  High-resolution laser-based angle-resolved photoemission measurements have
been carried out on Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) and
Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$ (Bi2201) superconductors. Unexpected
hybridization between the main band and the superstructure band in Bi2212 is
clearly revealed. In the momentum space where one main Fermi surface intersects
with one superstructure Fermi surface, four bands are observed instead of two.
The hybridization exists in both superconducting state and normal state, and in
Bi2212 samples with different doping levels. Such a hybridization is not
observed in Bi2201. This phenomenon can be understood by considering the
bilayer splitting in Bi2212, the selective hybridization of two bands with
peculiar combinations, and the altered matrix element effects of the hybridized
bands. These observations provide strong evidence on the origin of the
superstructure band which is intrinsic to the CuO$_2$ planes. Therefore,
understanding physical properties and superconductivity mechanism in Bi2212
should consider the complete Fermi surface topology which involves the main
bands, the superstructure bands and their interactions.
