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Stacking and twisting graphene unleashes an unusual form of magnetism.




Since the discovery of graphene more than 15 years ago, researchers have been competing globally to unleash its unique traits. Graphene (a single-atom-thick carbon sheet arranged in a hexagonal lattice) is not only the strongest and thinnest material known to humans, but also an excellent conductor of heat and electricity.

Currently, a team of researchers at Columbia University and the University of Washington have discovered that a variety of exotic electronic states, including rare forms of magnetism, can occur in a three-layer graphene structure.

The findings will appear in an article published on Nature Physics on October 12th.

This work is inspired by recent studies of twisted monolayers or twisted bilayers of graphene and consists of a total of two or four sheets. These materials have been found to host a series of anomalous electronic states driven by strong electron-electron interactions.

“I wondered what would happen if we combined graphene monolayers and bilayers into a twisted three-layer system,” said Cory De, a professor of physics at Columbia University and one of the lead authors of the paper. Mr. an says. “We have found that changing the number of graphene layers gives these composites some exciting new properties never seen before.”

In addition to Dean, Assistant Professor Matthew Jankowitz and Professor Xiaodong Shu of the Faculty of Physics and Materials Science and Engineering at the University of Washington are senior authors of this study. Shaowen Chen, a graduate student at Columbia University, and Minhao He, a graduate student at the University of Washington, are co-authors of this paper.

To carry out the experiment, the researchers stacked a single-layer sheet of graphene on a double-layer sheet and twisted it about once. The team observed a series of insulation states (no electricity) driven by strong interactions between electrons at temperatures several degrees above absolute zero. They also found that these conditions could be controlled by applying an electric field throughout the graphene sheet.

“I learned that the direction of the applied electric field is very important,” said Jankowitz, a former postdoctoral researcher in Dean’s group.

When the researchers directed the electric field to the single-layer graphene sheet, the system resembled a twisted double-layer graphene. However, when the direction of the electric field was reversed and directed toward the two-layer graphene sheet, it mimicked a twisted two-layer graphene (four-layer structure).

The team also discovered a new magnetic state in the system. They observed that, unlike traditional magnets, which are driven by a quantum mechanical property of electrons called “spins,” the collective swirling of electrons within the team’s three-layer structure underlies magnetism.

This form of magnetism has recently been discovered by other researchers in various structures of graphene on boron nitride crystals. The team has demonstrated that it can be observed even with a simpler system built entirely of graphene.

“Pure carbon is not magnetic,” Yankowitz said. “Surprisingly, this characteristic can be designed by placing three graphene sheets in the proper twist angle.”

In addition to magnetism, research has revealed signs of structural topology. Similar to tying various types of knots with ropes, the phase properties of materials “potentially become a platform for quantum computing or new types of energy efficient data storage applications” into a new form of information storage. There is a possibility of connecting.

For now, they are working on experiments to further understand the basic characteristics of the new states discovered on this platform. “This is really just the beginning,” said Jankowitz.

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