A current that flows between two through a thin barrier with no voltage at all, carried by pairs of electrons tunnelling across – slipping through a barrier that everyday physics says they cannot pass. How large that current is, and how it reacts to a magnetic field, is one of the sharpest ways to find out what kind of superconductor you are holding.
Going deeper
A Josephson junction carries a supercurrent set entirely by the phase difference between its two superconductors. How that current responds to magnetic flux, to a gate and to temperature is one of the most informative measurements available on a van der Waals stack.
A current carried by phase
Two superconductors joined by a thin barrier share a single quantum phase difference, and that difference alone sets the current: I = Ic sin Δφ, with no voltage across the junction. Apply a voltage and the phase winds instead of sitting still, so the current oscillates – at 2eV/h, or 483.6 MHz for every microvolt. That ratio involves only fundamental constants, which is why the volt is now realised with Josephson junctions rather than with a chemical cell.
Two junctions in a loop make a , whose critical current depends on the flux through the loop with a period of one flux quantum. The same sensitivity is what makes a single junction a diagnostic: everything about the weak link shows up in how Ic behaves.
Weak links made of layered crystals
The weak link can be anything that carries phase coherence: a tunnel barrier, a normal metal, a semiconducting , a twisted interface, or a graphene channel. Graphene is the best-studied case. Encapsulating it in hexagonal boron nitride and contacting it along its edge with molybdenum–rhenium gives a transparent interface and a channel clean enough for electrons to cross without scattering.
In those devices the supercurrent survives over distances up to 1.5 µm, and Ic oscillates as the gate changes the – the signature of a Fabry–Pérot cavity formed between the two contacts. Because MoRe stays superconducting to 8 T at 4 K, the same junction can be pushed into the regime with the contacts still superconducting, which is how broken-symmetry states and superconductivity were brought into contact in one sample.
What the pattern does and does not prove
Sweeping a perpendicular field maps the supercurrent’s spatial distribution. A current spread evenly across the junction gives the Fraunhofer pattern of single-slit diffraction, with lobes that decay; a current carried mostly along two edges gives a SQUID-like pattern whose lobes barely decay at all. That contrast is the usual argument for edge-dominated transport, and by extension for in candidate materials.
It is weaker evidence than it looks. Flux focusing by the superconducting leads, junction geometry, an uneven current distribution from ordinary , and self-field effects all distort the pattern. A serious claim combines the field pattern with the current–phase relation, the temperature dependence of Ic, and Shapiro steps under microwave drive – and treats a missing odd Shapiro step, on its own, with the same caution.
For specialists
Phase-coherent tunnelling of Cooper pairs across a weak link, giving a supercurrent I = I_c·sin Δφ and, under a voltage, an oscillation at 2eV/h. In the weak link can be a graphene channel, a semiconducting monolayer or a twisted interface, and the current–phase relation, the Fraunhofer pattern and the temperature dependence of the critical current report on ballistic transport, edge modes and pairing symmetry.