▎ 摘　　要

We study the time evolution of the electric current in ballistic graphene, after switching on an electric field. After a short transient, determined by high energy states, the current reaches an intermediate-time plateau. The "dc conductivity" coincides with the ac conductivity of graphene in this regime. For longer times, it increases further linearly with time and nonlinearly with the electric field as E(3/2). To test the predictions based on the Dirac equation, we simulate numerically the time evolution of the current on the honeycomb lattice. Our analytical results agree with numerics for both the intraband and interband contribution to the current. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim