THE SUPERSTRUCTURE EFFECT ON THE SUPERCONDUCTING PHASE-DIAGRAMS AND ON THE ELECTRON SPECTRUM SINGULARITIES NEAR VAN HOVE POINTS

We propose a possible mechanism of the superstructure effect on an ele ctron spectrum near its singular points. The two-dimensional electron spectrum of the tight-binding model and of the spin-polaron model, whi ch describes the motion of a hole in CuO2 planes, are considered. For the latter model we show, that the dependence of the superconducting t ransition temperature on the position of the Fermi level can have two maxima both near the bottom of the spin-polaron band and near its boun dary. This dependence has also a discontinuity of slope near the magne tic Brillouin band boundary. The existence of a discontinuity as well as the presence of the maxima are caused by the electron spectra trans formations under a long-wave perturbation.