Doped t-J model on a triangular lattice: Possible application to Na x CoO 2 ⋅yH 2 O and Na 1-x TiO 2

We report the finding of time-reversal-symmetry-breaking d x 2- x 2+id x y 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. There results are potentially relevant to doped Mott insulators Na x CoO 2 . y H 2 O and Na 1 - x TiO 2 .