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Infrared Conductivity and Charge Ordering in NaCoO2

S. Lupi·M. Ortolani·L. Baldassarre·P. Calvani·D. Prabhakaran·A. T. Boothroyd
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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.

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S. Lupi, M. Ortolani, L. Baldassarre, P. Calvani, D. Prabhakaran, A. T. Boothroyd. 2005-01-31. Infrared Conductivity and Charge Ordering in NaCoO2. https://doi.org/10.1103/physrevb.72.024550

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