Fermi surface evolution and Luttinger theorem in Na$_x$CoO$_2$: a systematic photoemission study

TL;DR

This systematic photoemission study of Na$_x$CoO$_2$ reveals a single hole-like Fermi surface consistent with Luttinger theorem across various doping levels, and shows the sinking of predicted small pockets below the Fermi level.

Contribution

It provides comprehensive experimental evidence on Fermi surface evolution and confirms Luttinger theorem applicability in Na$_x$CoO$_2$ over a wide doping range.

Findings

Single hole-like Fermi surface observed at all doping levels.

Fermi surface area changes with doping according to Luttinger theorem.

Predicted small Fermi surface pockets sink below the Fermi level regardless of doping.

Abstract

We report a systematic angle-resolved photoemission study on Na$_x$CoO$_2$ for a wide range of Na concentrations ($0.3 \leq x \leq 0.72$). In all the metallic samples at different $x$, we observed (i) only a single hole-like Fermi surface centered around $\Gamma$ and (ii) its area changes with $x$ according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The $e_g^\prime$ band and the associated small Fermi surface pockets near the $K$ points predicted by band calculations are found to ``sink'' below the Fermi energy in a manner almost independent of the doping and temperature.