Very high upper critical fields in MgB2 produced by selective tuning of impurity scattering

We report a significant enhancement of the upper critical field H-c2 of different MgB2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities rho, we show a clear trend of an increase in H-c2 as rho 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 Nb3Sn and Nb-Ti. Our high-field transport measurements give record values H-c2(perpendicular to) (0) approximate to34 T and H-c2(parallel to) (0) approximate to49 T for high resistivity films and H-c2(0) approximate to 29 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 gamma (T) = H-c2(parallel to)/H-c2(perpendicular to) opposite to that of single crystals: gamma (T) decreases as the temperature decreases, from gamma (T-c) approximate to 2 to gamma (0) approximate to 1.5. This remarkable H-c2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB2, 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 MgB2 can be made into a versatile, competitive high-field superconductor.