Condensed Matter Physics and Materials Science Division, Brookhaven National Laboratory, Upton, New York 11973
Low energy, laser-based ARPES with variable light polarization, including both linear and circularly polarized, is used to examine the Fe-based superconductor family, FeTe1-xSex. At the center of the Brillouin zone we observe the presence of a Dirac cones with helical spin structure as expected for a topological surface state and as previously reported in the related FeTe0.55Se0.45. These experimental studies are compared with theoretical studies that take account of the disordered local magnetic moments related to the paramagnetism observed in this system. Indeed including the magnetic contributions in the theoretical description is necessary to bring the chemical potential of the calculated electronic band structure into alignment with the experimental observations. In the bulk superconducting state for FeTe0.7Se0.3 the system appears to reflect the presence of some level of orbital selectivity in the pairing even though the system is in the tetragonal phase above and below the transition temperature Tc. At the same time the topological state appears to acquire mass at the superconducting transition, possibly indicative of time reversal symmetry breaking. These observations are discussed in detail.