Infrared spectra of superconducting $La_{2-x}Sr_xCuO_4$ and $YBa_2Cu_3O_7-\delta$

Infrared spectra of $La_{2-x}Sr_xCuO_4$ and $YBa_2Cu_3O_7-\delta$ are discussed, specially with respect to the disappearance of the high-frequency (\sim 650 cm) band of the superconducting compositions at 300K. Some of the bands persist at 300K event though the materials are fairly conducting. $YBa_2Cu_3O_7-\delta$ does not show evidence in the far IR spectrum for the presence of an optical gap