Revealing the superconducting edge of twisted bilayer graphene

Graphene is a simple material with only one layer of carbon atoms, but when two sheets of it are folded and folded at a small angle, this twisted bilayer material produces many interesting effects, especially superconductivity.

Now Cornell researchers are continuing to understand how the material achieves this level by knowing its highest temperature that can be reached – 60 Kelvin. The finding is mathematically correct, a rare feat in the field, and provides new insight into the factors that govern superconductivity.

“Looking forward, this opens the way to understanding which degrees of freedom one should try to control and improve in order to strengthen the tendency towards greater performance. of these two platforms,” ​​said Debanjan Chowdhury, Joyce A. Yelencsics Rosevear ’65 and Frederick M. Rosevear ’64 Associate Professor of physics in the College of Arts and Sciences (A&S).

Chowdhury is co-author of “Low-Energy Optical Sum-Rule in Moiré Graphene,” published in Physical Examination Letters. The study’s first author is Juan Felipe Mendez-Valderrama, Ph.D. ’24, now at Princeton University; the second author is Dan Mao, who was a Bethe/KIC Theory Fellow at Cornell’s Laboratory of Atomic and Solid State Physics from 2021–2024 and is now at the University of Zürich.

“Taking two layers of graphene and placing them at 1.1 degrees, the magic angle, leads to amazing results,” Chowdhury said. “One such explanation is that by simply changing the electric field, experimentalists can turn twisted bilayer graphene into a superconductor or insulator, with different electrical properties.

“Essentially, we want to know theoretically what is the maximum temperature that electrons can control in twisted layers of graphene, and what sets the interaction between them -different insulators and superconductors.”

In 2023, Chowdhury and Mao developed a new theoretical formalism to combine the highest temperature of the superconducting transition in any material obtained by stacking and twisting two-dimensional materials. For the current work, they applied it to twisted bilayer graphene.

“They created these hard statements in 2023, which at that time you could only read,” said Mendez-Valderrama. “What we tried to do here is to calculate this precisely in a real sample of twisted bilayer graphene, which leads to new insights into the factors that control superconductivity.”

One of the most sought-after objects in physics labs around the world, superconductivity is the phenomenon where electrons can flow through a material without loss of energy. Currently, this can only be achieved at very low temperatures.

Superconductivity is best understood in ordinary materials, such as aluminum, where electrons move with such high kinetic energy that they do not sense each other’s presence. This greatly simplifies the explanation of superconductivity, said Chowdhury. The temperatures at which ordinary materials transition to the superconducting state are also low compared to the material’s internal energy scale.

The situation is completely different with twisted bilayer graphene, as the motion of each electron is highly correlated with each electron, Chowdhury said. In addition, the transition temperature of the material—starting at about 5 Kelvin—is relatively high compared to the internal energy scale, which gives hope for designing superconductors with a temperature even higher in the future.

“One of the surprising properties of twisted bilayer graphene is its relative coupling,” said Chowdhury. “You have unprecedented control over the temperature and the windings—the tiny electric fields that are used to change the material from insulator to superconductor—making it easier to especially to explore all kinds of exciting regimes with this property.”

The theoretical framework the team developed could be applied to other devices in the future, according to Mao.

“We are considering other promising combinations of materials beyond twisted bilayer graphene to realize high-temperature superconductors, and we are also trying to extend these ideas to other desirable opto-electronic materials that can be measured experimentally,” said Mao.