997 research outputs found
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
Implementation of the Hierarchical Reference Theory for simple one-component fluids
Combining renormalization group theoretical ideas with the integral equation
approach to fluid structure and thermodynamics, the Hierarchical Reference
Theory is known to be successful even in the vicinity of the critical point and
for sub-critical temperatures. We here present a software package independent
of earlier programs for the application of this theory to simple fluids
composed of particles interacting via spherically symmetrical pair potentials,
restricting ourselves to hard sphere reference systems. Using the hard-core
Yukawa potential with z=1.8/sigma for illustration, we discuss our
implementation and the results it yields, paying special attention to the core
condition and emphasizing the decoupling assumption's role.Comment: RevTeX, 16 pages, 2 figures. Minor changes, published versio
Recent developments of the Hierarchical Reference Theory of Fluids and its relation to the Renormalization Group
The Hierarchical Reference Theory (HRT) of fluids is a general framework for
the description of phase transitions in microscopic models of classical and
quantum statistical physics. The foundations of HRT are briefly reviewed in a
self-consistent formulation which includes both the original sharp cut-off
procedure and the smooth cut-off implementation, which has been recently
investigated. The critical properties of HRT are summarized, together with the
behavior of the theory at first order phase transitions. However, the emphasis
of this presentation is on the close relationship between HRT and non
perturbative renormalization group methods, as well as on recent
generalizations of HRT to microscopic models of interest in soft matter and
quantum many body physics.Comment: 17 pages, 5 figures. Review paper to appear in Molecular Physic
Positive Resources for Flourishing: The Effect of Courage, Self-Esteem, and Career Adaptability in Adolescence
Flourishing is defined as an optimal state of functioning in which individuals pursue their goals and aspirations. Hence, flourishing seems to be a protective factor for career transitions in adolescence. This study aimed to analyze the predicting role of self-esteem, courage, and the four career adaptability dimensions, i.e., concern, control, curiosity, and confidence, on flourishing. The sample consisted of 221 Italian adolescents attending the last year of middle school. The preliminary analyses showed gender differences in courage and flourishing, reporting females higher scores than males on both variables. The SEM path model showed that courage, self-esteem, and confidence predict flourishing, and suggested that confidence partially mediates the relationship between courage, self-esteem, and flourishing. Findings have also permitted us to draw practical implications for interventions in adolescence
Exact Renormalization Group : A New Method for Blocking the Action
We consider the exact renormalization group for a non-canonical scalar field
theory in which the field is coupled to the external source in a special non
linear way. The Wilsonian action and the average effective action are then
simply related by a Legendre transformation up to a trivial quadratic form. An
exact mapping between canonical and non-canonical theories is obtained as well
as the relations between their flows. An application to the theory of liquids
is sketched
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
Properties of derivative expansion approximations to the renormalization group
Approximation only by derivative (or more generally momentum) expansions,
combined with reparametrization invariance, turns the continuous
renormalization group for quantum field theory into a set of partial
differential equations which at fixed points become non-linear eigenvalue
equations for the anomalous scaling dimension . We review how these
equations provide a powerful and robust means of discovering and approximating
non-perturbative continuum limits. Gauge fields are briefly discussed.
Particular emphasis is placed on the r\^ole of reparametrization invariance,
and the convergence of the derivative expansion is addressed.Comment: (Minor touch ups of the lingo.) Invited talk at RG96, Dubna, Russia;
14 pages including 2 eps figures; uses LaTeX, epsf and sprocl.st
Condensate Fraction of a Fermi Gas in the BCS-BEC Crossover
We investigate the Bose-Einstein condensation of Fermionic pairs in a uniform
two-component Fermi gas obtaining an explicit formula for the condensate
density as a function of the chemical potential and the energy gap. We analyze
the condensate fraction in the crossover from the Bardeen-Cooper-Schrieffer
(BCS) state of weakly-interacting Cooper pairs to the Bose-Einstein Condensate
(BEC) of molecular dimers. By using the local density approximation we study
confined Fermi vapors of alkali-metal atoms for which there is experimental
evidence of condensation also on the BCS side of the Feshbach resonance. Our
theoretical results are in agreement with these experimental data and give the
behavior of the condensate on both sides of the Feshbach resonance at zero
temperature.Comment: 5 pages, 2 figure
The Phase Diagram of Correlated Electrons in a Lattice of Berry Molecules
A model for correlated electrons in a lattice with local additional spin--1
degrees of freedom inducing constrained hopping, is studied both in the low
density limit and at quarter filling. We show that in both 1D and 2D two
particles form a bound state even in presence of a repulsive U<U_c. A picture
of a dilute Bose gas, leading to off-diagonal long range order (LRO) in 2D
(quasi-LRO in 1D), is supported by quantitative calculations in 1D which allow
for a determination of the phase diagram.Comment: 7 pages + 2 ps figures, published versio
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