Comparison of the influence of carbon substitution and neutron-induced defects on the upper critical field and flux pinning in MgB2 single crystals

The superconducting properties of pure and carbon-substituted MgB2 single crystals were studied prior to and after fast neutron irradiation. The C substitution significantly enhances the upper critical field Hc2. In comparison with an unsubstituted crystal, Hc2 parallel to the c-axis in Mg(B0.948C0.052)2, e.g. at 15 K, is enhanced by a factor of about 2 and the Hc2 anisotropy is reduced. After fast neutron irradiation very pronounced changes in flux pinning are observed, both for the pure and for the substituted crystal, and the impact of the induced defects on the critical current density is significantly stronger than that of substitution. On the other hand, the changes in Hc2 of the carbon-substituted crystal are much more pronounced than those caused by irradiation. The observed changes in Hc2 can be explained by taking the characteristic length scale of the defects introduced by C substitution and by neutron irradiation into account.