Flux motion in a two-dimensional single-crystal Nb film

We have studied thermally activated flux motion (flux creep) in the resistive transition of a single-crystal ultrathin (20 Å) Nb film in perpendicular magnetic fields. This thermally activated resistance extends over four decades from 0.5 Rn to 10−5 Rn. Possible pinning mechanisms and the field dependence of the activation energy are discussed. Comparisons between this system and the high Tc cuprate superconductors, which are quasi-two-dimensional in nature, are made.