Pressure Tuned Quantum Phase Transitions in Model Systems

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****Technical Abstract****There has been enormous activity devoted to quantum phase transitions in fascinating but complicated materials, most often disordered and almost all involving indirect probes of the order. Ideally, one would like to tune a simple, stoichiometric material to its quantum critical point and directly measure the disappearance of its order parameter, the effects of the quantum fluctuations, and the potential emergence of competing order. This project combines diamond anvil cell technology at cryogenic temperatures with synchrotron x-ray scattering, magnetotransport, and magnetic susceptibility in a set of model systems to attempt to isolate the key physical parameters and mechanisms at quantum phase transitions. These include the onset of modulated spin order in an elemental antiferromagnet, the fundamental quantum magnetism of spin singlets arranged on a square lattice, the ordering of charge and its interplay with superconductivity, and the transition from insulator to metal in a correlated material. The wide array of techniques, from x-ray scattering to magnetotransport, should train students well for careers in industry, the national laboratories, or academia. Advances in diamond anvil cell technology should influence condensed matter physicists and geophysicists alike. Helping guide a new program to train math and science teachers for urban schools is a personal priority.***Non-Technical Abstract****Phase transitions at the absolute zero of temperature involve fundamentally different physics than their finite temperature analogues. This can include the optical response of materials, the electronic character of devices, or the magnetic capacity of storage materials. In the limit of very low temperatures, quantum mechanics plays a central role, altering the universal response that has been mapped out for classical transitions. This project will probe the unique quantum characteristics in four related arenas: the onset of magnetism, the interplay of magnetism and superconductivity, the ordering of electronic charge, and the way in which an insulator can be transformed into a metal. In each we take advantage of the ability of diamond anvil cell technology to access pressures outside of everyday techniques and to transmit high energy x-rays. The wide array of techniques, from x-ray scattering at a synchrotron to electrical measurements in the laboratory, and the necessity to apply physics, chemistry and materials science to these studies, should train students well for careers in industry, the national laboratories, or academia. Bringing research perspectives to education is another emphasis, including public lectures, course development for both science and non-science majors, and a leadership role in developing a national database for science education outreach.