Researchers at the University of Manchester’s National Graphene Institute, led by lead author Julien Barrier and corresponding authors Professor Sir Andre Geim and Professor Alexey Berdyugin of the National University of Singapore, have demonstrated that superconductivity in magic-angle twisted bilayer graphene can be completely suppressed by screening the electrical interactions between its electrons, according to Graphene Info. The finding provides strong experimental evidence that electron-electron interactions drive the pairing behind the material’s superconductivity, rather than atomic vibrations, addressing a question open since the property’s discovery.
Researchers Solve Decade-Old Mystery in Magic-Angle Graphene Superconductivity
Magic-angle twisted bilayer graphene is formed by stacking two graphene sheets with a rotational offset of about 1.1 degrees, making it an intensely studied quantum material over the past decade. Previous experiments attempting to settle the pairing mechanism placed screening layers nanometers away separated by a dielectric spacer, which suppressed correlated-insulator states but left superconductivity largely intact or shifted its critical temperature by only 2 to 3 percent. The new device instead placed two twisted graphene bilayers directly on top of each other, separated by less than a nanometer while remaining electronically decoupled via a large relative twist angle. This geometry allowed the team to use one bilayer as an electrically tunable screening layer positioned immediately next to the magic-angle bilayer.
To make a difference, we had to solve two issues,
said Barrier, explaining the requirement to build a screening layer a fraction of a nanometer from the superconducting graphene while keeping it electronically separate, and making that layer tunable through atomic contact. Berdyugin added, When we switched on the screening, we were surprised to find that superconductivity was completely suppressed.
The team conducted the study at temperatures cold enough to turn helium liquid, noting that understanding the mechanism is crucial for advancing toward room-temperature superconductivity and commercial utility.
Ohio State Team Explores Environmental Control of Superconductivity
In a separate study published in sciencedaily.com, researchers led by Chun Ning (Jeanie) Lau, a physics professor at The Ohio State University, found that tuning the environment around twisted bilayer graphene can strengthen or weaken electron interactions to switch superconductivity on and off. Superconductivity allows certain materials to carry electricity with zero energy loss when cooled below a critical temperature.
Electrons normally repel each other, but in superconductors they form pairs; this pair formation is the key to a superconductor’s ability to conduct electricity without dissipation,
Lau said. The researchers observed that increasing certain adjustments within the material made superconductivity weaker instead of stronger, differing from conventional superconductors where reducing repulsive forces typically strengthens the phenomenon. Lead author Xueshi Gao, a physics PhD student at Ohio State, noted that the mechanism in this system remains not well understood.
MIT Physicists Measure Magic-Angle Tri-Layer Graphene Gap
Separately, MIT physicists published results in scitechdaily.com providing evidence of unconventional superconductivity in magic-angle twisted tri-layer graphene (MATTG), a material built by stacking three ultrathin carbon sheets at a precise angle. The team utilized a measurement system involving electron tunneling between layers to reveal a sharp, V-shaped superconducting gap, pointing to a pairing mechanism distinct from traditional superconductors.
