# FeTe$_{0.55}$Se$_{0.45}$ van der Waals Tunneling Devices

## TL;DR

This study demonstrates the fabrication and characterization of FeTe$_{0.55}$Se$_{0.45}$ van der Waals tunneling devices, revealing a persistent hard gap and defect-assisted tunneling phenomena at very low temperatures and high magnetic fields.

## Contribution

First successful integration of FeTe$_{0.55}$Se$_{0.45}$ with van der Waals materials for tunneling devices, showing robust hard-gap features and defect-related resonant states.

## Key findings

- Hard-gap tunneling persists up to 9 T in-plane magnetic field.
- Surface oxidation affects device yield but not bulk properties.
- Detection of defect-assisted resonant tunneling states.

## Abstract

We report on fabrication of devices integrating FeTe$_{0.55}$Se$_{0.45}$ with other van-der-Waals materials, measuring transport properties as well as tunneling spectra at variable magnetic fields and temperatures down to 35 mK. Transport measurements are reliable and repeatable, revealing temperature and magnetic field dependence in agreement with prior results, confirming that fabrication processing does not alter bulk properties. However, cross-section scanning transmission microscopy reveals oxidation of the surface, which may explain a lower yield of tunneling device fabrication. We nonetheless observe hard-gap planar tunneling into FeTe$_{0.55}$Se$_{0.45}$ through a MoS$_2$ barrier. Notably, a minimal hard gap of 0.5 meV persists up to a magnetic field of 9 T in the $ab$ plane and 3 T out of plane. This may be the result of very small junction dimensions, or a quantum-limit minimal energy spacing between vortex bound states. We also observed defect assisted tunneling, exhibiting bias-symmetric resonant states which may arise due to resonant Andreev processes.