Non-equilibrium steady state and current saturation of Dirac electrons in graphene

We investigate the strong-field transport phenomena in graphene, particularly concentrating on the case when the system is at the charge-neutral point. The non-equilibrium momentum distribution of electrons is shown for both samples which are on/off the Dirac point. It is revealed that the system away from the Dirac point shows current saturation due to optical phonon interaction, whereas the current does not saturate in the system on the Dirac point. On the contrary, the graphene sample on the Dirac point shows a crossover from “super-linear” $I \sim V^1.5$ behavior to an approximately linear behavior as optical phonon interaction is turned on. We also unveil the different behaviors of effective temperature while current saturation occurs.