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

## TL;DR

This study provides experimental evidence that FeTe$_{0.55}$Se$_{0.45}$ is a higher order topological superconductor hosting helical hinge zero modes, using a novel junction fabrication method and detailed zero-bias anomaly measurements.

## Contribution

The paper introduces a new method for creating normal metal/superconductor junctions with 2D atomic crystal heterostructures and demonstrates the presence of helical hinge zero modes in FeTe$_{0.55}$Se$_{0.45}$.

## Key findings

- Zero-bias anomaly observed in hinge contacts consistent with helical hinge zero modes.
- Suppression of the anomaly with temperature and magnetic field along the c-axis.
- Evidence supports FeTe$_{0.55}$Se$_{0.45}$ as a higher order topological superconductor.

## Abstract

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 FeTe$_{0.55}$Se$_{0.45}$ (FTS) may also be topologically non-trivial. Should the superconducting order be s$^{\pm}$, then FTS could be a higher order topological superconductor with Helical Hinge Zero Modes (HHZM).To test the presence of these modes we developed a new method for making normal metal/superconductor junctions via 2D atomic crystal heterostructures. As expected,junctions in contact with the hinge reveal a sharp zero-bias anomaly whose suppression with temperature and magnetic field only along the c-axis are completely consistent with the presence of HHZM. This feature is completely absent when tunneling purely into the c-axis, and its characteristics are also inconsistent with other origins of zero bias anomalies. Furthermore, 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 as well as a new method for probing 2D atomic crystals.