  We performed low-bias STM measurements on underdoped Bi2212 crystals, and
confirmed that a two-dimensional (2D) superstructure with a periodicity of four
lattice constants (4a) is formed within the Cu-O plane at T<Tc. This 4a X 4a
superstructure, oriented along the Cu-O bonding direction, is nondispersive and
more intense in lightly doped samples with a zero temperature pseudogap (ZTPG)
than in samples with a d-wave gap. The nondispersive 4a X 4a superstructure was
clearly observed within the ZTPG or d-wave gap, while it tended to fade out
outside the gaps. The present results provide a useful test for various models
proposed for an electronic order hidden in the underdoped region of high-Tc
cuprates.
