8,434 research outputs found
Dispersion and damping of multi-quantum well polaritons from resonant Brillouin scattering by folded acoustic modes
We report on confined exciton resonances of acoustic and folded acoustic
phonon light scattering in a GaAs/AlAs multi-quantum-well. Significant
variations of the line shifts and widths are observed across the resonance and
quantitatively reproduced in terms of the polariton dispersion. This high
resolution Brillouin study brings new unexpectedly detailed informations on the
polariton dynamics in confined systems
Dark Matter Prediction from Canonical Quantum Gravity with Frame Fixing
We show how, in canonical quantum cosmology, the frame fixing induces a new
energy density contribution having features compatible with the (actual) cold
dark matter component of the Universe. First we quantize the closed
Friedmann-Robertson-Walker (FRW) model in a sinchronous reference and determine
the spectrum of the super-Hamiltonian in the presence of ultra-relativistic
matter and a perfect gas contribution. Then we include in this model small
inhomogeneous (spherical) perturbations in the spirit of the Lemaitre-Tolman
cosmology. The main issue of our analysis consists in outlining that, in the
classical limit, the non-zero eigenvalue of the super-Hamiltonian can make
account for the present value of the dark matter critical parameter.
Furthermore we obtain a direct correlation between the inhomogeneities in our
dark matter candidate and those one appearing in the ultra-relativistic matter.Comment: 5 pages, to appear on Modern Physics Letters
Statistical mechanics of damage phenomena
This paper applies the formalism of classical, Gibbs-Boltzmann statistical
mechanics to the phenomenon of non-thermal damage. As an example, a non-thermal
fiber-bundle model with the global uniform (meanfield) load sharing is
considered. Stochastic topological behavior in the system is described in terms
of an effective temperature parameter thermalizing the system. An equation of
state and a topological analog of the energy-balance equation are obtained. The
formalism of the free energy potential is developed, and the nature of the
first order phase transition and spinodal is demonstrated.Comment: Critical point appeared to be a spinodal poin
Hole-LO phonon interaction in InAs/GaAs quantum dots
We investigate the valence intraband transitions in p-doped self-assembled
InAs quantum dots using far-infrared magneto-optical technique with polarized
radiation. We show that a purely electronic model is unable to account for the
experimental data. We calculate the coupling between the mixed hole LO-phonon
states using the Fr\"ohlich Hamiltonian, from which we determine the polaron
states as well as the energies and oscillator strengths of the valence
intraband transitions. The good agreement between the experiments and
calculations provides strong evidence for the existence of hole-polarons and
demonstrates that the intraband magneto-optical transitions occur between
polaron states
Genericity aspects in gravitational collapse to black holes and naked singularities
We investigate here the genericity and stability aspects for naked
singularities and black holes that arise as the final states for a complete
gravitational collapse of a spherical massive matter cloud. The form of the
matter considered is a general Type I matter field, which includes most of the
physically reasonable matter fields such as dust, perfect fluids and such other
physically interesting forms of matter widely used in gravitation theory. We
first study here in some detail the effects of small pressure perturbations in
an otherwise pressure-free collapse scenario, and examine how a collapse
evolution that was going to the black hole endstate would be modified and go to
a naked singularity, once small pressures are introduced in the initial data.
This allows us to understand the distribution of black holes and naked
singularities in the initial data space. Collapse is examined in terms of the
evolutions allowed by Einstein equations, under suitable physical conditions
and as evolving from a regular initial data. We then show that both black holes
and naked singularities are generic outcomes of a complete collapse, when
genericity is defined in a suitable sense in an appropriate space.Comment: 24 pages, 6 figures, some changes in text and figures to match the
version accepted for publication by IJMP
Irreversible magnetization switching using surface acoustic waves
An analytical and numerical approach is developped to pinpoint the optimal
experimental conditions to irreversibly switch magnetization using surface
acoustic waves (SAWs). The layers are magnetized perpendicular to the plane and
two switching mechanisms are considered. In precessional switching, a small
in-plane field initially tilts the magnetization and the passage of the SAW
modifies the magnetic anisotropy parameters through inverse magneto-striction,
which triggers precession, and eventually reversal. Using the micromagnetic
parameters of a fully characterized layer of the magnetic semiconductor
(Ga,Mn)(As,P), we then show that there is a large window of accessible
experimental conditions (SAW amplitude/wave-vector, field
amplitude/orientation) allowing irreversible switching. As this is a resonant
process, the influence of the detuning of the SAW frequency to the magnetic
system's eigenfrequency is also explored. Finally, another - non-resonant -
switching mechanism is briefly contemplated, and found to be applicable to
(Ga,Mn)(As,P): SAW-assisted domain nucleation. In this case, a small
perpendicular field is applied opposite the initial magnetization and the
passage of the SAW lowers the domain nucleation barrier.Comment: 11 pages, 4 figure
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