Infrared reflectance from oriented film samples of Y-Ba-Cu-O

We have studied infrared reflectivity from epitaxial films of YBaCuO, in which the c axes are oriented either perpendicular or parallel to the film surface. We classified phonons in superconducting phase to be two modes including E⊥C and E//C modes. The strong coupling between the phonons and free carriers reduces the infrared activity of phonons. The infrared activity of phonons increases in the process that the orthorhombic phase changed into tetragonal phase. The important role of phonon in superconductivity is shown.