Unified theory of double-time quantum transport and quantum diffusion

Based on the nonequilibrium Green function technique, a unified theory of quantum transport and quantum diffusion is developed from quantum-kinetic equations of the full carrier distribution function f(<) (k, kappa vertical bar T, t). The rigorous two-time approach is equally applicable to quantum transport as well as quantum diffusion that proceeds either via extended states or by hopping between localized carriers. The approach focuses on the entanglement between the microscopic and macroscopic time regimes and on the related non-Markovian evolution in both time channels. For a one-dimensional superlattice model subject to quantizing dc and ac electric fields, both phononless transport and diffusion mechanisms are identified that have a strict double-time origin. The experimental verification of this intricate quantum effect seems to be feasible by studying quantum-box superlattices at low temperatures.