Strong Proximity Josephson Coupling in Vertically Stacked NbSe₂-Graphene-NbSe₂ van der Waals Junctions

A layered two-dimensional superconducting material 2H-NbSe₂ is used to build a van der Waals heterostructure, where a proximity-coupled superconducting order can be induced in the interfacing materials. Vertically stacked NbSe₂-graphene-NbSe₂ is fabricated using van der Waals interlayer coupling, producing defect-free contacts with a high interfacial transparency. The atomically thin graphene layer allows the formation of a highly coherent proximity Josephson coupling between the two NbSe₂ flakes. The temperature dependence of the junction critical current (I_c) reveals short and ballistic Josephson coupling characteristics that agree with theoretical prediction. The strong Josephson coupling is confirmed by a large junction critical current density of 1.6 × 10⁴ A/cm², multiple Andreev reflections in the subgap structure of the differential conductance, and a magnetic-field modulation of I_c. This is the first demonstration of strongly proximity-coupled Josephson junctions with extremely clean interfaces in a dry-transfer-stacked van der Waals heterostructure.