Unconventional Charge Ordering in Na0.70CoO2 below 300 K

TL;DR

This study investigates charge and spin dynamics in Na0.70CoO2 between 50 and 340 K, revealing anomalies near 250-295 K that suggest unconventional charge ordering or electron localization phenomena.

Contribution

It provides new insights into charge ordering and spin dynamics in Na0.70CoO2, highlighting anomalies near 250-295 K without magnetic phase transitions.

Findings

Anomalies in NMR response near 250 and 295 K.

No evidence of magnetic phase transitions in susceptibility.

Indications of charge-density wave or electron localization phenomena.

Abstract

We present the results of measurements of the dc-magnetic susceptibility chi(T) and the 23Na-NMR response of Na_{0.70}CoO_{2} at temperatures between 50 and 340 K. The chi(T) data suggest that for T > 75 K, the Co ions adopt an effective configuration of Co^{3.4+}. The 23Na-NMR response reveals pronounced anomalies near 250 and 295 K, but no evidence for magnetic phase transitions is found in chi(T). Our data suggest the onset of a dramatic change in the Co 3d-electron spin dynamics at 295 K. This process is completed at 230 K. Our results maybe interpreted as evidence for either a tendency to electron localization or an unconventional charge-density wave phenomenon within the cobalt oxide layer, CoO_2, 3d electron system near room temperature.