Rare Form of Magnetism Unlocked by Stacking and Twisting Graphene

Stacking Monolayer and Bilayer Graphene Sheets

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Stacking monolayer and bilayer graphene sheets with a twist results in brand-new cumulative electronic states, consisting of an uncommon kind of magnetism. Credit: Columbia University

Twisting a monolayer and a bilayer sheet of graphene into a three-layer structure results in brand-new quantum mechanical states.

Since the discovery of graphene more than 15 years back, scientists have actually remained in an international race to open its special residential or commercial properties. Not just is graphene—a one-atom-thick sheet of carbon set up in a hexagonal lattice—the greatest, thinnest product understood to guy, it is likewise an exceptional conductor of heat and electrical energy.

Now, a group of scientists at Columbia University and the University of Washington has actually found that a range of unique electronic states, consisting of an uncommon kind of magnetism, can develop in a three-layer graphene structure.

The findings appear in a short article released on October 12, 2020, in Nature Physics.

The work was influenced by current research studies of twisted monolayers or twisted bilayers of graphene, making up either 2 or 4 overall sheets. These products were discovered to host a range of uncommon electronic states driven by strong interactions in between electrons.

“We wondered what would happen if we combined graphene monolayers and bilayers into a twisted three-layer system,” stated Cory Dean, a teacher of physics at Columbia University and among the paper’s senior authors. “We found that varying the number of graphene layers endows these composite materials with some exciting new properties that had not been seen before.”

In addition to Dean, Assistant Professor Matthew Yankowitz and Professor Xiaodong Xu, both in the departments of physics and products science and engineering at University of Washington, are senior authors on the work. Columbia college student Shaowen Chen, and University of Washington college student Minhao He are the paper’s co-lead authors.

To perform their experiment, the scientists stacked a monolayer sheet of graphene onto a bilayer sheet and twisted them by about 1 degree. At temperature levels a couple of degrees over outright no, the group observed a range of insulating states—which do not carry out electrical energy—driven by strong interactions in between electrons. They likewise discovered that these states might be managed by using an electrical field throughout the graphene sheets.

“We learned that the direction of an applied electric field matters a lot,” stated Yankowitz, who is likewise a previous postdoctoral scientist in Dean’s group.

When the scientists pointed the electrical field towards the monolayer graphene sheet, the system looked like twisted bilayer graphene. But when they turned the instructions of the electrical field and pointed it towards the bilayer graphene sheet, it imitated twisted double bilayer graphene—the four-layer structure.

The group likewise found brand-new magnetic states in the system. Unlike standard magnets, which are driven by a quantum mechanical residential or commercial property of electrons called “spin,” a cumulative swirling movement of the electrons in the group’s three-layer structure underlies the magnetism, they observed.

This kind of magnetism was found just recently by other scientists in different structures of graphene resting on crystals of boron nitride. The group has actually now shown that it can likewise be observed in an easier system built completely with graphene.

“Pure carbon is not magnetic,” stated Yankowitz. “Remarkably, we can engineer this property by arranging our three graphene sheets at just the right twist angles.”

In addition to the magnetism, the research study revealed indications of geography in the structure. Akin to connecting various kinds of knots in a rope, the topological residential or commercial properties of the product might result in brand-new kinds of details storage, which “may be a platform for quantum computation or new types of energy-efficient data storage applications,” Xu stated.

For now, they are dealing with experiments to even more comprehend the basic residential or commercial properties of the brand-new states they found in this platform. “This is really just the beginning,” stated Yankowitz.

Reference: “Electrically tunable correlated and topological states in twisted monolayer–bilayer graphene” by Shaowen Chen, Minhao He, Ya-Hui Zhang, Valerie Hsieh, Zaiyao Fei, K. Watanabe, T. Taniguchi, David H. Cobden, Xiaodong Xu, Cory R. Dean and Matthew Yankowitz, 12 October 2020, Nature Physics.
DOI: 10.1038/s41567-020-01062-6

The research study, “Electrically tunable correlated and topological states in twisted monolayer-bilayer graphene,” was established with assistance from Programmable Quantum Materials, an Energy Frontier Research Center moneyed by the United States Department of Energy (DOE), Office of Science and Basic Energy Sciences.

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