Critical Field of Al-Doped MgB2 Samples: Correlation with the   Suppression of Sigma-Band Gap
  In this Letter, the 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. For larger doping we observed the crossover to the dirty regime driven mostly by the strong decrease of Delta_sigma rather than by the increase of the sigma-band scattering rate 
M.Putti, C.Ferdeghini, M.Monni, I.Pallecchi, C.Tarantini,   P.Manfrinetti, A.Palenzona, D.Daghero, R.S.Gonnelli, V.A. Stepanov