Landau level degeneracy lifting by the emergence of massive quasiparticles in magic-angle twisted bilayer graphene*

In twisted bilayer graphene, when reducing the twist angle towards the magic angle, ~1.1°, a transition from 8-fold to 4-fold band degeneracy is observed by quantum oscillations in perpendicular magnetic fields exceeding several Tesla. Low-field quantum oscillations can provide direct insights into the origin of the lifted degeneracy. Here we report on magneto-transport measurements of a multi-terminal twisted bilayer graphene device with a twist angle of 1.17°±0.02° .¹ We find that even at very low magnetic fields, B=0.4T, the Landau sequence already exhibits 4-fold degeneracy. Furthermore, Shubnikov-de Haas oscillation on the Landau fan emanating from the charge neutrality point (CNP), reveal a zero Berry phase, contrary to the π/2 phase expected for massless Dirac fermions. This shows that the reduced degeneracy is a consequence of emergent interaction-induced massive quasi-particles, in contrast to non-interacting band structure calculations that find Dirac cones at the CNP. Thus,  a correct description of the system must include correlations effects even at the lowest fields.