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Chemical Potential and Quantum Hall Ferromagnetism in Bilayer Graphene

Bilayer graphene has a unique electronic structure influenced by a complex interplay between various degrees of freedom. We probe its chemical potential using double bilayer graphene heterostructures, separated by a hexagonal boron nitride dielectric. The chemical potential has a non-linear carrier density dependence, and bears signatures of electron-electron interactions. The data allow a direct measurement of the electric field-induced bandgap at zero magnetic field, the orbital Landau level (LLs) energies, and the broken symmetry quantum Hall state gaps at high magnetic fields. We observe spin-to-valley polarized transitions for all half-filled LLs, as well as emerging phases at filling factors ν= 0 and ν= +-2. Furthermore, the data reveal interaction-driven negative compressibility and electron-hole asymmetry in N = 0, 1 LLs.