Signatures of unconventional 2D superconductivity in a bulk van der Waals superlattice*

Two-dimensional systems are expected to support a variety of exotic superconducting states, however, these states can be acutely sensitive to disorder. While two-dimensional (2D) superconductors are readily available by thin-film growth techniques or mechanical exfoliation, it remains a challenge to design and realize clean 2D superconductors, presenting an impediment to realizing these unusual states. Here we report on a bulk van der Waals superlattice Ba₆Nb₁₁S₂₈ consisting of the transition metal dichalcogenide (TMD) superconductor H-NbS₂ and a commensurate spacer layer Ba₃NbS₅ that yields enhanced two-dimensionality, high electronic quality, and clean-limit 2D superconductivity. With application of magnetic field parallel to the superconducting layers we observe long-anticipated signatures of 2D finite-momentum superconductivity. In parallel, analysis of quantum oscillations in the metallic state reveals topological bands with π Berry phase originating from commensuration between the TMD and spacer layer, suggesting a potential coexistence of topological and finite-momentum superconductivity. Finally, we comment on prospects of translating this superlattice construction to TMDs beyond H-NbS₂ towards the realization of other unconventional 2D electronic states.