Origin of the Weak Pseudo-gap Behaviors in Na_{0.35}CoO_2: Absence of Small Hole Pockets

TL;DR

This study explains the weak pseudo-gap behaviors in Na_{0.35}CoO_2 by analyzing its electronic states and Fermi surface topology, supporting the absence of small hole pockets as indicated by recent experiments.

Contribution

It demonstrates that the absence of small hole pockets explains the pseudo-gap behaviors, contrasting with prior predictions of their existence.

Findings

Weak pseudo-gap behaviors reproduced without small hole pockets.

Presence of small hole pockets would lead to anti-pseudo-gap behaviors.

Supports recent ARPES findings of no small hole pockets.

Abstract

We analyze the ``normal electronic states'' of Na_{0.35}CoO_2 based on the effective d-p model with full d-orbital freedom using the fluctuation-exchange (FLEX) approximation. They sensitively depend on the topology of the Fermi surfaces, which changes as the crystalline electric splitting (CES) due to the trigonal deformation. We succeed in reproducing the weak pseudo-gap 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. When they exist, on the contrary, then ``anti-pseudo-gap 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 \Gamma-point would account for the superconductivity in water-intercalated samples.