1,928 research outputs found
Huge (but finite) time scales in slow relaxations: beyond simple aging
Experiments performed in the last years demonstrated slow relaxations and
aging in the conductance of a large variety of materials. Here, we present
experimental and theoretical results for conductance relaxation and aging for
the case-study example of porous silicon. The relaxations are experimentally
observed even at room temperature over timescales of hours, and when a strong
electric field is applied for a time , the ensuing relaxation depends on
. We derive a theoretical curve and show that all experimental data
collapse onto it with a single timescale as a fitting parameter. This timescale
is found to be of the order of thousands of seconds at room temperature. The
generic theory suggested is not fine-tuned to porous silicon, and thus we
believe the results should be universal, and the presented method should be
applicable for many other systems manifesting memory and other glassy effects.Comment: 4+ pages, 4 figure
A model of an expanding giant that swallowed planets for the eruption of V838 Monocerotis
In early 2002 V838 Monocerotis had an extraordinary outburst whose nature is
still unclear. The optical light curve showed at least three peaks and imaging
revealed a light echo around the object - evidence for a dust shell which was
emitted several thousand years ago and now reflecting light from the eruption.
Spectral analysis suggests that the object was relatively cold throughout the
event, which was characterized by an expansion to extremely large radii. We
show that the three peaks in the light curve have a similar shape and thus it
seems likely that a certain phenomenon was three times repeated. Our suggestion
that the outburst was caused by the expansion of a red giant, followed by the
successive swallowing of three relatively massive planets in close orbits,
supplies a simple explanation to all observed peculiarities of this intriguing
object.Comment: 5 pages, 1 LaTex file, 2 .eps figures, accepted for publication in
MNRA
Polyelectrolyte Persistence Length: Attractive Effect of Counterion Correlations and Fluctuations
The persistence length of a single, strongly charged, stiff polyelectrolyte
chain is investigated theoretically. Path integral formulation is used to
obtain the effective electrostatic interaction between the monomers. We find
significant deviations from the classical Odijk, Skolnick and Fixman (OSF)
result. An induced attraction between monomers is due to thermal fluctuations
and correlations between bound counterions. The electrostatic persistence
length is found to be smaller than the OSF value and indicates a possible
mechanical instability (collapse) for highly charged polyelectrolytes with
multivalent counterions. In addition, we calculate the amount of condensed
counterions on a slightly bent polyelectrolyte. More counterions are found to
be adsorbed as compared to the Manning condensation on a cylinder.Comment: 5 pages, 1 ps figur
On the Path Integral Representation for Spin Systems
We propose a classical constrained Hamiltonian theory for the spin. After the
Dirac treatment we show that due to the existence of second class constraints
the Dirac brackets of the proposed theory represent the commutation relations
for the spin. We show that the corresponding partition function, obtained via
the Fadeev-Senjanovic procedure, coincides with the one obtained using coherent
states. We also evaluate this partition function for the case of a single spin
in a magnetic field.Comment: To be published in J.Phys. A: Math. and Gen. Latex file, 12 page
Securing One Time Password (OTP) for Multi-Factor Out-of-Band Authentication through a 128-bit Blowfish Algorithm
Authentication and cryptography have been used to address security issues on various online services. However, researchers discovered that even the most commonly used multi-factor out-of-band authentication mechanism was vulnerable to attacks and traditional crypto-algorithms were characterized to have some drawbacks making it crucial to choose desirable algorithms for a particular purpose. This study introduces an innovative modification of the Blowfish algorithm designed to capitalize on its strengths but supports 128-bits block size text input using dynamic selection encryption method and reduction of cipher function execution through randomly determined rounds. Experimentation results on 128-bit input text revealed significant performance improvements with utmost 5.91 % in terms of avalanche effect, 38.97 % for integrity, and 41.02 % in terms of execution time. Results also showed that the modification introduced extra security layer, thus, displaying higher complexity and stronger diffusion at faster execution time making it more difficult and complex for an unauthorized individual to decipher the information and desirable to be used for applications with multiple users respectively. This is a good contribution to the continuous developments in the field of information security particularly in cryptography and towards providing a secure OTP for multifactor out-of-band authentication
Neutron Stars as Type-I Superconductors
In a recent paper by Link, it was pointed out that the standard picture of
the neutron star core composed of a mixture of a neutron superfluid and a
proton type-II superconductor is inconsistent with observations of a long
period precession in isolated pulsars. In the following we will show that an
appropriate treatment of the interacting two-component superfluid (made of
neutron and proton Cooper pairs), when the structure of proton vortices is
strongly modified, may dramatically change the standard picture, resulting in a
type-I superconductor. In this case the magnetic field is expelled from the
superconducting regions of the neutron star leading to the formation of the
intermediate state when alternating domains of superconducting matter and
normal matter coexist.Comment: 4 page
A Novel Mechanism for Type-I Superconductivity in Neutron Stars
We suggest a mechanism that may resolve a conflict raised by Link between the
precession of a neutron star and the standard picture in which its core is
composed of a mixture of a neutron superfluid and a type-II proton
superconductor. We will show that if there is a persistent, non-dissipating
current running along the magnetic flux tubes, the force between magnetic flux
tubes may be attractive, resulting in a type-I, rather than a type-II,
superconductor. If this is the case, the conflict between the observed
precession and the canonical estimation of the Landau-Ginzburg parameter (which
suggests type II behaviour) will be automatically resolved. Such a current
arises in some condensed matter systems and may also appear in QCD dense matter
as a consequence of quantum anomalies. We calculate the interaction between two
vortices carrying a current j and find a constraint on the magnitude of j where
a superconductor is always type-I, even when the cannonical Landau-Ginzburg
parameter indicates type-II behaviour. If this condition is met, the magnetic
field is expelled from the superconducting regions of the neutron star leading
to the formation of the intermediate state where alternating domains of
superconducting matter and normal matter coexist. We further argue that even
when the induced current is small the vortex Abrikosov lattice will
nevertheless be destroyed due to the helical instability studied previously in
many condensed matter systems. This would also resolve the apparent
contradiction with the precession of the neutron stars. We also discuss some
instances where anomalous induced current may play a crucial role, such as the
neutron star kicks, pulsar glitches and the toroidal magnetic field.Comment: 10 pages, Additional arguments are given supporting the idea that the
Abrikosov lattice will be destroyed in regions where longitudinal currents
are induce
Morphology and structure of ABS membranes filled with two different activated carbons
Mixed matrix-composite membranes (MMCM) for gas separation are prepared and characterized in this work. Acrylonitrile-butadiene-styrene (ABS) copolymer was used for the continuum phase of the membrane filled with two different activated carbons (AC). The so-obtained membranes have been characterized by gas permeability, optical microscopy, electronic microscopy and atomic force microscopy. The membranes have different roughness on both their surfaces but are always recovered by the polymeric material. Better ABS-AC adhesion has been always reached giving high selectivity and permeability for CO2 / CH4. Such intimate contact can be attributed to the rubber properties of the butadiene-styrene chains in ABS. The morphological characteristics and the increase in both permeability and selectivity with the volume fraction of the filler are explained in terms of the properties of pure activated carbons.Fil: Marchese, Jose. Universidad Nacional de San Luis; Argentina. Universidad Nacional de San Luis. Facultad de Ciencias Fisico Matematicas y Naturales. Departamento de Fisica. Laboratorio de Ciencias de Superficies y Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis; ArgentinaFil: Anson, M.. Universidad Nacional de San Luis; Argentina. Universidad Nacional de San Luis. Facultad de Ciencias Fisico Matematicas y Naturales. Departamento de Fisica. Laboratorio de Ciencias de Superficies y Medios Porosos; ArgentinaFil: Ochoa, Nelio Ariel. Universidad Nacional de San Luis; Argentina. Universidad Nacional de San Luis. Facultad de Ciencias Fisico Matematicas y Naturales. Departamento de Fisica. Laboratorio de Ciencias de Superficies y Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis; ArgentinaFil: Prádanos, P.. Universidad de Valladolid; EspañaFil: Palacio, L.. Universidad de Valladolid; EspañaFil: Hernández, A.. Universidad de Valladolid; Españ
Detailed study of SNR G306.3–0.9 using XMM-Newton and Chandra observations
Aims. We aim to study the spatial distribution of the physical and chemical properties of the X-ray emitting plasma of the supernova remnant (SNR) G306.3-0.9 in detail to obtain constraints on its ionization stage, the progenitor supernova explosion, and the age of the remnant.
Methods. We used combined data from XMM-Newton and Chandra observatories to study the X-ray morphology of G306.3-0.9 in detail. A spatially resolved spectral analysis was used to obtain physical and geometrical parameters of different regions of the remnant. Spitzer infrared observations, available in the archive, were also used to constrain the progenitor supernova and study the environment in which the remnant evolved.
Results. The X-ray morphology of the remnant displays a non-uniform structure of semi-circular appearance, with a bright southwest region and very weak or almost negligible X-ray emission in its northern part. These results indicate that the remnant is propagating in a non-uniform environment as the shock fronts are encountering a high-density medium, where enhanced infrared emission is detected. The X-ray spectral analysis of the selected regions shows distinct emission-line features of several metal elements, confirming the thermal origin of the emission. The X-ray spectra are well represented by a combination of two absorbed thermal plasma models: one in equilibrium ionization (VAPEC) with a mean temperature of ∼0.19 keV, and another out of equilibrium ionization (VNEI) at a higher temperature of ∼1.1 or 1.6-1.9 keV. For regions located in the northeast, central, and southwest part of the SNR, we found elevated abundances of Si, S, Ar, Ca, and Fe, typical of ejecta material. The outer regions located northwest and south show values of the abundances above solar but lower than to those found in the central regions. This suggests that the composition of the emitting outer parts of the SNR is a combination of ejecta and shocked material of the interstellar medium. The comparison between the S/Si, Ar/Si, and Ca/Si abundances ratios (1.75, 1.27, and 2.72 in the central region, respectively), favor a Type Ia progenitor for this remnant, a result that is also supported by an independent morphological analysis using the X-ray and 24 μm IR data.Facultad de Ciencias Astronómicas y GeofísicasInstituto Argentino de Radioastronomí
Why is the B -> eta' X decay width so large ?
New mechanism for the observed inclusive B -> \eta'X decay is suggested. We
argue that the dominant contribution to this amplitude is due to the Cabbibo
favored b -> \bar{c}cs process followed by the transition \bar{c}c -> \eta'. A
large magnitude of the "intrinsic charm" component of \eta' is of critical
importance in our approach. Our results are consistent with an unexpectedly
large Br(B -> \eta'+X) \sim 10^{-3} recently announced by CLEO. We stress the
uniqueness of this channel for 0^{-+} gluonia search.Comment: Comments on a mixing model for intrinsic charm and pre-asymptotic
effects and some references are added. Latex, 9 page
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