Electronic model for CoO₂ layer based systems: chiral resonating valence bond metal and superconductivity

Abstract

Takada et al. have reported superconductivity in layered Na_xCoO₂yH₂O (T_c≈5 K)and more recently Wen et al. in A_xCoO_2+δ (A = Na,K)(T_c≈31 K). We model a reference neutral CoO₂ layer as an orbitally nondegenerate spin-½ antiferromagnetic Mott insulator on a triangular lattice and Na_xCoO₂.yH₂O and A_xCoO_2+δ 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.