Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F29: First-principles Modeling of Excited-State 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 Pair-Density Functional Theory for Excited-States in Molecules and Materials Invited Speaker: Laura Gagliardi Multiconfiguration pair-density 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: Koopmans-compliant Spectral Functionals for Extended Systems Nicola Marzari, Linh Ngoc Nguyen, Nicola Colonna, Andrea Ferretti Koopmans-compliant 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 one-dimensional molecular systems of increasing length. Then, we consider the band gaps of 30 paradigmatic solid-state test cases, for which accurate experimental and computational results are available. The results are found to be comparable with the state-of-the-art in diagrammatic techniques (self-consistent many-body perturbation theory with vertex corrections), notably using just a functional formulation for spectral properties and the physics of the generalized-gradient 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 pi-conjugated 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 light-harvesting complexes act to convert light into chemical energy) and emerging technologies (where organic dyes are used as the active materials). In large linear π-conjugated materials, the lowest-energy 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 (one-electron) and double (two-electron) 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 pi-conjugated material polyacetylene using our newly developed functional based on the particle-particle random phase approximation (pp-RPA).This approach allows for several experimental observations to be rationalized and new predictions to be made in regards to the impact of state-dependent correlation lengths on higher-energy excited states. |
Tuesday, March 6, 2018 12:15PM - 12:27PM |
F29.00004: Multiple Exciton Generation in Chiral Single-Walled Carbon Nanotubes and Silicon Nanowires: DFT-Based Study Including Competition Between Carrier Multiplication and Phonon-Mediated 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 phonon-mediated thermalization, carrier multiplication, etc. Here, we study time evolution of a photo-excited state using Boltzmann transport equation (BE) that includes phonon emission/absorption together with the exciton multiplication and recombination. BE rates are computed using non-equilibrium finite-temperature many-body perturbation theory based on DFT simulations, including exciton effects using RPA-screened Coulomb interaction. We compute rates for both all-singlet MEG and Singlet Fission channels, which are of order 1014 s-1. For all-singlet MEG we calculate internal quantum efficiency (QE), the number of excitons generated from a single absorbed photon. Efficient MEG in chiral single-wall carbon nanotubes (SWCNTs), such as (6,2), both pristine and Cl-doped, (6,5), and in nm-sized amorphous H-passivated Si nanowires is present within the solar spectrum range. We predict QE≈1.3-1.6 at the excitation energy of 3 Egap 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 State-of-the-art methods for calculating neutral excitation energies, such as time-dependent density functional theory (TDDFT) and GW + Bethe-Salpeter, 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 open-shell Kohn-Sham [3]. We introduce a computationally light, generally applicable, first-principles approximation based on constrained DFT [4-5] for calculating neutral excitations, and our implementation in the linear-scaling 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: Electron-Defect Scattering from First-Principles Calculations I-Te Lu, Jin-Jian Zhou, Luis Agapito, Marco Bernardi Materials contain defects that can significantly impact charge transport. While ab initio calculations have focused on electron-phonon scattering and the phonon-limited mobility, electron-defect (e-d) 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 e-d scattering rate due to neutral defects. The formalism relies on 1st-order perturbation theory, where the perturbation is the difference of the Kohn-Sham potentials between a pristine material and the material with a defect. We discuss numerical treatments of the local and non-local parts of this perturbation potential, and effective computation of the associated e-d 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 e-d 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 defect-limited carrier mobility at low temperature. |
Tuesday, March 6, 2018 12:51PM - 1:03PM |
F29.00007: Non-Radiative Recombination at Dynamic Shallow Level Junhyeok Bang, Sheng Meng, Shengbai Zhang To date, the Shockley-Read-Hall (SRH) theory is the ruling theory explaining the defect-mediated non-radiative recombination (NRR) of excited carriers. However, recent first-principles 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 State-of-the-art experimental techniques allow the stimulation of excited electronic dynamics on an ultra-fast 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 non-radiative nature. As opposed to radiative decay, the nonradiative mechanisms take place on a much faster timescale, i.e. femto- to atto-seconds. 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 time-dependent density functional theory approaches. Within our method we solve the Kadanoff-Baym equations (KBE) in the nonequilibrium Green's function framework by using the generalized Kadanoff-Baym ansatz (GKBA), i.e. recasting the KBE into a computationally more convenient closed equation for the one-particle density matrix. As an illustration we simulate the emission of Auger electrons in real time in one-dimensional atomic systems. This paves the way towards the description of time-dependent Auger decay in realistic systems. |
Tuesday, March 6, 2018 1:15PM - 1:27PM |
F29.00009: Auger Recombination From First-principles in Group-III Nitride Alloys Andrew McAllister, Dylan Bayerl, Christina Jones, Emmanouil Kioupakis The group-III 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 non-radiative Auger recombination. Experimental measurements of Auger and other nonradiative processes are difficult, making first-principles atomisc simulations vital to gaining a better understanding of how carriers recombine. We use density functional theory and many-body perturbation theory to study Auger and radiative rates in group-III 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 Oxygen-rich 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 Oxygen-rich Strontium Niobate Oxides (SrNbO3+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: Time-resolved X-ray Spectroscopy in One-dimensional Strongly Correlated Systems Chen-Yen Lai, Jian-Xin Zhu In recent years, ultrafast pump-probe spectroscopy has provided insightful information about non-equilibrium dynamics of excitations in materials. In a typical experiment of time-resolved x-ray absorption spectroscopy, the systems are excited by a femtosecond laser pulse (pump pulse) following by an x-ray (probe pulse) after a time delay to measure the absorption spectra of the excited systems. In this talk, we present a theory for time-resolved x-ray absorption spectroscopy in one-dimensional strongly correlated systems. We consider a one-dimensional Hubbard model, and use the time-dependent density matrix renormalization group method to solve the ground state and non-equilibrium dynamics numerically. The core hole created by x-ray 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 x-ray absorption spectroscopy to the strength of Hubbard repulsion, and frequency and intensity of laser pump pulse, has also been studied. LA-UR-17-29749 |
Tuesday, March 6, 2018 1:51PM - 2:03PM |
F29.00012: Tackling Quantum Many-Body Problems in X-Ray Spectra via a Basic Graph Algorithm Yufeng Liang, David Prendergast The growth in access to detailed materials characterization using X-ray spectroscopy highlights the need for more accurate electronic-structure theory predictions of X-ray absorption near-edge fine structure. |
Tuesday, March 6, 2018 2:03PM - 2:15PM |
F29.00013: First Principles Simulations of ε-Ge/InxAl1-xAs Interfaces: Band Alignments and Interface Structure Gabriel Greene-Diniz, Myrta Gruening Tensile strained Ge (ε-Ge) grown on III-V 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 on-chip optical interconnects based on III-V/ε-Ge/III-V quantum well light emitters. In this work, we investigate the sensitivity of quasiparticle band offsets to interface structure for the ε-Ge/(001)InxAl1-xAs 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 ε-Ge on (001)In0.25Al0.75As substrates are explained in terms of the of diffusion of substrate species into the Ge film. |
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