Recently, physicists discovered superconductivity in graphene. (Image Credit: CoolVid-Shows/Pixabay)


In 2018, physicists made a unique discovery: stacking two carbon sheets with a small, 1.1-degree twist between them causes the material to behave like a superconductor. Researchers set out to determine why this was happening with a form of robust superconductivity. Theorists hoped the discovery would redefine how they understood superconductivity and allow the development of materials with high-temperature sustainability. Now, physicists from the University of California, Santa Barbara, announced they discovered superconductivity in a triple-decker stack of graphene with no twists.


The team’s ABC trilayer graphene stack, the cleanest and simplest produced, involves shifting the 2nd and 3rd layers instead of twisting them. This is achieved with a half-honeycomb, allowing the carbon atoms below to drop just above the center of the lattice.


It’s difficult to stack graphene, regardless of whether or not they’re twisted. Twisted layers contain several wrinkles that disrupt the magic angle in various zones, making each apparatus unique. While being manufactured, most ABC trilayer devices snapped into a different stacking pattern. However, the ones that didn’t change were very identical.

After their first ABC device was developed, the team utilized a tunable electric field to shuffle electrons between each layer. Once adjusted at cryogenic temperatures, they observed the device behaving similarly to twisted graphene. It jumped between various magnetic behavior types, which was shown based on how the system slowed down the current. After detailed observations of the shifts, the team identified superconductivity when it was approximately one-tenth of a degree above absolute zero. 


While they can’t directly see the electrons’ Cooper pairs, they discovered unique behavior. Electrons traveling between the three layers boosted the configurations for the electrons, also called the systems’ density of states. At higher densities of states, electrons can easily mingle among themselves. According to BCS theory, this likely helps Cooper pairs form. When the density states increased, the material exhibited two flickers of superconductivity. Now that the BCS equation seems correct, regular phonons could be causing superconductivity.


Not everyone is convinced. Some say that the evidence supporting phonons in ABC trilayer graphene isn’t conclusive. Superconductivity tracks with higher density states. However, that doesn’t mean the BCS equation works out. There is only a mild exotic type of superconductivity in the team’s data. Both superconductivity flickers showed up just before the electrons organized into ferromagnetic states. Once they started aligning, these unstable uniformity pockets could have shaped electrons into Cooper pairs in the same way as phonons.


Now, the team is conducting tests to determine whether ferromagnetism causes superconductivity. Otherwise, ordinary phonons could be the cause.


Physicists remain hopeful that this ABS trilayer could help them determine how electrons superconduct in graphene. The idiosyncrasies of each twisted graphene system made it impossible for a lab to imitate those results. ABC trilayer graphene helps to overcome such an obstacle.


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