A significant enhancement of the upper critical field, HC2, of various samples alloyed with non-magnetic impurities was reported. By studying films and bulk polycrystals with different resistivities ρ, a clear trend of increasing HC2 with increasing ρ was demonstrated. A particularly high-resistivity film had a zero-temperature HC2(0) which was well above the HC2 values of competing non-cuprate superconductors such as Nb3Sn and Nb–Ti. These high-field transport measurements furnished then record values of HC2(0)  34T and HC2||(0)  49T for high resistivity films and HC2(0)  29T for non-textured bulk polycrystals. The highest HC2 film also exhibited a significant upward curvature of HC2(T) and a temperature dependence of the anisotropy parameter, γ(T) = HC2||/HC2; opposite to that of single crystals: γ(T) decreased as the temperature decreased, from γ(TC)  2 to γ(0)  1.5. This remarkable HC2 enhancement and its anomalous temperature dependence were a consequence of the 2-gap superconductivity in MgB2, which offers special opportunities for further HC2 increased by tuning of the impurity scattering by selective alloying on Mg and B sites. The experimental results could be explained by a theory of 2-gap superconductivity in the dirty limit.

Very High Upper Critical Fields in MgB2 Produced by Selective Tuning of Impurity Scattering. A.Gurevich, S.Patnaik, V.Braccini, K.H.Kim, C.Mielke, X.Song, L.D.Cooley, S.D.Bu, D.M.Kim, J.H.Choi, L.J.Belenky, J.Giencke, M.K.Lee, W.Tian, X.Q.Pan, A.Siri, E.E.Hellstrom, C.B.Eom, D.C.Larbalestier: Superconductor Science and Technology, 2004, 17, 278-86