Graphene superconductivity switched off, revealing electron interactions drive it

Researchers at The University of Manchester’s National Graphene Institute have demonstrated the ability to completely suppress superconductivity in magic-angle graphene by weakening electron interactions. This finding provides significant evidence that electron interactions are central to the superconductivity observed in this material.

The team developed a device that allows precise control over electron interactions in magic-angle twisted bilayer graphene. This material is created by stacking two graphene sheets with a rotational offset of approximately 1.1 degrees, leading to unusual electronic states, including superconductivity. The cause of electron pairing in this context has been debated, with theories suggesting either lattice vibrations or electron interactions as the driving force.

To test these theories, the researchers constructed a structure with two twisted graphene bilayers separated by less than a nanometer. This setup enabled one bilayer to act as a tunable screening layer for the other. By increasing the carrier density in the adjacent bilayer, the Coulomb interactions between electrons in the magic-angle graphene were progressively weakened, leading to the suppression of superconductivity.

The experiment also affected the correlated insulating state of magic-angle graphene, which disappeared under the same conditions. Measurements indicated that the superconducting critical temperature could decrease by more than an order of magnitude. The researchers achieved this control at separations as short as 0.3 nanometers, enhancing the screening effect compared to previous experiments.

Professor Alexey Berdyugin from the National University of Singapore noted that the complete suppression of superconductivity upon screening provides clear evidence that strong electron-electron interactions are responsible for the phenomenon. This result challenges the conventional explanation involving phonons, as weakening Coulomb interactions would not typically diminish superconductivity if phonons were the primary driver.

While the experiment does not pinpoint a single pairing mechanism, it narrows down the possibilities, suggesting that collective electronic interactions play a significant role. The findings may also aid in the study of other materials where strong electron interactions are crucial, such as high-temperature superconductors.

The study, published in Physical Review X, contributes to the broader understanding of superconductivity, a step towards achieving room-temperature superconductivity. Professor Sir Andre Geim emphasized the importance of understanding the mechanisms behind exotic superconductivity in graphene to advance towards practical applications.