TUNNELING STUDIES OF VORTICES IN HIGH-SHEET-RESISTANCE GRANULAR SUPERCONDUCTING FILMS

We have measured the tunneling density of states in ultrathin high-sheet-resistance, 16 OMEGA < RN < 17 kOMEGA, granular Pb films in perpendicular magnetic fields, H. At the lowest R(N), states appear within the superconducting energy gap at low fields (H < 0.1 T) indicating the presence of vortex cores. At the highest R(N), no states appear in the gap in fields as high as 2 T indicating that the field-induced vortices do not have cores. A simple model implies that the average Josephson coupling energy between the grains of the film is comparable to the condensation energy of a single grain at the R(N) separating these two regimes.