Time Reversal Symmetry Breaking in the FeTe_1-xSe_x family of high Tc superconductors.

Laser-based ARPES with variable light polarization offers a powerful probe of the electronic structure near the center of the Brillouin zone. Here the technique is used to examine the Fe-based superconductor family, FeTe_1-xSe_x. At the zone center we observe the presence of Dirac cones with helical spin structure as expected for topological surface states and as previously reported in the related FeTe_0.55Se_0.45.[1] These 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 experimental observation. In the bulk superconducting state for FeTe_0.7Se_0.3 the topological state appears to acquire mass below T_c. With a single state at the center of the zone, mass acquisition is indicative of time reversal symmetry breaking which in turn suggests the potential formation of ferromagnetism in the surface layer.

[1] P. Zhang et al., Science 360, 182 (2018)