On the Nature of Charge Transport in Quantum-Cascade Lasers

The first global quantum simulation of semiconductor-based quantum-cascade lasers is presented. Our three-dimensional approach allows to study in a purely microscopic way the current-voltage characteristics of state-of-the-art unipolar nanostructures, and therefore to answer the long-standing controversial question: is charge transport in quantum-cascade lasers mainly coherent or incoherent? Our analysis shows that: (i) Quantum corrections to the semiclassical scenario are minor; (ii) Inclusion of carrier-phonon and carrier-carrier scattering gives excellent agreement with experimental results.Comment: 4 pages, 7 Postscript figures. Phys. Rev. Lett. (in press

Wigner-function formalism applied to semiconductor quantum devices: Need for nonlocal scattering models

In designing and optimizing new-generation nanomaterials and related quantum devices, dissipation versus decoherence phenomena are often accounted for via local scattering models, such as relaxation-time and Boltzmann-like schemes. Here we show that the use of such local scattering approaches within the Wigner-function formalism may lead to unphysical results, namely anomalous suppression of intersubband relaxation, incorrect thermalization dynamics, and violation of probability-density positivity. Furthermore, we propose a quantum-mechanical generalization of relaxation-time and Boltzmann-like models, resulting in nonlocal scattering superoperators that enable one to overcome such limitations.Comment: 12 pages, 7 figure

Microscopic theory of hot-carrier relaxation in semiconductor-based quantum-cascade lasers

A microscopic analysis of basic nonequilibrium phenomena in unipolar quantum devices is presented. In particular, energy-relaxation processes governing the hot-carrier dynamics in the active region of GaAs-based quantum-cascade lasers are investigated by means of a generalized ensemble Monte Carlo simulation. Such analysis is essential in determining the validity range and limitations of purely macroscopic models with respect to basic device parameters, like injection current and temperature

Carrier thermalization versus phonon-assisted relaxation in quantum-cascade lasers: A Monte Carlo approach

In this letter, we present a microscopic analysis of the hot-carrier dynamics governing intersubband light-emitting devices. In particular, a global Monte Carlo simulation scheme is proposed in order to directly access details of the three-dimensional carrier relaxation, without resorting to phenomenological parameters. The competition between intercarrier thermalization and phonon-assisted relaxation in quantum-cascade lasers is investigated and their relative importance on device performance is clearly identified

Energy Dissipation and Decoherence in Solid-State Quantum Devices: Markovian versus non-Markovian Treatments

The design and optimization of new-generation solid-state quantum hardware absolutely requires reliable dissipation versus decoherence models. Depending on the device operational condition, the latter may range from Markov-type schemes (both phenomenological- and microscopiclike) to quantum-kinetic approaches. The primary goal of this paper is to review in a cohesive way virtues versus limitations of the most popular approaches, focussing on a few critical issues recently pointed out (see, e.g., Phys. Rev. B 90, 125140 (2014); Eur. Phys. J. B 90, 250 (2017)) and linking them within a common framework. By means of properly designed simulated experiments of a prototypical quantum-dot nanostructure (described via a two-level electronic system coupled to a phonon bath), we shall show that both conventional (i.e., non-Lindblad) Markov models and density-matrix-based non-Markov approaches (i.e., quantum-kinetic treatments) may lead to significant positivity violations. While for the former case the problem is easily avoidable by choosing genuine Lindblad-type dissipation models, for the latter, a general strategy is still missing

Photoexcitation of electron wave packets in quantum spin Hall edge states: effects of chiral anomaly from a localised electric pulse

We show that, when a spatially localised electric pulse is applied at the edge of a quantum spin Hall system, electron wavepackets of the helical states can be photoexcited by purely intra-branch electrical transitions, without invoking the bulk states or the magnetic Zeeman coupling. In particular, as long as the electric pulse remains applied, the photoexcited densities lose their character of right- and left-movers, whereas after the ending of the pulse they propagate in opposite directions without dispersion, i.e. maintaining their space profile unaltered. Notably we find that, while the momentum distribution of the photoexcited wave packets depends on the temperature $T$ and the chemical potential $\mu$ of the initial equilibrium state and displays a non-linear behavior on the amplitude of the applied pulse, in the mesoscopic regime the space profile of the wave packets is independent of $T$ and $\mu$. Instead, it depends purely on the applied electric pulse, in a linear manner, as a signature of the chiral anomaly characterising massless Dirac electrons. We also discuss how the photoexcited wave packets can be tailored with the electric pulse parameters, for both low and finite frequencies.Comment: 15 pages, 5 figure

Wigner-function formalism applied to semiconductor quantum devices: Failure of the conventional boundary-condition scheme

The Wigner-function formalism is a well known approach to model charge transport in semiconductor nanodevices. Primary goal of the present article is to point out and explain intrinsic limitations of the conventional quantum-device modeling based on such Wigner-function paradigm, providing a definite answer to open questions related to the application of the conventional spatial boundary-condition scheme to the Wigner transport equation. Our analysis shows that (i) in the absence of energy dissipation (coherent limit) the solution of the Wigner equation equipped with given boundary conditions is not unique, and (ii) when decoherence/dissipation phenomena are taken into account via a relaxation-time approximation the solution, although unique, is not necessarily a physical Wigner function.Comment: 18 pages, 8 figures, accepted by Phys. Rev.

Interplay between energy dissipation and reservoir-induced thermalization in nonequilibrium quantum nanodevices

A solid state electronic nanodevice is an intrinsically open quantum system, exchanging both energy with the host material and carriers with connected reservoirs. Its out-of-equilibrium behavior is determined by a nontrivial interplay between electronic dissipation and decoherence induced by inelastic processes within the device, and the coupling of the latter to metallic electrodes. We propose a unified description, based on the density matrix formalism, that accounts for both these aspects, enabling us to predict various steady-state as well as ultrafast nonequilibrium phenomena, nowadays experimentally accessible. More specifically, we derive a generalized density-matrix equation, particularly suitable for the design and optimization of a wide class of electronic and optoelectronic quantum devices. The power and flexibility of this approach is demonstrated with the application to a photoexcited triple-barrier nanodevic

Quantum transport theory for semiconductor nanostructures: A density-matrix formulation

A general density-matrix formulation of quantum transport phenomena in semiconductor nanostructures is presented. More specifically, contrary to the conventional single-particle correlation expansion, we shall investigate separately the effects of the adiabatic or Markov limit and of the 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, e.g., N electrons-plus-quasiparticles, N electrons only, and single-particle picture. In addition to coherent transport phenomena characterizing the transient response of the system, the proposed theoretical description allows us to study scattering induced phase coherence in steady-state conditions. As a prototypical example, we shall investigate polaronic effects in strongly biased semiconductor superlattices