Superconductors usually don't get along with magnets: a strong field destroys the state in which current flows without resistance. But in thin layers of carbon, things are different. A team of physicists took six-layer graphene, six atomic planes stacked in a rhombohedral arrangement, and found that a magnetic field can not only quench superconductivity but also switch it on.

At zero field, this material already exhibits an unusual form of superconductivity — chiral superconductivity. Its electron pairs are twisted in one direction, like a spiral staircase, so the resistance changes depending on the direction of the current. But when the researchers applied a field of about 0.8 tesla, roughly the strength of an MRI machine, a second superconducting regime appeared. It persists up to 1.6 tesla and is controlled by three knobs: the field itself, the electron density, and the electrical displacement between layers.

Measurements showed that this field-induced superconductor emerges from a special magnetic state — a quarter-metal that is magnetized opposite to the original. Thus, in a single piece of graphene, the team obtained a quartet of superconducting phases: four different combinations of spin and valley (a quantum index reminiscent of electron flavor). Each can be selected by tuning external parameters. The authors attribute this to competition between two effects: spin-valley splitting (similar to the mechanism in topological insulators) and the interaction of magnetic moments with the field.