7,924 research outputs found
Non-Kramers Freezing and Unfreezing of Tunneling in the Biaxial Spin Model
The ground state tunnel splitting for the biaxial spin model in the magnetic
field, H = -D S_{x}^2 + E S_{z}^2 - g \mu_B S_z H_z, has been investigated
using an instanton approach. We find a new type of spin instanton and a new
quantum interference phenomenon associated with it: at a certain field, H_2 =
2SE^{1/2}(D+E)^{1/2}/(g \mu_B), the dependence of the tunneling splitting on
the field switches from oscillations to a monotonic growth. The predictions of
the theory can be tested in Fe_8 molecular nanomagnets.Comment: 7 pages, minor changes, published in EP
Representative galaxy age-metallicity relationships
The ongoing surveys of galaxies and those for the next generation of
telescopes will demand the execution of high-CPU consuming machine codes for
recovering detailed star formation histories (SFHs) and hence age-metallicity
relationships (AMRs). We present here an expeditive method which provides
quick-look AMRs on the basis of representative ages and metallicities obtained
from colour-magnitude diagram (CMD) analyses. We have tested its perfomance by
generating synthetic CMDs for a wide variety of galaxy SFHs. The representative
AMRs turn out to be reliable down to a magnitude limit with a photometric
completeness factor higher than 85 per cent, and trace the chemical
evolution history for any stellar population (represented by a mean age and an
intrinsic age spread) with a total mass within ~ 40 per cent of the more
massive stellar population in the galaxy.Comment: 12 pages, 11 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Disorder induced brittle to quasi-brittle transition in fiber bundles
We investigate the fracture process of a bundle of fibers with random Young
modulus and a constant breaking strength. For two component systems we show
that the strength of the mixture is always lower than the strength of the
individual components. For continuously distributed Young modulus the tail of
the distribution proved to play a decisive role since fibers break in the
decreasing order of their stiffness. Using power law distributed stiffness
values we demonstrate that the system exhibits a disorder induced brittle to
quasi-brittle transition which occurs analogously to continuous phase
transitions. Based on computer simulations we determine the critical exponents
of the transition and construct the phase diagram of the system.Comment: 6 pages, 6 figure
The age-metallicity relationship in the Fornax spheroidal dwarf galaxy
We produce a comprehensive field star age-metallicity relationship (AMR) from
the earliest epoch until ~ 1 Gyr ago for three fields in the Fornax dSph galaxy
by using VI photometric data obtained with FORS1 at the VLT. We find that the
innermost one does not contains dominant very old stars (age > 12 Gyr), whereas
the relatively outer field does not account for representative star field
populations younger than ~ 3 Gyr. When focusing on the most prominent stellar
populations, we find that the derived AMRs are engraved by the evidence of a
outside-in star formation process. The studied fields show bimodal metallicity
distributions peaked at [Fe/H] = (-0.95 +- 0.15) dex and (-1.15 or -1.25 +-
0.05) dex, respectively, but only during the first half of the entire galaxy
lifetime. Furthermore, the more metal-rich population appears to be more
numerous in the outer fields, while in the innermost Fornax field the
contribution of both metallicity populations seems to be similar. We also find
that the metallicity spread ~ 6 Gyr ago is remarkable large, while the
intrinsic metallicity dispersion at ~ 1-2 Gyr results smaller than that for the
relatively older generations of stars. We interpret these outcomes as a result
of a possible merger of two galaxies that would have triggered a star formation
bursting process that peaked between ~ 6 and 9 Gyr ago, depending on the
position of the field in the galaxy.Comment: 7 pages, 5 figures, MNRAS, in pres
A 3-3-1 model with low scale seesaw mechanisms
We construct a viable 3-3-1 model with two scalar triplets,
extended fermion and scalar spectrum, based on the family symmetry
and other auxiliary cyclic symmetries, whose spontaneous breaking yields the
observed pattern of SM fermion mass spectrum and fermionic mixing parameters.
In our model the SM quarks lighter than the top quark, get their masses from a
low scale Universal seesaw mechanism, the SM charged lepton masses are produced
by a Froggatt-Nielsen mechanism and the small light active neutrino masses are
generated from an inverse seesaw mechanism. The model is consistent with the
low energy SM fermion flavor data and successfully accommodates the current
Higgs diphoton decay rate and predicts charged lepton flavor violating decays
within the reach of the forthcoming experiments.Comment: 24 pages, 4 figures. Published versio
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