An August 5 Nature Physics paper describes work by the Nadj-Perge group and IQIM theorists, Electronic correlations in twisted bilayer graphene near the magic angle.

A scanning tunneling microscopy topographic image of twisted bilayer graphene

A scanning tunneling microscopy topographic image of twisted bilayer graphene, Credit S. Nadj-Perge

“Twisted bilayer graphene with a twist angle of around 1.1° features a pair of isolated flat electronic bands and forms a platform for investigating strongly correlated electrons. Here, 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. When the bands are half-filled, we observe the development of gaps originating from correlated insulating states. Near charge neutrality, we find a previously unidentified correlated regime featuring an enhanced splitting of the flat bands. We describe this within a microscopic model that predicts a strong tendency towards nematic ordering. Our results provide insights into symmetry-breaking correlation effects and highlight the importance of electronic interactions for all filling fractions in twisted bilayer graphene.”

Read more about the paper in the August 9 Caltech media story, Finding the Magic in the Magic Angle“Previously, it was thought that correlation effects do not play a major role in charge neutrality,” Nadj-Perge says. “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.”

This work is also featured in a “News and Views” commentary, Twisted Bilayer Graphene: A needle in a moiré stack: “Over a decade after its discovery, it seemed as if graphene had already revealed its magic. Then last year, much to everyone’s surprise, experiments demonstrated that giving two layers of graphene just the right small twist will give rise to correlated electronic phases (an insulator1 or a superconductor2) that do not exist when the twist is not there. Writing in Nature Physics, Choi and colleagues utilize a local probe to provide more clues into the correlated states in twisted bilayer graphene close to the magic angle, as they tune the Fermi level of the system3.”

Read the full paper: Youngjoon Choi, Jeannette Kemmer, Yang Peng, Alex Thomson, Harpreet Arora, Robert Polski, Yiran Zhang, Hechen Ren, Jason Alicea, Gil Refael, Felix von Oppen, Kenji Watanabe, Takashi Taniguchi, Stevan Nadj-Perge (2019) Electronic correlations in twisted bilayer graphene near the magic angle. Nature Physics