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: ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$, ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$, ${\mathrm{Pr}}_2$ ${\mathrm{CuO}}_4$, and ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_2$ ${\mathrm{Cl}}_2$. These absorption bands are shown to be weakly electric dipole allowed excitations of the ${\mathrm{CuO}}_2$ 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.