Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field

TL;DR

This study investigates the microwave surface resistance of pristine and neutron-irradiated MgB2 samples under magnetic fields, revealing how irradiation affects fluxon dynamics and the applicability of existing models.

Contribution

It provides new insights into the microwave response of MgB2 post-irradiation, highlighting anomalies and model applicability at different temperatures and fields.

Findings

Irradiated MgB2 shows merged superconducting gaps.

Strong irradiation effects are explained by a generalized Coffey and Clem model.

Pristine samples exhibit anomalies at low temperatures not explained by standard models.

Abstract

We report on the microwave surface resistance of two polycrystalline Mg11B2 samples; one consists of pristine material, the other has been irradiated at very high neutron fluence. It has already been reported that in the strongly irradiated sample the two gaps merge into a single value. The mw surface resistance has been measured in the linear regime as a function of the temperature and the DC magnetic field, at increasing and decreasing fields. The results obtained in the strongly irradiated sample are quite well justified in the framework of a generalized Coffey and Clem model, in which we take into account the field distribution inside the sample due to the critical state. The results obtained in the pristine sample show several anomalies, especially at low temperatures, which cannot be justified in the framework of standard models for the fluxon dynamics. Only at temperatures near Tc and for magnetic fields greater than 0.5Hc2(T) the experimental data can quantitatively be accounted for by the Coffey and Clem model, provided that the upper-critical-field anisotropy is taken into due account.