Based on the experimentally-found existence of two superconducting gaps in MgB2 (one gap associated with the B σ-states and the other with the B π-states), the different contributions to the transport properties, electrical conductivity and Hall coefficient were studied using the full potential-linearized augmented plane wave method and the generalized gradient approximation. Four different relaxation times were needed to adjust the electrical conductivity and Hall coefficient to experimental values. MgB2 doping was analyzed in the rigid band approximation; this permitted a detailed study of the partial substitution of Mg for Al (Mg1-xAlxB2). Other substitutions such as AB2 (A = Be, Sc, Zr, Nb and Ta) were also considered. The MgB2 σ-bands (B σ-states), which were associated with the large gap, were very anisotropic at EF, while the π bands have very little anisotropic character. In Mg1-xAlxB2, TC diminishes with Al content; the other compounds were not superconductors or have a low TC. It was found here that, with electron doping such as Al substitution, the σ-band conductivity decreased and the corresponding bands became less anisotropic. The σ-band contribution for BeB2 and ScB2 at EF was very small and the anisotropy was much lower. For Zr, Nb and Ta there were no σ-bands at EF. These results gave a clear connection between superconductivity and the character of the σ-band, band conductivity, and band anisotropy. This gave a plausible explanation for the diminution of TC with differing degrees of doping of MgB2.

Anisotropic Normal-State Properties of the MgB2 Superconductor. P.de la Mora, M.Castro, G.Tavizon: Journal of Physics - Condensed Matter, 2005, 17, 965-78