Na Induced Correlations in Na$_x$CoO$_2$

TL;DR

This paper investigates how sodium content influences electronic correlations in Na$_x$CoO$_2$, revealing that Na potential and ordering significantly affect the emergence of strong correlation effects in this material.

Contribution

The study introduces a minimal low-energy Hamiltonian and demonstrates the crucial role of Na potential and ordering in correlation phenomena using LDA+DMFT.

Findings

Strong correlations appear at high Na content ($x oughly0.7$) but not at low Na content ($x oughly0.3$).

Na potential causes a binary perturbation of Co sites, affecting electronic correlations.

Na ordering predicts a binary pattern of Co site perturbations linked to Na site positions.

Abstract

Increasing experimental evidence is building which indicates that signatures of strong correlations are present in the Na rich region of Na$_x$CoO$_2$ (ie. $x\approx0.7$) and absent in the Na poor region (ie. $x\approx0.3$). This is unexpected given that NaCoO$_2$ is a band insulator and CoO$_2$ has an integer filled open shell making it a candidate for strong correlations. We explain these experimental observations by presenting a minimal low-energy Hamiltonian for the cobaltates and solving it within LDA+DMFT. The Na potential is shown to be a key element in understanding correlations in this material. Furthermore, LDA calculations for the realistic Na ordering predict a \emph{binary} perturbation of the Co sites which correlates with the Na$_1$ sites (ie. Na sites above/below Co sites).