Electron Correlations at the Edge of Instability: Complex Materials and Restricted Geometries

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9705300 Hellman This is a new project by a PI who collaborates with a well established group of scientists a UC San Diego to study "highly correlated electron systems". These are complex systems, with superconductivity, magnetism, metallic conduction, and insulating behavior found in the same material system, often with only small changes in composition or structure. One electron bands are narrow, screening is limited, and electron-electron correlation energy is high, leading to a general instability of the electronic system to different competing ground states. In the neighborhood of such instabilities, the effects of reducing dimensionality, thereby increasing the correlation energy, or of adding magnetic moments can be extremely dramatic. This complexity is absent in more traditional materials, where properties can generally be quite successfully described using Fermi liquid theory, and dramatic effects seldom result from small changes. In these complex systems the importance of measuring all properties on the same sample cannot be overstated, since subtle and often nearly unmeasurable structural or chemical changes can drastically affect properties. Measurements of specific heat from low temperature up through any phase transitions reveal important characteristics of the electronic system. Our ability to measure the specific heat of thin films and microgram size single crystals allows measurements of magneto-transport, magnetization, IR and tunneling spectroscopy on the same samples. We plan collaborations with Maple, Schuller, Basov, and Dynes to prepare and measure samples of materials in which correlation energies can be varied, either by composition or reduced dimensionality, and with theorists Sham, Arovas, and Renn to develop models for these materials. %%% This is a new project by a PI who collaborates with a well established group of scientists a UC Sa n Diego to study "highly correlated electron systems". These are materials causing great excitement in condensed matter physics and with great potential for applications, such as high temperature superconductors or colossal magnetoresistance. The phase diagrams of these materials are complex, with superconductivity, magnetism, metallic conduction, or insulating behavior found in the same material system, often with only small changes in composition or structure. This complexity is absent in more traditional materials. The interactions between the electrons, which cause the magnetism, superconductivity, or insulating behavior, compete with each other, with one or another winning. It thus appears that the electrons in these new materials are perched on the edge of many different types of behavior, with the potential to go different ways depending on extremely subtle differences in structure or composition. With this in mind, the importance of measuring all properties on the same sample cannot be overstated, since subtle and often nearly unmeasurable structural or chemical changes can drastically affect properties. We have a unique capability here to make a basic measurement (specific heat) on samples which cannot be measured elsewhere. This grant will allow us to develop collaborations taking advantage of our unique expertise together with others' expertise in sample preparation, characterization, measurement and theoretical understanding. ***