Mid-infrared optical absorption in undoped lamellar copper oxides

Abstract

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.