Abstract
Microwave complex frequency shift has been measured for high quality niobium films in perpendicular static magnetic fields up to the upper critical field. Data have been analyzed by the effective conductivity model for type-II superconductors in the mixed state. Depinning frequencies and upper critical fields have been determined from the analysis. The film of thickness 10 nm has κ ≈ 9.5 and depinning frequency ≈20 GHz. It can serve as a model system for high- κ 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.
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