S. Tajima et al., OPTICAL EVIDENCE FOR STRONG ANISOTROPY IN THE NORMAL AND SUPERCONDUCTING STATES IN BI2SR2CACU2O8+Z, Physical review. B, Condensed matter, 48(21), 1993, pp. 16164-16167
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.