Origin of subgap states in normal-insulator-superconductor van der Waals heterostructures

TL;DR

This study investigates the origin of subgap states in van der Waals heterostructures involving superconducting NbSe₂, revealing defect-induced Andreev bound states at edges, with implications for understanding superconductivity in layered materials.

Contribution

It demonstrates that defect states in MoS₂ tunnel barriers cause subgap excitations in NbSe₂ heterostructures, highlighting the role of defects and spin-orbit coupling in subgap state formation.

Findings

Subgap states are linked to defect states at NbSe₂ edges.

Subgap states are present in MoS₂ but absent in hBN barriers.

Strong spin-orbit coupling influences the magnetic properties of subgap states.

Abstract

Superconductivity in van der Waals materials, such as NbSe$_{2}$ and TaS$_{2}$, is fundamentally novel due to the effects of dimensionality, crystal symmetries, and strong spin-orbit coupling. In this work we perform tunnel spectroscopy on NbSe$_{2}$ by utilizing MoS$_{2}$ or hexagonal Boron Nitride (hBN) as a tunnel barrier. We observe subgap excitations and probe their origin by studying various heterostructure designs. We show that the edge of NbSe$_{2}$ hosts many defect states, which strongly couple to the superconductor and form Andreev bound states. Furthermore, by isolating the NbSe$_{2}$ edge we show that the subgap states are ubiquitous in MoS$_{2}$ tunnel barriers, but absent in hBN tunnel barriers, suggesting defects in MoS$_{2}$ as their origin. Their magnetic nature reveals a singlet or a doublet type ground state and based on nearly vanishing g-factors or avoided-crossing of subgap excitations we highlight the role of strong spin-orbit coupling.