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\nmean-field variational theories with Gutzwiller approximation. We find that a\nsuperconducting state with broken time-reversal symmetry (d+id state) is\nrealized in the parameter region close to the triangular lattice. The\nfrustration enlarges the region of superconductivity when $t&lt;0$ for the\nhole-doped case, which is equivalent to $t&gt;0$ for electron doping. We also\ndiscuss the SU(2) degeneracy at half-filling. The d+id state probably\ncorresponds to the spin gap state at half-filling.\n