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Physics Magic Trick: Take 2 sheets of carbon and a twist



In the universe of office supplies, lead lead – a mixture of graphite and clay that does not include any lead – seems unusual beyond its ability to draw dark lines.

But 15 years ago, scientists discovered that a sheet of graphite – a layer of carbon atoms with a thickness of one atom, placed in a bee – was a miracle. This ultra-thin carbon, called graphene, is more flexible and lighter than paper, but 200 times stronger than steel. It is also a good conductor of heat and electricity.

Scientists imagined all the remarkable things in which graphene can be made: transistors, sensors, new materials. But after researching and cataloging its properties, scientists moved on to other problems. Practical use comes slowly, as part of what makes graphene enticing ̵

1; its strength, also makes it difficult to cut the material into precise shapes.

Last year, graphene exploded again in the physics research scene when the Massachusetts Institute of Technology physicists revealed that the arrangement of two sheets of material twisted at a small angle between them opened a box of treasures of strange phenomena. Launches a new field: twistronics.

Paper published on Wednesday in the journal Nature, examines in detail this material known as twisted two-layer graphene with a magic angle . An international team of scientists performed a series of experiments and showed that by adjusting the temperature, the magnetic field, and the number of electrons electrons that can move freely, keep this as an insulator where no electrical current is flowing to become a superconductor capable of transmitting electrical current without resistance.

The hope of twistronics is that researchers will be able to take advantage of superconductivity and other properties to create new electronics for quantum computers and other applications that cannot yet be represented.

" Our work is really kind of a richness showing the whole system, where we see all these effects at once, " "said Dmitry K. Efetov a physicist at the Barcelona Institute on Science and Technology in Spain and senior author of the article.

The ability to easily force graphene into different types of behavior gives scientists a simple system of inquiry as they attempt to understand the basic physics of its superconducting activity as well as other behaviors.

"He is the man who did this best," Andrea Young professor of physics at the University of California, San Diego, who did not participate in the research, said of Dr. Efetov and his contributors. "Somehow they have the magic touch."

Young said he and others were still deciding what was going on in a magically twisted two-layer graphene.

"There are many things that can happen, and what happens depends on many experimental details," he said, "We are just beginning to understand and map this space. But the hope is that there will be something that is not visible in any other system. ”

What if two pieces of graphene are stacked on top of each other? If the layers were perfectly aligned, two graphene layers would behave essentially the same as a single graphene sheet. But when one of the layers was slightly twisted over the other, the rotational displacement of the two lattices produced a repetitive moire model, spanning many atoms.

"That's where I started," Dr. MacDonald said. "What if they are almost aligned?"

The electrons could easily jump between the two sheets where their grids are arranged. But in places where they were leveled, the flow would be more difficult. In 2011, Dr. McDonald and Rafi Bistritzer a doctoral researcher, calculated that at a small angle the electronic structure would become "flat", with electrons clogging up like cars trying to get their way way through Times Square.

Slow moving electrons would be more likely to interact with each other – "strongly coupled", in the language of physics – and from experience, physicists knew that highly coupled systems were often surprising.

"We have rejected several assumptions," says Dr. McDonald.

The document was intriguing but largely ignored. Equations covering multiple particles at one time are generally too complex to solve accurately. So Dr. McDonald and Dr. Bistricer made some simplifications to come up with rough answers. Many scientists thought their results were an artifact of their approximations, not a likely description of what would actually be observed.

Philip Kim a Harvard physicist who did many of the early experiments with graphene – Dr. Efetov and Dr. Jarillo-Herrero worked in his lab, thinking that the details of the calculations would be important. "I was skeptical," he said.

But Dr. Jarillo-Herrero decided to test the forecast. "There was good theoretical motivation to see what would happen," he said.

The technique still includes a sticky tape for crushing a graphite crystal until only one layer of graphene remains. The graphene is then split in two to produce two scales with perfectly padded gratings. Then one of the flakes rotates about 1.3 degrees and presses on the other.

The layers are only loosely coupled, and sometimes scientists watch them return to perfect alignment. Other times, the sheet starts to rotate but stops before fully lining sometimes ending at the desired 1.1 degrees. The angle should not be accurate; the behavior appears to occur when the torsion angle is between 1.0 and 1.2 degrees.

Last year, Dr. Jarilo-Erero and his colleagues reported a startling finding. The two layers of graphene, now known as the magic corner twisted two-layer graphene, became a superconductor when cooled to a fraction of degree above absolute zero. (Dr. McDonald and Dr. Bistritzer did not anticipate this.)

"When we saw superconductivity, all hell broke up," says Dr. Jarillo-Herrero. "Then we realized it was a very big thing."

For all the amazing tricks of the original work with graphene, scientists have never been able to turn it into a superconductor. It was a revelation that his behavior could be transformed simply by putting another sheet on top and slightly distorting. It seems as if the color of two sheets of paper suddenly changed if one of them turned.

Other experimental physicists have jumped again in the study of graphene. "I was completely wrong," Dr. Kim admitted. "The theory of Alan MacDonald was correct."

In a new paper on nature, Dr. Efetov and his colleagues confirmed Dr. Jarillo-Herrero's findings, but found additional permutations of temperature, magnetic field, and electron density. graphene in a superconductor.

They have also discovered that graphene may also exhibit an unusual type of magnetism, arising from the movement of its electrons rather than the internal magnetism of its atoms, as seen in materials such as iron. This behavior is rarely observed.

p. Efetov said his improvement in the recipe for combining graphene layers is to roll the second layer when pressed, similar to how one presses a screen protector on a smartphone to prevent air bubbles from forming while applying it.

He also says that the cleaner boundary between the two layers leads to more detailed results. "What M.I.T. we saw, we reproduce, "he said. "But on top of that, we see many more states that were probably not noticed in his case due to dirty devices."

The new twistronics field goes beyond graphene. The electronic behavior of the material may depend on the material to which the graphene is placed, usually boron nitride. Trying other materials or configurations can lead to different results.

Scientists began looking at three layers of graphene and many other two-dimensional materials.

"I think this is just the beginning," said Dr. Kim of Harvard.

With such a wide variety of materials that he can work with, he believes that scientists can create new superconductors that are suitable for quantum computers. "I think that can be really exciting."


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