OPTICAL EVIDENCE FOR STRONG ANISOTROPY IN THE NORMAL AND SUPERCONDUCTING STATES IN BI2SR2CACU2O8+Z

The optical reflectivity spectra for the bc surface of a large Bi2Sr2C aCu208+z crystal were investigated over a wide energy range (30-30000 cm-1) at room temperature and 6 K. In the spectrum for E parallel-to c , only the phonon peaks were observed without any electronic contribut ion down to 30 cm-1, whereas for E parallel-to b a Drude-like spectrum was observed. Even below T(c) the spectrum for E parallel-to c does n ot show any remarkable change except for a small change in the phonon spectrum. This indicates an extremely small plasma frequency for E par allel-to c, less than 30 cm-1, and thus quite a large mass anisotropy m(c)/m(b)* > 10(4). Such a small plasma frequency supports the conduc tion model along the c axis, that the supercurrent flows by tunneling through insulating (semiconductor) layers such as the Bi-O layers.