New Study Reveals Clues Behind Twisted Graphene Superconductor

Scientists at The Ohio State University have produced new evidence of how graphene, when twisted to a precise angle, can become a superconductor. In a study published in the journal Nature, the team reported on 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, and in 2018, scientists 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 were twisted to a specific angle. This creates twisted bilayer graphene.

However, the conventional theory of superconductivity doesn't work in this situation. 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. But with great precision, the team was able to obtain a device so close to the magic angle that the electrons were nearly stopped by usual condensed matter physics standards. The sample nevertheless showed superconductivity. In their experiments, the research team demonstrated the slow speeds of the electrons and gave more precise measurements of electron movement than had been previously available. And they also found the first clues as to what makes this graphene material so special. The team discovered that quantum geometry plays a key role in allowing twisted graphene to become a superconductor. 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. Conventional equations could only explain about 10% of the superconductivity signal that was found, but experimental measurements suggest quantum geometry is responsible for 90% of what makes this a superconductor. The superconductive effects of this material can only be found in experiments at extremely low temperatures. However, 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. Overall, the study provides new evidence of how twisted graphene can become a superconductor, and the discovery of the key role that quantum geometry plays in this process could have significant implications for future technologies.