Deformation of electronic structures due to CoO₆ distortion and phase diagrams of Na_xCoO₂·yH₂O

Motivated by recently reported experimental phase diagrams, we study the effects of CoO₆ distortion on the electronic structure in Na _xCoO₂·yH₂O. We construct the multiband tight-binding model by employing the LDA result. Analyzing this model, we show the deformation of band dispersions and Fermi-surface topology as functions of CoO₂-layer thickness. Considering these results together with previous theoretical ones, we propose a possible schematic phase diagram with three successive phases: the extended s-wave superconductivity (SC), the magnetic order, and the spin-triplet SC phases when the Co valence number s is +3.4. A phase diagram with only one phase of spin-triplet SC is also proposed for the s = +3:5 case.