Physicists discover ‘family’ of strong, superconducting graphene structures

Physicists at the Massachusetts Institute of Technology have created twisted graphene as a new “family” of strong superconductors, each member consisting of alternating layers of graphene, stacked at precise angles. Credit: Courtesy of the researchers

When it comes to graphene, superconductivity seems to run in the family.

Graphene is a single-atom material that can be exfoliated from the same graphite found in pencil lead. The ultra-thin material is made entirely of carbon atoms arranged in a simple hexagonal pattern, similar to that of chicken wire. Since its isolation in 2004, graphene has been found to embody many remarkable properties in its monolayered form.

In 2018, MIT researchers found that if two layers of graphene are stacked at a very specific “magic” angle, the twisted bilayer structure can exhibit strong superconductivity, a widely desired physical state through which an electric current can flow without loss. energy. Recently, the same group found a similar superconducting state found in twisted three-layer graphene – a structure made of three layers of graphene stacked at a new, micro-magical angle.

The team has now reported that—you guessed it—four and five layers of graphene can be rolled and stacked at magical new angles to obtain strong superconductivity at lower temperatures. This latest discovery, published this week in nature materials, identifies the various twisted and stacked configurations of graphene as the first known ‘family’ of multilayer magic angle superconductors. The team also identified similarities and differences between members of the graphene family.

The results can serve as a blueprint for designing practical room-temperature superconductors. If the properties can be replicated among family members in other naturally conductive materials, they can be harnessed, for example, to conduct electricity without dissipation or build magnetic trains that operate without friction.

“The magic-angle graphene system is now a legitimate ‘family’, transcending two systems,” says lead author Jeong Min (Jin) Park, a graduate student in the MIT Department of Physics. “Having this family makes sense because it provides a way to design super-strong conductors.”

‘no limits’

The Jarillo-Herrero group was the first to discover magic-angle graphene, in the form of a two-layered structure of two sheets of graphene, placed one on top of the other and shifted slightly at a precise angle of 1.1 degrees. This twisted configuration, known as a superconductor, turns the material into a strong, continuous superconductor at extremely low temperatures.

The researchers also found that the material exhibited a type of electronic structure known as a “flat ribbon,” in which the material’s electrons have the same energy, regardless of their momentum. In this flat-band state, and at extremely cold temperatures, the superheated electrons usually collectively slow down enough to mate in what are known as Cooper pairs—key components of superconductivity that can flow through the material without resistance.

While the researchers noted that the twisted bilayer graphene exhibited both superconductivity and a flat band structure, it was not clear whether the former originated from the latter.

“There was no evidence that the flat band structure led to superconductivity,” Park says. “Other groups have since produced other twisted structures of other materials that have some flat band, but they don’t really have strong superconductivity. So we wondered: Can we produce another flat band superconducting device?”

When they looked at this question, a group from Harvard University deduced calculations that mathematically confirmed that three layers of graphene, twisted at 1.6 degrees, would also show flat bands, and suggested that they might be superconducting. They went on to show that there should be no limit to the number of graphene layers that exhibit superconductivity, if they are stacked and warped just the right way, at the angles they predicted, either. Finally, they demonstrated that they could mathematically relate each multilayer structure to the common flat band structure – strong evidence that flat banding may lead to strong superconductivity.

“They found that there might be this whole hierarchy of graphene structures, into infinite layers, which might correspond to a mathematical expression similar to a flat band structure,” says Park.

Soon after this work, Jarillo-Herrero’s group found that superconductivity and a flat band indeed appeared in twisted three-layer graphene—three sheets of graphene, stacked like a cheese sandwich, and the middle cheese layer shifted by 1.6° with respect to the outer confined layers. But the three-layer structure also showed subtle differences compared to its two-layer counterpart.

“This made us wonder, where do these two structures fit in in terms of the entire material class, and do they belong to the same family?” Park says.

unconventional family

In the current study, the team sought to scale up the number of graphene layers. They made two new structures, made of four and five layers of graphene, respectively. Each structure is stacked alternately, similar to a shifting cheeseburger of twisted three-layer graphene.

The team kept the structures in a refrigerator below 1 K (about -273 degrees Celsius), conducted an electrical current through each structure, and measured the output under different conditions, similar to tests for their two- and three-layer systems.

Overall, they found that both four- and five-layer twisted graphene also exhibit strong superconductivity and a flat band. The structures also share other similarities with their three-layered counterpart, such as their response under a magnetic field of varying strength, angle, and direction.

These experiments showed that twisted graphene structures can be considered as a new family, or class of common superconducting materials. Experiments also suggested that there might be black sheep in the family: the original bilayer structure, while sharing key characteristics, also showed slight differences from its siblings. For example, the group’s previous experiments showed that the structure’s superconductivity collapsed under lower magnetic fields and was more uneven with field rotation, compared to its multilayered siblings.

The team ran simulations of each type of structure, looking for an explanation for the differences between family members. They concluded that the fact that twisted bilayer graphene’s superconductivity vanishes under certain magnetic conditions is simply because all of its physical layers are present in an “inverted” form within the structure. In other words, no two layers in the structure are opposite to each other, while the siblings of multilayered graphene show some kind of mirror symmetry. These results indicate that the mechanism that drives electrons to flow in a strong superconducting state is the same across the twisted graphene family.

“This is very important,” Park notes. “Without knowing it, people might think bilayer graphene is more conventional compared to multilayer structures. But we’re showing that this whole family can be unconventional and powerful superconductors.”

Unusual superconductivity is observed in twisted three-layer graphene

more information:
Jeong Min Park et al, Strong superconductivity in the magic angle multilayer graphene family, nature materials (2022). DOI: 10.1038 / s41563-022-01287-1

Provided by the Massachusetts Institute of Technology

the quote: Physicists Discover ‘Family’ of Strong Superconducting Graphene Structures (2022, July 8) Retrieved July 9, 2022 from .html

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