9705442 Schlesinger This is a new project to study systems in which the effects of electron interactions extend the physics beyond the domain of simple non-interacting theories of solids. For such systems there is generally a characteristic energy scale in the infrared which separates a strongly correlated low-energy region from a high-energy region in which interaction effects are much less significant. The proposed research seeks to use spectroscopic measurements to probe the dynamics in this crucial infrared range and thereby to develop an understanding of correlated systems. Specific systems to be studied include Kondo insulators, mixed-valent systems, metal- insulator transitions driven by magnetic order, and superconductor- insulator transition systems. These experiments will elucidate the nature of the charge dynamics in these systems, and explore crucial relationships between charge dynamics, magnetism, disorder and localization. Results will provide both a test of and a stimulus for theoretical work in this area at the forefront of condensed matter physics. %%% This is a new project to study systems in which the effects of electron interactions extend the physics beyond the domain of simple non-interacting theories of solids. Although our basic understanding of metals and semiconductors is built on theories of non-interacting electrons, future electronic technologies will increasingly include materials in which interactions between electrons play a role. It is thus important to fully understand the electronic properties of these correlated electron systems, in which phenomena occur that cannot be readily predicted from a knowledge of the system components (the electrons and nuclei), but rather, arise in a non-obvious way from the interactions between them. (Examples include magnetism and superconductivity.) This proposal is directed toward the study o f electron dynamics in the infrared spectral region, which encompasses the energy range relevant to the formation of most correlated electron states and is thus particularly important. Systems to be studied include rare-earth compounds that can be viewed as correlated semiconductors, and oxide systems where an interplay between magnetism and electrical conductivity leads to striking effects (e.g., giant magnetoresistance). The experimental results can be expected to stimulate and test theoretical efforts in this area at the forefront of condensed matter physics. ***
