Physicists discover strong "family" of superconducting graphene structures

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

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

In 2018, MIT researchers discovered that if two layers of graphene are stacked at very specific “magic” angles, the bent bilayer structure can exhibit strong superconductivity, a much sought-after material state in which electric current can flow without loss of energy. Recently, the same group discovered a similar superconductive state that exists in bent trilayer graphene – a structure made of three layers of graphene stacked at new, precise magic angles.

Now the team reports that — you guessed it — four and five layers of graphene can be twisted and stacked at new magic angles to produce strong superconductivity at low temperatures. This latest discovery, published this week in Nature Materials, establishes various configurations of bent and stacked graphene as the first known “family” of multilayer magic-angle superconductors. The team also identified similarities and differences between members of the graphene family.

This finding could serve as a blueprint for designing practical room temperature superconductors. If the properties among family members could be replicated in other naturally conductive materials, they could be exploited, for example, to conduct electricity without dissipation or to build magnetically levitating trains that run without friction.

“The magic angle graphene system is now a legitimate ‘family’, outside of some systems,” said lead author Jeong Min (Jane) Park, a graduate student in MIT’s Department of Physics. “Having this family means a lot 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, Professors of Physics Cecil and Ida Green, along with Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Tsukuba, Japan.

“No limit”

The Jarillo-Herrero group was the first to discover magic-angle graphene, in the form of a bilayer structure of two graphene sheets placed one above the other and slightly offset at a precision angle of 1.1 degrees. This twisted configuration, known as superlattice moiré, turns the material into a strong and persistent superconductor at very low temperatures.

The researchers also found that matter exhibits a type of electronic structure known as a “flat band”, in which the electrons of the matter have the same energy, regardless of their momentum. In this flat band state, and at very cold temperatures, the normally frenetic electrons collectively slow down enough to pair up in what’s known as Cooper pairing – an essential ingredient of superconductivity that can flow through the material unimpeded.

While the researchers observed that the bent bilayer graphene exhibits superconductivity and a flat band structure, it is unclear whether the former emerged from the latter.

“There is no evidence that the flat band structure causes superconductivity,” said Park. “Other groups have since produced other bent structures from other materials that have flat bands, but they don’t really have strong superconductivity. So we wondered: Can we produce another flat-band superconducting device?”

As they considered this question, a group from Harvard University obtained calculations that confirmed mathematically that the three layers of graphene, rotated at 1.6 degrees, would also exhibit flat bands, and suggested that they might be superconductors. They went on to show that there should be no limit to the number of graphene layers exhibiting superconductivity, if stacked and twisted the right way, at the angles they also predicted. Finally, they proved that they could mathematically relate any multilayer structure to a common flat band structure – solid evidence that flat bands can cause strong superconductivity.

“They worked out there maybe this whole hierarchy of graphene structures, down to an infinite layer, which might fit a similar mathematical expression for flat band structures,” Park said.

Shortly after that work, the Jarillo-Herrero group discovered that, indeed, superconductivity and flat bands appear in bent trilayer graphene — three graphene sheets, stacked like a cheese sandwich, the middle cheese layer shifted 1.6 degrees with respect to the sandwiched outer layer. But the trilayer structure also shows subtle differences compared to its bilayer counterpart.

“That led us to ask, where do these two structures fit in in terms of all material classes, and are they from the same family?” said Park.

Unusual family

In the current study, the team looked to increase the number of layers of graphene. They created two new structures, made of four and five layers of graphene, respectively. Each structure is stacked in turn, similar to a cheese sandwich shifted from bent trilayer graphene.

The team stored the structures in a refrigerator below 1 kelvin (about -273 degrees Celsius), passed an electric current through each structure, and measured the output under various conditions, similar to tests for their bilayer and trilayer systems.

Overall, they found that the four- and five-layer bent graphene also exhibited strong superconductivity and flat bands. The structure also has other similarities with its three-layered counterpart, such as its response under magnetic fields of different strengths, angles, and orientations.

These experiments show that the bent graphene structure can be considered a new family, or class of general superconducting materials. The experiments also suggest there may be a scapegoat in the family: The original twisted bilayer structure, while sharing a key property, also shows subtle differences from its siblings. For example, the group’s previous experiments showed the superconductivity of the structure breaks down under a lower magnetic field and is more uneven when the field is rotated, compared to its multilayer sibling.

The team simulated each type of structure, seeking explanations for differences between family members. They conclude that the fact that superconductivity of twisted bilayer graphene dies under certain magnetic conditions is simply because all of its physical layers exist in a “non-mirror” form within the structure. In other words, no two layers in the structure are mirror opposites of each other, whereas the graphene’s multilayer sibling exhibits some kind of mirror symmetry. This finding suggests that the mechanism that drives electrons to flow in a strongly superconductive state is the same across the bent graphene family.

“That’s pretty important,” Park notes. “Without knowing this, one might think that bilayer graphene is more conventional compared to multilayer structures. But we show that this entire family may be unconventional and powerful superconductors.”

This research was supported, in part, by the US Department of Energy, 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.

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