Intertwined Topological and Time-reversal-symmetry-breaking States on the Surface of Fe-Chalcogenide Superconductors

Fe-Chalcogenide FeTe1 − xSex is a known topological superconductor with a topological surface state (TSS). Chiral Majorana modes could emerge when this Dirac surface state is spoiled by U(1)-gauge-symmetry-breaking (superconductivity) and time-reversal-symmetry-breaking (TRSB). Until recently, TRSB has only been accomplished in FeTe1 − xSex by applying an external magnetic field. Using a combination of transport, bulk magnetometry, and precision magneto-optic-Kerr effect (MOKE) measurements with a Sagnac interferometer microscope, we demonstrate at zero magnetic fields the existence of an intrinsic surface ferromagnetic state that intertwines with the superconducting order. It onsets at a temperature close to Tc, and has a perpendicular magnetization component, leading to the opening of a Dirac gap in the TSS. The bulk of FeTe1 − xSex is not TRSB, remaining non-magnetic to the lowest temperature. We will describe the detailed temperature evolutions of this surface magnetic state with multiple magnetic transitions and will show that at these transition temperatures the Andreev tunneling spectrum changes abruptly and eventually achieves a perfect flat plateau shape. These observations can be naturally explained by the existence of propagating chiral Majorana "hinge" modes arising from intertwined TRSB and superconducting orders in the topological surface state.