CoO2-layer-thickness dependence of magnetic properties and possible two different superconducting states in NaxCoO2·yH2O

In order to understand the experimentally proposed phase diagrams of NaxCoO2·yH2O, we theoretically study the CoO2-layer-thickness dependence of its magnetic and superconducting (SC) properties by analyzing a mulitorbital Hubbard model using the random phase approximation. When the Co valence s is +3.4, we show that the magnetic fluctuation exhibits a strong layer-thickness dependence, where it is enhanced at finite (zero) momentum in the thicker (thinner) layer system. A magnetic order phase appears sandwiched by two SC phases, consistent with the experiments. These two SC phases have different pairing states, where one is the singlet extended s-wave state and the other is the triplet p-wave state. On the other hand, only a triplet p-wave SC phase with a dome-shaped behavior of Tc is predicted when s=+3.5, which is also consistent with the experiments. Controversial experimental results on the magnetic properties are also discussed. (author)