Valley separation in graphene by polarized light
  We show that the optical excitation of graphene with polarized light leads to the pure valley current where carriers in the valleys counterflow. The current in each valley originates from asymmetry of optical transitions and electron scattering by impurities owing to the warping of electron energy spectrum. The valley current has strong polarization dependence, its direction is opposite for normally incident beams of orthogonal linear polarizations. In undoped graphene on a substrate with high susceptibility, electron-electron scattering leads to an additional contribution to the valley current that can dominate. 
L.E. Golub, S.A. Tarasenko, M.V. Entin, and L.I. Magarill
cond-mat.mes-hall cond-mat.dis-nn cond-mat.mtrl-sci