6,502 research outputs found
On the Photorefractive Gunn Effect
We present and numerically solve a model of the photorefractive Gunn effect.
We find that high field domains can be triggered by phase-locked interference
fringes, as it has been recently predicted on the basis of linear stability
considerations. Since the Gunn effect is intrinsically nonlinear, we find that
such considerations give at best order-of-magnitude estimations of the
parameters critical to the photorefractive Gunn effect. The response of the
system is much more complex including multiple wave shedding from the injecting
contact, wave suppression and chaos with spatial structure.Comment: Revtex, 8 pag., 4 fig. (jpg), submit to Physical Review
Chaotic motion of space charge wavefronts in semiconductors under time-independent voltage bias
A standard drift-diffusion model of space charge wave propagation in
semiconductors has been studied numerically and analytically under dc voltage
bias. For sufficiently long samples, appropriate contact resistivity and
applied voltage - such that the sample is biased in a regime of negative
differential resistance - we find chaos in the propagation of nonlinear fronts
(charge monopoles of alternating sign) of electric field. The chaos is always
low-dimensional, but has a complex spatial structure; this behavior can be
interpreted using a finite dimensional asymptotic model in which the front
(charge monopole) positions and the electrical current are the only dynamical
variables.Comment: 12 pages, 8 figure
A moment based approach to the dynamical solution of the Kuramoto model
We examine the dynamics of the Kuramoto model with a new analytical approach.
By defining an appropriate set of moments the dynamical equations can be
exactly closed. We discuss some applications of the formalism like the
existence of an effective Hamiltonian for the dynamics. We also show how this
approach can be used to numerically investigate the dynamical behavior of the
model without finite size effects.Comment: 6 pages, 5 figures, Revtex file, to appear in J. Phys.
Two mini-band model for self-sustained oscillations of the current through resonant tunneling semiconductor superlattices
A two miniband model for electron transport in semiconductor superlattices
that includes scattering and interminiband tunnelling is proposed. The model is
formulated in terms of Wigner functions in a basis spanned by Pauli matrices,
includes electron-electron scattering in the Hartree approximation and modified
Bhatnagar-Gross-Krook collision tems. For strong applied fields, balance
equations for the electric field and the miniband populations are derived using
a Chapman-Enskog perturbation technique. These equations are then solved
numerically for a dc voltage biased superlattice. Results include
self-sustained current oscillations due to repeated nucleation of electric
field pulses at the injecting contact region and their motion towards the
collector. Numerical reconstruction of the Wigner functions shows that the
miniband with higher energy is empty during most of the oscillation period: it
becomes populated only when the local electric field (corresponding to the
passing pulse) is sufficiently large to trigger resonant tunneling.Comment: 26 pages, 3 figures, to appear in Phys. Rev.
Chaos in resonant-tunneling superlattices
Spatio-temporal chaos is predicted to occur in n-doped semiconductor
superlattices with sequential resonant tunneling as their main charge transport
mechanism. Under dc voltage bias, undamped time-dependent oscillations of the
current (due to the motion and recycling of electric field domain walls) have
been observed in recent experiments. Chaos is the result of forcing this
natural oscillation by means of an appropriate external microwave signal.Comment: 3 pages, LaTex, RevTex, 3 uuencoded figures (1.2M) are available upon
request from [email protected], to appear in Phys.Rev.
Addressing culture in the EFL classroom: A dialogic proposal built up through dialogism
Language teaching has gone from a linguistic centered approach towards a lingocultural experience in which learning a language goes hand in hand with the understanding of, not only the target culture but the learner’s own culture. This paper intends to describe and reflect upon a collaborative and dialogical experience carried out between two teachers of the Languages Program of Universidad de la Salle. The bilateral enrichment of such a pedagogical experience not only helped the teachers to improve their language teaching contexts but also prompted the construction of a theoretical proposal to enhance intercultural awareness and develop critical intercultural competence in FL learners
Chapman-Enskog method and synchronization of globally coupled oscillators
The Chapman-Enskog method of kinetic theory is applied to two problems of
synchronization of globally coupled phase oscillators. First, a modified
Kuramoto model is obtained in the limit of small inertia from a more general
model which includes ``inertial'' effects. Second, a modified Chapman-Enskog
method is used to derive the amplitude equation for an O(2) Takens-Bogdanov
bifurcation corresponding to the tricritical point of the Kuramoto model with a
bimodal distribution of oscillator natural frequencies. This latter calculation
shows that the Chapman-Enskog method is a convenient alternative to normal form
calculations.Comment: 7 pages, 2-column Revtex, no figures, minor change
Dynamics of Electric Field Domains and Oscillations of the Photocurrent in a Simple Superlattice Model
A discrete model is introduced to account for the time-periodic oscillations
of the photocurrent in a superlattice observed by Kwok et al, in an undoped 40
period AlAs/GaAs superlattice. Basic ingredients are an effective negative
differential resistance due to the sequential resonant tunneling of the
photoexcited carriers through the potential barriers, and a rate equation for
the holes that incorporates photogeneration and recombination. The
photoexciting laser acts as a damping factor ending the oscillations when its
power is large enough. The model explains: (i) the known oscillatory static I-V
characteristic curve through the formation of a domain wall connecting high and
low electric field domains, and (ii) the photocurrent and photoluminescence
time-dependent oscillations after the domain wall is formed. In our model, they
arise from the combined motion of the wall and the shift of the values of the
electric field at the domains. Up to a certain value of the photoexcitation,
the non-uniform field profile with two domains turns out to be metastable:
after the photocurrent oscillations have ceased, the field profile slowly
relaxes toward the uniform stationary solution (which is reached on a much
longer time scale). Multiple stability of stationary states and hysteresis are
also found. An interpretation of the oscillations in the photoluminescence
spectrum is also given.Comment: 34 pages, REVTeX 3.0, 10 figures upon request, MA/UC3M/07/9
Current-voltage characteristic and stability in resonant-tunneling n-doped semiconductor superlattices
We review the occurrence of electric-field domains in doped superlattices
within a discrete drift model. A complete analysis of the construction and
stability of stationary field profiles having two domains is carried out. As a
consequence, we can provide a simple analytical estimation for the doping
density above which stable stable domains occur. This bound may be useful for
the design of superlattices exhibiting self-sustained current oscillations.
Furthermore we explain why stable domains occur in superlattices in contrast to
the usual Gunn diode.Comment: Tex file and 3 postscript figure
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