For something that largely exists in only 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, often amazing combination of thermal and electromagnetic properties.
Recently, many of the strangest experimental discoveries have been made in the study of graphene, 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 seems to create unusual and unexpected electronic states.
Now, a new study by researchers at Columbia University and the University of Washington has found another frequency of this behavior when graphene lattices with a thickness of one atom collide with each other.
“We wondered what would happen if we combined monolayers and bilayers of graphene into a twisted three-layer system,”; said Cory Dean, a physicist at Columbia University.
“We have found that changing the number of layers of graphene gives these composite materials new exciting properties that were not there before.”
In recent years, investigating the effects of a layer of graphene, scientists have found that the twisting of one of the layers is so slight – so that the two sheets lie at a slightly offset angle – creates a so-called twisted structure “magic angle” and a superconductor (either by blocking the electricity flowing through the material or by facilitating it without resistance).
In the new work, Dean and his team experimented with a three-layer graphene system built from a single single-layer sheet folded on top of a two-layer sheet and then twisted by about 1 degree.
Under the influence of extremely low temperatures, only a few degrees warmer than absolute zero, the system of twisted single-layer two-layer graphene (tMBG) demonstrated many insulating states that could be controlled by the electric field applied to the structure.
Depending on the direction of the applied electric field, the insulating ability of tMBG varied, resembling the strength of twisted two-layer graphene when the field was directed toward a single-layer sheet.
When the field was changed, 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 recently.
“We observe the appearance of electrically regulated ferromagnetism when filling a quarter of the conduction band and the associated abnormal Hall effect,” the researchers write in their work.
The Hall effect traditionally refers to when a voltage can be deflected 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 when the gain gains rise up quantized. steps, not in a straightforward, linear magnification.
Recent studies have found this magnetic behavior in graphene systems containing boron nitride crystals.
Here, however, for the first time physicists created the same anomaly, but this time they somehow did it with graphene on their own, which is quite something given the atoms we are dealing with.
“Pure carbon is not magnetic,” says Jankowitz. “It is noteworthy that we can design this property by placing our three graphene sheets at the right torsion angles.”
The findings are reported in Physics of nature.