Rare form of magnetism unlocked by stacking and twisting graphene



Stacking monolayer and bilayer graphene sheets with a twist leads to new collective electronic states, including a rare form of magnetism. Credit: Columbia University

Twist of a single layer sheet and a bilayer sheet of graphene in a three-layer structure leads to new states of quantum mechanics.

Since the discovery of graphene over 15 years ago, researchers have embarked on a global race to discover its unique properties. Not only is graphene aatom-Thick carbon foil arranged in a hexagonal lattice – the strongest and thinnest material known to man, it is also an excellent conductor of heat and electricity.

Now, a team of researchers from Columbia university and the Washington University discovered that a variety of exotic electronic states, including a rare form of magnetism, can appear in a three-layered graphene structure.

The results appear in an article published on October 12, 2020, in Physics of Nature.

The work was inspired by recent studies of twisted monolayers or twisted bilayers of graphene, comprising two or four total sheets. These materials have been shown to harbor a set of unusual electronic states driven by strong interactions between electrons.

“We wondered what would happen if we combined monolayers and bilayers of graphene in a twisted three-layer system,” said Cory Dean, professor of physics at Columbia University and one of the lead authors of the ‘article. “We have found that varying the number of graphene layers gives these composite materials exciting new properties that have never been seen before.”

In addition to Dean, Assistant Professor Matthew Yankowitz and Professor Xiaodong Xu, both from the Departments of Physics and Materials Science and Engineering at the University of Washington, are the main authors of the book. Columbia graduate student Shaowen Chen and Washington University graduate student Minhao He are the co-lead authors of the article.

To conduct their experiment, the researchers stacked a single-layered sheet of graphene on top of a double-layered sheet and twisted them about 1 degree. At temperatures above a few degrees absolute zero, the team observed a set of insulating states, which do not conduct electricity, caused by strong interactions between electrons. They also found that these states could be controlled by applying an electric field through the sheets of graphene.

“We’ve learned that the direction of an applied electric field matters a lot,” said Yankowitz, who is also a former postdoctoral fellow in Dean’s group.

When the researchers directed the electric field at the single-layered graphene sheet, the system looked like twisted bilayer graphene. But when they reversed the direction of the electric field and pointed it at the bilayer graphene sheet, it was mimicking the twisted bilayer graphene, the four-layer structure.

The team also discovered new magnetic states in the system. Unlike conventional magnets, which are driven by a quantum mechanical property of electrons called “spin,” a collective swirling motion of electrons in the team’s three-layered structure underlies magnetism, they observed.

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

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

In addition to magnetism, the study revealed signs of topology in the structure. As with tying different types of knots in a rope, the topological properties of the material can lead to new forms of information storage, which “can be a platform for quantum computing or new types of data storage applications. energy-saving data, ”Xu said.

For now, they are working on experiments to better understand the fundamental properties of the new states they have discovered in this platform. “This is really just the start,” Yankowitz said.

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, October 12, 2020, Physics of Nature.
DOI: 10.1038 / s41567-020-01062-6

The study, “Electrically Tunable Correlated and Topological States in Twisted Monolayer-Bilayer Graphene,” was developed with support from Programmable Quantum Materials, an Energy Frontier research center funded by the US Department of Energy (DOE), Office of Science and Basic Energy Sciences.



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