Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F29: Firstprinciples Modeling of ExcitedState Phenomena in Materials V: Density Functional Theory for Excited StatesFocus

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Sponsoring Units: DCOMP DMP DCMP DCP Chair: Shane Parker, University of California, Irvine Room: LACC 406A 
Tuesday, March 6, 2018 11:15AM  11:51AM 
F29.00001: Multiconfiguration PairDensity Functional Theory for ExcitedStates in Molecules and Materials Invited Speaker: Laura Gagliardi Multiconfiguration pairdensity functional theory[1],[2] is a generalization of Kohn−Sham density functional theory in that the electron kinetic energy and classical electrostatic energy are calculated from a reference wave function, with the rest of the energy obtained from a density functional. By combining the advantages of wave function theory and density functional theory it provides a better treatment of strongly correlated systems. Some recent applications of the new method in excited states both in molecules and materials will be presented. 
Tuesday, March 6, 2018 11:51AM  12:03PM 
F29.00002: Koopmanscompliant Spectral Functionals for Extended Systems Nicola Marzari, Linh Ngoc Nguyen, Nicola Colonna, Andrea Ferretti Koopmanscompliant functionals have been shown to provide accurate spectral properties for molecular systems; this accuracy is driven by the generalized linearization condition imposed on each charged excitation  i.e. on changing the occupation of any orbital in the system, while accounting for screening and relaxation from all other electrons. Here, we discuss the theoretical formulation and the practical implementation of this formalism to the case of extended systems, where a third condition, the localization of Koopmans' orbitals, proves crucial to reach seamlessly the thermodynamic limit. We illustrate the formalism by first studying onedimensional molecular systems of increasing length. Then, we consider the band gaps of 30 paradigmatic solidstate test cases, for which accurate experimental and computational results are available. The results are found to be comparable with the stateoftheart in diagrammatic techniques (selfconsistent manybody perturbation theory with vertex corrections), notably using just a functional formulation for spectral properties and the physics of the generalizedgradient approximation; when ionization potentials are compared, the results are roughly twice as accurate. 
Tuesday, March 6, 2018 12:03PM  12:15PM 
F29.00003: A Unified Treatment of Single and Double Electronic Excitations and Corresponding Delocalization Lengths in piconjugated Materials Christopher Sutton, Yang Yang, Du Zhang, Weitao Yang Understanding the lowest excited states of materials is of great interest for investigating biological processes (where lightharvesting complexes act to convert light into chemical energy) and emerging technologies (where organic dyes are used as the active materials). In large linear πconjugated materials, the lowestenergy singlet state has significant double excitation character, corresponding qualitatively to the promotion of two electrons from an occupied state to an unoccupied state. However, the accurate description of both the single (oneelectron) and double (twoelectron) excitations for complex materials has been a significant challenge for electronic structure methods. We present the accurate calculation of single and double excitation energies for the prototypical piconjugated material polyacetylene using our newly developed functional based on the particleparticle random phase approximation (ppRPA).This approach allows for several experimental observations to be rationalized and new predictions to be made in regards to the impact of statedependent correlation lengths on higherenergy excited states. 
Tuesday, March 6, 2018 12:15PM  12:27PM 
F29.00004: Multiple Exciton Generation in Chiral SingleWalled Carbon Nanotubes and Silicon Nanowires: DFTBased Study Including Competition Between Carrier Multiplication and PhononMediated Relaxation Deyan Mihaylov, Andrei Kryjevski, Svetlana Kilina, Dimitri Kilin The conclusion about multiple exciton generation (MEG) efficiency in a nanoparticle can only be made by comprehensive study of different relaxation channels, such as phononmediated thermalization, carrier multiplication, etc. Here, we study time evolution of a photoexcited state using Boltzmann transport equation (BE) that includes phonon emission/absorption together with the exciton multiplication and recombination. BE rates are computed using nonequilibrium finitetemperature manybody perturbation theory based on DFT simulations, including exciton effects using RPAscreened Coulomb interaction. We compute rates for both allsinglet MEG and Singlet Fission channels, which are of order 10^{14} s^{1}. For allsinglet MEG we calculate internal quantum efficiency (QE), the number of excitons generated from a single absorbed photon. Efficient MEG in chiral singlewall carbon nanotubes (SWCNTs), such as (6,2), both pristine and Cldoped, (6,5), and in nmsized amorphous Hpassivated Si nanowires is present within the solar spectrum range. We predict QE≈1.31.6 at the excitation energy of 3 E_{gap} in (6,2) and (6,5). However, QE=1 is found in CNT (10,5) which suggests strong chirality dependence of MEG. MEG efficiency in functionalized SWCNTs is enhanced compared to the pristine case. 
Tuesday, March 6, 2018 12:27PM  12:39PM 
F29.00005: Excitonic DFT: An Efficient and Flexible Constrained DFT Approach for Simulating Neutral Excitations in Isolated and Periodic Systems Subhayan Roychoudhury, Stefano Sanvito, David O'Regan Stateoftheart methods for calculating neutral excitation energies, such as timedependent density functional theory (TDDFT) and GW + BetheSalpeter, are computationally demanding and currently limited to moderate system sizes. Recent years have seen a growing interest in adapting density functional theory (DFT) to describe excited states, in the form of methods including ensemble DFT [1], constricted variational DFT [2], and restricted openshell KohnSham [3]. We introduce a computationally light, generally applicable, firstprinciples approximation based on constrained DFT [45] for calculating neutral excitations, and our implementation in the linearscaling DFT code ONETEP. We demonstrate close agreement with TDDFT for the Thiel molecular test set [6], and show that our method may also be successfully applied to extended systems like semiconductors and 2D materials. 
Tuesday, March 6, 2018 12:39PM  12:51PM 
F29.00006: ElectronDefect Scattering from FirstPrinciples Calculations ITe Lu, JinJian Zhou, Luis Agapito, Marco Bernardi Materials contain defects that can significantly impact charge transport. While ab initio calculations have focused on electronphonon scattering and the phononlimited mobility, electrondefect (ed) scattering controls the mobility at low temperature and at room temperature in materials with impurities, dislocations, and interfaces. We present a new ab initio approach to compute the ed scattering rate due to neutral defects. The formalism relies on 1storder perturbation theory, where the perturbation is the difference of the KohnSham potentials between a pristine material and the material with a defect. We discuss numerical treatments of the local and nonlocal parts of this perturbation potential, and effective computation of the associated ed scattering matrix elements. Using silicon as a case study, we show that the contribution to scattering from the nonlocal part of the pseudopotential, which was neglected in previous work, can be large and unpredictable a priori. We present converged ed scattering rates for electrons in silicon and graphene due to a range of defects including vacancies, interstitials, and impurities. Using the Boltzmann transport equation, we carry out the first fully ab initio computation of the defectlimited carrier mobility at low temperature. 
Tuesday, March 6, 2018 12:51PM  1:03PM 
F29.00007: NonRadiative Recombination at Dynamic Shallow Level Junhyeok Bang, Sheng Meng, Shengbai Zhang To date, the ShockleyReadHall (SRH) theory is the ruling theory explaining the defectmediated nonradiative recombination (NRR) of excited carriers. However, recent firstprinciples calculations show that the SRH theory seriously underestimates the NRR rate. While the potential importance of ionic relaxation has been implicated, the model has yet to be confirmed. Here, we present the theoretical formalism of the NRR that involves intermediate ionic relaxation. As a demonstration, we show that the DX center is an efficient NRR center by carrier capture at dynamics shallow level. As the concentration of DX center can be controlled in experiments, this NRR mechanism may be readily verified. 
Tuesday, March 6, 2018 1:03PM  1:15PM 
F29.00008: The Auger process from time dependent density matrix evolution in the GKBA Fabio Covito, Enrico Perfetto, Gianluca Stefanucci, Angel Rubio Stateoftheart experimental techniques allow the stimulation of excited electronic dynamics on an ultrafast timescale. For example, with XUV radiation it is possible to create highly unstable excited states, which give rise to different relaxation processes of radiative and/or nonradiative nature. As opposed to radiative decay, the nonradiative mechanisms take place on a much faster timescale, i.e. femto to attoseconds. A typical decay mechanism enabled by electron correlations is the Auger decay. In this process a secondary electron is expelled from the system to relax to a lower energy state. To describe the Auger mechanism it is key to include electronic correlations otherwise not accounted for in adiabatic timedependent density functional theory approaches. Within our method we solve the KadanoffBaym equations (KBE) in the nonequilibrium Green's function framework by using the generalized KadanoffBaym ansatz (GKBA), i.e. recasting the KBE into a computationally more convenient closed equation for the oneparticle density matrix. As an illustration we simulate the emission of Auger electrons in real time in onedimensional atomic systems. This paves the way towards the description of timedependent Auger decay in realistic systems. 
Tuesday, March 6, 2018 1:15PM  1:27PM 
F29.00009: Auger Recombination From Firstprinciples in GroupIII Nitride Alloys Andrew McAllister, Dylan Bayerl, Christina Jones, Emmanouil Kioupakis The groupIII nitrides are widely used in optoelectronic devices like LEDs and lasers. However, at high power these materials have a drop in efficiency. This has been attributed to nonradiative Auger recombination. Experimental measurements of Auger and other nonradiative processes are difficult, making firstprinciples atomisc simulations vital to gaining a better understanding of how carriers recombine. We use density functional theory and manybody perturbation theory to study Auger and radiative rates in groupIII nitride alloys. Our previous results have shown that in pure GaN, Auger primarily occurs through the assistance of phonons, while in pure InN, Auger occurs without assistance from other mechanisms. More interesting for optoelectronics are AlGaN and InGaN alloys, for which Auger is also assisted by alloy disorder. We will discuss results of our calculations on special quasirandom structures of AlGaN and InGaN over their complete composition range. Our findings provide insight into the microscopic origin of Auger and suggest approaches to reduce its impact on the efficiency of nitride devices. 
Tuesday, March 6, 2018 1:27PM  1:39PM 
F29.00010: Unconventional multiple plasmons generation in Oxygenrich Strontium Niobate mediated by Local Field Effects Tao Zhu, Paolo Trevisanutto, Teguh Asmara, Yuan Feng, Andrivo Rusydi Recently, an anomalous form of plasmons have been observed experimentally in Oxygenrich Strontium Niobate Oxides (SrNbO_{3+δ}). These new plasmons have multiple frequencies in the visible to ultraviolet ranges and were believed to be connected with the strongly correlated electrons induced by spatial confinement. Here we theoretically investigate the origin of plasmons generation by means of first principle calculations. In the framwork of the Random Phase Approximation (RPA), we infer that the spectral weight transfer and the multiple formation of plasmon frequencies can be ascribed to the dielectric function local fluctuations called Local Field Effects (LFEs). We proved that the LFEs are unusually predominant with formation of plasmons in materials with strong electrical inhomogeneity and high polarizability. 
Tuesday, March 6, 2018 1:39PM  1:51PM 
F29.00011: Timeresolved Xray Spectroscopy in Onedimensional Strongly Correlated Systems ChenYen Lai, JianXin Zhu In recent years, ultrafast pumpprobe spectroscopy has provided insightful information about nonequilibrium dynamics of excitations in materials. In a typical experiment of timeresolved xray absorption spectroscopy, the systems are excited by a femtosecond laser pulse (pump pulse) following by an xray (probe pulse) after a time delay to measure the absorption spectra of the excited systems. In this talk, we present a theory for timeresolved xray absorption spectroscopy in onedimensional strongly correlated systems. We consider a onedimensional Hubbard model, and use the timedependent density matrix renormalization group method to solve the ground state and nonequilibrium dynamics numerically. The core hole created by xray is modeled as an additional effective potential and interaction on the core hole site. Our study shows that, when the system is away from the half filling, the spectrum shows side peaks along with the major transitions; while the system moves closed to half filling, the spectrum starts to reveal the charge gap. The response of the xray absorption spectroscopy to the strength of Hubbard repulsion, and frequency and intensity of laser pump pulse, has also been studied. LAUR1729749 
Tuesday, March 6, 2018 1:51PM  2:03PM 
F29.00012: Tackling Quantum ManyBody Problems in XRay Spectra via a Basic Graph Algorithm Yufeng Liang, David Prendergast The growth in access to detailed materials characterization using Xray spectroscopy highlights the need for more accurate electronicstructure theory predictions of Xray absorption nearedge fine structure. 
Tuesday, March 6, 2018 2:03PM  2:15PM 
F29.00013: First Principles Simulations of εGe/In_{x}Al_{1x}As Interfaces: Band Alignments and Interface Structure Gabriel GreeneDiniz, Myrta Gruening Tensile strained Ge (εGe) grown on IIIV substrates is currently the focus of substantial research efforts due to the range of potential technological applications, from high performance, low power tunnel FETs, to onchip optical interconnects based on IIIV/εGe/IIIV quantum well light emitters. In this work, we investigate the sensitivity of quasiparticle band offsets to interface structure for the εGe/(001)In_{x}Al_{1x}As interface. Utilizing first principles methodologies for atomistic simulations, namely density functional theory (DFT) combined with many body perturbation theory within the GW approximation, valence and conduction band alignments are calculated for a range of interface configurations. Considering the combined effects of cation stoichiometry of the substrate, interface stoichiometry, diffusion of mixed layers across the interface, and the strain induced movement of quasiparticle valleys in Ge, band alignments recently measured from core level XPS spectra for epitaxial thin films of εGe on (001)In_{0.25}Al_{0.75}As substrates are explained in terms of the of diffusion of substrate species into the Ge film. 
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