Superconducting Bands Stabilizing Superconductivity in YBa2Cu3O7 and MgB2

It is shown that the ceramic superconductor YBa2Cu3O7 as well as the superconducting intermetallic compound MgB2 possesses a narrow, partly filled "superconducting band" with Wannier functions of special symmetry in their band structures. This result corroborates previous observations about the band structures of numerous superconductors and non-superconductors showing that evidently superconductivity is always connected with such superconducting bands. These findings are interpreted in the framework of a nonadiabatic extension of the Heisenberg model. Within this new group-theoretical model of correlated systems, Cooper pairs are stabilized by a nonadiabatic mechanism of constraining forces effective in narrow superconducting bands. The formation of Cooper pairs in a superconducting band is mediated by the energetically lowest boson excitations in the considered material that carry the crystal-spin angular momentum 1a <...a". These crystal-spin-1 bosons are proposed to determine whether the material is a conventional low-T (c) or a high-T (c) superconductor. This interpretation provides the electron-phonon mechanism that enters the BCS theory in conventional superconductors.