When sweeping the carrier concentration in monolayer graphene through the charge neutrality point, the experimentally measured Hall resistivity shows a smooth zero crossing. Using a two- component model of coexisting electrons and holes around the charge neutrality point, we unambiguously show that both types of carriers are simultaneously present. For high magnetic fields up to 30 T the electron and hole concentrations at the charge neutrality point increase with the degeneracy of the zero-energy Landau level which implies a quantum Hall metal state at \nu=0 made up by both electrons and holes.
@article{arxiv.1107.3929,
title = {Coexistence of electron and hole transport in graphene},
author = {S. Wiedmann and H. J. van Elferen and E. V. Kurganova and M. I. Katsnelson and A. J . M. Giesbers and A. Veligura and B. J. van Wees and R. V. Gorbachev and K. S. Novoselov and J. C. Maan and U. Zeitler},
journal= {arXiv preprint arXiv:1107.3929},
year = {2011}
}