  Two fermions occupying the same site of a lattice model with strongly
repulsive Hubbard-type interaction U form a doublon, a long-living excitation
the decay of which is suppressed because of energy conservation. By means of an
exact-diagonalization approach based on the Krylov-space technique, we study
the dynamics of a single doublon, of two doublons, and of a doublon in the
presence of two additional fermions prepared locally in the initial state of
the extended Hubbard model. The time dependence of the expectation value of the
double occupancy at the different sites of a large one-dimensional lattice is
analyzed by perturbative arguments. In this way the spatiotemporal evolution of
the doublon can be understood. The initial decay takes place on a short time
scale 1/U, and the long-time average of the decayed fraction of the total
double occupancy scales as 1/U^2 . We demonstrate how the dynamics of a doublon
in the initial state is related to the spectrum of two-fermion excitations
obtained from linear-response theory, we work out the difference between
doublons composed of fermions vs doublons composed of bosons, and we show that
despite the increase of phase space for inelastic decay processes, the
stability of a doublon is enhanced by the presence of additional fermions on an
intermediate time scale.
