Breakdown of the N=0 quantum Hall state in graphene: Two insulating regimes

We studied the unusual quantum Hall effect (QHE) near the charge neutrality point in high-mobility graphene sample for magnetic fields up to 18 T. We observe breakdown of the delocalized QHE transport and strong increase in resistivities ρxx, | ρxy | with decreasing Landau-level filling for ν<2, where we identify two insulating regimes. First, ρxx,xy increases nearly exponentially within the range of several resistance quanta RK, while the Hall effect gradually disappears and the off-diagonal resistivity ρxy eventually becomes independent of the direction of magnetic field, consistent with the Hall insulator with local transport. Then, at a filling ν=1/2, there is a cusp in ρxx (ν) and an onset of even faster growth with the decreasing ν, indicating transition to a collective insulator state. A likely candidate for this state is a pinned Wigner crystal.