For something that largely exists in only two dimensions, graphene seems to be everywhere. The ultra-thin “wonderful material” is known not only for its incredible strength, but also for its unique, often surprising combination of thermal and electromagnetic properties.
Recently, many of the strangest experimental discoveries have been made in graphene research, when scientists stack individual layers of graphene on top of each other. When ordinary materials are combined in this way, nothing special happens, but even the layering of several sheets of graphene together creates unusual and unexpected electronic states.
Now, a new study led by researchers at Columbia University and the University of Washington finds another frequency of this type of behavior when graphene-atom lattices come into contact with each other.
“We were wondering what would happen if we combined graphene single-layer and double-layer in a twisted three-layer system,”
“We have found that the change in the number of graphene layers gives these composite materials some new exciting properties that have not been seen before.”
In recent years, while investigating the effects of graphene layering, scientists have found that twisting one of the layers still slightly – so that the two leaves rest at a slightly offset angle – creates a so-called twisted “magic angle” structure that can alternate. be as an insulator and superconductor (either blocking the electricity flowing through the material, or facilitating without resistance).
In the new work, Dean and his team experimented with a three-layer graphene system made up of a single single-layer sheet arranged on top of a double-layer sheet and then twisted by about 1 degree.
When subjected to extremely low temperatures, only a few degrees warmer than absolute zero, the twisted monolayer-two-layer graphene system (tMBG) demonstrates a variety of insulation states that can be controlled by an electric field applied to the structure.
Depending on the direction of the applied electric field, the insulating capacity of tMBG changes, resembling that of twisted bilayer graphene, when the field was directed towards the single-layer sheet.
When the field was inverted, however, pointing to the two-layer sheet, the insulation state resembled the state of a four-layer graphene structure composed of a twisted double two-layer system.
However, this is not all that the team has found. During the experiments, the team discovered a rare form of magnetism, discovered only recently.
“We observe the appearance of electrically adjustable ferromagnetism at a quarter of the filling of the conductive band and the associated anomalous Hall effect,” the researchers wrote in their report.
The Hall effect traditionally refers to when the voltage can be diverted by the presence of a magnetic field, and a related phenomenon called the quantum Hall effect – observed in two-dimensional electronic systems such as graphene – creates an anomaly in which the effect gains jump up quantitatively. steps, not in straight, linear increments.
Recent studies have revealed this magnetic behavior in graphene systems involving boron nitride crystals.
Here, however, for the first time, physicists created the same anomaly, only this time they somehow did it with graphene itself, which is quite something considering the atoms we are dealing with.
“Pure carbon is not magnetic,” says Jankowitz. “It’s remarkable that we can design this property by arranging our three graphene sheets just below the right angles of rotation.”
The findings were reported in Physics of nature.