The quantum Hall (QH) effect in two-dimensional electrons and holes in high quality graphene samples is studied in strong magnetic fields up to 45 T. QH plateaus at filling factors $\nu =0,\pm 1,\pm 4$ are discovered at magnetic fields $B>20\mathrm{T}$, indicating the lifting of the fourfold degeneracy of the previously observed QH states at $\nu =\pm 4(|n|+1/2)$, where $n$ is the Landau-level index. In particular, the presence of the $\nu =0,\pm 1$ QH plateaus indicates that the Landau level at the charge neutral Dirac point splits into four sublevels, lifting sublattice and spin degeneracy. The QH effect at $\nu =\pm 4$ is investigated in a tilted magnetic field and can be attributed to lifting of the spin degeneracy of the $n=1$ Landau level.

• Received 1 January 2006

DOI:https://doi.org/10.1103/PhysRevLett.96.136806