7,314 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
Microscopic Model for Sequential Tunneling in Semiconductor Multiple Quantum Wells
We propose a selfconsistent microscopic model of vertical sequential
tunneling through a multi-quantum well.The model includes a detailed
description of the contacts,uses the Transfer Hamiltonian for expressions of
the current and it treats the Coulomb interaction within a mean field
approximation. We analyze the current density through a double well and a
superlattice and study the formation of electric field domains and
multistability coming from the Coulomb interaction. Phase diagrams of parameter
regions (bias, doping in the heterostructure and in the contacts,etc) where the
different solutions exist are given.Comment: 4 pages, 8 Postscript Figure
Wigner-Poisson and nonlocal drift-diffusion model equations for semiconductor superlattices
A Wigner-Poisson kinetic equation describing charge transport in doped
semiconductor superlattices is proposed. Electrons are supposed to occupy the
lowest miniband, exchange of lateral momentum is ignored and the
electron-electron interaction is treated in the Hartree approximation. There
are elastic collisions with impurities and inelastic collisions with phonons,
imperfections, etc. The latter are described by a modified BGK
(Bhatnagar-Gross-Krook) collision model that allows for energy dissipation
while yielding charge continuity. In the hyperbolic limit, nonlocal
drift-diffusion equations are derived systematically from the kinetic
Wigner-Poisson-BGK system by means of the Chapman-Enskog method. The
nonlocality of the original quantum kinetic model equations implies that the
derived drift-diffusion equations contain spatial averages over one or more
superlattice periods. Numerical solutions of the latter equations show
self-sustained oscillations of the current through a voltage biased
superlattice, in agreement with known experiments.Comment: 20 pages, 1 figure, published as M3AS 15, 1253 (2005) with
correction
Depinning transitions in discrete reaction-diffusion equations
We consider spatially discrete bistable reaction-diffusion equations that
admit wave front solutions. Depending on the parameters involved, such wave
fronts appear to be pinned or to glide at a certain speed. We study the
transition of traveling waves to steady solutions near threshold and give
conditions for front pinning (propagation failure). The critical parameter
values are characterized at the depinning transition and an approximation for
the front speed just beyond threshold is given.Comment: 27 pages, 12 figures, to appear in SIAM J. Appl. Mat
Theory of defect dynamics in graphene: defect groupings and their stability
We use our theory of periodized discrete elasticity to characterize defects
in graphene as the cores of dislocations or groups of dislocations. Earlier
numerical implementations of the theory predicted some of the simpler defect
groupings observed in subsequent Transmission Electron Microscope experiments.
Here we derive the more complicated defect groupings of three or four defect
pairs from our theory, show that they correspond to the cores of two pairs of
dislocation dipoles and ascertain their stability.Comment: 11 pages, 7 figures; replaced figure
Wavefront depinning transition in discrete one-dimensional reaction-diffusion systems
Pinning and depinning of wavefronts are ubiquitous features of spatially
discrete systems describing a host of phenomena in physics, biology, etc. A
large class of discrete systems is described by overdamped chains of nonlinear
oscillators with nearest-neighbor coupling and controlled by constant external
forces. A theory of the depinning transition for these systems, including
scaling laws and asymptotics of wavefronts, is presented and confirmed by
numerical calculations.Comment: 4 pages, 4 figure
Photo-excited semiconductor superlattices as constrained excitable media: Motion of dipole domains and current self-oscillations
A model for charge transport in undoped, photo-excited semiconductor
superlattices, which includes the dependence of the electron-hole recombination
on the electric field and on the photo-excitation intensity through the
field-dependent recombination coefficient, is proposed and analyzed. Under dc
voltage bias and high photo-excitation intensities, there appear self-sustained
oscillations of the current due to a repeated homogeneous nucleation of a
number of charge dipole waves inside the superlattice. In contrast to the case
of a constant recombination coefficient, nucleated dipole waves can split for a
field-dependent recombination coefficient in two oppositely moving dipoles. The
key for understanding these unusual properties is that these superlattices have
a unique static electric-field domain. At the same time, their dynamical
behavior is akin to the one of an extended excitable system: an appropriate
finite disturbance of the unique stable fixed point may cause a large excursion
in phase space before returning to the stable state and trigger pulses and wave
trains. The voltage bias constraint causes new waves to be nucleated when old
ones reach the contact.Comment: 19 pages, 8 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.
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