Infrared Conductivity and Charge Ordering in NaCoO2

TL;DR

This study investigates how doping and temperature affect infrared conductivity and charge localization in NaCoO2, revealing charge ordering, the impact of spin-density waves, and Na+ ion dynamics.

Contribution

It provides new insights into charge localization mechanisms and the effects of Na+ ion ordering in NaCoO2 across different doping levels.

Findings

Charge localization evidenced by a far-infrared peak

Long-range charge ordering at x=0.5 creates a gap

Spin-Density Wave shifts the FIP at high x

Abstract

The infrared conductivity of NaxCoO2 is studied as a function of doping and temperature for x between 0.5 and 1. Charge localization in CoO2 layers shows up through a far-infrared peak (FIP) in the infrared conductivity which coexists with a small Drude contribution. Long-range ordering at x = 0.5 is confirmed to create a far-infrared gap, in addition to the FIP. At high x, the formation of a Spin-Density Wave reported below 22 K dramatically shifts the FIP to higher energy when x is incommensurate with the lattice, indicating an abrupt deepening of the localizing potential. The in-plane E1u phonon lifetime is shown to be sensitive to both "freezing" and ordering of the mobile Na+ ions. A comparison with the behavior of the FIP shows that such "freezing" is not the only origin of charge localization in the CoO layers.