  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 and electron doped, whose Fermi surface
consists of six elliptical electron pockets. With decreased temperature, parts
of the bands shift downward to high binding energy while some bands shift
upwards to EF. 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 except some minor quantum size effect. Our paper reports
the first electronic structure of hexagonal FeSe, and shows that the possible
magnetic transition is driven by large scale electronic structure
reconstruction.
