Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G32: Focus Session: Orbital/Charge Order in Complex Oxides |
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Sponsoring Units: DMP GMAG Chair: Ray Osborn, Argonne National Laboratory Room: Baltimore Convention Center 329 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G32.00001: Raman scattering studies of field- and temperature-dependent melting of charge order in La$_{1-x}$Ca$_{x}$MnO$_{3}$ Minjung Kim, Harini Barath, S.L. Cooper, M. Ruebhausen, S.W. Cheong The La$_{1-x}$Ca$_{x}$MnO$_{3}$ is one of the well-known perovskite manganites that display charge ordering behavior below a charge ordering temperature T$_{co}$ for x $\ge $ 0.5. In particular, the strongest ordering is shown at commensurate doping x=0.5 with the transition temperature, T$_{CO}$=160K. We investigate the field-induced melting of charge ordering in this system using field-dependent Raman scattering at different temperatures. By monitoring the field- and doping-dependence of excitations specific to the charge-ordered state, we are able to sensitively monitor the breakdown of both long-range and local charge order, and to carefully examine the stability of the charge ordered state to both doping (x) and applied magnetic field. [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G32.00002: Theoretical study of charge, spin, and orbital order in half-doped $La_{0.5}Ca_{0.5}MnO_3$. Dmitri Volja, Weiguo Yin, Wei Ku Deep microscopic insights into the half-doped $La_{0.5}Ca_{0.5}MnO_3$ are obtained with a novel construction of many-body picture derived from LDA+U and Wannier state analysis. Experimentally observed zig-zag orbital order and CE-type magnetic structure is found to have only very weak charge disproportionation. The apparent contradiction between well-defined bridge (3+) and corner (4+) Mn sites and the weak charge disproportionation is resolved naturally with occupied Wannier states centered at 3+ sites extending to 4+ and O-sites (reflecting non-negligible charge-transfer nature of the system.) Contrary to current lore, electron-electron interaction is found crucial to the long-range order in ordering the orbital and the spin. Specific experimental means of verification (e.g.: measurement of pseudospin direction) are suggested. [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G32.00003: Charge versus orbital-occupancy ordering in manganites Weidong Luo, Maria Varela, Jing Tao, Stephen J. Pennycook, Sokrates T. Pantelides It is generally assumed that density-functional theory (DFT) in the local-spin-density approximation (LSDA) or the generalized- gradient approximation (GGA) is not adequate to describe mixed- valence manganites. Here we report benchmark DFT/GGA calculations for the ground-state structural, electronic and magnetic properties for both undoped and doped CaMnO$_3$ and find the results to be in excellent agreement with available data, including new atomic-resolution Z-contrast imaging and electron-energy loss spectra. More specifically, we found that the DFT results predict two inequivalent Mn atoms in both 0.33 and 0.5 electron-doped CaMnO$_3$, in agreement with experimental evidence of Mn$^{+3}$/Mn$^{+4}$ oxidation state ordering. The inequivalent Mn atoms are marked by their distinctive orbital occupancies, dissimilar local Jahn-Teller distortion and different magnetic moments from DFT calculations. We also show that the spherically integrated charges associated with the two inequivalent Mn atoms are the same, and they are actually the same as in the Mn metal. This charge neutrality with different orbital occupancies is the result of self-consistency and atomic relaxations in the crystal. We conclude that DFT without additional correlations can account for the observed properties of oxidation-state ordering in this system. The impact of the results on other mixed-valence systems will be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G32.00004: Orbital and oxygen vacancy ordering in La$_{1-x}$Sr$_{x}$MnO$_{3-\delta }$ (x$\ge $0.8, $\delta \ge $0.15) Leopoldo Suescun, Bogdan Dabrowski, James Mais, Stanislaw Kolesnik, James Richardson, James Jorgensen We have developed synthesis method, obtained, and studied the highly Sr substituted LSM manganites. New oxygen and orbital ordered perovskite type phase SrMnO$_{2.6}$ (nominally Sr$_{5}$Mn$_{5}$O$_{13})$ was found tetragonal P4/m, a=8.61328(18) and c=3.80997(11) {\AA}. Mn ion shows charge separation to Mn$^{3+}$ in a pyramidal and Mn$^{4+}$ in a regular octahedral coordination. Nonstoichiometry has been observed with oxygen compositions between 0.25$<\delta <$0.4; upon oxidation a monoclinic distortion is observed. Analogous tetragonal and monoclinic phases have been found for La$_{1-x}$Sr$_{x}$MnO$_{3-\delta }$ (0.3$<\delta <$0.5) with x=0.05 and 0.2, respectively. In-situ neutron powder diffraction as well as susceptibility measurements show different magnetic arrangements below RT depending on x and $\delta $. This work was supported by the U.S. Department of Transportation and NSF-DMR-0302617. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G32.00005: A realistic approach to effective Hamiltonians for strongly correlated electron materials: Study of orbital ordering in LaMnO$_3$ Wei-Guo Yin, Dmitri Volja, Wei Ku We present a general scheme to the realistic derivation of many- body effective Hamiltonians, $H^{\mathrm{eff}}$, for strongly correlated electron systems: Based on a novel Wannier state analysis of the LDA+$U$ electronic structure, relevant mechanisms can be clearly singled out and their strengths can be accurately determined by mapping $H^{\mathrm{eff}}$ to the low-energy LDA+$U$ Hamiltonian within self-consistent Hartree-Fock mean-field theory [1]. Applying this scheme to LaMnO$_3$, the parent compound of colossal magnetoresistance manganites, we have quantified the relative importance of the effective electron-electron interaction ($\sim 1.7$ eV) and the Jahn-Teller splitting ($\sim 0.9$ eV) in ordering orbitals in LaMnO$_3$. We find that beyond the conventional Jahn-Teller picture, the electron-lattice (electron-electron) interaction alone is insufficient (sufficient) to stabilize the orbital ordering. Furthermore, our analysis indicates certain competition between different mechanisms, allowing direct experimental determination of their relative strengths. [1] W.-G. Yin, D. Volja, and W. Ku, cond-mat/0509075. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G32.00006: In-plane anisotropy of the electronic structure for the charge/orbital ordered state in half-doped Mn-oxide Y.S. Lee, S. Onoda, Y. Tokunaga, J.P. He, Y. Kaneko, T. Arima, N. Nagaosa, Y. Tokura We have investigated the in-plane anisotropy of the electronic response for the charge/orbital ordered phase in a half-doped Mn-oxide. Compared with the theoretical calculation the optical measurement with a single domain of Eu$_ {1/2}$Ca$_{3/2}$MnO$_{4}$ reveals that the optical conductivity along the chain direction exhibits smaller optical gap and lower energy distribution of the spectral weight than along the stripe (interchain) direction. It is suggested that the electronic anisotropy reported here is attributed to the quasi-one-dimensional electron hopping which is subject to the zigzag chain-type $e_{g}$ orbital ordering. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G32.00007: Magnetic Excitations in the CE-type manganite Pr$_{0.5}$Ca$_{1.5}$MnO$_{4}$ J. A. Fernandez-Baca, Feng Ye, Songxue Chi, Pengcheng Dai, J. W. Lynn, R. Mathieu, Y. Tokura Charge and orbital ordering (CO-OO) in doped manganites can control bulk and magnetic properties of the material. Canonical examples of the CO-OO compounds are 3D R(1-x)A(x)MnO3 and 2D R(1-x)A(1+x)MnO4 manganites near x=0.5. The magnetic ions in those systems are usually arranged in a checkerboard order, where spins of Mn3+ and Mn4+ form zigzap ferromagnetic chains coupling antiferromagnetically. Recent theoretical calculations and experimental observations suggest that the stabilization of CO-OO state in CE-type magnetic structure might be magnetic in origin. In this talk we will present our recent results on the study of the spin-wave excitations in the layered compound Pr0.5Ca1.5MnO4 with CE-type magnetic structure using inelastic neutron scattering. ORNL is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725. The work at UT was supported by the U.S. NSF DMR-0453804 and DOE DE-FG02-05ER46202. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G32.00008: How Cation-Covalency controls Electron Localization in t2g Perovskites Ole Krogh Andersen, Atsushi Yamasaki, Eva Pavarini In the series of perovskites, SrVO3-CaVO3-LaTiO3-YTiO3, the transition-metal d electron becomes increasingly localized and undergoes a Mott transition. By defining a low-energy Hubbard Hamiltonian in the basis of NMTO Wannier functions for the t2g LDA band and solving it in the single-site dynamical mean-field approximation, it was recently shown [1] that simultaneously with the Mott transition, the orbital fluctuations are strongly suppressed. Here, we explain the underlying chemical mechanisms [2]: Proceeding through the series, covalency between the empty large-cation-d states and the occupied oxygen-p states causes the oxygen octahedra to rotate and tilt. Moreover, since those oxygen-p orbitals which bond to the cations are the same as those which bond to the transition-metal t2g orbitals, the t2g Wannier functions exhibit residual covalency. This splits the t2g levels and reduces the width of the lowest subband. Cation covalency thus causes the electron to localize in LaTiO3 and YTiO3. [1] E. Pavarini, S. Biermann, A. Poteryaev, A.I. Lichtenstein, A. Georges, O.K. Andersen, Phys. Rev. Lett. 92 (2004) 176403. [2] E. Pavarini, A. Yamasaki, J. Nuss, O.K. Andersen, New Journal of Physics 7 (2005) 188 [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G32.00009: Resonant soft x-ray diffraction from Fe$_{3}$O$_{4}$/MgO below the Verwey transition C. Schuessler-Langeheine, J. Schlappa, C. F. Chang, H. Ott, Z. Hu, L. H. Tjeng, E. Schierle, E. Weschke, G. Kaindl, A. Tanaka The electronic structure of the low-temperature phase of magnetite (Fe$_{3}$O$_{4}$) has been studied for more than 60 years. In particular the question whether or to which extend the low-temperature phase is characterized by a charge order on the octahedral Fe-sites is controversially discussed. We studied this system using resonant soft x-ray diffraction at the Fe $L_ {2,3}$ resonance, a technique particularly sensitive to spatial modulations of the electronic state. The resonance behaviour of the (001) peak, which is characteristic for the low temperature phase, is distinctly different from that of the also studied (001/2) diffraction peak; we assign the (001) peak to charge order with a modulation of about 16 percent of a unit charge, while the (001/2) peak is caused by a different order involving only Fe$^{2+}$ sites in octahedral environment. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:24AM |
G32.00010: Electronically smectic-like phase in a nearly half-doped manganite Invited Speaker: Charge/orbital ordering is one of the most important of phenomena in doped manganites [1-3]. In this talk, I am going to present our recent neutron scattering results on the charge/orbital ordering (CO-OO) and spin ordering in the nearly half-doped perovskite manganite Pr$_{0.55}$(Ca$_{0.8}$Sr$_{0.2})_{0.45}$MnO$_{3}$ (PCSMO) [4]. On cooling from room temperature, PCSMO first enters into a CO-OO state below T$_{CO}$ and then becomes a CE-type long-range ordered antiferromagnet below TN. At temperatures above TN but below T$_{CO}$ (T$_{N} \quad <$ T $<$ T$_{CO})$, the spins in PCSMO form highly anisotropic smectic liquid-crystal-like texture with ferromagnetic (FM) quasi-long-range ordered one-dimensional zigzag chains weakly coupled antiferromagnetically. Such a magnetic smectic-like phase results directly from the spin-orbit interaction and demonstrates the presence of textured `electronic soft' phases in doped Mott insulators [5,6]. This work was supported by the DOE No.DE-AC05-00OR22725 with UT/Battelle, LLC and U. S. NSF DMR-0139882. This work was performed under the US-Japan Cooperative Program on Neutron Scattering. \newline \newline [1] J. van den Brink et al., Phys. Rev. Lett. 83, 5118 (1999). \newline [2] I. V. Solovyev and K. Terakura, Phys. Rev. Lett. 83, 2825 (1999). \newline [3] I. V. Solovyev, Phys. Rev. Lett. 91, 177201 (2003). \newline [4] F. Ye et al., Phys, Rev. B. (in press) \newline [5] G. C. Milward et al., Nature 433, 607 (2005). \newline [6] S. A. Kivelson et al., Nature 393, 550 (1998). [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G32.00011: Spin moment over 10-300 K and the delocalization of magnetic electrons above the Verwey transition in magnetite P. A. Montano, Yinwan Li, B. Barbiellini, P. E. Mijnarends, S. Kaprzyk, A. Bansil In order to probe the magnetic ground state, we have carried out temperature dependent magnetic Compton scattering experiments on an oriented single crystal of magnetite (Fe$_3$O$_4$), together with the corresponding first-principles band theory computations to gain insight into the measurements. An accurate value of the magnetic moment $\mu_S$ associated with unpaired spins is obtained directly for the first time over the temperature range of 10-300K. $\mu_S$ is found to be non integer and to display an anomalous behavior with the direction of the external magnetic field near the Verwey transition. The anisotropy of the magnetic Compton profiles shows a dramatic jump through the Verwey temperature $T_v$ and indicates that magnetic electrons in the ground state of magnetite become delocalized above $T_v$. Work supported by the USDOE. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G32.00012: Dimensionality switching in electric conduction in FeS Tomohiro Takayama, Kohshi Takenaka, Hidenori Takagi Orbital degree of freedom plays an essential role in the properties of strongly correlated systems, and will be a key function in the next generation. The orbital state, namely, shape of electron cloud, can affect some macroscopic properties such as transport properties. We have indeed observed a dimensionality change in electric conduction in stoichiometric iron sulfide, FeS, as a consequence of change in orbital states. FeS crystallizes in a NiAs-based troilite structure and is an antiferromagnetic semiconductor below $T_{\rm N}$ = 600 K. At $T_{s} \sim$ 400 K, spin-axis transition, the rotation of antiferromagnetically ordered spins of iron atoms, occurs; the spins are directed along $c$-axis at low temperatures, and they rotate and lie in $c$-plane at $T_{s}$. This transition involves a change in orbital states of ${\rm Fe^{2+}}$ ions. The outermost electron of ${\rm Fe^{2+}}$ ions spreads in basal-plane below $T_{s}$, while it elongates in $c$-direction above $T_{s}$. This change strongly couples onto the electric conduction and induces its dimensionality change from two-dimensional below Ts to three-dimensional above $T_{s}$. We will discuss this change in orbital states and its appearance in electric conduction. [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G32.00013: First-order phase transition induced by disorder in a model for manganites Eduardo Castro, Jo{\~a}o Santos The role of disorder in the physics of manganese oxides has been unveiled by a series of experiments in half-doped ordered and disordered manganites. For the ordered family a multicritical behaviour was found in the temperature vs bandwidth phase diagram, where a charge-order insulator (COI) competes with a ferromagnetic metal (FM). Chemical disorder induces a strong suppression of the transition to the COI state, effectively turning an insulator into a FM and enhancing the colossal magnetoresistive effect above the Curie temperature. Recently, a one orbital double-exchange model with cooperative phonons and quenched disorder was proposed to explain this metal-insulator transition induced by disorder. Treating the electronic degrees of freedom exactly, and the local spins and lattice distortions classically, in a variational mean-field approach, we have investigated the thermodynamic behaviour of the model in 2D systems larger by one order of magnitude than in previous works. Not only the suppression of the COI state by disorder was obtained, but we have also found that the second-order charge-order phase transition turns into first-order in the presence of intermediate disorder. This picture is consistent with experiments: first-order phase transitions are known to be present in half-doped disordered manganites which show the spectacular phenomena of the collapse of a charge-ordered state under a magnetic field. [Preview Abstract] |
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