Researchers made a tiny, gate-defined channel in bilayer graphene and contacted it with a single aluminum superconductor. When the aluminum is superconducting, the channel’s conductance plateaus rise above their normal values. At larger voltages the team also finds a clear conductance anomaly that marks the loss of superconductivity and the drop of the so-called Andreev excess current.

The device is a bilayer graphene (BLG) flake sandwiched in hexagonal boron nitride, with a graphite back gate, a narrow split-gate gap of 75 nm, and a channel gate separated by an Al2O3 layer. A titanium/aluminum (Ti/Al, 5/40 nm) superconducting contact sits about 350 nm from the constriction. The authors measured conductance both above the superconducting transition (1.7 K) and well below it (10 mK). In the normal state the channel shows quantized conductance steps Gn = (4 e^2/h)N, where the factor 4 comes from spin and valley degeneracy in BLG.

The main change when the contact is superconducting is due to Andreev reflection. In Andreev reflection an electron hitting the superconductor is converted into a hole and a Cooper pair enters the superconductor; this process can raise conductance above the normal value. The measured plateaus grow by roughly 1.25 times the normal step height rather than the factor of two expected for a perfectly transparent interface. Interpreting this within the Blonder–Tinkham–Klapwijk (BTK) framework yields an effective interface transparency near τ ≈ 0.9, slightly below ideal because of residual scattering or momentum mismatch.

From bias-dependent measurements the authors extract a superconducting gap Δ ≈ 120 μeV from coherence peaks in the differential conductance. At larger bias voltages they see a pronounced dip above the gap. The dip is consistent with a collapse of the Andreev excess current and therefore a transition of the contact from superconducting to normal. The superconducting features are sensitive to magnetic field: they survive to much larger in-plane fields than out-of-plane fields (out-of-plane critical field Bc,⊥ ≈ 20 mT), as expected for thin-film superconductors.