Interplay of disorder and superconductivity in ultra-thin Sn films

Abstract

This thesis reports low-temperature, in-situ transport measurements on quench-condensed ultra-thin films of Sn. Film thicknesses are < 100 Å. These films exhibit an insulator-to-superconductor transition. Novel effects are expected in transport due to lower dimensionality (2D) and the presence of disorder. Disorder is known to induce localization in charge carrier states. Phase coherence, on the other hand, is responsible for superconductivity. These two opposite phenomena drive a 2D system towards incompatible extremes, viz., superconductivity and insulating behaviour. Several characteristics of this insulator-superconductor transition are believed to be universal or only weakly dependent on the film material and the substrate. This transition can be tuned either by disorder or by an external magnetic field. The resistive transition and current-voltage characteristics in presence and absence of transverse magnetic field up to 6 Tesla are studied. Films are grown on quartz substrates and on thin (~10 Å) Ge underlayers. The observations on Sn films are compared with results on quench-condensed Bi films, obtained earlier by our group using similar methods. The evolution of transport mechanism with gradual increase in film thickness is studied. These measurements give clues about the film structure which cannot be otherwise determined directly in our experimental apparatus. Sn films are found to be more granular when compared to Bi films. The grains are coupled to each other via Josephson junctions. As the films grow thicker, the Josephson coupling starts to percolate across the film. The appearance of zero resistance in transport occurs when the phase coherence is established across the entire sample by such percolation. The superconducting films show current-voltage characteristics similar to Josephson junction arrays. Using a resistively and capacitively shunted junction model, we estimate several relevant film parameters and the energy scales involved. The resistive transition in transverse magnetic field indicates presence of thermally activated flux-flow. Flux-flow resistance, induced by the bias current as well as by the external magnetic field, is studied. The latter suggests presence of vortex solid in the low-temperature and low-field regime. Although there are many qualitative similarities in the behaviour of Sn and Bi, the quantitative differences are significant to draw attention and merit further investigation. From this study, it is reasonable to conclude that microstructural differences in films of different materials are indeed important parameters in such disordered systems.