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

• Published in: arXiv.org 2003-09
• Main Authors: Gurevich, A, Patnaik, S, Braccini, V, Kim, K H, Mielke, C, Song, X, Cooley, L D, Bu, S D, Kim, D M, Choi, J H, Belenky, L J, Giencke, J, Lee, M K, Tian, W, Pan, X Q, Siri, A, Hellstrom, E E, Eom, C B, Larbalestier, D C
• Format: Article
• Language: English
• Subjects: Anisotropy; Critical field (superconductivity); Curvature; Electrical resistivity; Impurities; Magnesium compounds; Niobium; Polycrystals; Scattering; Single crystals; Superconductivity; Temperature dependence; Tuning
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.0305474

We report a significant enhancement of the upper critical field \(H_{c2}\) of different \(MgB_2\) samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities \(\rho\), we show a clear trend of \(H_{c2}\) increase as \(\rho\) increases. One particular high resistivity film had zero-temperature \(H_{c2}(0)\) well above the \(H_{c2}\) values of competing non-cuprate superconductors such as \(Nb_3Sn\) and Nb-Ti. Our high-field transport measurements give record values \(H_{c2}^\perp (0) \approx 34T\) and \(H_{c2}\|(0) \approx 49 T\) for high resistivity films and \(H_{c2}(0)\approx 29 T\) for untextured bulk polycrystals. The highest \(H_{c2}\) film also exhibits a significant upward curvature of \(H_{c2}(T)\), and temperature dependence of the anisotropy parameter \(\gamma(T) = H_{c2}\|/ H_{c2}^\perp\) opposite to that of single crystals: \(\gamma(T)\) decreases as the temperature decreases, from \(\gamma(T_c) \approx 2\) to \(\gamma(0) \approx 1.5\). This remarkable \(H_{c2}\) enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in \(MgB_2\), which offers special opportunities for further \(H_{c2}\) increase 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 \(MgB_2\) can be made into a versatile, competitive high-field superconductor.