Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J41: Correlated Electrons: Manganites |
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Sponsoring Units: GMAG Chair: Amlan Biswas, University of Florida Room: LACC 150A |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J41.00001: Temperature-Dependent X-Ray Absorption Spectroscopy of Colossal Magnetoresistive Perovskites Norman Mannella, Axel Rosenhahn, Masamitsu Watanabe, Brian Sell, Akira Nambu, Staphanie Ritchey, Elke Arenholz, Antony Young, Yasuhide Tomioka, Charles Fadley We have measured the temperature dependence of the O K-edge pre-edge structure in the x-ray absorption spectra of the perovskites La$_{1-x}$A$_{x}$MnO$_{3}$, (A = Ca, Sr; x = 0.3, 0.4). Our measurements reveal a correlation between the disappearance of the splitting in the pre-edge region and the presence of Jahn-Teller distortions. The different magnitudes of the distortions for different compounds are proposed to explain some dissimilarity in the line shape of the spectra taken above the Curie temperature. [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J41.00002: Resonant x-ray diffraction study of orbital and magnetic order in half-doped manganites Jessica Thomas, John Hill, Stephane Grenier, Michel Van Veenendaal, Peter Abbamonte Resonant x-ray diffraction performed near a transition metal $L$ absorption edge is directly sensitive to the chemical and magnetic environment of the important 3d electrons in transition metal oxides. This technique can be used to directly probe and compare the orbital and magnetic correlations which form the ground state in half-doped, insulating manganites [1]. In this talk, we compare resonant diffraction measurements as a function of temperature, energy and scattering geometry for three half or near-half doped insulating manganites, $Pr_{1-x}Ca_{x}MnO_{3}$ ($x$ = 0.4 and 0.5) and $Nd_{1-x}Sr_{x}MnO_{3}$ ($x$ = 0.5). In all three samples, the orbital scattering exhibits a characteristic resonant (energy dependent) line- shape and dependence on azimuthal angle. However, subtle changes in the resonant line-shapes measured in different samples may indicate different interactions with the surrounding lattice. Interestingly, both x=0.4 and 0.5 $Pr_{1-x}Ca_{x}MnO_{3}$ samples show a smaller correlation length associated with orbital order compared to magnetic order. [1] K. J. Thomas et al, Phys. Rev. Lett. 92 237204 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J41.00003: Resonant Soft X-ray Scattering on Layer Manganite: Observation of Orbital Ordering Effect Yi-De Chuang, Antonio Nissen, Zahid Hussain, Dong Qian, M. Zahid Hasan, Tsuyoshi Kimura, Yoshinori Tokura, John Mitchell We have developed an endstation to perform resonant soft X-ray scattering on highly correlated electron systems. With this instrument, we are able to study the photon energy and temperature dependence of superlattice reflection originating from orbital ordering on bilayer manganite. The intensity of superlattice reflection closely correlates to the resistivity behavior across phase transition boundary, indicating the intimate relationship between them. The correlation length is consistent with results reported previously in hard X-ray studies. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J41.00004: Glassy response to gate and magnetic fields in ultrathin manganite films M. Eblen-Zayas, A. Bhattacharya, N.E. Staley, A.L. Kobrinskii, A.M. Goldman We have studied the low temperature behavior of thin films of La$_{0.8}$Ca$_{0.2}$MnO$_{3}$ in a field effect geometry. The films exhibit the usual insulator-metal transition near the Curie temperature, but below 30K there is a re-entrant insulating regime. In this regime, we observe glassy dynamics and a hierarchical response in the resistance to both electronic and magnetic perturbations. In addition, the magnetization of the film responds to electrostatic gating. We interpret these results in a framework where the system dynamics are governed by strain relaxation. This work was supported by the National Science Foundation under grant NSF/DMR-0138209 and the Univ. of Minnesota MRSEC (NSF/DMR-0212032). [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J41.00005: Resonant Higher Order Scattering in Double Perovskites Jakob Andreasson, Joakim Holmlund, Mikael K\"all, Lars B\"orjesson, Stefan Naler, Joakim B\"ackstr\"om, Mikael R\"ubhausen, Abul K. Azad, Sten Eriksson Resonant and non-resonant higher order Raman scattering in the double perovskites La$_{2-x}$Sr$_{x}$FeCrO$_{6-}$ (x = 0, 0.33, 0.66,1) and Ba$_{2}$(Sr$_{2})$FeWO$_{6}$ is investigated. The B-site disordered compound La$_{2}$FeCrO$_{6}$ displays an exceptional series of resonant higher order excitations for =514 nm (2.42 eV). This feature is attributed to defects, in the form of oxygen vacancies, resulting in a localized resonant electron-phonon coupling effect similar to the Franck-Condon effect predicted in the perovskite structured manganites. The resonant state is critically sensitive to Sr doping and . Higher energy excitations in the compounds Ba$_{2}$(Sr$_{2})$FeWO$_{6 }$are shown to be of non-resonant multiphonon character. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J41.00006: Dynamical mean field study of Manganite superlattices Chungwei Lin, Andrew Millis A theoretical study of 001 manganite superlattices $[La Mn O_3]_n [Sr Mn O_3]_m$ is presented. The superlattice is defined by different charges of $La$ and $Sr$; the conduction band is modelled via a nearest neighbor tightly binding model. The interaction between conduction electrons and localized $t_{2g}$ spins is treated by the dynamical mean field approximation, while the long range Coulomb interaction is taken into account by the Hartree approximation. The magnetic phase diagram and physical properties including charge distribution, dc and optical conductivity are calculated at different layer geometries. A range of physical parameters is explored. The research is supported by Columbia University MRSC (CL) and DOE-ER46169(AM) [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J41.00007: First-principles study of x-ray linear dichroism at the Mn $K$ edge of LaMnO$_3$ Tatsuya Shishidou, Tamio Oguchi LaMnO$_3$ shows orbital ordering of Mn $3d$ ($3x^2-r^2/3y^2-r^ 2$) states associated with the cooperative Jahn-Teller distortion. Resonant x-ray scattering measured at the Mn $K$ edge by Murakami {\it et al.}\ shows peculiar azimuth-angle dependence, indicating that the degeneracy of the Mn $4p$ states should be lifted. For its mechanism, two models have been proposed. One is the Coulomb mechanism, that is, the $3d$ orbital ordering causes the level splitting of the $4p$ states through the intra-atomic $3d$-$4p$ Coulomb interaction. The other is the Jahn-Teller mechanism: the $4p$ bands are strongly influenced by the Oxygen displacements, not by the anisotropic $3d$ charge distribution resulting from the orbital ordering. The important point to note is that these two models yield opposite way of the $4p$ splitting. To resolve this argument, very recently, Maruyama {\it et al.}\ have carried out x-ray photoabsorption measurements at the Mn $K$ edge using linearly polarized light. The energy and polarization dependence of their spectra clear up the unoccupied Mn $p$ states, apparently supporting the Jahn-Teller mechanism. Furthermore, they found interesting azimuth-angle dependence as a function of photon energy. For its interpretation, detailed band-structure information is undoubtedly necessary. In this talk, using the first-principles FLAPW calculations, we will discuss the overall feature of the measured spectra and its relation to the $3d$ orbital ordering and Jahn-Teller distortion. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J41.00008: Electronic structures of non-half-metallic antiferromagnetic double perovskites ALaVMoO$_6$ (A = Ca, Sr, and Ba) I.G. Kim, A.J. Freeman, M.S. Park, B.I. Min Recently, double perovskites $A$LaVMoO$_6$ ($A=$ Ca and Sr) of the $Fm\bar{3}m$ space group were proposed experimentally to be half-metallic antiferromagnets.\footnote{Uehara, Yamada, and Kimishima, Solid St. Commun. \textbf{129}, 385 (2004).} The electronic structures and magnetism of the double perovskites $A$LaVMoO$_6$ ($A=$ Ca, Sr, and Ba) were determined within the generalized gradient approximation to density functional theory using the all-electron full-potential linearized augmented plane wave (FLAPW) method.\footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB \textbf{24}, 864 (1981).} The $A=$ Ca case shows \emph{metallic} ferrimagnetism as the most stable phase, with magnetic moments of $1.15\;\mu_B$ for V and $-0.53\;\mu_B$ for Mo, whereas the Sr and Ba cases are calculated to be almost non-magnetic metals. Comparing the calculated density of states, we find that the heavier $A$ implies stronger hybridization between the divalent atom $sp$ states and the transition metal atom $d$ states. The stronger $sp$-$d$ hybridization is considered to be responsible for the suppression of magnetism for the Sr and Ba cases. These results, at least for the $Fm\bar{3}m$ space group, are in contrast with the recent experimental result proposing half-metallic antiferromagnetism for $A=$ Ca and Sr. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J41.00009: Magneto-Optical Investigation of La$_{1-x}$Sr$_{x}$CoO$_{3}$\,($x$\,=\,0.15, 0.2) Ralf Rauer, Michael R\"ubhausen, John F. Mitchell A complete magneto-optical characterisation of the perovskite cobaltites La$_{0.8}$Sr$_{0.2}$CoO$_{3}$ and La$_{0.85}$Sr$_{0.15}$CoO$_{3}$ was performed using temperature dependent spectral generalised magneto-optical ellipsometry (SGME). The measurements cover the energy range from 1.5 to 5.5\,eV and temperatures between 175 and 25\,K. The complex diagonal and off-diagonal elements $\varepsilon_{xx}$ and $\varepsilon_{xy}$ of the dielectric tensor are determined simultaneously yielding enhanced sensitivity to the interplay between electronic and magnetic properties and thus to the electronic structure of the ferromagnetic phase. The investigated compositions are close to the phase boundary between unconventional ferromagnetism and a mixed phase displaying spin-glass as well as ferromagnetic behaviour at $x = 0.18$. While, for the 15\,\% doped sample, the amplitude of the magneto-optical response is found to be proportional to the net magnetisation, for the $x = 0.2$ sample this holds true only for temperatures $T\,\geq\, T'\,\approx\,100$\,K. Additionally, an energy shift of the order of 100\,meV of spectral features of $\varepsilon_{xy}$ is observed below $T'$. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J41.00010: Ultrafast photo-induced melting of charge and orbital order in the manganite Nd$_{0.5}$Sr$_{0.5}$MnO$_3$ Rohit Prasankumar, Antoinette Taylor, Richard Averitt, Konstantin Kamenev, G. Balakrishnan, D. Paul The physics of perovskite manganites is a subject of intense research in
condensed matter physics due to the interplay between charge, spin, lattice,
and orbital degrees of freedom in these materials. The manganite
Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ is ferromagnetic below its Curie temperature
($T_{c})$ of $\sim $250 K, exhibiting colossal magnetoresistance upon
application of a magnetic field. Below $T_{co}\sim $150 K, it is
antiferromagnetic with charge and orbital ordering, which can be ``melted''
upon application of a high magnetic field. Optical conductivity measurements
revealed an optical gap below $T_{co}$, with Drude-like behavior for
$T_{co} |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J41.00011: Instrinsic Insulators at the Surfaces of Layered Ferromagnetic Manganites J.W. Freeland, K.E. Gray, E. Badica, J.F. Mitchell, J.J. Kavich, R.H. Kodama To explore loss of ferromagnetic order at the surfaces of manganites, we brought together a powerful combination of two surface probes, tunneling and polarized x-ray interactions, to study the intrinsic electronic and magnetic surface states of a layered manganite, La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$, that is ferromagnetic and conducting in the bulk. These probes present clear evidence for an intrinsic insulating nonferromagnetic surface layer atop adjacent ferromagnetic subsurface layers. The presence of a nonferromagnetic surface layer of one bilayer thickness was observed with x-ray resonant magnetic scattering (XRMS), and point contact tunneling results show this layer is insulating (0.6 eV band gap), in agreement with the expectations of the double-exchange model. Temperature dependence shows the magnetization in the sub-surface bilayer retains the bulk value close to T$_c$. In addition the nonferromagnetic surface layer shows a linear response in fields up to 7T, consistent with antiferromagnetic order in the surface bilayer. This research, including the use of the Advanced Photon Source, was supported by the U.S.Department of Energy, Office of Science, under Contract No. W-31-109-Eng-38. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J41.00012: Lattice Distortion due to Charge-ordering effects in single-layer La$_{0.5}$Sr$_{1.5}$MnO$_4$ Derek Larson, F. Bridges, A. Mehta, S. Larochelle, M. Latimer We report measurements of the local structure of the single-layer manganite La$_{1-x}$Sr$_{1+x}$MnO$_4$ (x = 0.5) using polarized EXAFS at the Mn K-edge. Earlier diffraction data suggested a quadrupling of the simple perovskite unit cell, most likely from a charge/orbital ordering effect. From a careful analysis of the single crystal diffraction pattern, space group symmetry of the ordered structure is deduced to be B2mm (special space group settings employed to keep Mn-O planes perpendicular to the c-axis). In this space group symmetry there are 3 distinct Mn sites, but the diffraction data indicates that two of them are related by a pseudo-glide and the distinction between them is very subtle. For the EXAFS analysis we have imposed the glide symmetry and therefore assumed a more symmetric space group Bbmm, which contains only two unique Mn sites. We have further assumed that the predominant effect of the charge/orbital ordering is on the basal Mn-O bonds. Bbmm symmetry allows for 4 unique basal Mn-O bond distances, however, the EXAFS analysis shows that they are clustered in two distinct groups. Combining these results with analysis of single crystal diffraction data indicates that the primary mode of distortion of the basal Mn-O bonds is driven by the Jahn-Teller effect. (That is, each Mn has two close basal O neighbors and two distant basal O neighbors.) Support: NSF DMR0301971, and BES/DOE. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J41.00013: Orbiton features in YTiO$_3$ and LaTiO$_3$ probed by Raman light scattering C. Ulrich, A. Goessling, M. Grueninger, H. Roth, T. Lorenz, G. Khaliullin, M. Guennou, C. Frost, Y. Taguchi, Y. Tokura, B. Keimer The existence of collective orbital excitations, termed orbitons, in YTiO$_3$ has been proposed by G. Khaliullin et al. [1] and S. Ishihara [2]. We have performed Raman and inelastic neutron scattering experiments on LaTiO$_3$ and YTiO$_3$ single crystals in order to determine the excitation spectrum for energies above the range of optical phonons. There are two unexpected features in the high energy Raman spectrum of both materials. A sharp structure at 165 meV and a broad continuum like feature around 230 meV. Our theoretical calculations of the phonon dispersion relation have revealed, that the peak at 165 meV is right at the high energy cutoff of the two-phonon density of states, but it's unexpected large intensity cannot be explained with two-phonon processes alone. The temperature dependence, polarization dependence and the resonance behavior probed with different laser lines for excitation, yields that the feature at 230 meV can possibly be assigned to orbital excitations. [1] G. Khaliullin and S. Okamoto, PRB {\bf 68}, 205109 (2003). [2] S. Ishihara. PRB {\bf 69}, 075118 (2004). [Preview Abstract] |
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