Layers of graphene. Image: Ohio State University.
Layers of graphene. Image: Ohio State University.

Researchers have uncovered new evidence for how graphene, when twisted to a precise angle, can become a superconductor, moving electricity with no loss of energy. In a paper in Nature, a team led by physicists at Ohio State University report their finding of the key role that quantum geometry plays in allowing this twisted graphene to become a superconductor.

Graphene is a single layer of carbon atoms in a honeycomb pattern. In 2018, scientists at the Massachusetts Institute of Technology discovered that, under the right conditions, graphene could become a superconductor if one piece of graphene were laid on top of another piece and the layers twisted to a specific angle – 1.08° – to create twisted bilayer graphene.

Ever since, scientists have been studying twisted bilayer graphene and trying to figure out how this ‘magic angle’ works, said Marc Bockrath, professor of physics at Ohio State and co-author of the paper. “The conventional theory of superconductivity doesn’t work in this situation. We did a series of experiments to understand the origins of why this material is a superconductor.”

In a conventional metal, high-speed electrons are responsible for conductivity. But twisted bilayer graphene has a type of electronic structure known as a ‘flat band’, in which the electrons move very slowly – in fact at a speed that approaches zero if the angle is exactly at the magic one. Under the conventional theory of superconductivity, electrons moving this slowly should not be able to conduct electricity, said study co-author Jeanie Lau, also a professor of physics at Ohio State.

With great precision, Haidong Tian, first author of the paper and a student in Lau’s research group, produced twisted bilayer graphene so close to the magic angle that the electrons were nearly stopped by usual condensed matter physics standards. The sample nevertheless showed superconductivity.

“It is a paradox. How can electrons which move so slowly conduct electricity at all, let alone superconduct? It is very remarkable,” Lau said.

In their experiments, the research team demonstrated the slow speeds of the electrons and made more precise measurements of electron movement than had been previously possible. And they also found the first clues as to what makes this graphene material so special.

“We can’t use the speed of electrons to explain how the twisted bilayer graphene is working,” Bockrath said. “Instead, we had to use quantum geometry.”

As with everything quantum, quantum geometry is complex and non-intuitive. But the results of this study have to do with the fact that an electron is not only a particle, but also a wave – and thus has wavefunctions.

“The geometry of the quantum wavefunctions in flat bands, together with the interaction between electrons, leads to the flow of electrical current without dissipation in bilayer graphene,” said co-author Mohit Randeria, professor of physics at Ohio State.

“We found that conventional equations could explain maybe 10% of the superconductivity signal we found,” said Lau. “Our experimental measurements suggest quantum geometry is 90% of what makes this a superconductor.”

The superconducting properties of this material can only be found in experiments at extremely low temperatures. The ultimate goal is to be able to understand the factors that lead to high-temperature superconductivity, which will be potentially useful in real-world applications such as electrical transmission and communication.

“It would have a huge impact on society,” Bockrath said. “It is a long way off, but this research is definitely taking us forward in understanding how it could happen.”

This story is adapted from material from Ohio State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.