Nesting, Spin Fluctuations, and Odd-Gap Superconductivity inNaxCoO2·yH2O

We calculated the one-electron susceptibility of hydrated ${\mathrm{N}\mathrm{a}}_x{\mathrm{C}\mathrm{o}\mathrm{O}}_2$ and find strong nesting, involving about 70% of all electrons at the Fermi level and nearly commensurate with a $2\times 2$ superstructure. This nesting creates a tendency to a charge density wave compatible with the charge order often seen at $x\approx 0.5$ and usually ascribed to electrostatic repulsion of Na ions. In the spin channel, it leads to strong spin fluctuations, which should be important for superconductivity. The state most compatible with this nesting structure is an odd-gap triplet $s$-wave state.