ab-plane optical properties of Ni-doped Bi2Sr2CaCu2O8+delta

The ab-plane optical reflectance of single crystal of Bi2Sr2Ca(Cu1-xNix)(2)O8+d, for x = 0.0215, has been measured from 80 to 40 000 cm(-l) (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. (C) 1999 Elsevier Science B.V. All rights reserved.