Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J17: Focus Session: Bulk Properties of Complex Oxides - Ferrites + Vanadates |
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Sponsoring Units: DMP GMAG Chair: Jaime Fernandez-Baca, Oak Ridge National Laboratory Room: D174 |
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J17.00001: New structural phase transition in Bi$_{2}$(Fe$_{4-x}$Mn$_{x}$)O$_{10-x}$ complex oxides and it implications in the mullite family of materials Patricia Kalita, Andrew Cornelius, Stanislav Sinogeikin, Kristina Lipinska, Oliver Hemmers, Michael Lufaso, Zachary Kann, Hartmut Schneider Complex oxides with the mullite crystal structure belong to the most important phase in both traditional (porcelains and alumino silicate refractories) and advanced ceramics (heat exchangers, shock resistant composites, optical devices). New complex oxides in the mullite family Bi$_{2}$(Fe$_{4-x}$Mn$_{x})$O$_{10-x}$ were synthesized and characterized. Using synchrotron x-ray diffraction we demonstrate a new structural phase transition in Bi$_{2}$(Fe$_{4-x}$Mn$_{x})$O$_{10-x}$ induced by pressure. We contrast it with the structural stability for mullite senso stricto Al$_{4+2x}$Si$_{2-2x}$O$_{10-x}$ where we did not observe any phase transition. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J17.00002: The role of charge and orbital order for the Verwey transition in Fe$_3$O$_4$ S. de Jong, R. Kukreja, M. Hossain, C. Back, A. Scherz, D. Zhu, W. Schlotter, J. Turner, W. Lee, Y. Chuang, R. Moore, O. Krupin, M. Trigo, L. Patthey, H. D\"urr, N. Pontius, T. Kachel, A. F\"ohlisch, M. Beye, F. Sorgenfrei, W. Wurth, C. Chang, M. D\"ohler, C. Trabant, C. Sch\"ussler-Langeheine Magnetite, Fe$_3$O$_4$, displays a strong decrease in resistivity upon heating above $T_C=123$~K: the Verwey transition. Since long it has been proposed that charge and orbital order (CO/OO), via Fe$^{3+}$ and Fe$^{2+}$ charge disproportionation, play a crucial role. However, the mechanism behind the Verwey transition to date remains unclear. Using pump-probe O K-edge resonant soft X-ray scattering at the new LCLS SXR beamline, we have studied the role of CO/OO for the Verwey transition on ultra-fast time-scales. We focus on the structurally forbidden (00 $^1/_2$) peak. Upon excitation, the charge gap of 200~meV is quenched on resolution limited time-scales, $<250$~fs, while we still observe a residual CO/OO signal. This may indicate the existence of a new transient state of matter, displaying charge and orbital order in coexistence with metallic behavior. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J17.00003: Pseudogap Phase of Magnetite Yohanes Pramudya, Hanna Terletska, Efstratios Manousakis, Vladimir Dobrosavljevic Despite extensive experimental and theoretical work, the description of the electrical transport mechanism in magnetite (Fe$_{3}$O$_{4})$ is still an unresolved issue [1]. This unusual resistivity behavior close to the Verwey transition in magnetite has long been a matter of controversy. In our study, we focus on the temperature regime above Verwey transition and far below the magnetic phase transition, where a nearly charge ordering state (due to the long-range Coulomb frustration) is expected. Here, we expect similar behavior to what has been discussed in a nearly frozen Coulomb liquid [2] with the existence of a pseudogap phase. Following this line of thought, we use extended dynamical mean field theory (EDMFT) and Monte Carlo simulation to study the simplest spinless model describing this system. Our studies do capture the main transport trends in this temperature regime with a typical pseudogap--like behavior. \\[4pt] [1] N. F. Mott, ``Metal-Insulator Transitions'', Taylor{\&}Francis (1990). \\[0pt] [2] S. Pankov and V. Dobrosavljevic, Phys. Rev. Lett. \textbf{94}, 046402 (2005). [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J17.00004: Synchrotron x-ray single-crystal structure analysis of a spinel oxide FeV$_{2}$O$_{4}$ with spin and orbital degrees of freedom Yoichi Nii, Hajime Sagayama, Taka-hisa Arima, Riu Sakai, Shinobu Aoyagi, Eiji Nishibori, Hiroshi Sawa, Kunihisa Sugimoto, Hiroyuki Ohsumi, Masaki Takata It has been reported that FeV$_{2}$O$_{4}$, which has orbital and spin degrees of freedom both in tetrahedral Fe$^{2+}$(d$^{6})$ sites and octahedral V$^{3+}$(d$^{2})$ sites, exhibits successive structural phase transitions, accompanying a ferrimagnetic transition. The origin of the phase transitions is supposed to be a cooperation and/or competition between the orbital and spin degrees of freedom both in Fe$^{2+}$ and V$^{3+}$. By a synchrotron x-ray single-crystal structure analysis, we determined the space group and atomic coordinate of each phase (cubic- HT-tetra.- HT-ortho.- LT-tetra.). The results suggest that the HT-tetra. (a$>$c) and HT-ortho. phases should be ascribed to the FeO$_{4}$ local compression, whereas VO$_{6}$ elongation should be responsible for the LT-tetra. (c$>$a) phase. We also discuss the orbital ordering (OO) pattern assuming strong electron-lattice coupling. A conceivable OO pattern of V$^{3+}$ at the LT-tetra. (c$>$a) is \textit{ferroic} one with \textit{yz} and \textit{zx} orbitals occupied, which is unique among spinel-type vanadates. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J17.00005: Single-ion Anisotropy, Dzyaloshinskii-Moriya Interaction and Negative Magnetoresistance of the Spin-1/2 Pyrochlores R2V2O7 Hongjun Xiang, Erjun Kan, M.-H. Whangbo, C. Lee, Su-Huai Wei, X.G. Gong The electronic and magnetic properties of spin-1/2 pyrochlores R$_2$V$_2$O$_7$ were investigated on the basis of density-functional calculations. Contrary to the common belief, the spin-1/2 $V^{4+}$ ions are found to have a substantial easy-axis single-ion anisotropy. The $|D/J|$ ratio deduced from the magnon quantum Hall effect of Lu$_2$V$_2$O$_7$, where $J$ is the nearest-neighbor spin exchange and $D$ is the Dzyaloshinskii-Moriya parameter, is much greater than the value estimated from our calculations (i.e., 0.32 vs. 0.05). We show that this discrepancy is due to the neglect of the single-ion anisotropy of the V$^{4+}$ ions, and the negative magnetoresistance observed for R$_2$V$_2$O$_7$ arises from a new mechanism. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J17.00006: Stability of the Ni$_{3}$V$_{2}$O$_{8}$ phase diagram on substitution with magnetic and non-magnetic transition metal ions Akila Kumarasiri, Ambesh Dixit, Gavin Lawes There is considerable interest in understanding the materials properties underlying the development of simultaneous magnetic and ferroelectric order in multiferroics. Ni$_{3}$V$_{2}$O$_{8}$ develops strongly coupled ferroelectric and antiferromagnetic order simultaneously at low temperatures and has a rich magnetic phase diagram due to competing magnetic interactions. We investigated how the magnetic phases of Ni$_{3}$V$_{2}$O$_{8}$ were affected by systematic doping by transition metal ions. For these studies, polycrystalline Ni$_{3}$V$_{2}$O$_{8}$ samples substituted by various concentrations of transition metal ions M (M = Zn, Cu, Co, Mn, Fe) were prepared. Heat capacity, magnetization, dielectric, AC susceptibility, and pyrocurrent measurements were used to track the change in phase transition temperatures. On doping with spin-0 Zn, the system behaves as expected for site dilution consistent with 2-D spins, where the phase transition temperatures are suppressed linearly to lower temperatures. The modifications to the phase diagram for magnetic dopants (Co, Cu, Mn and Fe) show more variation, but the multiferroic phase transition appears to persist over a range of concentrations. This suggests that the specific spin structure in Ni$_{3}$V$_{2}$O$_{8}$ responsible for the development of ferroelectric order is relatively robust against perturbations produced by both magnetic and non-magnetic dopants. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J17.00007: Orbitally driven trimerization in LiVO$_2$ and LiVS$_2$: a ``partial Mott transition" Hua Wu, D.I. Khomskii Layered triangular-lattice transition-metal compounds often display interesting magnetic and electronic properties. Here we studied the formation of the trimerized spin-singlet state of the V$^{3+}$ ($S$=1) in vanadates LiVO$_2$ and LiVS$_2$ and their electronic structure with a special orbital order, using constrained LSDA+$U$ calculations combined with lattice optimization. The obtained results show that the trimerization distortion in LiVO$_2$ increases as the effective $U$ decreases, and the calculated distortion of $\sim$0.3 \AA~ at the small $U$=1 eV agrees well with the experiments, indicating that LiVO$_2$ is close to a metal-insulator transition. The corresponding distortion in LiVS$_2$ is even stronger, being $\sim$0.4 \AA~ at the $U$=1 eV, which is due to enhanced electron delocalization via increased V-S covalency, in spite of a lattice expansion. This agrees with the experimental finding that LiVS$_2$ has a metal-insulator transition. The calculated energy gain associated with the trimerization well accounts for the observed structural phase transition temperature in LiVO$_2$ and LiVS$_2$. We conclude that the trimerization in LiVO$_2$ and LiVS$_2$ is due to a partial delocalization of the orbitally ordered electrons---a ``partial Mott transition,'' occurring not in the whole system but in small clusters (here in trimers). This situation is contrasted with that in NaVO$_2$, which is further away from the localized-itinerant crossover and thus remains insulating with different orbital ordering. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J17.00008: Simultaneous electrical transport and Raman spectroscopic measurements on individual nanowires of $W_{x}V_{1-x}O_{2}$ Tai-Lung Wu, Luisa Whittaker, C.J. Patridge, S. Banerjee, G. Sambandamurthy Vanadium oxide is a well-know material to study the metal-insulator transition (MIT) in correlated electron systems. Upon heating to about 340 K, $VO_{2}$ undergoes orders of magnitude drop in resistance from an insulating phase (I) to a metallic phase (M) and accompanies a lattice structural phase transition from a low-temperature monoclinical phase (M1) to a high-temperature tetragonal phase (R). We present results from combined electrical transport and Raman spectroscopic measurements to discern the effects of doping in controllably tuning the MIT in individual nanowires of single crystal $W_{x}V_{1-x}O_{2}$. The MIT temperature ($T_{c}$) in our $W_{x}V_{1-x}O_{2}$ nanowires can be tuned through a wide range from 280 to 330 K by controlling the dopant concentration. The M-I transition can also driven electrically in these nanowires. Our simultaneous measurement of electrical transport and Raman spectroscopic measurement help us understand the role of structural transition in affecting the macroscopic electrical transition in individual wires. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J17.00009: Strain- and temperature-induced phase transitions in single crystalline VO$_2$ Joanna Atkin, Emily Chavez, Samuel Berweger, Jinbo Cao, Wen Fan, Junqiao Wu, Markus Raschke The metal-insulator transition (MIT) of VO$_2$ exhibits a rich phase behavior involving two monoclinic (M1, M2), triclinic, and tetragonal phases that can form a complex domain structure and accompany the electronic transition. The interplay between these structural variants arises from strain due to differing lattice constants, temperature-dependent phase stability, and possible external strain from the substrate; the coupling between these effects renders a systematic study of the phase behavior difficult. We report on phase mapping of the structural changes through independent control of temperature and uniaxial strain in individual single-crystal nanorods, using Raman spectroscopy and near-field imaging. This allows us to investigate the transformation between the various insulating phases, elucidating the nature of the triclinic phase as a continuously distorted variant of the M1 monoclinic phase, intermediate in the first-order transformation into the monoclinic M2 phase. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J17.00010: Discrepancy of structural and electronic transitions in the vicinity of the Metal-Insulator-transition in V$_{2}$O$_{3}$ Hyun-Tak Kim, Jun-Hwan Shin, Jung-Young Choi, Bong-Jun Kim Vanadium sesquioxide (V$_{2}$O$_{3})$, representative of strongly correlated electronic system, has been known as undergoing the MIT (Metal-Insulator-Transition) which is between rhombohedral paramagnetic metallic phase and monoclinic antiferromagnetic insulating phase near the transition temperature, (T$_{c})$ \textbf{$\approx $}150 K. In order to reveal a relation between electronic and structural atomic transition, we has measured the temperature dependence of DC conductivity and structural crystallographic characterization with various temperatures from 90 K to 300 K by using low-temperature X-Ray diffraction (LTXRD). The obtained results show a discrepancy of structural and electronic transitions. This discrepancy can be explained by forming of the metallic puddles whose the size and number increased by nucleation and percolation[1,2] during the electronic transition progress from 120 K to 180 K. The puddles have an insulating monoclinic structure before the structural phase transition at $\sim $185 K. These metallic puddles are induced by the MIT not related to the SPT (structure phase transition). (1. M. M. Qazilbash et al., Science 318, 1750 (2007); 2. B. J. Kim et al., Phys. Rev. B 77, 235401(2008)) [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J17.00011: Interplay of chemical pressure and spin degrees of freedom on the magnetic properties of the $A$Ag$_{2}M$[VO$_{4}$]$_{2}$ type of compounds Angela M\"{o}ller, Ngozi Amuneke, Phillip Daniel, Dana Gheorghe, Bernd Lorenz A series of layered compounds of the $A$Ag$_{2}M$[VO$_{4}$]$_{2}$ type of structures, featuring the magnetic ions on a triangular lattice, have been synthesized by solid state reactions. Studies on the interplay of i) the chemical pressure induced by the differences in cation sizes ($A^{II}$ = Sr, Ba) and ii) the spin system ($M^{II}$ = Mn, Co, Ni, Cu) provide further insights into the structure-properties relationships which have been investigated by specific heat and magnetization measurements. Furthermore, spectroscopic methods have been employed to evaluate vibrational and electronic structural aspects in detail. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J17.00012: Trimer Formation and Metal-Insulator Transition in Triangular-Lattice Systems LiV{\it{X}}$_2$ ({\it{X}}=O,Se,S) Junki Yoshitake, Yukitoshi Motome Geometrically-frustrated systems sometimes lift their degeneracy by spontaneous formation of multisite clusters via coupling to other degrees of freedom. A fascinating example is found in a triangular-lattice system LiVO$_2$, which exhibits a three-site trimer formation. The origin was argued to be an orbital ordering under strong electronic correlation [1], however, recent experiments on a series of LiV{\it{X}}$_2$ ({\it{X}}=O,Se,S) suggest that the system is intermediately correlated and rather close to a metal-insulator transition [2]. In this contribution,we revisit this problem within a multiorbital Hubbard model in a wide range of Coulomb interaction by strong-coupling perturbation and Hartree-Fock approximation. We find a new trimer state under substantial trigonal crystal-field splitting; it is located in the vicinity of a metal-insulator transition and not adiabatically connected to the trimer state previously proposed. We discuss the origin of this new trimerization and the relation to experimental results.\\[4pt] [1] H. F. Pen {\it et al}., Phys. Rev. Lett. {\bf 78}, 1323 (1997).\\[0pt] [2] N. Katayama {\it et al}., Phys. Rev. Lett. {\bf 103}, 146405 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J17.00013: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J17.00014: Electronic Excitations in Vanadates William Thornton, Anton Kozhevnikov, Adolfo Eguiluz Vanadates represent an electronic analogue to the cuprates, which have one hole in the 3d shell. There are many realizations of the vanadates, as the solid-state chemistry of the V-O system allows various coordination numbers. Here we probe the electronic structure of vanadates by studying neutral electronic excitations computed within a time-dependent density functional theory framework. We evaluate the dynamical electronic response in both a Bloch basis and a Wannier basis, for both large momentum transfers, and in the optical limit. We compare our results with available experimental data, and assess the importance of many-body (excitonic) interactions. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J17.00015: A microscopic view on the Mott transition in chromium-doped V$_2$O$_3$ G. Sangiovanni, S. Lupi, L. Baldassarre, D. Nicoletti, M. Marsi, P. Hansmann, N. Parragh, A. Toschi, T. Saha-Dasgupta, O.-K. Andersen, K. Held V$_2$O$_3$ is the prototype system for the Mott transition, one of the most fundamental phenomena of electronic correlation. Temperature, doping or pressure induce a metal-to-insulator transition (MIT) between a paramagnetic metal (PM) and a paramagnetic insulator. This or related MITs have a high technological potential, among others, for intelligent windows and field effect transistors. However the spatial scale on which such transitions develop is not known in spite of their importance for research and applications. Here we unveil for the first time the MIT in Cr-doped V$_2$O$_3$ with submicron lateral resolution: with decreasing temperature, microscopic domains become metallic and coexist with an insulating background. This explains why the associated PM phase is actually a poor metal. The phase separation can be associated with a thermodynamic instability near the transition. This instability is reduced by pressure, that promotes a genuine Mott transition to an eventually homogeneous metallic state. \\[4pt] Nature Communications {\bf 1}, 105 (2010) doi: 10.1038/ncomms1109 [Preview Abstract] |
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