Microwave surface resistance and upper-critical-field anisotropy of MgB2 superconductor

摘要

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 T-c and it takes on different values for the two samples, even though they have similar shape. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 33]