Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X36: Focus Session: Bulk Properties of Complex Oxides -- Novel Systems |
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Sponsoring Units: DMP GMAG Chair: Ram Seshadri, University of California, Santa Barbara Room: E146 |
Thursday, March 18, 2010 2:30PM - 3:06PM |
X36.00001: Hollandites: a novel class of oxides with unusual properties Invited Speaker: Hollandite with a general chemical formula $A_{x}M_{8}$O$_{16}$ ($x\le $2) is one of minerals. Its crystal structure consists of the $M_{8}$O$_{16}$-framework and $A$-cations at tunnel sites in the $M_{8}$O$_{16}$-framework. The most popular crystal symmetry is a tetragonal $I$4/$m$ in which the crystallographycally independent $M$ site is unique, hence, $A^{+}_{2}M_{8}$O$_{16}$ is a mixed valent oxide with $M^{3+}$/$M^{4+}$ = 1/3. We have synthesized K$_{2}$V$_{8}$O$_{16}$ [1] and K$_{2}$Cr$_{8}$O$_{16}$ [2] in both powder and single crystal form under high pressure and found metal-insulator (MI) transitions. The manner of MI transitions, however, is very different between both compounds. K$_{2}$V$_{8}$O$_{16}$ exhibits a first order MI transition at around 170 K. The magnetic susceptibility is reduced to a small value at the transition, suggesting the formation of V$^{4+}$-V$^{4+}$ singlet pairs and V$^{3+}$-V$^{3+ }$pairs in the low temperature insulator phase. The transition is accompanied by the structural change from a tetragonal to a monoclinic structure. The low temperature phase has a superlattice of $\surd $2 x $\surd $2 x 2, suggesting a charge ordering of V$^{4+}$ and V$^{3+}$. On the other hand, K$_{2}$Cr$_{8}$O$_{16}$ is a ferromagnetic metal (or half-metal) with $T_{C}$ = 180 K and shows a transition to an insulator at 95 K without any apparent structural change but retaining ferromagnetism. K$_{2}$Cr$_{8}$O$_{16}$ is quite unique in three aspects: It has a rare mixed valence of Cr$^{3+}$ and Cr$^{4+}$; it has a metal (or half-metal) to insulator transition in a ferromagnetic state; and the resulting low temperature phase is a rare case of a ferromagnetic insulator. This discovery could open a new frontier on the relation of magnetism and conducting properties in strongly correlated electron systems. \\[4pt] References:\\[0pt] [1] M. Isobe, S. Koishi, N. Kouno, J. Yamaura, T. Yamauchi, H. Ueda, H. Gotou, T. Yagi, and Y. Ueda, J. Phys. Soc. Jpn. \textbf{75} (2006) 073801. \\[0pt] [2] K. Hasegawa, M. Isobe, T. Yamauchi, H. Ueda, J-I. Yamaura, H. Gotou, T. Yagi, H. Sato, and Y. Ueda, Phys. Rev. Lett. \textbf{103 }(2009) 146403. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X36.00002: Spin and Orbital fluctuations in antiferromagnetic RbO$_2$ Erik Ylvisaker, Rajiv Singh, Warren Pickett We examine magnon and orbiton behavior in the Mott insulating RbO$_2$, where the magnetic moment is confined to localized O$_2$ anti-bonding molecular $\pi^*$ orbitals, and the orbital ordering is frustrated. We derive an effective Kugel-Khomskii Hamiltonian from a two band Hubbard model with hopping parameters taken from {\em ab initio} density functional calculations. The ground state orbital order is frustrated, and due to the rather different interband and intraband hoppings this leads to a magnon excitation spectrum which is strongly dependent on the orbital ground state. This strong coupling between the magnon spectrum and the orbital ground state, along with the near degeneracy of the orbital ordering leads to a quenching of the spin order, resulting in a N\'{e}el temperature significantly lower than mean field theory predicts. [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X36.00003: Electronic excitations in YTiO3 William Thornton, Anton Kozhevnikov, Adolfo Eguiluz, Robert Harrison We study the electronic excitations across the Mott gap in YTiO3 using time dependent density functional theory, implemented in a Wannier basis. The leading excitation, lying at $\sim $2eV, involves orbitally ordered states. In the optical limit, the excitation is a dipole-allowed d-d off site feature which we analyze in view of recent optical data and model calculations (PRB 76, 155125 (2007), and PRB 78, 075122 (2008)). For large wave vectors, the loss feature switches over to on-site d-d character; its strength and energy location depends sensitively on the d-d excitonic interaction. This on-site excitonic feature may be observable via inelastic scattering of x-rays. [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 3:42PM |
X36.00004: Exchange an Correlation effects in the electronic properties of transition metal oxides: the example of NiO Matteo Guzzo, Matteo Gatti, Lucia Reining NiO, as a prototype for strongly-correlated materials, has been extensively studied experimentally and theoretically. Its Antiferromagnetic phase is reasonably well described in GW (a many-body approximated approach), but self-consistency is needed to obtain proper wavefunctions for the system. Still, agreement with experiment is not perfect. In particular, the unoccupied d-states result too high in energy by about 1 eV. In the present work we investigate the effects of vertex corrections derived from time-dependent density-functional theory on this result, starting from a simple LDA correction and going to more complex vertices including non-locality. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X36.00005: Raman study of the Verwey transition in Magnetite at high-pressure and low-temperature; effect of doping L. Gasparov, V. Struzhkin, A. Gavriliuk, H. Berger We report high-pressure low-temperature Raman studies of the Verwey transition in pure and Al --doped magnetite (Fe$_{3}$O$_{4})$. Low temperature phase of magnetite displays a number of additional phonon modes that serve as transition markers. The diamond anvil cell in combination with these transition markers allows us to investigate the effect of hydrostatic pressure on the transition temperature. Pure magnetite displays nearly linear decrease of the transition temperature with increase of pressure yielding dTv/dp= -5 K/GPa. In contrast, Al-doped magnetite (T$_{V}$=123K) displays reduced slope of the PT curve at low temperatures and high pressures with overall dT$_{v}$/dp around -10 K/GPa. These dependences are directly related to the changes of the molar entropy and molar volume at the transition. We compare the data obtained in our Raman experiment with that obtained from the ambient pressure specific heat measurements. [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X36.00006: Spin density functional study on magnetism of potassium-loaded Zeolite A Yoshiro Nohara, Kazuma Nakamura, Ryotaro Arita In order to clarify the mechanism of spin polarization in potassium-loaded zeolite A, we perform {\em ab initio} density-functional calculations. We find that (i) the system comprising only non-magnetic elements (Al, Si, O and K) can indeed exhibit ferromagnetism, (ii) the host cage makes a confining quantum-well potential in which superatom $s$- and $p$-like states are formed, the latter $p$ states are responsible for the spin polarization, and (iii) the size of the magnetic moment depends sensitively on atomic positions of potassium clusters. We show that the spin polarization can be described systematically in terms of the confining potential and the crystal-field splitting in the superatom $p$ states. (arXiv 0909.4432 (2009)) [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X36.00007: Crystal field and magnetic structure of UO$_{2}$: a computational probe of the multi-determinantal ground and excited states Fei Zhou, Vidvuds Ozolins The properties of UO$_{2}$ is the result of complex $f$-electron physics, characterized by localization of orbitals, strong electronic interactions and spin-orbit effects, crystal field potential, magnetic and electron-lattice couplings. We present a comprehensive theoretical study of the electronic structure of UO$_{2}$ with combined applications of a newly improved density functional method and a model Hamiltonian. The model, unambiguously parametrized with DFT calculations, predict the excitation energies of the $f^{2}$ states, including the whole spectrum with different $L,S,J$ as well as the low energy crystal field excitations $\Gamma_{5} \rightarrow \Gamma_{3,4,1}$, with good agreement to experiment. The $\Gamma_{5}$ triplet is confirmed as the lowest multiplet. We also investigated different non-collinear magnetic structures, in particular the 3-k ground state, of UO$_{2}$. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X36.00008: Moment configuration of SrLaFeO$_{4}$ at low temperatures A.M. Alsmadi, P. Piccoli, A.J. Schultz, M. Torikachvili, S. Adak, K. Kothapalli, F. Nasreen, H. Nakotte We determined the configuration of the Fe$^{3+}$ moments in SrLaFeO$_{4}$ by single-crystal neutron diffraction at low temperatures. Previous magnetic and neutron-diffraction studies provided evidence that this compound exhibits antiferromagnetic order at room temperature. The magnetic peaks could be indexed using an orthorhombic structure of Cmca symmetry with two propagation vectors q$_{1}$ = (1/2, 1/2, 0) and q$_{2}$ = (-1/2, 1/2, 0), representing the two possible domains[1]. Irregularities in some of the magnetic intensities provided evidence for a second magnetic transition or domain repopulation effects at $\sim $30 K. We performed additional in-depth neutron-diffraction studies at 15, 65, and 150 K, using the SCD diffractometer at the Los Alamos Neutron Science Center. Using a theoretical form factor for Fe$^{3+}$ and assuming collinear moments, we were able to fit the observed magnetic intensities to magnetic models at all three temperatures. Our data are indicative of moment rotation rather than domain repopulation effects at $\sim $ 30 K. At 15 K, we find evidence that the moments point along the [100] direction, while the magnetic moments are rotated by $\sim $ 25\r{ } from the [100] direction at 65 and 150 K. [1] M. H. Jung et al., J. Appl. Phys. 79 10A926 (2005) [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X36.00009: Spin-Orbital-Lattice coupling in KO$_{2}$ superoxide Minjae Kim, Beom Hyun Kim, Hong Chul Choi, B. I. Min In KO$_{2}$ superoxide, the magnetic ordering from the incomplete pi level emerges concomitantly with a symmetry lowering such as gyration of anions. The simultaneous structural and magnetic phase transitions suggest the spin-orbital-lattice coupling in correlated 2$p$ electrons system. We have studied the interplay of the spin-orbital-lattice degrees of freedom in KO$_{2}$ by employing the first-principles electronic structure theory. We used generalized gradient approximation (GGA) for the exchange correlation potential incorporating the Coulomb interaction U and the spin-orbit coupling (GGA+U+SO). For the high symmetry phase of KO$_{2}$, we have found that the band gap opens by the spin-orbit coupling which splits the degenerate pi anti-bonding level. For the low symmetry phase of KO$_{2}$, we have found that the band gap opens by the strong Coulomb interaction and the crystal field effect from alkali cations. The simultaneous antiferro-spin and ferro-orbital orderings are found to occur with the band gap opening. [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X36.00010: Electronic Structure and Properties of CePO$_{4}$ Nicole Adelstein, H. Ray, L.C. De Jonghe, B.S. Mun, P.N. Ross Jr., M. Zboray, J.B. Neaton Structural and electronic properties of cerium orthophosphate (CePO$_{4})$, a mixed electronic and protonic conducting electrolyte, are calculated using density functional theory (DFT) and beyond, and compared with experiments. A Hubbard-like parameter U is employed to reduce the self-interaction for localized Ce 4f states. As U is varied from 1-5 eV, the band structure is found to undergo significant changes, with the Kohn-Sham gap varying between 0.8 and 4.3 eV. Interestingly, the lattice parameters are unchanged, indicating structural properties are far less sensitive to U. The valence band energies and the optical band gap of CePO$_{4}$ are measured using photoemission and optical reflectance spectroscopy. The structure was determined using XRD. The choice of U = 4 eV provides the best match between the calculated density of states and the experimental photoemission and optical reflectance spectra. The activation energy of the minimum energy path for proton transfer will be calculated and compared to that of LaPO$_{4}$, another proton conducting electrolyte. [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X36.00011: Structural Integrity and Microstructure of NA$^{+}$ Conducting Ceramics Kristina Lipinska, Patricia Kalita, Oliver Hemmers, Stanislav Sinogeikin, Olga Shebanova, Wenge Yang, Gino Mariotto Oxides with the general formula of Na$_{1+x }$Zr$_{2 }$Si$_{x}$ P$_{3-x }$O$_{12 }$, known as Nasicon, are fast Na+ ion-conducting materials with important electrochemical applications and many functional properties, often attributed to their unique structural features. Comparative, in situ studies of the limits of structural integrity were performed for selected Nasicon materials, using synchrotron x-ray diffraction and diamond anvil cell technology. We show how different processing conditions produce crystalline structures with specific morphology. We discuss the bulk modulus, the compressibility and the influence of the volume fraction of primary and secondary crystalline phases on the overall Nasicon structural integrity. [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X36.00012: Atomic Order Modeling of Nanodomains in Solid-Oxide Fuel Cell Materials Paul Dalach, Don Ellis, Axel Van de Walle Yttria-stabilized zirconia (YSZ), (La$_{x}$,Sr$_{1-x})$(Fe$_{y}$,Cr$_{1-y})$O$_{3-z}$ (LSFCr), and (La$_{x}$,Sr$_{1-x})$(Ru$_{y}$,Cr$_{1-y})$O$_{3-z}$ (LSRCr) were modeled using first principles Density Functional theory (DFT) and DFT-derived thermodynamic statistical cluster expansion methods. Cluster expansions were trained using theoretical structural energies to develop semi-analytical energy expressions for each material's configuration space.\footnote{ A. van de Walle, Nature Materials 7, 455 - 458 (2008)} A lattice Monte Carlo algorithm was used to thermally equilibrate structures of fixed concentration. Thermally equilibrated atomic ordering data were analyzed to reveal nanodomains below current experimental diffraction resolution limits and correlate local atomic features to material properties, such as aging and ionic diffusion in YSZ and B-site metal precipitation in the lanthanum perovskites. [Preview Abstract] |
Thursday, March 18, 2010 5:18PM - 5:30PM |
X36.00013: Spin-charge coupled phenomena in Mo pyrochlore oxides under pressure: Monte Carlo study of the double-exchange model on a frustrated pyrochlore lattice Yukitoshi Motome, Nobuo Furukawa Mo pyrochlore oxides $R_2$Mo$_2$O$_7$ is known to show an interesting competition between ferromagnetic metal and spin-glass insulator by changing the reare-earth element $R$. Recent experiments by applying external pressure revealed some new aspects on the phase competition. In particular, it was shown that the ferromagnetic metal turns into a peculiar diffusive metal under pressure, with showing an intervening spin-glass metallic state [1]. We theoretically investigate the origin of these complex behaviors by studying the pyrochlore double-exchange model. We find that the experimental results are well explained by the keen competition between the double-exchange ferromagnetic interaction and the super-exchange antiferromagnetic interaction as well as the interplay between spin and charge on the frustrated lattice: The system exhibits an incoherent metallic state with extremely suppressed spin correlation and an electronic phase separation between two metallic states which potentially corresponds to the spin-glassy metallic behavior.\\[4pt] [1] S. Iguchi et al., Phys. Rev. Lett. {\bf 102}, 136407 (2009). [Preview Abstract] |
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