Effect of ferromagnetic spin fluctuations on the electronic states in Na_x CoO₂ based on two-dimensional triangular lattice 11-band d-p model

Electronic states of the CoO₂ plane in Na_x CoO₂ are investigated on the basis of the triangular lattice 11-band d-p model by using the second order perturbation (SOP) and the random phase approximation (RPA). Due to the effect of the in-plane ferromagnetic fluctuations enhanced for 0.6 ≲ x ≲ 0.75, the quasiparticle dispersion near the Γ-point is strongly reduced to yield an almost flat dispersion just above the fermi level. With increasing x, the electronic specific heat coefficient γ increases towards the critical doping x_c ∼ 0.75 above which the in-plane ferromagnetism takes place. We also find that the six small hole pockets near the K-point are stabilized due to the electron correlation effect within both the SOP and the RPA.