ABSTRACT
In the previous chapters, our consideration has been based on a
model of noninteracting chiral fermions with a one-half pseudospin
and the absence of an energy gap in graphene. Strictly speaking,
the validity of the model is limited within the conic approximation.
Since the real electron dispersion always deviates from linear
form, the electron-hole binding becomes possible. In particular,
the trigonal warping of the electron energy spectrum imposes
additional selection rules for the electron-electron scattering in
the neutrality point. Such selection rules represent the conditions
for the electron-hole binding in graphene. Thus, the appearance of
excitons actually is related with deviation of the electron dispersion
from the conic shape.
