20,677 research outputs found
Coercivity reduction in a two-dimensional array of nanoparticles
We report on theoretical investigation of the magnetization reversal in
two-dimensional arrays of ferromagnetic nano-particles with parameters of
cobalt. The system was optimized for achieving the lowest coercivity in an
array of particles located in the nodes of triangular, hexagonal and square
grids. Based on the numerical solution of the non-stochastic
Landau-Lifshitz-Gilbert equation we show that each particle distribution type
is characterized with a proper optimal distance, allowing to lower the
coercivity values for approximately 30% compared with the reference value
obtained for a single nano-particle. It was shown that the reduction of
coercivity occurs even if the particle position in the array is not very
precise. In particular, the triangular particle arrangement maintained the same
optimal distance between the particles under up to 20% random displacements of
their position within the array.Comment: 7 pages, 5 figure
Solving the kilo-second QPO problem of the intermediate polar GK Persei
We detect the likely optical counterpart to previously reported X-ray QPOs in
spectrophotometry of the intermediate polar GK Persei during the 1996 dwarf
nova outburst. The characteristic timescales range between 4000--6000 s.
Although the QPOs are an order of magnitude longer than those detected in the
other dwarf novae we show that a new QPO model is not required to explain the
long timescale observed. We demonstrate that the observations are consistent
with oscillations being the result of normal-timescale QPOs beating with the
spin period of the white dwarf. We determine the spectral class of the
companion to be consistent with its quiescent classification and find no
significant evidence for irradiation over its inner face. We detect the white
dwarf spin period in line fluxes, V/R ratios and Doppler-broadened emission
profiles.Comment: 14 pages, 11 figures. Accepted for publication in MNRA
Dark Matter and Vector-like Leptons From Gauged Lepton Number
We investigate a simple model where Lepton number is promoted to a local
gauge symmetry which is then spontaneously broken, leading to a viable
thermal DM candidate and vector-like leptons as a byproduct. The dark matter
arises as part of the exotic lepton sector required by the need to satisfy
anomaly cancellation and is a Dirac electroweak (mostly) singlet neutrino. It
is stabilized by an accidental global symmetry of the renormalizable Lagrangian
which is preserved even after the gauged lepton number is spontaneously broken
and can annihilate efficiently to give the correct thermal relic abundance. We
examine the ability of this model to give a viable DM candidate and discuss
both direct and indirect detection implications. We also examine some of the
LHC phenomenology of the associated exotic lepton sector and in particular its
effects on Higgs decays.Comment: References and a few comments adde
Optimization of soliton ratchets in inhomogeneous sine-Gordon systems
Unidirectional motion of solitons can take place, although the applied force
has zero average in time, when the spatial symmetry is broken by introducing a
potential , which consists of periodically repeated cells with each cell
containing an asymmetric array of strongly localized inhomogeneities at
positions . A collective coordinate approach shows that the positions,
heights and widths of the inhomogeneities (in that order) are the crucial
parameters so as to obtain an optimal effective potential that yields
a maximal average soliton velocity. essentially exhibits two
features: double peaks consisting of a positive and a negative peak, and long
flat regions between the double peaks. Such a potential can be obtained by
choosing inhomogeneities with opposite signs (e.g., microresistors and
microshorts in the case of long Josephson junctions) that are positioned close
to each other, while the distance between each peak pair is rather large. These
results of the collective variables theory are confirmed by full simulations
for the inhomogeneous sine-Gordon system
Inhomogeneous soliton ratchets under two ac forces
We extend our previous work on soliton ratchet devices [L. Morales-Molina et
al., Eur. Phys. J. B 37, 79 (2004)] to consider the joint effect of two ac
forces including non-harmonic drivings, as proposed for particle ratchets by
Savele'v et al. [Europhys. Lett. 67}, 179 (2004); Phys. Rev. E {\bf 70} 066109
(2004)]. Current reversals due to the interplay between the phases, frequencies
and amplitudes of the harmonics are obtained. An analysis of the effect of the
damping coefficient on the dynamics is presented. We show that solitons give
rise to non-trivial differences in the phenomenology reported for particle
systems that arise from their extended character. A comparison with soliton
ratchets in homogeneous systems with biharmonic forces is also presented. This
ratchet device may be an ideal candidate for Josephson junction ratchets with
intrinsic large damping
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