Electronic Model for CoO₂ Layer Based Systems: Chiral Resonating Valence Bond Metal and Superconductivity

Takada et al. have reported superconductivity in layered Na_xCoO₂yH₂O (T_c≈5   K). We model a reference neutral CoO₂ layer as an orbitally nondegenerate spin-1/2 antiferromagnetic Mott insulator on a triangular lattice and Na_xCoO₂yH₂O as electron doped Mott insulators described by a t-J model. It is suggested that at optimal doping chiral spin fluctuations enhanced by the dopant dynamics lead to a gapful d-wave superconducting state. A chiral resonating valence bond (RVB) metal, a parity and time (PT) reversal violating state with condensed RVB gauge fields, with a possible weak ferromagnetism, and low temperature p-wave superconductivity are also suggested at higher dopings.