Electronic structure and magnetic phase transition of hexagonal FeSe thin films studied by photoemission spectroscopy*

Hexagonal FeSe thin films were grown on SrTiO₃ substrates and the temperature and thickness dependence of their electronic structures were studied. The hexagonal FeSe is found to be metallic, with a Fermi surface consisting of six elliptical electron pockets. With decreased temperature, parts of the bands shift downward to high binding energy while some bands shift upward to E_F. The shifts of these bands begin around 300 K and saturate at low temperature, indicating a magnetic phase-transition temperature of about 300 K. With increased film thickness, the Fermi surface topology and band structure show no obvious change. Our paper reports the electronic structure of hexagonal FeSe for the first time, and shows that the possible magnetic transition is driven by large-scale electronic structure reconstruction.