Hartman effect and spin precession in graphene

Spin precession has been used to measure the transmission time \tau over a distance L in a graphene sheet. Since conduction electrons in graphene have an energy-independent velocity v, one would expect \tau > L/v. Here we calculate that \tau < L/v at the Dirac point (= charge neutrality point) in a clean graphene sheet, and we interpret this result as a manifestation of the Hartman effect (apparent superluminality) known from optics.