  We have performed systematic angle-resolved photoemission spectroscopy
(ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6 to elucidate the
origin of shadow band. We found that the shadow band is exactly the c(2x2)
replica of the main band irrespective of the carrier concentration and its
intensity is invariable with respect to temperature, doping, and substitution
constituents of block layers. This result rules out the possibility of
antiferromagnetic correlation and supports the structural origin of shadow
band. ARPES experiments on optimally doped La1.85Sr0.15CuO4 also clarified the
existence of the c(2x2) shadow band, demonstrating that the shadow band is not
a unique feature of Bi-based cuprates. We conclude that the shadow band is
related to the orthorhombic distortion at the crystal surface.
