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

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.

16 pages, 7 figures. Includes Supporting Information

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