Signatures of random matrix theory in metallic clusters, inert gas tunneling devices and vortices in a type II superconductor.

摘要

This dissertation reports on three different low temperature measurements of superconductors. The first and most significant portion of this dissertation is concerned with low temperature scanning tunneling microscope measurements on sub-nanometer size Pb clusters fabricated using the technique of buffer layer assisted growth. Discrete energy levels were resolved in current-voltage characteristics as current peaks rather than current steps. Distributions of peak voltage spacings and peak current heights were consistent with Wigner-Dyson and Porter-Thomas distributions respectively, suggesting the relevance of random matrix theory to the description of the electronic eigenstates of the clusters. The observation of peaks rather than steps in the current-voltage characteristics is attributed to a resonant tunneling process involving the discrete energy levels of the cluster, the tip, and the states at the interface between the cluster and the substrate surface.; The second part of this dissertation pertains to metal-insulator-superconductor junctions which were fabricated with insulating barriers of Xe. The current-voltage characteristics of these junctions when unshorted were consisted with tunneling theory while shorted junctions produced Andreev reflection characteristics. The fabrication of junctions with solid inert gas insulating barriers may make it possible to carry out tunneling spectroscopy of complex compounds that are susceptible to chemical and mechanical damage.; The third part of the dissertation is a study of the vortices in a type II superconductor, V3Si. The magnetic field and temperature dependencies of the magnetic moments were studied. In a constant magnetic field and at temperatures somewhat below the superconducting transition temperature, the moments are hysteretic in temperature. However, the magnetic moment- magnetic field isotherms are reversible and exhibit features that formally resemble the pressure-volume isotherms of the liquid-gas transition. This suggests the existence of a first-order phase transition, a two-phase regime, and a critical point in the superconducting vortex densities. The entropy change, determined from the data using the Clausius-Clapeyron equation, is consistent with estimates based on the difference in the vortex densities of the two phases.