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

A layered two-dimensional superconducting material 2H-NbSe is used to build a van der Waals heterostructure, where a proximity-coupled superconducting order can be induced in the interfacing materials. Vertically stacked NbSe-graphene-NbSe 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 NbSe flakes. The temperature dependence of the junction critical current (I) 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 × 10 A/cm, multiple Andreev reflections in the subgap structure of the differential conductance, and a magnetic-field modulation of I. This is the first demonstration of strongly proximity-coupled Josephson junctions with extremely clean interfaces in a dry-transfer-stacked van der Waals heterostructure.