Stacking and twisting of graphene releases a rare form of magnetism

Since the discovery of graphene more than 15 years ago, researchers have embarked on a global race to unlock its unique properties. Not only is graphene – an atom-thick sheet of carbon arranged in a hexagonal lattice – the strongest and thinnest material known to man, it is also an excellent conductor of heat and light. ‘electricity.

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

The results appear in an article published on October 12 in Natural Physics.

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

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

In addition to the 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 lead authors of the work. Columbia graduate student Shaowen Chen and University of Washington graduate student Minhao He are co-lead authors of the paper.

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

“We learned that the direction of an applied electric field is very important,” said Yankowitz, who is also a former postdoctoral researcher in Dean’s group.

When the researchers directed the electric field at the single-layer 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 mimicked the twisted double 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-layer structure underlies the 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 with graphene.

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

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

For now, they are working on experiments to better understand the fundamental properties of the new states they discovered in this platform. “It’s really just the beginning,” Yankowitz said.