Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N9: Invited Session: Computational Spectroscopy |
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Sponsoring Units: DCOMP Chair: Giulia Galli, University of California, Davis Room: 308 |
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N9.00001: Recent developments in time-dependent density-functional theory within and beyond linear response Invited Speaker: E.K.U. Gross Time-dependent density functional theory (TDDFT) is a popular and rather successful method in the description of photo-absorption spectra of atoms and molecules in the linear response regime. In extended solids, however, a satisfactory description of excitonic effects has become possible only recently with the advent of advanced approximations for the exchange-correlation kernel f$_{\mathrm{xc}}$. One of these advanced approximations is the so-called bootstrap kernel [S. Sharma et al, PRL \textbf{107}, 186401 (2011)]. We shall explore the performance of this kernel in the long-wavelength limit and for finite values of q, looking at electron-loss as well as photo-absorption spectra. We find, in particular, that excitonic effects in LiF and Ar are enhanced for values of q away from the $\Gamma $-point [S. Sharma et al, New J Phys \textbf{14}, 053052 (2012)]. Then we present two recent developments in TDDFT beyond the linear-response regime: (i) By using a geometrical partitioning, we calculate the angle and energy resolved photo-electron spectra of finite systems including multi-photon effects [De Giovannini, et al, A. Rubio, PRA \textbf{86}, 062515 (2012)]. (ii) Finally we show how the dynamics of many-electron systems can be controlled with lasers by marrying TDDFT with optimal control theory [A. Castro et al, PRL \textbf{109}, 153603 (2012)]. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N9.00002: Computational spectroscopy using many-body perturbation theory: Large scale calculations without virtual orbitals Invited Speaker: Dario Rocca An accurate description of electronic excitations is essential to model and understand the properties of several materials of fundamental and technological interest. First principles, many-body techniques based on Green's functions are promising approaches that can provide an accurate description of excited state properties; however their applicability has long been hindered by their numerical complexity. In this talk we will summarize some recent methodological developments based on many-body perturbation theory for the efficient calculation of optical absorption spectra [1], photoemission spectra [2], and multiple exciton generation rates [3]. Several applications to realistic materials will be presented, with emphasis on materials for solar energy applications; these include silicon nanowires and bulk tungsten oxide, that are promising photoelectrode materials in water splitting solar cells, molecules used in organic photovoltaics, and semiconductor nanoparticles with potential use in third generation photovoltaic cells based on multiple exciton generation. Work done in collaboration with Y. Ping, T. A. Pham, M. Voros, D. Lu, H.-V. Nguyen, S. Wippermann, A. Gali, G. T. Zimanyi, and G. Galli.\\[4pt] $^*$Present address\\[4pt] [1] D. Rocca, D. Lu,G. Galli, J. Chem. Phys. 133, 164109 (2010); D. Rocca, Y. Ping, R. Gebauer, G. Galli, Phys. Rev. B 85, 045116 (2012).\\[0pt] [2] H.-V. Nguyen, T.A. Pham, D. Rocca, G. Galli, Phys. Rev. B 85, 081101 (2012).\\[0pt] [3] M. Voros, D. Rocca, G. Galli, G.T. Zimanyi, A. Gali, submitted (2012). [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N9.00003: Many-electron interactions and first-principles studies of spectral functions: spin multiplets and plasmon satellites in photoemission spectra Invited Speaker: Johannes Lischner The photoemission spectrum of an interacting system is often simply thought to be qualitatively similar to the corresponding non-interacting system: interactions only cause a shift and a broadening of the quasiparticle peak and result in a transfer of spectral weight into an incoherent background. We discuss two cases where this simple quasiparticle picture of photoemission fails and interactions result in a more drastic, qualitative difference from the non-interacting system. For electronic systems with unfilled shells, the coupling of angular momenta results in a multiplet structure in the photoemission spectrum. We describe how accurate calculations of multiplet splittings are possible within the GW approximation and present results for several magnetic molecules and defects, such as the negatively charged nitrogen-vacancy defect (NV$^{-})$ center in diamond. We also discuss plasmon satellite structures in photoemission spectra. We show for bulk silicon and doped graphene that the \textit{ab initio}GW approximation overestimates the quasiparticle-satellite separation significantly and falsely predicts a plasmaron excitation. By including significant vertex corrections via the \textit{ab initio}GW$+$cumulant approximation, we improve the description of plasmon satellites and find good agreement with experimental photoemission spectra.\\[4pt] Work was done in collaboration with Jack Deslippe, Manish Jain, Derek Vigil-Fowler and Steven G. Louie. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N9.00004: Theory for Time-Domain Photon Spectroscopy Invited Speaker: Thomas Devereaux In this talk I will present some recent work concerning the development of theories for time-domain photon spectroscopies, with a focus on studying non-equilibrium pump-probe dynamics. Studies of several model systems will be presented, including non-equilibrium dynamics across of metal-insulator transition in correlated systems, strong electron-phonon interactions, and spectral properties in a charge density wave state. The similarities and differences between equilibrium dynamics will be highlighted [1]. \\[4pt] [1] Phys. Scripta T151, 014062 (2012); arXiv:1210.3088; arXiv:1207.3835; Phys. Rev. Lett. 109, 176402 (2012); Nature Communications 3, 838 (2012); arXiv:1204.1803 [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 2:15PM |
N9.00005: Computational Spectroscopy for Nanoscale Photovoltaics Invited Speaker: Marco Bernardi Nanoscale photovoltaic (PV) systems employ nanomaterial interfaces to dissociate bound excitons formed upon sunlight absorption. This mechanism results in a correlated electron, hole, and exciton interface dynamics whose accurate determination is challenging both theoretically and experimentally. In this talk, I will discuss approaches available to compute and combine relevant spectroscopic quantities to predict efficient nanoscale PV systems. Further, I will present our recent work on two novel families of nanoscale PV devices based on: 1) Nanocarbon materials, achieving 1.3\% efficiency, tunable infra-red optical absorption, and superior photostability compared to organic solar cells 2) Two-dimensional monolayer semiconductors such as Graphene-BN and MoS$_2$, capable of absorbing a significant fraction of sunlight within just $\approx10$nm, and showing tunable absorption, band offsets, and power conversion efficiency (PCE).\\[4pt] In closing, I will discuss the errors and necessary accuracy in predicting PCE from first-principles calculations, and propose a suitable figure of merit to quantify absorption solar-matchedness to be used in large-scale searches of nanoscale PV materials. [Preview Abstract] |
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