Anisotropic optical response of the mixed-valent Mott-Hubbard insulator NaCu ₂O ₂

We report the results of a comprehensive spectroscopic ellipsometry study of NaCu ₂O ₂, a compound composed of chains of edge-sharing Cu2 ⁺O ₄ plaquettes and planes of Cu1 ⁺ ions in a O-Cu1 ⁺-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 Cu1 ⁺ 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 Cu2 ⁺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 Cu1 ⁺ 3d _xz(d _yz) and Cu2 ⁺ 3d _xy orbitals, strongly hybridized with O p states. The major contribution to the background response comes from transitions between Cu1 ⁺ 3d _z2 and 4p _x(p _y) bands. This assignment accounts for the measured in-plane anisotropy. The dielectric response along the Cu2 ⁺O ₂ chains develops a weak two-peak structure centered at 2.1 and 2.65 eV upon cooling below ∼100 K, along with the appearance of spin correlations along the Cu2 ⁺O ₂ chains. These features bear a striking resemblance to those observed in the single-valent Cu2 ⁺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 Cu2 ⁺ 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.