4,315 research outputs found
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.
Temperature dependence of current self-oscillations and electric field domains in sequential tunneling doped superlattices
We examine how the current--voltage characteristics of a doped weakly coupled
superlattice depends on temperature. The drift velocity of a discrete drift
model of sequential tunneling in a doped GaAs/AlAs superlattice is calculated
as a function of temperature. Numerical simulations and theoretical arguments
show that increasing temperature favors the appearance of current
self-oscillations at the expense of static electric field domain formation. Our
findings agree with available experimental evidence.Comment: 7 pages, 5 figure
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
The primosomal protein DnaD inhibits cooperative DNA binding by the replication initiator DnaA in Bacillus subtilis
DnaA is an AAA+ ATPase and the conserved replication initiator in bacteria. Bacteria control the timing of replication initiation by regulating the activity of DnaA. DnaA binds to multiple sites in the origin of replication (oriC) and is required for recruitment of proteins needed to load the replicative helicase. DnaA also binds to other chromosomal regions and functions as a transcription factor at some of these sites. Bacillus subtilis DnaD is needed during replication initiation for assembly of the replicative helicase at oriC and during replication restart at stalled replication forks. DnaD associates with DnaA at oriC and at other chromosomal regions bound by DnaA. Using purified proteins, we found that DnaD inhibited the ability of DnaA to bind cooperatively to DNA and caused a decrease in the apparent dissociation constant. These effects of DnaD were independent of the ability of DnaA to bind or hydrolyze ATP. Other proteins known to regulate B. subtilis DnaA also affect DNA binding, whereas much of the regulation of Escherichia coli DnaA affects nucleotide hydrolysis or exchange. We found that the rate of nucleotide exchange for B. subtilis DnaA was high and not affected by DnaD. The rapid exchange is similar to that of Staphylococcus aureus DnaA and in contrast to the low exchange rate of Escherichia coli DnaA. We suggest that organisms in which DnaA has a high rate of nucleotide exchange predominantly regulate the DNA binding activity of DnaA and that those with low rates of exchange regulate hydrolysis and exchange.United States. Public Health Service (Grant GM41934
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.
Practica de las ceremonias de la Missa : rezada, y cantada solemne, segun las rubricas del Missal Romano, con otros tratados pertenecientes al Culto Divino
Sign.: *8, 2*4, 3*2, A-Z8, Aa-Bb
Algunas consideraciones acerca de fray Pedro Ponce de León y Juan Pablo Bonet
Separata del núm. extraordinario de "La Paraula", publicado con motivo del IV centenario del nacimiento de Fray P. Ponce de León y III de la publicación del libro "Reducción de las letras" de Joan. P. BonetCopia digital. Valladolid : Junta de Castilla y León. Consejería de Cultura y Turismo, 2009-201
Nonlinear stochastic discrete drift-diffusion theory of charge fluctuations and domain relocation times in semiconductor superlattices
A stochastic discrete drift-diffusion model is proposed to account for the
effects of shot noise in weakly coupled, highly doped semiconductor
superlattices. Their current-voltage characteristics consist of a number stable
multistable branches corresponding to electric field profiles displaying two
domains separated by a domain wall. If the initial state corresponds to a
voltage on the middle of a stable branch and a sudden voltage is switched so
that the final voltage corresponds to the next branch, the domains relocate
after a certain delay time. Shot noise causes the distribution of delay times
to change from a Gaussian to a first passage time distribution as the final
voltage approaches that of the end of the first current branch. These results
agree qualitatively with experiments by Rogozia {\it et al} (Phys. Rev. B {\bf
64}, 041308(R) (2001)).Comment: 9 pages, 12 figures, 2 column forma
Synchronization in populations of globally coupled oscillators with inertial effects
A model for synchronization of globally coupled phase oscillators including
``inertial'' effects is analyzed. In such a model, both oscillator frequencies
and phases evolve in time. Stationary solutions include incoherent
(unsynchronized) and synchronized states of the oscillator population. Assuming
a Lorentzian distribution of oscillator natural frequencies, , both
larger inertia or larger frequency spread stabilize the incoherent solution,
thereby making harder to synchronize the population. In the limiting case
, the critical coupling becomes independent of
inertia. A richer phenomenology is found for bimodal distributions. For
instance, inertial effects may destabilize incoherence, giving rise to
bifurcating synchronized standing wave states. Inertia tends to harden the
bifurcation from incoherence to synchronized states: at zero inertia, this
bifurcation is supercritical (soft), but it tends to become subcritical (hard)
as inertia increases. Nonlinear stability is investigated in the limit of high
natural frequencies.Comment: Revtex, 36 pages, submit to Phys. Rev.
Pattern Competition in the Photorefractive Semiconductors
We analytically study the photorefractive Gunn effect in n-GaAs subjected to
two external laser beams which form a moving interference pattern (MIP) in the
semiconductor. When the intensity of the spatially independent part of the MIP,
denoted by , is small, the system has a periodic domain train (PDT),
consistent with the results of linear stability analysis. When is large,
the space-charge field induced by the MIP will compete with the PDT and result
in complex dynamics, including driven chaos via quasiperiodic route
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