Polaron-Induced Local Distortions in Correlated Electron Systems

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9705117 Bridges This is a new project which proposes X-Ray absorption fine structure (XAFS) measurements on systems exhibiting metal- insulator transitions and Colossal Magnetoresistance (CMR) effects, such as La1-xCaxMnO3. In these oxides, the large magnetoresistance near the ferromagnetic transition temperature, Tc, is now attributed to a novel coupling between magnetism, the electron system and local lattice distortions and, in a recent paper, we have shown that the transition from metallic to insulating behavior correlates well with a rapid increase in the amplitude of the local distortions which are believed to be polaronic in origin. However, the nature, symmetry and extent of the atomic displacements are not yet known and are needed for modeling these unusual transport effects. XAFS measurements as a function of temperature and magnetic field will enable us to probe the details of the local structural distortions about the metal atoms and provide this crucial information. %%% This is a new project which proposes X-Ray absorption fine structure (XAFS) measurements on systems such as La1-xCaxMnO3 which exhibit novel features in their magnetic and electrical behavior that appear to be related to the atomic structure of the material. Recent theoretical models suggest that the arrangement of the atoms become distorted at the same temperature at which the magnetic properties disappear and it is suggested that these distortions contribute to the large observed increase in the resistance to the flow of electrical current. Our initial study supports such a model, but details about the arrangements of the atoms are not yet known. We will use x-ray absorption techniques to investigate the distortions about a given atom at various temperatures and magnetic fields. We need to determine which atoms are distorted and the range and magnitude of the distortions to test and improve the models of this unusual behavior. This novel connection between the microscopic atomic arrangement and the electrical properties involves new physics and may lead to new applications since such materials are potentially useful to make devices for reading and writing information on computer disk. ***