18 January 2023

The Magic Angle

Graphene is an uncommonly versatile material. This versatility is based in part on the unusual properties of its components. As an isolated monolayer, its low-energy electrons behave as massless Dirac fermions and move with extremely high velocity. But when two graphene sheets are twisted atop one another at a ‘magic angle’ of 1.1°, the electrons slow to a relative crawl and behave collectively. The twisted layers exhibit superconductivity and a stunning array of other correlated states, caused by strongly interacting electrons. Effects like these have made graphene one of the most intriguing materials in condensed-matter physics.

© D. Efetov

Mechanically, it is possible to further improve the layer system by stacking extra monolayers of graphene with alternating twist angles between the layers. However, an additional energy band conceals the electron movement. This has prevented researchers before now from studying the strongly correlated states. In a novel spectroscopic approach, LMU physicists led by Dmitri Efetov have probed the strongly correlated states of trilayer graphene twisted at the magic angle of 1.5°. To do this, they exploited the overlapping of electronic bands. By studying the exact intersection points between the bands, they were able to identify the topological characteristics of the correlated states and determine their energy gaps in detail. The findings shed a whole new light on the underlying properties of twisted graphene systems.

Source: LMU Website

Publication

Dirac spectroscopy of strongly correlatedphases in twisted trilayer graphene.
C.Shen, P. J. Ledwith, K. Watanabe, T. Taniguchi, E. Khalaf, A. Vishwanath & D. K. Efetov.
Natural Materials (2022)
DOI: doi.org/10.1038/s41563-022-01428-6

The results have been featured in an News&Views article on Nature Materials.