Twisted Graphene Reveals Quantum Geometry’s Key Role in Superconductivity
The study published in the journal Nature on Feb. 15, 2023, by a team of physicists at The Ohio State University provided new evidence of how graphene can become a superconductor with no loss of energy when twisted to a specific angle.
The team discovered that quantum geometry plays a crucial role in enabling the twisted graphene to become a superconductor.
Graphene is a single layer of carbon atoms that is commonly found in pencils.
In 2018, scientists at the Massachusetts Institute of Technology found that graphene could become a superconductor if one piece of graphene was placed on top of another and the layers were twisted to a precise angle of 1.08 degrees. This created twisted bilayer graphene.
Since then, scientists have been studying this “magic angle” to understand how it works.
According to Marc Bockrath, the conventional theory of superconductivity does not apply to twisted bilayer graphene. To understand the reasons behind the material’s superconductivity, the team conducted a series of experiments.
In a regular metal, fast-moving electrons are responsible for conductivity. However, in twisted bilayer graphene, there is a flat band electronic structure in which electrons move slowly, and at the magic angle, they can approach a speed of zero.
According to Jeanie Lau, a co-author of the study and a professor of physics at Ohio State, according to conventional superconductivity theory, electrons that move at such slow speeds should not be able to conduct electricity.
Haidong Tian, a student in Lau’s research group, was able to obtain a device with exceptional precision that was almost at the magic angle, causing the electrons to slow down to an extent where they were nearly stopped by the standards of condensed matter physics. However, the sample still demonstrated superconductivity.
Jeanie Lau stated that it is paradoxical that electrons that move so slowly are still able to conduct electricity, let alone superconduct. This phenomenon is impressive.
The research team provided more precise measurements of electron movement than previously available and demonstrated the slow speeds of the electrons in their experiments.
The team also discovered the initial evidence that makes this graphene material unique.
Marc Bockrath said that the team cannot use the speed of electrons to explain how twisted bilayer graphene operates. Instead, they had to use quantum geometry.
The study’s results are linked to the fact that an electron is not just a particle but also a wave and has wave functions, and the geometry of the quantum wave functions in flat bands, along with the interaction between electrons, results in electrical current flow without dissipation in bilayer graphene, according to co-author Mohit Randeria, a professor of physics at Ohio State.
According to Jeanie Lau, conventional equations can only explain approximately 10% of the superconductivity signal observed, while experimental measurements indicate that quantum geometry accounts for 90% of what makes this material a superconductor.
The material’s superconductivity can only be observed at extremely low temperatures, and the aim is to understand the factors that lead to high-temperature superconductivity, which could have significant real-world applications, such as in electrical transmission and communication, according to Marc Bockrath.
Bockrath stated that such a discovery could have a significant impact on society, although it is a long way off. Nonetheless, this research is advancing our understanding of how it could occur.
More information: Chun Lau, Evidence for Dirac flat band superconductivity enabled by quantum geometry, Nature (2023). DOI: 10.1038/s41586-022-05576-2. www.nature.com/articles/s41586-022-05576-2