Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D2: Spin, Charge and Orbital Excitations in Manganites |
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Sponsoring Units: DCMP Chair: H. Dennis Drew, University of Maryland Room: LACC 151 |
Monday, March 21, 2005 2:30PM - 3:06PM |
D2.00001: \emph{d-d} electronic excitations in manganites by resonant inelastic x-ray scattering. Invited Speaker: We report a study of the electronic excitations in a range of magnetic and electronic ground states in manganites utilizing Mn \emph{K}-edge resonant inelastic x-ray scattering. The spectra were measured between 1.5 and 16 eV of energy loss, at finite \emph{q}. Excitations with energy as high as 10 eV exhibits a temperature dependence which is systematicaly related to the magnetism. On the basis of LDA+U, we calculated the electronic structure, the Wannier functions of the highest occupied and lowest unoccupied states, and finite-\emph{q} response functions. The finite-\emph{q} dynamical structure factor reproduce the main features and the magnetic dependence of the spectra. We identify the temperature dependent excitations as the intersite electron transfers between spin-polarized states. These excitations are enhanced (reduced) for ferro-magnetic (anti-) Mn neighbors. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:42PM |
D2.00002: Orbital excitations in LaMnO$_{3}$ Invited Speaker: We have investigated the excitation spectrum of the orbital ordered LaMnO$_{3}$ by resonance Raman scattering and by spectral ellipsometry. By resonance Raman scattering for incident photon energies between 1.8 eV and 5.0 eV we probe the orbital exciton and its coupling to the lattice degrees of freedom. From the variation of the intensity of the first and second order scattering by the Jahn-Teller phonons as a function of incident photon energy, we derive the so called resonance profile and the corresponding exitonic states. In comparison with the dielectric function we show that the resonant behavior of the Jahn-Teller phonon cross section corresponds to the absorption due to the gap in LaMnO$_{3}$ at 2 eV and to charge transfer processes at 4.4 eV. We assign the 2 eV resonance behavior to the orbital exciton in contrast to lower energy-scales which have been estimated for the orbital excitation previously (E. Saitoh et al. Nature 410, 180 (2001)). The strong second order scattering of the phonons can be understood by theoretical calculations based on the Franck-Condon mechanism activating multi-phonon Raman scattering in first order of the electron-phonon coupling, strongly outlining the importance of the electron-phonon interaction. However, the estimated width of the resonance is too broad compared to the experimental results, possibly indicating other contributions such as electronic correlations within a Mott-Hubbard type picture as has been recently suggested (N.N. Kovaleva et al. Phys. Rev. Lett. 93, 147204 (2004)). \\\\In collaboration with R. Rauer, B. Schulz, S. Naler, D. Budelmann, J. Baeckstroem, M. Ruebhausen, Institut fuer Angewandte Physik, Universitaet Hamburg, Germany, K.H. Kim, School of Physics and CSCMR, Seoul National University, Seoul 151-747, S. Korea, S-W.Cheong, Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA, V. Perebeinos, Brookhaven National Laboratory, Upton, NY 11973, USA. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 4:18PM |
D2.00003: STM probe of charge ordering and 2D carrier confinement in mixed valence manganites Invited Speaker: One property common to most layered transition metal perovskite oxides is their highly anisotropic transport properties. The in-plane resistivity is metallic, whereas electron transport perpendicular to the layers is more characteristic of a semiconductor. We present vacuum tunneling spectroscopy on cleaved La$_{1.4}$Sr$_{1.6}$Mn$_{2}$O$_{7}$ single crystals using a scanning tunneling microscope (STM) in ultrahigh vacuum. We find a gap E$_{g}$ that is associated with the room temperature c-axis conductivity. Surprisingly, E$_{g}$ remains finite when the temperature is lowered through the metal-insulator transition at 90 Kelvin. This remarkable result suggests that the intrinsic electron transport between adjacent perovskite bilayers is thermally activated at all temperatures, with a characteristic activation energy E$_{g}$. We further find no evidence of electronic phase separation, as E$_{g}$ is spatially homogeneous at all temperatures. We shall discuss these spectroscopy data and atomically resolved STM micrographs in the context of colossal magnetoresistance (CMR), and their contribution to our understanding of CMR manganites. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:54PM |
D2.00004: Electronic soft matter in the manganites Invited Speaker: |
Monday, March 21, 2005 4:54PM - 5:30PM |
D2.00005: Structural insight of the charge-ordering phenomena in manganites Invited Speaker: Recent experiments using x-ray absorption spectroscopy (XAS) and x-ray resonant scattering (XRS) techniques show that the conventional description of the so-called charge ordering phases of manganites in terms of Mn$^{3+}$/Mn$^{4+}$ ionic ordering is far from reality. I present here the XRS study of the low temperature phase of Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ manganite. Strong resonances are observed in the energy dependent spectra of (300), (030) and (05/20) reflections. Their azimuthal and polarization dependencies are well explained by the anisotropy of the local geometrical structure. Two different Mn sites were found. One of them is surrounded by a tetragonal distorted oxygen octahedron, whereas the other site has a nearly regular octahedral environment. The charge separation between the intermediate valence states is less than 0.2 e-. The analysis performed resolves some of the apparent contradictions with previous XRS and XAS experiments in manganites. These results joined to those recently obtained on the Verwey transition in magnetite indicate that the electronic states in transition-metal oxides need to be described in terms of band states instead of localized ones. Colaborators: G. Sub\'{\i}as, J. Blasco, M. G. Proietti, M. S\'{a}nchez and J. Herrero-Martin [Preview Abstract] |
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