Why Twisted Graphene is one of the most exciting physics stories of the year



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A Moiré pattern on twisted double-layer graphene.
Image: NIST

Just a year ago, scientists presented results that seemed almost too good to be true: carbon sheets with only one atom of thickness, called graphene, acquired a pair of important physical properties when twisted at the correct "magic" angle to another. If this month's atmosphere at the world's largest physics conference was an indication, distorted graphene has now spawned an entirely new field of physics research.

Despite cool Boston temperatures and a late winter snowstorm, physicists filled the March meeting of the American Physical Society, many standing out in the hallway, hoping to hear the latest results on this magical angle graphene. The result has sparked interest from physicists worldwide who hope to understand the strange phenomena trapped in carbon sheets.

"Fields that were relatively connected before are now joined together a type of material, "Pablo Jarillo-Herrero, a professor of physics at MIT and principal investigator of graphene papers last year, told Gizmodo. "This has created a huge amount of exciting interactions."

In 2004, scientists Andre Geim and Konstantin Novoselov first isolated the graphene by stripping the single atom layers of graphite (also known as graphite graphite) using adhesive tape, creating a two-dimensional material. Since then, graphene has become known for its flexibility, conductivity and the ability to store electricity.

Last year, a team of physicists led by graduate student Yuan Cao made a discovery as shocking as science can get. They stacked a couple of sheets of graphene on top of each other, cooled the system to almost absolute zero, and twisted one of the sheets to an angle of 1.1 degrees to the other. They added a voltage, and the system became a type of insulator in such a way that the interactions between the particles themselves prevent the electrons from moving. When they added more electrons, the system became a superconductor, a type of system in which electric charge can move without resistance.

"It was incredible," Jarillo-Herrero told Gizmodo. "We thought it was too good to be true … At first we were so dubious that we wondered if we should spend more time on this, but when we saw the results, we were surprised."

They knew their outcome would be important and tried to do as many experiments as quickly as they could to present solid evidence of what they had found. "We were very worried that we would get caught," said Jarillo-Herrero. "But if you announce something important and many people are paying attention, you'd better make sure the basics are correct."

These magical angle effects are related to the Moiré patterns that develop in the braided sheets. When you stack two hexagonal sheets one over the other, larger hexagonal patterns begin to form. These larger hexagons become the individual units rather than the small hexagons drawn by the carbon atoms.

The results were replicated by several teams, and a year after the discovery, physicists are researching the mass material. Although early theorists predicted that new physical effects would manifest in these distorted layered graphene systems almost a decade ago, more than a hundred new theoretical articles appeared on the arXiv preprint server last year, citing MIT staff documents. Physicists have long understood the origin of superconductivity and the nature of insulating states.

But why was this system deactivated? Jarillo-Herrero explained that it combines already flourishing physics fields, including those that study graphene and other two-dimensional materials, topological properties (characteristics that do not change despite certain physical transformations), super-cold matter and unusual electronic behaviors. which arise from the way the electrons are distributed in certain materials.

In addition, the stacked graphene sheets are controllable and accessible in a way that other materials are not, since they are relatively easy to produce. And the ability to switch between various effects with just one twist, one voltage and some electrons allows a higher level of control than other materials. Researchers continue to use this platform to discover more extraneous properties of the material.

The survey saw an influx of postgraduate and post-doctoral students searching for a field in which they could impact. "Being able to contribute something so exciting and seeing these interesting news was a lot of fun," Aaron Sharpe, a Ph.D. student in applied physics at Stanford University, told Gizmodo. Sharpe's team recently presented their own measurements of material properties at the March APS meeting.

The field also attracted experienced specialists; I participated in a lecture by the famous Harvard graphene scientist, Philip Kim, on the characterization of twisted leaves with various scientific tools. Other researchers stood tiptoe in the hallway to hear what he had to say.

Even if physicists are full of excitement, it will probably take decades before you see two-layer graphene on your smartphone or any consumer device, though this is obviously hard to predict. Researchers have realized that much of the graphene on the market today is actually just expensive graphite. Two-dimensional sheets are difficult to work with: they should be maintained at 1.7 degrees above absolute zero, and the sheets would prefer not to be held at that angle of 1.1 degrees (similar to how two bar magnets would prefer not to have north poles touching ). It is understandably difficult to manipulate a material with only one atom of thickness.

The excitement of two-layer graphene stems from the physics behind it, not from the promise that it will become useful in technology like quantum computers or solar panels. But the camp is not likely to die soon. Jarillo-Herrero said, "This kind of field of twistronics is something with great potential in terms of scientific discovery and intellectual interest."

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