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

We report a significant enhancement of the upper critical field Hc2 of different MgB2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities ρ ,w e sho wac lear trend of a ni ncrease in Hc2 as ρ increases. One particular high resistivity film had a zero-temperature Hc2(0) well above the Hc2 values of competing non-cuprate superconductors such as Nb3Sn and Nb–Ti. Our high-field transport measurements give record values H ⊥ c2 (0) ≈ 34 T and H || c2 (0) ≈ 49 T for high resistivity films and Hc2(0) ≈ 29 T for untextured bulk polycrystals. The highest Hc2 film also exhibits a significant upward curvature of Hc2(T ) and a temperature dependence of the anisotropy parameter γ( T ) = H || c2 /H ⊥ c2 opposite to that of single crystals: γ( T ) decreases as the temperature decreases, from γ( Tc) ≈ 2t o γ( 0) ≈ 1.5. This remarkable Hc2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB2 ,w hich offers special opportunities for further Hc2 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 Hc2(T ) observed suggest that MgB2 can be made into a versatile, competitive high-field superconductor.