Optical Properties and Correlation Effects in NaxCoO2

TL;DR

This study calculates the optical spectra of NaxCoO2 for various x values using LDA, compares results with experiments, and discusses the effects of correlation and hybridization on optical properties.

Contribution

It demonstrates that LDA reproduces key features but overestimates peak positions, and shows LDA+U is unsuitable for this material due to increased discrepancies.

Findings

LDA captures main spectral features but with shifted peaks.

Overhybridization and spin fluctuations affect optical spectra.

LDA+U worsens agreement with experimental data.

Abstract

We have calculated the optical spectra of Na$_{x}$CoO$_2$ for $x$=0.3, 0.5, and 0.7 within the LDA. We compare our results to available experimental data and show that the important features and trends are reproduced well, but there is a nearly uniform shift of peak positions and poor agreement in intensities. We show, through application of a simple model, that these differences can be attributed to overhybridization between Co and O orbitals and spin fluctuations which renormalize the bandwidth. Applying the LDA+U procedure shifts the optical peaks further from their experimental locations, indicating that this method of incorporating correlation effects is ill-suited for the case NaxCoO2.