat capacity at low temperature is originated from the phonon contribution, whereas the magnon contribution for heat capacity is absent at low temperature due to the magnon gap. At high temperature, the magnetic heat capacity from Monte Carlo calculations shows a peak, which is associated with the N\'eel temperature. The energy dispersion and heat capacity of antiferromagnetic L_{10}type MnPt provide insight into studies of other properties such as temperaturedependent magnetooptics. 
Monday, March 15, 2021 9:12AM  9:24AM Live 
A22.00005: LowEnergy Polaron Spectra in the Doped Fröhlich Model Nikolaus Kandolf, Carla Verdi, Feliciano Giustino We investigate the lowenergy electron quasiparticle spectrum of the doped 
Monday, March 15, 2021 9:24AM  9:36AM Live 
A22.00006: Vibronic Spectra from First Principles: Capturing the FranckCondon Effect without BornOppenheimer Surfaces Kevin Lively, Aaron Kelly, Shunsuke Sato, Guillermo Albareda, Angel Rubio We simulate electronnuclear vibronic spectra using the semiclassical MultiTrajectory Ehrenfest (MTEF) dynamics method, without relying on the use of excited Born Oppenheimer (BO) energy surfaces. We find that the vibrational energy spectra matches the energy profile of the initial state of the electronic system. We explore the roles of both the initial state preparation and the approximate semiclassical time evolution, and show how the MTEF approach allows for the inclusion of electronnuclear correlation in the initial conditions. We apply this approach to a onedimensional model for the Hydrogen molecule and with an ab initio treatment of Benzene using timedependent density functional theory, demonstrating that this first principles approach, besides being efficient and scalable, performs well in comparison with experiment. These results show promise for the applicability of this realtime method to capture electronnuclear correlated phenomena in timeresolved spectra, and in nonlinear driving regimes, for systems where the BO framework is computationally intractable. 
Monday, March 15, 2021 9:36AM  9:48AM Live 
A22.00007: First Principles Density Functional Theory Study of Polarons in Transition Metal Oxides Hori Pada Sarker, Muhammad Huda Understanding of charge transport in complex materials is crucial for fundamental and technological relevance. In strongly correlated materials, such as transition metal oxides (TMO), charge carriers (electron and hole) interact strongly with phonons (quanta of lattice vibrations) and as a consequence, the resulting lattice distortions trap the carriers and form quasiparticles known as polarons. In such polaron forming materials, polarons play a decisive role towards determining the transport behavior of these materials. We have benchmarked the applied computational techniques by calculating polaron in binary TMO’s such as TiO_{2} and Fe_{2}O_{3}. In the present work, we have studied the polaron formation in BiVO_{4}, one of the best metaloxide photoanode materials for PEC H_{2}O splitting. In this study, we have theoretically characterized the polaron formation as well as calculated the sitetosite polaron hopping barrier and mobility along a specific hopping pathway within the framework of density functional theory (DFT). Finally, we have calculated the supercell size dependency towards the polaron formation in BiVO_{4}. 
Monday, March 15, 2021 9:48AM  10:00AM Live 
A22.00008: Toward precise simulations of the coupled ultrafast dynamics of electrons and atomic vibrations in materials Xiao Tong, Marco Bernardi Ultrafast spectroscopies can access the dynamics of electrons and nuclei at short timescales, shedding light on nonequilibrium phenomena in materials. However, development of accurate calculations to interpret these experiments has lagged behind as widely adopted simulation schemes are limited to subpicosecond timescales or employ simplified interactions lacking quantitative accuracy. Here we show a precise approach to obtain the timedependent populations of nonequilibrium electrons and atomic vibrations (phonons) up to tens of picoseconds, with a femtosecond time resolution. Combining firstprinciples electronphonon and phononphonon interactions with a parallel numerical scheme to timestep the coupled electron and phonon Boltzmann equations, our method provides unprecedented microscopic insight into scattering mechanisms in excited materials. Focusing on 2D materials, we demonstrate calculations of ultrafast electron and phonon dynamics in graphene, including simulated transient optical absorption, structural snapshots and diffuse Xray scattering. We additionally present results for the ultrafast dynamics of chiral phonons in monolayer WSe2. Our firstprinciples approach paves the way for quantitative atomistic simulations of ultrafast dynamics in materials. 
Monday, March 15, 2021 10:00AM  10:12AM Live 
A22.00009: Ab initio ultrafast spin dynamics in solds Junqing Xu, Adela Habib, Ravishankar Sundararaman, Yuan Ping We present a firstprinciples realtime densitymatrix approach [1] to simulate ultrafast spinorbitmediated dynamics in solids with arbitrary crystal symmetry. Through the complete theoretical descriptions of pump, probe and scattering processes including electronphonon, electronimpurity and electronelectron scattering, our method can directly simulate the nonequilibrium ultrafast pumpprobe measurements for coupled spin and electron dynamics and is applicable to any temperatures and doping levels. We use this method to simulate spin dynamics of GaAs and obtain excellent agreement with experiments. It is found that the relative contributions of different scattering mechanisms and phonon modes vary considerably between spin and carrier relaxation processes. Importantly, we point out that at low temperatures the electronelectron scattering becomes very important and causes the strong reduction of spin relaxation time under inplane magnetic fields. 
Monday, March 15, 2021 10:12AM  10:24AM Live 
A22.00010: Ab initio signatures of phononmediated hydrodynamic transport in semimetals Yaxian Wang, Georgios Varnavides, Prineha Narang Hydrodynamic electron flow in condensed matters has been one of the most active research areas recently, where many open questions regarding the underlying mechanisms still remain. We utilize ab initio techniques to treat the electron scattering events explicitly, and show in combination with Boltzmann transport equation a more applicable metric of hydrodynamic transport taking into account temperature, channel width, and impurity length, which can be directly verified by various experimental techniques. By investigating different scattering lengthscales in PdCoO2, ZrSiS, and TaAs2, we show that that phonon mediated electronelectron interaction could lead to much shorter momumtem conserving mean free path, facilitating hydrodynamic behavior in systems where the direct Coulomb interaction is largely screened. 
Monday, March 15, 2021 10:24AM  10:36AM Live 
A22.00011: Conditional wavefunction approach to the structure and dynamics of manybody systems Guillermo Albareda, Kevin Lively, Shunsuke Sato, Aaron Kelly, Angel Rubio The interacting conditional wavefunction approach is a recently introduced method for performing quantum dynamics simulations that is multiconfigurational by construction and that is able to capture quantitative accuracy for situations where meanfield theory fails. The technique is highly parallelizable and reformulates the traditional “curse of dimensionality” by using a stochastic wavefunction ansatz that is based on an interacting set of singleparticle conditional wavefunctions. Here, we put forth an imaginarytime version of the method and demonstrate its ability for capturing correlated properties in systems made of interacting electrons and nuclei. This is illustrated for three highlycorrelated problems: a model system of electronHydrogen scattering, a photoexcited protoncoupled electron transfer problem, and the strongfield ionization dynamics of a model H_{2} molecule. These examples highlight the ability of the method to capture electronelectron, electronnuclear and fieldinduced correlations respectively. This work paves the way for applications to systems driven out of equilibrium. 
Monday, March 15, 2021 10:36AM  10:48AM Live 
A22.00012: VibroPolariton States from First Principles John Bonini, Johannes Flick In recent years there has been a number of exciting experimental and theoretical developments in the study of light matter coupled systems. In the strong coupling regime the normal modes of the system can become hybrid states which mix nuclear, electronic, and photon degrees of freedom. Altering the vibrational modes of molecules through strong light matter coupling has been demonstrated to influence chemical reactions in molecular systems [1]. First principles methods capable of treating the light and matter degrees of freedom on the same level of theory are an important tool in understanding the physics of such systems. In this talk we develop and apply a generalized force constant matrix approach to the study of mixed vibrationphoton states of molecules within the quantum electrodynamical density functional theory framework. With this method the IR and Raman spectra can be computed via linear response in a manner analogous to techniques widely used for conventional phonons. This enables more efficient computation than previous time dependent approaches [2], with access to additional properties. Extensions of the method to solids and the nonlinear regime will also be discussed. 
Monday, March 15, 2021 10:48AM  11:00AM Live 
A22.00013: FirstPrinciples Investigation of Thermally Activated Delayed Fluorescence Processes Tommaso Francese, Francois Gygi, Giulia Galli Thermally Activated Delayed Fluorescence (TADF) compounds are promising materials for the realization of next generation Organic Light Emitting Diodes^{ [1]}. Among several TADF systems recently proposed in the literature, NAIDMAC^{[2]}[C_{37}H_{32}N_{2}O_{2}] represents one of the few available orangered emitters, with a remarkable 30% external quantum efficiency. We present a detailed first principle study of the NAIDMAC compound, and we consider both the single molecule (dilute limit) and the crystal (highpacking limit). We carried out FirstPrinciples Molecular Dynamics simulations at finite temperature with the Qbox^{[3]} code (http://qboxcode.org) and we analyzed the structural motifs that lead to the desired energy splitting between the singlet and triplet excited states. This splitting is the key driving force of the TADF process. We also computed diabatic coupling parameters associated to the charge mobility^{[4]} and we discuss how charge transfer processes occur in NAIDMAC. 
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