Manchester Physicists Switched Off the Electron Repulsion in Magic-Angle Graphene, and the Superconductivity Died With It

Physicists at the University of Manchester have found a way to turn superconductivity in magic-angle graphene up and down like a dial, and what happened when they turned it down settles a question that has been open since the material was discovered.

Stack two sheets of graphene and rotate one by about 1.1 degrees — the "magic angle" — and the electrons in the stack slow almost to a halt and start behaving collectively. The material becomes an insulator at some electron densities and a superconductor at others. Since 2018 there have been two competing explanations for the superconductivity. Either it works the way superconductivity works in ordinary metals, with vibrations of the crystal lattice called phonons gluing electrons into pairs, or it is driven by the electrons' own repulsion for one another, which is what happens in high-temperature copper-oxide superconductors and is far less understood.

The Manchester group, working at the National Graphene Institute, built a device that could tell the two apart. They placed a second, separate graphene bilayer less than a nanometer away from the magic-angle stack and used it as a screening layer. Loading that neighbor with charge carriers weakens the Coulomb repulsion between electrons in the magic-angle layer without changing anything else about the sample — no new device, no new twist angle, no thermal cycling.

Then they turned the screening up.

"When we switched on the screening, we were surprised to find that superconductivity was completely suppressed," said Alexey Berdyugin, now a professor at the National University of Singapore and one of the paper's corresponding authors.

The superconducting critical temperature dropped by more than an order of magnitude before vanishing entirely at high screening densities. The correlated insulating states disappeared under the same conditions. That pairing is the tell. If phonons were doing the work, screening the electron-electron interaction should have left the superconductivity largely intact, and might even have strengthened it by removing a competing order. Instead both the insulating and superconducting phases died together, which is the signature of a single underlying cause: the repulsion itself.

The work was led by Julien Barrier, with Sir Andre Geim — who shared the 2010 Nobel Prize in Physics for isolating graphene — as the other corresponding author. Collaborators came from the Henry Royce Institute, Washington University in St. Louis, the University of Pennsylvania, the University of Antwerp, Japan's National Institute for Materials Science and the National University of Singapore. The results appeared in Physical Review X on Sept. 4 under DOI 10.1103/z9qg-287y.

The practical value is the technique as much as the answer. Screening is a knob that can be applied to any two-dimensional stack, which gives experimentalists a way to interrogate a whole family of twisted materials without the sample-to-sample variation that has made this field so hard to reproduce. Magic-angle graphene superconducts only at a few kelvin, so nothing here changes what can be built. What it changes is which theory people build on.