Critical field of Al-doped MgB(2) samples: Correlation with the suppression of the sigma-band gap

Abstract: In this paper, a study of the effect of Al substitution on the upper critical field, B(c2), in Al(x)Mg(1-x)B(2) samples is presented. We find a straightforward correlation between B(c2) and the sigma-band gap, Delta(sigma), evaluated by point-contact measurements. Up to x=0.2, B(c2) can be well described within a clean limit model and its decrease with x is directly related to the suppression of Delta(sigma).