A flat band at the chemical potential of a Fe1.03Te0.94S0.06 superconductor observed by angle-resolved photoemission spectroscopy

The electronic structure of superconducting Fe1.03Te0.94S0.06 has been studied by angle resolved photoemission spectroscopy (ARPES). Experimental band topography is compared to the calculations using the methods of Korringa-Kohn-Rostoker (KKR) with coherent potential approximation (CPA) and linearized augmented plane wave with local orbitals (LAPW+LO). The region of the Gamma point exhibits two hole pockets and a quasiparticle peak close to the chemical potential with undetectable dispersion. This flat band with mainly dz2 orbital character is formed most likely by the top of the outer hole pocket or is an evidence of the third hole band. It may cover up to 3 % of the Brillouin zone volume and should give rise to a Van Hove singularity. Studies performed for various photon energies indicate that at least one of the hole pockets has a two-dimensional character. The apparently nondispersing peak at the chemical potential is clearly visible for 40 eV and higher photon energies, due to an effect of photoionisation cross section rather than band dimensionality. Orbital characters calculated by LAPW+LO for stoichiometric FeTe do not reveal the flat dz2 band but are in agreement with the experiment for the other dispersions around Gamma in Fe1.03Te0.94S0.06.