Researchers report a superconducting diode that steers Cooper‑pair currents more easily one way than the other, and does so without applying any external magnetic field. The diode effect comes from electron–electron interactions inside tiny superconducting lead (Pb) islands placed on proximitized bilayer graphene. By changing the island’s electrostatic charge with a gate, the team can switch the diode polarity and control nonreciprocal superconducting currents.

The device is built from Pb islands that form on bilayer graphene grown on silicon carbide. A superconducting scanning tunneling microscope tip makes a second junction to the island, so the experiment is effectively two Josephson junctions in series. The authors measure conductance and current–voltage curves at low temperature (T = 1.2 K). Large islands show a usual zero‑bias Josephson peak. Small islands show Coulomb blockade: the zero‑bias peak disappears and two resonant Cooper‑pair tunneling peaks appear, separated by a voltage gap that grows as the island area shrinks and can reach about 1.5 mV for the smallest islands.

At a qualitative level the effect comes from charging energy. In a small island the number of Cooper pairs becomes quantized and adding or removing a pair costs an energy EC. The system is described by a charging term HC = EC(n̂ − n0)2, where n0 is a gate‑induced fractional charge. Cooper pairs tunnel only when bias provides the required charging energy, producing resonant Cooper‑pair tunneling (RCT) peaks. If the junctions are asymmetric (different capacitances) and the gate sets a nonzero n0, particle–hole symmetry is broken. That combination produces different critical currents in opposite directions and thus a diode for Cooper pairs.