3,619 research outputs found
Validity of the N\'{e}el-Arrhenius model for highly anisotropic Co_xFe_{3-x}O_4 nanoparticles
We report a systematic study on the structural and magnetic properties of
Co_{x}Fe_{3-x}O_{4} magnetic nanoparticles with sizes between to nm,
prepared by thermal decomposition of Fe(acac)_{3} and Co(acac)_{2}. The large
magneto-crystalline anisotropy of the synthesized particles resulted in high
blocking temperatures ( K \leqq K for d nm ) and large coercive fields ( kA/m for K).
The smallest particles ( nm) revealed the existence of a magnetically
hard, spin-disordered surface. The thermal dependence of static and dynamic
magnetic properties of the whole series of samples could be explained within
the N\'{e}el-Arrhenius relaxation framework without the need of ad-hoc
corrections, by including the thermal dependence of the magnetocrystalline
anisotropy constant through the empirical Br\"{u}khatov-Kirensky
relation. This approach provided values very similar to the bulk
material from either static or dynamic magnetic measurements, as well as
realistic values for the response times ( s).
Deviations from the bulk anisotropy values found for the smallest particles
could be qualitatively explained based on Zener\'{}s relation between
and M(T)
Probing the stability of superheavy dark matter particles with high-energy neutrinos
Two of the most fundamental properties of the dark matter particle, the mass
and the lifetime, are only weakly constrained by the astronomical and
cosmological evidence of dark matter. We derive in this paper lower limits on
the lifetime of dark matter particles with masses in the range 10 TeV-10^15 TeV
from the non-observation of ultrahigh energy neutrinos in the AMANDA, IceCube,
Auger and ANITA experiments. For dark matter particles which produce neutrinos
in a two body or a three body decay, we find that the dark matter lifetime must
be longer than O(10^26-10^28) s for masses between 10 TeV and the Grand
Unification scale. Finally, we also calculate, for concrete particle physics
scenarios, the limits on the strength of the interactions that induce the dark
matter decay.Comment: 17 pages, 6 figures; v2: references added, discussion improved,
matches the version published at JCA
Small steps towards Grand Unification and the electron/positron excesses in cosmic-ray experiments
We consider a small extension of the standard model by adding two Majorana
fermions; those are adjoint representations of the SU(2)_L and SU(3)_c gauge
groups of the standard model. In this extension, the gauge coupling unification
at an energy scale higher than 10^{15} GeV is realized when the masses of the
triplet and the octet fermions are smaller than 10^4 GeV and 10^{12} GeV,
respectively. We also show that an appropriate symmetry ensures a long lifetime
of the neutral component of the triplet fermion whose thermal relic density
naturally explains the observed dark matter density. The electron/positron
excesses observed in recent cosmic-ray experiments can be also explained by the
decay of the triplet fermion.Comment: 11 pages, 5 figure
Computing with cells: membrane systems - some complexity issues.
Membrane computing is a branch of natural computing which abstracts computing models from the structure and the functioning of the living cell. The main ingredients of membrane systems, called P systems, are (i) the membrane structure, which consists of a hierarchical arrangements of membranes which delimit compartments where (ii) multisets of symbols, called objects, evolve according to (iii) sets of rules which are localised and associated with compartments. By using the rules in a nondeterministic/deterministic maximally parallel manner, transitions between the system configurations can be obtained. A sequence of transitions is a computation of how the system is evolving. Various ways of controlling the transfer of objects from one membrane to another and applying the rules, as well as possibilities to dissolve, divide or create membranes have been studied. Membrane systems have a great potential for implementing massively concurrent systems in an efficient way that would allow us to solve currently intractable problems once future biotechnology gives way to a practical bio-realization. In this paper we survey some interesting and fundamental complexity issues such as universality vs. nonuniversality, determinism vs. nondeterminism, membrane and alphabet size hierarchies, characterizations of context-sensitive languages and other language classes and various notions of parallelism
Higgs Decays in the Low Scale Type I See-Saw Model
The couplings of the low scale type I see-saw model are severely constrained
by the requirement of reproducing the correct neutrino mass and mixing
parameters, by the non-observation of lepton number and charged lepton flavour
violating processes and by electroweak precision data. We show that all these
constraints still allow for the possibility of an exotic Higgs decay channel
into a light neutrino and a heavy neutrino with a sizable branching ratio. We
also estimate the prospects to observe this decay at the LHC and discuss its
complementarity to the indirect probes of the low scale type I see-saw model
from experiments searching for the decay.Comment: 15 pages, 8 figures; references added and results unchanged; matched
with the published version on PL
- …