AB-plane optical spectra of iodine-intercalated Bi_1.9Pb_0.1Sr₂CaCu₂O_8+δ: Normal and superconducting properties

We report on the ab-plane optical reflectance of an iodine-intercalated Bi_1.9Pb_0.1Sr₂CaCu₂O_8+δ single crystal in the 80-40 000 cm^-1 (10 meV-5 eV) frequency range and at temperatures between 10 and 300 K. As compared to the iodine-free Bi₂Sr₂CaCu₂O_8+δ, we find that the visible-ultraviolet interband transitions are strongly modified after intercalation. Estimates of the low-frequency spectral weight indicate that there is an increase of hole concentration in the CuO₂ planes. This behavior is a consequence of charge transfer between intercalated iodine atoms and the CuO₂ sheets leading to an ionized iodine species. The ab-plane optical conductivity is analyzed in both the one-component and the two-component pictures, suggesting that the intercalated iodine does not have any significant effect on the in-plane scattering rate. In the superconducting state, a sum-rule evaluation finds that the superfluid contains about 25% of the total doping-induced, or nearly 86% of the free-carrier oscillator strength in the normal state. The value of the superconducting penetration depth is estimated to be 1980 Å, slightly larger than the 1860 Å found in iodine-free Bi₂Sr₂CaCu₂O_8+δ.