Observation of supermodulation in Lao_0.5F_0.5BiSe₂ by scanning tunneling microscopy and spectroscopy

We observed the surface and electronic structure of single-crystal LaO_0.5F_0.5BiSe₂ by scanning tunneling microscopy= spectroscopy at 4.2 K. A square lattice composed of Bi atoms was observed at a positive sample bias voltage for the surface prepared by cleavage. At a negative sample bias voltage, a stripe structure along the Bi-Bi directions was observed as in a previous report on NdO_0.7F_0.3BiS₂. Furthermore, we observed a supermodulation along the diagonal directions with a period of about five times the lattice constant. This seems to be indicative of the structural instability of this system rather than the electronic instability attributed to a nesting picture.