Transient regime in nonlinear transport through many-level quantum dots

We investigate nonstationary electronic transport in noninteracting nanostructures driven by a finite bias and time-dependent signals applied at their contacts to the leads. The systems are modeled by a tight-binding Hamiltonian, and the transient currents are computed from the nonequilibrium Green-Keldysh formalism. The numerical implementation is not restricted to weak coupling to the leads and does not imply the wideband limit assumption for the spectral width of the leads. As an application of the method we study in detail the transient behavior and the charge dynamics in single and double quantum dots connected to leads by a steplike potential, but the method allows as well consideration of nonperiodic potentials or short pulses. We show that when the higher-energy levels of the isolated system are located within the bias window of the leads, the transient current approaches the steady state in a nonoscillatory smooth fashion. At moderate coupling to the leads and fixed bias the transient acquires a steplike structure, the length of the steps increasing with system size. The number of levels inside a finite bias window can be tuned by a constant gate potential. We find also that the transient behavior depends on the specific way of coupling the leads to the mesoscopic system.