Mid-infrared optical absorption in undoped lamellar copper oxides

Common optical absorption features are observed near 0.5 eV in four undoped single-crystal copper oxides: La₂{CuO}₄$, ${Nd}₂CuO₄, Pr₂{CuO}₄$, and ${Sr}₂CuO₂{Cl}₂$. These absorption bands are shown to be weakly electric dipole allowed excitations of the ${CuO}₂$ layers. The features are ascribed to an exciton near 0.4 eV, probably of crystal field origin, with strong multimagnon sidebands. Our observations provide an important basis for understanding the doped materials.