631 research outputs found
Depletion interaction between spheres of unequal size and demixing in binary mixtures of colloids
The possibility to induce demixing in a colloidal mixture by adding small
polymers, or other equivalent depletant agents, is theoretically investigated.
By use of Mean Field Theory, suitably generalized to deal with short range
effective interactions, the phase diagram of a binary mixture ofcolloidal
particles (modelled as hard spheres) in a solvent is determined as a function
of the polymer concentration on the basis of the Asakura-Oosawa model.The
topology of the phase diagram changes when the relative size of the colloidal
particles is reduced: the critical line connecting the liquid-vapour critical
points of the two pure fluids breaks and the branch starting from the critical
point of the bigger particles bends to higher volume fractions, where
concentration fluctuations drive the transition. The effects of a softer
colloid-polymer interaction is also investigated: Even the presence of a small
repulsive tail in the potential gives rise to a significant lowering of the
stability threshold. In this case, phase transitions may take place by adding
just a few percent of depletant in volume fraction. These results may be
relevant for the interpretation of recent experiments of solidification
kinetics in colloidal mixtures.Comment: To be published in Molecular Physic
Shell Effects and Phase Separation in a Trapped Multi-Component Fermi System
Shell effects in the coordinate space can be seen with degenerate Fermi
vapors in non-uniform trapping potentials. In particular, below the Fermi
temperature, the density profile of a Fermi gas in a confining harmonic
potential is characterized by several local maxima. This effect is enhanced for
"magic numbers" of particles and in quasi-1D (cigar-shaped) configurations. In
the case of a multi-component Fermi vapor, the separation of Fermi components
in different spatial shells (phase-separation) depends on temperature, number
of particles and scattering length. We derive analytical formulas, based on
bifurcation theory, for the critical density of Fermions and the critical
chemical potential, which give rise to the phase-separation.Comment: to be published in the Proceedings of the VIII Meeting on Problems in
Theoretical Nuclear Physics, Cortona, October 18-20, 2000, Ed. G. Pisent, A.
Fabrocini and L. Canton (World Scientific
Effective wave-equations for the dynamics of cigar-shaped and disc-shaped Bose condensates
Starting from the 3D Gross-Pitaevskii equation and using a variational
approach, we derive an effective 1D wave-equation that describes the axial
dynamics of a Bose condensate confined in an external potential with
cylindrical symmetry. The trapping potential is harmonic in the transverse
direction and generic in the axial one. Our equation, that is a time-dependent
non-polynomial nonlinear Schr\"odinger equation (1D NPSE), can be used to model
cigar-shaped condensates, whose dynamics is essentially 1D. We show that 1D
NPSE gives much more accurate results than all other effective equations
recently proposed. By using 1D NPSE we find analytical solutions for bright and
dark solitons, which generalize the ones known in the literature. We deduce
also an effective 2D non-polynomial Schr\"odinger equation (2D NPSE) that
models disc-shaped Bose condensates confined in an external trap that is
harmonic along the axial direction and generic in the transverse direction. In
the limiting cases of weak and strong interaction, our approach gives rise to
Schr\"odinger-like equations with different polynomial nonlinearities.Comment: 7 pages, 5 figures, to be published in Phys. Rev.
Bosonic clouds with attractive interaction beyond the local interaction approximation
We study the properties of a Bose-Einstein condensed cloud of atoms with
negative scattering length confined in a harmonic trap. When a realistic non
local (finite range) effective interaction is taken into account, we find that,
besides the known low density metastable solution, a new branch of Bose
condensate appears at higher density. This state is self-bound but its density
can be quite low if the number of atoms is not too big. The transition
between the two classes of solutions as a function of can be either sharp
or smooth according to the ratio between the range of the attractive
interaction and the length of the trap. A tight trap leads to a smooth
transition. In addition to the energy and the shape of the cloud we study also
the dynamics of the system. In particular, we study the frequencies of
collective oscillation of the Bose condensate as a function of the number of
atoms both in the local and in the non local case. Moreover, we consider the
dynamics of the cloud when the external trap is switched off.Comment: Latex, 6 pages, 2 figure, 1 table, presented to the International
Symposium of Quantum Fluids and Solids 98, Amherst (USA), 9-14 June 199
Fluctuations and Pattern Formation in Fluids with Competing Interactions
One of the most interesting phenomena in the soft-matter realm consists in
the spontaneous formation of super-molecular structures (microphases) in
condition of thermodynamic equilibrium. A simple mechanism responsible for this
self-organization or pattern formation is based on the competition between
attractive and repulsive forces with different length scales in the microscopic
potential, typically, a short-range attraction against a longer-range
repulsion.
We analyse this problem by simulations in 2D fluids. We find that, as the
temperature is lowered, liquid-vapor phase separation is inhibited by the
competition between attraction and repulsion, and replaced by a transition to
non-homogeneous phases. The structure of the fluid shows well defined
signatures of the presence of both intra- and inter-cluster correlations.
Even when the competition between attraction and repulsion is not so strong
as to cause microphase formation, it still induces large density fluctuations
in a wide region of the temperature-density plane. In this large-fluctuation
regime, pattern formation can be triggered by a weak external modulating field.Comment: To appear in the proceedings of the "International workshop on
collective phenomena in macroscopic systems", 2006 Villa Olmo (Como), Ital
Quantum Monte Carlo study of a vortex in superfluid He and search for a vortex state in the solid
We have performed a microscopic study of a straight quantized vortex line in
three dimensions in condensed He at zero temperature using the Shadow Path
Integral Ground State method and the fixed-phase approximation. We have
characterized the energy and the local density profile around the vortex axis
in superfluid He at several densities, ranging from below the equilibrium
density up to the overpressurized regime. For the Onsager-Feynman (OF) phase
our results are exact and represent a benchmark for other theories. The
inclusion of backflow correlations in the phase improves the description of the
vortex with respect to the OF phase by a large reduction of the core energy of
the topological excitation. At all densities the phase with backflow induces a
partial filling of the vortex core and this filling slightly increases with
density. The core size slightly decreases for increasing density and the
density profile has well defined density dependent oscillations whose wave
vector is closer to the wave vector of the main peak in the static density
response function rather than to the roton wave vector. Our results can be
applied to vortex rings of large radius and we find good agreement with the
experimental value of the energy as function of without any free parameter.
We have studied also He above the melting density in the solid phase using
the same functional form for the phase as in the liquid. We found that
off-diagonal properties of the solid are not qualitatively affected by the
velocity field induced by the vortex phase, both with and without backflow
correlations. Therefore we find evidence that a perfect He crystal is not a
marginally stable quantum solid in which rotation would be able to induce
off-diagonal long-range coherence.Comment: 15 pages, 8 figure
Thermodynamics of Bose-Condensed Atomic Hydrogen
We study the thermodynamics of the Bose-condensed atomic hydrogen confined in
the Ioffe-Pritchard potential. Such a trapping potential, that models the
magnetic trap used in recent experiments with hydrogen, is anharmonic and
strongly anisotropic. We calculate the ground-state properties, the condensed
and non-condensed fraction and the Bose-Einstein transition temperature. The
thermodynamics of the system is strongly affected by the anharmonicity of this
external trap. Finally, we consider the possibility to detect Josephson-like
currents by creating a double-well barrier with a laser beam.Comment: 11 pages, 4 figures, to be published in European Physical Journal
Bounds for the Superfluid Fraction from Exact Quantum Monte Carlo Local Densities
For solid 4He and solid p-H2, using the flow-energy-minimizing one-body phase
function and exact T=0 K Monte Carlo calculations of the local density, we have
calculated the phase function, the velocity profile and upper bounds for the
superfluid fraction f_s. At the melting pressure for solid 4He we find that f_s
< 0.20-0.21, about ten times what is observed. This strongly indicates that the
theory for the calculation of these upper bounds needs substantial
improvements.Comment: to be published in Phys. Rev. B (Brief Reports
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