Optical Properties of Doped High Critical Transition Temperature Superconductors

Measurements are presented of the infrared reflectance of Ni-doped superconducting rm YBa_2Cu _3O_{7-delta} films over the frequency range 50-4,000 cm^{ -1}. From the residual resistivity, Ni doping increases the scattering rate tau^ {-1} for the conduction electrons to the dirty limit hbartau^{-1 }>k_{B}T_{c}. This scattering should enhance the absorption edge at the gap both by increasing the normal-state absorption and by reducing anisotropy in the superconducting energy gap. However, no gap is evident in the reflectance data. Some phenomenological models are proposed to explain the absence of a gap.