A water pipe: the wider it is, the more water flows through. In superconducting contacts — Josephson junctions (bridges through which current flows without resistance)—it's usually the same: the better the material conducts in its normal state, the larger the superconducting current that flows through it. But a recent experiment with junctions made of twisted bilayer graphene (two graphene sheets rotated relative to each other by a magic angle) showed the opposite: the current grew while conductivity dropped. It's like turning the tap on stronger, and the pressure decreases.
The key lies in the unusual geometry of electronic states. In the flat energy bands of twisted graphene, electrons move very slowly, and the ordinary Josephson current nearly vanishes. But another mechanism emerges — the quantum-metric current. Its magnitude depends not on the speed of electrons but on the "distance" between their states in a special space — the so-called quantum metric. If this distance is comparable to the junction length, current can flow even when ordinary conductivity is small.
The authors built a simple model based on the Lieb lattice, and then a realistic six-band model of twisted graphene. In both cases, calculations showed that for certain parameters, the critical current indeed grows while conductivity drops — exactly as in the experiment. This is the first direct indication that the quantum-metric current, previously neglected, can dominate in systems with flat bands.
This anomaly is not just a curiosity. It opens a path to controlling superconducting properties through geometry, not just material composition. Perhaps in the future, this will enable ultra-sensitive sensors or quantum computer components where current is switched on and off by twisting layers, rather than by changing temperature or magnetic field.
Twisted bilayer graphene is called "magic" because of the twist angle of about 1.1 degrees: at this angle, electrons behave as if their speed dropped to zero and begin to interact strongly with each other, giving rise to superconductivity and other unusual effects.
In Arthur C. Clarke's novel "Childhood's End," aliens manipulated matter at the quantum level, changing its properties with a simple twist. The anomalous current in twisted graphene is a step toward such control over superconductivity.