Spectroscopic Studies on Iron-Chalcogenide Superconductors

Superconductivity in F-doped LaFeAsO, Tc= 26 K was first reported by Kamihara et al in 2008. Following that, more iron-based superconductors, e.g., (Ba,K)Fe2As2, LiFeAs, and Fe1+yTe1-xSex were discovered. The new class of high Tc superconductors have attracted a tremendous interest. Iron-chacogenides do not involve arsenic but share several common features with the iron-pnictides. They are characterized by FeCh layers which are isostructural to the FeAs or FeP layers in the iron-pnictides. The electronic structure near the Fermi energy derives from the Fe 3d orbitals. Both families exhibit a spin density wave (SDW) ground state. Se substation for Te in FeTe suppresses the SDW and leads to superconductivity. From a more practical point of view, the iron-chacogenides have a rather simple structure, and they are less toxic and easier to obtain in large single crystals than the arsenide counterparts. For all these properties, the iron-chalcogenides are ideal compounds for angle-resolved photoemission (ARPES) studies. In this thesis, I have investigated by ARPES the normal state electronic structure of Fe1+yTe1-xSex superconductors with various doping level. I have studied the multi-orbital Fermi surface of Fe1+yTe1-xSex, mainly by means of polarized photons from synchrotron radiation sources. Individual bands of different orbital characters could then be distinguished by exploiting ARPES selection rules. I have also investigated the 3D electronic structure of these materials by performing normal emission measurements as a function of the incident photon energy. Namely, by using photon energies as large as 150 eV, I have performed an unusually broad ARPES survey of momentum space. The data reveal two inequivalent Fermi contours at nominally equivalent Gamma points in different Brillouin zones. The observed periodicity corresponds to a unit cell containing only one Fe atom, rather than the standard tetragonal unit cell containing two Fe atoms. I also observe a strong and selective suppression of spectral weight around perpendicular lines at the corner of the Brillouin zone.