The As-surface of an iron-based superconductor CaKFe4As4

As a new type of iron-based superconductor, CaKFe4As4 has recently been demonstrated to be a promising platform for observing Majorana zero modes (MZMs). The surface of CaKFe4As4 plays an important role in realizing the MZM since it hosts superconducting topological surface states. However, due to the complicated crystal structure, the terminal surface of CaKFe4As4 has not been determined yet. Here, by using scanning tunneling microscopy/spectroscopy (STM/S), we find that there are two types of surface structure in CaKFe4As4. Bias-dependent atomic resolution images show an evolvement from $$\sqrt 2 \times \sqrt 2 $$ superstructure with respect to the As lattice into 1 × 1 when the tip is brought close to the surface, revealing the sublattice of missing As atoms. Together with the first-principles calculations, we show that the surface As layer has a buckled structure. Our findings provide insight to future surface study of CaKFe4As4 as well as other iron-pnictide superconductors.