Superconducting transition temperature and the thickness of CoO2 planes of NaxCoO2·yH2O

The superconductivity of Na_xCoO₂·yH₂O appears in two regions of ν_Q divided by a narrow nonsuperconducting phase in the T_c–ν_Q phase diagram, where T_c and ν_Q are the superconducting transition temperature and ⁵⁹Co-nuclear quadrupole frequency, respectively. It suggests that the existence/nonexistence of the superconductivity depends on the local structure around Co sites or on the thickness of the CoO₂ planes, through the change of the crystal field. We have carried out specific heat measurements on several samples with different ν_Q values distributing over both the superconducting ν_Q regions, and found that the electronic specific heat coefficient γ does not change significantly with ν_Q. It suggests that a topological change of the Fermi surface which has been proposed as a possible origin of the existence of the two superconducting regions, does not take place with the change of the local structure around Co sites or thickness of the CoO₂ planes. We have also carried out neutron inelastic measurements on aligned crystals of Na_xCoO₂·yD₂O, and found that ferromagnetic fluctuations with two-dimensional character observed in the high temperature region lose their intensities with decreasing T and becomes inappreciable below 25 K. It indicates that the hole-pockets near the K points in the reciprocal space do not exist in these crystals. Combining these results, we can exclude, in the entire region of ν_Q, the existence of the hole-pockets, on which arguments of the possible triplet superconductivity are based. The present results are consistent with the singlet pairing which we showed in previous papers.