The Fermi surface of Na$_{x}$CoO$_2$

TL;DR

This study systematically investigates how the Fermi surface topology of Na$_x$CoO$_2$ evolves with doping, revealing the presence or absence of hole pockets depending on the computational method, and confirms the measured Fermi surface aligns with LSDA+U predictions.

Contribution

It provides a comprehensive analysis of the doping-dependent Fermi surface of Na$_x$CoO$_2$ using multiple computational approaches, clarifying the role of electron correlations.

Findings

Large Fermi surface and small hole pockets predicted by LDA/LSDA at x~0.5.

Hole pockets are absent in LSDA+U calculations across doping levels.

Measured Fermi surface of Na$_{0.7}$CoO$_2$ matches LSDA+U predictions and shows no violation of Luttinger's rule.

Abstract

Doping evolution of the Fermi surface topology of Na$_x$CoO$_2$ is studied systematically. Both local density approximation (LDA) and local spin density approximation (LSDA) predict a large Fermi surface as well as small hole pockets for doping levels $x\sim$ 0.5. In contrast, the hole pockets are completely absent for all doping levels within LSDA+U. More importantly, we find no violation of Luttinger's rule in this system, contrary to a recent suggestion. The measured Fermi surface of Na$_{0.7}$CoO$_2$ can be explained by its half-metallic behavior and agrees with our LSDA+U calculations.