Anomalous optical properties of the mixed-valent lithium cuprateLiCu2O2

We show that the optical properties of ${\mathrm{LiCu}}^{1+}{\mathrm{Cu}}^{2+}{\mathrm{O}}_{2}$ in the spectral range of $0.6\text{--}5.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ radically differ from those of all other known ${\mathrm{Cu}}^{1+}$, ${\mathrm{Cu}}^{2+}$, and mixed-valent oxide cuprates. An extremely strong, sharp, and highly anisotropic optical feature with ${\ensuremath{\epsilon}}_{2}^{xx}=26$ is observed at $3.27\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ which we assign to an excitonlike transition in the ${\mathrm{O}}^{2\ensuremath{-}}\text{\ensuremath{-}}{\mathrm{Cu}}^{1+}\text{\ensuremath{-}}{\mathrm{O}}^{2\ensuremath{-}}$ dumbbells. Our findings thoroughly disagree with reported ab initio calculations and can be explained by an exciton-type model that includes strong electron-hole correlations and a crystal-field splitting of the ${\mathrm{Cu}}^{1+}$ states. The excitonic effects in ${\mathrm{LiCu}}_{2}{\mathrm{O}}_{2}$ appear strongly enhanced due to the shortening of the dumbbell lattice spacing which is the shortest one among known cuprates. Our experimental data along with the model reveal a previously unknown regularity in the electronic structure of cuprates.