New aspects of microwave properties of Nb in the mixed state

TL;DR

This study investigates the microwave vortex dynamics in Nb thin films across various frequencies, revealing new phenomena such as field-dependent depinning, vortex creep deviations, and switching instabilities, enhancing understanding of superconductor behavior.

Contribution

It introduces a swept-frequency Corbino-disk technique to analyze vortex dynamics in Nb films, uncovering previously unnoticed features in microwave response.

Findings

Field-dependent depinning frequency observed

Deviations from standard frequency dependence suggest vortex creep

Switching phenomena indicative of vortex instabilities

Abstract

We present a study of the frequency dependence of the vortex dynamics in a conventional superconductor. We have employed a swept-frequency, Corbino-disk technique to investigate the temperature (3.6K-Tc) and high-field (from Hc2/2 to Hc2) microwave complex resistivity in Nb thin (20-40 nm) films as a function of the frequency (1-20 GHz). We have found several previously unnoticed features: (i) a field-dependent depinning frequency in the GHz range; (ii) deviations from the accepted frequency dependence, that can be ascribed to some kind of vortex creep; (iii) the presence of switching phenomena, reminiscent of vortex instabilities. We discuss the possible origin of the features here reported.