Very high upper critical fields in MgB2produced by selective tuning of impurity scattering

We report a significant enhancement of the upper critical field H c2 of different MgB 2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities ρ, we show a clear trend of an increase in H c2 as ρ increases. One particular high resistivity film had a zero-temperature H c2 (0) well above the H c2 values of competing non-cuprate superconductors such as Nb 3 Sn and Nb–Ti. Our high-field transport measurements give record values T and T for high resistivity films and T for untextured bulk polycrystals. The highest H c2 film also exhibits a significant upward curvature of H c2 ( T ) and a temperature dependence of the anisotropy parameter opposite to that of single crystals: γ( T ) decreases as the temperature decreases, from to . This remarkable H c2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB 2 , which offers special opportunities for further H c2 increases by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of H c2 ( T ) observed suggest that MgB 2 can be made into a versatile, competitive high-field superconductor.