Electronic structure and Fermi surface tolopogy of Na$_x$CoO$_2$

TL;DR

This paper develops an effective Hamiltonian for Na_xCoO_2, revealing a Fermi surface topology consistent with experiments and highlighting significant band renormalization effects due to strong correlations.

Contribution

It introduces a multiband model with exact solutions for CoO6 clusters, providing a more accurate description of the electronic structure than local-density approximation methods.

Findings

Fermi surface topology matches experimental observations

Significant band renormalization due to correlations

Effective hopping parameters quantified

Abstract

We construct an effective Hamiltonian for the motion of T2g highly correlated states in NaxCoO2. We solve exactly a multiband model in a CoO6 cluster with electronic occupation corresponding to a nominal Co valence of either +3 or +4. Using the ensuing ground states, we calculate the effective O mediated hopping t=0.10 eV between many-body T2g states, and estimate the direct hopping t'~0.04 eV. The trigonal splitting 3D=0.315 eV is taken from recent quantum chemistry calculations. The resulting effective Hamiltonian is solved using a generalized slave-boson mean-field approximation. The results show a significant band renormalization and a Fermi surface topology that agrees with experiment, in contrast to predictions using the local-density approximation.