Science 352, 966–969 (2016)

Josephson junctions based on graphene are an ideal environment for the development of exotic physical phenomena. In the presence of an external magnetic field, graphene enters the quantum Hall state, where its gapped nature prevents the propagation of conventional Andreev states within the bulk. Theoretical arguments suggest that two conducting chiral channels confined to opposite edges are able to sustain supercurrents. This is different from topological insulators, where a single edge is enough to conduct carriers in two directions. However, these phenomena have not yet been demonstrated experimentally.

Now, François Amet and Gleb Finkelstein at Duke University, along with colleagues from institutions in Japan and the US, demonstrate the interplay of superconductivity and the quantum Hall effect in graphene-based heterostructures with superconducting MoRe contacts. The researchers performed back-gated magnetotransport measurements in the sub-kelvin limit, where the thermal energy is comparable to the characteristic Josephson coupling strength for the chosen current values. Magnetic fields of ∼2 T ensured that the radius of the cyclotron orbit was smaller than both the junction length and the mean free path. At the same time, the opposite free edges were far enough apart to prevent edge coupling through conventional Andreev reflections. Remarkably, the behaviour of the differential resistance unambiguously showed signatures of edge-confined, spatially inhomogeneous supercurrents.