Tunable Fe-vacancy disorder-order transition in FeSe thin films

Various Fe-vacancy orders have been reported in tetragonal $\ensuremath{\beta}\ensuremath{-}\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{Se}$ single crystals and nanowires/nanosheets, which are similar to those found in alkali-metal intercalated ${\mathrm{A}}_{1\ensuremath{-}x}\mathrm{F}{\mathrm{e}}_{2\ensuremath{-}y}\mathrm{S}{\mathrm{e}}₂$ superconductors. Here we report the in situ angle-resolved photoemission spectroscopy study of Fe-vacancy disordered and ordered phases in FeSe multilayer thin films grown by molecular beam epitaxy. Low-temperature annealed FeSe films are identified to be Fe-vacancy disordered phase and electron doped. A further long-time low-temperature anneal can change the Fe-vacancy disordered phase to the ordered phase, which is found to be semiconductor/insulator with $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ superstructure and can be reversely changed to the disordered phase with a high-temperature anneal. Our results reveal that the disorder-order transition in FeSe thin films can be simply tuned by a vacuum anneal and the FeSe thin films provide rich information to understand the complex electronic structure and superconductivity of FeSe.