The results (The Magic angle of superconducting graphene structures) can inform the design of actual superconducting devices.
When it comes to graphene, it seems like superconductors are in the family. Graphene is a thin monoatomic material that can be exfoliated from the same graphite found in pencils. The ultrathin material is made entirely of carbon atoms arranged in a simple hexagon, similar to chicken wire. Since it was isolated in 2004, graphene has been shown to have many remarkable properties in its monolayer form (Discover the “family” of strong and superconducting graphene structures).
In 2018, MIT researchers discovered (The Magic angle of superconducting graphene structures) that if two graphene layers are stacked at a very specific “magic” angle, the twisted bilayer structure can exhibit strong superconductivity, a widely studied state of matter in which electric current can flow without loss of energy. Recently, the same team discovered that a similar superconducting state exists in twisted three-layer graphene – a structure made up of three graphene layers stacked at a new, precise magic angle.
Now the team reports that – you guessed it – four and five graphene layers can be twisted and stacked at new magic angles to achieve strong low-temperature superconductivity. This latest discovery, published this week in the journal Nature Materials, establishes the twisted and stacked configurations of graphene as the first known “family” of angle-matter multilayer superconductors. art. The team also identified similarities and differences between members of the graphene family.
The results (The Magic angle of superconducting graphene structures) can be used as a model for the design of practical room-temperature superconductors. If the assets of family members can be regenerated in other naturally conductive materials, they could be exploited, for example, to provide electricity without dissipation or to build ships. Magnetic flight operates without friction.
“The magic-angle graphene system is now a legitimate ‘family’ that goes beyond just a few systems,” said lead author Jeong Min (Jane) Park, a PhD student in MIT’s Department of Physics. “Having this family is particularly important because it provides a way to design powerful superconductors.”
Park’s co-authors at MIT include Yuan Cao, Li-Qiao Xia, Shuwen Sun and Pablo Jarillo-Herrero, Cecil and Ida Green Professors of Physics, and Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science at the National Institutes of Materials Science. Tsukuba, Japan. . Jarillo-Herrero’s group was the first to discover magic angle graphene, as a bilayer structure of two sheets of graphene superimposed and slightly apart by an angle of exactly 1.1 degrees.
This twisted configuration, known as a moiré superlattice, transformed the material into a strong, stable superconductor at extremely low temperatures. The researchers also discovered that the material exhibits a type of electronic structure known as a “flat band,” in which the material’s electrons have the same energy regardless of their momentum. In this flat band state and at extremely cold temperatures, electrons that normally gather madly slow down enough to pair up in so-called Cooper pairs – the essential component of superconductivity that can flow through materials without any interference. have resistors.
While the researchers observed that the twisted bilayer graphene exhibits both superconductivity and a planar band structure, it remains unclear whether the former is derived from the latter. “There is no evidence that a flat band structure leads to superconductivity,” says Park. “Since then, other groups have produced other twisted structures from other materials with flat bands, but they are not really strong superconductors. So we asked ourselves: Can we manufacture another flat band superconductor?
While investigating this question, a team at Harvard University made calculations that mathematically confirmed that three layers of graphene, twisted by 1.6 degrees, would also exhibit flat bands and suggested that they could be is a superconductor. They then show that there is no limit to the number of graphene layers that exhibit superconductivity, if properly stacked and twisted, at the angles they also predicted. Finally, they demonstrated that they could mathematically relate each multilayer structure to the usual flat band structure – strong evidence that flat banding can lead to strong superconductivity.
“They found that there could be an entire hierarchy of graphene structures, infinitely layered, that could fit a similar mathematical expression for the planar band structure,” says Park. Not long after this work, Jarillo-Herrero’s team discovered that indeed superconducting and a flat band appeared in twisted three-layer graphene – three sheets of graphene, stacked like a cheese sandwich, layers the cheese in the center deviates 1.6 degrees from the sandwiched outer layers. But the three-layer structure also shows subtle differences from its two-layer structure. “It led us to ask, where do these two structures fit in terms of overall materials, and do they belong in the same family?” Park said. An atypical family
In the current study, the team sought to increase the number of graphene layers. They created two new structures, consisting of four and five layers of graphene. Each structure is interlaced, similar to a twisted graphene three-layer cheese sandwich. The team kept the structures in a refrigerator below 1 kelvin (about -273 degrees Celsius), passed a current through each structure, and measured the output under different conditions, similar to testing a two-layer system. and their three classes. Overall, they found that the four- and five-layer twisted graphene also exhibited strong superconductivity and flat banding.
The structures also share other similarities with their three-layer counterparts, such as their response under a magnetic field of varying magnitude, angle, and direction. These experiments show that the twisted graphene structure can be considered as a new family or class of common superconducting materials. Experiments also show that there may be a black sheep in the family: the original twisted bilayer structure, while sharing key characteristics, also shows slight differences from its siblings. For example, previous experiments by the group showed that the superconductivity of the broken structure under magnetic fields is weaker and more uneven as the field rotates, compared to its multi-layered siblings.
The team ran simulations of each type of structure, looking for an explanation for the differences between family members. They conclude that the fact that the superconductivity of double-helical graphene quenches under certain magnetic conditions is simply because all of its physical layers exist in an “unreflected” form in the structure. In other words, no two layers in the structure oppose each other, while graphene’s multilayer siblings exhibit a pattern of mirror symmetry. These results suggest that the mechanism for conduction of moving electrons in the strong superconducting state is the same throughout the whole family of twisted graphene.
“That’s pretty important, “Park noted. “Involuntarily, people might think that bilayer graphene is more common than a multilayer structure. But we show that this entire family can be powerful and unique superconductors.
