Critical current of Nb and NbN thin-film structures:: The cross-section dependence

We present results of a systematic study of the critical current in superconducting Nb and NbN thin film bridges. The bridges with a width from 50 nm to 10 mu m were patterned from thin superconducting films by means of electron-beam lithography and ion milling. For both materials the nominal critical current density extrapolated to zero temperature varied with the bridge width and thickness. We attribute these variations to a fabrication-enhanced reduction of the effective, superconducting cross-section of the bridges with respect to their geometric cross-section and to an uneven distribution of the super-current over the superconducting core of the bridge. In very thin bridges, i.e. 5 nm and 8 nm for NbN and Nb, respectively, the nominal current density increased drastically when the bridge width became smaller than 500 nm. We associate the enhancement of the critical current density in narrow bridges with the crossover from depinning of magnetic vortices to either their nucleation or breaking of Cooper pairs. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.