Image of graphene magic angle revealed


Rebecca Pool

Tuesday, August 13, 2019 - 22:15
A scanning tunnelling microscopy topographic image of twisted bilayer graphene. [Stevan Nadj-Perge]
Using scanning tunnelling microscopy, researchers from Caltech, US, have taken a direct image of the magic-angle of twisted graphene sheets opening the door to room-temperature superconductors.
The results come just over a year after researchers at MIT stunned the physics world with the discovery of the "magic angle" for stacked sheets of graphene.
Understanding the so-called magic angle - a specific orientation between sheets of graphene that yields special electric properties - could pave the way to realizing the dream of room-temperature superconductors, which could transmit enormous electric currents while producing zero heat.
In early 2018, researchers at MIT discovered that at a certain orientation, about 1.1 degrees of relative twist, the bilayer material becomes superconducting and the superconducting properties can be controlled with the electric fields.
The discovery launched a new field of research into magic angle-oriented graphene, known as “twistronics.”
Engineers and physicists at Caltech have built upon that discovery by generating an image of the atomic structure and electronic properties of magic angle-twisted graphene, yielding new insight into the phenomenon by offering a more direct way of studying it.
"This pulls back the shroud on twistronics," says Caltech's Professor Stevan Nadj-Perge from the Division of Engineering and Applied Science.
Research on the magic angle requires an extreme level of precision to get the two sheets of graphene aligned at just the right angle.
When two layers of graphene are rotated relative to each other, electrons become localized at specific places in the crystal and give rise to a periodic height profile.
The periodicity of this so-called Moiré pattern is set by the rotation angle and at the magic angle; for this rotation angle, the correlation effects between electrons are maximized.
As Nadj-Perge and colleagues write in Nature Physics: “We use scanning tunnelling microscopy to probe the local properties of highly tunable twisted bilayer graphene devices and show that the flat bands deform when aligned with the Fermi level.”
According to the researchers, when the bands are half-filled, they observed the development of gaps originating from correlated insulating states. 
Near charge neutrality, they found a previously unidentified correlated regime featuring an enhanced splitting of the flat bands. 
"Previously, it was thought that correlation effects do not play a major role in charge neutrality," says Nadj-Perge. "Closer, more detailed examination of samples like this could help us to explain why the exotic electronic effects near the magic angle exist.”
“Once we know that, we could help pave the way for useful applications of it, perhaps even leading to room-temperature superconductivity one day," he adds.
Research is published in Nature Physics.
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