THE T-J MODEL .2. INSTABILITY OF THE MAGNETIC ORDER AND OF THE FERMI-SURFACE UNDER DOPING

The behaviour of the two-dimensional t-J model under doping near the half-filled state is considered. We demonstrate analytically the instability of the long-range Neel order and of the Fermi surface of the holes with respect to a special collective excitation. The exchange of this excitation leads to an attraction between the holes in the singlet state. An estimate of the superconducting gap is presented.