Effects of ferromagnetic fluctuations on the electric and thermal transport properties in Na_xCoO₂

We investigate the electronic states and the transport properties of the CoO₂ plane in Na_xCoO₂ on the basis of the two-dimensional triangular lattice 11-band d-p model by using the fluctuation exchange approximation, where we consider the Coulomb interaction between the t_2g electrons on a Co site. It is found that all of the effective mass of quasiparticles, the resistivity, the thermoelectric power and the uniform spin susceptibility increase with increasing x for x > 0.6. This implies that the ferromagnetic fluctuations play significant roles on determining the electronic states. These results are qualitatively consistent with the experiments.