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Physics A magic trick: take 2 sheets of coal and spin



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In the universe of office supplies, pencil lead – a mixture of graphite and clay that does not contain lead – seems to be unique except for its ability to draw dark lines.

But 15 years ago, scientists discovered that a single graphite sheet – a layer of carbon atoms one atom thick arranged in a honeycomb – is a miracle. This ultra-thin carbon, called graphene, is 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 unusual things graphene could be made of: transistors, sensors, new materials. But after studying and cataloging its properties, scientists went on to other problems. Practical applications are very slow, because part of what makes graphene attractive – its strength – also makes it difficult to cut the material into precise shapes.

Last year, graphene reappeared on the physics research scene when physicists from the Massachusetts Institute of Technology discovered that stacking two sheets of material, twisted at a slight angle between them, opened a treasure chest with strange phenomena. She started a new field: twistronics.

Dr. Young said that he and others are still determining what is happening in a two-layered magically twisted graphene.

"There are many things that can happen and what happens will depend on many experimental details," he said. "We are just beginning to understand and map this space. We hope, however, that there will be something that cannot be seen in any other system. "

What happens if two pieces of graphene are stacked on top of each other? If the layers were perfectly aligned, the two layers of graphene would behave essentially the same as a single graphene sheet. But when one of the layers was slightly twisted compared to the other, the rotational misalignment of the two networks causes repeated "moire pattern"Extending to many atoms.

"I just started there," said Dr. MacDonald. "What if they were almost even?"

Electrons can easily jump between two sheets in which their grids were lined up. But in places where they were positioned incorrectly, the flow would be more difficult. In 2011, Dr. MacDonald and Rafi Bistritzer, PhD scientist, he calculated that from a small angle the electronic structure would become "flat" and the electrons would block like cars trying to get through Times Square.

Slowly moving electrons will interact more – "strongly correlated" in the language of physics – and from experience physicists knew that strongly correlated systems are often surprising.

"We threw out a few guesses," said Dr. MacDonald.

The article was intriguing, but largely ignored. Equations, covering many particles at once, are generally too complex to be solved exactly. So Dr. MacDonald and Dr. Bistritzer introduced some simplifications to get rough answers. Many scientists believed that their results were an artifact of their approximations, not a likely description of what could actually be observed.

Philip Kim, a Harvard physicist who conducted many early experiments with graphene – both Dr. Efetow and Dr. Jarillo-Herrero worked in their laboratory – believed that glazed details in calculations were 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.

This technique still requires sticky tape to separate the graphite crystal until only one layer of graphene remains. Then graphene is torn in half to get two flakes with perfectly arranged grids. Then one of the petals is rotated about 1.3 degrees and pressed against the other.

Layers are only loosely bonded and sometimes scientists have noticed that they are returning to perfect alignment. Other times the sheet begins to rotate but stops before it is completely aligned, sometimes it ends up with the desired 1.1 degree. The angle does not have to be exact; behavior appears to occur when the steering angle is between 1.0 and 1.2 degrees.

Last year, Dr. Jarillo-Herrero and his colleagues reported a surprising discovery. Two layers of graphene, now known as magically twisted bilayer graphene, became a superconductor after cooling to a fraction of a degree above absolute zero. (Dr. MacDonald and Dr. Bistritzer did not foresee this.)

"When we saw superconductivity, all hell broke loose," said Dr. Jarillo-Herrero. "Then we realized it was a very important matter."

Despite all the amazing tricks of the original work with graphene, scientists have never been able to transform it into a superconductor. It was a revelation that his behavior can be changed by simply placing another sheet on top and slightly twisting it. It was as if the color of two sheets of paper suddenly changed if one was rotated.

Other experimental physicists have returned to the study of graphene. "I was totally wrong," admitted Dr. Kim. "Allan MacDonald's theory was correct."

In a new Nature article, Dr. Efetov and his colleagues confirmed Dr. Jarillo-Herrero's findings, but found additional permutations of temperature, magnetic field and electron density, which also turn graphene into a superconductor.

They also found that graphene can also show an unusual kind of magnetism, arising from the movement of its electrons, not from the internal magnetism of its atoms, as seen in materials such as iron. This behavior has rarely been observed.

Dr. Efetov said that improving his recipe for combining graphene layers was to roll up the second layer after pressing it, as was the pressure on the smartphone screen protector to prevent the formation of air bubbles during its application.

He also says that the cleaner border between the two layers leads to its more detailed results. "What M.I.T. I saw, we are multiplying – he said. "In addition, we see many more states that most likely were not visible in his case due to dirty devices."

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

Scientists began to look 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 range of work materials, he thought that scientists could develop new superconductors that would be suitable for quantum computers. "I think it can be really exciting."

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