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.

Automated summary

The study reveals that subgap excitations emerge on NbSe2 when MoS2 or hexagonal boron nitride (hBN) is used as the tunnel barrier. By investigating various heterostructure designs, the origin of these excitations is probed. Defect states at the edge of NbSe2 strongly couple to the superconductor, forming Andreev bound states. Moreover, the absence of subgap states in hBN tunnel barriers compared to the widespread presence in MoS2 tunnel barriers suggests defects in MoS2 as their origin. The magnetic nature of these excitations indicates a singlet- or a doublet-type ground state, emphasizing the role of strong spin-orbit coupling based on nearly vanishing g factors or avoided crossings.