Electrical transport properties of ultrathin amorphous bismuth films near the superconductor-insulator transition.

Abstract

A combination of thickness- and perpendicular magnetic field-tuning of SI transitions has been performed on quench-deposited homogeneous a-Bi thin films with a 14.67 Angstrom a-Sb underlayer. Transport properties, including measurements of resistance and of I-V characteristics have been studied in both the insulating and superconducting regimes. In the insulating regime, the resistance exhibits an Arrhenius type of conduction and the magnetoresistance (MR) exhibits a peak in perpendicular magnetic field. Furthermore, a possible quantum phase transition is found in the insulating regime. Presumably this transition is between the Bose and Fermi insulators discussed in the literature. The I-V characteristics exhibit strong non-linearities in the insulating regime at low temperatures. These non-linear curves can be well described by a heating model involving the decoupling of the electronic and phononic degrees of freedom at low temperatures. On the conductive or superconducting side of the transition, the transport properties are found to be remarkably similar to those of an overdamped random Josephson junction array, and vortex dynamics dominates the conductive behavior in both zero and non-zero magnetic fields. These observations suggest that isolated superconducting islands or localized Cooper pairs exist in both the insulating and conductive regimes. An AFM scan of the last film in the sequence has revealed that this series of films although continuous, has thickness variations on a mesoscopic length scale. Therefore, it is not surprising that there may be superconducting islands. The AFM scan also suggests that some of the thick, nominal granular films grown by quench condensed deposition are directly connected with large thickness variations. These insulating granular films also exhibit an Arrhenius type conduction at low temperatures, which reveals the existence of a hard gap in the electronic density of states, which is consistent with the theory of Feigel'man et al.. However, the activation energy of the Arrhenius type conduction found in the thickness tuning homogeneous a-Bi films doesn't follow this model. Therefore, the model may not completely explain the hard gap.