Radiation effects on MgB2: a review and a comparison with A15 superconductors

摘要

The study of the effects of irradiation damage in superconducting materials is crucial for the twofold aim of helping to understand the nature of superconductivity and of improving the superconducting properties for applications. In this paper we review the main experiments performed on irradiated MgB2, with the main purpose being to investigate the suppression, or in some cases enhancement, of the superconducting properties. However, we do not treat the important but substantially different topic of irreversible properties such as magnetization and critical current densities. All the results obtained recently on damaged MgB2, primarily using neutrons and a particles, are compared with similar experiments performed in the past on A15 superconductors. We discuss the suppression of the critical temperature ( T-c) with disorder, showing the existence of a universal dependence of T-c on the residual resistivity. We review the experiments investigating the defect structure and the effect of annealing after irradiation. The behaviours of the upper critical fields, specific heat, resistivity and magnetoresistivity upon irradiation are also presented. The experiments of specific heat, tunnelling and point contact spectroscopies, which provide an evaluation of the energy gaps as a function of disorder, are summarized. Finally, phenomenological models for the changes in the superconducting properties are discussed in connection with the experimental evidence outlined before. The differences, as well as the important similarities, arising from the comparison between the behaviour of irradiated MgB2 and A15 samples provide significant indications of an overall understanding of radiation damage in superconducting materials.