Effective Interaction between the inter-penetrating Kagome lattices in Na_xCoO_2

TL;DR

This paper develops a multi-orbital model for Na_xCoO_2, revealing how inter-penetrating Kagome lattices influence electronic interactions, leading to various symmetry-broken metallic states and complex ordering patterns.

Contribution

It introduces a novel multi-orbital model capturing the inter-penetrating Kagome lattice structure and analyzes resulting charge, spin, and orbital instabilities in Na_xCoO_2.

Findings

Multiple metallic states with broken symmetry are identified.

Charge, orbital, and spin orderings are characterized.

Superstructure formation at x=0.5 is explained within the model.

Abstract

A multi-orbital model for a CoO_2-layer in Na_xCoO_2 is derived. In this model the kinetic energy for the degenerate t_2g-orbitals is given by indirect hopping over oxygen, leading naturally to the concept of four inter-penetrating Kagome lattices. Local Coulomb interaction couples the four lattices and an effective Hamiltonian for the interaction in the top band can be written in terms of fermionic operators with four different flavors. Focusing on charge and spin density instabilities, a big variety of possible metallic states with spontaneously broken symmetry are found. These states lead to different charge, orbital, spin and angular momentum ordering patterns. The strong superstructure formation at x=0.5 is also discussed within this model.