Quantum-Hall activation gaps in graphene

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