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Physicists have discovered a family of superconducting graphene structures

This discovery of superconducting graphene structures could encourage the development of practical superconducting devices

[Translation by Dr. Moshe Nachmani]

Illustration showing superconducting electron pairs within a multilayer graphene structure at the magic angle. Courtesy: [Ella Maru Studio]
Illustration showing superconducting electron pairs within a multilayer graphene structure at the magic angle. Courtesy: [Ella Maru Studio]

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

Graphene is a one-atom-thick two-dimensional material that can be produced by a chemical process (exfoliation) from the same raw material found in pencil tips - graphite. The extremely thin material consists entirely of only carbon atoms arranged in a simple hexagonal structure, similar to a chicken cage. Since the isolation of this special material in 2004, scientists have discovered that graphene has many unusual properties due to its single-layer structure.  

Graphene, a monolayer of carbon atoms arranged in a two-dimensional honeycomb lattice nanostructure, is one of the best-known two-dimensional materials. When you take two layers of graphene and move them towards each other at the "magic angle" (From Wikipedia) particularly special properties emerge such as superconductivity and ferromagnetism.

In 2018, scientists from the Massachusetts Institute of Technology (MIT) discovered that if you take two layers of graphene and tilt them towards each other at a very specific angle (the “magic angle”), this double layer can exhibit stable superconductivity. In such a situation, an electric current can move within this double layer with zero loss of energy. Recently, the same team of researchers found similar superconductivity in a shifted trilayer of graphene - a structure consisting of three layers of graphene stacked towards each other at a particularly defined angle.

Now, the research team reports that they believe that even four and five shifted layers of graphene, if shifted at specific angles, could exhibit the stable superconductivity property at low temperatures. This latest discovery, published on July 2022, XNUMX, in the scientific journal Nature Materials, establishes the existence of an entirely new superconducting material family of multilayer graphene. This new discovery could serve as a basis for the development of practical superconductors, stable at room temperature. If the properties of this family of materials could be duplicated into natural conductive materials, they could be harnessed, for example, to transmit electric current without resistance or energy loss, for example, to develop magnetically levitating train cars without any friction.

"The existence of this family is important because it provides a way for the development of stable superconductors," said the lead researcher. 

Jarillo-Herrero's research group was the first ever to discover the graphene with the magic angle, in the form of a two-layered structure of two graphene sheets placed one above the other at an angular offset of exactly 1.1 degrees. This shifted configuration, known as 'moiré superlattice', turns the material into a strong and durable superconductor at very low temperatures. The researchers also discovered that this material exhibits a type of electronic knowledge structure called a 'flat band', in which the material's electrons have the same energy, regardless of their angular momentum. In this flat-band electronic state, and at super-cold temperatures, the normal electrons slow down their movement cooperatively enough that they pair together to form Cooper pairs - essential components of superconductivity that are able to move through matter without any resistance. Although the researchers observed that shifted graphene does indeed exhibit both superconductivity and a flat band structure, they were not clear as to which caused the formation of the other.

"There was no proof that the flat band is the cause of superconductivity," says the lead researcher. "Other research groups have since been able to produce diverted structures from other materials with flat stripes, but they did not have stable superconductivity. And so we pondered this: can we produce another device with flat band superconductivity”?

While pondering this question, another research group from Harvard University came up with calculations that proved mathematically that three layers of graphene, tilted at an angle of 1.6 degrees, would also exhibit flat stripes, suggesting that these structures might be superconducting. They went on to show that there is no limit to the number of graphene layers that may exhibit superconductivity, if they are stacked and deflected towards each other in the appropriate structure, at angles that the researchers also evaluated. In the end, they demonstrated that they could mathematically match any multilayer structure to a common flat-band structure—solid proof that a flat-band might lead to stable superconductivity.

Shortly after this study, the Jarillo-Herrero research team found that, indeed, superconductivity and a flat band are formed in the state of Shifted trilayer graphene - Three graphene sheets placed in the configuration of a cheese sandwich, where the middle layer is shifted at an angle of 1.6 degrees relative to the two layers above and below. In addition, the three-layer structure also exhibited slight differences compared to its two-layer counterpart. "This finding led us to the question - how are these two structures related to the more general group of materials and are they indeed part of the same family of materials?", said the lead researcher.

As part of the current research, the team planned to increase the number of graphene layers. They created two completely new structures, consisting of four and five layers of graphene, respectively. Each structure consists of several offset layers, similar to the familiar two- and three-layer structures. The researchers kept the structures in a freezer at a temperature of minus 273 degrees Celsius, passed an electric current through each of the structures and measured the results under varying conditions, similar to the tests conducted with the familiar two- and three-layer structures.    

In the end, the researchers found that even the new graphene structures, with the four- and five-layer structure, exhibit stable superconductivity as well as a flat band. The structures also shared other similar properties with their two- and three-layer counterparts, such as their response to magnetic fields of varying strengths, angles, and orientations. 

These experiments showed that shifted graphene structures can be considered as a new family of materials, or a group of common superconducting materials. The experiments also suggest that there may be a 'black sheep' in the family: the original shifted bilayer structure, which still shares several key features, also exhibits subtle differences from its family counterparts. For example, the research group's previous experiments showed that the structure's superconductivity disappears when weak magnetic fields are applied to it and it is more uneven as the field rotates around it, compared to its multi-layer counterparts.  

The researchers conducted computer simulations for each of the types of buildings, while looking for the explanation for the differences between the group members. They concluded that the fact that the superconductivity of the shifted bilayer graphene fades under certain magnetic conditions is simply because all the physical layers exist in a way that is "not mirror images of each other" within the structure. In other words, there are no two layers within the structure that are mirror images of each other, while the other graphene structures do include a certain type of mirror symmetry. These findings suggest that the mechanism that drives the electrons to flow with a stable conductivity is common to the entire family of shifted graphenes. "This finding is quite important," explains the lead researcher. "If we didn't discover this, people would think that bilayer graphene is more conventional compared to multilayer structures. However, we have shown that both belong to the same material family of stable superconductors."

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