Wednesday, November 30, 2022

Physicists discover a ‘family’ of robust, superconducting graphene structures

Graphene is a thin, single-atom material that can be exfoliated from the same graphite found in a pencil lead. The ultra-thin material is made entirely of carbon atoms arranged in a simple, wire-like hexagonal pattern. Since its isolation in 2004, graphene has been found to embody numerous remarkable properties in its single-layer 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 robust superconductivity, a widely sought-after material state in which an electrical current can flow with loss of energy. zero energy. Recently, the same group discovered that a similar superconducting state exists in twisted three-layer graphene — a structure made of three graphene layers stacked at a new magically precise angle.

Now, the team reports that – you guessed it – four and five layers of graphene can be twisted and stacked at magical new angles to achieve robust superconductivity at low temperatures. This latest discovery, published in Materials from Nature (“Robust superconductivity in the magic-angle multilayer graphene family”) establishes 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 findings could serve as a blueprint for designing practical room-temperature superconductors. If properties among family members could be replicated in other naturally conductive materials, they could be harnessed, for example, to provide electricity without dissipation or to build magnetically levitating trains that run without friction.

“The magic-angle graphene system is now a legitimate ‘family’, in addition to a few systems,” says lead author Jeong Min (Jane) Park, a graduate student in the MIT Department of Physics. “Having this family is particularly significant because it provides a way to design robust superconductors.”

Park’s co-authors at MIT include Yuan Cao, Li-Qiao Xia, Shuwen Sun, and Pablo Jarillo-Herrero, physics professor Cecil, and Ida Green, along with Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Tsukuba, Japan. .


Jarillo-Herrero’s group was the first to discover magic-angle graphene, in the form of a bilayer structure of two sheets of graphene placed one on top of the other and slightly offset at a precise angle of 1.1 degrees. This twisted configuration, known as a moiré superlattice, turned the material into a strong, persistent superconductor at ultra-low temperatures.

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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 ultracold temperatures, the normally frenetic electrons collectively slow down enough to form pairs in what are known as Cooper pairs — essential ingredients of superconductivity that can flow through the material without resistance. Twisted Graphene MIT physicists established twisted graphene as a new “family” of robust superconductors, each member consisting of alternating layers of graphene, stacked at precise angles. (Image courtesy of the researchers)

While the researchers observed that twisted bilayer graphene exhibited superconductivity and a flat band structure, it was unclear whether the former arose from the latter.

“There was no evidence that a flat band structure led to superconductivity,” says Park. “Other groups have since produced other twisted structures from other materials that have some flat band, but they don’t really have robust superconductivity. So we asked ourselves: could we produce another flat-band superconducting device?”

In considering this question, a group at Harvard University deduced calculations that mathematically confirmed that three layers of graphene, twisted by 1.6 degrees, would also exhibit flat bands and suggested that they might superconduct. They went on to show that there should be no limit to the number of graphene layers that exhibit superconductivity, if stacked and twisted the right way, at angles they also predicted. Finally, they proved that they could mathematically relate each multilayer structure to a common flat band structure – strong proof that a flat band can lead to robust superconductivity.

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

Shortly after this work, Jarillo-Herrero’s group discovered that, in fact, superconductivity and a flat band emerged in graphene from three twisted layers – three sheets of graphene, stacked like a cheese sandwich, the middle layer of cheese displaced. by 1.6 degrees from the interleaved outer layers. But the three-layer structure also showed subtle differences compared to its two-layer counterpart.

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“That made us ask, where do these two structures fit in in terms of the whole class of materials and are they in the same family?” Park says.

An unconventional family

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

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

Overall, they found that four- and five-layer twisted graphene also exhibit robust superconductivity and a flat band. The structures also shared other similarities with their three-layer counterparts, such as their response under a magnetic field of varying strength, angle, and orientation.

These experiments showed that twisted graphene structures can be considered a new family or class of common superconducting materials. The experiments also suggested that there might be a black sheep in the family: the original twisted two-layer structure, while sharing important properties, also showed subtle differences from its siblings. For example, the group’s previous experiments showed that the structure’s superconductivity broke down under lower magnetic fields and became more uneven as the field rotated, compared to its multilayered brethren.

The team performed simulations of each type of structure, seeking an explanation for the differences between family members. They concluded that the fact that the superconductivity of twisted bilayer graphene disappears under certain magnetic conditions is simply because all of its physical layers exist in an “unmirrored” form within the structure. In other words, there are no two layers in the structure that are opposite mirrors of each other, while graphene’s multilayered sisters exhibit some sort of mirror symmetry. These findings suggest that the mechanism that causes electrons to flow in a robust superconducting state is the same throughout the twisted graphene family.

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

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