Breakdown of the N=0 Quantum Hall State in graphene: two insulating regimes

Abstract

We studied the unusual Quantum Hall Effect (QHE) near the charge neutrality point (CNP) 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 $\rho_{xx},|\rho_{xy}|$ with decreasing Landau level filling for $\nu < 2$, where we identify two insulating regimes. For $1 \gtrsim |\nu| \gtrsim 1/2$ we find an exponential increase of $\rho_{xx,xy} \sim e^{a(H-H_c)}$ within the range up to several resistance quanta $R_K$, while the Hall effect gradually disappears, consistent with the Hall insulator (HI) with local transport. Then, at $\nu \approx 1/2$ a cusp in $\rho_{xx}(H)$ followed by an onset of even faster growth indicates transition to a collective insulator (CI) state. The likely candidate for this state is a pinned Wigner crystal.