| Bilayer graphene has attracted considerable interest due to the important role played by many-body effects, particularly at low energies. The exceptional quality of suspended devices has enabled the observation of interaction-driven broken-symmetry states and the fractional quantum Hall effect. Here we focus on local compressibility measurements of a suspended graphene bilayer. We find that the energy gaps at filling factors ν = ±4 do not vanish at low fields, but instead merge into an incompressible region near the charge neutrality point at zero electric and magnetic field. This is the first observation of a zero-field ordered state and is consistent with the formation of either an anomalous quantum Hall state or a nematic phase with broken rotational symmetry. At higher fields, we measure the intrinsic energy gaps of broken-symmetry states at ν = 0, ±1 and ±2, and find that they scale linearly with magnetic field, yet another manifestation of the strong Coulomb interactions in bilayers. |
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