26,272 research outputs found
Ion specificity and the theory of stability of colloidal suspensions
A theory is presented which allow us to accurately calculate the critical
coagulation concentration (CCC) of hydrophobic colloidal suspensions. For
positively charged particles the CCC's follow the Hofmeister (lyotropic)
series. For negatively charged particles the series is reversed. We find that
strongly polarizable chaotropic anions are driven towards the colloidal surface
by electrostatic and hydrophobic forces. Within approximately one ionic radius
from the surface, the chaotropic anions loose part of their hydration sheath
and become strongly adsorbed. The kosmotropic anions, on the other hand, are
repelled from the hydrophobic surface. The theory is quantitatively accurate
without any adjustable parameters. We speculate that the same mechanism is
responsible for the Hofmeister series that governs stability of protein
solutions.Comment: Phys. Rev. Lett. (in press
Cultivo da soca de arroz irrigado.
A soca de arroz oferece oportunidade para aumentar a produção de grãos por unidade de área cultivada pois apresenta menor duração de crescimento que um novo cultivo. A sua importância como uma alternativa para aumentar a produção sem acrescer a área de cultivo e com menor custo de produção deve ser enfatizada.bitstream/CNPAF/19416/1/circ_40.pd
Influence of chirping the Raman lasers in an atom gravimeter: phase shifts due to the Raman light shift and to the finite speed of light
We present here an analysis of the influence of the frequency dependence of
the Raman laser light shifts on the phase of a Raman-type atom gravimeter.
Frequency chirps are applied to the Raman lasers in order to compensate gravity
and ensure the resonance of the Raman pulses during the interferometer. We show
that the change in the Raman light shift when this chirp is applied only to one
of the two Raman lasers is enough to bias the gravity measurement by a fraction
of Gal (Gal~=~~m/s). We also show that this effect is
not compensated when averaging over the two directions of the Raman wavevector
. This thus constitutes a limit to the rejection efficiency of the
-reversal technique. Our analysis allows us to separate this effect from the
effect of the finite speed of light, which we find in perfect agreement with
expected values. This study highlights the benefit of chirping symmetrically
the two Raman lasers
Disordered two-dimensional superconductors: roles of temperature and interaction strength
We have considered the half-filled disordered attractive Hubbard model on a
square lattice, in which the on-site attraction is switched off on a fraction
of sites, while keeping a finite on the remaining ones. Through Quantum
Monte Carlo (QMC) simulations for several values of and , and for system
sizes ranging from to , we have calculated the
configurational averages of the equal-time pair structure factor , and,
for a more restricted set of variables, the helicity modulus, , as
functions of temperature. Two finite-size scaling {\it ansatze} for have
been used, one for zero-temperature and the other for finite temperatures. We
have found that the system sustains superconductivity in the ground state up to
a critical impurity concentration, , which increases with , at least up
to U=4 (in units of the hopping energy). Also, the normalized zero-temperature
gap as a function of shows a maximum near , for . Analyses of the helicity modulus and of the pair structure factor
led to the determination of the critical temperature as a function of , for
4 and 6: they also show maxima near , with the highest
increasing with in this range. We argue that, overall, the observed
behavior results from both the breakdown of CDW-superconductivity degeneracy
and the fact that free sites tend to "push" electrons towards attractive sites,
the latter effect being more drastic at weak couplings.Comment: 9 two-column pages, 14 figures, RevTe
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