Visualization of Macroscopic Ising Superconducting State in Superconductor-Graphene Junctions

Abstract

Graphene-based two-dimensional van der Waals Josephson junctions with superconductors are hopeful for realizing versatile quantum devices. However, research on the junctions with Ising superconductors in atomic-layer transition metal dichalcogenides, whose superconductivity is resilient to strong in-plane magnetic fields, has remained elusive. Here the scanning tunneling microscopy study of Ising superconductivity in single-layer (SL) NbSe2 island and its long-range proximity effect to the adjacent graphene with high spatial and energy resolution is reported. It is found that SL NbSe2 island manifests as an Ising superconductor while the island area is larger than about 800 nm2. Moreover, the superconducting proximity effect from the NbSe2 island generates a superconducting gap in graphene, which can extend to over several tens of nanometers without obvious decay. Such a long-range proximity effect makes the intrinsically non-superconducting graphene as a superconductor. More intriguingly, such a superconducting gap can survive under strong in-plane magnetic fields, following the Ising behavior of that in SL NbSe2. This work reveals a brand-new macroscopic Ising superconducting regime, opening perspectives for superconductor-based quantum devices with outstanding functionality.