Microwave response of thin niobium films under perpendicular static magnetic fields

Abstract

The microwave response of high quality niobium films in a perpendicular static magnetic field has been investigated. The complex frequency shift was measured 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. Thin niobium films can serve as model system for the upper critical field determination of high-κ superconducting samples, and can be used with high-temperature superconductors even at temperatures much below Tc. It is shown that for samples with high values of depinning frequency, one should measure both components of the complex frequency shift in order to determine the flow resistivity. The thick Nb film (160 nm) has low resistivity at 10 K, comparable to the best single crystals, and low κ value. In contrast, the thinnest (10 nm) film has κ ≈ 9.5 and exhibits a high depinning frequency (≈ 20 GHz) . The upper critical field determined from microwave measurements is related to the radius of nonoverlaping vortices, and appears to be larger than the one determined by the transition to the normal state