Tunneling and Transport Studies of Strongly Disordered Ultra-Thin Films of Metals and Superconductors

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w:\awards\awards96\*.doc 9801983 Valles This experimental research project is concerned with the mechanism of the superconductor-insulator quantum phase transition exhibited in ultra-thin quench condensed metal films. Such a transition may occur for a disordered superconducting film so thin that its normal resistance per square is on the order of h/4e2, and the insulator like state is also promoted by a magnetic field. The research project involves in-situ evaporation of ultra thin layers onto cryogenic substrates, and in-situ measurements of magnetotransport, STM topography and spectroscopy. The goals are to establish correlations between the macroscopic transport exhibiting the SIT transition and the microscopic properties as revealed by the STM measurements. A key issue to resolve is the relative importance of phase and amplitude fluctuations of the superconducting order parameter in the vicinity of the transition. This research program is interdisciplinary in nature and has typically involved several undergraduate and graduate students in its activities. These involvements are beneficial in the preparation of students for further study and for careers in industry, government laboratories or academia. %%% This experimental research project is concerned with electrical conduction in extremely thin layers of metal, at very low temperature. If the metal is a superconductor, like lead, the usual behavior is a transition between a metallic state and a superconducting state, that is the resistance of the metal film will go to zero below a transition temperature. However, when the films are only a few atomic layers thick, an entirely different behavior can be observed, in which a transition occurs between an insulating state and a superconducting state. This unusual behavior is the focus of this project. The experimental apparatus allows vacuum deposit of a metal layer onto a liquid-helium- temperature substrate, so called quench condensing, which permits the ultra thin and even-thickness films that show the effect of interest. The experimental apparatus is set up to allow electrical resistance measurements of the film to be made in the same apparatus, and also scanning tunneling microscope topographs and electrical measurements to be made without disturbing the as deposited metal film. The temperature and magnetic field can also be varied. The goal of this basic research is to understand the mechanism of the unusual insulator-superconductor transition. Such understanding will add to the overall understanding of electrical conduction in materials and also in microelectronic and other devices. This research program is interdisciplinary in nature and has typically involved several undergraduate and graduate students in its activities. These involvements are beneficial in the preparation of students for further study and for careers in industry, government laboratories or academia. ***