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Why twisted graphene is one of the year's most exciting physics stories



A moiré pattern of twisted two-ply graphene.
Image: NIST

Only a year ago, the scientists presented results that seemed almost too good to be true: carbon plates that were only a single atom thick assumed graphene called a pair of important physical properties, when they are rotated at exactly the right "magic" angle. When the atmosphere at the world's largest physics conference this month was indicative, the twisted graphene has now spawned a whole new field of physics research.

Despite freezing temperatures in Boston and a late winter storm, physicists had rooms in premises at the March meeting. The American Physical Society, many of them standing in the corridor, peering in, hoping to get the latest results from this magical angle graph hear. The result has garnered much interest from physicists around the world who want to understand the peculiar phenomena involved in carbon films.

"Fields that were previously relatively interconnected are now merging to study this one type of material ." Pablo Jarillo-Herrero, MIT physics professor and senior investigator behind the twisted graphene papers of the past year, told Gizmodo. "It has created a tremendous amount of exciting interactions."

Already in 2004, the scientists Andre Geim and Konstantin Novoselov first isolated the graphene by detaching the single atom layers of graphite (also called pencil lead) as a two-dimensional material. Since then, graphene has been known for its flexibility, conductivity and ability to store electricity.

Last year, a team of physicists headed by graduate student Yuan Cao made a discovery as close to science as it can shock science. They stacked a pair of graphene sheets on top of each other, cooling the system down to absolute zero, and twisting one of the blades at an angle of 1.1 degrees relative to the other. They added tension, and the system became a kind of insulator, in which the interactions between the particles themselves prevent the movement of the electrons. As they added more electrons, the system became a superconductor, a kind of system in which electrical charge can move without resistance.

"It was amazing," said Jarillo-Herrero to Gizmodo. "We thought it was too good to be true … We were so dubious at first that we wondered if we should spend more time with it, but when we saw the results, we were overwhelmed."

That you knew your result would be important, and you tried to do as many experiments as you could to provide substantiated evidence of what you had found. "We were very worried that we would be scooped," said Jarillo-Herrero. "But if you're announcing something important and a lot of people are paying attention, make sure the basics are right."

These magical angle effects refer to the moiré patterns that form in the twisted leaves. If you stack two hexagonal sheets on top of each other, you will see larger hexagonal patterns. These larger hexagons become the single units, not the small hexagons, that are caused by the carbon atoms.

The results have since been replicated by several teams and a year after the discovery physicists explore the material in droves. Although theorists predicted for the first time nearly a decade ago that new physical effects would manifest in these twisted layered graph systems, over a hundred new theory lessons emerged on the arXiv Preprint server last year. There is still a lot that physicists do not understand about the origin of superconductivity and the nature of the insulating states.

But why did this system become established? Jarillo-Herrero explained that he already combines blossoming areas of physics, including those that study graphene and other two-dimensional materials, topological properties (properties that do not change despite certain physical transformations), over-cold matter, and unusual electronic behaviors the way electrons are distributed in certain materials.

In addition, stacked graphene sheets are controllable and accessible in a way that other materials do not offer because they are relatively easy to manufacture. The ability to toggle between different effects with just one turn, one voltage, and a few electrons allows for a greater degree of control than other materials. Researchers continue to use this platform to discover more strange features of the material.

Research has seen an influx of doctoral students and postdocs looking for a field in which to make an impact. "Being able to do something so exciting and seeing this interesting new material was really fun," said Aaron Sharpe, Ph.D. An undergraduate student of applied physics at Stanford University, Gizmodo said. The Sharpe team recently presented its own measurements of the material's properties at the March APS meeting.

The field has also attracted seasoned experts. I participated in a lecture by the famous Harvard graphene scientist Philip Kim, in which he characterized the twisted leaves with various scientific aids. Other researchers stood on tiptoe down the hall to hear what he had to say.

Even if physicists are buzzing with excitement, it will likely take decades for you to see twisted bilayer graphs in your smartphone or other device, though this is obviously difficult to predict. Researchers have realized that much graphene on the market today is actually just an expensive pencil lead is. It is difficult to work with the two-dimensional plates: they must be kept at 1.7 degrees above absolute zero, and the plates would prefer not to be held at that 1.1 degree angle (similar to two Bar magnets that they have no north pole touching). It is understandably difficult to process a material that is only one atom thick.

The excitement of two-ply graphene comes from the underlying physics, not from the promise that it will be useful in technologies such as quantum computers or solar panels. But the field probably will not die soon. Jarillo-Herrero said, "This kind of Twistronics is something that has great potential in terms of scientific discovery and intellectual interest."


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