Structural Distortions in the Paramagnetic Insulating Phase of La 0.7 Ca 0.3 MnO 3
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Structural Distortions in the Paramagnetic Insulating Phase of La0.7Ca0.3MnO3 C.S. Nelson,1 Y.J. Kim,1 J.P. Hill,1 Doon Gibbs,1 V. Kiryukhin,2 T.Y. Koo,2,3 and S-W. Cheong2,3 1
Department of Physics, Brookhaven National Laboratory, Upton, NY 11973-5000 Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 3 Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974 2
ABSTRACT We report x-ray scattering studies of polarons and tilt ordering in the hightemperature, paramagnetic insulating phase of La0.7Ca0.3MnO3. The temperature dependence of scattering due to individual and correlated polarons was measured, and the latter was found to disappear at ~400 K. The scattering due to tilt ordering, which was observed by tuning the incident photon energy near the La L1 edge, was also measured as a function of temperature. The destruction of tilt ordering at ~690 K is believed to be associated with the orthorhombic-rhombohedral structural transition.
INTRODUCTION The colossal magnetoresistance (CMR) effect exhibited by some manganese oxides—or manganites—has led to a renewed interest in these complex materials over the past decade [1]. One prototypical CMR manganite is the 3D perovskite of chemical formula La1-xCaxMnO3. In the doping range of 0.2 < x < 0.5, La1-xCaxMnO3 is a paramagnetic insulator at room temperature and becomes a ferromagnetic metal—with a concomitant drop in resistivity—at low temperatures. The metallic behavior in the ferromagnetic state has long been ascribed to double exchange between mixed-valence Mn ions [2]. In this interaction, the hopping of an eg electron from a Mn3+ ion to a Mn4+ ion requires ferromagnetic ordering due to a large Hund coupling between the eg electron and the localized t2g electrons. In the past several years, double exchange has been supplemented with additional ingredients such as electron-phonon coupling [3,4], but a complete description of the CMR effect remains to be formulated. One recent avenue of investigation into the origin of the CMR effect concerns the role played by phase separation. In this picture, the physics of CMR manganites is dominated by inhomogeneities arising from the coexistence of competing phases [5]. When applied to La1-xCaxMnO3, phase separation studies emphasize the importance of understanding not only the ferromagnetic metallic phase, but the paramagnetic insulating phase as well. With this goal in mind, we report x-ray scattering studies of the paramagnetic insulating phase of La0.7Ca0.3MnO3. Two different structural distortions are believed to occur in the paramagnetic insulating phase of La0.7Ca0.3MnO3: one is associated with the formation of polarons, which are caused by strong electron-phonon coupling, and the other arises from tilt ordering of the MnO6 octahedra. Considering first the polarons, evidence of polarons in
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La1-xCaxMnO3, for x ≈ 0.3, has been obtained using a variety of techniques [6-9], and more recently, interest has turned to studies of polaron-polaron interactions [10-15]. X-ray scattering t
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