Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U23: Semiconductors: Theory and Spectra I |
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Sponsoring Units: FIAP Room: 325 |
Thursday, March 21, 2013 11:15AM - 11:27AM |
U23.00001: Atomic Multiplets in X-ray Spectroscopies of Solids Bernard Delley, Anne-Christine Uldry The electronic structures of compounds involving open d- and f- shell are studied frequently by X-ray and electron spectroscopies. For a better understanding of the multiplets arising in spectra involving one or more open shells, we have developed recently an easy to use program multiX,\footnote{Systematic computation of crystal field multiplets for x-ray core spectroscopies, A. Uldry, F. Vernay and B. Delley, Phys. Rev. B 85, 125133 (2012).} which is available to download.\footnote{http://people.web.psi.ch/uldry/multiplets/} This first step allows the inclusion of the crystal environment as a crystal field entered simply as positions and charges of a cluster of atoms around the core hole site. This often gives valuable insights in the case of x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray spectroscopy (RIXS) measurements. However, in many cases it is desirable to allow for hybridization of the open shell electrons with the orbitals of neighbor atoms. This requires dealing with a significantly larger active Hilbert space. This is addressed with our recent Lanczos-based procedure to calculate spectra. First results will be discussed. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U23.00002: The Physical Content of Eigenvalues from Density Functional Theory (DFT) D. Bagayoko, L. Franklin, C. Ekuma, Y. Malozovsky The density functional theory (DFT) of Hohenberg and Kohn rests on the energy functional E$_{\mathrm{v}}$[n] assuming its minimum for the \textit{correct density} n(\textbf{r}), with the admissible functions restricted by the condition$N\left[ n \right]=\int {n\left( r \right)} dr=N$, where N is the number of particles in the system under study. We show that, for such a system, there is an infinite number of basis sets (of localized orbitals) for which N is fixed while the density is not necessarily the correct one. Consequently, the eigenvalues obtained with self consistent DFT calculations using a single basis set do not necessarily have any particular physical content. The physical content is ensured only by the search and utilization of \textit{the optimal basis set} that yields \textit{the minima of the occupied energies} and physically meaningful values of low laying unoccupied energies. Further, by virtue of the Rayleigh theorem, there exist many basis sets larger than the optimal one [and that contain it] for which some unoccupied energies are lowered on account of a mathematical artifact. We illustrate these points in the cases of ZnO, TiO$_{2}$, and SrTiO$_{3}$. The calculated band gaps and other properties of these materials are in excellent agreement with experiment. Work funded by in part by the National Science Foundation, through LASiGMA [NSF AwardEPS-1003897, No. NSF (2010-15)-RII-SUBR, and No. HRD-1002541], LONI [Award No. 2-10915], and the Louisiana Space Consortium (LaSPACE). [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U23.00003: Beyond the \textit{GW} approximation: a second-order screened exchange correction Patrick Rinke, Fabio Caruso, Xinguo Ren, Matthias Scheffler, Noa Marom Despite the success of the $GW$ method in describing the photoemission spectra of solids, molecules and clusters, challenges remain. For aromatic molecules for example absolute as well as relative positions of ionisation energies and affinities are not well reproduced in perturbative $G_0W_0$ schemes with different starting points as well as in self-consistent $GW$ [1], sometimes even giving the wrong orbital order. Motivated by renormalized second-order perturbation theory [2] for the ground-state energy, we propose a second-order screened exchange correction (SOSEX) to the $GW$ self-energy. This correction follows the spirit of the SOSEX correction to the random-phase approximation for the electron correlation energy and reduces the self-correlation error. The performance of the $GW$+SOSEX scheme has been benchmarked for a set of molecular systems, including the G2 set, commonly used acceptor molecules, benzene and the azabenzene molecules. We find that the SOSEX correction improves the description of the spectral properties including the orbital order with respect to the different $GW$ schemes, highlighting the importance of reducing the self-correlation error.\\[4pt] [1] N. Marom {\it et al.}, arXiv:1211.0416\\[0pt] [2] X. Ren {\it et al.}, J. Mater. Sci. \textbf{47}, 7447 (2012) [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U23.00004: GW calculations of the bandgap of pyrite under various conditions Brian Kolb, Alexie Kolpak Iron pyrite holds great promise as a solar cell material because of it's near optimal bandgap (0.95 eV) and its high optical absorbance. Nevertheless, real solar cells made from this material suffer from poor performance. In particular, the low open circuit voltage of around 200 meV precludes pyrite's use in effective solar cell devices. Several theories have been proposed to explain this low open-circuit voltage including bulk defects, intrinsic surface states within the gap, and surface defects. Careful DFT calculations have shown that bulk defects are exceedingly rare. Further, the calculations do not exhibit intrinsic surface states within the gap. Researchers disagree about the effect of surface defects, particularly sulfur deficiencies, on the bandgap. This work combines DFT with GW calculations of the bandgap to address some of the most fundamental and important questions about the cause of the low open-circuit voltage of pyrite solar cells including the true role of surface defects, the nature of the interface with metal electrodes, and the effect of phonons on the bandgap. This investigation is undertaken with an eye toward engineering a pyrite-based material that can perform well in real solar cell applications. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U23.00005: $GW$ at the interface: CH$_3$OH and H$_2$O on TiO$_2$(110) Duncan Mowbray, Annapaola Migani, Amilcare Iacomino, Jin Zhao, Hrvoje Petek, Angel Rubio Electronic level alignment at the interface between an adsorbed molecular layer and a semiconducting substrate determines the activity and efficiency of many photocatalytic and photovoltaic materials. However, a quantitative description of the states at the interface remains elusive, due to the computational complexity of quasiparticle $GW$ based algorithms. We compare density functional theory (DFT) calculations and quasiparticle techniques with ultraviolet photoelectron spectra and two photon photoemission spectra to determine the level of theory required to obtain an accurate description of occupied and unoccupied states at the interface. Specifically, we consider GGA DFT, hybrid DFT and $G_0W_0$, $scGW1$, $scGW_0$, and $scGW$ quasiparticle calculations for the interface between rutile TiO$_2$(110) and methanol or water. We find the quasiparticle energy shifts $\Delta$ are linearly dependent on the fraction of the wave function density within the molecular layer $f_{mol}$ and the bulk substrate $f_{bulk}$. For the unoccupied states, the same correlation holds for all the molecular layers studied. This allows one to describe the quasiparticle energy shifts semi-quantitatively for larger molecular layers on TiO$_2$(110) based on more tractable DFT calculations. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U23.00006: \textit{Ab-initio} Calculations of Electronic Properties of InP and GaP Yuriy Malozovsky, Lashounda Franklin, Chinedu Ekuma, Guang-Lin Zhao, Diola Bagayoko We present results from \textit{ab-initio}, self consistent local density approximation (LDA) calculations of electronic and related properties of zinc blende indium and gallium phosphides (InP {\&} GaP) We employed a local density approximation (LDA) potential and implemented the linear combination of atomic orbitals (LCAO) formalism. This implementation followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). This method searches for the optimal basis set that yields the minima of the occupied energies. This search entails methodically increasing the size of the basis set, up to the optimal one, and the accompanying enrichment of angular symmetry and of radial orbitals. Our calculated, direct band gap of 1.398 eV (1.40 eV) for InP, at the $\Gamma $ point, is in excellent agreement with experimental values. We discuss our preliminary results for the indirect band gap, from $\Gamma $ to X, of GaP. We also report calculated electron and hole effective masses for both InP and GaP and the total (DOS) and partial (pDOS) densities of states. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U23.00007: Computational study of the Effect of Sulfur Passivation on GaAs Heterojunction Solar Cells Ted Yu, Ramesh Laghuvamarapu, Liang Yan, Wei You, Diana Huffaker, Christian Ratsch We report DFT calculations that study the effect of sulfur passivation ((NH$_{4})_{2}$S and octanethiol) on GaAs surfaces. Sulfur passivation of GaAs solar cells is an area of interest, as it improves the I-V characteristics of heterojunctions by decreasing the density of surface states. We elucidate the fundamental mechanism of sulfur passivation on GaAs by showing how the sulfur species react with different reconstructed GaAs (100) and (111B) surfaces. Using state of the art hybrid functionals to calculate band structures and density of states, we find that a reconstructed GaAs surface does not have mid-gap surface states. Therefore, we show that sulfur passivation does not reduce surface states on reconstructed surfaces. We also study arsenic vacancies and adatoms on these surfaces to determine the energies of creating these imperfections. They lead to mid-gap surface states that are shown to be energetically plausible in certain GaAs surface reconstruction. We study the most energetically favorable surface reconstructions with As vacancies and show how sulfur passivation plays a role in removing surface states. These results will guide in the selection of passivating agents for GaAs solar cells and lead to a better understanding of such systems. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U23.00008: CsSnX3 (X= Cl, Br, I) band structure calculations by the QSGW method Ling-yi Huang, Walter R.L. Lambrecht CsSnX$_3$ (X=Cl,Br,I) perovskite compounds are of interest because of their strong photoluminescence and their potential application to solar cells. We present quasiparticle self-consistent GW (QSGW) calculations for the cubic ($\alpha$-phase) including spin-orbit coupling and study the changes in band structures from the $\alpha$-phase to the $\beta$- and $\gamma$-phases in LDA. The QSGW gaps are in good agreement with experiment. An analysis of the orbital character of the bands shows that they have an ``inverted'' band structure: the VBM has a non-degenerate s-like character (Sn-s and X-p antibonding), while the (CBM) has Sn-p character. The strongly intra-atomic dipole allowed nature of the direct gap explains the high photoluminescent intensity. The low hole mass indicates high hole mobility in agreement with experiment. The pressure dependence of the gap is found to be anomalous: the band gap decreases when the lattice constant is decreased. Effective masses and the Kohn-Luttinger type Hamiltonian of the CBM are extracted from the band structures and subsequently used to estimate exciton binding energies using our calculated dielectric constants. These indicate a much lower exciton binding energy for CsSnI3 than recently proposed. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U23.00009: Quasiparticle band structures and interface physics of SnS and GeS Brad Malone, Efthimios Kaxiras Orthorhombic SnS and GeS are layered materials made of earth-abundant elements which have the potential to play a useful role in the massive scale up of renewable power necessary by 2050 to avoid unmanageable levels of climate change. We report on first principles calculations of the quasiparticle spectra of these two materials, predicting the type and magnitude of the fundamental band gap, a quantity which shows a strong degree of scatter in the experimental literature. Additionally, in order to evaluate the possible role of GeS as an electron-blocking layer in a SnS-based photovoltaic device, we investigate the band offsets of the interfaces between these materials along the three principle crystallographic directions. We find that while the valence-band offsets are similar along the three principle directions, the conduction-band offsets display a substantial amount of anisotropy. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U23.00010: Ab Initio Study of Quasiparticle and Excitonic Properties of MoS2 Diana Qiu, Felipe Jornada, Steven Louie MoS2 is a layered, transition-metal dichalcogenide that can be cleaved into single-layer sheets, in a manner similar to graphene. Monolayer MoS2 has a direct band gap, strong spin-orbit coupling and strongly enhanced photoluminescence, compared with the bulk. MoS2's interesting electronic and optical properties mean that it could have many applications in single-layer electronic devices, but on the theoretical level, when many-electron interaction effects are included, there is still some uncertainty about the quasiparticle and excitonic properties of MoS2. We use first-principles calculations to study the quasiparticle band structure and optical absorption spectrum of MoS2 at the GW$+$BSE level. We include spin-orbit coupling as a perturbation either before or after the GW calculation of the band structure, and we demonstrate that our calculations are fully converged with respect to the dielectric cutoff and summation over empty bands. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NERSC. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U23.00011: Strain Modulation on electric-optical properties of Graphene and ZnO micro/nanowires Xuewen Fu, Zhimin Liao, Hanchun Wu, Yang-Bo Zhou, Jun Xu, Wanlin Guo, Dapeng Yu Strain increasingly prevails in micro- and nano-structures, and has important influence on the crystal and electronic structures. But its role in these structures remains unclear. The strain dependence of conductance of monolayer graphene has been studied here. The results illustrate the notable transitions: the slight increase, the dramatic decrease, and the sudden dropping of the conductance by gradually increasing the uniaxial strain. The graphene conductance behaves reversibly by tuning of the elastic tensile strain up to 4.5\%, while it fails to recover after the plastic deformation at 5\%. We also investigated the bending strain effect on the photoresponse of ZnO micro/nanowires and found larger photoconductivity and faster rising speed when photo-excitation is localized at the bending region in atmospheric environment, while the rising speeds are almost the same when photo-excitations are localized at the bending and straight regions under vacuum. The bending strain induced improvement of the UV photoresponse in air was well explained by the coupling of piezoelectric effect and surface oxygen adsorption/desorption procedure on the bent ZnO microwire.\\[0pt] [1] Xue-Wen Fu, Zhi-Min Liao, Jian-Xin Zhou, Yang-Bo Zhou, Han-Chun Wu, Appl. Phys. Lett. 99, 213107 (2011). [2] Xue-Wen Fu, Zhi-Min Liao, Jun Xu, Xiao-Song Wu, Wanlin Guo and Da-Peng Yu, Nanoscale, 2013, 5, 916–920. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U23.00012: Electronic Structure of N-doped TiO James Lewis, Barry Haycock, Gary Lander, Binay Prasai, David Drabold Via \textit{ab-initio}density functional theory calculations, we present evidence of the most energetically stable atomic configuration for nitrogen-doped amorphous TiOand analysis of the electronic structure. This material receives much attention in the literature due to it's proposed photocatalytic applications, however synthesis of the crystalline form is an unfavorable process. Nitrogen doping has previously been shown to enable absorption in the visible in crystalline TiO2. As compared to crystalline TiO2, thin films of a- TiOdo not need thermal treatment and have other production advantages such as less dependence on substrate materials. With control of the electronic structure of the amorphous phase via doping the electronic characteristics can be taken advantage of without the costly production of crystalline TiO. N-doping of the amorphous phase introduces tail states to the valance band. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U23.00013: Majorana fermions in vortex lattices Rudro Biswas We consider Majorana fermions tunneling between vortices, within an array of such vortices in a 2D chiral p-wave superconductor. We calculate that the tunneling amplitude for Majorana fermions in a pair of vortices is proportional to the sine of half the difference between the global order parameter phases at the two vortices. Using this result we study tight-binding models of Majorana fermions in vortices arranged in a triangular or square lattice. In both cases we find that this phase-tunneling relationship leads to the creation of superlattices where the Majorana fermions form macroscopically degenerate `flat' bands at zero energy, in addition to other dispersive bands. This finding suggests that in vortex arrays tunneling processes do not change the energies of a finite fraction of Majorana fermions and hence brighten the prospects of topological quantum computing with a large number of Majorana states. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U23.00014: Ultrafast intersystem crossing in nickel porphyrins Javier Fernandez-Rodriguez, Jun Chang, A.J. Fedro, Michel van Veenendaal We study the relaxation dynamics to the metastable state in laser-pumped nickel porphyrins. We use a ligand-field model which takes into account the crystal field created by the porphyrin ring and axial ligands. We propose different decay pathways in terms of charge-transfer and metal-center intermediate states By accounting for the energy redistribution of the lattice vibrations we get an irreversible decay to the metastable state within the order of a few hundred femtoseconds. We show how non-equilibrium time-dependent x-ray absorption at the Ni K-edge measurements can elucidate the nature of the intermediate states involved in the decay. Understanding radiationless transitions in transition-metal complexes is of interest for their relevance for the design of photocatalytic systems and photothermal sensitizers for cancer treatment. [Preview Abstract] |
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