Anomalous optical properties of the mixed-valent lithium cuprate LiCu2 O2

Abstract

We show that the optical properties of LiCu1+ Cu2+ O2 in the spectral range of 0.6-5.8 eV radically differ from those of all other known Cu1+, Cu2+, and mixed-valent oxide cuprates. An extremely strong, sharp, and highly anisotropic optical feature with ε2 xx =26 is observed at 3.27 eV which we assign to an excitonlike transition in the O2- - Cu1+ - O2- 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 Cu1+ states. The excitonic effects in LiCu2 O2 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. © 2006 The American Physical Society.