Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D32: Focus Session: Density Functional Theory of Bulk Complex Oxides |
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Sponsoring Units: GMAG DMP Chair: Antonio Cammarata, Czech Technical University, Prague Room: 207B |
Monday, March 2, 2015 2:30PM - 2:42PM |
D32.00001: First-principles study of transport in SrTiO3 Burak Himmetoglu, Anderson Janotti, Hartwin Peelaers, Audrius Alkauskas, Chris G Van de Walle As a wide-band-gap semiconductor, SrTiO3 has attracted great interest for electronic device applications. While high electron mobilities of around 30,000 cm2 V-1 s-1 have been observed at low temperatures [1], room temperature mobilities are only on the order of a few cm2 V-1 s-1. These low mobilities pose a significant limitation for electronic device applications. In our work, we investigate the transport properties of n-doped SrTiO3 using first-principles calculations. We compute scattering of electrons with longitudinal optical modes in order to determine the scattering rates for the three conduction bands of SrTiO3. These scattering rates are invoked in Boltzmann transport integrals to calculate room-temperature transport coefficients. Our results indicate the strong impact of longitudinal optical phonon scattering as the main mechanism that leads to small electron mobilities at room temperature. In addition, our analysis provides valuable insights into designing high-mobility and high-thermopower materials, based on band-structure and strain engineering. Work supported by ONR and NSF. [1] J. Son, P. Moetakef, B. Jalan, O. Bierwagen, N. J. Wright, R. Engel-Herbert, and S. Stemmer, Nat. Mater. 9, 482 (2010) [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D32.00002: Many body effects on the formal charge state of 3d - Transition Metal Doped BaTiO$_3$ Subhasish Mandal, R.E. Cohen, K. Haule Using density functional theory in combination with dynamical mean field theory in Mn doped BaTiO$_3$, we find a different charge state and 3d - orbital occupations than obtained from either DFT or DFT+U. We find that the explicit treatment of many-body effects induced by the Hund's rule coupling in Mn shows a donor charge state of Mn$^{2+} $, instead of usual acceptor charge state of Mn$^{4+} $ as is found in both DFT and DFT+U. The differences in electron density reveal that charge transfer due to strong Hubbard interactions is not sufficient to describe the electron correlations in transition metal doped ferroelectrics. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D32.00003: Pressure induced iron spin state changes in MgGeO$_3$ Perovskite and Post-perovskite Kanchan Sarkar, Gaurav Shukla, Mehmet Topsakal, Renata Wentzcovitch MgGeO$_3$-perovskite is a low pressure analog of MgSiO$_3$-perovskite, the main Earth forming phase, and is used to shed light on several phenomena that occur in MgSiO3, particularly the post-perovskite transition. As such, experimental investigations of spin state changes in Fe-bearing MgGeO$_3$ might help to clarify some aspects of this phenomenon in MgSiO$_3$. Using DFT+U calculations, we have investigated pressure induced spin state changes in Fe$^{2+}$ and Fe$^{3+}$ in MgGeO$_3$ perovskite and post-perovskite and their effect on the post-perovskite transition. We uncover a direct relationship between average Fe-O bond-lengths and spin transition pressures in all cases. The effect of iron on the post-perovskite transitions in these phases can also be related to the average Fe-O bond lengths. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:42PM |
D32.00004: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 3:42PM - 3:54PM |
D32.00005: Oxygen vacancy induced localized state in rutle TiO$_{2}$ Donghan Shin, Chungwei Lin, Alexander A. Demkov Titanium dioxide (TiO$_{2})$ is as a promising material for several applications including photocatalysis, solar cells, spintonics and memory devices. Oxygen vacancies (OV) act as active sites for water dissociation and induce ferromagnetism in the bulk. Using density functional theory (DFT) and model Hamiltonian analysis, we investigate the localized states induced by an OV in rutile TiO$_{2}$. We identify two classes of localized states: a hybrid and polaron states. The hybrid state is caused by the orbital overlap between three Ti atoms next to a vacancy and is mainly derived from the Ti e$_{\mathrm{g}}$ orbitals. The polaron state is caused by the local lattice distortion and is mainly composed of one particular t$_{\mathrm{2g}}$ orbital from a single Ti atom. The first principles calculation shows that the polaron state is energetically favored, and the tight-binding analysis reveals the underlying connection between the bulk band structure and the orbital character of the polaron. Their respective spin moments are deduced from the on-site electron correlation. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D32.00006: The Correlation between Structure, Electronic Structure, and Shift Current in Visible-Light Ferroelectrics from First Principles Fenggong Wang, Steve Young, Fan Zheng, Ilya Grinberg, Andrew Rappe Shift current, as a dominant mechanism of the bulk photovoltaic effect (BPVE), remains not well understood, especially in terms of its connection to material's structure and electronic structure. This holds especially for the recently designed and demonstrated visible-light ferroelectric photovoltaics, Pb(Ni,Ti)O$_{3-\delta}$ and (K,Ba)(Ni,Nb)O$_{3-\delta}$, that have great structural and electronic tunabilities. Here, we study the BPVE of the visible-light-absorbing ferroelectrics by calculating shift current from first principles. The effects of phase, lattice distortion, oxygen vacancy, cation arrangement, composition, and strain on BPVE are systematically studied. The wavefunction nature of the contributing electronic states dictates the eventual shift current yield, which can be significantly affected by the change of the O vacancy location, cation arrangement, and strain. Consequently, under broad spectrum illumination, the total current can be greatly enhanced by reducing the cancellation of counter propagating currents and by increasing the shift vector magnitude. This not only is helpful for understanding other photovoltaic mechanisms that relate to the motion of the photocurrent carriers, but also provides guidelines for the design of the photovoltaic converters. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D32.00007: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 4:18PM - 4:30PM |
D32.00008: First-Principles Physics of Nanocheckerboard Formation in ZnMnGaO Spinels Mordechai Kornbluth, Chris Marianetti Using first-principles calculations, we present the physics behind spinel nanocheckerboards in $\mathrm{ZnMn_{x}Ga_{2-x}O_{4}}$. Previously, experiments discovered a group of Mn-based spinels that spontaneously phase-separate into nanocheckerboards. We analyze their origin in the Jahn-Teller (JT) effect, which couples local atomic distortions to an electronic degeneracy (here, the $e_g$ manifold of the Mn d-orbital). Using density functional theory, we show that the interaction between cubic Mn-poor and tetragonal Mn-rich regions causes phase separation, but diffusion prevents the thermodynamic ground state of bulk separation. We demonstrate that the energetics and geometry mandate a nanocheckerboard configuration. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D32.00009: A DFT$+$DMFT study of Orbital Physics in a Spin Orbital Lattice Coupled 2$p$ Electron Mott System: KO$_{2}$ Minjae Kim, B.I. Min We have investigated the temperature ($T)$-dependent orbital physics in a typical spin-orbital-lattice coupled 2$p$ electron Mott system KO$_{2}$, based on the electronic structures obtained by the dynamical mean-field theory as well as the density functional theory. KO$_{2}$ consists of K$^{+}$ cations and O$_{2}^{-}$ molecule anions, and there are three electrons in the fourfold degenerate pi anti-bonding orbital of O$_{2}^{-}$ anions. Hence, the orbital degeneracy occurs in a O$_{2}^{-}$ anion with a magnetic moment. We have shown that KO$_{2}$ exhibits the orbital fluctuation phenomenon at high $T$ due to the degenerate pi anti-bonding orbital. Upon cooling, this orbital fluctuation is suppressed by the Jahn-Teller (JT) type crystal field with lowering of the crystal structure symmetry, and then the ferro-orbital (FO) ordering emerges at low $T$. This FO ordering is compatible with the experimental antiferromagnetic spin order at low $T$ in KO$_{2}$. We suggest that the suppression of the orbital fluctuation in KO$_{2}$ upon cooling is similar that in 3$d$ transition-metal oxides such as LaVO$_{3}$. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D32.00010: Quantum Monte Carlo simulations of Ti4O7 Magn\'{e}li phase Anouar Benali, Luke Shulenburger, Jaron Krogel, Xiaoliang Zhong, Paul Kent, Olle Heinonen Ti4O7 is ubiquitous in Ti-oxides. It has been extensively studied, both experimentally and theoretically in the past decades using multiple levels of theories, resulting in multiple diverse results. The latest DFT$+$SIC methods and state of the art HSE06 hybrid functionals even propose a new anti-ferromagnetic state at low temperature. Using Quantum Monte Carlo (QMC), as implemented in the QMCPACK simulation package, we investigated the electronic and magnetic properties of Ti4O7 at low (120K) and high (298K) temperatures and at different magnetic states. This research used resources of the Argonne Leadership Computing Facility at Argonne National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under contract DE-AC02-06CH11357. L.S, J.K and P.K were supported through Predictive Theory and Modeling for Materials and Chemical Science program by the Office of Basic Energy Sciences (BES), Department of Energy (DOE) Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D32.00011: Systematics of spin crossovers across the RECoO3 family Mehmet Topsakal, Chris Leighton, Renata Wentzcovitch We have investigated structural and electronic properties of rare-earth cobaltites (RECoO3) using ab initio DFT + U calculations. A structurally consistent and self-consistent Hubbard U treatment is shown to be essential for the proper description of strongly correlated cobalt-d electrons. We successfully capture the experimentally observed structural variations and explain the trend in the spin transition temperature in cobalt across the RE series. We believe that Hubbard U values presented in this study will allow further predictive studies of RE cobaltites. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D32.00012: Spin-orbit effects in iridates via electronic structure calculations: effects of tension and dimensionality Victor Pardo, Jose L. Lado Ab initio calculations have been performed in 5d$^5$-electron-based oxides in the large spin-orbit coupling limit. Our work tries to analyze the effects of strain and dimensionality in the electronic structure properties of iridates with Ir$^{4+}$:5d$^5$ electronic configuration in order to understand the different set of properties these materials present: they can be either metals or insulators, e.g. We focus on studying how close to the fully ionic j$_{eff}$=1/2 limit the system is by analyzing the L$_z$/S$_z$ ratio. We observe that it varies continuously as a function of strain or pressure, changing drastically with relatively small variations. We also analyze what effects on the band structure accompany this variation. In order to do this, we needed to include a full non-collinearity in the calculation of spin-orbit interaction. We have explored SrIrO$_3$, Sr$_2$IrO$_4$, Sr$_3$Ir$_2$O$_7$, thin films of SrIrO$_3$ so as to analyze the dimensionality effects and the structural implications. [Preview Abstract] |
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