Superconductivity seems to run in the family as far as graphene is concerned.
An illustration showing superconducting Cooper pairs in magic-angle multilayer graphene family. The adjacent layers are twisted in an alternating fashion. Image Credit: Ella Maru Studio
Graphene is a single-atom-thin material that can be separated from graphite, the same material found in pencil lead. The ultrathin material is composed entirely of carbon atoms organized in a simple hexagonal pattern, equivalent to chicken wire. Since its discovery in 2004, graphene has been discovered to have a number of exceptional properties in its single-layer form.
MIT scientists determined in 2018 that by stacking two graphene layers at a very particular “magic” angle, the twisted bilayer structure could demonstrate sturdy superconductivity, a material condition in which an electrical current can flow through with zero energy loss. The same group recently discovered an equivalent superconductive position in twisted trilayer graphene—a structure composed of three graphene layers piled at an accurate, new magic angle.
The team now observes that 4 and 5 graphene layers can be distorted and stacked at new magical angles to produce sturdy superconductivity at cold temperatures.
The recent revelation, published in Nature Materials this week, sets up graphene’s numerous twisted and stacked setups as the first known “family” of multilayer magic-angle superconductors. The researchers also discovered commonalities and dissimilarities among graphene family members.
The results could be used to create functional room-temperature superconductors. If the features of family members could be reproduced in other normally conductive materials, they could be used to produce electricity without degradation or to construct magnetically levitating trains that run without friction.
The magic-angle graphene system is now a legitimate ‘family,’ beyond a couple of systems. Having this family is particularly meaningful because it provides a way to design robust superconductors.
Jeong Min (Jane) Park, Study Lead Author and Graduate Student, Department of Physics, Massachusetts Institute of Technology
Park’s MIT co-authors include Yuan Cao, Li-Qiao Xia, Shuwen Sun, and Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics, along with Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Tsukuba, Japan.
“No Limit”
Jarillo-group Herrero’s team was the first to explore magic-angle graphene, which takes the form of a bilayer structure composed of two graphene sheets stacked one on top of the other and offset by 1.1°. At ultralow temperatures, this tangled setup, recognized as a moiré superlattice, changed the material into a robust and persistent superconductor.
The researchers also noted that the material had a type of electronic structure called a “flat band,” which means that the electrons in the material have the same energy irrespective of their momentum.
The normally frenetic electrons jointly slow down enough in this flat band state and at ultracold temperatures to team up in what is identified as Cooper pairs—essential additives of superconductivity that can move through the material without resistance.
While the researchers discovered both superconductivity and a flat band structure in twisted bilayer graphene, it was unclear whether the former resulted from the latter.
There was no proof a flat band structure led to superconductivity. Other groups since then have produced other twisted structures from other materials that have some flattish band, but they did not really have robust superconductivity. So we wondered: Could we produce another flat band superconducting device?
Jeong Min (Jane) Park, Study Lead Author and Graduate Student, Department of Physics, Massachusetts Institute of Technology
As they regarded this question, a team from Harvard University developed calculations that proved arithmetically that three graphene layers twisted at 1.6° would display flat bands as well, and that they might superconduct.
They went on to show that there is no cap on the number of graphene layers that demonstrate superconductivity if piled and distorted at the anticipated angles. Eventually, researchers demonstrated that they could mathematically connect every multilayer structure to a common flat band structure, providing strong evidence that a flat band can lead to rigorous superconductivity.
“They worked out there may be this entire hierarchy of graphene structures, to infinite layers, that might correspond to a similar mathematical expression for a flat band structure,” Park says.
Jarillo-group Herrero’s discovered superconductivity and a flat band in twisted trilayer graphene—three graphene sheets piled like a cheese sandwich, with the middle cheese layer moved by 1.6° with respect to the squished outer layers. However, the trilayer structure differed slightly from its bilayer counterpart.
“That made us ask, where do these two structures fit in terms of the whole class of materials, and are they from the same family?” Park says.
An Unconventional Family
The study, therefore, seeks to increase the number of graphene layers. They created two new structures, each with four and five graphene layers. Each structure is piled conversely, related to a twisted trilayer graphene cheese sandwich.
The team kept the structures in a refrigerator with a temperature of less than 1 K (about −273 °C), drove an electrical current over each structure, and evaluated the output under different conditions, equivalent to tests for their bilayer and trilayer systems.
Overall, they discovered that four- and five-layer twisted graphene display strong superconductivity as well as a flat band. Other commonalities between the structures and their three-layer counterpart included their response to a magnetic field of different strengths, angles, and orientations.
These experiments revealed that twisted graphene structures could be classified as a new family or class of prevalent superconducting materials. The experimental studies also recommended that the family might have a black sheep: while the original twisted bilayer structure shared key properties with its siblings, it also displayed subtle differences.
Prior studies by the group, for example, revealed that the structure’s superconductivity degraded at lower magnetic fields and became more uneven as the field rotated, especially in comparison to its multilayer siblings.
The team simulated each structure type to find an explanation for the disparities between family members. They stated that the loss of superconductivity in twisted bilayer graphene within certain magnetic conditions is purely due to all of its physical layers existing in a “nonmirrored” form inside the structure.
In other phrases, there are no two layers in the structure that are opposites of one another, whereas graphene’s multilayer siblings do. These results suggest that the process that drives electrons to flow in a rigorous superconductive state is consistent across the twisted graphene family.
“That’s quite important,” Park notes. “Without knowing this, people might think bilayer graphene is more conventional compared to multilayer structures. But we show that this entire family may be unconventional, robust superconductors.”
The National Science Foundation, the Air Force Office of Scientific Research, the Gordon and Betty Moore Foundation, the Ramon Areces Foundation, and the CIFAR Program on Quantum Materials all provided funding in part for this study.
Journal Reference:
Park, J. M., et al. (2022) Robust superconductivity in magic-angle multilayer graphene family. Nature Materials. doi.org/10.1038/s41563-022-01287-1.