Physics' newest twisted graphene family

Just when you would think graphene could not possibly be more amazing than it already is, researchers at the Massachusetts Institute of Technology prove that to be furthest from the truth, discovering that if two or more graphene layers are stacked at an offset angle, the conductive properties of graphene become superconductive.

Initially discovered in 2018, the MIT researchers recently published a new finding in Nature Materials that if you just keep stacking graphene with a slight offset, kind of like stacking alternating pieces of paper so that they form a six-pointed star, it configures into the first known "family" of multilayer magic-angle superconductors.

Why is this so significant? A widely sought material state, superconductors allow an electrical current to flow through with zero energy loss. This stacked graphene discovery could provide scientists with a blueprint for designing practical room-temperature superconductors, which would be a major advantage over the required supercooling required to keep all functioning superconductors today, superconductive.

"The magic-angle graphene system is now a legitimate 'family,' beyond a couple of systems," said lead author Jeong "Jane" Min Park, a graduate student in MIT's department of physics. "Having this family is particularly meaningful because it provides a way to design robust superconductors."

Infinite possibilities

By taking two sheets of graphene, placing them atop one another, and slightly offsetting them at precisely 1.1 degrees, this twisted configuration, known as a moiré superlattice, transformed the material into a strong and persistent superconductor at ultralow temperatures.

The researchers also found that the material exhibited a type of electronic structure known as a "flat band," in which the material's electrons have the same energy, regardless of their momentum.

In this flat band state, and at ultracold temperatures, the normally frenetic electrons collectively slow down enough to pair up in what is known as Cooper pairs, essential ingredients of superconductivity that can flow through the material without resistance – think of the electrons as a couple skating on an ice rink.

While the researchers observed that twisted bilayer graphene exhibited both superconductivity and a flat band structure, it was unclear whether the former arose from the latter.

"There was no proof a flat band structure led to superconductivity," said Park. "Other groups since then have produced other twisted structures from other materials that have some flattish band, but they didn't really have robust superconductivity. So we wondered: Could we produce another flat band superconducting device?"

As they considered this question, a group from Harvard University derived calculations that confirmed mathematically that three graphene layers, twisted at 1.6 degrees, would exhibit flat bands as well, and suggested they may superconduct.

Further experimentation by the Harvard team demonstrated that there should be no feasible limit to the number of graphene layers that exhibit superconductivity if stacked and twisted in the right way – angles they also predicted.

The scientists also proved they could mathematically relate every multilayer structure to a common flat band structure – essentially providing strong proof that a flat band may lead to a robust superconductor.

"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," continued Park.

The MIT group found that, indeed, superconductivity and a flat band emerged in twisted trilayer graphene. However, the trilayer structure showed subtle differences compared to its bilayer relative.

"That made us ask, where do these two structures fit in terms of the whole class of materials, and are they from the same family?" added the lead author.

What makes a family?

What is meant by a family of superconductive graphene layers? Are they not all the same but just have more layers stacked on top of one another?

That is where things become even more interesting.

In the study, the MIT scientists looked to increase the number of graphene layers and produced two new structures, made from four and five layers-each structure stacked alternatingly.

Keeping the structures in a super-fridge with the temperature at one kelvin (minus 273 degrees Celsius or minus 459 degrees Fahrenheit), they then ran an electrical current through each structure and measured the output under various conditions, similar to the bi- and tri-layer systems.

What makes a family? They are still related but ultimately different individuals. Hence, the researchers found that both the four- and five-later twisted graphene exhibited robust superconductivity and a flat band, as well as other similarities with their three-layer sibling, such as their response under a magnetic field of varying strength, angle, and orientation.

Ultimately, these experiments showed that twisted graphene structures could be considered a new class of common superconducting materials. By adding new layers, each new addition would show slightly differing results – but like any family, there is a black sheep – the original twisted bilayer structure.

While it shares key properties, it also showed subtle differences from its brethren. For example, 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.

With more simulations on each structure type, the team wanted an explanation for the differences between this new family. Their conclusion: that the twisted bilayer dies out under certain magnetic conditions simply because all its physical layers exist in a "nonmirrored" form within the structure.

In layman, there were no two parts in the structure that had a counterpart, leading to destabilization. In contrast, graphene's multilayer siblings exhibit one or more counterparts to provide a balance of sorts.

These findings suggest that the mechanism driving electrons to flow in a robust superconductive state is the same across the twisted graphene family.

"That's quite important," Park pointed out. "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."

What does this all mean in a practical sense? By understanding the design of atoms in particular structures, we may be one step closer to fabricating natural superconductors simply by knowing where all the puzzle pieces go instead of blindly adding Legos or Lincoln Logs to the mix.