1,115 research outputs found
Microscopic theory of quantum-transport phenomena in mesoscopic systems: A Monte Carlo approach
A theoretical investigation of quantum-transport phenomena in mesoscopic
systems is presented. In particular, a generalization to ``open systems'' of
the well-known semiconductor Bloch equations is proposed. The presence of
spatial boundary conditions manifest itself through self-energy corrections and
additional source terms in the kinetic equations, whose form is suitable for a
solution via a generalized Monte Carlo simulation. The proposed approach is
applied to the study of quantum-transport phenomena in double-barrier
structures as well as in superlattices, showing a strong interplay between
phase coherence and relaxation.Comment: to appear in Phys. Rev. Let
Modeling of open quantum devices within the closed-system paradigm
We present an alternative simulation strategy for the study of nonequilibrium carrier dynamics in quantum devices with open boundaries. We propose to replace the usual modeling of open quantum systems based on phenomenological injection/loss rates with a kinetic description of the system-reservoir thermalization process. In this simulation scheme the partial carrier thermalization induced by the device spatial boundaries is treated within the standard Boltzmann-transport approach via an effective scattering mechanism between the highly nonthermal device electrons and the thermal carrier distribution of the reservoir. Applications to state-of-the-art semiconductor nanostructures are discussed. Finally, the proposed approach is extended to the quantum-transport regime; to this end, we introduce an effective Liouville superoperator, able to describe the effect of the device spatial boundaries on the time evolution of the single-particle density matrix
Mode Coupling and Cavity-Quantum-Dot Interactions in a Fiber-Coupled Microdisk Cavity
A quantum master equation model for the interaction between a two-level
system and whispering-gallery modes (WGMs) of a microdisk cavity is presented,
with specific attention paid to current experiments involving a semiconductor
quantum dot (QD) embedded in a fiber-coupled, AlGaAs microdisk cavity. In
standard single mode cavity QED, three important rates characterize the system:
the QD-cavity coupling rate g, the cavity decay rate kappa, and the QD
dephasing rate gamma_perpendicular. A more accurate model of the microdisk
cavity includes two additional features. The first is a second cavity mode that
can couple to the QD, which for an ideal microdisk corresponds to a traveling
wave WGM propagating counter to the first WGM. The second feature is a coupling
between these two traveling wave WGMs, at a rate beta, due to backscattering
caused by surface roughness that is present in fabricated devices. We consider
the transmitted and reflected signals from the cavity for different parameter
regimes of {g,beta,kappa,gamma_perpendicular}. A result of this analysis is
that even in the presence of negligible roughness induced backscattering, a
strongly coupled QD mediates coupling between the traveling wave WGMs,
resulting in an enhanced effective coherent coupling rate g = sqrt(2)*g0
corresponding to that of a standing wave WGM with an electric field maximum at
the position of the QD. In addition, analysis of the second-order correlation
function of the reflected signal from the cavity indicates that regions of
strong photon antibunching or bunching may be present depending upon the
strength of coupling of the QD to each of the cavity modes. Such intensity
correlation information will likely be valuable in interpreting experimental
measurements of a strongly-coupled QD to a bi-modal WGM cavity.Comment: rev4: updated references and added additional correlation function
calculations; to appear in Phys. Rev. A in Feb 200
Self-sustained current oscillations in the kinetic theory of semiconductor superlattices
We present the first numerical solutions of a kinetic theory description of
self-sustained current oscillations in n-doped semiconductor superlattices. The
governing equation is a single-miniband Boltzmann-Poisson transport equation
with a BGK (Bhatnagar-Gross-Krook) collision term. Appropriate boundary
conditions for the distribution function describe electron injection in the
contact regions. These conditions seamlessly become Ohm's law at the injecting
contact and the zero charge boundary condition at the receiving contact when
integrated over the wave vector. The time-dependent model is numerically solved
for the distribution function by using the deterministic Weighted Particle
Method. Numerical simulations are used to ascertain the convergence of the
method. The numerical results confirm the validity of the Chapman-Enskog
perturbation method used previously to derive generalized drift-diffusion
equations for high electric fields because they agree very well with numerical
solutions thereof.Comment: 26 pages, 16 figures, to appear in J. Comput. Phy
Modeling of diffusion of injected electron spins in spin-orbit coupled microchannels
We report on a theoretical study of spin dynamics of an ensemble of
spin-polarized electrons injected in a diffusive microchannel with linear
Rashba and Dresselhaus spin-orbit coupling. We explore the dependence of the
spin-precession and spin-diffusion lengths on the strengths of spin-orbit
interaction and external magnetic fields, microchannel width, and orientation.
Our results are based on numerical Monte Carlo simulations and on approximate
analytical formulas, both treating the spin dynamics quantum-mechanically. We
conclude that spin-diffusion lengths comparable or larger than the
precession-length occur i) in the vicinity of the persistent spin helix regime
for arbitrary channel width, and ii) in channels of similar or smaller width
than the precession length, independent of the ratio of Rashba and Dresselhaus
fields. For similar strengths of the Rashba and Dresselhaus fields, the
steady-state spin-density oscillates or remains constant along the channel for
channels parallel to the in-plane diagonal crystal directions. An oscillatory
spin-polarization pattern tilted by 45 with respect to the channel
axis is predicted for channels along the main cubic crystal directions. For
typical experimental system parameters, magnetic fields of the order of Tesla
are required to affect the spin-diffusion and spin-precession lengths.Comment: Replaced with final version (some explanations and figures improved).
8 pages, 6 figure
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