Not only is graphene — a one-atom-thick sheet of carbon arranged in a hexagonal lattice — the strongest, thinnest material known to man, it is also an excellent conductor of heat and electricity. Now, a team of researchers has discovered that a variety of exotic electronic states, including a rare form of magnetism, can arise in a three-layer graphene structure.
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.
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.
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 three-layer structure underlies the magnetism.
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. 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.