Non-equilibrium Green’s function transport theory for molecular junctions with general molecule-lead coupling and temperatures

In quantum transport across molecular junctions, time-dependent effects arise mainly due to interactions with external perturbations such as pulsed laser fields or fluctuating environments. While the calculation of the charge dynamics in such an open quantum system is a complex problem, it is highly relevant for engineering nanoscale devices. Several theoretical approaches to this problem including some based on quantum master equations, hierarchical schemes, or non-equilibrium Green's functions (NEGFs) rely on assuming a molecule-lead coupling composed of Lorentzian functions and a decomposition of the Fermi function. The underlying assumptions in this strategy lead to limitations in the functional form of the molecule-lead coupling and to an inefficiency at low temperatures. To overcome these restrictions, a Chebyshev expansion is applied to derive a set of coupled ordinary differential equations within the NEGF formalism. The numerical efficiency of this approach is independent of temperature and band structure of the electrodes. Moreover, since the scheme deals with a single particle basis set, it is possible to replace all auxiliary matrices present in the equations by vectors. Test setups for this new scheme include molecular junctions under the influence of strong time-dependent perturbations leading, for example, to coherent destruction of tunneling.