The electronic structure of the Na$_x$CoO$_2$ surface

TL;DR

This study investigates how surface effects and potential contamination influence the electronic structure of Na$_{1/3}$CoO$_2$, aiming to reconcile discrepancies between theoretical calculations and ARPES experimental observations.

Contribution

The paper provides the first detailed calculation of the surface electronic structure of Na$_{1/3}$CoO$_2$, highlighting its impact on Fermi surface features and potential contamination effects.

Findings

Surface formation significantly alters the Fermi surface compared to bulk.

Contamination like hydroxyl groups may influence surface electronic properties.

Results suggest surface effects could explain ARPES and theory discrepancies.

Abstract

The idea that surface effects may play an important role in suppressing $e_g'$ Fermi surface pockets on Na$_x$CoO$_2$ $(0.333 \le x \le 0.75)$ has been frequently proposed to explain the discrepancy between LDA calculations (performed on the bulk compound) which find $e_g$' hole pockets present and ARPES experiments, which do not observe the hole pockets. Since ARPES is a surface sensitive technique it is important to investigate the effects that surface formation will have on the electronic structure of Na$_{1/3}$CoO$_2$ in order to more accurately compare theory and experiment. We have calculated the band structure and Fermi surface of cleaved Na$_{1/3}$CoO$_2$ and determined that the surface non-trivially affects the fermiology in comparison to the bulk. Additionally, we examine the likelihood of possible hydroxyl cotamination and surface termination. Our results show that a combination of surface formation and contamination effects could resolve the ongoing controversy between ARPES experiments and theory.