6,811 research outputs found
On the double and triple-humped fission barriers and half-lives of actinide elements
The deformation barriers standing in the quasi-molecular shape path have been
determined in the actinide region within a macroscopic-microscopic energy
derived from a generalized liquid drop model, the algebraic droplet model shell
corrections and analytic expressions for the pairing energies. Double and
triple-humped fission barriers appear. The second barrier corresponds to the
transition from one-body shapes to two touching ellipsoids. The third minimum
and third peak, when they exist, come from shell rearrangements in the deformed
fragment. The shape of the other almost magic one is close to the sphere. The
barrier heights agreewith the experimental results, the energy of the second
minimum being a little too high. The predicted half-lives follow the
experimental data trend
Multiquantum well spin oscillator
A dc voltage biased II-VI semiconductor multiquantum well structure attached
to normal contacts exhibits self-sustained spin-polarized current oscillations
if one or more of its wells are doped with Mn. Without magnetic impurities, the
only configurations appearing in these structures are stationary. Analysis and
numerical solution of a nonlinear spin transport model yield the minimal number
of wells (four) and the ranges of doping density and spin splitting needed to
find oscillations.Comment: 11 pages, 2 figures, shortened and updated versio
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.
Super-energy tensor for space-times with vanishing scalar curvature
A four-index tensor is constructed with terms both quadratic in the Riemann
tensor and linear in its second derivatives, which has zero divergence for
space-times with vanishing scalar curvature. This tensor reduces in vacuum to
the Bel-Robinson tensor. Furthermore, the completely timelike component
referred to any observer is positive, and zero if and only if the space-time is
flat (excluding some unphysical space-times). We also show that this tensor is
the unique that can be constructed with these properties. Such a tensor does
not exist for general gravitational fields. Finally, we study this tensor in
several examples: the Friedmann-Lema\^{\i}tre-Robertson-Walker space-times
filled with radiation, the plane-fronted gravitational waves, and the Vaidya
radiating metric.Comment: 13 pages, LaTeX 2.09. To be published in Journal of Mathematical
Physic
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
Effects of disorder on the wave front depinning transition in spatially discrete systems
Pinning and depinning of wave fronts 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 subject to random external
forces. The presence of weak randomness shrinks the pinning interval and it
changes the critical exponent of the wave front depinning transition from 1/2
to 3/2. This effect is derived by means of a recent asymptotic theory of the
depinning transition, extended to discrete drift-diffusion models of transport
in semiconductor superlattices and confirmed by numerical calculations.Comment: 4 pages, 3 figures, to appear as a Rapid Commun. in Phys. Rev.
Magnetoswitching of current oscillations in diluted magnetic semiconductor nanostructures
Strongly nonlinear transport through Diluted Magnetic Semiconductor
multiquantum wells occurs due to the interplay between confinement, Coulomb and
exchange interaction. Nonlinear effects include the appearance of spin
polarized stationary states and self-sustained current oscillations as possible
stable states of the nanostructure, depending on its configuration and control
parameters such as voltage bias and level splitting due to an external magnetic
field. Oscillatory regions grow in size with well number and level splitting. A
systematic analysis of the charge and spin response to voltage and magnetic
field switching of II-VI Diluted Magnetic Semiconductor multiquantum wells is
carried out. The description of stationary and time-periodic spin polarized
states, the transitions between them and the responses to voltage or magnetic
field switching have great importance due to the potential implementation of
spintronic devices based on these nanostructures.Comment: 14 pages, 4 figures, Revtex, to appear in PR
Closure of the Monte Carlo dynamical equation in the spherical Sherrington-Kirkpatrick model
We study the analytical solution of the Monte Carlo dynamics in the spherical Sherrington-Kirkpatrick model using the technique of the generating function. Explicit solutions for one-time observables (like the energy) and two-time observables (like the correlation and response function) are obtained. We show that the crucial quantity which governs the dynamics is the acceptance rate. At zero temperature, an adiabatic approximation reveals that the relaxational behavior of the model corresponds to that of a single harmonic oscillator with an effective renormalized mass
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