Origin of Weak Pseudogap Behaviors in Na_0.35CoO₂: Absence of Small Hole Pockets

Abstract

We analyze the normal electronic states of Na_0.35CoO₂ based on the effective d–p model with a full d-orbital freedom using the fluctuation-exchange (FLEX) approximation. The electronic states sensitively depend on the topology of Fermi surfaces, which changes with the crystalline electric splitting (CES) due to the trigonal deformation. We succeed in reproducing the weak pseudogap behaviors in the density of states (DOS) and in the uniform magnetic susceptibility below 300 K, assuming that six small hole pockets predicted by LDA band calculations are absent. If these pockets exist, on the contrary, “anti-pseudogap behaviors” should inevitably appear. Thus, the present study strongly supports the absence of the small hole pockets in Na_0.35CoO₂, as reported by recent ARPES measurements. A large Fermi surface around the Γ-point would account for the superconductivity in water-intercalated samples.