Strongly interacting σ-electrons and MgB2 superconductivity

MgB(pnpn ) 2 is classified as a system with strongly interacting σelectrons and non-correlated π-electrons of boron ions. The kinematic and Coulomb interaction V between the orbitally degenerated σ-electrons provide the superconducting state with an anisotropic gap of s*-wave symmetry. The critical temperature Tc has a non-monotonic dependence on the distance r between the centers of gravity of σand π-bands. MgB2 corresponds to r = 0.085 eV and V = 0.45 eV in our model with flat bands. The derived superconducting density of electronic states is in good agreement with available experimental and theoretical data. The possibilities for increasing Tc are discussed.