Since superconductivity was discovered in magic-angle twisted bilayer graphene, physicists have argued over whether its electron pairing is conventional, mediated by lattice vibrations, or driven by electron-electron interactions. Researchers at Manchester's National Graphene Institute, with the Henry Royce Institute and partners in the United States, Belgium, Japan and Singapore, built a device designed to settle the question: a second twisted graphene bilayer positioned less than a nanometre above the magic-angle layer, close enough to screen Coulomb interactions yet electronically decoupled.
The screening layer acts as a tunable dial on the strength of electron-electron repulsion. As screening increased, superconductivity in the magic-angle layer, with its twist of about 1.1 degrees, progressively weakened and was fully suppressed at high carrier density, with the critical temperature falling by more than an order of magnitude. The correlated insulating states that accompany superconductivity in these systems disappeared under the same conditions.
Conventional phonon-mediated pairing would predict little sensitivity to such screening, so the observations point squarely towards unconventional superconductivity driven by electron interactions, although the authors are careful to note the experiment does not pin down a single definitive pairing mechanism. The work, published as Coulomb Screening of Superconductivity in Magic-Angle Graphene in Physical Review X, gives theorists a rare controlled experiment against which to test competing models of this much-studied material.