MIDINFRARED EXCITATIONS IN UNDOPED LAYERED COPPER OXIDES

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