Mid-infrared excitations in undoped layered copper oxides

Optical absorption spectra are presented for four undoped single-crystal copper oxides: La₂CuO₄, Nd₂CuO₄, Pr₂CuO₄ and Sr₂CuO₂Cl₂. Common absorption features are observed near 0.5 eV, which are shown from polarization measurements to be weakly electric-dipole allowed excitations of the CuO₂ layers. The detailed polarization and spectral dependencies are consistent with normally forbidden valence-conserving crystal-field transitions with multi-magnon and phonon sidebands. Such transitions provide a natural origin for the mid-infrared band observed in doped copper oxides.