Vortex dynamics and upper critical fields in ultrathin Bi films

Current-voltage (formula presented) characteristics of quench condensed, superconducting, ultrathin Bi films in a magnetic field are reported. These (formula presented) show hysteresis for all films, grown both with and without thin Ge underlayers. Films on Ge underlayers, close to superconductor-insulator transition, show a peak in the critical current, indicating a structural transformation of the vortex solid. These underlayers, used to make the films more homogeneous, are found to be more effective in pinning the vortices. The upper critical fields (formula presented) of these films are determined from the resistive transitions in perpendicular magnetic field. The temperature dependence of the upper critical field is found to differ significantly from Ginzburg-Landau theory, after modifications for disorder.