Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session N46: Excited State IV: exciton dynamicsFocus Recordings Available
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Sponsoring Units: DCOMP DMP Chair: Serdar Ogut, UIC Room: McCormick Place W-470A |
Wednesday, March 16, 2022 11:30AM - 12:06PM |
N46.00001: Photo-induced phase-transitions and coherent phenomena in realistic materials: an ab-initio Many-Body approach. Invited Speaker: Andrea Marini It is widely known that a monochromatic electro-magnetic wave can travel forever without losing information. It is similarly common knowledge among experimentalists that after a material has been photo-excited an additional, induced, electric field gets super-imposed to the external probe. This field is generated by the electronic charge oscillations. The key point is that this induced field appears only after the excitation. The system, at rest, is not able to produce it. |
Wednesday, March 16, 2022 12:06PM - 12:18PM |
N46.00002: An ab-initio framework for phonon-mediated exciton diffusion in crystals Jonah B Haber, Felipe H da Jornada, Sivan Refaely-Abramson, Diana Y Qiu, Gabriel Antonius, Steven G Louie, Jeffrey B Neaton The dynamics of excitons in complex materials upon photoexcitation are important for energy applications, e.g., light-emitting diodes and photovoltaics. As a specific example, in organic photovoltaics, strongly-bound photo-excited excitons must diffuse to donor-acceptor interfaces where charge separation may occur before the rest of the energy conversion process can proceed. Describing phonon-mediated exciton transport in these materials is complicated by the fact that exciton bandwidth and exciton-phonon coupling strengths are similar in magnitude. In turn it is unclear a priori whether exciton diffusion is best described by phonon-limited Boltzmann-like or thermally activated hopping-like theories. In this talk, using density functional perturbation theory and the ab initio Bethe-Salpeter equation approach, we describe a self-contained framework for computing exciton diffusion coefficients in both the band-like and hopping regimes. Our reciprocal-space based, linear response method explicitly takes into account the entire crystalline environment and can seamlessly be applied to study both spin-singlet and -triplet excitations. We apply our method to a select set of acene crystals shedding additional light on microscopic origins of exciton diffusion in these classic materials. |
Wednesday, March 16, 2022 12:18PM - 12:30PM |
N46.00003: First-principles calculations of phonon-mediated exciton relaxation in two-dimensional semiconductors Xiaowei Zhang, Kaichen Xie, Enge Wang, Ting Cao, Xinzheng Li Exciton-phonon coupling (ExPC) is crucial for energy relaxation in semiconductors, yet the first-principles calculation of such coupling remains challenging, especially for low-dimensional systems. Here, we present a newly developed algorithm for calculating and analyzing ExPC and apply the algorithm in the exciton relaxation problem of monolayer transition metal dichalcogenides. We find that the interplay between the exciton wave functions and electron-phonon coupling (EPC) results in distinct selection rules from the ones of EPC. We further generalize these selection rules to Wannier exciton-phonon couplings in two-dimensional semiconductors. To verify our theory and algorithm, we calculate inter-valley exciton relaxation time, which agrees well with a recent experiment. |
Wednesday, March 16, 2022 12:30PM - 12:42PM |
N46.00004: Free carrier absorption in doped silicon from first-principles Xiao Zhang, Guangsha Shi, Emmanouil Kioupakis Optoelectronic applications of silicon (Si) always involve doping to inject free carriers. It is thus important to understand the effects of free carrier absorption (FCA) on Si-based devices. In this work, we developed and applied first-principles computational approaches based on density functional and many-body perturbation theory to quantify the different free-carrier absorption mechanisms in Si, including direct, phonon-assisted, charged-impurity-assisted and conductivity-induced absorptions. We show that our calculated FCA coefficients agree with experimental measurements over a wide range of carrier densities and photon wavelengths for free electrons and free holes. We show that the different mechanisms of FCA dominate different photon wavelengths. Quantifying the effects of FCA on electron-hole induced current density, we show that FCA is an essential source of optical loss in Si especially for IR applications. |
Wednesday, March 16, 2022 12:42PM - 12:54PM |
N46.00005: Study of Doping-dependent Multiparticle Excitation Spectrum in Monolayer TMDCs from First Principles Supavit Pokawanvit, Aurelie Champagne, Jonah B Haber, Diana Y Qiu, Jeffrey B Neaton, Felipe H da Jornada Monolayers of transition-metal dichalcogenides (TMDC), such as MoS2, WSe2, MoTe2, have opened the door to the theoretical and experimental studies of multiparticle excitations, such as trions and biexcitons, with relatively large binding energies. Upon carrier doping or strong optical pump fluence, these excitation complexes dominate the optical properties of these materials owing to the strong Coulomb interactions and reduced electronic screening in such systems. In this work, we study, from first principles, how carrier doping affects the many-electron screening and renormalizes the exciton and trion linewidth in monolayer TMDCs. We employ interacting Green’s-function-based approaches, including the Bethe-Salpeter equation and beyond, to compute these multiparticle excitations. We show the valley- and momentum-resolved multiparticle excitations for various amounts of carrier dopings, and comment on the applicability of the traditional trion picture of one exciton plus an electron compared to the hybrid exciton plus Fermi-sea description to capture such excitations from first principles. |
Wednesday, March 16, 2022 12:54PM - 1:06PM |
N46.00006: Long-range magnetic ordering coupling with bulk excitonic effect and defect related sharp peak in van der Waals antiferromagnet NiPS3 Chunhao Guo, Junqing Xu, Dario Rocca, Yuan Ping The antiferromagnetic van der Waals material NiPS3 has recently emerged as a novel system for its versatile long-range magnetic ordering and its strong correlation with electronic and optical properties. We first studied the quasiparticle and optical properties of pristine NiPS3, and revealed the mechanism of the coupling between excitonic effect and magnetic ordering from first-principles many-body perturbation theory. We found the experimental sharp peak below the main absorption edge around 1.48 eV cannot be explained by the excitonic excitation from bulk states. Then we studied the optoelectronic properties of S vacancy defect states in different magnetic ordering phases. We also studied the photoluminescence lineshape, and exciton lifetime through radiative and nonradiative decay, and emitted photon polarization of the defect related sharp peak. We found that the defect state induces disparate transitions and photon polarization while varying long-range magnetic ordering. Our results and proposed mechanisms are in good agreement with recent experiment data. We propose that the appearance of this sharp peak below the band gap could be a unique probe for the magnetic ordering of this system. |
Wednesday, March 16, 2022 1:06PM - 1:18PM |
N46.00007: Delocalization of dark and bright strongly bound excitons in flat-band materials: optical properties of V2O5 Vitaly Gorelov, Lucia Reining, Martin Feneberg, Ruediger Goldhahn, Andre Schleife, Walter R Lambrecht, Matteo Gatti The common picture of excitons in materials with atomic-like localization of electrons is that of Frenkel excitons, where electrons and holes stay close together, which is associated with a large binding energy. Here, using the example of the layered oxide V2O5, we highlight another kind of exciton: it also has a huge binding energy but, at the same time, a large electron-hole distance. We explain that this seemingly contradictory finding is rooted in the charge transfer nature of the excitation. The anisotropy of the exciton delocalization is determined by the local anisotropy of the structure, whereas the exciton extends orthogonally to the chains formed by the crystal structure. Moreover we show that the bright exciton goes together with a dark exciton of even larger binding energy and more pronounced anisotropy. These findings are obtained by combining first principles many-body perturbation theory calculations, ellipsometry experiments, and tight binding modelling, leading to excellent agreement and a consistent picture. Our explanation is general and can be extended to other materials. |
Wednesday, March 16, 2022 1:18PM - 1:30PM |
N46.00008: Ab initio study of quasiparticle bandstructure and chiral exciton in the topological insulator Bi2Se3 Bowen Hou, Dan Wang, Diana Y Qiu Most known bound excitons are composed of massive electron-hole pairs, while optical excitations arising from massless quasiparticle bands, such as the Dirac cone in graphene, tend to be short-lived resonant exciton states. However, researchers have recently measured novel long-lived chiral excitons in the 3D topological insulator Bi2Se3, which are composed of massive holes and massless electrons, both residing on the surface state and subject to strong spin-orbit interactions. These chiral excitons exhibit both circular dichroism and almost perfectly preserved circular polarization in the photoluminescence emission caused by chiral exciton recombination. Here, we utilize the ab initio GW and GW plus Bethe Salpeter equation (GW+BSE) method to study the quasiparticle bandstructure and optical absorption spectrum including excitonic effects in both the bulk and surface of Bi2Se3. Our calculations are based on the fully-relativistic spinor GW-BSE formalism and reveal the nature of both excitons near the Fermi level and those arising from transitions between higher-energy surface states. |
Wednesday, March 16, 2022 1:30PM - 1:42PM |
N46.00009: Optical properties of HgxCd1-xS and HgxCd1-xSe alloys from first principles Erick I Hernandez Alvarez, Andrew M Smith, Andre Schleife Mercury-cadmium chalcogenide alloys are materials of interest for applications in the visible to near-infrared region of the electromagnetic spectrum. Their tunable optical properties make them candidates for use as NIR detectors and quantum dot emitters. In this work we present results from first principles density functional theory calculations of zincblende HgxCd1-xS and HgxCd1-xSe alloys. We compare the band structures and band gap energies from DFT-PBE and HSE hybrid exchange-correlation functionals. We consider excitonic effects on the optical spectra of the binary end components and discuss the influence of spin-orbit coupling on the optical spectra. We also employ the generalized quasi-chemical approximation to predict the optical spectra of intermediate alloy compositions in thermodynamic equilibrium. We find a higher Hg:Cd ratio results in a larger effect on band gap energies from spin-orbit coupling in DFT-PBE, with a less pronounced effect in HSE. We also find minimal changes in the simulated optical spectra due to spin-orbit coupling. |
Wednesday, March 16, 2022 1:42PM - 1:54PM |
N46.00010: Frenkel excitons in LiCoO2: QSGW calculations including ladder diagrams and BSE approach Walter R Lambrecht, Santosh K Radha, Brian Cunningham, Myrta GrĂ¼ning, Dimitar Pashov, Mark van Schilfgaarde While rhombohedral (R-3m) LiCoO2 has been widely studied in the context of its battery applications, the relation between its band structure and optical properties is not yet clear. The basic understanding is that a gap exists between t2g filled and eg empty Co-d states of the octahedral environment and gives rise to optical transitions near 2 eV and with an onset near 1.4 eV, which at first seems reasonably close to the LDA gap of 1.2 eV found in the literature. However, we show that in the much more reliable quasiparticle self-consistent (QS) GW method, the gap is as high as 4.125 eV. In this work, we apply a recently developed QSGW method including ladder-diagram (electron-hole) corrections to the screening of W, implemented in the linearized muffin-tin orbital basis set. Even this gives a gap or 3.76 eV. However, when the optical dielectric function is calculated at the Bethe Salpeter equation (BSE) level, we find a huge change in the optical response with exciton binding energies in excesss of 2 eV. The density of states of BSE eigenvalues as a whole is shifted rigidly from the one-particle joint density of states. The lowest exciton state is found to be dark but several bright peaks exist. The optical spectrum should thus be interpreted in terms of excitons instead of interband transitions. The exciton wavefunctions are found to be very delocalized in k-space and hence localized in real space and can thus be labeled Frenkel excitons. A systematic study of the dielectric function at different levels of theory is presented. |
Wednesday, March 16, 2022 1:54PM - 2:06PM |
N46.00011: First-Principles Calculations of Optical Spectra in Liquid Water using GW plus Bethe-Salpeter Equation approach Fujie Tang, Zhenglu Li, Chunyi Zhang, Diana Y Qiu, Xifan Wu Optical spectroscopy is a powerful experimental technique to probe the electronic structure of liquid water. An accurate modeling of optical spectra demands accurate descriptions of both molecular structures and electron-hole excitation processes from first principles. We simulate the water structure from deep potential path-integral molecular dynamics simulations, where the neural network potential is trained on the state-of-the-art density-functional theory data with the SCAN0 hybrid functional. Using the equilibrated liquid structure, we compute optical spectra of liquid water by solving the GW-Bethe-Salpeter equation as implemented in the BerkeleyGW software package. Our theoretical optical spectrum agrees well with existing experiments. We further analyze the effects from quantum nuclei and thermal fluctuations on the optical spectrum. |
Wednesday, March 16, 2022 2:06PM - 2:18PM |
N46.00012: Hidden Selection Rule for Exciton Coupling in Organic Crystals Aaron R Altman, Sivan Refaely-Abramson, Felipe H da Jornada Organic molecular crystals host unique exciton-exciton interactions, allowing the formation of multiexcitons through exciton fission and long-lived exciton energy carriers. However, a general understanding of how the crystal structure affects exciton fission is lacking, requiring computationally demanding calculations to classify each candidate material. Here, we present a DFT-derived, effective Hamiltonian approach to understand structural effects on exciton-exciton interactions in molecular crystals directly from their electronic band structure. We use this model to derive hidden selection rules on crystal pentacene and predict that the common bulk polymorph supports fast Coulomb-mediated singlet fission, with a coupling that is at least one order of magnitude larger than that of the thin-film polymorph, a result we confirm with explicit calculations based on many-body perturbation theory. Our approach can be used to understand a variety of hidden symmetries involving electronic and optical excitations in complex materials from DFT calculations, and provides design principles for the experimental and computational discovery of new materials with efficient non-radiative exciton decay rates. |
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