The work was inspired by recent studies of twisted monolayers or twisted bilayers of graphene, comprising either two or four total sheets. These materials were found to host an array of unusual electronic states driven by strong interactions between electrons.
"We wondered what would happen if we combined graphene monolayers and bilayers into a twisted three-layer system," explained Cory Dean, a professor of physics 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."
To conduct their experiment, the researchers stacked a monolayer sheet of graphene onto a bilayer sheet and twisted them by about 1 degree. At temperatures a few degrees over absolute zero, the team observed an array of insulating states - which do not conduct electricity - driven by strong interactions between electrons. They also found that these states could be controlled by applying an electric field across the graphene sheets.
"We learned that the direction of an applied electric field matters a lot," said Assistant Professor Matthew Yankowitz one of the research's senior authors.
When the researchers pointed the electric field toward the monolayer graphene sheet, the system resembled twisted bilayer graphene. But when they flipped the direction of the electric field and pointed it toward the bilayer graphene sheet, it mimicked 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 the electrons in the team's three-layer structure underlies the magnetism, they observed.
This form of magnetism was discovered recently by other researchers in various structures of graphene resting on crystals of boron nitride. The team has now demonstrated that it can also be observed in a simpler system constructed entirely with graphene.
"Pure carbon is not magnetic," said 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 study uncovered signs of topology in the structure. Akin to tying different types of knots in a rope, the topological properties of the material may lead to new forms of information storage, which "may be a platform for quantum computation or new types of energy-efficient data storage applications," said Professor Xiaodong Xu, another senior author involved with the research.
The findings appear in an article published Oct. 12 in Nature Physics.
