Observation of Reentrant Correlated Insulators and Interaction-Driven Fermi-Surface Reconstructions at One Magnetic Flux Quantum per Moiré Unit Cell in Magic-Angle Twisted Bilayer Graphene

Abstract

The discovery of flat bands with non-trivial band topology in magic angle twisted bilayer graphene (MATBG) has provided a unique platform to study strongly correlated phenomena including superconductivity, correlated insulators, Chern insulators and magnetism. A funda mental feature of the MATBG, so far unexplored, is its high magnetic field Hofstadter spectrum. Here we report on a detailed magneto-transport study of a MATBG device in external magnetic fields of up to B = 31 T, corresponding to one magnetic flux quantum per moiré unit cell Φ0. At Φ0, we observe a re-entrant correlated insulator at a flat band filling factor of ν = +2, and inter action-driven Fermi surface reconstructions at other fillings, which are identified by new sets of Landau levels originating from these. These experimental observations are supplemented by the oretical work that predicts a new set of 8 well-isolated flat bands at Φ0 , of comparable band width but with different topology than in zero field. Overall, our magneto-transport data reveals a qual itatively new Hofstadter spectrum in MATBG, which arises due to the strong electronic correla tions in the re-entrant flat bands.