By determining the material’s maximum achievable superconducting temperature, 60 Kelvin, Cornell researchers are advancing the comprehension of how it reaches this state, according to a study published in Physical Review Letters.
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Graphene is a simple material composed of only one layer of carbon atoms, but when two sheets of it are stacked together and offset at a slight angle, the resulting twisted bilayer material produces various intriguing effects, most notably superconductivity.
The discovery is mathematically precise, a rare feat in the field, and it is sparking new research into the factors that fundamentally control superconductivity.
Looking ahead, this paves the way for understanding what are the possible degrees of freedom that one should try to control and optimize in order to enhance the tendency towards superconductivity in these two-dimensional material platforms.
Debanjan Chowdhury, Assistant Professor, Physics Department, Cornell University
Chowdhury is a co-author of “Low-Energy Optical Sum-Rule in Moiré Graphene," which was published on November 4th, 2024 in Physical Review Letters. The study’s first author is Juan Felipe Mendez-Valderrama, Ph.D. '24, who is 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 to 2024 and is now at the University of Zürich.
Chowdhary added, “Taking two layers of graphene and setting them at 1.1 degrees, a magic angle, leads to dramatic effects. One such effect is that by simply varying an electric field, experimentalists can turn twisted bilayer graphene into either a superconductor or an insulator, which have wildly different electrical properties. Of course, we want to know theoretically what is the highest possible temperature at which the electrons can superconduct in twisted layers of graphene, and what sets the interplay between the various insulators and superconductors.”
A new theoretical formalism was created in 2023 by Chowdhury and Mao to calculate the maximum superconducting transition temperature that can be achieved in any material by stacking and twisting two-dimensional materials. They used it on twisted bilayer graphene for the current study.
They had developed these rigorous expressions in 2023, which at the time you could only calculate approximately. What we tried to do here is precisely calculate this in a realistic model of twisted bilayer graphene, which leads to new insights into the factors that fundamentally control superconductivity.
Juan Felipe Mendez-Valderrama, PCCM Postdoctoral Research Fellow, Princeton University
The ability of electrons to move through a material without losing energy is known as superconductivity, and it is one of the most desired characteristics at physics labs around the world. This is currently limited to extremely low temperatures.
Conventional materials like aluminum, where electrons move with such high kinetic energies that they hardly notice one another, are well known for their superconductivity. Chowdhury claimed that this significantly simplifies the explanation of superconductivity. In addition, the temperatures at which conventional materials become superconducting are low in relation to the materials' inherent energy scales.
In contrast, according to Chowdhury, every electron's motion in twisted bilayer graphene is highly coordinated with every other electron. Furthermore, compared to the intrinsic energy scales, the material's transition temperature, which starts at about 5 Kelvin, is comparatively high, offering hope for developing superconductors with even higher temperatures in the future.
“One of the remarkable properties of twisted bilayer graphene is the associated tunability. You have unprecedented control over temperature and the twist angle – the tiny electric fields that are applied to switch the material from being an insulator versus a superconductor – making it very easy to explore all sorts of exciting regimes in this material,” Chowdhary stated.
According to Mao, the theoretical framework developed by the team will apply to other materials in the future.
We are thinking about other promising material combinations beyond twisted bilayer graphene to identify possibly higher temperature superconductors, and also trying to extend these ideas to other desirable opto-electronic properties that can be measured via experiment.
Dan Mao, Postdoctoral Researcher, University of Zurich
This research was partially funded by a National Science Foundation grant and a Sloan Research Fellowship.
Journal Reference:
Mendez-Valderrama, J. F. et. al. (2024) Low-Energy Optical Sum Rule in Moiré Graphene. Physical Review Letters. doi.org/10.1103/PhysRevLett.133.196501