Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y13: Focus Session: Charge/Orbital Order in Complex Oxides |
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Sponsoring Units: DMP GMAG Chair: Igor Solovyev, National Institute for Materials Science Room: Colorado Convention Center Korbel 4C |
Friday, March 9, 2007 11:15AM - 11:27AM |
Y13.00001: Commensurate and incommensurate charge order in Fe$_2$OBO$_3$ Manuel Angst, Raphael Hermann, Jong-Woo Kim, Peter Khalifah, Brian C. Sales, David G. Mandrus Charge order CO in the form of a Wigner crystal had been proposed by Attfield {\em et al}. [Nature {\bf 396}, 655 (1998)] based on measurements, particularly M\"{o}ssbauer spectroscopy, on polycrystalline Fe$_2$OBO$_3$, but no superstructure due to the CO had been detected. We have grown the first single crystals of pure Fe$_2$OBO$_3$, and resistivity and thermal analysis indicate not one, but two transitions associated with CO. To elucidate the nature of these two transitions a synchrotron study was performed. At low $T$ a superstructure corresponding to a doubling of the $a$ axis was observed for the first time. The phase between the two phase transitions, in contrast, exhibits an incommensurate modulation with propagation vector ($\frac{1}{2}$,$0$,$\tau$), $\tau$ increasing with $T$ towards $\frac{1}{2}$. Resonances in the energy-dependence of the scattered intensity around the Fe $K$ edge suggest that the modulations in both phases are indeed associated with CO. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y13.00002: Optical study of CuIr2S4 and MgTi2O4: experimental support for orbital-Peierls transitions Nan Lin Wang, Jun Zhou, Guanghan Cao, Jing Shi In metals with low-dimensional electronic structure, Fermi surface instability can drive a system into a symmetry-breaking insulating state. However, such instability is not expected to develop in a three-dimensional (3D) system. Recently two types of highly exotic orderings were discovered by Radaelli et al. in two spinel compounds: an octamer ordering in CuIr2S4 and a helical ordering in MgTi2O4. In both cases, sharp metal-insulator transitions (MIT) and spin-dimerizations associated with the structural distortions occur simultaneously. It was suggested that the orbital degree of freedom plays a key role in such transition: the ordering of the orbitals makes the electrons travel exclusively along certain chains which effectively reduces the dimension from 3D to 1D, then is susceptible to Peierls instability at low T. Here we present optical measurement on spinels CuIr2S4 and MgTi2O4, and show that the spectral change across the MIT can be well understood from the picture of 1D Peierls transitions driven by the $d_{xy}$ orbital ordering in CuIr2S4, and the $d_{yz}$ and $d_{zx}$ orbital orderings in MgTi2O4, respectively, proposed by Khomskii et al. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y13.00003: Orbital ordering in spinel AV$_{2}$O$_{4}$ (A=Mn,Fe) T. Katsufuji, T. Suzuki, M. Katsumura, H. Takei, K. Taniguchi, T. Arima In spinel vanadates, AV$_{2}$O$_{4}$ (A:divalent ion), there is an orbital degree of freedom in the V$^{3+}$ ion, which contains two $d$ electrons in the triply degenerate $t_{2g}$ orbital. We measured the x-ray diffraction of the single crystal of MnV$_{2}$O$_{4}$, which exhibits simultaneous ferrimagnetic and a structural phase transition at 57 K, and found that a diamond-glide symmetry is broken in the ferrimagnetic tetragonal phase in this compound, indicating the antiferro-type ordering of the V $t_{2g}$ orbitals. This orbital ordering can be explained by the enhancement of the antiferro-orbital interaction caused by the ferromagnetic alignment of the V spins in the ferrimagnetic phase. We also measured the x-ray diffraction of the FeV$_{2}$O$_{4}$ single crystal, and found that a different type of orbital ordering from that of MnV$_{2}$O$_{4}$ occurs in this compound. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y13.00004: Magnetic fluctuations and orbital orderings in ferrimagnetic spinels Jae-Ho Chung, Jung Hwa Kim, Seung-Hun Lee, Taku S. Sato, Takuro Katsufuji We report our inelastic neutron scattering studies of two related spinels, AB$_2$O$_4$ (A = Mn, B = Mn and V), with e$_g$ and t$_{2g}$ orbital degeneracy, respectively. Both systems undergo noncollinear triangular ferrimagnetic orderings at low temperatures, where the lattice symmetries are tetragonal. Characteristics of the phase transitions, however, are different. In the case of Mn$_3$O$_4$, the tetragonal distortion with $c > a$ exists below T = 1443 K, and upon cooling a noncollinear ferrimagnetic ordering occurs at 42 K, followed by two more magnetic transitions into incommensurate (40 K) and commensurate cell doublings (34 K). Those magnetic phases exhibit magnetocapacitance. In the case of MnV$_2$O$_4$, on the other hand, a collinear ferrimagnetic ordering occurs at 65 K, followed by the tetragonal distortion with $c < a$ and a noncollinear ordering at 58 K. Our single crystal inelastic neutron scattering data show magnetic excitations up to 20 meV for Mn$_3$O$_4$ and up to 40 meV for MnV$_2$O$_4$. We have performed linear spin wave calculations to obtain their effective Hamiltonians by comparing the calculated dispersions of spin waves to the observed ones. The implications of the spin Hamiltonians to their orbital states, and the polarization of spin waves will be discussed. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:39PM |
Y13.00005: Electronic structure of complex oxides using DMFT Invited Speaker: |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y13.00006: Orbital ordering in striped phases of La$_{1-x}$Ca$_x$MnO$_3$: GGA+U versus GGA results Nadia Binggeli, Giancarlo Trimarchi We have investigated the orbital ordering in La$_{1-x}$Ca$_x$MnO$_3$, with x=2/3 and 1/2, within the generalized gradient approximation (GGA) of density functional theory and within the GGA plus onsite Coulomb interaction (GGA+U) approach. The calculations were performed using the pseudopotential plane-waves scheme and include structural relaxation. In the half-doped case, similar results are obtained for the orbital and atomic structure within the two approaches, with structural properties consistent with experiment. For the two-third-doped system, instead, the results differ between the two approaches. Only the GGA+U yields an insulating striped phase with structural properties in agreement with experiment. Our results show the importance of the cooperative effects of the Jahn-Teller distortion, strain modulation, and electronic localization in stabilizing the orbital-ordered striped phases, and provide new insight into the atomic-scale structure of the orbital order in La$_{1/3}$Ca$_{2/3}$MnO$_3$. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y13.00007: Direct evidence of charge inhomogeneity during the commensurate-incommensurate phase transition in charge ordered La$_{1-x}$Ca$_{x}$MnO$_{3}$ J. Tao, M. Varela, S. J. Pennycook, J. M. Zuo Commensurate-incommensurate (C-IC) phase transitions occurring in charge ordered (CO) manganites have been measured extensively by neutron scattering, x-ray diffraction and electron diffraction. However, these measurements only provide an average picture of the CO transition. Direct observations at the nano-scale are lacking, and the exact mechanism underlying the CO phase transitions remains unknown. Here, we report our \textit{in-situ} electron microscopic studies of the CO C-IC phase transition in La$_{0.33}$Ca$_{0.67}$MnO$_{3}$. Scanning an electron probe about 1.7 nm in size over the sample, the local CO structures are recorded in the electron nano-diffraction patterns and the super-reflections associated with the CO phase are used to study the local effective doping. The mapping of the CO phase clearly shows inhomogeneous patterns of phase separation during the phase transition. The C-IC phase transition is interpreted by the change of the distribution of electrons/holes in La$_{1-x}$Ca$_{x}$MnO$_{3}$. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y13.00008: Competing Ferromagnetic and Charge-Ordered States in Realistic Models for Manganites: the Origin of the CMR Effect Cengiz \c{S}en, Gonzalo Alvarez, Elbio Dagotto The one-orbital model for manganites with cooperative phonons has been investigated via large-scale Monte Carlo (MC) simulations. Focusing on electronic density $n=0.75$, a regime of competition between ferromagnetic (FM) metallic and charge-ordered (CO) insulating states was identified. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects were observed. These effects appear even in the clean limit, but only by fine tuning parameters, while adding quenched disorder makes the CMR effects robust. The CMR magnitude is much larger than recently reported when randomly distributed polarons form the competing insulator. The crucial role of the superexchange coupling $J_{\rm AF}$ is discussed. The appearance of CMR effects is shown to be associated with the development of correlations among polarons above the spin ordering temperatures, in agreement with early neutron scattering investigations. These polarons tend to form small regions resembling the charge and spin arrangement of the low-temperature CO insulating state. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y13.00009: Fe-doping-induced charge-orbital ordering in manganese oxides Hideaki Sakai, Kiminori Ito, Yoshinori Tokura We investigated Fe-doping effects on a ferromagnetic metallic crystal, (La$_{0.7}$Pr$_{0.3}$)$_{0.65}$Ca$_{0.35}$MnO$_3$, which locates near the phase boundary to the charge-orbital ordered insulator. It was found that the competing charge-orbital ordering correlation is induced by substituting a small amount of Fe atoms for Mn ones. Such a tendency nicely contrasts with the impurity-induced ferromagnetic metallic phase appearing in the charge-orbital ordered manganites, for example, Cr-doped Nd$_{0.5}$Ca$_{0.5} $MnO$_3$ as intensively investigated for past years. Furthermore, we observed glassy magnetotransport properties as well as diffuse insulator-metal transition, such as magnetic- field annealing effects and long-time relaxation, like a ``relaxor ferromagnet". [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y13.00010: Dirty Peierls transition to stripe phase in manganites Susan Cox, Jason Lashley, Edward Rosten, Anthony Williams, John Singleton, Peter Littlewood The nature of the phase transitions in La$_{0.48}$Ca$_{0.52} $MnO$_3$ and Pr$_{0.48}$Ca$_{0.52}$MnO$_3$ has been probed using heat capacity and magnetisation measurements. The phase transition associated with the onset of the stripe phase has been identified as a second order transition which can be very well fitted by the model of a Peierls transition in a disordered system (a `dirty' Peierls transition). We demonstrate that this model can also be applied to other systems (such as alpha- Uranium) in which Peierls transitions occur. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y13.00011: Weak charge disproportion and leading mechanisms in half-doped manganites. Dmitri Volja, Wei Ku, Wei-Guo Yin The puzzling very weak charge disproportionation found in half- doped manganites such as La$_{1/2}$Ca$_{1/2}$MnO$_3$ is reconciled with the well-accepted Mn$^{3+}$/Mn$^{4+}$ picture of charge and orbital orders via our novel first-principles Wannier function analysis. The strong electron itinerancy is found to delocalize the ``Mn$^{3+} $'' Wannier states significantly, producing remarkable charge leaking into the ``Mn$^{4+}$'' sites. Thus, it is necessary to distinguish for this charge-transfer system actual charge from the occupation number. Finally, a realistic low-energy effective Hamiltonian is derived, revealing the interesting role of electron-electron interactions in the charge and orbital channels, which can be applied to other doping regions including the CMR phase. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y13.00012: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y13.00013: Experimental and theoretical determination of the anisotropic anomalous scattering tensor at the Mn K edge in LaMnO$_{3}$ Joaquin Garcia, Gloria Subias, M.C. Sanchez, J. Herrero-Martin, Keisuke Hatada, C.R. Natoli, S. Di Matteo, C. Mazzoli, J. Blasco A resonant x-ray scattering (RXS) study of (h 0 0), (0 k 0) and (0 0 l) forbidden reflections (h, k, l odd) at the Mn K- edge of LaMnO$_{3 }$was performed between 10 and 300 K. We observed strong resonant peaks at the three reflections. The azimuth angle dependence of all these reflections showed a characteristic sine evolution of $\pi $-period. The energy dependence of the intensity for (h 0 0) and (0 k 0) reflections was identical while different structures were observed for (0 0 l) reflections. We did not observe any change either on the resonance intensity or on the line shape when crossing the N\'{e}el temperature T$_{N}\sim $ 140 K. The energy, azimuth angle and polarization dependences of the three reflections are originated by off-diagonal terms of the scattering tensor. The theoretical analysis show that the principal axes of the anomalous scattering tensor depends on the photon energy. Moreover, resonant scattering in LaMnO3 is not a probe of d-orbital ordering. [Preview Abstract] |
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