ABSENCE OF A GAP IN THE INFRARED REFLECTANCE OF NI-DOPED YBA2CU3O7-DELTA FILMS

We study the effects of Ni doping on the infrared reflectance of YBa2( Cu1-xNix)3O7-delta films with up to 4% substitution of Cu by Ni. Norma l-state spectra indicate that the primary effect of Ni on the conducti on carriers is to increase their elastic scattering rate. At 4 at. % N i, the elastic-scattering rate is calculated to be large enough to gen erate a large absorption onset at the gap edge in the superconducting state, if a gap exists. The superconducting-state spectra show no evid ence for such a feature. Comparisons with phenomenological models for the optical conductivity show that the conductivity, and hence the sup erconducting density of states, is very gapless.