Tunneling and Transport in Ordered and Disordered Superconductors

9876816

Goldman

This research is focussed on the investigation of the properties of ultra-thin films, which undergo superconductor-insulator transitions controlled by disorder or magnetic field. These films are members of a class of materials exhibiting behavior governed by a zero-temperature critical point, which separates two distinct quantum mechanical ground states. Such quantum phase transitions are an important paradigm in contemporary Condensed Matter Physics. Among other systems exhibiting such transitions are high-mobility two-dimensional electron gases, certain magnetic compounds, high-Tc superconductors, and liquid helium adsorbed on random substrates. The underlying physical model of the superconductor-insulator transition is unresolved, as competing pictures have been developed. A series of electrical transport, tunneling and structural investigations are proposed to provide experimental tests of these competing theories. An important feature of the work is the extension of the experimental parameter space to low temperatures (T ~ 20mK) and high frequencies (f ~ 50 GHz) so as to explore the critical regime of the transition, which is essential to determining its character. Also the insulating state will be probed in search of superconducting fluctuations.

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This work is directed at the study of the transition of superconductors into insulators, driven by disorder and magnetic field, at the limits of zero temperature and two dimensions. The resultant phase transitions are quantum transitions. Other materials exhibiting quantum phase transitions, and thus potentially explainable within a common theoretical framework, include helium-4 in porous media, high temperature superconductors, superconducting junction arrays, two-dimensional electron gases exhibiting the quantum Hall effect, and various magnetic compounds. Electrical measurements and structural investigations using scanning tunneling microscopy are proposed to provide experimental tests of competing theories. The work breaks new ground in combining ultra-low temperature cryogenic technology, ultra-high vacuum thin film growth techniques, and scanning tunneling microscopy. The work will shed light on the formation of amorphous films, a problem of technological importance. The understanding of superconductivity in two dimensions is also critical to the understanding of high temperature superconductors. Students in this program receive rigorous training in physics and materials, and can pursue careers in either academic or industrial science.