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 Ω<RN<17 kΩ, granular Pb films in perpendicular magnetic fields, H. At the lowest RN, states appear within the superconducting energy gap at low fields (H<0.1 T) indicating the presence of vortex cores. At the highest RN, 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 RN separating these two regimes.