Elucidating the Interplay between Ferromagnetism and Superconductivity in Fe-Chalcogenide Superconductor FeTe_1−xSe_x

The high-T_c family of superconductors, FeTe_1−xSe_x, have recently been shown to display topological superconductivity, and evidence has been found for the presence of Majorana fermions, which carry the potential for the advancement of quantum-computing technology. The presence of time-reversal symmetry breaking (TRSB) is an important factor in understanding the pairing mechanisms in strongly correlated superconductors, and so it is of great interest to study the magnetic properties of FeTe_1−xSe_x. Recent high-resolution laser-based photoemission measurements have shown the opening of a second gap at the Dirac point, suggesting the onset of TRSB which is suspected to be associated with the formation of magnetism in the surface layer. In addition, NV optical detection of magnetic resonance (ODMR) measurements have observed the coexistence of superconductivity and ferromagnetism in FeTe_1−xSe_x. Here we perform high resolution magneto-optic Kerr effect (MOKE) and angle-resolved-photoemission (ARPES) measurements on superconducting and non-superconducting FeTe_1−xSe_x crystals, and use these results to elucidate the relation between the onset of ferromagnetism and superconductivity in FeTe_1−xSe_x .