8,541 research outputs found
Overcharging: The Crucial Role of Excluded Volume
In this Letter we investigate the mechanism for overcharging of a single
spherical colloid in the presence of aqueous salts within the framework of the
primitive model by molecular dynamics (MD) simulations as well as
integral-equation theory. We find that the occurrence and strength of
overcharging strongly depends on the salt-ion size, and the available volume in
the fluid. To understand the role of the excluded volume of the microions, we
first consider an uncharged system. For a fixed bulk concentration we find that
upon increasing the fluid particle size one strongly increases the local
concentration nearby the colloidal surface and that the particles become
laterally ordered. For a charged system the first surface layer is built up
predominantly by strongly correlated counterions. We argue that this a key
mechanism to produce overcharging with a low electrostatic coupling, and as a
more practical consequence, to account for charge inversion with monovalent
aqueous salt ions.Comment: 7 pages, 3 figs (4 EPS files). To appear in Europhysics Letter
A criterion for entanglement in two two-level systems
We prove a necessary and sufficient condition for the occurrence of
entanglement in two two-level systems, simple enough to be of experimental
interest. Our results are illustrated in the context of a spin star system
analyzing the exact entanglement evolution of the central couple of spins.Comment: 4 pages, Submitted to Physical Review Letter
Determination of rotation periods in solar-like stars with irregular sampling: the Gaia case
We present a study on the determination of rotation periods (P) of solar-like
stars from the photometric irregular time-sampling of the ESA Gaia mission,
currently scheduled for launch in 2013, taking into account its dependence on
ecliptic coordinates. We examine the case of solar-twins as well as thousands
of synthetic time-series of solar-like stars rotating faster than the Sun. In
the case of solar twins we assume that the Gaia unfiltered photometric passband
G will mimic the variability of the total solar irradiance (TSI) as measured by
the VIRGO experiment. For stars rotating faster than the Sun, light-curves are
simulated using synthetic spectra for the quiet atmosphere, the spots, and the
faculae combined by applying semi-empirical relationships relating the level of
photospheric magnetic activity to the stellar rotation and the Gaia
instrumental response. The capabilities of the Deeming, Lomb-Scargle, and Phase
Dispersion Minimisation methods in recovering the correct rotation periods are
tested and compared. The false alarm probability (FAP) is computed using Monte
Carlo simulations and compared with analytical formulae. The Gaia scanning law
makes the rate of correct detection of rotation periods strongly dependent on
the ecliptic latitude (beta). We find that for P ~ 1 d, the rate of correct
detection increases with ecliptic latitude from 20-30 per cent at beta ~
0{\deg} to a peak of 70 per cent at beta=45{\deg}, then it abruptly falls below
10 per cent at beta > 45{\deg}. For P > 5 d, the rate of correct detection is
quite low and for solar twins is only 5 per cent on average.Comment: 12 pages, 18 figures, accepted by MNRA
Reentrant transitions in colloidal or dusty plasma bilayers
The phase diagram of crystalline bilayers of particles interacting via a
Yukawa potential is calculated for arbitrary screening lengths and particle
densities. Staggered rectangular, square, rhombic and triangular structures are
found to be stable including a first-order transition between two different
rhombic structures. For varied screening length at fixed density, one of these
rhombic phases exhibits both a single and even a double reentrant transition.
Our predictions can be verified experimentally in strongly confined charged
colloidal suspensions or dusty plasma bilayers.Comment: 4 pages, 3 eps figs - revtex4. PRL - in pres
Statistics of polymer adsorption under shear flow
Using non-equilibrium Brownian dynamics computer simulations, we have
investigated the steady state statistics of a polymer chain under three
different shear environments: i) linear shear flow in the bulk (no walls), ii)
shear vorticity normal to the adsorbing wall, iii) shear gradient normal to the
adsorbing wall. The statistical distribution of the chain end-to-end distance
and its orientational angles are calculated within our monomer-resolved
computer simulations. Over a wide range of shear rates, this distribution can
be mapped onto a simple theoretical finite-extensible-nonlinear-elastic
dumbbell model with fitted anisotropic effective spring constants. The tails of
the angular distribution functions are consistent with scaling predictions
borrowed from the bulk dumbbell model. Finally, the frequency of the
characteristic periodic tumbling motion has been investigated by simulation as
well and was found to be sublinear with the shear rate for the three set-ups,
which extends earlier results done in experiments and simulations for free and
tethered polymer molecules without adsorption.Comment: 10 figure
W-like states of N uncoupled spins 1/2
The exact dynamics of a disordered spin star system, describing a central
spin coupled to N distinguishable and non interacting spins 1/2, is reported.
Exploiting their interaction with the central single spin system, we present
possible conditional schemes for the generation of W-like states, as well as of
well-defined angular momentum states, of the N uncoupled spins. We provide in
addition a way to estimate the coupling intensity between each of the N spins
and the central one. Finally the feasibility of our procedure is briefly
discussed.Comment: 8 pages, 2 figures, accepted for publication in European Physical
Journal Special Topic
Binary crystals in two-dimensional two-component Yukawa mixtures
The zero-temperature phase diagram of binary mixtures of particles
interacting via a screened Coulomb pair potential is calculated as a function
of composition and charge ratio. The potential energy obtained by a Lekner
summation is minimized among a variety of candidate two-dimensional crystals. A
wealth of different stable crystal structures is identified including
structures [ particles correspond to large
(small) charge.] Their elementary cells consist of triangular, square or
rhombic lattices of the particles with a basis comprising various
structures of and particles. For small charge asymmetry there are no
intermediate crystals besides the pure and triangular crystals.Comment: RevTeX 4 - 17 pages - 6 main figure
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