Of late, many of the strangest experimental discoveries in graphene research were made when scientists stacked separate layers of graphene on top of each other. Not much happens when common materials are combined like this, but even layering a few layers of graphene together seems to create unusual and unexpected electronic states.
A new study conducted by researchers from Columbia University and the University of Washington found another incidence of this type of behavior when graphene an atom-thick lattice comes into contact with one another.
“We were wondering what would happen if we combined graphene monolayers and bilayers into a twisted three-layer system,” says Cory Dean, a physicist at Columbia University.
“We found that varying the number of graphene layers gives these composites some exciting new properties that have not been seen before.”
In recent years, when investigating the effects of graphene layers, scientists have found that slightly twisting one of the layers – so that the two layers rest at a slightly offset angle – creates a so-called twisted “magic angle” structure that can change You between an insulator and a superconductor (either block the electricity flowing through the material or facilitate it without resistance).
In the new work, Dean and his team experimented with a three-layer graph system that was built from a single single layer on top of a two-layer layer and then rotated about 1 degree.
At extremely cold temperatures, just a few degrees warmer than absolute zero, the twisted monolayer-double-layer graphene (tMBG) system exhibited a range of isolation states that could be controlled by an electric field applied to the structure.
Depending on the direction of the applied electric field, the insulation capacitance of the tMBG changed and resembled that of a twisted bilayer graph when the field was directed towards the monolayer film.
However, when the field was reversed and pointed at the bilayer, the state of insulation resembled that of a four-layer graphene structure made up 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 occurrence of an electrically tunable ferromagnetism when the conduction band is quarter full and an associated anomalous Hall effect,” the researchers write in their work.
The Hall effect traditionally refers to when the voltage can be deflected by the presence of a magnetic field and a related phenomenon called the quantum Hall effect, which occurs in two-dimensional electron systems like graphene, creates an anomaly in which amplifications of the Effect increase in quantized steps, not in a straight, linear increase.
Recent research has discovered this magnetic behavior in graph systems with boron nitride crystals.
This is where physicists created the same anomaly for the first time, only this time they kind of did it with graphs all by themselves, which is quite a lot given the atoms we’re dealing with.
“Pure carbon is not magnetic,” says Yankowitz. “Remarkably, we can construct this property by arranging our three graphene layers at exactly the right twist angle.”
The results are reported in Natural physics.
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