  Strongly correlated systems of fermions have an interesting phase diagram
arising from the Hubbard gap. Excitation across the gap leads to the formation
of doubly occupied lattice sites (doublons). This state offers interesting
electronic and optical properties. Moreover, when the system is driven out of
equilibrium interesting collective dynamics may arise that are related to the
spatial propagation of doublons. Here, a novel mechanism that was recently
proposed by us [Balzer \textit{et al.}, submitted for publication] is verified
by exact diagonalization and nonequilibrium Green functions (NEGF)
simulations---fermionic doublon creation by the impact of energetic ions. We
report the formation of a nonequilibrium steady state with homogeneous doublon
distribution. A physically intuitive picture is given in terms of an analytical
model for a two-site system where the doublon formation is explained in terms
of a two-fold passage of an avoided crossing (Landau-Zener picture). The effect
should be particularly important for strongly correlated finite systems, such
as graphene nanoribbons, and directly observable with fermionic atoms in
optical lattices. We demonstrate that doublon formation and propagation in
correlated lattice systems can be accurately simulated with NEGF. In addition
to two-time results we present single-time results within the generalized
Kadanoff-Baym ansatz (GKBA) with Hartree-Fock propagators (HF-GKBA), and we
present systematic improvements that use correlated propagators (correlated
GKBA).
