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 rho(xx),vertical bar rho(xy)vertical bar with decreasing Landau-level filling for nu < 2, where we identify two insulating regimes. First, rho(xx,xy) increases nearly exponentially within the range of several resistance quanta R-K, while the Hall effect gradually disappears and the off-diagonal resistivity rho(xy) eventually becomes independent of the direction of magnetic field, consistent with the Hall insulator with local transport. Then, at a filling nu approximate to 1/2, there is a cusp in rho(xx)(nu) and an onset of even faster growth with the decreasing nu, indicating transition to a collective insulator state. A likely candidate for this state is a pinned Wigner crystal.