# Proximity-induced superconducting gap in the quantum spin Hall edge state of monolayer WTe$_2$

## TL;DR

This study demonstrates proximity-induced superconductivity in the quantum spin Hall edge states of monolayer WTe2 by placing it on a superconductor, paving the way for realizing 1D topological superconductors and Majorana states.

## Contribution

It introduces a novel dry-transfer flip technique and provides experimental evidence of superconductivity in WTe2's QSH edge states using STM/STS.

## Key findings

- Proximity-induced superconducting gap observed in WTe2 monolayers.
- Superconductivity coexists with QSH edge state signatures.
- Evidence supports potential for 1D topological superconductivity.

## Abstract

The quantum spin Hall (QSH) state was recently demonstrated in monolayers of the transition metal dichalcogenide 1T'-WTe$_2$ and is characterized by a band gap in the two-dimensional (2D) interior and helical one-dimensional (1D) edge states. Inducing superconductivity in the helical edge states would result in a 1D topological superconductor, a highly sought-after state of matter. In the present study, we use a novel dry-transfer flip technique to place atomically-thin layers of WTe$_2$ on a van der Waals superconductor, NbSe$_2$. Using scanning tunneling microscopy and spectroscopy (STM/STS), we demonstrate atomically clean surfaces and interfaces and the presence of a proximity-induced superconducting gap in the WTe$_2$ for thicknesses from a monolayer up to 7 crystalline layers. At the edge of the WTe$_2$ monolayer, we show that the superconducting gap coexists with the characteristic spectroscopic signature of the QSH edge state. Taken together, these observations provide conclusive evidence for proximity-induced superconductivity in the QSH edge state in WTe$_2$, a crucial step towards realizing 1D topological superconductivity and Majorana bound states in this van der Waals material platform.