PARITY OF THE ORDER-PARAMETER IN HIGH-TEMPERATURE SUPERCONDUCTORS WITH RESPECT TO A PI 2 ROTATION/

In order to establish the parity of the order parameter in the model b ased on the idea of a strong phonon-mediated long-range attraction, en hanced by the extended saddle-point singularities in the electron spec trum, plus a weak short-range repulsion, a comparison of critical temp eratures and free energies of the even and odd solutions for the order parameter is performed. A criterion is established distinguishing the two options. It is shown, particularly, that superconductivity, induc ed by phonons, can, nevertheless, lead to an odd order parameter. On t he other hand, contrary to the previous statements, this is the only p ossibility for the order parameter to change its sign within this mode l. The theory is influenced by the latest experimental results.