Obtaining tetragonal FeAs layer and superconducting K_xFe₂As₂ by molecular beam epitaxy

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.