ab - plane optical properties of Ni - doped Bi Sr CaCu

Ž . The ab-plane optical reflectance of single crystal of Bi Sr Ca Cu Ni O , for xs0.0215, has been measured 2 2 1yx x 2 8qd y1 Ž . from 80 to 40 000 cm 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, 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 . An estimate of the low-frequency spectral weight shows that Ni reduces c the carrier density in the CuO plane: each Ni removes one carrier. Below T , a sum-rule evaluation finds the 2 c 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 . This decreased superfluid density in the CuO plane could be connected with the c 2 effect of impurity-induced disorder. q 1999 Elsevier Science B.V. All rights reserved. PACS: 74.25.Gz; 74.62.Dh; 74.72.Hs; 78.30.y j