engleski

Mixed state conductivity of thin niobium films in perpendicular magnetic fields

Microwave complex frequency shift has been measured for high quality niobium films in perpendicular static magnetic fields up to the upper critical fields. The data have been analyzed by the effective conductivity model for the type-II superconductors in the mixed state. The depinning frequencies and the upper critical fields have been determined from the analysis. The film of thickness 10 nm has $\kappa \approx 9.5$ and depinning frequency $\approx 20$ GHz. It can serve as a model system for high-$\kappa$ superconductors. The upper critical fields were determined by the flow resistivity in low fields and by the crossover to the normal state. The former method gives slightly higher upper critical field values, reflecting the Ginzburg-Landau coherence length of nonoverlaping vortices.

Nb thin films; upper critical fields; microwave response

Proceedings of the 8th International Conference on Materials and Mechanisms of Superconductivity and High Temperature Superconductors - M2S-HTSC VIII, Dresden, Germany, 9-14 July 2006, Edited by B. Büchner and J. Fink