Comparison of $H_{c2}(T,\theta)$ in Mg$_{1-x}$Al$_x$B$_2$ single   crystals with the dirty-limit two-gap theory
  We studied the temperature and the angular dependences of the upper critical field ($H_{c2}(T,\theta$)) of Mg$_{1-x}$Al$_x$B$_2$ single crystals ($x = 0.12$ and 0.21) and compared with the dirty-limit two-gap theory. We found that $H_{c2}(T,\theta)$'s were well described in a unified way by this theory. The obtained values of the parameters indicated that as the Al concentration was increased, anisotropic impurity scattering increased, making the $\sigma$ bands less anisotropic. Accordingly, the temperature dependence of the anisotropy ratio of $H_{c2}$ ($\gamma_H$) systematically decreased, and for $x=0.21$, $\gamma_H$ was nearly constant. Our results imply that Mg$_{1-x}$Al$_x$B$_2$ single crystals are in dirty-limit and that two-gap nature survives until $x = 0.21$. 
Heon-Jung Kim, Hyun-Sook Lee, Byeongwon Kang, Woon-Ha Yim, Younghun   Jo, Myung-Hwa Jung, and Sung-Ik Lee