A Moiré pattern in twisted bilayer graphene.Image: NIST
Just a year ago, scientists presented results that seemed almost too good to be true: Carbon sheets only a single atom thick, called graphene, took on a pair of important physical properties when they were twisted at just the right “magic” angle relative to one another. If the atmosphere this month at the world’s largest physics conference was any indication, twisted graphene has now spawned an entirely new field of physics research.
Despite frigid Boston temperatures and a late-winter snowstorm, physicists swarmed rooms at the March meeting of the American Physical Society, many standing out in the hallway looking in, hoping to hear the latest results about this magic-angle graphene. The result has drawn interest from physicists around the world who hope to understand the strange phenomena locked into the carbon sheets.
“Fields that were relatively connected before are now joined studying this one type of material,” Pablo Jarillo-Herrero, MIT professor of physics and principal investigator behind last year’s twisted graphene papers, told Gizmodo. “It has created an enormous amount of exciting interactions.”
Back in 2004, scientists Andre Geim and Konstantin Novoselov first isolated graphene by peeling the single-atom layers off of graphite (also known as pencil lead) using adhesive tape, creating a two-dimensional material. Since then, graphene has become well-known for its flexibility, conductivity, and ability to store electricity.
Last year, a team of physicists led by graduate student Yuan Cao made a discovery as close to shocking as science can get. They stacked a pair of graphene sheets on top of one another, cooled the system down to near absolute zero, and twisted one of the sheets to a 1.1-degree angle relative to the other. They added a voltage, and the system became a kind of insulator such that the interactions between the particles themselves prevent electrons from moving. When they added more electrons, the system became a superconductor, a kind of system in which electrical charge can move without resistance.
“It was amazing,” Jarillo-Herrero told Gizmodo. “We thought it was too good to be true… We were so doubtful at first that we wondered if we should spend more time on it, but when we saw the results, we were blown away.”
They knew that their result would be important, and tried to do as many experiments as quickly as they could to present rock-solid evidence of what they had found. “We were very worried that we would get scooped,” Jarillo-Herrero said. “But if you announce something important and a lot of people are paying attention, you better be sure that the basics are correct.”
These magic-angle effects are related to the Moiré patterns that develop in the twisted sheets. When you stack two hexagonal sheets on top of each other, larger hexagonal patterns begin to form. These larger hexagons become the individual units, rather than the small hexagons traced out by the carbon atoms.
The results have since been replicated by several teams, and a year after the discovery, physicists are researching the material in droves. Though theorists first predicted that new physical effects would manifest in these twisted layered graphene systems almost a decade ago, over a hundred new theory papers have appeared on the arXiv preprint server in the past year, citing the MIT team’s papers. There’s still plenty that physicists don’t understand about the origin of the superconductivity and the nature of the insulating states.
But why has this system taken off? Jarillo-Herrero explained that it combines already-flourishing fields of physics, including those that study graphene and other two-dimensional materials, topological properties (characteristics that don’t change despite certain physical transformations), super-cold matter, and unusual electronic behaviors that come about from the way electrons are distributed in certain materials.
On top of that, stacked graphene sheets are controllable and accessible in a way that other materials aren’t, given that they are relatively easy to produce. And the ability to switch between various effects with just a twist, a voltage, and some electrons allows a higher level of control than other materials. Researchers continue to use this platform to discover more strange properties of the material.
The research has seen an influx of graduate students and post-docs looking for a field where they can make an impact. “To be able to contribute to something this exciting and see this interesting new stuff has been really fun,” Aaron Sharpe, Ph.D. student in applied physics at Stanford University, told Gizmodo. Sharpe’s team recently presented their own measurements of the material’s properties at the APS March meeting.
The field has also attracted seasoned experts; I sat in on a talk by famed Harvard graphene scientist Philip Kim on characterizing the twisted sheets with various scientific tools. Other researchers stood on tiptoe out in the hallway to hear what he had to say.
Even as physicists are buzzing with excitement, it will probably be decades before you see twisted bilayer graphene in your smartphone or any consumer device, though obviously that’s hard to predict. Researchers have realized that lots of the graphene on the market today is actually just expensive pencil lead. The two-dimensional sheets are difficult to work with: They must be held at 1.7 degrees above absolute zero, and the sheets would prefer not to be held at that 1.1-degree angle (similar to how two bar magnets would prefer not to have their north poles touching). It’s understandably hard to manipulate a material that’s only one atom thick.
Excitement for bilayer graphene stems from the physics that underlies it, not the promise that it will become useful in tech like quantum computers or solar panels. But the field likely won’t die soon. Jarillo-Herrero said: “This sort of field of ‘twistronics’ is something with great potential in terms of scientific discovery and intellectual interest.”
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