Abstract
We report the results of a comprehensive spectroscopic ellipsometry study of , a compound composed of chains of edge-sharing plaquettes and planes of Cu ions in a -- dumbbell configuration, in the spectral range eV at temperatures K. The spectra of the dielectric function for light polarized parallel to the 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 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 3() and 3 orbitals, strongly hybridized with O states. The major contribution to the background response comes from transitions between 3 and 4() bands. This assignment accounts for the measured in-plane anisotropy. The dielectric response along the 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 chains. These features bear a striking resemblance to those observed in the single-valent chain compound , 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 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.
5 More- Received 23 August 2011
DOI:https://doi.org/10.1103/PhysRevB.84.245116
©2011 American Physical Society