6,781 research outputs found
Approximated integrability of the Dicke model
A very approximate second integral of motion of the Dicke model is identified
within a broad region above the ground state, and for a wide range of values of
the external parameters. This second integral, obtained from a Born Oppenheimer
approximation, classifies the whole regular part of the spectrum in bands
labelled by its corresponding eigenvalues. Results obtained from this
approximation are compared with exact numerical diagonalization for finite
systems in the superradiant phase, obtaining a remarkable accord. The region of
validity of our approach in the parameter space, which includes the resonant
case, is unveiled. The energy range of validity goes from the ground state up
to a certain upper energy where chaos sets in, and extends far beyond the range
of applicability of a simple harmonic approximation around the minimal energy
configuration. The upper energy validity limit increases for larger values of
the coupling constant and the ratio between the level splitting and the
frequency of the field. These results show that the Dicke model behaves like a
two-degree of freedom integrable model for a wide range of energies and values
of the external parameters.Comment: 6 pages, 3 figures. Second version with added text, references and
some new figure
Molecular gas and stars in the translucent cloud MBM 18 (LDN 1569)
Seven of ten candidate H-alpha emission-line stars found in an objective
grism survey of a 1 square degree region in MBM 18, were observed
spectroscopically. Four of these have weak H-alpha emission, and 6 out of 7
have spectral types M1-M4V. One star is of type F7-G1V, and has H-alpha in
absorption. The spectra of three of the M-stars may show an absorption line of
LiI, although none of these is an unambiguous detection. For the six M-stars a
good fit is obtained with pre-main-sequence isochrones indicating ages between
7.5 and 15Myr. The molecular cloud mass, derived from the integrated 12CO(1-0)
emission, is 160Mo (for a distance of 120pc), much smaller than the virial mass
(10^3Mo), and the cloud is not gravitationally bound. Nor are the individual
clumps we identified through a clump-finding routine. Considering the relative
weakness or absence of the H-alpha emission, the absence of other emission
lines, and the lack of clear LiI absorption, the targets are not T Tauri stars.
With ages between 7.5 and 15Myr they are old enough to explain the lack of
lithium in their spectra. Based on the derived distances (60-250pc), some of
the stars may lie inside the molecular cloud (120-150pc). From the fact that
the cloud as a whole, as well as the individual clumps, are not gravitationally
bound, in combination with the ages of the stars we conclude that it is not
likely that (these) stars were formed in MBM 18.Comment: Accepted for publication in Astronomy & Astrophysics (20 pages
Simulation of low-energy antiproton annihilation in silicon
The simulation of the interaction of low-energy antiprotons in a silicon target is of primary importance to the AEGIS Collaboration at CERN. In Geant4 two different models, called CHIPS and FTFP, are implemented for the simulation
of ¯p-nucleus annihilation at rest, but their validation is currently incomplete. The study presented in the following aims at describing their behavior while dealing with annihilation at rest in silicon, comparing their results on the one hand with data available in the literature and on the other hand with data collected by the AEGIS Collaboration using a silicon prototype
Enhancement of rare-earth--transition-metal exchange interaction in PrFe probed by inelastic neutron scattering
The fundamental magnetic interactions of PrFe are studied by
inelastic neutron scattering and anisotropy field measurements. Data analysis
confirms the presence of three magnetically inequivalent sites, and reveals an
exceptionally large value of the exchange field. The unexpected importance of
-mixing effects in the description of the ground-state properties of
PrFe is evidenced, and possible applications of related compounds
are envisaged.Comment: 4 RevTeX pages, 4 EPS figures. Accepted for publication by Appl.
Phys. Lett. (will be found at http://apl.aip.org
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