Ba sub 2 YCu sub 3 O sub (7-delta): Electrodynamics of crystals with high reflectivity.

We present results for the reflectivity of two bulk crystals of Ba sub 2 YCu sub 2 O sub (7-delta) which, below the superconducting transition at T sub c, show behavior consistent with perfect reflectivity up to an energy gap of 3 to 4 k sub B T sub c. Results above T sub c show reproducibly that the charge carriers have an enhanced effective mass at low] frequency and temperature and an unusual variation of the scattering rate, indicating a strong interaction with some excitation.