Infrared spectra of superconducting La 2-x Sr x CuO 4 and YBa 2 Cu 3 O 7-δ

Infrared spectra of La 2-x Sr x CuO 4 and YBa 2 Cu 3 O 7-δ are discussed, specially with respect to the disappearance of the high-frequency (~650 cm) band of the superconducting compositions at 300K. Some of the bands persist at 300K eventhough the materials are fairly conducting. YBa 2 Cu 3 O 7-δ does not show evidence in the far IR spectrum for the presence of an optical gap.