Transport, magneto-transport and electron tunnelling studies on disordered superconductors

I will present an experimental investigation of the effect of atomic scale homogenous disorder on the electronic and superconducting properties of NbN thin films. At strong disorder, these properties: electrical resistivity, Hall effect, magnetoresistance, superconducting transition temperature and the superconducting energy gap  undergo  drastic  changes,  to  the  extent  that  both metallicity and superconductivity are destroyed almost completely. In the vicinity of these transitions, a number of novel phenomena are seen to emerge. The temperature coefficient of resistance is reversed, Cooper pairing persists above the transition temperature and into the region where global superconductivity is destroyed, physically homogenous samples show a spontaneous inhomogeneity in tunneling spectra and a peak is seen in the magnetoresistance. These observations come together to form a phenomenological phase diagram, the different parts of which are explained using a combination of multiple effects, namely the localization of wave functions, Coulomb repulsion, Cooper pairing and fluctuations in the phase of the order parameter.