Vortex dynamics and upper critical fields in ultrathin Bi films

Current-voltage $(I\ensuremath{-}V)$ characteristics of quench condensed, superconducting, ultrathin Bi films in a magnetic field are reported. These $I\ensuremath{-}V\mathrm{'}\mathrm{s}$ 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 ${(B}_{c2})$ 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.