Origin of Weak Pseudogap Behaviors in Na_<0.35>CoO_2 : Absence of Small Hole Pockets(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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We analyze the normal electronic states of Na_<0.35>CoO_2 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 300K, assuming that six small hole pockets predicted by LDA band calculations are absent. If these pockets exist, on the contrary, "antipseudogap behaviors" should inevitably appear. Thus, the present study strongly supports the absence of the small hole pockets in Na_<0.35>CoO_2, as reported by recent ARPES measurements. A large Fermi surface around the Γ-point would account for the superconductivity in water-intercalated samples.
2005-08-15