Landau-level splitting in graphene in high magnetic fields

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,+/- 1,+/- 4 are discovered at magnetic fields B > 20 T, indicating the lifting of the fourfold degeneracy of the previously observed QH states at nu=+/- 4(parallel to n parallel to+1/2), where n is the Landau-level index. In particular, the presence of the nu=0,+/- 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=+/- 4 is investigated in a tilted magnetic field and can be attributed to lifting of the spin degeneracy of the n=1 Landau level.