Spin and Valley Splitting of Two-Dimensional Electrons in Graphene in the High Magnetic Field Limit

The quantum Hall effect in high quality graphene, a sinlge atomic layer of graphite, is studied in strong magnetic fields up to 45 Tesla. The splitting of Landau levels $n$ = 0 and $\pm $1, caused by the lifting of the spin and valley degeneracies in strong magnetic fields, is observed at T $<$ 5 K. In particular, the quantum Hall state $\nu =\pm $4 is found to arise from the spin splitting of Landau level n = $\pm $1. The effective Lande g-factor measured at this state is close to 2. The spin origin of $\nu =\pm $4 is further confirmed in magnetotransport experiments performed in the presence of an in-plane magnetic field. While the exact origin of the valley degeneracy lifting is not yet clear, we propose several possibilities.