Tunable Fe-Vacancy Disorder-Order Transition In Fese Thin Films

Various Fe-vacancy orders have been reported in tetragonal beta-Fe1-xSe single crystals and nanowires/nanosheets, which are similar to those found in alkali-metal intercalated A(1-x)Fe(2-y)Se(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 root 5 x root 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.