Magnetic fluctuation and pairing instability controlled by the CoO₆ distortion in the sodium cobalt oxide

We study how the CoO₆ distortion along the c-axis affects the magnetic properties and superconductivity in the novel Co-oxide superconductor Na_x CoO₂ · y H₂ O by analyzing the multiorbital Hubbard model using the fluctuation-exchange approximation. It is shown that through generating the trigonal crystal field, the distortion pushes the Co e_g^′ bands up and consequently gives rise to the hole-pocket Fermi surfaces, which have been predicted in the band calculations. As the distortion increases, the hole pockets are enlarged and the ferromagnetic fluctuation as well as the pairing instability increases, which is in good agreement with recent NQR results.