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

Abstract

We report a significant enhancement of the upper critical field $H_{c2}$ of different $Mg{B}_2$ 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 $N{b}_{3}Sn$ and Nb-Ti. Our high-field transport measurements give record values $H_{c2}^{\perp }(0)\approx 34T$ and $H_{c2}\Vert (0)\approx 49T$ for high resistivity films and $H_{c2}(0)\approx 29T$ 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 $\gamma (T)=H_{c2}\Vert /H_{c2}^{\perp }$ 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 $Mg{B}_2$, 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 $Mg{B}_2$ can be made into a versatile, competitive high-field superconductor.

An updated version of the paper (12/12/2002)that was placed on cond-mat on May 7 2003

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