engleski

Mixed state conductivity of thin niobium films in perpendicular magnetic fields

Cavity perturbation technique was used to measure temperature and magnetic field dependence of microwave complex frequency shift on high quality niobium thin films in perpendicular static magnetic field. Sample surface was oriented parallel to the microwave electric field and the static magnetic field varied up to the upper critical field. Data have been analysed by effective conductivity model for type II superconductors in mixed state which enabled us to determine depinning frequency of the sample. The analysis showed that depinning frequency is comparable to the microwave driving frequency and decreases with increasing film thickness indicating that pinning is dominated by surface deffects. Values of upper critical fields were determined by means of flow resistivity in low fields and by crossover to the normal state. Comparison of the two methods shows that the former always gives slightly higher upper critical field values, reflecting the Ginzburg– Landau coherence length of nonoverlaping vortices. We have shown that thin niobium films can serve as a model system for determination of upper critical field of high– $kappa$ superconductor from microwave measurements.

Nb thin films; upper critical fields; microwave response

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