Aluminium and carbon doped MgB2: band filling, band shift, and   anisotropy loss
  In this work the effect of Carbon and Aluminium doping on the multiband MgB2 superconductor is analyzed. Using the rigid band and virtual crystal approximations (RBA and VCA), it was found that the main effect of doping on the band structure is band filling and a relative band-shift. If this band-shift is eliminated with an appropriate change of scale, then the RBA provides a good description of the band structure as function of doping. With this procedure both the inplane electrical conductivity of the C- and Al-doped MgB2 and the superconducting critical temperature follow the same curve. The Tc graph approximately follows the sigma-band density of states; the differences between these two can be explained by loss of anisotropy which plays an important role in these systems. 
Sabina Ruiz-Chavarria, Gustavo Tavizon and Pablo de la Mora