When it comes to graphene, it turns out that superconductivity runs in the family.
Graphene is a one atom thick material that can be separated from the same graphite as pencil lead. The ultra-thin material is made entirely of carbon atoms arranged in a simple hexagon, similar to wire mesh. Since its isolation in 2004, graphene has been found to have numerous remarkable properties in its single-layer form.
In 2018, MIT researchers found that when two layers of graphene are stacked on top of each other at a very specific “magic” angle, the twisted two-layer structure can exhibit robust superconductivity—the widely sought-after state of a material in which an electric current can flow with zero energy loss. Recently, the same group discovered that a similar superconducting state exists in twisted three-layer graphene, a structure made up of three layers of graphene stacked on top of each other at exactly the new magic angle.
Now the team reports that, you guessed it, four and five layers of graphene can be twisted and folded at magical new angles to achieve reliable superconductivity at low temperatures. This is the latest discovery published this week in natural materials, establishes 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 obtained can serve as a basis for the development of practical superconductors at room temperature. If the properties of family members could be reproduced in other naturally conductive materials, they could be used, for example, to supply electricity without leakage or to create maglev trains that move without friction.
“The magic-angle graphene system is now a legitimate ‘family’ beyond a couple of systems,” says lead author Jung Min (Jane) Park, a graduate student in the MIT Department of Physics. “Having this family is of particular importance because it allows the development of reliable superconductors.”
The Jarillo-Herrero group was the first to discover graphene at the magic angle, in the form of a two-layer structure of two sheets 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 has a type of electronic structure known as a “flat stripe” in which the material’s electrons have the same energy, regardless of their momentum. In this flat-band state and at ultra-low temperatures, the normally frenzied electrons are collectively slowed down enough to form what are known as Cooper pairs—essential components of superconductivity that can flow through a material without resistance.
Although the researchers observed that twisted bilayer graphene exhibited both superconductivity and a planar band structure, it was not clear whether the former originated from the latter.
“There was no evidence that a flat band structure leads to superconductivity,” says Park. “Since then, other groups have produced other twisted structures from other materials that have some kind of flat band, but in fact they did not have reliable superconductivity. So we asked ourselves: can we create another superconducting device with a flat strip?”
In considering this issue, a group at Harvard University performed calculations that mathematically confirmed that three layers of graphene twisted by 1.6 degrees would also have flat bands, and suggested that they could be superconducting. They went on to show that there should be no limit to the number of layers of superconducting graphene if they were folded and twisted in the right way, at the angles they also predicted. Finally, they proved that they could mathematically relate any multilayer structure to the overall flat band structure—strong evidence that a flat band could lead to reliable superconductivity.
“They figured out that there could be this whole hierarchy of graphene structures, up to infinite layers, which could correspond to a similar mathematical expression for a flat ribbon structure,” says Park.
Shortly after this work, the Jarillo-Herrero group discovered that superconductivity and a flat strip actually appeared in twisted three-layer graphene – three sheets of graphene stacked on top of each other like a cheese sandwich, the middle layer of cheese is offset by 1.6 degrees with respect to those located between them to the outer layers. . But the three-layer structure also showed subtle differences compared to its two-layer counterpart.
“This led us to ask where do these two structures fit in the whole class of materials, and do they belong to the same family?” Park says.
In the current study, the team aimed to increase the number of graphene layers. They made two new structures, consisting of four and five layers of graphene, respectively. Each structure is stacked in turn, like a cheese sandwich made from twisted three-layer graphene.
The team kept the structures in a refrigerator below 1 Kelvin (about -273 degrees Celsius), ran an electrical current through each structure, and measured the power output under various conditions, similar to the tests for their two-layer and three-layer systems.
Overall, they found that both four-layer and five-layer twisted graphene also exhibit stable superconductivity and a flat band. The structures also shared other similarities with their three-layer counterparts, such as their response to a magnetic field of varying strength, angle, and orientation.
These experiments have shown that twisted graphene structures can be considered as a new family or class of conventional superconducting materials. Experiments also showed that there could be a black sheep in the family: the original twisted two-layer structure, while sharing key properties, also showed subtle differences from its siblings. For example, the group’s previous experiments have shown that the structure’s superconductivity breaks down at lower magnetic fields and becomes more uneven as the field rotates compared to its multilayer counterparts.
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 two-layer graphene disappears under certain magnetic conditions is simply due to the fact that all of its physical layers exist in a “non-mirror” form within the structure. In other words, there are no two layers in the structure that are mirror-opposite to each other, while graphene’s multi-layer siblings exhibit a kind of mirror symmetry. These results show that the mechanism that brings electrons to a stable superconducting state is the same across the family of twisted graphenes.
“It’s very important,” says Park. “Without knowing this, people might think that two-layer graphene is more conventional compared to multilayer structures. But we show that this entire family can be unconventional, reliable superconductors.”
Unusual superconductivity discovered in twisted three-layer graphene
Chong Min Park et al., Robust superconductivity in a magic-angle family of multilayer graphenes, natural materials (2022). DOI: 10.1038/s41563-022-01287-1
Provided by the Massachusetts Institute of Technology
Quote: Physicists discover ‘family’ of robust superconducting graphene structures (July 8, 2022) retrieved July 9, 2022 from https://phys.org/news/2022-07-physicists-family-robust-superconducting-graphene.html .
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