10 April 2024

Twisted bilayer graphene’s gallery of phases

A joint paper by Dmitri Efetov, MCQST member and Chair of Solid Stated Physics at LMU München and Andrei Bernevig of Princeton University on the many phases of twisted bilayer graphene has been featured in an overview article in Physics Today.

Twisted bilayer graphene’s gallery of phases © C. Hohmann / MCQST
In a groundbreaking discovery that shook the world of condensed-matter physics in 2018, researchers found that a simple twist between two sheets of carbon crystals could unlock a host of extraordinary properties. This twist, known as the magic angle, led to the emergence of superconductivity and other intriguing phenomena. What's remarkable is that this system, dubbed magic-angle twisted bilayer graphene (MATBG), consists solely of light and basic carbon atoms, unlike traditional strongly correlated materials.

Traditionally, the electronic properties of a material are the result of its atomic composition and the arrangement of its atoms. So-called strongly correlated materials are one of the richest and yet least understood material classes. Their electrons do not behave as individual particles but are highly entangled with one another. Such compounds include, for example, heavy-fermion systems and cuprate high-temperature superconductors that exhibit a rich phase diagram governed by a mysterious interplay between magnetic and superconducting phases, in which current flows without resistance. Typically, strongly correlated materials are complex, multicomponent systems that often contain large atoms with f- and d-shell electrons.

The observation of strongly correlated electrons and multiple many-body ground states in magic-angle twisted bilayer graphene (MATBG) devices shocked and excited the physics community. The surprise lies in the fact that it was achieved by an entirely novel and previously unthinkable approach and in the unlikeliest of materials. In stark contrast to traditional strongly correlated systems, MATBG consists entirely of light and simple carbon atoms, and its building blocks, the single-layer graphene sheets, show no signs of strongly correlated electron effects. Those properties can, however, be turned on by a conceptually simple trick—stacking two graphene sheets, one on top of another, and twisting them. Just like when a combination lock is opened by turning to the right sequence of symbols, strong electronic correlations are unlocked when the twist angle between the layers is set to a well-defined value, 1.1°, the so-called magic angle.

Read the full article in Physics Today.


Twisted bilayer graphene’s gallery of phases.
B. Andrei Bernevig; Dmitri K. Efetov.
Physics Today 77 (4), 38–44 (2024)
DOI: doi.org/10.1063/pt.jvsd.yhyd

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