Fluxon dynamics by microwave surface resistance measurements in MgB 2

Abstract

Field-induced variations of the microwave surface resistance, R s( H), have been investigated in high-density ceramic MgB 2. At low temperatures, several peculiarities of the R s( H) curves cannot be justified in the framework of models reported in the literature. We suggest that they are ascribable to the unconventional vortex structure in MgB 2, related to the presence of two gaps. On the contrary, the results near T c can be accounted for by 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.