Origin of Subgap States in Normal-Insulator-Superconductor van der Waals Heterostructures

Superconductivity in van der Waals materials, such as NbSe₂ and TaS₂, is fundamentally novel due to the effects of dimensionality, crystal symmetries, and strong spin-orbit coupling. In this work, we perform tunnel spectroscopy on NbSe₂ by utilizing MoS₂ 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 NbSe₂ hosts many defect states, which strongly couple to the superconductor and form Andreev bound states. Furthermore, by isolating the NbSe₂ edge we show that the subgap states are ubiquitous in MoS₂ tunnel barriers but absent in hBN tunnel barriers, suggesting defects in MoS₂ as their origin. Their magnetic nature reveals a singlet- or a doublet-type ground state, and based on nearly vanishing g factors or avoided crossings of subgap excitations, we highlight the role of strong spin-orbit coupling.