Subgap states in superconducting 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. Large spin-orbit coupling and the crystal symmetry in these materials leads to effects such as the survival of superconductivity up to tens of Teslas of applied in-plane magnetic field.

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-crossing of subgap excitations we highlight the role of strong spin-orbit coupling.