  Dynamical mean-field theory (DMFT) studies frequently observe a fine
structure in the local spectral function of the SU(2) Fermi-Hubbard model at
half filling: In the metallic phase close to the Mott transition, subpeaks
emerge at the inner edges of the Hubbard bands. Here we demonstrate that these
subpeaks originate from the low-energy effective interaction of doublon-holon
pairs, by investigating how the correlation functions of doublon and holon
operators contribute to the subpeaks. A mean-field analysis of the low-energy
effective Hamiltonian provides results consistent with our DMFT calculation
using the numerical renormalization group as an impurity solver. In the SU(3)
and SU(4) Hubbard models, the subpeaks become more pronounced due to the
increased degeneracy of doublon-holon pair excitations.
