Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B15: Electronic Structure: Theory and Spectra II |
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Sponsoring Units: FIAP Chair: David Strubbe, Massachusetts Institute of Technology Room: 008B |
Monday, March 2, 2015 11:15AM - 11:27AM |
B15.00001: Dynamical and anharmonic effects on the electron-phonon coupling and the zero-point renormalization of the band structure Gabriel Antonius, Samuel Ponc\'e, \'Etienne Lantagne-Hurtubise, Gabriel Auclair, Michel C\^ot\'e, Xavier Gonze The electron-phonon coupling in solids renormalizes the band structure, reducing the band gap by several tenths of an eV in light-atoms semiconductors. Using the Allen-Heine-Cardona theory (AHC), we compute the zero-point renormalization (ZPR) as well as the quasiparticle lifetimes of the full band structure in diamond, BN, LiF and MgO. We show how dynamical effects can be included in the AHC theory, and still allow for the use of a Sternheimer equation to avoid the summation over unoccupied bands. The convergence properties of the electron-phonon coupling self-energy with respect to the Brillouin zone sampling prove to be strongly affected by dynamical effects. We complement our study with a frozen-phonon approach, which reproduces the static AHC theory, but also allows to probe the phonon wavefunctions at finite displacements and include anharmonic effects in the self-energy. We show that these high-order components tend to reduce the strongest electron-phonon coupling elements, which affects significantly the band gap ZPR. [Preview Abstract] |
Monday, March 2, 2015 11:27AM - 11:39AM |
B15.00002: Raman spectra calculations for nanostructures using \textit{ab initio} real-space methods N. Scott Bobbitt, James R. Chelikowsky We use a real-space pseudopotential method within density functional theory to calculate Raman spectra for Si nanocrystals. We examine the effects of quantum confinement and the presence of impurities, including dopant concentration and location of dopant within the nanocrystal. The ability to predict the effects of dopant concentration and location on a Raman spectrum from first principles suggests that this calculation technique could be coupled with spectroscopic experiments to identify the size and nature of doped nanocrystals. [Preview Abstract] |
Monday, March 2, 2015 11:39AM - 11:51AM |
B15.00003: The atomistic limit of envelope function theory Craig Pryor, Mats-Eric Pistol Electronic properties of semiconductor nanostructures and impurity states are typically calculated using one of three different methods: tight-binding models, pseudopotentials, or envelope function theory. The first two are well suited to modeling atomistic scale structures, however their parameters must be fit to bulk properties which can be a complicated procedure. In contrast, envelope function theory is best at describing larger scales in which the placement of individual atoms is not important and the parameters are directly related to experimentally determined quantities. As usually implemented, envelope function theory is insensitive to atomic scale structure. We show that this does not need to be the case, and construct an atomistic envelope function theory. This is advantageous for nanostructure modeling because it provides an atomistic model parameterized in terms of physical matrix elements rather than by complicated fitting procedures. [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B15.00004: Quantum Mechanics of Chemisorption on GaAs Clusters Frank Naranjo, Ajit Hira, Ruben Rivera, Oliver Oviedo This research focuses on the theoretical study of molecular clusters to examine the chemical properties of small Ga$_{n}$As$_{n}$ clusters (n $=$ 2 - 10). We study the chemisorption of different atomic and molecular species on small clusters of metallic elements, by examining the interactions of H, H$_{2,}$ Li and Be adsorbates with the GaAs clusters. Semiconductor clusters are of interest for the study of quantum size effects and for metallization phenomena, Hybrid ab initio methods of quantum chemistry (particularly the DFT-B3LYP model) are used to derive optimal geometries for the clusters of interest. We compare calculated binding energies, bond-lengths, ionization potentials, electron affinities and HOMO-LUMO gaps for these clusters. Mapping of the singlet, triplet, and quintet, potential energy surfaces is performed. Implications for fundamental mechanisms of atomistic assembly on the GaAs surfaces are examined. [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B15.00005: Stress effects on Raman spectroscopy of aSi:H -- theory and experiment David A. Strubbe, Eric C. Johlin, Timothy R. Kirkpatrick, Tonio Buonassisi, Jeffrey C. Grossman Raman microscopy has proven to be a very useful technique for inferring stress distributions in materials, since the positions of vibrational peaks are sensitive to local stress. This method has been applied extensively for crystalline silicon, and would be useful for amorphous silicon as well, particularly for studying local stress and composition of nanostructured amorphous/crystalline devices. Toward that goal, we have simulated the Raman spectrum of hydrogenated amorphous silicon with density-functional perturbation theory, using atomistic structures from the WWW algorithm with different stress states. We obtain a spectrum in good agreement with experimental results, and calculate a coefficient for the change in the TO peak position proportional to stress. We then performed Raman experiments on an a-Si:H film deposited on a c-Si wafer, in which stress was applied with a 4-point bending setup, and found a stress coefficient consistent with the simulations, in contrast to inconsistent previous results in the literature. These results can be used to map out local stress distributions, and also relate to thermal expansion and vibrational anharmonicity via the Gr\"uneisen parameter. [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:27PM |
B15.00006: Angle resolved photoemission studies of the Rashba states in ferroelectric GeTe Beomyoung Kim, Wonshik Kyung, Garam Han, Yeongkwan Kim, Jonathan Denlinger, In Chung, Changyoung Kim There has been significant increase in the research of spin orbit coupling (SOC) induced exotic phenomena. The Rashba effect, theoretically predicted to exist, is one of the SOC related phenomena. The phenomenon was later experimentally observed in the surface states of metals and topological insulators as well as interfaces of hetero structures that have inversion symmetry breaking (ISB). Even bulk states with intrinsic ISB such as BiTe is found to have Rashba split bands. It was very recently proposed that ferroelectric GeTe has Rashba effect in the bulk. This is a unique situation where ISB is provided not by the structure ISB but by an electrical polarization. We have performed angle-resolved photoemission spectroscopy (ARPES) on GeTe single crystals to investigate the unique bulk Rashba state. Our results indeed show the existence of a Rashba-type band splitting as theoretically predicted. We discuss various aspect of the Rashba state in GeTe. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 12:39PM |
B15.00007: Electronic structure of parabolically confined quantum wire with Rashba and Dresselhaus spin-orbit interactions under the influence of perpendicular magnetic field Sevil Sarikurt, Serpil Sakiroglu, Kadir Akgungor, Ismail Sokmen We have investigated the effect of spin-orbit (SO) coupling on the energy level spectrum and spin texturing of parabolically confined quantum wire that is subjected to an externally applied perpendicular magnetic field. Additionally we have also taken into account exchange-correlation contribution.Highly accurate numerical calculations have been carried out by finite element method. Our results have been revealed that the interplay of the SO coupling with effective magnetic field significantly modifies the band structure, producing additional subband extrema and energy gaps. Energy subband structure varies depending on which type of SO coupling strength is considered and also the magnitude of SO coupling. We also obtain that spatial modulation of spin density along the wire width can be considerably modified by SO coupling strength, magnetic field and charge carrier concentration. Besides, we have observed that the presence of exchange-correlation contribution leads to a softening behavior in the local maxima at subbands and shifts all energy subbands to lower energy values. Numerical results point out that the combined effect of exchange-correlation and SO coupling produces asymmetry in the dispersion relations. [Preview Abstract] |
Monday, March 2, 2015 12:39PM - 12:51PM |
B15.00008: Strongly Anisotropic Ballistic Magnetoresistance in Compact Three-Dimensional Semiconducting Nanoarchitectures Carmine Ortix, Ching-Hao Chang, Jeroen van den Brink In this talk, I will show that in non-magnetic semiconducting bilayer or multilayer thin film systems rolled-up into compact quasi-one-dimensional nanoarchitectures, the ballistic magnetoresistance is very anisotropic: conductances depend strongly on the direction of an externally applied magnetic field. This phenomenon originates from the curved open geometry of rolled-up nanotubes, which leads to a tunability of the number of one-dimensional magnetic subbands crossing the Fermi energy. The experimental significance of this phenomenon is illustrated by a sizable anisotropy that scales with the inverse of the number of windings, and persists up to a critical temperature that can be strongly enhanced by increasing the strength of the external magnetic field or the characteristic radius of curvature, and can reach room temperature. [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B15.00009: Electronic Structure Evolution of Fullerene on CH3NH3PbI3 Chenggong Wang, Congcong Wang, Xiaoliang Liu, Cheng Bi, Yuchuan Shao, Zhengguo Xiao, Jinsong Huang, Yongli Gao The thickness dependence of fullerene on CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$ perovskite film surface has been investigated by using ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS) and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) can be observed directly with IPES and UPS. It is observed that the HOMO level in fullerene shifts to lower binding energy. The XPS results show an initial shift of core levels to lower binding energy in the perovskite, which indicates that electrons transfer from the perovskite film to fullerene molecules. We observed that the WF of the perovskite is 5.0 eV and the VBM is 0.6 eV. The band gap of the perovskite is 1.66 eV, which is in accordance with previous reports. We also observed the HOMO level of C$_{\mathrm{60}}$ shifts to lower binding energy, indicating a band bending in the C$_{\mathrm{60}}$ region. The perovskite core levels show a strong initial shift to lower binding energy, indicating electron transfer from the perovskite film to fullerene molecules. The strongest electron transfer happened at 1/4 monolayer of fullerene, and further deposition reduced the transfer as forms fullerene forms C$_{\mathrm{60}}$ solid film. [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B15.00010: Auger recombination in scintillator materials from first principles Andrew McAllister, Emmanouil Kioupakis, Daniel \r{A}berg, Andr\'{e} Schleife Scintillators convert high energy radiation into lower energy photons which are easier to detect and analyze. One of the uses of these devices is identifying radioactive materials being transported across national borders. However, scintillating materials have a non-proportional light yield in response to incident radiation, which makes this task difficult. One possible cause of the non-proportional light yield is non-radiative Auger recombination. Auger recombination can occur in two ways - direct and phonon-assisted. We have studied both types of Auger recombination from first principles in the common scintillating material sodium iodide. Our results indicate that the phonon-assisted process, assisted primarily by short-range optical phonons, dominates the direct process. The corresponding Auger coefficients are $5.6 \pm 0.3\times10^{-32} \mbox{cm}^6\mbox{s}^{-1}$ for the phonon-assisted process versus $1.17 \pm 0.01 \times 10^{-33} \mbox{cm}^6\mbox{s}^{-1}$ for the direct process. At higher electronic temperatures the direct Auger recombination rate increases but remains lower than the phonon-assisted rate. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B15.00011: Pressure Dependent Electronic Properties of Organic Semiconductors from First Principles Franz Knuth, Christian Carbogno, Volker Blum, Matthias Scheffler The electronic properties of organic semiconductors typically exhibit a significant dependence on the strain, stress, and pressure~[1]. In this contribution, we present the theoretical background, assessment of approximations, and results of electronic and transport properties in the framework of density-functional theory. Our implementation considers the analytical strain derivatives~(stress tensor) including the contributions that stem from (a)~van-der-Waals interactions~[2] and (b)~the Fock-exchange in hybrid functionals. We validate our approach by investigating the geometric and electronic changes that occur in polyacetylene and anthracene under hydrostatic pressure. We show that the fraction of exact exchange included in the calculations is critical -- and non-trivial to choose -- for a correct description of these systems. Furthermore, we point out trends for the electrical conductivity under pressure and identify the dominant charge carriers and transport directions.\newline [1] J. H. Kim, S. Seo, and H. H. Lee, Appl. Phys. Lett. \textbf{90}, 143521 (2007); G. Giri \textit{et al.}, Nature. \textbf{480}, 504 (2011).\newline [2] A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. \textbf{102}, 073005 (2009). [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B15.00012: Investigation of Observating the role of orbital angular momentum with in Rashba and Dresselhaus effects system Soohyun Cho, Wonsig Jung, Beomyoung Kim, Garam Han, Mats Leandersson, Balasubramanian Thiagarajan, Changyoung Kim In our previous studies of Au(111) surface states with circular dichroism angle-resolved photoemission spectroscopy (CD-ARPES), we found strong CD signal, indicating probable existence and role of orbital angular momentum (OAM) in the Rashba effect. We proposed that OAM plays a key role in the electronic structures of such systems in the presence of inversion symmetry breaking and spin-orbit coupling (SOC). Semiconductors with the zinc blende structure with an inversion symmetry breaking have band splitting near the $\Gamma $-point as predicted by the band calculation. In addition, the overall spin structure can be understood in within the Dresselhaus effect. While the net OAM in the bulk should be normally zero for semiconductors~with an inversion symmetry, non-zero OAM can appear in the bulk of zinc-blende structures due to the inversion symmetry breaking. Moreover, as for ARPES experiments, the sample surface provide an additional inversion symmetry breaking. To investigate the OAM structure, we have performed CD-ARPES experiments on InSb, CdTe and GaAs. Our results show CD signal in the J$_{\mathrm{eff}}=$3/2 states (heavy and light hole band). We attribute the CD modulations of the three bands (heavy hole, light hole and split-off bands) to the existence of OAM. The OAM structure deduced from the CD-ARPES results is explained if we assume both the Rashba (from surface) and Dresselhaus (from bulk) effects are present. These results suggest that OAM also plays a role in the Dresselhaus effect through SOC. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B15.00013: Towards simple orbital-dependent density functionals for molecular dissociation Igor Ying Zhang$^1$, Patrick Richter, Matthias Scheffler Density functional theory (DFT) is one of the leading first-principles electronic-structure theories. However, molecular dissociation remains a challenge, because it requires a well-balanced description of the drastically different electronic structure at different bond lengths. One typical and well-documented case is the dissociation of both H$_2^+$ and H$_2$, for which all popular DFT functionals fail [1,2]. We start from the Bethe-Goldstone equation to propose a simple orbital-dependent correlation functional which generalizes the linear adiabatic connection approach. The resulting scheme is based on second-order perturbation theory (PT2), but includes the self-consistent coupling of electron-hole pairs, which ensures the correct H$_2$ dissociation limit and gives a finite correlation energy for systems with a (near)-degenerate energy gap. This coupling PT2-like (CPT2) approximation delivers a significant improvement over all existing functionals for both H$_2$ and H$_2^+$ dissociation. We will demonstrate the reason for this improvement analytically for H$_2$ in a minimal basis. [1] A. J. Cohen {\it et al.}, \emph{Chem. Rev.} \textbf{112} 289 (2012), [2] F. Caruso {\it et al.}, \emph{Phys. Rev. Lett.} \textbf{110} 146403 (2013). [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B15.00014: DFT Predictions of Electronic, Transport, and Bulk Properties of Li$_{2}$S Yuriy Malozovsky, Lashounda Franklin, Chinedu Ekuma, Diola Bagayoko We present results from \textit{ab-initio, }self consistent calculations of electronic, transport, and bulk properties of cubic antifluorite (anti-CaF$_{2})$ lithium sulfide (Li$_{2}$S). Our computations employed the local density approximation (LDA) potential of Ceperley and Alder and the linear combination of atomic orbital (LCAO) formalism. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams method, as enhanced by Ekuma and Franklin (BZW-EF). Consequently, we solved self consistently both the Kohn-Sham equation and the one giving the ground state charge density in terms of the wave functions of the occupied states. For a low temperature lattice constant, our calculated, indirect gap, from $\Gamma $ to X, is 3.723. The predicted LDA band gap is 3.702 eV. We discuss the total and partial densities of states, electron and hole effective masses, and the predicted bulk modulus of 45.57 GPa that agrees with a low temperature measurement of 45.7 GPa. Acknowledgments: This work was funded in part by the National Science Foundation (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 -- National, Nuclear Security Administration (NNSA) (Award Nos. DE-NA0001861 and DE- NA0002630), LaSPACE, and LONI-SUBR. [Preview Abstract] |
Monday, March 2, 2015 2:03PM - 2:15PM |
B15.00015: Metallic 2D Surface State of Silicon by Ionic Liquid gating and observation of Reentrant Insulating behavior J.J. Nelson, A.M. Goldman Metal insulator transitions are usually observed in high mobility and low carrier density 2D electron systems. There are several open questions regarding the metallic state including its existence in the limit of zero temperature. The current experimental focus is on the production of higher mobility samples to push the critical carrier density to even lower values, which will increase the effects of the Coulomb interaction. Here we report an unexpected result, the observation of the onset of a metallic state at high carrier densities in silicon gated with the ionic liquid DEME-TFSI. In addition we have observed a return to the insulating state as the carrier density was further increased. This reentrant insulting behavior is an effect that was recently predicted [Das Sarma, S. and Hwang, E. H., PRB 89 121413, 2014]. [Preview Abstract] |
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