The origin of electron-hole asymmetry in graphite
  The electron hole asymmetry has been measured in natural graphite using magneto-optical absorption measurements. A splitting is observed for the transitions at both the $K$-point and the $H$-point of the Brillouin zone of graphite where the effect of trigonal warping vanishes. This result is fully consistent with the SWM Hamiltonian providing the free electron kinetic energy terms are taken into account. An identical electron-hole asymmetry should be present in graphene. 
P. Plochocka, P. Y. Solane, R. J. Nicholas, J. M. Schneider, B. A.   Piot, D. K. Maude, O. Portugall, and G. L. J. A. Rikken
cond-mat.mes-hall