Anisotropic normal-state properties of the MgB2 superconductor

Based on the experimentally-found existence of two superconducting gaps in M-B, (one gap associated to the boron g-states and the other to the boron pi-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 analysed in the rigid band approximation; this permitted a detailed study of the partial Substitution of magnesium for aluminium (Mg1-xAlxB2). Other substitutions such as AB(2) (A = Be, Sc, Zr, Nb and Ta) are also discussed. The MgB2 or-bands (boron or-states), which are associated to the large gap, are very anisotropic at E-F, while the pi bands have very little anisotropic character. In Mg1-xAlxB2, T-c diminishes with Al content; the other compounds are not superconductors or have a low T-c. In this work it was found that with electron doping, such as Al substitution, the or-band conductivity decreases and the corresponding bands become less anisotropic. The sigma-band contribution for BeB2 and ScB2 at EF is very small and the anisotropy is much lower. For Zr, Nb and Ta there are no sigma-bands at EF. These results give a clear connection between superconductivity and the character of the sigma-band, band conductivity, and band anisotropy. This gives a plausible explanation for the diminution of T-c with different doping of MgB2.