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A sheet of magic-angle twisted bilayer graphene was found to be capable of hosting new topological phases of matter.

First discovered in 2018, magic-angle twisted bilayer graphene (MATBG) is made when two layers of graphene - a special form of solid carbon whose atoms are arranged in a honeycomb-shaped lattice structure - are placed on top of one another and twisted at precisely 1.05 degrees from each other. As a result, MABTG has unique electronic properties, such as acting as a semiconductor material whose ability to insulate or conduct depends on the number of electrons added to the surface.

However, theoretical studies postulating its existence have been published since 2007. A study published in APS Physics has theoretically calculated the electronic structure of a "graphene bilayer with a relatively small angle rotation between the layers."

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Twistronics and Quantum Phases

The discovery of the new material led to an increase in the material's studies, creating a new field of study called "twistronics." At the California Insitute of Technology (Caltech), one such researcher into the field is Stevan Nadj-Perge, assistant professor of applied physics and materials science. He was part of the team that conducted direct imaging of electronic properties of magic-angle twisted bilayer graphene at the atomic level in 2019. In the following year, their team demonstrated superconductivity in MABTG can exist away from the magic angle, with help from a two-dimensional superconductor.

Now, the Caltech team has found something new in magic-angle twisted bilayer graphene: unexpected topological quantum phases, reporting their findings in the journal Nature. Traditionally, materials can be classified as insulators that do not conduct electricity, metals that conduct well, and semiconductors that allow electrons to pass through at capacities between the first two.

Additionally, strong magnetic fields applied to different materials causes electrons to act differently, producing other new and unique conditions - and topological quantum phases. For example, the bulk of material could become an insulator while its surfaces (for 3D) or edges (for 2D materials) remain highly conductive. Similar quantum phenomena from these topological phases have applications, such as in the development of quantum information processing.

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Finding Quantum Phases in MATBGs

Nadj-Perge and his colleagues conducted scanning tunneling microscopy to obtain direct imaging on the MATBG at the atomic scale in their new study. They found that a strong interaction between electrons in the twisted bilayer graphene allows for the quantum topological phases' appearance, even without a strong magnetic field. Researchers also observed graphene layers twisted at other angles, only to confirm that the quantum topological phases only appear when twisting at the magic angle or "for a small range of twist angles" around it.

"The discovery of topological phases in magic-angle twisted bilayer graphene opens up yet another chapter about this amazing material and brings us closer to understanding its electronic properties," said Nadj-Perge. He adds that the findings could lead to engineering topological phases in the future.

 

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