Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session G59: FirstPrinciples Simulations of ExcitedState Phenomena: Excitons and BetheSalpeter Equation IIFocus

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Sponsoring Units: DCOMP Chair: Li Yang, Washington University, St. Louis Room: Room 301 
Tuesday, March 7, 2023 11:30AM  12:06PM 
G59.00001: Intra and interlayer excitons of twodimensional semiconductors on substrates and in magnetic fields Invited Speaker: Michael Rohlfing Excitonic states in twodimensional semiconductors, in particular in transitionmetal dichalcogenides (TMDC), exhibit characteristic optical properties combining spin selectivity with reduced dimension. They can be described by standard manybody perturbation theory (MBPT), notably by the GW method in combination with the BetheSalpeter equation. Due to their lowdimensional structure the excitons in these systems can easily be manipulated and tuned by external stimuli. Here we discuss in particular (i) energetic redshifts of TMDC monolayer excitons due to a supporting substrate [1], (ii) the layer dependence of polarizabilityrelated shifts at TMDC surfaces [2], and (iii) magneticfield induced shift and splitting of intra and interlayer excitons [3,4,5]. 
Tuesday, March 7, 2023 12:06PM  12:18PM 
G59.00002: Strong BlochFloquet effects driven by excitonic fields in monolayer transition metal dichalcogenides from a timedependent GW approach Felipe H da Jornada, Diana Y Qiu, Yanghao Chan A longsought goal in condensedmatter physics is to create nontrivial phases of matter by optically driving materials out of equilibrium. Within the BlochFloquet formalism, a system driven by a perturbation with period T experiences a coupling of bands separated by energies multiple of ?(2π/T) and proportional to the intensity of the external field. Although most experimental realizations of BlochFloquet physics have been realized with strong optical fields, recent theoretical works have proposed the utilization of other bosonic fields, such as phonons, magnons, and excitons, as external periodic driving fields. In this work, we present firstprinciples calculations of BlochFloquet effects in a transition metal dichalcogenide (TMD) monolayer using a recently develop timedependent GW method. Contrary to the optically driven case, we observe strong band renormalizations and hybridization effects even for moderate exciton concentrations. We explain this uniquely strong effect in 2D materials in light of their strong exciton binding energy, and efforts towards the experimental interpretation of such a signal from timeresolved angleresolved photoemission spectroscopy (TRARPES). We also connect our work with recent discussions regarding nonequilibrium excitonic insulators and condensates. 
Tuesday, March 7, 2023 12:18PM  12:30PM 
G59.00003: Optical excitation energies of Co@S defect of WS_{2} computed with the spinflip BetheSalpeter equation approach Arabi Seshappan, Bradford A Barker, Nolan Kelly, David A Strubbe Defected transition metal dichalcogenides are exciting materials as potential single photon emitters, quantum light sources, and roomtemperature solidstate qubits. Accurate calculations of the exciton interactions in these materials is imperative to understanding these applications. Monolayer cobaltatsulfursite substituted WS_{2} (Co@S:WS_{2}) has an openshell electronic structure due to the spin of the cobalt atom, as found from our DFT calculations of magnetism and JahnTeller distortions in this system. While quasiparticle energies for openshell systems have been calculated previously–and are known to have multiplet solutions–there is at present no similar approach available for the BetheSalpeter equation in openshell systems. Instead, we investigate the vertical excitation energies through the SpinFlip BetheSalpeter equation approach [arXiv:2207.04549], which allows for the simultaneous calculation of ground and excitedstate energies for multiconfigurational openshell systems. Previously, this approach has been used to calculate excitation energies for defected bulk semiconductors, and we now leverage methodological development for the calculation of optical excitations in 2D systems to calculate excitation energies for Co@S:WS_{2}. 
Tuesday, March 7, 2023 12:30PM  12:42PM 
G59.00004: First Principles Calculation of Electronic Circular Dichroism Including Exciton Effects Nicholas G Richardson, Bowen Hou, Victor Chang Lee, Diana Y Qiu Electronic circular dichroism (CD)—the differential absorption of left and righthand circularly polarized light—is a powerful tool for identifying chiral molecules and probing chirality transfer at heterogeneous interfaces between molecules and crystalline materials. Timedependent density functional theory (TDDFT) is currently the most popular method for calculating CD, but its accuracy is limited by the choice of the exchangecorrelation functional. In particular, it can be challenging to accurately describe systems with a mix of localized and delocalized states, such as those at a moleculecrystal interface, in TDDFT. Ab initio manybody perturbation theory (MBPT) is an alternative approach that can describe optical excitations, including excitonic effects, in both molecules and crystals with great accuracy, but it is not yet widely applied to the study of electronic CD. Here, we demonstrate how to calculate electronic CD within the GW plus BetheSalpeter equation (GWBSE) approach in MBPT through the application of Fermi’s Golden Rule. We then apply this approach to calculate the electronic CD of various chiral molecules. 
Tuesday, March 7, 2023 12:42PM  12:54PM Author not Attending 
G59.00005: Accelerating the calculation of absorption spectra of complex materials at finite temperature Andrew C Xu, Marco Govoni, Giulia Galli In a recent work [1] we developed a framework to accelerate the calculation of the dielectric screening required for absorption spectra calculations at finite temperature, by using techniques based on regression analyses. Here, we discuss improvements to this framework using neural networks and present the calculations of accurate absorption spectra for complex heterogeneous systems, including aqueous interfaces. 
Tuesday, March 7, 2023 12:54PM  1:06PM 
G59.00006: Realtime TDDFT for excitons in solids: timedependent screening Carsten A Ullrich, Jared R Williams Timedependent densityfunctional theory (TDDFT) is a promising alternative to the BetheSalpeter equation (BSE) for calculating optical spectra in solids, including excitonic effects. By using simple global hybrid functionals where the admixture of nonlocal exchange is controlled by the dielectric constant, optical spectra with accuracies comparable to the BSE can be achieved, at a fraction of the computational cost. As a first step towards extending this hybrid TDDFT approach into the realtime domain, we present a theory of timedependent dielectric screening. The method will be illustrated for twodimensional model solids. 
Tuesday, March 7, 2023 1:06PM  1:18PM 
G59.00007: Linear response in relativistic quantumelectrodynamical density functional theory Lukas Konecny, Valeriia P Kosheleva, Heiko Appel, Angel Rubio, Michael Ruggenthaler The quest to control the properties of matter has produced ingenious techniques utilizing, for example, lasers or optical cavities. In cavities, light is squeezed into a small volume resulting in strong coupling between the cavity modes and matter. Unlike intense laser radiation, the interaction with light in cavities does not heat up and damage molecules and materials. Proper theoretical description of cavity control of materials necessitates quantized treatment of cavity modes due to the strong coupling. Therefore, the theoretical framework of choice is quantum electrodynamics (QED). However, the stateoftheart in practice is based on nonrelativistic QED or even further simplified fewlevel models. Quantum electrodynamical density functional theory (QEDFT) [1, 2] has emerged as a bridge between quantum chemistry and quantum optics, combining an ab initio description of matter with a quantized description of light. In this work, we introduce relativistic QEDFT [3] in the linear response regime for molecules in optical cavities. The developed methodology combines a fourcomponent DiracKohnSham treatment of electrons with a quantized description of photons included as dynamical variables. The relativistic level of theory allows for studying heavy elementcontaining molecules. The linear response formalism is used to describe their excitation spectra as influenced by cavities. In addition, the method can be employed to describe radiative decay from first principles via the interaction of molecules with a photonic bath. 
Tuesday, March 7, 2023 1:18PM  1:30PM 
G59.00008: Exciton delocalization in layered perovskites from first principles Yinan Chen, Marina R Filip Optoelectronic properties of layered organicinorganic halide perovskites can be tuned by leveraging their remarkable structural diversity [1]. In this work, we study the impact of several structural parameters of layered perovskites on their photophysics from first principles. We calculate the excited state properties of a series of layered perovskites with different structural features within the GW [2] and BetheSalpeter Equation (BSE) [3] frameworks (GW+BSE). We find that the key optoelectronic properties (i.e. band gaps, band structures, and optical spectra) depend strongly on the separation and alignment of the perovskite layers, and rationalize our findings from a tight binding perspective. We visualize the exciton with the electronhole correlation function [4] and systematically link the structural features with the extent of exciton delocalization across the quasi2D perovskites layers. Our study points to structural and chemical intuition by which we can tune the optoelectronic properties of layered perovskites by engineering changes to their crystal structure. 
Tuesday, March 7, 2023 1:30PM  1:42PM 
G59.00009: Cavity coupled molecules and solids from finite field DFT John R Bonini, Iman Ahmadabadi, Samantha O'Sullivan, Johannes Flick Strong lightmatter coupling in optical cavities can alter the dynamics of molecular and material systems resulting in polaritonic excitation spectra and modified reaction pathways. For strongly coupled photon modes close in energy to nuclear vibrations the Cavity Born Oppenheimer Approximation (CBOA) in the context of quantumelectrodynamical density functional theory (QEDFT) has been demonstrated to be an appropriate description of the coupled lightmatter system. In this work we show that results equivalent to those from a CBOA functional can be obtained from the response of the molecule(s) or material to electric fields, with the cavity parameters (mode frequency and coupling strength) entering only in a post processing step. This procedure enables properties at a range of cavity frequencies and coupling strengths to be obtained from a single set of electric field calculations. The formalism can be applied using existing techniques for characterizing insulating solids in electric fields enabling first principles modeling of cavity coupled systems in periodic boundary conditions. Results for IR spectra and other properties of cavity coupled molecules and solids will be presented. 
Tuesday, March 7, 2023 1:42PM  1:54PM 
G59.00010: Theory of PolaritonMediated Photophysics in J Aggregates Yu Zhang Localization and delocalization are the centers of many photophysical phenomena. In general, the disorders in the molecular aggregates lead to exciton localization. However, when the molecular aggregates are coupled to a plasmonic cavity, the strong coupling between the molecular exciton and cavity photon could lead to the delocalized hybrid lightmatter states, i.e., excitonpolaritons. Hence, there is competition between disorderinduced localization and polaritoninduced delocalization, enabling new photophysical dynamics. In this talk, I will present our recent work on understanding the disorder effect in the formation of excitonpolaritons in J aggregated and polariton/disorderinduced photophysical dynamics via multiscale and multiphysics modeling. 
Tuesday, March 7, 2023 1:54PM  2:06PM 
G59.00011: Optically enhanced dissociation of divacancies in diamond Yifan Yao, Andre Schleife Nitrogen vacancy (NV) centers in diamonds are one of the promising examples of solidstate spin defects, with remarkable applications in quantum sensing and optics. While ion implantation with thermal annealing is extensively employed to generate NV centers in diamonds, the detrimental formation of divacancy (VV) centers degrades the resulting spin and optical properties by introducing a source of decoherence. As designing an annealing time and temperature progression for the targeted removal of VV centers is challenging, in this project, we demonstrate that the optical excitation of the VV centers can be an additional method to mediate its dissociation. Using firstprinciples simulations, we predict that the binding energy of VV centers is reduced by 1.7 eV due to the excitation of electrons from the VVrelated defect states. Thus, a reduced annealing temperature may be sufficient to target specifically the removal of VV centers, without activating the dissociation of any other defect complexes. With experimental validation, we will provide a quantitative understanding and practical process of eliminating VV centers, which can improve the engineering of quantum devices. 
Tuesday, March 7, 2023 2:06PM  2:18PM 
G59.00012: Firstprinciples calculation of excitonic effects on shift current in the ferroelectric perovskite BaTiO_{3} Xian Xu, Yanghao Chan, Diana Y Qiu Shift current is a bulk photovoltaic effect (BPVE) that arises in noncentrosymmetric materials and results in the generation of a direct current under continuous wave illumination. It holds promise for next generation solar cells, but the role of electronhole interactions is just beginning to be explored, with some theories suggesting it will suppress the shift current and others seeing an enhancement. Here, we calculate the shift current including excitonic effects using a new ab initio approach based on a perturbative sumoverstates formalism in the exciton basis, where the exciton states are obtained within the ab initio GW plus Bethe Salpeter equation (GWBSE) approach. We show that the sumoverstates formalism gives results identical to a realtime nonequilibrium Green's function approach within the adiabatic GW approximation. We apply this method to calculate the shift current of the ferroelectric perovskite BaTiO3 including quasiparticle and exciton effects and see that electronhole interactions give rise to both a red shift and enhancement of the shift current near the band edge. Our calculation demonstrate a rigorous and efficient method for studying excitonic effects in the shift current. 
Tuesday, March 7, 2023 2:18PM  2:30PM 
G59.00013: The Ccenter in silicon: an Lband emitter with memory Péter Udvarhelyi, Anton Pershin, Péter Deák, Adam Gali Silicon is a technologically mature material that has attracted renewed interest as a host for quantum defects. Its smaller band gap mainly enables excitonic states, nevertheless, stable emitters operating in the telecommunication wavelengths may exist in this material. 
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