This paper examines the phonon-induced electron decoherence in semiconductor nanostructures and nanodevices within the Wigner transport equation, solved using a particle Monte Carlo technique. The Wigner-Boltzmann formalism is first established as a relevant and original approach to modeling electron quantum decoherence in semiconductors. The simulation of the time evolution of a free wave-packet then allows analyzing the competition between decoherence and wave-packet expansion in a semiconductor. It is additionally argued that decoherence occurs faster than in the widely studied case of quantum Brownian motion. The simulation of a wave packet interacting with a tunnel barrier allows studying the electron localization induced by decoherence. The case of a wave packet interacting with a double barrier puts forward the mechanism of decoherence-induced transition from resonant to sequential transport through a bound state. Finally the simulation of resonant tunneling devices shows how these phenomena take place in nanodevices and highlights the transition from the quantum transport regime to the semiclassical transport regime induced by phonon scattering.
This paper examines the phonon-induced electron decoherence in semiconductor nanostructures and nanodevices within the Wigner transport equation, solved using a particle Monte Carlo technique. The Wigner-Boltzmann formalism is first established as a relevant and original approach to modeling electron quantum decoherence in semiconductors. The simulation of the time evolution of a free wave-packet then allows analyzing the competition between decoherence and wave-packet expansion in a semiconductor. It is additionally argued that decoherence occurs faster than in the widely studied case of quantum Brownian motion. The simulation of a wave packet interacting with a tunnel barrier allows studying the electron localization induced by decoherence. The case of a wave packet interacting with a double barrier puts forward the mechanism of decoherence-induced transition from resonant to sequential transport through a bound state. Finally the simulation of resonant tunneling devices shows how these phenomena take place in nanodevices and highlights the transition from the quantum transport regime to the semiclassical transport regime induced by phonon scattering.
We have modeled the transmission of an electron wave packet through a resonant tunneling semiconductor nanostructure by solving the time-dependent Schr\"odinger equation using the finite-difference method. We have found in all cases that the passage of the electron wave packet through the tunneling barrier is accompanied by a propagation delay relative to the propagation of an undisturbed wave packet. Tunneling transport is shown to be causal, and no evidence of superluminal behavior is seen, either for resonant or for off-resonant tunneling. In the case of off-resonant tunneling, the peak of the transmitted wave packet is observed to exit a double resonant tunneling barrier before the peak of the incident wave packet enters the structure. However, these two peaks are not directly related, and the appearance of a well-formed peak in the transmitted intensity is shown to be a result of the transient behavior of the tunneling event.
We study electron-phonon interaction induced decoherence between two-electron singlet and triplet states in a semiconductor double quantum dot using a spin-boson model. We investigate the onset and time evolution of this dephasing, and study its dependence on quantum dot parameters such as dot size and double dot separations, as well as the host materials (GaAs and Si). We find that electron-phonon interaction causes an initial Gaussian decay of the off-diagonal density matrix element in the singlet-triplet Hilbert space, and a long-time exponential decay originating from phonon relaxation.
The Wigner formulation of quantum mechanics based on the concept of Wigner function (WF), is particularly suitable for the study of quantum transport in mesoscopic systems since it allows to describe quantum mechanical effects in an (r,p) phase space, in analogy to what is done for classical systems. Recently we have introduced the concept of Wigner paths (WPs) in phase space which both provide a pictorial representation of the quantum evolution of the system of interest and constitute a useful tool for providing numerical solutions of the quantum equation describing the time evolution of the system. The theory has been applied to the following cases: (a) The study of the transport properties of the transient regime for homogeneous systems. (b) The comparison between quantum and classical treatment of the scattering mechanism. (c) The study of the reflection of the WF from an infinite potential barrier. Furthermore, the Wigner approach, originally formulated in terms of single-time WF has been extended by introducing the two-time WF. The Fourier transform of this function contains independent information about both momentum and energy of the electron. The Monte Carlo procedure based on the generation of WPs can be used to evaluate the two-time WF of an electron interacting with the phonon gas.
A general density-matrix description of energy-dissipation and decoherence phenomena in open quantum systems is presented. More specifically, contrary to the conventional single-particle correlation expansion, we shall investigate the effect of the adiabatic or Markov limit, before considering/performing any reduction procedure. Our fully operatorial approach allows us to better identify the general properties of the scattering superoperators entering our effective quantum-transport theory at various description levels. In particular, we shall show that—contrary to the semiclassical case—the conventional Markov limit does not preserve the positive-definite character of the corresponding density matrix, thus leading to highly non-physical results. To overcome this serious limitation, originally pointed out and partially solved by Davies and co-workers almost three decades ago, we shall propose an alternative and more general adiabatic procedure, which (i) in the semiclassical limit reduces to the standard Fermi's golden rule, and (ii) describes a genuine Lindblad evolution, thus providing a reliable/robust treatment of energy-dissipation and dephasing processes in semiconductor quantum devices
The Wigner formalism is a convenient reformulation of the Schrödinger equation that allows the simulation of transient behavior of quantum systems in the presence of general boundary conditions. Recently, a Wigner Monte Carlo technique, based on particles signs, has been generalized to two-dimensional evolution problems. In this paper, we apply this technique to study the time reversibility of the quantum evolution of a wave packet colliding with a potential wall in the presence of interface roughness, elastic, inelastic, and diffusive interactions with the environment. We show that a wall surface roughness does not necessarily involve time irreversibility. The dynamics of the packet is indeed influenced, but remains coherent, until the boundaries of the system begin to absorb information from the system. Finally, it is shown that in the presence of inelastic scattering or diffusive processes, the time-reversibility of a quantum system is destroyed, whatever the shape of the wall interface is. In particular, we show that the random nature of a process, elastic or inelastic, is responsible for the appearance of quantum decoherence.