Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q16: Electronic Structure III |
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Sponsoring Units: DCOMP Chair: Pierre Darancet, Lawrence Berkeley National Laboratory Room: D173 |
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q16.00001: Oxides -- a challenge for (theoretical) spectroscopy P. Rinke, H. Jiang, M. Scheffler, A. Greuling, M. Rohlfing, A. Janotti, E. Kioupakis, C. G. Van de Walle Oxides are of tremendous technological importance, yet challenging materials to characterize. In many cases the agreement between experimental and theoretical spectroscopy observed for other material classes has not been attained. We use rutile TiO$_2$ as an example to illustrate some of the problems. Many-body perturbation theory in the $G_0W_0$ approach based on density-functional theory in the local-density approximation gives a fundamental band gap of 3.3~eV in seemingly good agreement with the 3.3$\pm$0.5~eV measured in direct and inverse photoemission [1]. However, the lowest exciton computed in Bethe-Salpeter calculations for the optical spectrum is found at an energy of 3.21~eV, while optical experiments only give 3.03~eV [2]. Polaronic effects, i.e. the renormalization of the band edges due to electron-phonon coupling, reduce the band gap, but it remains a challenge to include the ionic contribution to the dielectric function, which can be substantial in oxides, in the $G_0W_0$ calculations and to incorporate both effects consistently into Bethe-Salpeter calculations. Another aspect to consider is the role of electron correlations. [1] Y. Tezuka \textit{et al.}, J.\ Phys.\ Soc.\ Jpn.\textbf{63}, 347 (1994). [2] J. Pascual \textit{et al.}, Phys.\ Rev.\ B \textbf{18}, 5606 (1978). [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q16.00002: Calculated Electronic Properties of Rutile TiO$_{2}$ and Cubic SrTiO$_{3 }$ Chinedu Ekuma, Diola Bagayoko We present preliminary, calculated, electronic properties of rutile titanium dioxide (TiO$_{2})$ and of cubic strontium titanate (SrTiO$_{3})$. Our computations employed local density approximation (LDA) and generalized gradient Approximation (GGA) potentials for TiO$_{2}$ and SrTiO$_{3}$, respectively. We implemented the linear combination of atomic orbitals (LCAO) within the framework of the Bagayoko, Zhao, and Williams (BZW) method. In doing so, we solved, self-consistently, both the Kohn-Sham equation and the equation giving the ground state charge density in terms of the wave functions of the occupied states. Our preliminary findings indicate that TiO$_{2}$ has an indirect band gap of 2.95 eV, from $\Gamma $ to R. The direct gap at $\Gamma $ is 0.10 eV larger. The indirect band gap of SrTiO$_{3}$, from L to $\Gamma $ or X, is 3.05 eV. \\[4pt] Work funded in part by the Louisiana Optical Network Initiative (LONI), the National Science Foundation (NSF) and the Louisiana Board of Regents [Award Nos. EPS-1003897 and NSF (2010-15)-RII-SUBR], and and Ebonyi State, Federal Republic of Nigeria [Award No: EBSG/SSB/FSA/040/VOL. VIII/039] [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q16.00003: The confinement error corrections for the exchange energy in transition metal oxides Feng Hao, Rickard Armiento, Ann E. Mattsson We present some recent advances towards a straightforward scheme to correct for the confinement errors of the exchange energy of the transition metal oxides (TMO). This approach includes two steps: (i) identifying the spatial regions where the confinement errors exist, using local density and kinetic energy density information, and (ii) mapping these spatial regions to harmonic-oscillator (HO) models [1], and quantifying and correcting the relative confinement errors based on the model system. The scheme has been applied to calculations with several local and semi-local functionals, and a trend of improvement for the equilibrium structure is obtained after applying these confinement error corrections. Sandia is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.\\[4pt] [1] Hao et al, PRB {\bf 82}, 115103 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q16.00004: First principles study of electronic and structural properties of CuO Burak Himmetoglu, Matteo Cococcioni The ground state of CuO is particularly challenging to study with DFT-based computational techniques even below its Neel temperature. This situation is due to the inability of most approximate DFT energy functionals to describe electronic regimes hat are dominated by many-body effects. In this study, we show how a description of the ground state of this material in better agreement with observations can be obtained using extended Hubbard-based corrective energy functionals (DFT+U and DFT+U+V). In particular we uncover an orbitally ordered insulating ground state for the cubic phase of CuO (that was expected, but never reported before) whose appearance is determined by a fine interplay between correlation effects and magnetic interactions. Starting from this ground state we also study the tetrahedral distortion of the unit cell (recently reported in experiments), characterizing the reorganization of the electronic states and identifying all the equilibrium structures. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q16.00005: Transition metal dioxides: a case for the intersite term in Hubbard-model functionals Heather Kulik, Nicola Marzari Triatomic transition-metal oxides in the ``inserted dioxide'' (O-M-O) structure represent one of the simplest examples of systems that undergo qualitative geometrical changes via subtle electronic-structure modulation. We discuss three transition-metal dioxide molecules (MO$_2$ where M = Mn, Fe, or Co), for which equilibrium structural (eg bent or linear geometry) and electronic (eg spin or symmetry) properties have been challenging to assign both theoretically and experimentally. Augmenting a standard density-functional theory (DFT) approach with a Hubbard term (DFT+$U$) occasionally over-localizes the $3d$ manifold, leading to incorrect bond elongation and, in turn, poor equilibrium geometries for MO$_2$ molecules, while preserving good spin-state splittings. We recover a proper description of both geometry and energetics for these molecules through either calculating DFT+$U$ relaxations at fixed M-O bond lengths or by inclusion of an inter-site interaction term $V$ that favors M($3d$)-O($2p$) interactions. In the latter case, both $U$ and $V$ are calculated fully from first-principles and are not fitting parameters. Finally, we present an approach that more accurately determines the Hubbard $U$ over a coordinate in which the character of bonding varies. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q16.00006: On the origins of the deficiencies of density functional theory exchange-correlation functionals for transition metal oxides Ann E. Mattsson, Rickard Armiento, Feng Hao The transition metal oxides (TMO) are a class of compounds that are difficult to treat in density functional theory (DFT) with simple local and semi-local functionals. Especially for CuO, they failed to give the correct equilibrium monoclinic structure. The major source of the deficiency is attributed to the imperfect cancellation of the electronic self-interaction (SI) in the approximated exchange energy. Previous studies [1] show that a large part of the SI error is connected to the confinement error that can be modeled by harmonic-oscillator (HO) systems. We discuss recent advances towards a simple methodology to quantify the confinement errors in real TMO systems. Our results show that these confinement errors may account for the deficiencies of DFT functionals in obtaining the correct equilibrium structure of the TMO. Sandia is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.\\[4pt] [1] Hao et al, PRB {\bf 82}, 115103 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q16.00007: Ab initio calculation of the orbital magnetization by Wannier interpolation Graham Lopez, David Vanderbilt, Ivo Souza, Timo Thonhauser We present an analytic, first-principles scheme to efficiently calculate exactly the spontaneous orbital magnetization of ferromagnetic crystals [1,2]. This is in contrast to the standard method of integrating inside muffin-tin spheres which, while a good approximation in practice, is still an approximation. The method uses Wannier interpolation to perform the necessary Brillouin-zone integrals in a similar way as was done previously for the anomalous Hall conductivity [3]. The method has been implemented to work with a plane-wave density-functional code, and calculations were done on iron, cobalt and nickel. We compare our calculations of the orbital magnetization in these systems to recent ab initio and experimental results and find good agreement with both [4]. \\[4pt] [1] T. Thonhauser et al., Phys. Rev. Lett. {\bf 95}, 137205 (2005).\newline [2] D. Xiao et al., Phys. Rev. Lett. {\bf 95}, 137204 (2005).\newline [3] X. Wang et al., Phys. Rev. B, {\bf 74}, 195118 (2006).\newline [4] D. Ceresoli et al., Phys. Rev. B {\bf 81}, 060409(R) (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q16.00008: Origin for the disorder-induced quantum criticality in NbFe$_2$ Aftab Alam, Duane Johnson Using KKR-CPA ab-initio electronic-structure method founded on an optimal site-centered basis-set, we investigate the key features giving rise to the quantum critical transitions observed in NbFe$_2$ upon doping with 1.75{\%} Nb [1]. These phase transitions involve lowest-energy excitations at/near the Fermi surface. In particular, it is suggested to arise due to an accidental unconventional band critical point (uBCP) with vanishing quasi-particle velocity [2]. Moving off-stoichiometry by increasing Nb, or reducing electrons (e/a), we find the Fermi level E$_f$ increases (rather than decreases based only on band-filling) and meet the uBCP to produce excitations driving the anomalies. We detail the concentration-dependence electronic dispersion, density of state, E$_f$ shift, and energies for NbFe$_2$, and why disorder increase the E$_f$ with electron loss. At stoichiometry all our results agree with those from full potential calculations, including itinerant magnetism.\\[4pt] [1] D. Moroni-Klementowicz et al., Phys. Rev B 79, 224410 (2009).\\[0pt] [2] Brian Neal, Erik R. Ylviskar, and Warren E. Pickett, private communication (2009). [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q16.00009: Electronic structures of superionic conductor Li$_3$N Masaru Aoki, Yoshiyuki Ode, Kazuo Tsumuraya Lithium nitride is a superionic conductor with high Li conductivity. The compound has been studied extensively because of its potential utility as electrolyte in solid-state batteries. Though the mobility of the cations within the crystalline solid is high comparable to that of molten salts, the mechanism of the high mobility of the cations remains unsolved. To clarify the origin of the mobility we investigate the electronic states of the Li cations in the Li$_3$N crystal with the first principles electronic structure analysis, focusing a correlation between the cations and the ionicities of the constituent atoms. We have found the existence of the covalent bonding between the Li atoms in the Li$_3$N crystal in spite of the ionized states of the constituent atoms. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q16.00010: Why are the copper cations superionic in the $\alpha $-CuI crystal? Kazuo Tsumuraya, Takamitsu Ohtsuka, Hidekazu Tomono The mechanism of the superionic conduction is an unresolved issue in the solid-state physics. The cations are mobile species in $\alpha $-CuI and $\alpha $-AgI crystals. In these conductors, the constituent atoms are ionized. The clarification of the mechanism of the high mobility of the cations needs to investigate the electronic structures in the $\alpha $-CuI crystal. We obtain the dynamically-averaged local (DAL) positions of the mobile copper cations in the crystal from the pair distribution function and the angle distribution functions, which we calculate from the first principles molecular dynamics simulations at 700 K\@. The positions predict the existence of a correlation among the cations in the $\alpha $-CuI. The static electronic structure analysis, of the DAL structure, allows us to clarify the correlation. The correlation enables us to clarify the mechanism of the migration and the difference in the electronic structures between the conductors and the ionic crystals. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q16.00011: First-principles investigation of band offsets and dielectric properties of Silicon-Silicon Nitride interfaces Tuan Anh Pham, Tianshu Li, Francois Gygi, Giulia Galli Silicon Nitride (Si3N4) is a possible candidate material to replace or be alloyed with SiO2 to form high-K dielectric films on Si substrates, so as to help prevent leakage currents in modern CMOS transistors. Building on our previous work on dielectric properties of crystalline and amorphous Si3N4 slabs [1], we present an analysis of the band offsets and dielectric properties of crystalline-Si/amorphous Si3N4 interfaces based on first principles calculations. We discuss shortcomings of the conventional bulk-plus line up approach in band offset calculations for systems with an amorphous component, and we present the results of band offsets obtained from calculations of local density of states. Finally, we describe the role of bonding configurations in determining band edges and dielectric constants at the interface.\\[4pt] [1] T. Anh Pham et al., Appl. Phys. Lett., 96, 062902 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q16.00012: STEM-EELS calculations including both fine structure and diffraction M.P. Prange, M.P. Oxley, S.J. Pennycook, S.T. Pantelides Electron energy loss spectroscopy in scanning transmission electron microscopy (STEM-EELS) probes electronic excitations with high spatial and energy resolution. Interpretation of the spectra requires accurate treatment of both the diffraction of the electron probe and the electronic excitation of the sample. We present a theory of core loss STEM-EELS based on a detailed calculation of the mixed dynamic form factor (MDFF) using DFT which informs a Bloch wave treatment of the probe/sample interaction. No dipole approximation is made. The probe diffraction is computed using the Bloch wave method which includes the microscope geometry, multiple elastic, and thermal diffuse scattering. We illustrate the method with calculations of complex oxide materials. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q16.00013: Extended Pi-Sigma tilde orbital model for CO adsorption on Pt and Ru Thomas Mion, Nicholas Dimakis, Faisal Alamgir, Cherno Jaye, Daniel Fischer, Paul McGinn, James Cooper, Steve Greenbaum, Eugene Smotkin Several discrepencies between the predicted Blyholder-type adsobtion models and experimental, as well as DFT calculated infared spectra have been addressed for atop CO on Pt in contrast to Ru. This model correlates increased Near Edge X-Ray Absorbtion Fine Structure intensity as the result of a sub-eV downshift from CO on Ru compared to CO on Pt thereby forming a weaker C-O bond. The model accounts for the hybrid orbitals electron transfer between the CO - metal bonds while taking in to consideration the orbital polarization within the CO itself. The charge redistribution of the s-tilde orbitals and reduced charge donation from CO to the surface results in a weaker internal CO bond upon Ru relative to Pt. The extended Pi-Sigma model explains why atop C-O stretching frequencies do not correlate with carbon p-type vacancies. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q16.00014: Chirality and Electronic Structure of the Thiolate-Protected Au$_{38}$ Nanocluster Olga Lopez-Acevedo, Hironori Tsunoyama, Tatsuya Tsukuda, Hannu H\"akkinen, Christine M. Aikens Our joint computational and experimental investigation of the structural properties of the Au$_{38}$(SR)$_{24}$ gold protected nanocluster will be presented [1]. We have identified a new low-energy, chiral, D$_3$ symmetric structure that yields an excellent match between computed and measured powder XRD function. We have characterized the electronic shell structure of this nanocluster in terms of a particle-in-a-cylinder model. The CD response in the low-energy region (below 2.2 eV) of the new structure is very similar to the one reported several years ago from experiments for Au$_{38}$(SG)$_{24}$. The mechanism of the chiral response for low excitation energies is related to the chiral arrangement of the gold-thiolate ligand shell around the bi-icosahedral Au$_{23}$ core. The determination of the total structure of Au$_{38}$(SC$_2$H$_4$Ph)$_{24}$ nanoparticles by single crystal X-ray crystallography confirmed our results [2]. \\[4pt] [1] O. Lopez-Acevedo et al J. Am. Chem. Soc., 2010, 132 (23)\\[0pt] [2] Qian et al J. Am. Chem. Soc., 2010, 132 (24) [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q16.00015: Pseudospectral Calculation of Helium Wave Functions, Expectation Values, and Oscillator Strength Paul Grabowski, David Chernoff We extend the pseudospectral method from the solution of Schr\"{o}dinger's equation for two-electron atom S states to arbitrary angular momentum states. We evaluate the oscillator strength for the helium $1^1$S $\to$ $2^1$P transition. The result, 0.27616499(27), compares favorably to the best determination in the literature. The length, velocity, and acceleration expressions all have roughly the same accuracy in a pseudospectral treatment. We evaluate leading order finite-nuclear-mass and relativistic corrections for the helium ground state. The pseudospectral method achieves near state-of-the-art accuracy without requiring the implementation of any special-purpose numerical treatments. All the relevant quantities tested converge exponentially with increasing resolution and at roughly the same rate. Quantum mechanical matrix elements are directly and reliably calculable with pseudospectral methods. A general prescription is given for choosing coordinates and subdomains to achieve exponential convergence when two-particle Coulomb singularities are present. [Preview Abstract] |
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