Charge Ordering and Magnetopolarons in ${\mathrm{N}\mathrm{a}}_{0.82}{\mathrm{C}\mathrm{o}\mathrm{O}}_2$

Using spectral ellipsometry, we measured the dielectric function of a ${\mathrm{N}\mathrm{a}}_{0.82(2)}{\mathrm{C}\mathrm{o}\mathrm{O}}_2$ crystal that exhibits bulk antiferromagnetism with $T_N=19.8\mathrm{K}$. We identify two prominent transitions as a function of temperature. The first one at 280 K involves marked changes of the electronic and lattice responses that are indicative of charge ordering in the ${\mathrm{C}\mathrm{o}\mathrm{O}}_2$ layers. The second transition occurs around $T_N=19.8\mathrm{K}$ and reveals sizable spin-charge coupling. The data are discussed in terms of charge ordering and formation of magnetopolarons due to a charge-induced spin-state transition of adjacent ${\mathrm{C}\mathrm{o}}^{3+}$ ions.

Figure 1

Real parts of the in-plane optical conductivity, ${\sigma }_{1ab}=1/4\pi \nu {\epsilon }_{2ab}$, and dielectric function, ${\epsilon }_{1ab}$. The spectra of ${\sigma }_{1ab}$ (a) for the entire spectral range at 300 and 25 K (arrows mark interband transitions), (b) below $6000{\mathrm{c}\mathrm{m}}^{-1}$, and (c) below $650{\mathrm{c}\mathrm{m}}^{-1}$ (dc values from Ref. 11 are shown by solid circles, low-frequency extrapolations by dotted lines). Corresponding spectra of ${\epsilon }_{1ab}$ are shown in (d) and (e). The black dashed line in (d) shows the Drude-Lorentz fit at 25 K as outlined in the text. The inset of (e) gives a sketch of the suggested charge ordering pattern at $x=0.75$. (f) Splitting of the strongest phonon mode. Offsets have been added to the spectra for clarity.

Figure 2

The $c$-axis optical conductivity, ${\sigma }_{1c}$, for polarization perpendicular to the ${\mathrm{C}\mathrm{o}\mathrm{O}}_2$ layers. (a) Detailed view of the low-frequency range below $650{\mathrm{c}\mathrm{m}}^{-1}$ and (b) the full range below $5000{\mathrm{c}\mathrm{m}}^{-1}$. The inset shows the integrated spectral weight $\mathrm{S}\mathrm{W}={\int }_{80{\mathrm{c}\mathrm{m}}^{-1}}^{\omega }{\sigma }_{1c}({\omega }^{\prime })d{\omega }^{\prime }$ at different $T$.