1,766 research outputs found
Bose-Einstein condensation of positronium: modification of the s-wave scattering length below the critical temperature
The production of a Bose-Einstein condensate made of positronium may be
feasible in the near future. Below the condensation temperature, the
positronium collision process is modified by the presence of the condensate.
This makes the theoretical description of the positronium kinetics at low
temperature challenging. Based on the quasi-particle Bogoliubov theory, we
describe the many-body particle-particle collision in a simple manner. We find
that, in a good approximation, the full positronium-positronium interaction can
be described by an effective scattering length. Our results are general and
apply to different species of bosons. The correction to the bare scattering
length is expressed in terms of a single dimensionless parameter that
completely characterizes the condensate
Ultrafast Magnetization Dynamics in Diluted Magnetic Semiconductors
We present a dynamical model that successfully explains the observed time
evolution of the magnetization in diluted magnetic semiconductor quantum wells
after weak laser excitation. Based on the pseudo-fermion formalism and a second
order many-particle expansion of the exact p-d exchange interaction, our
approach goes beyond the usual mean-field approximation. It includes both the
sub-picosecond demagnetization dynamics and the slower relaxation processes
which restore the initial ferromagnetic order in a nanosecond time scale. In
agreement with experimental results, our numerical simulations show that,
depending on the value of the initial lattice temperature, a subsequent
enhancement of the total magnetization may be observed within a time scale of
few hundreds of picoseconds.Comment: Submitted to PR
Two-Body Random Ensembles: From Nuclear Spectra to Random Polynomials
The two-body random ensemble (TBRE) for a many-body bosonic theory is mapped
to a problem of random polynomials on the unit interval. In this way one can
understand the predominance of 0+ ground states, and analytic expressions can
be derived for distributions of lowest eigenvalues, energy gaps, density of
states and so forth. Recently studied nuclear spectroscopic properties are
addressed.Comment: 8 pages, 4 figures. To appear in Physical Review Letter
Experimental research activity on additive manufacturing of microwave passive waveguide components
All metal passive waveguide components are key building-blocks of several RF systems used for telecommunications, navigation, imaging, radio-astronomy, and cosmology. The accurate manufacture of these devices in Additive Manufacturing (AM) technologies can open the way to a high integration level of microwave functionalities with a significant cost and mass reduction. In the paper, after an introduction on the most common AM technologies with particular detail on selective laser melting (SLM) and stereo-lithography apparatus (SLA) processes, the results on the on-going research activity are discussed. Measured performances are reported for AM prototypes of Ku/K/Ka-band rectangular and circular waveguide lines, microwave filters and a smooth wall horn
From Regular to Chaotic States in Atomic Nuclei
An interesting aspect of nuclear dynamics is the co--existence, in atomic
nuclei, of regular and chaotic states. In the first part of the present work,
we review the state of the art of nuclear dynamics and use a schematic shell
model to show how a very simple and schematic nucleon--nucleon interaction can
produce an orderchaos transition. The second part is devoted to a
discussion of the wave function behaviour and decay of chaotic states using
some simple models (to be published in Rivista Nuovo Cimento).Comment: 65 pages, LaTex (the figures are not included), Preprint
DFPD/94/TH/26, University of Padov
Macroscopic description for a quantum plasma micro-instability: the quantum Weibel solution
The Weibel instability in the quantum plasma case is treated by means of a
fluid-like (moments) approach. Quantum modifications to the macroscopic
equations are then identified as effects of first or second kind. Quantum
effects of the first kind correspond to a dispersive term, similar to the Bohm
potential in the quantum hydrodynamic equations for plasmas. Effects of the
second kind are due to the Fermi statistics of the charge carriers and can
become the dominant influence for strong degeneracy. The macroscopic dispersion
relations are of higher order than those for the classical Weibel instability.
This corresponds to the presence of a cutoff wave-number even for the strong
temperature anisotropy case
The Onset of Chaos with a Quadrupole--Quadrupole Interaction
The transition from order to chaos in atomic nuclei has been studied
analytically and numerically using a quadrupole--quadrupole residual
interaction. This interaction leads to chaotic behaviour, but the critical
energy MeV, corresponding to the onset of chaos, is higher
than that of the experimental one.Comment: 14 pages, 5 figures (available upon request to the authors), LaTex,
DFPD/93/TH/73, to be published in Nuovo Cimento
Study of the quenched lifetime of an interacting positronium gas
Abstract By using a kinetic approach, we study the evolution of a gas composed of interacting ortho-and para-positronium atoms. We calculate the total lifetime of the gas and study the ortho-positronium quenching effect induced by the bi-atomic scattering mechanisms with spin exchange. We analyze a realistic situation where the positronium is formed by highly energetic positrons impinging on a solid surface. In the case of a spin-polarized source of positrons, the spin-polarization time of the final positronium gas is estimated
- âŠ