Charge ordering and magneto-polarons in Na$_{0.82}$CoO$_2$

TL;DR

This study investigates charge ordering and magneto-polaron formation in Na$_{0.82}$CoO$_2$ using spectral ellipsometry, revealing two key temperature-driven transitions related to charge and spin interactions.

Contribution

It provides experimental evidence of charge ordering and magneto-polaron formation in Na$_{0.82}$CoO$_2$, highlighting the interplay between charge, lattice, and spin degrees of freedom.

Findings

Charge ordering occurs at 280 K.

A spin-charge coupling transition at 19.8 K.

Formation of magneto-polarons due to charge-induced spin-state transitions.

Abstract

Using spectral ellipsometry, we have measured the dielectric function of a Na$_{0.82(2)}$CoO$_2$ crystal that exhibits bulk antiferromagnetism with T$_{N}$=19.8 K. We identify two prominent transitions as a function of temperature. The first one at 280 K involves marked changes of the electronic and the lattice response that are indicative of charge ordering in the CoO$_{2}$ layers. The second transition coincides with T$_{N}$=19.8 K and reveals a sizeable spin-charge coupling. The data are discussed in terms of charge ordering and formation of magneto-polarons due to a charge-induced spin-state transition of adjacent Co$^{3+}$ ions.