Is it weird for graphene, the group velocity and momentum

[PRB147]

everyone knows, there exists the relation between the group velocity and energy dispersion.
a question is how to expression the relation between the velocity and momentum k?
it seems that the Dirac electron in graphene is massless.

 

[DrDu]

it seems that the Dirac electron in graphene is massless.

Yes, the effective mass is zero. That's one of the reasons for the hpye about graphene.