▎ 摘　　要

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.