Doped t-J Model on a Triangular Lattice: Possible Application to Na_xCoO₂cdot yH₂0 and Na_{1-x}TiO₂

We report the finding of time-reversal-symmetry-breaking $d_{x^2-y^2}+ id_{xy}$ superconducting ground state in the slave-boson mean-field theory for the t-J model on triangular lattice. For $t/J=-5$ ($t/J=-9$) pairing exists for $x < 13%$ ($x<8%$) upon electron doping, and $x <56%$ ($x<13%$) upon hole doping. These results are potentially relevant to doped Mott insulators Na$_x$CoO$_2$.yH$_2$O and Na$_{1-x}$TiO$_2$.