Lab experiment switches off superconductivity in magic-angle graphene

Inside a device built by scientists at The University of Manchester's National Graphene Institute and collaborators, one graphene system was used to screen another. As the team increased the carrier density in the neighbouring layer, superconductivity in the magic-angle graphene weakened and then disappeared completely.

Magic-angle graphene gets its name from the way two graphene sheets are stacked with a rotational offset of about 1.1 degrees. At that angle, the material can develop unusual electronic states, including superconductivity, as electrons pair up. The central question has been what makes those pairs form: vibrations of the atomic lattice, called phonons, or interactions between the electrons themselves.

The new device was built to weaken those electron interactions without merging the two systems. Its twisted graphene bilayers sat less than a nanometre apart, while the researchers achieved control over the interactions at distances as short as 0.3 nanometres. Increasing the neighbouring layer's carrier density progressively strengthened the screening effect. Measurements showed that the superconducting critical temperature could fall by more than an order of magnitude, while the correlated insulating state disappeared under the same conditions.

The outcome points away from a simple phonon-based explanation. If phonons were the main driver, weakening the electron interactions would be expected to leave superconductivity largely unchanged or even strengthen it slightly. Instead, the opposite happened. Professor Alexey Berdyugin, the study's corresponding author at the National University of Singapore, said the result provides clear experimental evidence that strong electron-electron interactions drive superconductivity in this system. The researchers do not claim to have identified one definitive pairing mechanism; several theories involving collective electronic interactions remain possible.

For now, the concrete payoff is a more discriminating laboratory tool, not a room-temperature material or a commercial device. The study was conducted at temperatures so low that, as Professor Sir Andre Geim noted, even helium turns liquid. But by tightening the limits on how superconductivity can arise in magic-angle graphene, the experiment could help researchers investigate other strongly interacting materials, including high-temperature superconductors. The study was published in Physical Review X.