For something that basically exists in only two dimensions, the graph seems to be everywhere. The super-thin ‘wonderful material’ is not only famous for its incredible strength, but also its unique, often surprising blend of thermal and electromagnetic properties.
In recent times, many of the strangest experimental discoveries in graphene research have been made when researchers stack separate layers of graphene on top of each other. When ordinary materials are combined like this, not much happens, but even putting a few sheets of graphene together seems to produce unusual and unexpected electronic states.
Now, a new study led by researchers at Columbia University and the University of Washington has found a different occurrence of this type of behavior when the graphene̵
“We wondered what would happen if we combined graphene monolayers and double layers into a twisted three-layer system,” said Columbia University physicist Cory Dean.
“We found that varying numbers of graphene layers give these composite materials some exciting new properties that had not been seen before.”
In recent years, while studying the effects of graphene layering, researchers discovered that twisting one of the layers ever so easily – so the two sheets rest at a slightly offset angle – produces what is known as a twisted ‘magic angle’ structure. , which can alternately be an insulator and a superconductor (either blocks current flowing through the material or facilitates it without resistance).
In the new work, Dean and his team experimented with a three-layer graphene system, constructed of a single monolayer sheet stacked on top of a double-layer sheet and then twisted with approx. 1 degree.
When exposed to extremely cold temperatures, only a few degrees warmer than absolute zero, the twisted monolayer double-layer graphene system (tMBG) showed a series of insulating conditions that could be controlled by an electric field applied to the structure.
Depending on the direction of the applied electric field, the insulation capacity of tMBG changed and resembled that of the twisted double layer graph when the field was pointed towards the monolayer sheet.
When the field was inverted but pointing toward the bilayer sheet, the insulating state resembled that of a four-layer graphene structure consisting of a twisted bilayer system.
However, that’s not all the team found. During the experiments, the team discovered a rare form of magnetism that was only recently discovered.
“We observe the appearance of electrically adjustable ferromagnetism at a quarter fill of the wiring harness and an associated anomalous Hall effect,” the researchers write in their paper.
The Hall effect traditionally refers to when the voltage can be deflected in the presence of a magnetic field, and a related phenomenon called the quantum-Hall effect – seen in two-dimensional electron systems as the graph – produces an anomaly in which amplifications of the effect spring up. quantized steps, not in a straight, linear increase.
Recent research has uncovered this magnetic behavior in graphene systems that incorporate crystals of boron nitride.
Here, physicists have for the first time created the same anomaly, only this time they have somehow done it with the graph itself, which is something considering the atoms we are dealing with.
“Pure carbon is not magnetic,” says Yankowitz. “It is remarkable that we can construct this property by arranging our three graph sheets in just the right angles of rotation.”
The results are reported in Natural physics.