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.
@article{arxiv.cond-mat/0501746,
title = {Infrared Conductivity and Charge Ordering in NaCoO2},
author = {S. Lupi and M. Ortolani and L. Baldassarre and P. Calvani and D. Prabhakaran and A. T. Boothroyd},
journal= {arXiv preprint arXiv:cond-mat/0501746},
year = {2009}
}