Six-Layer Graphene Hosts a New Superconductor and Orbital Magnet Where Electrons and Holes Coexist
A Cornell-led team found the states by accident while chasing twisted graphene, in a device where two three-layer sheets relaxed into a rhombohedral stack.
Researchers at Cornell University have found a new superconducting state and a new kind of magnet in six stacked layers of graphene, and the discovery began as an accident. The team, working with colleagues at Japan's National Institute for Materials Science and the Max Planck Institute for the Structure and Dynamics of Matter in Germany, reported the results in Nature Physics.
Graphene is a single sheet of carbon atoms in a honeycomb pattern, and stacking sheets can change its behavior dramatically. The group studied rhombohedral hexalayer graphene, in which six layers are shifted sideways in a repeating pattern. "Our discovery was somewhat unexpected," said Xiaomeng Liu, the paper's senior author. "We originally set out to study twisted rhombohedral graphene, hoping to create isolated topological bands." In one device built from two three-layer sheets, the layers relaxed into alignment and formed the six-layer rhombohedral stack instead.
Early measurements hinted at behavior not seen in earlier work on five-layer graphene, so the team mapped the material's phase diagram, the chart of what states it enters under different conditions. To build the device they peeled thin flakes from graphite, hunted for the right stacking among them, and sandwiched the six-layer stack between insulating hexagonal boron nitride with graphite gates above and below.
They then measured how electricity flowed while changing temperature, magnetic field, the number of charge carriers and an electric field applied across the layers. "A vanishing electrical resistance revealed superconductivity, while an anomalous Hall response provided evidence of orbital magnetism arising from electrons' orbital motion," Liu explained. They also measured quantum oscillations in resistance, which let them reconstruct the Fermi surface and identify which charge carriers were present.
The team found several electronic states, including insulating states driven by interactions, regions of superconductivity, and two different types of orbital magnetism. Two of the superconducting-like states appeared only where electrons and holes, the missing-electron carriers that act like positive charges, coexist, hinting that both may take part in forming them.
"Our main finding is the discovery of a new superconducting state and a new multiferroic orbital magnet in rhombohedral six-layer graphene," Liu said. "Both emerge in a strongly correlated semimetallic regime, where electrons and holes coexist and interact. Our results suggest that interactions between these two types of charge carriers play a central role in the formation of these unusual states."
Why it matters is the long-running puzzle of unconventional superconductivity, in which electrons pair up without the usual vibration-based glue. Graphene stacks offer a clean, tunable setting for testing ideas, because researchers can switch states on and off with an electric field. A superconductor that arises when electrons and holes interact gives theorists a new case to explain. The result does not yet point to a practical device, but it widens the map of what stacked graphene can do.
