ab-plane optical properties of Ni-doped Bi₂Sr₂CaCu₂O_8+δ

The ab-plane optical reflectance of single crystal of Bi₂Sr₂Ca(Cu_1-xNi_x)₂O _8+δ, for x=0.0215, has been measured from 80 to 40000 cm^-1 (10 meV-5 eV) and at temperatures between 20 and 300 K. Through Ni substitution, the optical conductivity shows a broad peak at about 400 cm^-1, which grows in intensity and shifts to lower frequency as the temperature is reduced. We suggest that this peculiar low-frequency feature is associated with the significant disorder induced by Ni impurities. Analysis of the conductivity within the framework of the two-component and the one-component pictures indicates that the main effect of Ni substitution is to increase impurity scattering rate. There is no evidence of a rapid drop in the free carrier scattering rate near T_c. An estimate of the low-frequency spectral weight shows that Ni reduces the carrier density in the CuO₂ plane: each Ni removes one carrier. Below T_c, a sum-rule evaluation finds the superconducting condensate contains about a fifth of the total doping-induced, or two thirds of the free-carrier oscillator strength in the normal state. The strength of the condensate is horizontally offset from the Uemura line, with a smaller superfluid density than implied by T_c. This decreased superfluid density in the CuO₂ plane could be connected with the effect of impurity-induced disorder.