For something that largely exists in exactly two dimensions, graphene seems to be everywhere. The super-thin 'wonder material' is known not just for its incredible strength, but also its unique, often surprising mixture of thermal and electromagnetic properties.
In recent times, many of the strangest experimental discoveries in graphene research are made when scientists stack separate layers of graphene on top of 1 another. When ordinary materials combine like this, nothing much happens, but even layering some sheets of graphene together seems to supply unusual and unexpected electronic states.
Now, a brand new study led by researchers at Columbia University, and therefore the University of Washington has found another incidence of this sort of behavior when graphene's one-atom-thick lattices acquire contact with one another.
"We wondered what would happen if we combined graphene monolayers and bilayers into a twisted three-layer system," says Columbia University physicist Cory Dean.
"We found that varying the number of graphene layers endows these composite materials with some exciting new properties that had not been seen before."
In recent years while investigating the results of graphene layering, scientists discovered that twisting one in all the layers ever slightly – in order that the 2 sheets are resting at a rather offset angle – produces what's called a twisted 'magic angle' structure, which may alternate between being an insulator and a superconductor (either blocking electricity flowing through the fabric or facilitating it with no resistance).
In the new work, Dean and his team experimented with a three-layer graphene system, constructed from one monolayer sheet stacked on top of a bilayer sheet, and so twisted by about 1 degree.
When subjected to extremely cold temperatures, just some degrees warmer than temperature, the twisted monolayer–bilayer graphene (TBG) system, demonstrated an array of insulating states, which can be controlled by an electrical field that was applied to the structure.
Depending on the direction of the applied force field, the tMBG's insulation capacity altered, resembling that of twisted bilayer graphene when the sector was pointed towards the monolayer sheet.
When the sector was reversed, though, pointing towards the bilayer sheet, 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 rare variety of magnetism only recently discovered.
"We observe the emergence of electrically tunable ferromagnetism at one-quarter filling of the conduction band, and an associated anomalous Hall effect," the researchers write in their paper.
The Hall effect traditionally refers to when voltage is often deflected by the presence of a field, and a related phenomenon called the quantum Hall effect – seen in two-dimensional electron systems like graphene – produces an anomaly where amplification of the effect jump up in quantized steps, not in an exceedingly straight, linear increase.
Recent research has uncovered this magnetic behavior in graphene systems incorporating crystals of boron nitride.
Here, for the primary time, though, physicists have created the identical anomaly, only now, they've somehow done it with graphene all by itself, which is kind of something given the atoms we're addressing.
"Pure carbon isn't magnetic," says Yankowitz. "Remarkably, we will engineer this property by arranging our three graphene sheets at just the correct twist angles."
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