Superconducting States in Frustrating<i>t</i>–<i>J</i>Model: A Model Connecting High-<i>T</i>_cCuprates, Organic Conductors and Na_<i>x</i>CoO₂

Abstract

The two-dimensional t-J model on a frustrating lattice is studied using mean-field variational theories with Gutzwiller approximation. We find that a superconducting state with broken time-reversal symmetry (d+id state) is realized in the parameter region close to the triangular lattice. The frustration enlarges the region of superconductivity when $t<0$ for the hole-doped case, which is equivalent to $t>0$ for electron doping. We also discuss the SU(2) degeneracy at half-filling. The d+id state probably corresponds to the spin gap state at half-filling.