6,263 research outputs found
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.
Axisymmetric pulse recycling and motion in bulk semiconductors
The Kroemer model for the Gunn effect in a circular geometry (Corbino disks)
has been numerically solved. The results have been interpreted by means of
asymptotic calculations. Above a certain onset dc voltage bias, axisymmetric
pulses of the electric field are periodically shed by an inner circular
cathode. These pulses decay as they move towards the outer anode, which they
may not reach. As a pulse advances, the external current increases continuously
until a new pulse is generated. Then the current abruptly decreases, in
agreement with existing experimental results. Depending on the bias, more
complex patterns with multiple pulse shedding are possible.Comment: 8 pages, 15 figure
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
Free boundary problems describing two-dimensional pulse recycling and motion in semiconductors
An asymptotic analysis of the Gunn effect in two-dimensional samples of bulk
n-GaAs with circular contacts is presented. A moving pulse far from contacts is
approximated by a moving free boundary separating regions where the electric
potential solves a Laplace equation with subsidiary boundary conditions. The
dynamical condition for the motion of the free boundary is a Hamilton-Jacobi
equation. We obtain the exact solution of the free boundary problem (FBP) in
simple one-dimensional and axisymmetric geometries. The solution of the FBP is
obtained numerically in the general case and compared with the numerical
solution of the full system of equations. The agreement is excellent so that
the FBP can be adopted as the basis for an asymptotic study of the
multi-dimensional Gunn effect.Comment: 19 pages, 9 figures, Revtex. To appear in Phys. Rev.
Exact solution of the Schr\"{o}dinger equation for an hydrogen atom at the interface between the vacuum and a topologically insulating surface
When an hydrogen atom is brought near to the interface between
-media, the quantum-mechanical motion of the electron will be affected
by the electromagnetic interaction between the atomic charges and the
-interface, which is described by an axionic extension of Maxwell
electrodynamics in the presence of a boundary. In this paper we investigate the
atom-surface interaction effects upon the energy levels and wave functions of
an hydrogen atom placed at the interface between a -medium and the
vacuum. In the approximation considered, the Schr\"{o}dinger equation can be
exactly solved by separation of variables in terms of hypergeometic functions
for the angular part and hydrogenic functions for the radial part. In order to
make such effects apparent we deal with unrealistic high values of the
-parameter. We also compute the energy shifts using perturbation theory
for a particular small value of and we demonstrate that they are in a
very good agreement with the ones obtained from the exact solution.Comment: 20 pages, 17 figures, 6 tables, Accepted for publication in the
European Physics Journal
Vacuum stability with spontaneous violation of lepton number
The vacuum of the Standard Model is known to be unstable for the measured
values of the top and Higgs masses. Here we show how vacuum stability can be
achieved naturally if lepton number is violated spontaneously at the TeV scale.
More precise Higgs measurements in the next LHC run should provide a crucial
test of our symmetry breaking scenario. In addition, these schemes typically
lead to enhanced rates for processes involving lepton flavour violation .Comment: 9 pages, 4+2 figures; some references added, some textual
modifications: 2 figures added, appendices added. Results unchanged. Matches
published versio
A model for assembly sequence planning in a multirobot environment
2002 IFAC15th Triennial World Congress, Barcelona, SpainThis paper presents a model for the selection of optimal assembly sequences for a product in multirobot systems. The objective of the plan is the minimization of the total assembly time (makespan). To meet this objective, the model takes into account, in addition to the assembly times and resources for each task, the times needed to change tools in the robots, and the delays due to the transportation of intermediate subassemblies between different machines. An A* algorithm that solves the problem is also presented, which starts from the And/Or graph for the product (compressed representation of all feasible assembly plans)
El cuento y la creatividad como preparación a la resolución de problemas matemáticos
La aproximación a la realidad es, hoy en día, una de las estrategias más eficaces para motivar al alumno hacia las matemáticas y más concretamente hacia la resolución de problemas matemáticos (RPM). En el primer ciclo de primaria, las narraciones y los cuentos pertenecen a la realidad del niño y comparten con la RPM fases y estructuras de conocimiento. La manera de aproximarnos a un problema, determina el éxito de su solución y las emociones forman parte de ese proceso. El cuento, a su vez, prepara al niño para el conocimiento y posterior control de las emociones. El arte, la imaginación y el juego son características compartidas entre los cuentos y los problemas matemáticos. La enseñanza basada en el trabajo con los cuentos, durante el primer ciclo de primaria, permitirá abordar la RPM desde una perspectiva reconocible y motivadora en los cursos posteriores. El trabajo previo con los cuentos, pedagógicamente organizados, es un soporte fundamental y una condición necesaria para la “matematización” de la vida real
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