Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session V23: Focus Session: Charge/Orbital Ordering in Complex Oxides |
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Sponsoring Units: DMP GMAG Chair: Jeff Lynn, National Institute of Standards and Technology Room: Morial Convention Center 215 |
Thursday, March 13, 2008 11:15AM - 11:27AM |
V23.00001: Evolution of the `Orbital Peierls State' with doping C. Ulrich, G. Khaliullin, B. Keimer, M. Reehuis, K. Schmalzl, A. Ivanov, K. Hradil, J. Fujioka, Y. Tokura Orbital degrees of freedom play an important role in the physics of strongly correlated electron systems. Our extensive investigation of insulating vanadates by neutron scattering has led to the discovery of an unusual magnetic ground state. YVO$_3$ exhibits two magnetic phases, a C-type phase between 116 K and 77 K and a G-type phase below 77 K. While the magnetic properties of the G-type phase are in accordance with standard theories, the C-type phase shows highly unusual static and dynamic spin correlations. Based on the idea of orbital fluctuations we were able to identify this phase as a theoretically predicted 'orbital Peierls state' [1]. Neutron scattering experiments on Y$_{1-x}$Ca$_x$VO$_3$ show that the C-type phase, i.e. the `orbital Peierls phase', is stabilized upon doping, while the orbitally ordered G-type phase is quite unstable and disappears at x = 2 \%. Furthermore, with doping this phase also exhibits a highly unusual spin wave dispersion. These leads us to the conclusions, that the 'orbital Peierls state' becomes more robust with Ca-doping, whereas the formerly well defined G-type phase exhibits a more complex behaviour, probably as a consequence of an increase in orbital fluctuations. [1] C. Ulrich et al., PRL {\bf 91}, 257202 (2003). [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V23.00002: Magnetic interactions and orbital physics in RVO$_{3}$ perovskites J.-Q. Yan, S. Chang, C. Brown, M. Hehlen, F. Trouw, R.J. McQueeney We have performed inelastic neutron scattering study on high quality YVO3 and LaVO3 powders. The magnetic interactions determined from the scattering spectra for YVO3 agree with a previous single crystal study. [1] For LaVO3, a --Jab $>$ Jc is in sharp contrast to the Jc $>$ -Jab in the C-type magnetically (C-AF) ordered state of YVO3. The mechanism that greatly suppresses Jab in C-AF state of YVO3 will be discussed together with thermal conductivity [2] and Raman spectroscopy [3] results. \newline [1] C. Ulrich, et al., Phys. Rev. Lett. \textbf{91}, 257202 (2003). \newline [2] J.-Q. Yan, et al., Phys. Rev. Lett. \textbf{93}, 235901 (2004). \newline [3] S. Miyasaka, et al., Phys. Rev. B \textbf{73}, 224436 (2006). [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V23.00003: Pressure-temperature phase diagram for orbital and spin states in $R$VO$_{3}(R$=Y,Tb) Daisuke Bizen, Keisuke Nakatsuka, Tetsuya Murata, Hironori Nakao, Kazuaki Iwasa, Youichi Murakami, Toyotaka Osakabe, Shigeki Miyasaka, Yoshinori Tokura Perovskite-type vanadium oxides $R$VO$_{3}$ ($R$=Y, La-Lu) show various physical properties coupled with the orbital and spin states. Orbitally ordered states of V 3$d^{2}$ in YVO$_{3}$ have been systematically investigated by X-ray scattering technique under high-pressure and low-temperature (HP-LT). The pressure-temperature phase diagram for the orbital state was clearly determined from the crystal parameters, i.e. the lattice constants and the reflection conditions. It indicates that the $C$-type orbital ordering ($C$-OO) is stabilized as compared with the $G$-type orbital ordering ($G$-OO) by applying hydrostatic pressure. Based on the result, we succeeded in controlling the ground state of 3$d$-orbital in TbVO$_{3}$ from $G$-OO to $C$-OO by applying pressure. The spin state coupled with the orbital was also studied by neutron scattering under HP-LT. It elucidated that the magnetic ground state changed from the $C$-type spin ordering to the $G$-type one. This result indicates the strong coupling between orbital and spin states. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V23.00004: Stabilization of Charge Ordering by Magnetic Exchange Invited Speaker: The magnetic exchange energies in charge ordered La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ (LSFO) and its parent compound LaFeO$_{3}$ (LFO) have been determined by inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic exchange between Fe$^{3+}$-Fe$^{5+}$ pairs ($J_{F})$ and antiferromagnetic exchange between Fe$^{3+}$-Fe$^{3+}$ pairs ($J_{AF})$ fulfills the criterion for charge ordering driven by magnetic interactions ($J_{F}/J_{AF} \quad >$ 1). The 30{\%} reduction of $J_{AF}$ as compared to LFO indicates that doped holes are delocalized, and charge ordering occurs without a dominant influence from Coulomb interactions. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V23.00005: Electronic Raman Scattering in Magnetite: Spin vs. Charge gap Lev Gasparov, G. Guntherodt, H. Berger We report Raman scattering data on single crystals of magnetite (Fe$_{3}$O$_{4})$ with the Verwey transition temperature of 123 and 117K. Both single crystals reveal broad electronic background extending up to 900 wavenumbers ($\sim $110 meV). Redistribution of this background is observed when the samples are cooled below the transition temperature. In particular, spectra of the low temperature phase show diminished background below 300 wavenumbers followed by an enhancement of the electronic background between 300 and 400 wavenumbers. $\Downarrow $ To enhance the effect of the background distribution we divide the spectra just below the transition by the spectra just above the transition. The resultant broad peak-like feature is centered at 368 $\pm $5 wavenumbers (45 meV). The peak position of this feature does not scale with the transition temperature. We discuss two alternative assignments of this feature to a spin or charge gap in magnetite. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V23.00006: Electrically-driven phase transition in magnetite nanostructures D. Natelson, S. Lee, A. Fursina, J.T. Mayo, C.T. Yavuz, V.L. Colvin, R.M.S. Sofin, I.V. Shvets In 1939 Verwey found that bulk magnetite undergoes a first-order transition at $T_{V} \approx$120~K from a high temperature conducting phase to a low-temperature insulating phase. High-$T$ conduction occurs via the fluctuating valences of the octahedral iron atoms, and the transition comes from the interplay of charge ordering and structural distortion upon cooling. The Verwey transition mechanism and charge ordering remain highly controversial. We will present data on magnetite nanocrystals and single-crystal thin films demonstrating an electrically driven phase transition below the Verwey temperature. We find sharp conductance switching that is hysteretic in source-drain voltage, and show that this transition is not due to local heating, but instead is due to the breakdown of the correlated insulating state when driven out of equilibrium by electrical bias. Scaling of switching voltage with electrode spacing in thin film samples shows that the switching is driven by a critical temperature-dependent electric field. Further studies of this newly observed transition and its low-temperature conducting phase should shed light on how charge ordering and vibrational degrees of freedom determine the ground state of this important compound. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V23.00007: Pressure-Induced Intermetallic Valence Transition in BiNiO$_{3}$ Masaki Azuma, Masahiko Tsujimoto, Shintaro Ishiwata, Seiji Isoda, Yuichi Shimakawa, Mikio Takano, Sandra Carlsson, Jennifer Rodgers, J. Paul Attfield, Matthew G. Tucker The valence state change of BiNiO$_{3}$ perovskite under pressure has been investigated by a powder neutron diffraction study and electronic state calculations. At ambient pressure, BiNiO$_{3}$ has the unusual charge distribution Bi$^{3+}_{0.5}$Bi$^{5+}_{0.5}$Ni$^{2+}$O$_{3}$ with ordering of Bi$^{3+}$ and Bi$^{5+}$ charges on the A sites of a highly distorted perovskite structure. High pressure neutron diffraction measurements and Bond valence sum calculations show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high pressure phase is metallic Bi$^{3+}$Ni$^{3+}$O$_{3}$. This unprecedented charge transfer between A and B site cations coupled to electronic instabilities at both sites leads to a variety of ground states, and it is predicted that a Ni-charge disproportionated state should also be observable. \newline [1] M. Azuma \textit{et al}.\textit{, J. Am. Chem. Soc.}, \textbf{129}, (2007) 14433. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V23.00008: Phonon Anomaly across Charge/Orbital Ordering Transition in Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$. Jiandi Zhang, Hao Sha, F. Ye, M.D. Lumsden, P.C. Dai, J. A. Fernandez-Baca, Y. Tomioka, Y. Tokura The lattice dynamics, especially the Jahn-Teller active optical phonon modes across the charge/orbital ordering (CO/OO) transition in the single crystal Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$ has been investigated using inelastic neutron scattering (INS) technique. Three phonon peaks (around 36, 58 and 74 meV near the Brillouin zone center) appear in the scattering spectra, which are associated with the bond-stretching and bond-breathing modes of MnO$_{6}$. Both intensity and phonon energies show changes when the system undergoes the CO/OO transition indicating a strong coupling between the lattice and orbital degrees of freedom. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V23.00009: Charge Ordering in Half-Doped Manganites: Small Charge Disproportion and Leading Mechanisms Dmitri Volja, Wei-Guo Yin, Wei Ku The apparent contradiction between the recently observed weak charge disproportion and the traditional Mn$^{3+}$/Mn$^{4+}$ picture of the charge-orbital orders in half-doped manganites is resolved by a novel Wannier states analysis of the LDA$+U$ electronic structure. Strong electron itinerancy in this charge-transfer system significantly delocalizes the occupied low-energy ``Mn$^{3+}$'' Wannier states such that charge leaks into the Mn$^{4+}$-sites. Moreover, this feature is found to be generic in doped manganites. Based on a realistic effective Hamiltonian derived from first-principles calculations, we further quantify the leading mechanisms of the charge-orbital orders and find that both electron-lattice and electron-electron interactions are essential. \hspace{0.2 cm} {\it Preprint} arXiv:0704.1834. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V23.00010: Orbital ordered phases of Mn$_3$O$_4$ and MnV$_2$O$_4$ investigated by NMR Jeong Hyun Shim, Soonchil Lee, Takuro Katsufuji Compared to spin and charge, investigating orbital state is relatively difficult because it requires higher sensitivity of microscopic experimental tools. In the present study, we have demonstrated that the orbital state of Mn$_3$O$_4$ can be determined by a field-angle resolved NMR technique and the technique also applied to the investigation of MnV$_2$O$_4$ that was reported to have a strong orbital influence to spin and lattice.[1] The orbital ordered phase of MnV$_2$O$_4$ has been controversial, because lattice symmetry alone can not determine the orbital states of V$^{3+}$ ions. From our NMR results, we found that the orbital configuration of MnV$_2$O$_4 $ appears to support the Motome's antiferro-type model with an unexpected tiling of orbital direction within a-b plane, or another explanation is a phase coexistence of antiferro-type and ferro-type orbital ordered domains. \newline [1] T. Suzuki, et al., Phys. Rev Lett. 98, 127203 (2007). [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V23.00011: Orbital Order and Metal-Insulator Transition in PbRuO$_{3}$ Simon Kimber, Jennifer Rodgers, J. Paul Attfield, Dimitri Argyriou We have prepared the previously uncharacterised perovskite, PbRuO$_{3}$, using a high P/T synthesis technique (10 GPa, 1000 \r{ }C) and performed synchrotron powder x-ray diffraction (ID31, ESRF), powder neutron diffraction (GEM, ISIS) and physical propety measurements. PbRuO$_{3}$ undergoes a metal insulator transition at $\sim $ 90 K at which the resistivity jumps by four orders of magnitude. The susceptibility of PbRuO$_{3}$ shows a paramagnetic-paramagnetic anomaly at 90 K, and at lower temperatures, a broad maximum. At RT, the diffraction profile of PbRuO$_{3}$ is well fitted by a distorted perovskite structure in the space group \textit{Pnma}. On cooling through $\sim $ 90 K, PbRuO$_{3}$ undergoes a structural transition, the low temperature structure is well fitted in the \textit{Imma} space group. The \textit{Imma} phase shows layered Ru$^{4+}$ orbital order, we speculate that this dimensional reduction results in the broad maximum seen in the magnetic susceptibility measurements. The structural transition is first order and phase separation is seen at 75 K. The possible role of Pb -- O covalency in inducing orbital order will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V23.00012: Incommensurate Magnetic Structure of ZnCr$_{2}$Se$_{4}$ and ZnCr$_{2}$S$_{4}$ Fabiano Yokaichiya, Heloisa Nunes Bordallo, Dimitri Argyriou, A Krimmel, A Loidl, V Tsurkan Recent studies of chalcogenide chromium spinels have shown a coupling between ferroelectricity and magnetism. The motivation of this work is to determine the magnetic ground state, (including its symmetry properties), to comprehend the coupling of magnetic and ferroelectric order parameters in the spinels ZnCr$_{2}$Se$_{4}$ and ZnCr$_{2}$S$_{4}$. The incommensurate magnetic structures through the N\'{e}el transition in these systems have been studied by high-resolution powder neutron diffraction. Below T$_{N}$ ($\sim $22K), for both cases, the magnetic structure is described as ferromagnetic layers in the \textbf{\textit{ab}}-plane stacked in a spiral arrangement along the \textbf{c}-axis with a propagation vector \textbf{k} = (0,0,$\sim $0.46). In ZnCr$_{2}$Se$_{4}$ and ZnCr$_{2}$S$_{4, }$ the magnetic phase transition is of first order. Therefore to use the irreducible co-representation theory, for symmetry analysis, the magnetic phase is described by a linear combination of irreducible representations. In this talk we present results of Rietveld analysis on the magnetic and crystal structure through the magnetic transition. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V23.00013: ABSTRACT WITHDRAWN |
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