Unsaturated Drift Velocity of Monolayer Graphene.

We observe that carriers in graphene can be accelerated to the Fermi velocity without heating the lattice. At large Fermi energy | E _F | > 110 meV, electrons excited by a high-power terahertz pulse E _THz relax by emitting optical phonons, resulting in heating of the graphene lattice and optical-phonon generation. This is owing to enhanced electron-phonon scattering at large Fermi energy, at which the large phase space is available for hot electrons. The emitted optical phonons cause carrier scattering, reducing the drift velocity or carrier mobility. However, for | E _F | ≤ 110 meV, electron-phonon scattering rate is suppressed owing to the diminishing density of states near the Dirac point. Therefore, E _THz continues to accelerate carriers without them losing energy to optical phonons, allowing the carriers to travel at the Fermi velocity. The exotic carrier dynamics does not result from the massless nature, but the electron-optical-phonon scattering rate depends on Fermi level in the graphene. Our observations provide insight into the application of graphene for high-speed electronics without degrading carrier mobility.