Superconductivity is a family trait when it comes to graphene.
Graphene, a single-atom-thin material, can be exfoliated using the same graphite as pencil lead. This ultrathin material is entirely made from carbon atoms and has a hexagonal arrangement, similar to chicken wire. Graphene, in its single-layer form, has been discovered to possess many remarkable properties since its isolation in 2004.
2018 MIT researchers discovered that if two graphene layer are stacked at a specific angle, the twisted bilayer structure can exhibit strong superconductivity. This is a sought-after material state in which an electric current can flow through without any energy loss. The same team recently discovered a similar superconductive condition in twisted trilayer graphene, a structure consisting of three graphene layers that are stacked at a specific, new angle.
The team now reports that four to five graphene layers can easily be twisted or stacked at new magic angles to produce strong superconductivity at low temperature. This latest discovery, published this week in Nature Materials,establishes the various twisted and stacked configurations of graphene as the first known "family" of multilayer magic-angle superconductors. The graphene family members were also examined by the team.
These findings could be used as a guideline for the design of practical superconductors at room temperature. The family members' properties could be reproduced in other naturally conductive materials to produce electricity or magnetically-levitating trains without friction.
Lead author Jeong Min Jane (Jane), a graduate student at MIT's Department of Physics, said that the magic-angle graphene systems is now a 'family'. This family is especially meaningful as it allows us to design superconductors that are strong.
'No limit'
Jarillo-Herrero was the first group to discover magic angle graphene. This bilayer structure consisted of two graphene sheets placed on top of each other at an exact angle of 1.1 degrees. This unusual configuration is known as a moire supralattice. It transformed the material into an extremely strong and persistent superconductor at low temperatures.
Researchers also discovered that the material had a flat band electronic structure. This means that all electrons in the material have the same energy regardless of their momentum. The flat band state and ultracold temperatures cause the electrons to slow down enough to form what is known as Cooper pairs. These are essential ingredients in superconductivity, which allows the material to flow freely without resistance.
Researchers observed that twisted bilayer graphene had superconductivity as well as a flat band structure. However, it was not clear if the former resulted from the latter.
Park states that there was no evidence that a flat-band structure leads to superconductivity. Park says that other groups have since produced other twisted structures made from other materials with a flattish band but didn't have superconductivity. We wondered: "Can we make another flat-band superconducting device?"
While they were considering this question, a Harvard University group devised calculations that proved mathematically that three graphene layers twisted at 1.6 degrees would also exhibit flat bands. They suggested that they might superconduct. The graphene layers can be superconductive if they are stacked in the correct way and at the angles predicted by them. They also proved that they could mathematically link every multilayer structure to one common flat band structure, which is strong evidence of superconductivity.
Park says that they discovered there could be an entire hierarchy of graphene structures to infinity layers. This might correspond to a similar mathematical expression to describe a flat band structure.
Jarillo-Herrero and his group discovered that superconductivity was indeed present in twisted trilayer graphene. This graphene sheet, which is stacked like a cheese sandwich with three graphene sheets, had shifted the middle layer by 1.6 degrees relative to the outer layers. The trilayer structure showed subtle differences to its bilayer counterpart.
"That led us to ask: where do these two structures fit within the whole class materials? Are they part of the same family?" Park agrees.
A family with an unusual name
The team sought to increase the number of graphene layer in the current study. Two new structures were constructed, each with four and five graphene layer, respectively. The stacking of each structure is similar to the shifted cheese sandwich made from twisted trilayer graphene.
The structures were kept in a refrigerator at -1 kelvin (around -273 degrees Celsius), and the team ran an electrical current through them. They then measured the output under different conditions similar to those used for testing their trilayer and bilayer systems.
They found that both the four- and five-layers of twisted graphene had strong superconductivity, and a flat band. They also had similar characteristics to their three-layer counterparts, including their responses under magnetic fields of different strength, angle and orientation.
These experiments revealed that twisted graphene structures can be considered a new class or family of superconducting materials. These experiments suggested that there might be a black sheep within the family. The original twisted bilayer structure showed subtle differences to its siblings, despite sharing many key properties. The group's previous experiments revealed that the structure's superconductivity was less stable under lower magnetic fields than its multilayer siblings. It also showed that the field rotation made it more uneven.
Each structure type was simulated by the team to find out why there were differences among family members. The team concluded that twisted bilayer graphene's magnetic superconductivity is lost under certain magnetic conditions. This is because the structure contains all its physical layers in an "unmirrored" state. The structure has no layers that are opposite to each other. However, graphene's multilayer siblings display some degree of mirror symmetry. These results suggest that electrons flow in a strong superconductive state across the entire twisted graphene family.
"That's quite important," Park notes. People might mistakenly believe that bilayer graphene is more common than multilayer graphene. We show that the entire family of superconductors may be unusual and robust.
