Giant microwave absorption in fine powders of superconductors

TL;DR

This paper reports the observation of enhanced microwave absorption in fine powders of type-II superconductors, consistent with vortex motion theory, and provides quantitative analysis of vortex pinning forces.

Contribution

It demonstrates that microwave absorption enhancement can be observed in small grains, overcoming skin-depth limitations, and applies the Coffey--Clem theory for analysis.

Findings

Enhanced microwave absorption observed in fine superconductor powders.

Quantitative agreement with vortex motion theory.

Determination of vortex pinning force constant.

Abstract

Enhanced microwave absorption, larger than that in the normal state, is observed in fine grains of type-II superconductors (MgB$_2$ and K$_3$C$_{60}$) for magnetic fields as small as a few $\%$ of the upper critical field. The effect is predicted by the theory of vortex motion in type-II superconductors, however its direct observation has been elusive due to skin-depth limitations; conventional microwave absorption studies employ larger samples where the microwave magnetic field exclusion significantly lowers the absorption. We show that the enhancement is observable in grains smaller than the penetration depth. A quantitative analysis on K$_3$C$_{60}$ in the framework of the Coffey--Clem (CC) theory explains well the temperature dependence of the microwave absorption and also allows to determine the vortex pinning force constant.