The superconductivity of layered graphene is surprisingly strange

Why do cold thin sheets of carbon offer no resistance to electric currents? Two experiments are bringing us closer to an answer – and maybe even to practical room-temperature superconductors.

Kin Chung Fong at Northeastern University in Massachusetts was stunned when another physicist, Abhishek Banerjee at Harvard University, told him a number over dinner. They were studying different aspects of graphene – sheets of carbon only one atom thick – but both made the same estimate about how hard it should be for an electric current flowing through graphene to suddenly change.

Past experiments have shown that very cold stacks of two or three layers of graphene can superconduct, or perfectly conduct electricity without resistance and energy loss, if some of the sheets are rotated by a special angle. But why this happens remained mysterious. The two physicists thought the property they estimated at dinner, called kinetic inductance, might illuminate the answer.

“The feeling was like when you are in a wood hiking [through] the forest, and suddenly you find, well, wait a minute, I’m not the only person in this deep forest,” says Fong.

Together with other colleagues, they turned their idea into two experiments. One group measured kinetic inductance for two layers of stacked-and-twisted graphene; a second group focused on three layers.

Joel Wang at the Massachusetts Institute of Technology, who was in the group that worked on two-ply graphene, says that measuring kinetic inductance had previously been prohibitively difficult. Because multilayer graphene can only be produced in very small pieces, standard techniques for measuring its superconducting currents – such as exposing it to particles or magnetic fields – produced excessively weak signals. Instead, both teams had to innovate a setup where the tiny graphene flakes were exposed to microwaves while the researchers slowly varied properties like temperature, which must be kept very low for superconductivity to occur at all.

We know that multilayer graphene superconducts because the electrons inside of it pair up, and these pairs flow in currents more easily than individual particles. But electrons typically repel each other. How exactly the particles come together and what properties these pairs have is still not understood.

“Theory is [running] behind experiments here,” says Miuko Tanaka at the University of Tokyo, who was also in the two-ply group.

For two layers of graphene, her team found that the superconducting current is much “stiffer” – it resists change more – than is predicted by any conventional theory of superconductivity. They traced this anomaly back to something called quantum geometry. Specifically, the shape of the electrons’ wavefunctions, which encode all their properties and possible behaviours, seemed to drive this exotic type of superconductivity.

In trilayer graphene, researchers found surprising similarities between the kinetic inductance of their sample and the behaviour of a family of completely different superconductors – ones that maintain their special properties at much higher temperatures.

Because of this, both Banerjee and Tanaka say these experiments may do more than shed light on why graphene superconducts – they could also reveal key properties required for room-temperature superconductors. Physicists have been searching for such materials for decades in the hope that using them could radically decrease the energy consumption of many devices.

“We are finding interesting laws which seem to emerge in both these material systems. Maybe what we are uncovering is something deeper,” says Banerjee. Both teams are planning on performing similar experiments with other very thin superconductors.

“Recently, there have been so many new two-dimensional superconductors that are interesting, surprising and kind of unusual,” says Zeyu Hao, also at Harvard University, who was on the team researching three-layer graphene. For example, earlier this month a different team published research showing that two-layered crystals of a material called tungsten diselenide exhibit superconductivity when the layers are twisted relative to one another.

In the meantime, Hao’s colleague Mary Kreidel, now at NASA Jet Propulsion Laboratory in California, already has an application in mind for stacked-and-twisted graphene. She is working on particle detectors for space missions, many of which use superconductors. They could be made smaller and lighter – a crucial advantage in space flight – if they were made from multilayer graphene, she says.