We have calculated the one-electron susceptibility of hydrated NaxCoO2 and find strong nesting nearly commensurate with a 2X2 superstructure. The nesting involves about 70% of all electrons at the Fermi level and is robust with respect to doping. This nesting creates a tendency to a charge density wave compatible with the charge order often seen at x approx 0.5, which is usually ascribed to electrostatic repulsion of Na ions. In the spin channel, it gives rise to strong spin-fluctuations, which should be important for superconductivity. The superconducting state most compatible with this nesting structure is an odd-gap triplet s-wave state.
@article{arxiv.cond-mat/0403135,
title = {Nesting, spin-fluctuations, and odd-gap superconductivity in NaxCoO2 yH2O},
author = {M. D. Johannes and I. I. Mazin and D. J. Singh and D. A. Papaconstantopoulos},
journal= {arXiv preprint arXiv:cond-mat/0403135},
year = {2016}
}