Critical field of MgB2: Crossover from clean to dirty regimes

We have studied the upper critical field Bc2 in polycrystalline MgB2 samples in which disorder was varied in a controlled way to carry selectively pi and sigma bands from clean to dirty limit. We have found that the clean regime survives when pi bands are dirty and sigma bands are midway between clean and dirty. In this framework we can explain the anomalous behavior of Al doped samples, in which Bc2 decreases as doping increases.