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

By A. Gurevich, S. Patnaik, V. Braccini, K. Kim, C. Mielke, Xueyan Song, L. Cooley, S. D. Bu, D. M. Kim, J. Choi and 9 more. 2003

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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.

Subfields: Biomedical Engineering, Condensed Matter Physics

View Paper: https://www.semanticscholar.org/paper/266e3648d3d5a2b3da9dd0bc70c8a1f679a59340