  Heavy-fermion or Kondo lattice materials are considered to be typical
strongly correlated systems, for which mean-field approximations have shown
that the Coulomb interaction increases the effective mass and narrows the band
gap. In this paper, to clarify interaction effects on the electronic
excitation, the spectral function of the Kondo lattice and periodic Anderson
models is studied around Kondo insulators in the strong-coupling regime, by
using the non-Abelian dynamical density-matrix renormalization group method and
perturbation theory. Upon doping a Kondo insulator, an electronic mode emerges
in the Kondo insulating gap, exhibiting the momentum-shifted magnetic
dispersion relation. Although the ground-state properties are similar to those
of a doped band insulator, the emergence of the electronic mode reflecting
spin-charge separation of the Kondo insulator is a crucial interaction effect
that allows us to regard the Kondo-insulator--metal transition as a type of
Mott transition. In addition, electronic modes emerge even in the high-energy
regime by doping in the periodic Anderson model. These strong-correlation
effects have not been expected in conventional mean-field approximations and
would bring a different perspective on heavy-fermion or Kondo lattice systems.
