Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A15: Electronic Structure: Theory and Spectra I |
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Sponsoring Units: FIAP Chair: Barbara Jones, IBM Almaden Research Center Room: 008B |
Monday, March 2, 2015 8:00AM - 8:12AM |
A15.00001: First-principles interpretation of attosecond time-resolved XUV absorption spectroscopy of laser excited Silicon Sri Chaitanya Das Pemmaraju, Shunsuke Sato, Kazuhiro Yabana, Krupa Ramasesha, Martin Schultze, Stephen R. Leone, David Prendergast The availability of ultrafast x-ray pulses both from powerful free electron laser light-sources as well as table top high-harmonic generation, has significantly enhanced the utility of core-level spectroscopies as probes for investigating dynamical processes in functional materials. Theoretical approaches to complement these time-domain experiments are therefore actively being developed. In this study we employ a combination of real-time time-dependent density functional theory (TDDFT), occupancy-constrained density functional theory and many-body perturbation theory approaches to help interpret spectral signatures observed in attosecond time-resolved core-level spectroscopic measurements on laser-excited silicon. Using non-equilibrium electron-hole densities obtained from real-time TDDFT simulations of the valence electronic structure we estimate the transient modulation of L-edge absorption in femtosecond infrared pump - attosecond XUV probe experiments. We further estimate the contribution of electron-phonon and electron-electron scattering mechanisms to the lifetime broadening observed in measured L-edge spectra using occupation-constrained density functional theory and GW calculations respectively. [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A15.00002: First-principles calculation of electronic structure and optical absorption of BN ZnO Xiao Zhang, Andre Schleife The $\alpha$-BN structure of ZnO, a nonequilibrium phase with a transition pressure of 25 GPa, has been found in nano structures of ZnO. The structural difference between the BN structure and the equilibrium wurtzite structure can play an important role for applications of nanostructured ZnO. In order to understand the difference, first principles calculations have been performed on both phases. The electronic structure is computed using the $GW$ method based on Density Functional Theory and HSE hybrid functional calculations. The $GW$ method includes the quasiparticle effects due to the screened electron-electron interaction which gives an accurate description of the electronic band structure and density of states. After that, by solving the Bethe-Salpeter Equation for the optical polarization function, which take excitonic effects into account, we have achieved an accurate description of optical absorption spectra for both structures. We find a good agreement with experimental and previous computational results for WZ structure, and predict the absorption for the BN structure. The BN structure shows a larger band gap and we found a very large optical anisotropy: The gap for extraordinary light polarization is almost 0.7eV larger than that for ordinary light polarization. [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A15.00003: Wannier function analysis of charge states in transition metal oxides Yundi Quan, Warren Pickett The charge (or oxidation) state of a cation has been a crucial concept in analyzing the electronic and magnetic properties of oxides as well as interpreting ``charge ordering'' metal-insulator transitions. In recent years a few methods have been proposed for the objective identification of charge states, beyond the conventional (and occasionally subjective) use of projected densities of states, weighted band structures (fatbands), and Born effective charges. In the past two decades Wannier functions (WFs) and particularly maximally localized WFs (MLWFs), have become an indispensable tool for several different purposes in electronic structure studies. These developments have motivated us to explore the charge state picture from the perspective of MLWFs. We will illustrate with a few transition metal oxide examples such as AgO and YNiO3 that the shape, extent, and location of the charge centers of the MLWFs provide insights into how cation-oxygen hybridization determines chemical bonding, charge distribution, and ``charge ordering.'' [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A15.00004: DFT+\textit{U} invstigation of doped-PbPdO$_2$ spin gapless semiconductors Han Hsu, Sheng-Chieh Huang Spin gapless semiconductors (SGSs), with a zero gap in one spin channel and a finite gap in the other, have attracted considerable attention due to its potential in spintronics. Ever since PbPdO$_2$ was predicted a gapless semiconductor by local density approximation (LDA) calculations and confirmed by experiments afterward, it is anticipated to be a backbone material for SGS. Using density functional theory + self-consistent Hubbard \textit{U} (DFT+\textit{U}$_{sc}$) calculations, we investigate doped PbPdO$_2$ with different kinds of transition-metal dopants, including V, Cr, Mn, Fe, Co, and Ni, at different dopant concentration. We show that doped PbPdO$_2$ can be a SGS with a proper choice of dopant and dopant concentration. Furthermore, different dopant and dopant concentration can lead to different types of SGS. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A15.00005: Final-state effect on X-ray photoelectron spectrum of n-doped SrTiO$_3$ Chungwei Lin, Agham Posadas, Alexander Demkov X-ray photoelectron spectroscopy (XPS) is a widely used technique to determine the oxidation states of chemical elements. In stoichiometric SrTiO$_3$, the Ti$^{4+}$ peak appears at a binding energy of about 459.0 eV for photoelectrons ejected from the Ti $2p$ core level. In lightly n-doped SrTiO$_3$, a weak shoulder at a binding energy of about 1.5 eV lower than the Ti$^{4+}$ peak appears in the XPS spectrum that has been conventionally interpreted as a Ti$^{3+}$ signal. By taking the final-state effect into account, i.e. by considering the response of the valence electrons in the presence of a core hole, we argue that such a Ti$^{3+}$ peak does not necessarily imply the existence of spatially localized Ti$^{3+}$ ions, and explicitly show that a spatially uniform Ti$^{(4-x)+}$ distribution also leads to the multi-peak structure. Spectra from metallic n-doped SrTiO$_3$ (e.g. La replacing Sr, Nb replacing Ti, or even oxygen vacancy doping) should be interpreted as the latter case. Several experiments based on this interpretation are discussed. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A15.00006: Test set for materials science and engineering Toktam Morshedloo, Norina A. Richter, Fawzi Mohamed, Xinguo Ren, Sergey V. Levchenko, Luca M. Ghiringhelli, Igor Ying Zhang, Matthias Scheffler Understanding of the applicability and limitations of electronic-structure methods needs detailed comparison with highly accurate data of representative test sets. A variety of highly valuable test sets have been established in quantum chemistry for small molecules. However, for crystalline solids they are still lacking. We present a representative test set for materials science and engineering (MSE) which includes first and second row elements and their binaries, comprising various crystal structures. This allows for unbiased benchmarking for various chemical interactions. In the MSE test set, we consider cohesive energy, lattice constant, bulk modulus, electronic, band structures and phonons etc. A big effort is made to produce systematically converged results with respect to basis set[1] and \textbf{k} mesh for a hierarchy of electronic-structure methods, ranging from the local-density approximation to advanced orbital-dependent functionals implemented in the all-electron, full-potential FHI-aims code. Furthermore, we use incremental schemes to obtain benchmark values calculated with coupled-cluster approaches.\\[4pt] [1] I.Y. Zhang {\it et al.}, \emph{NJP} \textbf{15} 123033 (2013) [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:24AM |
A15.00007: Accurate Electronic, Transport, and Structural Properties of Disodium Sulfide (Na$_{2}$S) Walter Lynn, Bethuel Khamala, Yuriy Malozovsky, Diola Bagayoko We present results of \textit{ab-initio}, self consistent calculations of electronic, transport, and structural properties of cubic antifluorite disodium sulfide (Na$_{2}$S). We used a local density approximation (LDA) potential and the linear combination of Gaussian orbitals (LCGO) formalism. We followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF), in our implementation of the LCGO. We discuss electronic energy bands, total (DOS) and partial (pDOS) densities of states, effective masses that are pertinent to transport properties, and the bulk modulus. For a room temperature lattice constant of 6.539 {\AA}, our calculated, direct band gap of the material, at $\Gamma $, is 2.83 eV. We predict a direct band gap of 3.05 eV for the calculated equilibrium lattice constant of 6.395 {\AA}. The significant decrease of the lattice constant, for a drop in temperature from 300 to 0 K, is understandable with the relatively small, predicted bulk modulus of 43.9 GPa. Acknowledgments: This work was funded in part by the NSF and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy -- NNSA (Award Nos. DE-NA0001861 and DE- NA0002630), LaSPACE, and LONI-SUBR. [Preview Abstract] |
Monday, March 2, 2015 9:24AM - 9:36AM |
A15.00008: Electronic structure, spin--orbit coupling, and interlayer interaction in bulk MoS$_2$ and WS$_2$ Drew Latzke, Wentao Zhang, Sefaattin Tongay, Tay-Rong Chang, Hsin Lin, Horng-Tay Jeng, Aslihan Suslu, Junqiao Wu, Arun Bansil, Alessandra Lanzara Transition metal dichalcogenides (TMDs) (MX$_2$ where M = Mo or W and X = S, Se, or Te) are theorized to possess unique spin-split valence bands along with rare spin--valley coupling, making them attractive for applications within the growing fields of spintronics and valleytronics. Despite the importance of the split valence band that governs the unique spin- and valley-physics of TMDs, there remain many questions regarding its origin and properties in bulk TMDs. In this talk, I will present high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements of the electronic band structure of bulk TMDs MoS$_2$ and WS$_2$. Detailed comparison with first principle calculations will be shown. The role of the valence band splitting and how it can be controlled will be discussed. [Preview Abstract] |
Monday, March 2, 2015 9:36AM - 9:48AM |
A15.00009: Barium disilicides (BaSi$_{2})$ a low-cost, earth-abundant material for thin-film solar cells Mukesh Kumar, Naoto Umezawa, Motoharu Imai In order to meet the clean energy requirement, materials consisting of abundant, eco-friendly, and low-cost elements are of great interest. Therefore in this study, we discussed the importance of BaSi$_{2}$ and other similar semiconducting compounds which contain inexpensive and earth abundant elements, for solar cell applications. Employing first-principles modeling within the density function theory, we analyze the structural, electronic and optical properties and find that these compounds have fundamental indirect band gaps and the gap energies are in the region of 0.9--1.3 eV, which is suitable for solar cell applications. Furthermore, a lower energy dispersion of the conduction band (CB), which results in a flat shape of the CB minimum, implies a large optical absorption. In fact, our calculations reveal that the photoabsorption of these compounds is stronger than other common PV materials like Si and Cu(Ga,In)Se$_{2}$. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A15.00010: Quantifying defect levels in hexagonal boron nitride from simulated x-ray absorption spectroscopy Sebastiaan Huber, Robbert van de Kruijs, Fred Bijkerk, Eric Gullikson, David Prendergast X-ray Absorption Spectroscopy is a technique that is highly sensitive to the local structural and chemical environment of the probed species. This quality can be exploited to investigate the characteristics of localized structural imperfections such as point defects and grain boundaries. In this study the X-ray absorption spectra for various hexagonal ($h$-BN) and amorphous boron nitride ($a$-BN) samples have been recorded. Simulation of X-ray absorption spectra from first principles allows for characteristic features in the measured spectra to be attributed to local defects of the planar hexagonal structure of the material. Analysis of the relative intensities of these features can subsequently provide a quantitative analysis of the levels of various defects in the analyzed structure. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A15.00011: Accurate, Electronic and Transport Properties of Wurtzite Aluminum Nitride (w-AlN) Ifeanyi Nwigboji, Yuriy Malozovsky, Lashounda Franklin, Chinedu Ekuma, Diola Bagayoko We present results from \textit{ab-initio}, self-consistent calculations of electronic and transport properties of wurtzite aluminum nitride (w-AlN). We utilized a local density approximation (LDA) potential, the linear combination of Gaussian orbitals (LCGO), and the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). With multiple oxidation states of Al and N, the method led to several sets of calculations with different ionic species as input. LDA requires, for the description of w-AlN, the results of the calculation leading to the lowest, occupied energies. With Al$^{3+}$ and N$^{3-}$ as input, the binding energy was 1.5 eV larger, in magnitude, than those for other ionic inputs; hence, the description of w-AlN is provided by a calculation with these ionic species as input. Our calculated, direct band gap for w-AlN, at the $\Gamma $ point, is 6.28 eV, in excellent agreement with the 6.28 eV experimental value at 5 K. We discuss the bands, total and partial densities of states, and calculated electron and hole effective masses. Funded in part by the NSF and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy -- NNSA (Award No. DE-NA0001861), LaSPACE, and LONI-SUBR. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A15.00012: Electronic properties of perovskite absorbers for solar cell applications Marina Filip, Feliciano Giustino Metal halide perovskite absorbers have captured the attention of the photovoltaics research community in the past 3 years, reaching efficiencies over 19\%. Despite this unprecedented progress, the remarkable physical properties of these materials are not yet fully understood. In this work we show an exhaustive computational study of CH$_3$NH$_3$PbI$_3$ within density functional theory and the GW approximation. We show the effect of semicore states and spin-orbit coupling on the quasiparticle band gap of CH$_3$NH$_3$PbI$_3$ and describe a straightforward ``self-consistent scissor'' method to correct the underestimated dielectric screening in the G0W0 approach [1]. Finally, we model the interplay between the structural and electronic properties of lead-iodide perovskites and propose novel lead-iodide peroskite absorbers with different cations at the center of the cuboctahedral cavity facilitating the tunning of the fundamental band gap [2].\\[4pt] [1] Filip, M. R. \& Giustino, F., http://arxiv.org/abs/1410.2029 (2014)\\[0pt] [2] Filip, M. R., Eperon, G., Snaith, H. J. \& Giustino, F., http://arxiv.org/abs/1409.6478 (2014) [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A15.00013: Chemical trends in halide perovskite electronic properties Walter Lambrecht, Ling-yi Huang The halide perovskites ABX$_3$ with $B=$Pb or Sn, $X$=I, and $A$=Cs or methylammionium (MA), have recently attracted attention as solar cell materials. We discuss the basic bonding, stability and electronic band structure of these materials for different chemical substitutions using first-principles calculations. An important feature of the Pb and Sn based halides is that these element's $s$-electrons strongly hybridize with the halogen $p$-orbitals leading to a valence band maximum with strong Sn or Pb-$s$ character and small effective mass. The conduction band minimum is Sn or Pb $p$-like. We present trends in the electronic band structure with the halogen $X=$I, Br, Cl and the $B$ cation Pb, Sn, Ge, Si. The gap is remarkably insensitive because of the opposing trends of the increased spin-orbit coupling for heavier elements (reducing the gap) and the decreased valence band width for heaver elements due to the larger B-X distance, which increases the gap. The stability of the perovskite structure {\sl vs.} competing structures is influenced by the tolerance factor $t=R_{AC}/\sqrt{2}R_{BC}$. The smaller this factor, the least stable is the perovskite structure. CsSiI$_3$ is found to be a topological insulator. Its stability with respect to CsI and SiI$_n$ is discussed. [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A15.00014: Electronic structure studies of MX$_{2} $(M$=$Mo, W; X$=$S, Se) by ARPES and the band evolution of hydrogen gas exposed MoS$_{2}$ Beom Seo Kim, Soohyun Cho, Beomyoung Kim, Jonathan Denlinger, Byung Hoon Kim, Seung Ryong Park, Changyoung Kim The physics associated with transition metal dichalcogenides (TMDCs) is one of the most intriguing issues in condensed matter physics. These materials have several interesting aspects inter physical properties, especially the direct to indirect band gap transition and spin-orbit coupling induced spin band splitting at the K-point. However, thorough systematic studies on the electronic structures of TMDCs regarding those issues have not been done. We present the electronic structure studies of MX$_{2}$ using angle resolved photoemission spectroscopy(ARPES). We investigated the indirect band gap and spin-band splitting sizes of MX$_{2}$. In addition, we present ARPES results from MoS$_{2}+$H$_{2}$(10h), performed to investigate the evolution of the band structure as a function of hydrogen exposure (from MoS$_{2}$ to MoS$_{2}+$H$_{2}$(10h)). Photon energy dependence and potassium dosing were performed for each system. The results from MX$_{2}$ are quite consistent with the published band calculation results. In the MoS$_{2}+$H$_{2}$ (10h) case, there are two interesting observations from the results. The first is that the indirect band gap size decreases upon exposure to hydrogen. The other observation is that the $\Gamma $ band appears to have quantum confinement effect. [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A15.00015: Discrete energy bands in bulk semiconductors Maohua Du, Hongliang Shi Bulk semiconductors typically have continuous valence and conduction bands. Discrete energy levels and bands have been sought after for various applications. For instance, discrete energy levels existing in semiconductor nanocrystals, or quantum does (QDs) have been proposed as a mechanism to suppress hot carrier thermalization and to enhance carrier multiplication in QD solar cells. Impurity bands in the band gap have been introduced for intermediate-band solar cells and for efficient visible light absorption and photocatalysis. In this talk, we show by first principles calculations that, in a multinary compound, a combination of large electronegativity difference between different cations (anions) and large nearest-neighbor distances in cation (anion) sublattices can lead to the splitting of the conduction (valence) band, resulting in several discrete and narrow energy bands separated by large energy gaps. We also discuss applications that may benefit from such electronic structure. [Preview Abstract] |
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