Quantum twisting microscopy of phonons in twisted bilayer graphene

• Published in: Nature (London) Vol. 641; no. 8062; pp. 345 - 351
• Main Authors: Birkbeck, J., Xiao, J., Inbar, A., Taniguchi, T., Watanabe, K., Berg, E., Glazman, L., Guinea, F., von Oppen, F., Ilani, S.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 08.05.2025; Nature Publishing Group
• Subjects: 639/301/357/918; 639/766/119/995; 639/766/930/328/968; Acoustics; Bias; Bilayers; Coupling; Dispersions; Electrons; Energy; Graphene; Graphite; Humanities and Social Sciences; Interfaces; Interlayers; Magnons; Microscopy; Momentum; multidisciplinary; Phonons; Plasmons; Science; Science (multidisciplinary); Spectrum analysis; Superconductivity; Thermal conductivity; Twisting
• DOI: 10.1038/s41586-025-08881-8
• ISSN: 0028-0836; 1476-4687

The coupling between electrons and phonons is one of the fundamental interactions in solids, underpinning a wide range of phenomena, such as resistivity, heat conductivity and superconductivity. However, direct measurements of this coupling for individual phonon modes remain a substantial challenge. In this work, we introduce a new technique for mapping phonon dispersions and electron–phonon coupling (EPC) in van der Waals (vdW) materials. By generalizing the quantum twisting microscope 1 (QTM) to cryogenic temperatures, we demonstrate its capability to map not only electronic dispersions through elastic momentum-conserving tunnelling but also phononic dispersions through inelastic momentum-conserving tunnelling. Crucially, the inelastic tunnelling strength provides a direct and quantitative measure of the momentum and mode-resolved EPC. We use this technique to measure the phonon spectrum and EPC of twisted bilayer graphene (TBG) with twist angles larger than 6°. Notably, we find that, unlike standard acoustic phonons, whose coupling to electrons diminishes as their momentum tends to zero, TBG exhibits a low-energy mode whose coupling increases with decreasing twist angle. We show that this unusual coupling arises from the modulation of the interlayer tunnelling by a layer-antisymmetric ‘phason’ mode of the moiré system. The technique demonstrated here opens the way for examining a large variety of other neutral collective modes that couple to electronic tunnelling, including plasmons 2 , magnons 3 and spinons 4 in quantum materials. Generalization of a quantum twisting microscope to cryogenic temperatures in twisted bilayer graphene shows the ability to map phononic dispersions through inelastic momentum-conserving tunnelling and reveals an angle-dependent coupling between electrons and phonons.