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 $\ensuremath{\nu}=0,\ifmmode\pm\else\textpm\fi{}1,\ifmmode\pm\else\textpm\fi{}4$ are discovered at magnetic fields $B>20\text{ }\text{ }\mathrm{T}$, indicating the lifting of the fourfold degeneracy of the previously observed QH states at $\ensuremath{\nu}=\ifmmode\pm\else\textpm\fi{}4(|n|+1/2)$, where $n$ is the Landau-level index. In particular, the presence of the $\ensuremath{\nu}=0,\ifmmode\pm\else\textpm\fi{}1$ QH plateaus indicates that the Landau level at the charge neutral Dirac point splits into four sublevels, lifting…