  We observed surface and electronic structure of LaO0.5F0.5BiSe2 single
crystal by scanning tunneling microscopy/spectroscopy (STM/STS) at 4.2 K.
Square lattice composed of Bi atoms was observed at a positive sample bias
voltage on the surface prepared by cleavage. At a negative sample bias voltage,
a stripe structure running along Bi-Bi directions was observed as in the
previous report on NdO0.7F0.3BiS2. Furthermore, we observed a supermodulation
running along the diagonal directions with the period of about 5 times of the
lattice constant. This seems to be indicative of structural instability of this
system rather than electronic instability attributed to a nesting picture.
