5,003 research outputs found

### AIDS, Economic Growth and the HIPC Initiative in Honduras

AIDS, Heavily indebted poor countries, Economic growth, Foreign capital flows

### Phase behavior of hard-core lattice gases: A Fundamental Measure approach

We use an extension of fundamental measure theory to lattice hard-core fluids
to study the phase diagram of two different systems. First, two-dimensional
parallel hard squares with edge-length $\sigma=2$ in a simple square lattice.
This system is equivalent to the lattice gas with first and second neighbor
exclusion in the same lattice, and has the peculiarity that its close packing
is degenerated (the system orders in sliding columns). A comparison with other
theories is discussed. Second, a three-dimensional binary mixture of parallel
hard cubes with $\sigma_{\rm{L}}=6$ and $\sigma_{\rm{S}}=2$. Previous
simulations of this model only focused on fluid phases. Thanks to the
simplicity introduced by the discrete nature of the lattice we have been able
to map out the complete phase diagram (both uniform and nonuniform phases)
through a free minimization of the free energy functional, so the structure of
the ordered phases is obtained as a result. A zoo of entropy-driven phase
transitions is found: one-, two- and three-dimensional positional ordering, as
well as fluid-ordered phase and solid-solid demixings.Comment: 14 pages, 16 figure

### Neutral networks of genotypes: Evolution behind the curtain

Our understanding of the evolutionary process has gone a long way since the
publication, 150 years ago, of "On the origin of species" by Charles R. Darwin.
The XXth Century witnessed great efforts to embrace replication, mutation, and
selection within the framework of a formal theory, able eventually to predict
the dynamics and fate of evolving populations. However, a large body of
empirical evidence collected over the last decades strongly suggests that some
of the assumptions of those classical models necessitate a deep revision. The
viability of organisms is not dependent on a unique and optimal genotype. The
discovery of huge sets of genotypes (or neutral networks) yielding the same
phenotype --in the last term the same organism--, reveals that, most likely,
very different functional solutions can be found, accessed and fixed in a
population through a low-cost exploration of the space of genomes. The
'evolution behind the curtain' may be the answer to some of the current puzzles
that evolutionary theory faces, like the fast speciation process that is
observed in the fossil record after very long stasis periods.Comment: 7 pages, 7 color figures, uses a modification of pnastwo.cls called
pnastwo-modified.cls (included

### Fundamental measure theory for mixtures of parallel hard cubes. II. Phase behavior of the one-component fluid and of the binary mixture

A previously developed fundamental measure fucntional [J. Chem. Phys.
vol.107, 6379 (1997)] is used to study the phase behavior of a system of
parallel hard cubes. The single-component fluid exhibits a continuous
transition to a solid with an anomalously large density of vacancies. The
binary mixture has a demixing transition for edge-length ratios below 0.1.
Freezing in this mixture reveals that at least the phase rich in large cubes
lies in the region where the uniform fluid is unstable, hence suggesting a
fluid-solid phase separation. A method is develop to study very asymmetric
binary mixtures by taking the limit of zero size ratio (scaling the density and
fugacity of the solvent as appropriate) in the semi-grand ensemble where the
chemical potential of the solvent is fixed. With this procedure the mixture is
exactly mapped onto a one-component fluid of parallel adhesive hard cubes. At
any density and solvent fugacity the large cubes are shown to collapse into a
close-packed solid. Nevertheless the phase diagram contains a large
metastability region with fluid and solid phases. Upon introduction of a slight
polydispersity in the large cubes the system shows the typical phase diagram of
a fluid with an isostructural solid-solid transition (with the exception of a
continuous freezing). Consequences about the phase behavior of binary mixtures
of hard core particles are then drawn.Comment: 14 pages, 6 eps figures, uses revtex, amstex, epsfig, and multicol
style file

### Phase diagrams of Zwanzig models: The effect of polydispersity

The first goal of this article is to study the validity of the Zwanzig model
for liquid crystals to predict transitions to inhomogeneous phases (like
smectic and columnar) and the way polydispersity affects these transitions. The
second goal is to analyze the extension of the Zwanzig model to a binary
mixture of rods and plates. The mixture is symmetric in that all particles have
equal volume and length-to-breadth ratio, $\kappa$. The phase diagram
containing the homogeneous phases as well as the spinodals of the transitions
to inhomogeneous phases is determined for the cases $\kappa=5$ and 15 in order
to compare with previous results obtained in the Onsager approximation. We then
study the effect of polydispersity on these phase diagrams, emphasizing the
enhancement of the stability of the biaxial nematic phase it induces.Comment: 11 pages, 12 figure

### The Shared Reward Dilemma

One of the most direct human mechanisms of promoting cooperation is rewarding
it. We study the effect of sharing a reward among cooperators in the most
stringent form of social dilemma, namely the Prisoner's Dilemma. Specifically,
for a group of players that collect payoffs by playing a pairwise Prisoner's
Dilemma game with their partners, we consider an external entity that
distributes a fixed reward equally among all cooperators. Thus, individuals
confront a new dilemma: on the one hand, they may be inclined to choose the
shared reward despite the possibility of being exploited by defectors; on the
other hand, if too many players do that, cooperators will obtain a poor reward
and defectors will outperform them. By appropriately tuning the amount to be
shared a vast variety of scenarios arises, including traditional ones in the
study of cooperation as well as more complex situations where unexpected
behavior can occur. We provide a complete classification of the equilibria of
the $n$-player game as well as of its evolutionary dynamics.Comment: Major rewriting, new appendix, new figure

### First-principles derivation of density functional formalism for quenched-annealed systems

We derive from first principles (without resorting to the replica trick) a
density functional theory for fluids in quenched disordered matrices (QA-DFT).
We show that the disorder-averaged free energy of the fluid is a functional of
the average density profile of the fluid as well as the pair correlation of the
fluid and matrix particles. For practical reasons it is preferable to use
another functional: the disorder-averaged free energy plus the fluid-matrix
interaction energy, which, for fixed fluid-matrix interaction potential, is a
functional only of the average density profile of the fluid. When the matrix is
created as a quenched configuration of another fluid, the functional can be
regarded as depending on the density profile of the matrix fluid as well. In
this situation, the replica-Ornstein-Zernike equations which do not contain the
blocking parts of the correlations can be obtained as functional identities in
this formalism, provided the second derivative of this functional is
interpreted as the connected part of the direct correlation function. The
blocking correlations are totally absent from QA-DFT, but nevertheless the
thermodynamics can be entirely obtained from the functional. We apply the
formalism to obtain the exact functional for an ideal fluid in an arbitrary
matrix, and discuss possible approximations for non-ideal fluids.Comment: 19 pages, uses RevTeX

### Weak-Scale Hidden Sector and Energy Transport in Fireball Models of Gamma-Ray Bursts

The annihilation of pairs of very weakly interacting particles in the
neibourghood of gamma-ray sources is introduced here as a plausible mechanism
to overcome the baryon load problem. This way we can explain how these very
high energy gamma-ray bursts can be powered at the onset of very energetic
events like supernovae (collapsars) explosions or coalescences of binary
neutron stars. Our approach uses the weak-scale hidden sector models in which
the Higgs sector of the standard model is extended to include a gauge singlet
that only interacts with the Higgs particle. These particles would be produced
either during the implosion of the red supergiant star core or at the aftermath
of a neutron star binary merger. The whole energetics and timescales of the
relativistic blast wave, the fireball, are reproduced.Comment: 4 pp, 1 ps fig, text revised and improve

- …