PHASE-DIAGRAM AND PAIRING SYMMETRY OF THE 2-DIMENSIONAL T-J MODEL BY A VARIATION THEORY

Two-dimensional t-J model is studied by a variational Monte Carlo meth od, using Gutzwiller-Jastrow-type wave functions. Various kinds of sup erconducting pairing symmetries are compared in order to determine the phase diagram of the ground state in the full J/t-n plane. Near the h alf filling where the high temperature superconductivity is expected, the d(x2-y2)-wave pairing state is always the most stable among variou s symmetries. The three-site term hardly changes the phase diagram in this regime. In the low electron density, the extended s-type wave bec omes a quantitatively good state for large J/t although the energy gai n is small. The Gutzwiller wave function is shown to be the exact grou nd state in the low-electron-density limit for the supersymmetric case (J/t = 2).