Charge ordering and magneto-polarons in Na$_x$CoO$_2$

Using spectral ellipsometry, we measured the dielectric function of a Na$_x$CoO$_2$ single crystals. In particular, we investigated samples with x$>$0.75 that exhibit bulk AF with T$_{N}$=20K. We identify two prominent transitions in the optical response as a function of temperature. The first one at 260-280 K involves marked changes of the electronic and lattice responses that are indicative of charge ordering. Besides the formation of a very narrow Drude-peak we observe a strong collective mode and the formation of a pseudogap with a large energy scale of about 0.5 eV. The second transition occurs at T$_N$, it also involves a surprisingly large energy scale of about 0.4 eV and gives clear evidence for a strong spin-charge coupling. The data are discussed in terms of charge ordering and formation of magneto-polarons due to a charge-induced spin-state transition of adjacent Co$^{+3}$ ions. \\ C.Bernhard et al., PRL 93, 167003 (2004)