Phase Diagram of the Zero-Energy Landau-Level Octet in Bernal-Stacked Bilayer Graphene

The eight-fold degenerate zero-energy Landau levels in Bernal-stacked bilayer graphene are interesting owing to its spin, valley isospin, and orbital degrees of freedom. From prior studies, the ordering of LLs is fully known at large electric displacement D fields (Li et al., Phys. Rev. Lett. 120, 047701 (2018)). At small D-fields, the spin degeneracy is broken first, followed by the N = 0, 1 degeneracy (Huang et al., Phys. Rev. X 12, 049901 (2022)). However, this picture changes when the magnetic B field is sufficiently low. Our recent high-quality dual-gated devices enhance exchange interactions in all electronic degrees of freedom. In this work, we have systematically studied the nature of the = 1, 2, and 3 Landau levels and their associated collective excitations. We measured their energy gaps and looked for Landau-level coincidence points in a wide range of B-fields up to 31 T and D-fields down to 0. We construct a new, complete (B, D) phase diagram for the E = 0 octet and discuss its implications. We will also address the open question of whether valley skyrmions exist at = 1 and 2.