Microwave Surface Resistance and Upper-Critical-Field Anisotropy of MgB2 Superconductor

TL;DR

This study investigates the microwave surface resistance and upper-critical-field anisotropy in MgB2 superconductors, revealing a temperature-independent anisotropy factor that varies between samples with similar shapes.

Contribution

It provides the first detailed analysis of the anisotropy factor in MgB2 using microwave surface resistance measurements and the Coffey-Clem model.

Findings

Anisotropy factor remains constant below Tc within 3 K.

Different anisotropy values observed in similar-shaped samples.

Microwave surface resistance data consistent with anisotropic Ginzburg-Landau theory.

Abstract

The field-induced variations of the microwave surface resistance are investigated in two different samples of powdered MgB2. The experimental results can be justified in the framework of the Coffey and Clem model with fluxons moving in the flux-flow regime, provided that the anisotropy of the upper critical field is taken into due account. Assuming the angular dependence of the upper critical field expected from the anisotropic Ginzburg-Landau theory, we determine the anisotropy factor by fitting the experimental data of the field dependence of the microwave surface resistance. We show that the anisotropy factor is constant in a range of temperatures of about 3 K below Tc and it takes on different values for the two samples, even though they have similar shape.