Optical evidence for strong anisotropy in the normal and superconducting states in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{z}}$

The optical reflectivity spectra for the bc surface of a large ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathit{z}}$ crystal were investigated over a wide energy range (30–30 000 ${\mathrm{cm}}^{\mathrm{-}1}$) at room temperature and 6 K. In the spectrum for E∥c, only the phonon peaks were observed without any electronic contribution down to 30 ${\mathrm{cm}}^{\mathrm{-}1}$, whereas for E∥b a Drude-like spectrum was observed. Even below ${\mathit{T}}_{\mathit{c}}$ the spectrum for E∥c does not show any remarkable change except for a small change in the phonon spectrum. This indicates an extremely small plasma frequency for E∥c, less than 30 ${\mathrm{cm}}^{\mathrm{-}1}$, and thus quite a large mass anisotropy ${\mathit{m}}_{\mathit{c}}^{\mathrm{*}}$/${\mathit{m}}_{\mathit{b}}^{\mathrm{*}}$>${10}^4$. Such a small plasma frequency supports the conduction model along the c axis, that the supercurrent flows by tunneling through insulating (semiconductor) layers such as the Bi-O layers.