21,517 research outputs found
The Redner - Ben-Avraham - Kahng cluster system
We consider a coagulation model first introduced by Redner, Ben-Avraham and
Krapivsky in [Redner, Ben-Avraham, Kahng: Kinetics of 'cluster eating', J.
Phys. A: Math. Gen., 20 (1987), 1231-1238], the main feature of which is that
the reaction between a j-cluster and a k-cluster results in the creation of a
|j-k|-cluster, and not, as in Smoluchowski's model, of a (j+k)-cluster. In this
paper we prove existence and uniqueness of solutions under reasonably general
conditions on the coagulation coefficients, and we also establish
differenciability properties and continuous dependence of solutions. Some
interesting invariance properties are also proved. Finally, we study the
long-time behaviour of solutions, and also present a preliminary analysis of
their scaling behaviour.Comment: 24 pages. 2 figures. Dedicated to Carlos Rocha and Luis Magalhaes on
the occasion of their sixtieth birthday
Flux-tube geometry and solar wind speed during an activity cycle
The solar wind speed at 1 AU shows variations in latitude and in time which
reflect the evolution of the global background magnetic field during the
activity cycle. It is commonly accepted that the terminal wind speed in a
magnetic flux-tube is anti-correlated with its expansion ratio, which motivated
the definition of widely-used semi-empirical scaling laws relating one to the
other. In practice, such scaling laws require ad-hoc corrections. A predictive
law based solely on physical principles is still missing. We test whether the
flux-tube expansion is the controlling factor of the wind speed at all phases
of the cycle and at all latitudes using a very large sample of wind-carrying
open magnetic flux-tubes. We furthermore search for additional physical
parameters based on the geometry of the coronal magnetic field which have an
influence on the terminal wind flow speed. We use MHD simulations of the corona
and wind coupled to a dynamo model to provide a large statistical ensemble of
open flux-tubes which we analyse conjointly in order to identify relations of
dependence between the wind speed and geometrical parameters of the flux-tubes
which are valid globally (for all latitudes and moments of the cycle). Our
study confirms that the terminal speed of the solar wind depends very strongly
on the geometry of the open magnetic flux-tubes through which it flows. The
total flux-tube expansion is more clearly anti-correlated with the wind speed
for fast rather than for slow wind flows, and effectively controls the
locations of these flows during solar minima. Overall, the actual asymptotic
wind speeds attained are also strongly dependent on field-line inclination and
magnetic field amplitude at the foot-points. We suggest ways of including these
parameters on future predictive scaling-laws for the solar wind speed.Comment: Accepted for publicaton on Astronomy & Astrophysic
The Redner - Ben-Avraham - Kahng coagulation system with constant coefficients: the finite dimensional case
We study the behaviour as of solutions to the
Redner--Ben-Avraham--Kahng coagulation system with positive and compactly
supported initial data, rigorously proving and slightly extending results
originally established in [4] by means of formal arguments.Comment: 13 pages, 1 figur
Critical current of a superconductor measured via injection of spin polarized carriers
In this paper we report a direct evidence of the suppression of critical
current due to pair-breaking in a superconducting micro-bridge when the
measurement is carried out by injecting spin polarised carriers instead of
normal electrons. A thin layer of La0.7Ca0.3MnO3 was used as the source of spin
polarised carriers. The micro-bridge was formed on the DyBa2Cu3O7-d thin film
by photo-lithographic techniques. The design of our spin-injection device
allowed us to inject spin-polarised carriers from the La0.7Ca0.3MnO3 layer
directly to the DyBa2Cu3O7- d micro-bridge (without any insulating buffer
layer) making it possible to measure the critical current when polarised
electrons alone are injected into the superconductor. Our results confirm the
role of polarised carriers in breaking the Cooper pairs in the superconductor.Comment: 8 pages, 4 figure
Tanaka-Tagoshi Parametrization of post-1PN Spin-Free Gravitational Wave Chirps: Equispaced and Cardinal Interpolated Lattices For First Generation Interferometric Antennas
The spin-free binary-inspiral parameter-space introduced by Tanaka and
Tagoshi to construct a uniformly-spaced lattice of templates at (and possibly
beyond) order is shown to work for all first generation interferometric
gravitational wave antennas. This allows to extend the minimum-redundant
cardinal interpolation techniques of the correlator bank developed by the
Authors to the highest available order PN templates. The total number of 2PN
templates to be computed for a minimal match is reduced by a
factor 4, as in the 1PN case.Comment: 9 pages, 8 figures, 3 tables, accepted for publication in Phys. Rev.
A phenomenological model for magnetoresistance in granular polycrystalline colossal magnetoresistive materials: the role of spin polarised tunnelling at the grain boundaries
It has been observed that in bulk and polycrystalline thin films of collossal
magnetoresistive (CMR) materials the magnetoresistance follows a different
behaviour compared to single crystals or single crystalline films below the
ferromagnetic transition temperature Tc. In this paper we develop a
phenomenological model to explain the magnetic field dependence of resistance
in granular CMR materials taking into account the spin polarised tunnelling at
the grain boundaries. The model has been fitted to two systems, namely,
La0.55Ho0.15Sr0.3MnO3 and La1.8Y0.5Ca0.7Mn2O7. From the fitted result we have
separated out, in La0.55Ho0.15Sr0.3MnO3, the intrinsic contribution from the
intergranular contribution to the magnetoresistance coming from spin polarised
tunnelling at the grain boundaries. It is observed that the temperature
dependence of the intrinsic contribution to the magnetoresistance in
La0.55Ho0.15Sr0.3MnO3 follows the prediction of double exchange model for all
values of field.Comment: 14 pages + 5 figures, postscript (to appear in Journal of Applied
Physics
Thermal and non-thermal emission from reconnecting twisted coronal loops
Twisted magnetic fields should be ubiquitous in flare-producing active
regions where the magnetic fields are strongly non-potential. It has been shown
that reconnection in helical magnetic coronal loops results in plasma heating
and particle acceleration distributed within a large volume, including the
lower coronal and chromospheric sections of the loops. This scenario can be an
alternative to the standard flare model, where particles are accelerated only
in a small volume located in the upper corona. We use a combination of MHD
simulations and test-particle methods, which describe the development of kink
instability and magnetic reconnection in twisted coronal loops using resistive
compressible MHD, and incorporate atmospheric stratification and large-scale
loop curvature. The resulting distributions of hot plasma let us estimate
thermal X-ray emission intensities. The electric and magnetic fields obtained
are used to calculate electron trajectories using the guiding-centre
approximation. These trajectories combined with the MHD plasma density
distributions let us deduce synthetic HXR bremsstrahlung intensities. Our
simulations emphasise that the geometry of the emission patterns produced by
hot plasma in flaring twisted coronal loops can differ from the actual geometry
of the underlying magnetic fields. The twist angles revealed by the emission
threads (SXR) are consistently lower than the field-line twist present at the
onset of the kink-instability. HXR emission due to the interaction of energetic
electrons with the stratified background are concentrated at the loop
foot-points in these simulations, even though the electrons are accelerated
everywhere within the coronal volume of the loop. The maximum of HXR emission
consistently precedes that of SXR emission, with the HXR light-curve being
approximately proportional to the temporal derivative of the SXR light-curve.Comment: (accepted for publication on A&A
Large classical universes emerging from quantum cosmology
It is generally believed that one cannot obtain a large Universe from quantum
cosmological models without an inflationary phase in the classical expanding
era because the typical size of the Universe after leaving the quantum regime
should be around the Planck length, and the standard decelerated classical
expansion after that is not sufficient to enlarge the Universe in the time
available. For instance, in many quantum minisuperspace bouncing models studied
in the literature, solutions where the Universe leave the quantum regime in the
expanding phase with appropriate size have negligible probability amplitude
with respect to solutions leaving this regime around the Planck length. In this
paper, I present a general class of moving gaussian solutions of the
Wheeler-DeWitt equation where the velocity of the wave in minisuperspace along
the scale factor axis, which is the new large parameter introduced in order to
circumvent the abovementioned problem, induces a large acceleration around the
quantum bounce, forcing the Universe to leave the quantum regime sufficiently
big to increase afterwards to the present size, without needing any classical
inflationary phase in between, and with reasonable relative probability
amplitudes with respect to models leaving the quantum regime around the Planck
scale. Furthermore, linear perturbations around this background model are free
of any transplanckian problem.Comment: 8 pages, 1 figur
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