  We present a calculation of the effective geometry-induced quantum potential
for the carriers in graphene shaped as a helicoidal nanoribbon. In this
geometry the twist of the nanoribbon plays the role of an effective transverse
electric field in graphene and this is reminiscent of the Hall effect. However,
this effective electric field has a different sign for the two iso-spin states
and translates into a mechanism to separate the two chiral species on the
opposing rims of the nanoribbon. Iso-spin transitions are expected with the
emission or absorption of microwave radiation which could be adjusted to be in
the THz region.
