For something that largely exists in just two dimensions, graphene seems to be everywhere. The ultra-thin “miracle material” is famous not only for its incredible strength, but also for its unique and often surprising blend of thermal and electromagnetic properties.
In recent times, many of the strangest experimental discoveries in graphene research have been made when scientists stack separate layers of graphene on top of each other. When ordinary materials combine in this way, not much happens, but even the superposition of a few sheets of graphene seems to produce unusual and unexpected electronic states.
Now, a new study by researchers at Columbia University and the University of Washington has found another incidence of this kind of behavior when graphene’s atom-thick lattices come into contact with each other. others.
“We wondered what would happen if we combined monolayers and bilayers of graphene into a twisted three-layer system,” says Cory Dean, a physicist from Columbia University.
“We discovered that varying the number of graphene layers gives these composite materials exciting new properties that have never been seen before.”
In recent years, while investigating the effects of graphene layering, scientists have discovered that very slight twisting of one of the layers – so that the two sheets lie at a slightly offset angle – produces what this is called a “magic angle” twisted structure, which can alternate between being an insulator and a superconductor (either blocking electricity flowing through the material or facilitating it without resistance).
In the new work, Dean and his team experimented with a three-layer graphene system, constructed from a single monolayer sheet stacked on top of a bilayer sheet, then twisted about 1 degree.
When subjected to extremely cold temperatures, just a few degrees warmer than absolute zero, the twisted monolayer-bilayer graphene (tMBG) system demonstrated an array of insulating states, which could be controlled by an electric field applied to the structure.
Depending on the direction of the applied electric field, the insulating ability of tMBG changed, resembling that of twisted bilayer graphene when the field was directed towards the monolayer sheet.
When the field was reversed, however, pointing at the bilayer foil, the insulating state resembled that of a four-layer graphene structure composed of a twisted double bilayer system.
That’s not all the team found, however. During the experiments, the team detected a recently discovered rare form of magnetism.
“We observe the emergence of an electrically tunable ferromagnetism at a quarter fill of the conduction band, and an associated anomalous Hall effect,” the researchers write in their paper.
The Hall effect traditionally refers to when voltage can be deflected by the presence of a magnetic field, and a related phenomenon called the quantum Hall effect – seen in two-dimensional electron systems like graphene – produces an anomaly where the Effect amplifications jump in quantized steps, not in a straight, linear increase.
Recent research has discovered this magnetic behavior in graphene systems incorporating boron nitride crystals.
Here, for the first time, however, physicists have created the same anomaly, only this time they’ve kind of done it with graphene all by itself, which is quite something considering the atoms we have case.
“Pure carbon is not magnetic,” says Yankowitz. “Remarkably, we can engineer this property by arranging our three graphene sheets at the right twist angles.”
The findings are reported in Natural Physics.