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 $\sqrt{2}$ × $\sqrt{2}$ 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.