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.

Dislocation Majorana bound states in iron-based superconductors

“…Recent experimental breakthroughs have revealed hidden topological Dirac surface states for several high-Tc iron-based SCs 31,32 . Among these tFeSC candidates, FeTe 1−x Se x is of particular interest to us, as it additionally harbors surface ferromagnetism that coexists with bulk superconductivity below its superconducting T c ~14.5 K [46][47][48][49] . Furthermore, screw dislocations for FeTe 1−x Se x can be generated in a highly controllable manner during the growth process 45 .…”

Superconducting Stiffness and Coherence Length of FeSe0.5Te0.5 Measured in a Zero-Applied Field

“…Additional properties that can be deduced from this measurement are the magnetization saturation, retentivity (remanence), and coercivity values: m sat = 1.58 A•mm 2 , m remanence = 0.22 A•mm 2 , and µ 0 H coercivity = 0.0153 T, respectively. Although this ferromagnetism is sometimes ascribed to the topological surface state [17], we analyze it as a bulk property. From the magnetization saturation and the magnetic moment of a free Fe ion m Fe 2+ = 5.4 µ B or m Fe 3+ = 5.9 µ B , where µ B is the Bohr magneton [18], we can deduce that the fraction of the free iron ions per unit formula in the sample is y = 0.009 or y = 0.008, respectively.…”

“…(a) The right Y-axis shows the penetration depth as a function of the temperature in blue and emerald triangles, for the different methods.The left Y-axis shows the temperature dependence of the coherence length. The red-circles are taken from the critical current measurement in Figure2b-inset through Equation (14) with the measured λ, and the black-squares are from the second critical field in Figure3b-inset with Equation(17). Panels (b,c) are log-log plots of the stiffness λ −2 and 1/ξ vs. 1 − T/T c , respectively.…”

In Situ Local Imaging of Ferromagnetism and Superconductivity in RbEuFe4As4

“…Besides the competition and coexistence with antiferromagnetic fluctuations, superconductivity in iron-based superconductors has also been found to directly coexist with static magnetic order. For example, multiple measurements of Fe­(Te,Se) reveal that bulk superconductivity can coexist with ferromagnetism in the surface layer. − …”