Dirac revivals drive a resonance response in twisted bilayer graphene

• Published in: Nature physics 2023-08, Vol.19 (8), p.1156-1162
• Main Authors: Morissette, Erin, Lin, Jiang-Xiazi, Sun, Dihao, Zhang, Liangji, Liu, Song, Rhodes, Daniel, Watanabe, Kenji, Taniguchi, Takashi, Hone, James, Pollanen, Johannes, Scheurer, Mathias S., Lilly, Michael, Mounce, Andrew, Li, J. I. A.
• Format: Article
• Language: English
• Subjects: 639/766/119/1003; 639/766/119/995; 639/766/119/997; Atomic; Bilayers; Classical and Continuum Physics; Complex Systems; Condensed Matter Physics; Couplings; Electron paramagnetic resonance; Electron spin; Electrons; Energy bands; Excitation; Flavors; Graphene; Mathematical and Computational Physics; Microwave resonance; Molecular; Optical and Plasma Physics; Phase transitions; Physics; Physics and Astronomy; Resonance; Spin resonance; Theoretical
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/s41567-023-02060-0

Collective excitations contain key information regarding the electronic order of the ground state of strongly correlated systems. Various collective modes in the spin and valley isospin channels of magic-angle graphene moiré bands have been alluded to by a series of recent experiments. However, a direct observation of collective excitations has been impossible due to the lack of a spin probe. Here we observe low-energy collective excitations in twisted bilayer graphene near the magic angle, using a resistively detected electron spin resonance technique. Two independent observations show that the generation and detection of microwave resonance relies on the strong correlations within the flat moiré energy band. First, the onset of the resonance response coincides with the spontaneous flavour polarization at moiré half-filling, but is absent in the isospin unpolarized density range. Second, we perform the same measurement on various systems that do not have flat bands and observe no indication of a resonance response in these samples. Our explanation is that the resonance response near the magic angle originates from Dirac revivals and the resulting isospin order. Phase transitions during which electrons recover their Dirac nature are shown to produce a spin resonance response that allows the characterization of spin and valley couplings in twisted bilayer graphene.