Quantum Phase Transitions in Model Magnets and Switchable Mirrors

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NON-TECHNICAL ABSTRACTAt the lowest temperatures, quantum effects dominate the physics. However, even at temperatures as high as room temperature, the effects of quantum mechanics can be felt when a material changes state from metal to insulator, from magnet to non-magnet, from ordinary conductor to superconductor. This proposal will investigate two such changes of state in model and useful quantum systems. The first is a thin film of yttrium hydride, which can switch from shiny mirror to transparent window by simply changing the hydrogen concentration or by illuminating with ultraviolet light. The second is common chromium, whose magnetic character can be tuned with pressure or by alloying. This proposal highlights efforts to integrate research and education through Ph.D. training across a broad spectrum of skills and facilities, with an emphasis on recruiting women and minorities, undergraduate curriculum development, and personal commitments to outreach to the general public and neighborhood schools. It also describes the development and oversight of institutional programs that match students from the physical sciences and business to study problems of industrial import as well as a partnership between the University of Chicago and the 100% African-American Charter School that the University operates on the South Side of Chicago.TECHNICAL ABSTRACT This proposal aims to probe the fundamental nature of the quantum phase transition and fluctuations in two model experimental systems. The first is the metal-insulator transition in hydrogenated films of elemental Y and La, select materials with strong correlations but without a structural transformation tied to the localization of charge. The emphasis will be on defining the roles played by electron-electron interactions, disorder, and the non-linear dynamics. The insulating state also serves as a model for the interacting Coulomb glass, and studies of the long-time relaxation, memory effects and noise spectra will be pursued. The second system involves single crystals of the only elemental antiferromagnet, Cr. The experimental focus will be on high-pressure diamond anvil cell studies which can depress pure Cr's spin-density-wave transition smoothly to T = 0. A combination of transport and x-ray techniques should illuminate the interplay of the charge-density-wave, spin-density-wave and potentially superconducting instabilities at the magnetic quantum critical point. This proposal highlights efforts to integrate research and education through Ph.D. training across a broad spectrum of skills and facilities, with an emphasis on recruiting women and minorities, undergraduate curriculum development, and both personal and institutional outreach to the general public and neighborhood schools.