Researchers created a “superlattice” of two graphene sheets stacked together — not precisely on top of each other, but rotated ever so slightly, at a “magic angle” of 1.1 degrees. The hexagonal honeycomb pattern is offset slightly, creating a precise moiré configuration that is predicted to induce strange, “strongly correlated interactions” between the electrons in the graphene sheets. In any other stacked configuration, graphene prefers to remain distinct, interacting very little, electronically or otherwise, with its neighboring layers.
The team, led by Pablo Jarillo-Herrero, an associate professor of physics at MIT, found that when rotated at the magic angle, the two sheets of graphene exhibit nonconducting behavior, similar to an exotic class of materials known as Mott insulators. When the researchers then applied voltage, adding small amounts of electrons to the graphene superlattice, they found that, at a certain level, the electrons broke out of the initial insulating state and flowed without resistance, as if through a superconductor.
“We can now use graphene as a new platform for investigating unconventional superconductivity,” Jarillo-Herrero says. “One can also imagine making a superconducting transistor out of graphene, which you can switch on and off, from superconducting to insulating. That opens many possibilities for quantum devices.”
The behavior of strongly correlated materials, and in particular unconventional superconductors, has puzzled physicists for decades. Such difficulties have stimulated new research paradigms, such as ultracold atom lattices for simulating quantum materials. Here we report on the realization of intrinsic unconventional superconductivity in a two-dimensional superlattice created by stacking two graphene sheets with a small twist angle. For angles near 1.1°, the first ‘magic’ angle, twisted bilayer graphene exhibits ultraflat bands near charge neutrality, which lead to correlated insulating states at half-filling. Upon electrostatic doping away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature Tc up to 1.7 kelvin. The temperature–density phase diagram shows similarities with that of the cuprates, including superconducting domes. Moreover, quantum oscillations indicate small Fermi surfaces near the correlated insulating phase, in analogy with underdoped cuprates. Its relatively high Tc, given such a small Fermi surface (corresponding to a record-low two-dimensional carrier density of about 1011 per square centimetre), puts twisted bilayer graphene among the strongest coupling superconductors, in a regime close to the crossover between the Bardeen–Cooper–Schrieffer regime and a Bose–Einstein condensate (BCS–BEC). These results establish twisted bilayer graphene as the first purely carbon-based two-dimensional superconductor, providing a highly tunable platform with which to investigate strongly correlated phenomena, which could lead to insights into the physics of high-Tc superconductors and quantum spin liquids.