115 research outputs found
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
Symmetry protected topological phases of 1D interacting fermions with spin-charge separation
The low energy behavior of a huge variety of one-dimensional interacting
spinful fermionic systems exhibits spin-charge separation, described in the
continuum limit by two sine-Gordon models decoupled in the charge and spin
channels. Interaction is known to induce, besides the gapless Luttinger liquid
phase, eight possible gapped phases, among which are the Mott, Haldane,
charge-/spin-density, and bond-ordered wave insulators, and the Luther Emery
liquid. Here we prove that some of these physically distinct phases have
nontrivial topological properties, notably the presence of degenerate protected
edge modes with fractionalized charge/spin. Moreover, we show that the eight
gapped phases are in one-to-one correspondence with the symmetry-protected
topological (SPT) phases classified by group cohomology theory in the presence
of particle-hole symmetry P. The latter result is also exploited to
characterize SPT phases by measurable nonlocal order parameters which follow
the system evolution to the quantum phase transition. The implications on the
appearance of exotic orders in the class of microscopic Hubbard Hamiltonians,
possibly without P symmetry at higher energies, are discussed.Comment: latest version: 8 pages, 1 Tabl
Terahertz detection schemes based on sequential multi-photon absorption
We present modeling and simulation of prototypical multi bound state quantum
well infrared photodetectors and show that such a detection design may overcome
the problems arising when the operation frequency is pushed down into the far
infrared spectral region. In particular, after a simplified analysis on a
parabolic-potential design, we propose a fully three-dimensional model based on
a finite difference solution of the Boltzmann transport equation for realistic
potential profiles. The performances of the proposed simulated devices are
encouraging and support the idea that such design strategy may face the
well-known dark-current problem.Comment: 3 pages, 2 figures; submitted to Applied Physics Letter
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 and the chemical potential 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 and . 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.
Sequential multi-photon strategy for semiconductor-based terahertz detectors
A semiconductor-based terahertz-detector strategy, exploiting a
bound-to-bound-to-continuum architecture, is presented and investigated. In
particular, a ladder of equidistant energy levels is employed, whose step is
tuned to the desired detection frequency and allows for sequential multi-photon
absorption. Our theoretical analysis demonstrates that the proposed
multi-subband scheme could represent a promising alternative to conventional
quantum-well infrared photodetectors in the terahertz spectral region.Comment: Submitted to Journal of Applied Physic
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
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