Quantum Critical Behavior at Metal-Insulator and Magnetic Transitions

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Phase transitions at zero temperature involve fundamentally new physics. The Heisenberg uncertainty principle inextricably intertwines the static and dynamical response of the material changing state, introducing new critical exponents, new scaling laws, and a new relationship between interactions and disorder. This project will probe the fundamental nature of the quantum phase transition in two model experimental systems. The first is the metal-insulator transition in hydrogenated films of elemental Y and La. Rare earth hydride films can be converted reversibly from metallic mirrors to transparent, insulating windows simply by changing the hydrogen content or via UV exposure. The physics of the Mott-Hubbard MI transition in metal hydride films will be pursued with an emphasis on the roles played by electron-electron interactions, by different types of disorder, and by the (non-linear) dynamical response in electric and magnetic fields. The second model system involves single crystals of the only elemental antiferromagnet, Cr. Dilution of chromium with small amounts of vanadium, smoothly depresses Cr's spin-density-wave transition to T = 0. Will a simple antiferromagnet display the anomalous behavior demonstrated by the related high-Tc superconductors and heavy fermion compounds? The experimental focus will be on high resolution pressure studies at ultra-low temperatures, broadly developing the technical skills of students at the undergraduate, doctoral and postdoctoral levels. %%%Materials most commonly undergo changes of state with changing temperature. Non-thermal processes like pressure and magnetic field also can induce magnetic, electronic, and optical changes. When these transitions occur at absolute zero, where all motion stops, new quantum physics arises. Moreover, the effects can be felt up to surprisingly high temperatures. This proposal aims to probe the fundamental nature of the quantum phase transition in two model experimental systems. The first of these is the metal-insulator transition in hydrogenated films of elemental Y and La. Rare earth hydride films can be converted reversibly from metallic mirrors to transparent, insulating windows simply by changing the surrounding hydrogen gas pressure or through illumination with ultraviolet light. Switchable mirrors with both homogeneous and pixilated optical properties will be investigated, with applications in mind. The second model system involves single crystals of the only elemental antiferromagnet, Cr. Dilution of chromium with small amounts of vanadium, its neighboring element in the Periodic Table, smoothly depresses the magnetic transition temperature to absolute zero. Will a simple antiferromagnet display the anomalous behavior demonstrated by the related high-temperature superconductors? The experimental focus will be on high resolution pressure studies at ultra-low temperatures, broadly developing the technical skills of students at the undergraduate, doctoral and postdoctoral levels.***