Optical study of an untwinned (Bi1.57Pb0.43)Sr2CaCu2O8+δ single crystal: ab-plane anisotropy

We report on the ab-plane polarized reflectance of an untwinned (Bi1.57Pb0.43)Sr2CaCu2O8+δ single crystal over the frequency range from 80 to 32 000 cm (10 meV–4 eV) at temperatures between 10 and 300 K. We find a clear anisotropy in the ab-plane optical conductivity above and below Tc, which is very similar to that formerly published data of Bi2Sr2CaCu2O8 (M.A. Quijada et al., Z. Phys. B 94, 255 (1994)). We employ both the one-component and two-component analyses to the optical data, which suggest that the normal-state infrared anisotropy of (Bi1.57Pb0.43)Sr2CaCu2O8+δ originates not only from the mass anisotropy, but also from the scattering rate anisotropy. Our results provide evidence that the electronic structures within the CuO2 plane are anisotropic. In the superconducting state, there is a definite ab-plane anisotropy to the far-infrared absorption. This anisotropy could be due either to anisotropy of the superconducting gap or to anisotropy of the mid-infrared component to the conductivity. We also observe the superconducting condensate is anisotropic: The value of the superconducting penetration depth in the a-direction is slightly smaller than that along the b-axis. PACS. 74.25.Gz Optical properties – 74.72.Hs Bi-based cuprates – 78.30.-j Infrared and Raman spectra