Observation of an electron band above the Fermi level in FeTe$_{0.55}$Se$_{0.45}$ from emph{in-situ} surface doping

We used emph{in-situ} potassium (K) evaporation to dope the surface of the iron-based superconductor FeTe$_{0.55}$Se$_{0.45}$. The systematic study of the bands near the Fermi level confirms that electrons are doped into the system, allowing us to tune the Fermi level of this material and to access otherwise unoccupied electronic states. In particular, we observe an electron band located above the Fermi level before doping that shares similarities with a small three-dimensional pocket observed in the cousin, heavily-electron-doped KFe$_{2-x}$Se$_2$ compound.