  The nonadiabatic Heisenberg model (NHM) proposed as an extension of the
Heisenberg model makes a contribution to the eigenstate problem of
superconductivity. The Hamiltonian H^n derived within this group-theoretical
model has superconducting eigenstates if and only if the considered material
possesses a narrow, roughly half-filled "superconducting" energy band of
special symmetry in its band structure. This paper shows that the
high-temperature superconductor YBa_2Cu_3O_7 possesses such a superconducting
band. This new result together with previous observations about other
superconductors and non-superconductors corroborates the theoretical evidence
within the NHM that stable superconducting states are connected with
superconducting bands. It is proposed that the type of superconductivity, i.e.,
whether the material is a conventional low-T_c or a high-T_c superconductor, is
determined by the energetically lowest boson excitations that carry the crystal
spin 1*hbar and are sufficiently stable to transport this crystal spin-angular
momentum through the crystal. This mechanism provides the electron-phonon
mechanism that enters the BCS theory in conventional superconductors.
