Superconductivity in Na_xCoO₂·yH₂O: Protection of a d₁+id₂ state by spin-charge separation

Superconductivity in Na_xCoO₂·yH₂O is likely to be a p or d wave; however, experiments are unable to pinpoint the symmetry. A simple estimate of pair breaking effects from an unavoidable “Na⁺ vacancy disorder” in an ordered Na⁺ lattice, at an optimal x_opt≈0.30 is shown to destroy a Fermi liquid-based p- or d-wave superconductivity. However, a robustness of superconducting and normal states, seen in experiments, is pointed out and argued to imply the presence of a “quantum protectorate,” possibly a spin-charge decoupling that protects a d₁+id₂ and not a p state. A calculation of Knight shift and 1∕T₁ in the framework of resonating valence bond mean-field theory and a fit to the data of Kobayashi et al. (e-print, cond-mat/0305649; cond-mat/0306264) is made.