PERPENDICULAR UPPER CRITICAL-FIELD OF GRANULAR PB FILMS NEAR THE SUPERCONDUCTOR-TO-INSULATOR TRANSITION

We have measured the perpendicular upper critical field, H(c2)(T), in ultrathin granular Pb films with normal-state sheet resistances, R(N), that range from 10 OMEGA to 11 kOMEGA. The films with the highest R(N ) are near the threshold for the superconductor-to-insulator (S-I) tra nsition. We compare our results to calculations for granular supercond uctors and obtain values for the grain size and coherence length chara cteristic of a single grain. In addition, we present a measurement of the ratio of the intergrain Josephson-coupling energy to the single-gr ain condensation energy near the S - I transition in these films. The small value of this ratio, 0.05, at R(N) = 11 kOMEGA is consistent wit h the picture that fluctuations in the phase of the order parameter do minate the S - I transition in these films.