Physicists Discover a 'Family' of Robust, Superconducting Graphene Structures

In 2018, MIT researchers found that if two graphene layers are stacked at a very specific 'magic' angle, the twisted bilayer structure could exhibit robust superconductivity, a widely sought material state in which an electrical current can flow through with zero energy loss. Now the team reports that four and five graphene layers can be twisted and stacked at new magic angles to elicit robust superconductivity at low temperatures. Phys.Org reports: 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 team also identified similarities and differences between graphene family members. The findings could serve as a blueprint for designing practical, room-temperature superconductors. If the properties among family members could be replicated in other, naturally conductive materials, they could be harnessed, for instance, to deliver electricity without dissipation or build magnetically levitating trains that run without friction.

In the current study, the team looked to level up the number of graphene layers. They fabricated two new structures, made from four and five graphene layers, respectively. Each structure is stacked alternately, similar to the shifted cheese sandwich of twisted trilayer graphene. The team kept the structures in a refrigerator below 1 kelvin (about -273 degrees Celsius), ran electrical current through each structure, and measured the output under various conditions, similar to tests for their bilayer and trilayer systems. Overall, they found that both four- and five-layer twisted graphene also exhibit robust superconductivity and a flat band. The structures also shared other similarities with their three-layer counterpart, such as their response under a magnetic field of varying strength, angle, and orientation.

These experiments showed that twisted graphene structures could be considered a new family, or class of common superconducting materials. The experiments also suggested there may be 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 field rotated, compared to its multilayer siblings. The team carried out simulations of each structure type, seeking an explanation for the differences between family members. They concluded that the fact that twisted bilayer graphene's superconductivity dies out under certain magnetic conditions is simply because all of its physical layers exist in a 'nonmirrored' form within the structure. In other words, there are no two layers in the structure that are mirror opposites of each other, whereas graphene's multilayer siblings exhibit some sort of mirror symmetry. These findings suggest that the mechanism driving electrons to flow in a robust superconductive state is the same across the twisted graphene family.

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