  We report electron-hole conduction asymmetry in monolayer graphene.
Previously, it has been claimed that electron-hole conduction asymmetry is due
to imbalanced carrier injection from metallic electrodes. Here, we show that
metallic contacts have negligible impact on asymmetric conduction and may be
either sample or device-dependent phenomena. Electrical measurements show that
monolayer graphene based devices exhibit suppressed electron conduction
compared to hole conduction due to the presence of donor impurities which
scatter electrons more efficiently. This can be explained by the relativistic
nature of charge carriers in a graphene monolayer and can be reconciled with
the fact that in a relativistic quantum system transport cross section does
depend on the sign of scattering potential in contrast to a nonrelativistic
quantum system.
