Precise Control of Band Filling in NaxCoO2

TL;DR

This study systematically investigates how varying sodium content in NaxCoO2 influences its electronic properties, revealing a critical concentration where the material's magnetic and electronic behaviors change dramatically.

Contribution

It provides precise control over band filling in NaxCoO2 and identifies a critical Na concentration causing a significant electronic and magnetic transition.

Findings

Electronic properties change dramatically at x* between 0.58 and 0.59.

Fermi level touches the bottom of the a1g band at x*.

Emergence of a small electron pocket for x > x*.

Abstract

Electronic properties of the sodium cobaltate NaxCoO2 are systematically studied through a precise control of band filling. Resistivity, magnetic susceptibility and specific heat measurements are carried out on a series of high-quality polycrystalline samples prepared at 200 C with Na content in a wide range of 0.35 =< x =< 0.70. It is found that dramatic changes in electronic properties take place at a critical Na concentration x* that lies between 0.58 and 0.59, which separates a Pauli paramagnetic and a Curie-Weiss metals. It is suggested that at x* the Fermi level touches the bottom of the a1g band at the gamma point, leading to a crucial change in the density of states across x* and the emergence of a small electron pocket around the gamma point for x > x*.