The Surface Structure of FeTe$_{1-x}$Se$_{x}$: A LEED/STM Study*

We have utilized low energy electron diffraction (LEED $I-V)$ and scanning tunneling microscopy (STM) to investigate the structural properties of FeTe$_{1-x}$Se$_{x}$ ($x$ = 0 and 0.45) surface. The LEED pattern indicates there is no surface reconstruction on both parent and doped compounds. However, the detailed surface structure calculations from LEED $I-V$ show the cleaved surface of FeTe is Te termination with a 0.06 {\AA} compression on the top layer. The STM topography shows the extreme flatness of the FeTe surface with a corrugation of less than 8 pm. The high resolution STM topography indicates that nanoscale chemical phase separation between Te and Se atoms is unambiguously revealed on the surface of FeTe$_{0.55}$Se$_{0.45}$. Chemical phase separation on the nanoscale makes the LEED $I-V$ surface structural analysis of FeTe$_{1-x}$Se$_{x}$ (001) very challenging. We will discuss the solution of the LEED analysis and the STM observations as a function of $x.$