| The unifying theme of this thesis is the study of the electronic properties of poorly conducting disordered insulators and superconductors. I use transport measurements to examine the motion of electrons and vortices in two completely different highly disordered "model" systems, whose long experimental histories in our group at Stanford provide the foundation for the conclusions drawn here. Part I focuses upon resonant tunnelling and inelastic hopping transport through thin amorphous silicon tunnel barriers. Magnetoresistance measurements together with theoretical arguments prove that the Coulomb interaction correlates the motion of the electrons as they traverse the barrier. I first discuss the single-particle picture and show that a few simple models of electron transport via localized states in the insulating barrier describe the experimental data exceptionally well. I then address the addition of the interactions in detail and isolate the correlations through a careful study of the magneto-transport data. Part II examines vortex motion in thin films of an amorphous molybdenum-germanium alloy poised between superconducting and insulating behavior. In relatively low magnetic fields the sample resistance, expected to disappear as the temperature approaches zero and the vortex configuration enters a "vortex glass" phase, instead becomes temperature independent, suggesting the observation of the macroscopic quantum tunnelling of the vortices. I also describe the non-linear behavior at high current bias. Finally, I examine the effect of a ground plane placed close to the film as a direct experimental assessment of the effect of screening on the superconducting-insulating transition. |
