ARPES on Na$_{0.6}$CoO$_{2}$: Fermi surface, extended flat dispersion, and unusual band splitting

TL;DR

This ARPES study of Na$_{0.6}$CoO$_2$ reveals a consistent Fermi surface with theoretical predictions, but shows highly renormalized dispersion, an extended flat band above the Fermi level, and an unusual band splitting near the Fermi surface.

Contribution

First ARPES investigation of Na$_{0.6}$CoO$_2$ revealing detailed electronic structure, including flat bands and unexpected band splitting.

Findings

Fermi surface matches local density approximation predictions

Energy dispersion is highly renormalized and anisotropic

Extended flat band observed above the Fermi level near $\Gamma$-$K$

Abstract

The electronic structure of single crystals Na$_{0.6}$CoO$_2$, which are closely related to the superconducting Na$_{0.3}$CoO$_2$.$y$H$_2$O ($T_c \sim 5K$), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface is found to be consistent with the prediction of a local density band theory, the energy dispersion is highly renormalized, with an anisotropy along the two principle axes ($\Gamma$-$K$, $\Gamma$-$M$). Our ARPES result also indicates that an extended flat band is formed slightly above $E_F$ along $\Gamma$-$K$. In addition, an unusual band splitting is observed in the vicinity of the Fermi surface along the $\Gamma$-$M$ direction, which differs from the predicted bilayer splitting.