MAGNETIC TRANSITIONS AND SUPERCONDUCTIVITY IN THE T-J MODEL

With the use of the spin-wave and Born approximations the energy spect rum of the two-dimensional t-J model is self-consistently calculated i n the range of hole concentrations, 0 less than or equal to x less tha n or similar to 0.3. The anomalous magnon Green's functions, which ari se due to the hole-magnon interaction, are taken into consideration. T hey lead to a sharp transition from short-range antiferromagnetic orde r to a completely disordered paramagnetic state at x approximate to 0. 19, in addition to the transition from long-range to short-range antif erromagnetic order at x approximate to 0.02-0.04. In the region of hol e concentrations 0.04 less than or similar to x less than or similar t o 0.19 the obtained shape of the Fermi surface, the hole dispersion ne ar the Fermi level, and the density of states on it are in satisfactor y agreement with experiment in La2-xSrxCuO4 and Bi2212. The Eliashberg formalism is used for calculating T-c. The hole-magnon interaction is found to be unable alone to give rise to superconductivity. By adding a moderate interaction with apex oxygen vibrations high T-c's are obt ained for even-frequency d(x2-y2) pairing in the range 0.04 less than or similar to x less than or similar to 0.19. For larger hole concentr ations the odd-frequency s-wave solution becomes the leading one which can lead to s-wave superconductivity in the overdoped regime with the participation of a hole-phonon interaction of the respective symmetry .