  Low-energy properties of the one-dimensional Kondo lattice model are
investigated by using bosonization and renormalization group methods. The
formation of the spin and charge excitation gaps at half-filling is discussed
in detail both for antiferromagnetic and ferromagnetic Kondo couplings. Also,
the effect of the interaction between conduction electrons on the gap formation
is studied both for the Kondo insulating phase and for the Haldane gap phase.
It is found that while the repulsive interaction changes the dependence of the
excitation gap on the Kondo exchange coupling, the attractive interaction can
drive the Kondo (or Haldane) insulating phase to a completely different phase
of massless metallic states.
