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

TL;DR

This study identifies the origin of time-reversal symmetry breaking in FeTe1-xSex as occurring in the topological surface state, revealing a surface ferromagnetic order that influences Majorana zero modes relevant for quantum computing.

Contribution

The paper demonstrates that TRSB in FeTe1-xSex originates in the topological surface state and uncovers a surface ferromagnetic order linked to the material's composition.

Findings

TRSB is localized in the topological surface state.

Surface ferromagnetic order exists even in non-superconducting samples.

TRSB's sensitivity to composition suggests pathways for optimizing Majorana modes.

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.