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 ν=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 ν=±4(|n|+1/2), where n is the Landau-level index. In particular, the presence of the ν=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 ν=±4 is investigated in a tilted magnetic field and can be attributed to lifting of the spin degeneracy of the $n=1$ Landau level.