Ion Impact Induced Ultrafast Electron Dynamics in Correlated Materials and Finite Graphene Clusters

Abstract: 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).