We investigated the effect of alloying on the upper critical field $H_{ c2}$ in 12 $MgB_2$ films, in which disorder was introduced by growth, carbon doping or He-ion irradiation, finding a significant $H_{ c2}$ enhancement in C-alloyed films, and an anomalous upward curvature of $H_{ c2}(T)$. Record high values of $H_{ c2}^{ \perp}(4.2) \simeq 35T$ and $H_{ c2}\|(4.2) \simeq 51T$ were observed perpendicular and parallel to the ab plane, respectively. The temperature dependence of $H_{ c2}(T)$ is described well by a theory of dirty two-gap superconductivity. Extrapolation of the experimental data to T=0 suggests that $H_{ c2}\|(0)$ approaches the paramagnetic limit of $\sim 70T$.
Categories: cond-mat.supr-con