Tunneling studies of vortices in high-sheet-resistance granular superconducting films.

Abstract

We have measured the tunneling density of states in ultrathin high-sheet-resistance, 16 {Omega}{lt}{ital R}{sub {ital N}}{lt}17 k{Omega}, granular Pb films in perpendicular magnetic fields, {ital H}. At the lowest {ital R}{sub {ital N}}, states appear within the superconducting energy gap at low fields ({ital H}{lt}0.1 T) indicating the presence of vortex cores. At the highest {ital R}{sub {ital 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 {ital R}{sub {ital N}} separating these two regimes.