This grant supports theoretical research on models which mimic the behavior of real three-dimensional solids. Recent developments in mesoscopic physics have provided powerful new computer algorithms for studying electrical or heat transport in strongly disordered systems. The phenomenon of "resistivity saturation" will be studied, in order to test the hypothesis that "band-mixing" effects are involved and cause the excess conduction beyond the naive prediction of Boltzmann transport theory. A series of models will be studied numerically with increasingly complex band structures. An analog problem in lattice heat transport is the thermal conductivity of glasses. A realistic atomistic model for amorphous silicon will be treated using the tools of mesoscopic physics.
There are many important oxide materials with the perovskite (ABO3) or related crystal structures, including ferroelectrics, high temperature superconductors, ionic conductor electrolytes, etc. Although most perovskites are insulators, a few are metals, and quite a few have metal-to-insulator transitions as a function of doping or of temperature. The systems Ba1-xKxBiO3 and La1-xCaxMnO3 are particularly nice examples. The pure end-member (x=0) compounds are prototype examples of materials which are insulating because of a broken symmetry in the ground state, namely charge ordering in BaBiO3 , orbital ordering in LaMnO3 . Both systems become interesting metals when the doping level x increases to a critical value 0.2-0.4. Good models exist for the electons which participate in this metal-to-insulator transition. It is proposed to study self-trapped states in these materials. In particular, the opposite end-member (x=1) CaMnO3 , when lightly doped with electrons (x=1-epsilon) will be chosen as a model system for study of the competition between spin-polaron effects and lattice-polaron effects. The lowest electronic excitations of both BaBiO3 and LaMnO3 in this model are self-trapped excitons. These will be studied and their novel optical properties (adsorption spectra, resonant Raman spectra, etc.) will be predicted. When more heavily doped, holes should self-organize into planar anti-phase boundary charged defects (the three-dimensional version of "stripes"). These will be studied and the properties predicted. The metal-insulator transition as a function of doping (polaron-glass collapse) will be modeled and carefully characterized. %%% This grant supports theoretical research on the electrical and thermal properties of oxide materials which are of great fundamental interest due to their complex behavior as the temperature and/or concentration of additives is changed. These materials exhibit a wealth of effects including ferroelectricity, high temperature superconductivity and ionic conduction which may lead to widespread applications. ***
