Revealing the Origin of Time-reversal Symmetry Breaking in Fe-chalcogenide Superconductor FeTe1-xSex

Abstract

Recently evidence has emerged in the topological superconductor Fe-chalcogenide FeTe1-xSex for time-reversal symmetry breaking (TRSB), the nature of which has strong implications on the Majorana zero modes (MZM) discovered in this system. It remains unclear however whether the TRSB resides in the topological surface state (TSS) or in the bulk, and whether it is due to an unconventional TRSB superconducting order parameter or an intertwined order. Here by performing in superconducting FeTe1-xSex crystals both surface-magneto-optic-Kerr effect (SMOKE) measurements using a Sagnac interferometer and bulk magnetic susceptibility measurements, we pinpoint the TRSB to the TSS, where we also detect a Dirac gap. Further, we observe surface TRSB in non-superconducting FeTe1-xSex of nominally identical composition, indicating that TRSB arises from an intertwined surface ferromagnetic (FM) order. The observed surface FM bears striking similarities to the two-dimensional (2D) FM found in 2D van der Waals crystals, and is highly sensitive to the exact chemical composition, thereby providing a means for optimizing the conditions for Majorana particles that are useful for robust quantum computing.

accepted version at Physical Review Letters

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• Superconducting phenomena in systems with unconventional magnets
• Orbital Selective Mott Transition Effects and Non-Trivial Topology of Iron Chalcogenide
• Nematic Bogoliubov Fermi surfaces from magnetic toroidal order in FeSe${}_{1-x}$S${}_x$
• Proximity effect of time-reversal symmetry broken non-centrosymmetric superconductors
• Modified Martin-Puplett interferometer for magneto-optical Kerr effect measurements at sub-THz frequencies
• Discovery of an Intermediate Nematic State in a Bilayer Kagome Metal ScV6Sn6
• Superconducting Stiffness and Coherence Length of FeSe${}_{0.5}$Te${}_{0.5}$ Measured in Zero-Applied Field
• In-situ local imaging of ferromagnetism and superconductivity in RbEuFe${}_4$As${}_4$
• Pairing-induced Momentum-space Magnetism and Its Implication In Optical Anomalous Hall Effect In Chiral Superconductors
• Potentiometric detection of spin polarization expected at the surface of FeTe0.6Se0.4 in the effective p-wave superconducting state