Thesis presented November 30, 2007

Abstract:
This thesis presents a scanning tunneling spectrocsopy study of thin disordered superconducting films close to the superconductor-insulator transition (SIT), namely titanium nitride and amorphous indium oxide films. In the superconducting phase, both films can be driven into an insulating state by increasing disorder or by applying a perpendicular magnetic field. Our tunneling spectroscopy study performed at a temperature of 50 mK revealed strong disorder-enhanced inhomogeneities of the superconducting gap, despite an homogeneous disorder. Close to the SIT, such inhomogeneities yield a picture of superconducting islands connected by Josephson coupling. Consequently, an applied magnetic field breaks these weak links and restores the underlying insulating states. This thesis presents also a study of the superconducting fluctuations regime above the critical temperature. Fluctuations broaden the superconducting transition of thin films and open a pseudogap in the one-electron density of states. In TiN films, we measured a strong pseudogag in a wide temperature range. This pseudogap is in good agreement with a superconducting fluctuation pseudogap combined with a disorder-enhanced Coulomb anomaly.

Keywords:
Superconductor-insulator transition, scanning tunneling microscopy and spectroscopy, localization, Coulomb interaction and superconductivity, superconducting fluctuations, quantum phase transition

On-line thesis.