Ferromagnetic and triplet-pairing instabilities controlled by trigonal distortion of CoO₆ octahedra in Na_xCoO₂· yH₂O

In the recently discovered Co-oxide superconductor Na_xCoO ₂·yH₂O, the edge-shared CoO₆ octahedra are trigonally contracted along the c-axis in the CoO₂-plane. We study how this CoO₆ distortion affects the magnetic properties and superconductivity in this compound 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.