Coexistence of electron and hole transport in graphene
  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. 
S. Wiedmann, H. J. van Elferen, E. V. Kurganova, M. I. Katsnelson, A.   J .M. Giesbers, A. Veligura, B. J. van Wees, R. V. G...
cond-mat.mes-hall