Abstract
We report a significant enhancement of the upper critical fieldHc2 of differentMgB2 samplesalloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivitiesρ, we show a cleartrend of an increase in Hc2 as ρ increases. One particular high resistivity film had a zero-temperatureHc2(0) wellabove the Hc2 values of competing non-cuprate superconductors such asNb3Sn 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 highestHc2 film also exhibits a significant upward curvature ofHc2(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 Hc2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivityin MgB2, which offers special opportunities for furtherHc2 increases by tuning of the impurity scattering by selective alloying onMg and B sites. Our experimental results can be explained by a theoryof two-gap superconductivity in the dirty limit. The very high values ofHc2(T) observedsuggest that MgB2 can be made into a versatile, competitive high-field superconductor.
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