Van der Waals stacking of two-dimensional crystals with rotation or mismatch in lattice constants gives rise to rich physical phenomena that are closely related to the strong correlations and band topology. Twisted graphene and silicene heterobilayers have been theoretically predicted to host a tunable transport gap due to the mismatch of Dirac cones in the graphene and silicene layers. However, experimental realization of such twisted structure is challenging. Here, we report the formation of twisted graphene/silicene bilayers on Ru (0001) crystal via intercalation. Different moiré patterns form as single-crystalline graphene grows over different grains of the Ru surface. After silicon intercalation, graphene/silicene bilayers are observed with different twisting angles on top of different grains of the Ru substrate. Our work provides a new pathway towards construction of graphene based twisted heterobilayers.

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.