Absence of a gap in the infrared reflectance of Ni-dopedYBa2Cu3O7−δfilms

Abstract

We study the effects of Ni doping on the infrared reflectance of ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ni}}_{\mathit{x}}$ ${)}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films with up to 4% substitution of Cu by Ni. Normal-state spectra indicate that the primary effect of Ni on the conduction carriers is to increase their elastic scattering rate. At 4 at. % Ni, the elastic-scattering rate is calculated to be large enough to generate a large absorption onset at the gap edge in the superconducting state, if a gap exists. The superconducting-state spectra show no evidence for such a feature. Comparisons with phenomenological models for the optical conductivity show that the conductivity, and hence the superconducting density of states, is very gapless.