Graphene is a highly unconventional substance. After all, how many other Nobel-Prize-winning breakthroughs are made by scientists messing around on a Friday night with some sticky tape? Since then, graphene—and a whole family of additional 2D materials such as germanene and silicene—have been a source of fascination and excitement due to their fascinating properties.
Despite all of the speculation, perhaps the most unusual and exciting property of graphene, first isolated in 2004, took another 14 years to discover. Increasingly, scientists realized that stacking together layers of 2D materials in constructs known as Van der Waals heterostructures could allow for a greater degree of control over their optical and electronic properties. It was this kind of stacking that led to a bizarre and exciting discovery.
Take two layers of graphene. Twist them so that they are at a very slight angle to each other—1.1 degrees, to be precise—and then stack them together. (This is easier said than done, as the idea was first proposed in 2007 but only realized in 2018.) The resulting bilayer graphene is a superconductor: when the temperature is dropped below a critical threshold, the material has no electrical resistance at all.
Layers a single atom thick are naturally lightweight, yet also surprisingly tough and flexible. This led to initial speculation for graphene being used as the ideal building material or protective body armor of the future. Yet it is the electrical properties of graphene, which arise from the unique behavior of electrons in such a thin layer, that have led to the first use cases for graphene in sensors and LEDs. Superconductivity, on top of everything else, is the icing on the cake for this remarkable material.
A Physicist’s Playground
Of course, twisted bilayer graphene (TBG) is not the first substance to exhibit superconducting properties. Superconductors, which can, amongst other things, generate extremely high magnetic fields without losing energy to electrical resistance, are already widely in use. Striking examples include the magnets at ITER, the world’s largest fusion device, currently under construction.
Yet there are fascinating unsolved questions in superconductivity, even as it’s discovered in more and more different materials and under different conditions. The discovery of unconventional, high-temperature superconductors led to speculation that a room-temperature superconductor may one day be discovered. But with no closed physical theory that explains how unconventional superconductors work, it’s extremely difficult to predict which materials might exhibit this behavior, as well as to search for better materials which can generate larger magnetic fields, operate at higher temperatures, or are easier to build with. If discovered, they could enable smaller fusion reactors or even lossless power transmission, as well as frictionless, high-speed transportation, helping to solve the energy and climate crisis. But knowing where to look has proved difficult.
This is where twisted bilayer graphene is most exciting for physicists: it provides a unique way of testing theories of superconductivity. We know that superconductivity is caused by materials with highly correlated electrons: at these low temperatures, with less noise due to random particle motions, the electrons can exert a strong influence on each other and it’s this strong interaction that can lead to superconductivity.
Unlike unconventional superconductors like YCBO (Yttrium Barium Copper Oxide, chemical formula YBa2Cu3O7), twisted bilayer graphene has a relatively simple structure. Even producing a crystal of YCBO that will exhibit superconductivity is difficult enough, and the crystals remain poorly understood, with all of the mathematical models that attempt to explain them impossible to solve exactly. To get a YCBO crystal to become superconducting, it’s necessary to “dope” it with impurities that add free electrons to the system. But these same free electrons can be easily added to graphene.
Twisted Logic
The reasoning behind graphene’s “magic angle” arises due to the energy barrier for quantum tunnelling between the two layers of graphene. As you approach the precise angle of rotation (1.1 degrees), the energy barrier becomes very small, allowing electrons to strongly interact and become correlated between the layers. Fabricating this material wasn’t easy—it took the lab at MIT that discovered it several years to learn how to produce layers of graphene where the twist angle was this precisely controlled.
It was discovered that one of their test devices was a perfect insulator; it didn’t allow any electrons to be transmitted. Apply a small voltage, however, adding free electrons to the system, and there is a sudden transition to superconductivity. Once twisted bilayer graphene has been manufactured, its electronic and superconducting properties can be tuned simply by applying electrical fields or pressure to the layers.
Twisted bilayer graphene provides a wonderful playground for experiments into superconducting systems because its properties are so easy to tune and change. As Pablo Jarillo-Herrero, whose MIT lab first discovered twisted bilayer graphene’s superconductivity in 2018, noted to Quanta Magazine: “If there’s any system where we can hope to understand strongly correlated electrons, it’s this one. Instead of having to grow different crystals, we just turn a voltage knob, or apply more pressure with the stamps, or change the rotation angle.”
Testing Physics at the Limits
With this new wealth of experimental data for unconventional superconductors, physicists can get new insight into how strongly correlated electronic materials work. Understanding quantum condensed matter, the properties of electrons in solids, will be key to all manner of future developments, from ever-smaller circuitry to more efficient renewable energy sources.
It may be possible to find quantum states that can make qubits that aren’t so easily destroyed by heat fluctuations, which would help us build the next generation of quantum computers. And, of course, the search for room-temperature superconductors would know where to look. It’s no surprise that, since the initial announcement in 2018, dozens of scientists have flocked to the new research field of “twistronics”—and there’s already talk of a Nobel prize. Recent research has linked the properties of twisted bilayer graphene to another area of active physics research, the quantum Hall effect.
Naturally, experiments are expanding to consider other single atomic layers, and investigating the range of material properties that can arise from different twists to different stacks of material. Materials where small twists can dramatically change their optical, electronic, and mechanical properties could, for example, act as semiconductors and therefore be used in computing and telecommunication.
Nearly fifteen years after it was originally discovered, the field of 2D materials that began with graphene continues to hold surprises for physicists. As we inch closer to a deeper understanding of the weird and wonderful properties of these nanoscale materials, the prospects for applications and devices that exploit the fundamental, quantum nature of matter hoves into view. Yet, as this surprising discovery demonstrates, there’s still an awful lot of fascinating physics at the limits.
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