Quantum-hall activation gaps in graphene

We have measured the quantum-Hall activation gaps in graphene at filling factors ν=2 and ν=6 for magnetic fields up to 32 T and temperatures from 4 to 300 K. The ν=6 gap can be described by thermal excitation to broadened Landau levels with a width of 400 K. In contrast, the gap measured at ν=2 is strongly temperature and field dependent and approaches the expected value for sharp Landau levels for fields B>20T and temperatures T>100K. We explain this surprising behavior by a narrowing of the lowest Landau level.