Transport studies of disordered metallic and superconducting one dimensional wires

The effect of dimensionality on electron localization and superconductivity has been a subject of study for decades. Numerous experiments have been performed investigating the superconductor-insulator and metal-insulator transitions in quench-condensed ultrathin two dimensional films, where an increase in the disorder, i.e. normal state sheet resistance, drives these transitions. Highly disordered films can be prepared with morphologies in two extreme limits, an amorphous uniform film and a granular film. Despite these substantial morphological differences the normal state transport characteristics as a function of disorder are very similar. On the other hand, the superconducting properties strongly reflect the structural differences.;We have extended the study of the superconductor-insulator and metal-insulator transition in disordered two dimensional films to disordered one dimensional wires of both morphologies. The wires are created by quench-condensing through a prefabricated metallic stencil structure, containing a narrow opening defining the wire width, onto an insulating substrate. The thickness of the wire can be changed incrementally, and its resistance as a function of temperature and/or magnetic field can be measured in situ, throughout the superconductor/metal-insulator transition.;A systematic study of the superconductor-insulator transition in uniform Pb wires was carried out and it was found to look qualitatively similar to two dimensional uniform films. Nevertheless, there were many signatures of a dimensional cross-over into the one dimensional regime, both in terms of normal electron transport and superconductivity. In particular, an excess broadening of the superconducting resistive transitions, beyond that predicted by the one dimensional superconducting fluctuation models, is observed. In conjunction with this excess broadening a negative magnetoresistance is measured, where it is found that low fields actually enhance the superconductivity in the wire.;A further series of experiments studying the transport properties of granular metal and superconducting wires was performed. In the normal metal wires a discontinuous localization transition is observed between the strongly localized and weakly localized states. A large gap in resistance exists wherein no stable wire is observed. In two dimensional films and uniform wires the localization transition is continuous. The magnetoresistance along the resistive transition of granular superconducting wires was also measured and magnetoresistance oscillations are observed. These are believed to result from oscillating screening currents around phase coherent loops of grains. Generally, the superconducting properties of the granular wires are not found to be substantially altered from those of the two dimensional case.