129 research outputs found
Dynamical arrest and replica symmetry breaking in attractive colloids
Within the Replica Symmetry Breaking (RSB) framework developed by M.Mezard
and G.Parisi we investigate the occurrence of structural glass transitions in a
model of fluid characterized by hard sphere repulsion together with short range
attraction. This model is appropriate for the description of a class of
colloidal suspensions. The transition line in the density-temperature plane
displays a reentrant behavior, in agreement with Mode Coupling Theory (MCT), a
dynamical approach based on the Mori-Zwanzig formalism. Quantitative
differences are however found, together with the absence of the predicted
glass-glass transition at high density. We also perform a systematic study of
the pure hard sphere fluid in order to ascertain the accuracy of the adopted
method and the convergence of the numerical procedure.Comment: 7 pages, 6 figure
Smooth cutoff formulation of hierarchical reference theory for a scalar phi4 field theory
The phi4 scalar field theory in three dimensions, prototype for the study of
phase transitions, is investigated by means of the hierarchical reference
theory (HRT) in its smooth cutoff formulation. The critical behavior is
described by scaling laws and critical exponents which compare favorably with
the known values of the Ising universality class. The inverse susceptibility
vanishes identically inside the coexistence curve, providing a first principle
implementation of the Maxwell construction, and shows the expected
discontinuity across the phase boundary, at variance with the usual sharp
cutoff implementation of HRT. The correct description of first and second order
phase transitions within a microscopic, nonperturbative approach is thus
achieved in the smooth cutoff HRT.Comment: 8 pages, 4 figure
Quantized vortices in two dimensional solid 4He
Diagonal and off-diagonal properties of 2D solid 4He systems doped with a
quantized vortex have been investigated via the Shadow Path Integral Ground
State method using the fixed-phase approach. The chosen approximate phase
induces the standard Onsager-Feynman flow field. In this approximation the
vortex acts as a static external potential and the resulting Hamiltonian can be
treated exactly with Quantum Monte Carlo methods. The vortex core is found to
sit in an interstitial site and a very weak relaxation of the lattice positions
away from the vortex core position has been observed. Also other properties
like Bragg peaks in the static structure factor or the behavior of vacancies
are very little affected by the presence of the vortex. We have computed also
the one-body density matrix in perfect and defected 4He crystals finding that
the vortex has no sensible effect on the off-diagonal long range tail of the
density matrix. Within the assumed Onsager Feynman phase, we find that a
quantized vortex cannot auto-sustain itself unless a condensate is already
present like when dislocations are present. It remains to be investigated if
backflow can change this conclusion.Comment: 4 pages, 3 figures, LT26 proceedings, accepted for publication in
Journal of Physics: Conference Serie
A model colloidal fluid with competing interactions: bulk and interfacial properties
Using a simple mean-field density functional theory theory (DFT), we
investigate the structure and phase behaviour of a model colloidal fluid
composed of particles interacting via a pair potential which has a hard core of
diameter , is attractive Yukawa at intermediate separations and
repulsive Yukawa at large separations. We analyse the form of the asymptotic
decay of the bulk fluid correlation functions, comparing results from our DFT
with those from the self consistent Ornstein-Zernike approximation (SCOZA). In
both theories we find rich crossover behaviour, whereby the ultimate decay of
correlation functions changes from monotonic to long-wavelength damped
oscillatory decay on crossing certain lines in the phase diagram, or sometimes
from oscillatory to oscillatory with a longer wavelength. For some choices of
potential parameters we find, within the DFT, a -line at which the
fluid becomes unstable with respect to periodic density fluctuations. SCOZA
fails to yield solutions for state points near such a -line. The
propensity to clustering of particles, which is reflected by the presence of a
long wavelength , slowly decaying oscillatory pair correlation
function, and a structure factor that exhibits a very sharp maximum at small
but non zero wavenumbers, is enhanced in states near the -line. We
present density profiles for the planar liquid-gas interface and for fluids
adsorbed at a planar hard wall. The presence of a nearby -transition
gives rise to pronounced long-wavelength oscillations in the one-body densities
at both types of interface.Comment: 14 pages, 11 figure
Modulational Instability and Complex Dynamics of Confined Matter-Wave Solitons
We study the formation of bright solitons in a Bose-Einstein condensate of
Li atoms induced by a sudden change in the sign of the scattering length
from positive to negative, as reported in a recent experiment (Nature {\bf
417}, 150 (2002)). The numerical simulations are performed by using the 3D
Gross-Pitaevskii equation (GPE) with a dissipative three-body term. We show
that a number of bright solitons is produced and this can be interpreted in
terms of the modulational instability of the time-dependent macroscopic wave
function of the Bose condensate. In particular, we derive a simple formula for
the number of solitons that is in good agreement with the numerical results of
3D GPE. By investigating the long time evolution of the soliton train solving
the 1D GPE with three-body dissipation we find that adjacent solitons repel
each other due to their phase difference. In addition, we find that during the
motion of the soliton train in an axial harmonic potential the number of
solitonic peaks changes in time and the density of individual peaks shows an
intermittent behavior. Such a complex dynamics explains the ``missing
solitons'' frequently found in the experiment.Comment: to be published in Phys. Rev. Let
Liquid-vapor transition from a microscopic theory: Beyond the Maxwell construction
A smooth cut-off formulation of the Hierarchical Reference Theory (HRT) is
developed and applied to a Yukawa fluid. The HRT equations are derived and
numerically solved leading to: the expected renormalization group structure in
the critical region, non classical critical exponents and scaling laws, a
convex free energy in the whole phase diagram (including the two-phase region),
finite compressibility at coexistence, together with a fully satisfactory
comparison with available numerical simulations. This theory, which also
guarantees the correct short range behavior of two body correlations,
represents a major improvement over the existing liquid state theories.Comment: 4 pages, 5 figure
Periodic Quantum Tunneling and Parametric Resonance with Cigar-Shaped Bose-Einstein Condensates
We study the tunneling properties of a cigar-shaped Bose-Einstein condensate
by using an effective 1D nonpolynomial nonlinear Schr\"odinger equation (NPSE).
First we investigate a mechanism to generate periodic pulses of coherent matter
by means of a Bose condensate confined in a potential well with an oscillating
height of the energy barrier. We show that is possible to control the periodic
emission of matter waves and the tunneling fraction of the Bose condensate. We
find that the number of emitted particles strongly increases if the period of
oscillation of the height of the energy barrier is in parametric resonance with
the period of oscillation of the center of mass of the condensate inside the
potential well. Then we use NPSE to analyze the periodic tunneling of a
Bose-Einstein condensate in a double-well potential which has an oscillating
energy barrier. We show that the dynamics of the Bose condensate critically
depends on the frequency of the oscillating energy barrier. The macroscopic
quantum self-trapping (MQST) of the condensate can be suppressed under the
condition of parametric resonance between the frequency of the energy barrier
and the frequency of oscillation through the barrier of the very small fraction
of particles which remain untrapped during MQST.Comment: latex, 23 pages, 10 figures, to be published in J. Phys. B (Atom.
Mol.), related papers can be found at
http://www.mi.infm.it/salasnich/tdqg.htm
Microphase morphology in two dimensional fluids under lateral confinement
We study the effects of confinement between two parallel walls on a two
dimensional fluid with competing interactions which lead to the formation of
particle micro-domains at the thermodynamic equilibrium (microphases or
microseparation). The possibility to induce structural changes of the
morphology of the micro-domains is explored, under different confinement
conditions and temperatures. In presence of neutral walls, a switch from
stripes of particles to circular clusters (droplets) occurs as the temperature
decreases, which does not happen in bulk. While the passage from droplets to
stripes, as the density increases, is a well known phenomenon, the change of
the stripes into droplets as an effect of temperature is rather unexpected.
Depending on the wall separation and on the wall-fluid interaction parameters,
the stripes can switch from parallel to perpendicular to the walls and also a
mixed morphology can be stable.Comment: accepted by Physical Review E (rapid communications
Bosons in a Toroidal Trap: Ground State and Vortices
We study the Bose-Einstein condensate (BEC) in a 3-D toroidal Mexican hat
trap. By changing the parameters of the potential, or the number of bosons, it
is possible to modify strongly the density profile of the BEC. We consider the
ground state properties for positive and negative scattering length and
calculate the spectrum elementary excitations. We also discuss the macroscopic
phase coherence and superfluidity of the BEC by analyzing vortex states and
their stability.Comment: 15 pages + 4 figures, to be published in Phys. Rev.
Phase transitions in simple and not so simple binary fluids
Compared to pure fluids, binary mixtures display a very diverse phase
behavior, which depends sensitively on the parameters of the microscopic
potential. Here we investigate the phase diagrams of simple model mixtures by
use of a microscopic implementation of the renormalization group technique.
First, we consider a symmetric mixture with attractive interactions, possibly
relevant for describing fluids of molecules with internal degrees of freedom.
Despite the simplicity of the model, slightly tuning the strength of the
interactions between unlike species drastically changes the topology of the
phase boundary, forcing or inhibiting demixing, and brings about several
interesting features such as double critical points, tricritical points, and
coexistence domains enclosing `islands' of homogeneous, mixed fluid.
Homogeneous phase separation in mixtures can be driven also by purely repulsive
interactions. As an example, we consider a model of soft particles which has
been adopted to describe binary polymer solutions. This is shown to display
demixing (fluid-fluid) transition at sufficiently high density. The nature and
the physical properties of the corresponding phase transition are investigated.Comment: 6 pages + 3 figures, presented at the 5th EPS Liquid Matter
Conference, Konstanz, 14-18 September 200
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