Evidence for Helical Hinge Zero Modes in an Fe-Based Superconductor

Combining topology and superconductivity provides
a powerful tool for investigating fundamental physics as well as a
route to fault-tolerant quantum computing. There is mounting
evidence that the Fe-based superconductor FeTe0.55Se0.45 (FTS)
may also be topologically nontrivial. Should the superconducting
order be s±, then FTS could be a higher order topological
superconductor with helical hinge zero modes (HHZMs). To test
the presence of these modes, we have fabricated normal-metal/
superconductor junctions on different surfaces via 2D atomic crystal
heterostructures. As expected, junctions in contact with the hinge
reveal a sharp zero bias anomaly that is absent when tunneling
purely into the c-axis. Additionally, the shape and suppression with
temperature are consistent with highly coherent modes along the hinge and are incongruous with other origins of zero bias anomalies. Additional measurements with soft-point contacts in bulk samples with various Fe interstitial contents demonstrate the intrinsic nature of the observed mode. Thus, we provide evidence that FTS is indeed a higher order topological superconductor.

Nanoletters 19, 4890 (2019)