Molecular beam epitaxy and superconductivity of stoichiometric FeSe and K_xFe_2−ySe₂ crystalline films

Our recent progress in the fabrication of FeSe and K_xFe_2−ySe₂ ultra thin films and the understanding of their superconductivity properties is reviewed. The growth of high-quality FeSe and K_xFe_2−ySe₂ films is achieved in a well controlled manner by molecular beam epitaxy. The high-quality stoichiometric and superconducting crystalline thin films allow us to investigate the intrinsic superconductivity properties and the interplay between the superconductivity and the film thickness, the local structure, the substrate, and magnetism. In situ low-temperature scanning tunneling spectra reveal the nodes and the twofold symmetry in FeSe, high-temperature superconductivity at the FeSe/SrTiO₃ interface, phase separation and magnetic order in K_xFe_2−ySe₂, and the suppression of superconductivity by twin boundaries and Fe vacancies. Our findings not only provide fundamental information for understanding the mechanism of unconventional superconductivity, but also demonstrate a powerful way of engineering superconductors and raising the transition temperature.