A novel mechanism has been determined by researchers at the University of Texas at Dallas and their collaborators at the Ohio State University.
This mechanism develops superconductivity in a material in which the speed of electrons is almost zero, possibly setting the stage for the design of new superconductors.
Their study illustrates a new method to quantify electron speed and marks the first time that quantum geometry has been determined as the chief contributing mechanism to superconductivity present in any material.
The study outcomes were reported in Nature on February 15th, 2023.
Graphene is a single layer of carbon atoms that have been arranged in a honeycomb pattern in a periodic manner. In twisted bilayer graphene, two sheets of graphene are piled up on top of one another with a minor angular twist.
In principle, at a particular “magic” twist angle, the electrons’ speed in the material goes to zero, stated Dr. Fan Zhang, Associate Professor of Physics in the School of Natural Sciences and Mathematics at UT Dallas and an author of the study.
Zhang, a theorist, and his collaborators, earlier published a review article regarding the special physical properties of such systems.
In a conventional metal, the average velocity of electrons is responsible for conductivity, and in a superconductor, the electrons pair into Cooper pairs to flow uniformly with no resistance or dissipation.
Dr. Fan Zhang, Study Author and Associate Professor, Physics, School of Natural Sciences and Mathematics, The University of Texas at Dallas
Zhang added, “By contrast, in twisted bilayer graphene, the electrons move very, very slowly, with a speed approaching zero. But this produces a paradox: How can these slow electrons conduct electricity, let alone superconduct. The superconductivity must come from something else. We determined that it arises from quantum geometry.”
The newly performed study offers considerable knowledge into how superconductivity exhibits the potential to emerge in materials with almost “frozen” electrons.
A device of magic-angle twisted bilayer graphene was fabricated by the scientists of Ohio State, headed by physics professors and study authors Dr. Marc Bockrath, Dr. Jeanie Lau, and Dr. Mohit Randeria. Also, they were able to measure the electrons’ speed.
The Schwinger effect was leveraged by condensed matter physicists, in which electron-positron pairs are made instantly in the existence of an electric field, to quantify the velocity of electrons in the material and also its contribution to superconductivity.
The study outcomes mark the first time that the Schwinger effect, a phenomenon anticipated but not yet noted in relativistic particle physics, has been noticed in any superconductor.
It turns out that the speed is the slowest to date among all graphene systems. Surprisingly, superconductivity can still arise.
Tianyi Xu, Study Author and Physics Doctoral Student in Zhang’s Theory Group, The University of Texas at Dallas
Xu added, “Figuring out how to measure electron velocity was important because determining the velocity was a key step that allowed us to calculate its contribution to superconductivity. It turns out that the contribution is tiny.”
The theoretical analyses and experimental measurements illustrated the fact that the dominant contribution to superconductivity is rather from quantum geometry. This is known to be similar to ordinary geometry but originates from quantum many-body physics.
“Consider a balloon in our normal, three-dimensional space. All its geometric properties can be determined by metrics and the curvature defined on its surface,” stated Patrick Cheung MS’22, PhD’22, a former graduate student of Zhang’s and an author of the article.
Patrick added, “The same is true for the space in which the quantum electrons live. In this so-called Hilbert space, quantum geometry can give rise to unbelievable material properties and applications, such as the superconductivity discussed in this study and intelligent quantum sensing, which we demonstrated in previous work.”
Superconductivity allowed by quantum geometry is considered an untraditional mechanism. The new outcomes could be a foundation for identifying and designing new superconductors that function at higher temperatures compared to established ones, which function below 150 K (–123 °C or –190 °F) at ambient pressure.
Quantum geometry can give rise to unbelievable material properties and applications, such as the superconductivity discussed in this study and intelligent quantum sensing, which we demonstrated in a previous work.
Patrick Cheung MS’22, PhD’22
Xu stated, “A high-temperature superconductor, operating at room temperature, has long been the holy grail of condensed matter and materials physics. If it could be developed, our lives and society would be completely reshaped because, for example, we could transport electricity much more efficiently and run maglev trains much less expensively.”
Quantum geometry is amazing and leads to rich and unexpected consequences. Much more exciting physics waits to be discovered.
Dr Fan Zhang, Study Author and Associate Professor, Physics, School of Natural Sciences and Mathematics, The University of Texas at Dallas
Besides the scientists at Ohio State University, other authors included researchers from the National Institute for Materials Science in Japan.
In this study, financial support for the UTD scientists was provided by the National Science Foundation and the Army Research Office, a component of the US Army Combat Capabilities Development Command Army Research Laboratory.
Tian, H., et al. (2023) Evidence for Dirac flat band superconductivity enabled by quantum geometry. Nature. doi.org/10.1038/s41586-022-05576-2.