Critical field of Al-doped MgB2 samples: Correlation with the suppression of the sigma-band gap

In this paper, a study of the effect of Al substitution on the upper critical field, Bc2, in AlxMg1-xB2 samples is presented. We find a straightforward correlation between Bc2 and the sigma-band gap, Delta_sigma, evaluated by point-contact measurements. Up to x=0.2, Bc2 can be well described within a clean limit model and its decrease with x is directly related to the suppression of Delta_sigma.