Spectroscopic evidence for Landau degeneracy and massive quasiparticles in magic-angle twisted bilayer graphene

Transport, scanning tunneling microscopy/spectroscopy and compressibility measurements in magic-angle twisted bilayer graphene (MATBG) have revealed the emergence of strongly correlated quantum phases as the flat band is being filled. The appearance of these correlated phases is attributed to the interplay between the strong Coulomb interactions and the nontrivial narrow band topology. In the strong coupling limit, where the Coulomb interaction dominates, theoretical predictions of a parabolic band minimum at the center of the moire Brillouin zone, led to the expectation of massive quasiparticles at every integer filling. ^1,2 Experimentally however, reports of massive quasiparticle excitations have been scant.³ Here we report on planar tunneling spectroscopy measurements of the differential conductance in the presence of a small magnetic field, which provides access to the Landau level sequence, indicating a zero Berry phase, in MATBG. At the charge neutrality point we find nearly equally spaced Landau levels whose energy extrapolates to zero in vanishing magnetic field. These finding demonstrate the existence of massive quasiparticle excitations at the charge neutrality point with mass ~0.2m_e.



1 Kang, J., Bernevig, B. A. & Vafek, O. Physical Review Letters 127, 266402, doi:10.1103/PhysRevLett.127.266402 (2021).

2 Song, Z.-D. & Bernevig, B. A. Physical Review Letters 129, 047601, doi:10.1103/PhysRevLett.129.047601 (2022).

3 Wu, S., Zhang, Z., Watanabe, K., Taniguchi, T. & Andrei, E. Y. Nature Materials 20, 488-494, doi:10.1038/s41563-020-00911-2 (2021).

Work supported by DOE-FG02-99ER45742 and the Gordon and Betty Moore Foundation GBMF9453