  We present a theory for the single particle excitations and Fermi surface of
the Kondo lattice. Thereby we construct an effective Hamiltonian describing the
creation and propagation of single particle-like charge fluctuations on an
`RVB-background' of local singlets. The theory may be viewed as a `Fermionic
version' of linear spin wave theory and is of comparable simplicity, so that
the calculations for the strong-coupling limit can be performed analytically.
We calculate the single particle spectral function for the standard Kondo
lattice as well as for several extended versions: with a Coulomb repulsion
between conduction and f-electrons, Coulomb repulsion between conduction
electrons, and a `breathing' f-orbital. In all cases we study the evolution of
the spectrum in going from the Kondo insulator to the Heavy electron metal. We
compare our results to exact diagonalization of small clusters and find
remarkable agreement in nearly all cases studied. In the metallic case the
f-electrons participate in the Fermi surface volume even if they are replaced
by localized Kondo-spins and the number of bands, their dispersion and spectral
character, and the nontrivial (i.e. non-rigid band-like) doping dependence
including a pronounced transfer of spectral weight are reproduced by the
theory.
