In 2018, MIT scientists found that the twisted bilayer structure could exhibit robust superconductivity by stacking two graphene layers at a particular “magic” angle. A similar superconductive state was also found in twisted trilayer graphene.
The same group has discovered that by stacking four and five graphene layers at a specific “magic” angle, the twisted and stacked exhibit robust superconductivity at low temperatures. This discovery demonstrates several configurations of graphene as the first known “family” of multilayer magic-angle superconductors. The team also identified similarities and differences between graphene family members.
Lead author Jeong Min (Jane) Park, a graduate student in MIT’s Department of Physics, said, “The magic-angle graphene system is now a legitimate ‘family,’ beyond a couple of systems. This family is significant because it provides a way to design robust superconductors.”
The researchers found the superconductivity and flat band structure of twisted bilayer graphene, but it was unclear whether the former resulted from the latter.
Lead author Jeong Min (Jane) Park, a graduate student in MIT’s Department of Physics, said, “There was no proof a flat band structure led to superconductivity. Other groups since then have produced other twisted structures from other materials with some flattish band, but they didn’t have robust superconductivity. So we wondered: Could we produce another flat band superconducting device?”
A group from Harvard University considered the questions and derived the calculations mathematically. They found three graphene layers may superconduct when twisted at 1.6 degrees. They also showed that there should be no limit to the number of graphene layers that exhibit superconductivity if stacked and twisted in just the right way at the predicted angles. Finally, they proved they could mathematically relate every multilayer structure to a common flat band structure, proving that a flat band may lead to robust superconductivity.
Park said, “They worked out there may be this entire hierarchy of graphene structures, to infinite layers, that might correspond to a similar mathematical expression for a flat band structure.”
In this new study, the team looked to level up the number of graphene layers. Two novel structures were created by them using four and five graphene layers, respectively. Like the shifting cheese sandwich of twisted trilayer graphene, each structure is stacked alternately.
The team kept the structures in a refrigerator below 1 kelvin (about -273 degrees Celsius), an electrical current through each structure, and measured the output under various conditions, similar to tests for their bilayer and trilayer systems.
They discovered that twisted graphene with four and five layers also had robust superconductivity and a flat band. The responses of the structures to a magnetic field with varied strength, angle, and orientation likewise matched those of their three-layer equivalent.
These experiments showed that twisted graphene structures could be considered a new family or class of common superconducting materials. The experiments also suggested a black sheep in the family: The original twisted bilayer structure, while sharing key properties, also showed subtle differences from its siblings. For instance, the group’s previous experiments showed the structure’s superconductivity broke down under lower magnetic fields and was more uneven as the area rotated compared to its multilayer siblings.
When the team conducted simulations of each structure type, they concluded that twisted bilayer graphene’s superconductivity dies out under certain magnetic conditions because all of its physical layers exist in a “nonmirrored” form within the structure.
In contrast to its multilayer siblings, which have some degree of mirror symmetry, graphene does not contain any layers that are mirror opposites of one another. These results imply that the twisted graphene family shares a common mechanism that causes electrons to flow in a robust superconductive state.
Park noted, “That’s quite important. Without knowing this, people might think bilayer graphene is more conventional compared to multilayer structures. But we show that this entire family may be unconventional, robust superconductors.”