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 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 (O) 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^⊥(O) ≈ 34 T and H_c2^∥ (0) ≈ 49 T for high resistivity films and H_c2(0) ≈ 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 γ(T) = H_c2^∥/H_c2^⊥ opposite to that of single crystals: γ(T) decreases as the temperature decreases, from γ(T_c) ≈ 2 to γ(0) ≈ 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.