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 B59: FirstPrinciples Simulations of ExcitedState Phenomena: Excitons and BetheSalpeter Equation IFocus

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Sponsoring Units: DCOMP Chair: Talat Rahman, University of Central Florida Room: Room 301 
Monday, March 6, 2023 11:30AM  12:06PM 
B59.00001: Towards a unified ab initio description of excitations and their dynamics Invited Speaker: Claudia Draxl The interplay between electronelectron interaction, electronvibrational coupling, electronhole correlation, and (potentially) spinorbit coupling (SOC) may give rise to exciting phenomena in the response of matter to light. Manybody perturbation theory (MBPT), in particular the BetheSalpeter equation, is the method of choice for tackling such problems to describe what is happening in experimental probes like photoemission, optical and xray absorption, electronloss spectroscopy, etc. In this talk, I will provide examples along these lines, providing indepth analysis of the underlying processes. These examples include excitonexciton coupling in resonant inelastic xray scattering (RIXS) in oxides, the structural relaxation in optically excited molecules, or the impact of vibrations and/or SOC in the optical absorption spectra of different materials. Furthermore, I will discuss first steps towards the description of timeresolved processes. All methods are implemented in the fullpotential code exciting whose allelectron nature allows for treating valence and core excitations on the same footing, also fully accounting for SOC. 
Monday, March 6, 2023 12:06PM  12:18PM 
B59.00002: Firstprinciples calculations of excitons including radiative recombination and polaritonic effects: the retarded BetheSalpeter equation Zachary N Mauri, Christopher J Ciccarino, Felipe H da Jornada Recent research in 2D materials has revealed a host of phenomena only present in lowdimensional systems, such as the presence of strongly bound excitons due to the electronic confinement and weak dielectric screening in such systems. The electronic and optical properties in such systems have been largely captured with success through firstprinciples techniques based on interacting Green's function formalisms, such as the ab initio GW and BetheSalpeter equation approaches, respectively. However, these methods are typically derived assuming instantaneous manyelectron interactions. Here, we describe a formalism to include retardation effects into the BetheSalpeter equation that is computationally efficient and gauge invariant. While previous efforts have focused on the effect of retardation effects in the direct (screened) electronhole interactions, we find here that retardation effects in the (bare) exchange interactions dominate in low dimensions. Our work predicts a renormalization of the exciton dispersion and a broadening of the exciton dispersion when retardation effects are included. More broadly, our approach can be applied to the study of polaritonic effects directly from standard interacting Green'sfunction formalisms. 
Monday, March 6, 2023 12:18PM  12:30PM 
B59.00003: Phononassisted Photoluminescence and Exciton Lifetime in Solids from First Principles Chunhao Guo, Junqing Xu, Yuan Ping Timeresolved photoluminescence spectroscopy (TRPL) reveals many important dynamical manybody phenomena including phononassisted recombination of indirect excitons. However, such signatures in the PL spectrum and their dynamical processes are not yet been fully understood at an atomistic level. We present a universal firstprinciples methodology based on Heisenberg equation of motion to calculate the phononassisted photoluminescence spectrum and exciton lifetime at finite temperature. In particular, this approach can describe the exciton lifetime including both radiative recombination and excitonphonon scattering simultaneously from firstprinciples. We further analyze the phonon mode contribution and emphasize the relaxation pathway and dephasing lifetime through each scattering channel to explain the experimental measurements in vdW layered materials. We emphasize that firstprinciples phononassisted transitions are crucial for an indepth understanding of exciton dynamics in solids. 
Monday, March 6, 2023 12:30PM  12:42PM 
B59.00004: Excitedstate force calculations from GW/BSE and DFPT: development and application to organic metal halide perovskites Rafael R Del Grande, David A Strubbe While absorption of light has been long studied in electronic structure, the interactions of the resulting excited states with the lattice that cause lightinduced structural changes remain hard to handle. This is an underlying phenomenon for photodegradation, Stokes shifts, exciton transport, and other photophysics, which can be studied via excitedstate forces. IsmailBeigi and Louie [Phys. Rev. Lett. 90, 076401 (2003)] developed an approximate theory combining quasiparticle and excitonic effects from GW and BetheSalpeter Equation (BSE) with electronphonon interactions from Density Functional Perturbation Theory, but this approach has been little used. We revisit this theory, with improvements to the underlying approximations, and implement it in a practical workflow for BerkeleyGW. We make detailed tests of the validity of these approximations and demonstrate favorable convergence properties that make the forces less timeconsuming than ordinary GW/BSE. Then we explore some applications to methylammonium lead iodide perovskites, whose use in solar cells is limited by photodegradation. We study the equilibrium structures of the excited states, and coupling between different excitons due to the phonons, to give insight into the photophysics of perovskites. 
Monday, March 6, 2023 12:42PM  12:54PM 
B59.00005: Excitonic effects on nonlinear optical responses from first principles Yanghao Chan, Steven G Louie, Jiawei Ruan

Monday, March 6, 2023 12:54PM  1:06PM 
B59.00006: Vertical excitation energies of the SiV(0) defect in diamond computed by the SpinFlip BetheSalpeter Equation approach Bradford A Barker, David A Strubbe The neutral siliconvacancy defect in diamond (“SiV(0)”) is a promising candidate for qubit and nanosensing applications, and unlike the wellstudied NV^{} center, possesses inversion symmetry. As a potential qubit, optical transition energies from the defect’s triplet ground state to its triplet excited states are important quantities to calculate. Compared to the NV^{} center, however, there are fewer calculations of these transition energies. As the defect has an openshell electronic structure, computational methods must capture the multiconfigurational nature of its ground and excited states. We present calculations of singlet and triplet vertical excitation energies (that is, relative to the ground state energy at the ground state atomic coordinates) computed via the SpinFlip BetheSalpeter Equation approach (“SFBSE,” arXiv:2207.04549). SFBSE is a method based on manybody perturbation theory that allows for the simultaneous computation of ground and excited state energies, from a basis of “target” states constructed from exciting an occupied upspin electron to an unoccupied downspin empty orbital. While some excited states for the NV^{} defect have contributions from doubleexcitations that are inaccessible via singlereference spinflip methods, the excited states of interest for SiV(0) do not, allowing for improved quantitative agreement for vertical excitation energies. Results from this approach are compared to other multireference methods in the literature. 
Monday, March 6, 2023 1:06PM  1:18PM 
B59.00007: Engineering intermediate band states in Cuintercalated 2D transition metal chalcogenides Srihari M Kastuar, Chinedu E Ekuma Intercalated layered transition metal chalcogenides and metal atom hybrids have been shown to have remarkable properties with a high degree of tunability. Herein, we have studied the optoelectric properties of hybrid structures derived from Cuintercalated atomically thin GeSe/SnS heterostructures. Using the Green's function and Coulomb interaction and BetheSalpeter equation approaches, we determined the singleparticle electronic and twoparticle absorption properties. In the hybrid materials, we find intermediate band (IB) states with subband gap values of 0.78 and 1.26 eV, which are remarkably close to the subband gaps (~0.71 and 1.24 eV) for IB solar cell materials predicted by the LuqueMarti model. 
Monday, March 6, 2023 1:18PM  1:30PM 
B59.00008: A theoretical design of twodimensional thermally activated delayed fluorescence material Siyu Gao Organic lightemitting diodes (OLEDs) have attracted great interest for display and lighting applications. Now, Thermally Activated Delayed Fluorescence (TADF) based OLEDs may possess a nearly 100% internal quantum efficiency. In this work, a twodimentional TADF material is designed based on multiple resonance (MR) effect and density functional theory (DFT) grid search of the potential energy surface (PES) at an optimum interlayer distance. An energy barrier of 0.2 eV is observed between singlet and triplet state. The projected band structure by DFT shows the distribution of donor and acceptor. The quasipartical band structure and exciton wavefunctions simulated by GW approximation and the BetheSalpeter equation is reported. 
Monday, March 6, 2023 1:30PM  1:42PM 
B59.00009: ExchangeDriven Intermixing of Bulk and Topological Surface State by Chiral Excitons in Bi_{2}Se_{3} Bowen Hou, Dan Wang, Bradford A Barker, Diana Y Qiu Topological surface states (TSS) in the topological insulator Bi_{2}Se_{3} are often characterized using lightbased probes, such as the circular photoelectric effect. However, microscopic theory considering electronhole interactions and their effect on the optical response of TSS and surface localization has not yet been explored. Here, we study excitonic effects in the bulk and surface of Bi_{2}Se_{3} using the ab initio GW and BetheSalpeter equation (GWBSE) approach with a fullyrelativistic spinor formalism. We identify bright excitons with plike symmetry, as well as multiple series of excitons with chiral optical dipole selection rules spread over a wide energy range of 02.82 eV. The electronhole pair forming the chiral excitons exhibit the characters of both bulk states and topologically protected surface states, which are intermixed by the Coulomb exchange interaction. Our results address fundamental questions about the degree to which electronhole interactions can relax the topological protection of surface states in topological insulators by elucidating the complex intermixture of bulk and surface states excited in optical measurements and their coupling to light. 
Monday, March 6, 2023 1:42PM  1:54PM 
B59.00010: Flatband induced excitonic insulator in a carbonbased, triangulene Kagome lattice Jingwei Jiang, Aidan Delgado, Carolin Dusold, Adam Cronin, Felix R Fischer, Steven G Louie Excitonic insulator (EI) is a novel cooperative phase of matter formed by coherent excitons. This phase occurs when the bound electronhole excitations of the system have a lower energy than the normal band insulating ground state. Namely, when an electron from the valence band is put in the conduction band, the binding energy between the excited electrons and holes is larger than the bandgap of the normal state, leading to simultaneous formation and subsequent condensation of excitons. A previous work shows that a specific 2dimensional crystal of finitesize graphene triangles, a [4]triangulene Kagome lattice, possesses negative excitation energies of triplet excitons via a GWBSE on top of DFTLDA calculation[1], and thus could be a candidate for EI. Here we study the coherent ground state formed by excitons using a BCSlike theory, and explore the experimental signatures of such EI state (such as the local density of state (LDOS) and other electronic properties) from firstprinciples calculations. Our results agree well with STM measurements performed by our experimental collaborators. We also discuss the interplay between possible magnetic state, exciton condensation state, and normal state, as well as considering substrate effects. 
Monday, March 6, 2023 1:54PM  2:06PM 
B59.00011: Excitonic near resonanceenhancement of shift currents in monolayer NbOCl2 MingRui Lai The shift current is a static nonlinear photocurrent exhibited by crystals lacking centerofinversion symmetry. Such photocurrents have been widely studied for their potential photovoltaic applications and have potential to go beyond the ShockleyQueisser limit. As such, an accurate theoretical description of the phenomenon is essential for material design. Density functional theory is often utilized in first principles calculations of the shift current, treating the electrons independently. In 2D materials however, the reduced screening results in signficant manybody interactions and photoexcitations are more accurately described with excitons. We have developed a theoretical framework which captures these excitonic effects using manybody perturbation theory within the framework of nonlinear optical response. We derive an expression for the exciton shift current from the static nonlinear response. The manybody excited states are obtained using the GWBSE formalism by solving the BetheSalpeter Equation with quasiparticle energies. Using the manybody excited states, we then compute the excitonic shift current. We show that there is a large enhancement in the shift current when considering manybody interactions, mainly due to the nearresonance condition between two neardegenerate excited states. 
Monday, March 6, 2023 2:06PM  2:18PM 
B59.00012: Excitonic properties of monolayer and bulk hexagonal boron nitride Woncheol Lee, Ping Wang, Qiannan Wen, Zetian Mi, Mackillo Kira, Emmanouil Kioupakis In this work, we investigate the excitonic properties of monolayer and bulk hexagonal boron nitride (hBN) based on density functional theory and manybody perturbation theory. Our simulation results indicate that monolayer hBN on Highly Ordered Pyrolytic Graphite (HOPG) exhibit a giant renormalization of the electronic band gap and the exciton binding energy due to the strong screening induced by the HOPG substrate. Our photoluminescence measurements and reflectance measurements confirm our theoretical prediction on the electronic and optical gap. In addition, we perform firstprinciples calculations on the excitonphonon coupling matrix elements and reveal that the strong excitonphonon interaction contributes to the effective indirect optical transitions and the bright luminescence in hBN. As a result, we propose that both the strong screening from the substrate and the strong excitonphonon interaction within hBN should be considered as an important factor for exciton engineering in hBN. The work is supported by the University of Michigan College of Engineering Blue Sky Research Program. W.L. was partially supported by the Kwanjeong Educational Foundation Scholarship. Computational resources were provided by the DOE NERSC facility. 
Monday, March 6, 2023 2:18PM  2:30PM 
B59.00013: FirstPrinciples Study of the Dopingdependent Exciton and Trion Linewidth in Monolayer MoTe_{2} Supavit Pokawanvit, Aurelie Champagne, Jonah B Haber, Diana Y Qiu, Jeffrey B Neaton, Felipe H Jornada The linewidth of excitonic complexes provides direct insights into the nature of optical excitations in materials and their decay pathways. In doped semiconducting monolayers of transition metal dichalcogenides (TMDs), the linewidth associated with an exciton resonance is sensitive to extra charge carriers due to the nontrivial dielectric screening from the Fermi sea and the variety of states an exciton can scatter to, even in the weakdoping limit. Therefore, computational approaches and firstprinciples calculations can provide unique insights into the microscopic origin of such interactions and the nature of the associated scattering events. In this talk, we present results from firstprinciples calculations of Dysonlike equations associated with 3 and 4body interacting particle problems (involving electrons and holes) to address this problem. We compare our results with perturbative calculations based on the scattering of excitons to Fermisea electronhole pairs and assess the importance of manybody screening, bandfilling effects, and the ab initio description of electronhole coupling terms. 
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