Depinning frequency in a heavily neutron-irradiated MgB2 sample

The magnetic-field-induced variations of the microwave surface resistance have been investigated in a heavily neutron-irradiated MgB2 sample, in which the irradiation has caused the merging of the two gapsinto a single value. The experimental results have been analyzed in the framework of the Coffey and Clem model. By fitting the experimental data, we have determined the field dependence of the depinning frequency,w0, at different values of the temperature. Although the pinning is not particularly effective, the value of w0 obtained at low temperatures is considerably higher than that observed in conventionallow-temperature superconductors.