Modeling time-dependent current through electronic open channels using a mixed time-frequency solution to the electronic equations of motion

A nonequilibrium Green's function model based on time-dependent perturbation theory is developed to propagate electronic structure and molecular conductance of extended electrode-molecule-electrode nanostructures. In this model, we use the two-time variable nature of the Kadanoff-Baym equations of motion to formulate a mixed time-frequency representation for the electronic density expressed by the appropriate Green's function (G(<)). This allows for the dynamical treatment of open systems. Furthermore, highly informative time-dependent Wigner distributions are used to shed light on the features of dynamical observables, such as electron current. Calculations, performed on model systems, resolve the dynamic current into direct and alternating components. The direct current is due to electronic open channels near the Fermi level and the alternating response is due to interference fringes from a superposition of extended states. We analyze the transient conductance with respect to the fundamental system's parameters, the effect of bound states, and the conductance driven by laser-induced coherence affected by detuning due to an applied dc bias. The amplitude of the alternating transient current can be adjusted by reshaping the bias pulse or by controlling the electronic coupling terms. Bound states may yield a persisting oscillating response depending on their relative electronic densities. In the analysis we utilize the calculated highly informative time-dependent current distributions.