Optical anisotropy of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$

The optical anisotropy of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ in the 0.08–0.5-eV region is investigated by polarized reflectance measurements on single crystals. A very large anisotropy is found in this spectral region. The in-plane reflectance exhibits metallic behavior, while the c-axis reflectance exhibits insulatorlike behavior. This result is consistent with the large anisotropy found in the resistivity of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$. Our spectroscopic data suggest that ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ is a quasi-two-dimensional metal similar to ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$.