Atomic characterization on tetragonal FeAs layer and engineering FeAs superlattices is highly desirable to get deep insight into the multi-band superconductivity in iron-pnictides. We fabricate the tetragonal FeAs layer by topotactic reaction of FeTe films with arsenic and then obtain K_xFe₂As₂ upon potassium intercalation using molecular beam epitaxy. The in-situ low-temperature scanning tunneling microscopy/spectroscopy investigations demonstrate characteristic $\sqrt{2}\times \sqrt{2}$ reconstruction of the FeAs layer and stripe pattern of K_xFe₂As₂, accompanied by the development of a superconducting-like gap. The ex-situ transport measurement with FeTe capping layers shows a superconducting transition with an onset temperature of 10 K. This work provides a promising way to characterize the FeAs layer directly and explore rich emergent physics with epitaxial superlattice design.

Spatially uniform high-temperature superconducting films are highly desirable for exploring novel properties and popularizing applications. To improve the uniformity, we fabricate monolayer FeSe_xTe_1−x (0 < x ≤ 1) films on SrTiO₃(001) by topotactic reaction of monolayer FeTe films with selenium. Using in situ low-temperature scanning tunneling microscopy/spectroscopy, we demonstrate atomic-level uniformity of element distribution and well-defined superconducting gaps of ~ 15 meV in FeSe_xTe_1−x films. In particular, the monolayer FeSe films exhibit fewer line defects and higher superfluid density as evidenced by sharper coherence peaks than those prepared by the co-evaporation method. Our results provide a promising way to optimize sample quality and lay a foundation for studying new physics and drawing reliable conclusions.