Stacking and twisting a set of graphene sheets on top of each other produces a rare form of magnetism. (Image Credit: Columbia University)
Graphene has excellent thermal and electromagnetic properties, aside from being the world’s strongest and thinnest material. Now, researchers at Columbia University and the University of Washington have discovered that stacking and twisting a three-layer graphene structure can produce a rare form of magnetism.
“We wondered what would happen if we combined graphene monolayers and bilayers into a twisted three-layer system,” said Cory Dean, a physicist at Columbia University and one of 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.”
When observing the effects of graphene layering in previous studies, scientists discovered that slightly twisting one of the layers - so that both layers are resting at a balanced angle - created a twisted angle structure. The structure is capable of changing back and forth between an insulator and a superconductor, which obstructs the electricity flowing through the material or encourages it with no resistance.
In their experiments, the team stacked a single monolayer sheet on top of a bilayer sheet and twisted them by one degree. When the twisted monolayer-bilayer graphene (tMBG) system became exposed to temperatures a few degrees above absolute zero, it exhibited an array of insulating states. These states could be controlled by applying an electric field to the structure.
Pointing the electric field towards the monolayer sheet caused the bilayer graphene sheet to become twisted. However, when the electric field’s direction was reversed and pointed towards the bilayer graphene sheet, it mimicked a four-layer graphene structure comprised of a twisted double bilayer system.
Additionally, the researchers detected a rare form of magnetism in the three-layer graphene structure. They observed a collective swirling motion of electrons underlying the magnetism.
Another team discovered that graphene, when bonded with boron nitride, produced a magnetic field that rose from carbon’s molecular bonds in graphene and the boron in boron nitride. Dean and his team’s new study demonstrated that this form of magnetism could occur in a structure made entirely out of graphene due to interactions between carbon molecules.
“Pure carbon is not magnetic,” said Matthew Yankowitz, a physicist at the University of Washington. “Remarkably, we can engineer this property by arranging our three graphene sheets at just the right twist angles.”
The study also unveiled signs of topology in the structure. The topological properties of graphene could lead to new types of information storage, which “may be a platform for quantum computation or new types of energy-efficient data storage applications,” Professor Xiaodong Xu said.
The team is performing additional experiments to learn more about the properties of the new states they discovered in this system. “This is really just the beginning,” said Yankowitz.
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