Magnetic hysteresis in the microwave surface resistance of Nb samples in the critical state

TL;DR

This paper investigates the hysteretic behavior of microwave surface resistance in Nb superconductors in the critical state, using experiments and a model based on flux distribution to determine critical current density.

Contribution

It introduces a model incorporating flux distribution effects in the critical state to explain hysteresis in microwave surface resistance, validated by experimental data.

Findings

Quantitative agreement between model and experiments

Critical current density and its field dependence can be extracted from data

Hysteresis behavior explained by flux distribution in the critical state

Abstract

We discuss the hysteretic behavior of the field-induced variations of the microwave surface resistance in superconductors in the critical state. Measurements have been performed in a bulk sample of Nb and a powdered one at different values of the temperature. We discuss a model, based on the Coffey and Clem theory, in which we take into account the flux distribution inside the sample, due to the critical state. The experimental results are justified quantitatively in the framework of our model. We show that by fitting the experimental data it is possible to determine the value of the critical current density and its field dependence.