Observation of supermodulation in LaO0.5F0.5BiSe2 by scanning tunneling microscopy and spectroscopy

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.