Microwave response of thin niobium films under perpendicular static magnetic fields

TL;DR

This study investigates the microwave response of high-quality niobium films under perpendicular magnetic fields, analyzing complex frequency shifts to understand superconducting properties and vortex behavior.

Contribution

It applies the effective conductivity model to niobium films, demonstrating its effectiveness in analyzing superconducting parameters at various temperatures and film thicknesses.

Findings

Thick Nb film shows low resistivity comparable to single crystals.

Thin Nb film exhibits high kappa (~9.5) and depinning frequency (~20 GHz).

Microwave-determined upper critical field exceeds the transition to the normal state.

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. This model is found to yield consistent results for the coherence lengths in high-kappa superconducting samples, and can be used with HTSC even at temperatures much below T_c. 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 kappa value. In contrast, the thinnest (10 nm) film has kappa ~ 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.