Physicists discover a “family” of robust, superconducting graphene structures

With graphene, superconductivity seems to run in the family.

Graphene is an atom thin material that can be stripped from the same graphite found in pencil leads. 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 in its monolayer form has been found to embody numerous remarkable properties.

In 2018, MIT researchers found that if two layers of graphene were stacked at a very specific “magic” angle, the twisted double-layer structure could exhibit robust superconductivity, a widely sought-after material state in which an electric current can flow without loss of energy. Recently, the same group found that a similar superconducting state exists in twisted trilayer graphene – a structure of three graphene layers stacked at a precise, new magic angle.

Now the team reports that, you guessed it, four and five layers of graphene can be twisted and stacked at new magic angles to induce robust superconductivity at low temperatures. This latest discovery, published this week in natural materials, established 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 could serve as a blueprint for the development of practical room-temperature superconductors. If the properties of family members could be reproduced in other, naturally conductive materials, they could be used, for example, to supply electricity without dissipation or to build magnetic levitation trains that run without friction.

“The magic angle graph system is now a legitimate ‘family’ that extends beyond a few systems,” says lead author Jeong Min (Jane) Park, a graduate student in MIT’s Department of Physics. “Having this family is particularly useful because it offers a way to design robust superconductors.”

‘No limit’

Jarillo-Herrero’s group was the first to discover magic angle graphene, in the form of a bilayer structure made up of two layers of graphene stacked on top of each other and slightly offset at a precise angle of 1.1 degrees. This twisted configuration, known as the Moiré superlattice, turned the material into a strong and durable superconductor at ultra-low temperatures.

The researchers also found that the material exhibited 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 ultra-cold temperatures, the normally frenetic electrons slow together enough to pair up into so-called Cooper pairs — essential components of superconductivity that can flow through the material without resistance.

While the researchers observed that twisted bilayer graphene exhibited both superconductivity and a flat band structure, it wasn’t clear whether the former evolved from the latter.

“There was no evidence that a flat band structure led to superconductivity,” says Park. “Other groups have since made other twisted structures from other materials that have some flat band, but they didn’t have really robust superconductivity.

Considering this question, a Harvard University group derived calculations that mathematically confirmed that three layers of graphene twisted 1.6 degrees would also exhibit flat ribbons, 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 when stacked and twisted just right, at angles that they also predicted. Finally, they proved that they could mathematically relate each multilayer structure to a common flat-band structure – strong evidence that a flat-band can lead to robust superconductivity.

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

Shortly after this work, Jarillo-Herrero’s group found that superconductivity and a flat ribbon did indeed emerge in twisted three-layer graphene — three graphene sheets stacked like a cheese sandwich, with the middle cheese layer rotated 1.6 degrees with respect to the sandwiched arranged outer layers was shifted. But the three-layer structure also showed subtle differences compared to its two-layer counterpart.

“That got us to wonder, where do these two structures fit in terms of the overall class of materials and are they from the same family?” Park says.

An unconventional family

In the current study, the team attempted to increase the number of graphene layers. They produced two new structures consisting of four and five layers of graphene, respectively. Each structure is alternately stacked, similar to the shifted cheese sandwich made of twisted three-layer graphene.

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

Overall, they found that both four- and five-layer twisted graphene also exhibit robust superconductivity and a flat ribbon. The structures also shared other similarities with their three-layer counterpart, such as B. their response to a magnetic field of different strength, angle and orientation.

These experiments showed that twisted graphene structures could be considered as a new family or class of common superconducting materials. The experiments also suggested there may be a black sheep in the family: the original twisted bilayer structure, while sharing key features, also showed subtle differences from its siblings. For example, previous experiments by the group showed that the structure’s superconductivity collapsed at lower magnetic fields and was more uneven as the field rotated than its multilayer siblings.

The team ran simulations for each type of structure, looking for an explanation for the differences between family members. They concluded that the fact that the superconductivity of twisted bilayer graphene dies off under certain magnetic conditions is simply because all of its physical layers exist in a “non-mirrored” form within the structure. In other words, there are no two layers in the structure that are mirror images of each other, while graphene’s multilayered siblings exhibit some sort of mirror symmetry. These results suggest that the mechanism that causes electrons to flow in a robust superconducting state is common in the twisted graphene family.

“That’s pretty important,” Park notes. “Without knowing this, people might think that two-layer graphene is more conventional compared to multi-layer structures. But we show that this entire family can be unconventional, robust superconductors.”

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