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Abstract

We report the results of a comprehensive spectroscopic ellipsometry study of NaCu(2)O(2), a compound composed of chains of edge-sharing Cu(2+)O(4) plaquettes and planes of Cu(1+) ions in a O-Cu(1+)-O dumbbell configuration, in the spectral range 0.75-6.5 eV at temperatures 7-300 K. The spectra of the dielectric function for light polarized parallel to the Cu(1+) planes reveal a strong in-plane anisotropy of the interband excitations. Strong and sharp absorption bands peaked at 3.45 eV (3.7 eV) dominate the spectra for polarization along (perpendicular) to the Cu(2+)O(2) chains. They are superimposed on flat and featureless plateaux above the absorption edges at 2.25 eV (2.5 eV). Based on density-functional calculations, the anomalous absorption peaks can be assigned to transitions between bands formed by Cu(1+) 3d(xz) (d(yz)) and Cu(2+) 3d(xy) orbitals, strongly hybridized with O p states. The major contribution to the background response comes from transitions between Cu(1+) 3d(z)2 and 4p(x) (p(y)) bands. This assignment accounts for the measured in-plane anisotropy. The dielectric response along the Cu(2+) O(2) chains develops a weak two-peak structure centered at 2.1 and 2.65 eV upon cooling below similar to 100 K, along with the appearance of spin correlations along the Cu(2+) O(2) chains. These features bear a striking resemblance to those observed in the single-valent Cu(2+) O(2) chain compound LiCuVO(4), which were identified as an exciton doublet associated with transitions to the upper Hubbard band that emerges as a consequence of the long-range Coulomb interaction between electrons on neighboring Cu(2+) sites along the chains. An analysis of the spectral weights of these features yields the parameters characterizing the on-site and long-range Coulomb interactions.