25 research outputs found
Anomalous phase behavior of a soft-repulsive potential with a strictly monotonic force
We study the phase behavior of a classical system of particles interacting
through a strictly convex soft-repulsive potential which, at variance with the
pairwise softened repulsions considered so far in the literature, lacks a
region of downward or zero curvature. Nonetheless, such interaction is
characterized by two length scales, owing to the presence of a range of
interparticle distances where the repulsive force increases, for decreasing
distance, much more slowly than in the adjacent regions. We investigate, using
extensive Monte Carlo simulations combined with accurate free-energy
calculations, the phase diagram of the system under consideration. We find that
the model exhibits a fluid-solid coexistence line with multiple re-entrant
regions, an extremely rich solid polymorphism with solid-solid transitions, and
water-like anomalies. In spite of the isotropic nature of the interparticle
potential, we find that, among the crystal structures in which the system can
exist, there are also a number of non-Bravais lattices, such as cI16 and
diamond.Comment: 21 pages, 7 figures, in press on Phys. Rev.
Liquid-liquid phase transition for an attractive isotropic potential with wide repulsive range
We investigate how the phase diagram of a repulsive soft-core attractive potential, with a liquid-liquid phase transition in addition to the standard gas-liquid phase transition, changes by varying the parameters of the potential. We extend our previous work on short soft-core ranges to the case of large soft-core ranges, by using an integral equation approach in the hypernetted-chain approximation. We show, using a modified van der Waals equation we recently introduced, that if there is a balance between the attractive and repulsive part of the potential this potential has two fluid-fluid critical points well separated in temperature and in density. This implies that for the repulsive (attractive) energy
U
R
(
U
A
)
and the repulsive (attractive) range
w
R
(
w
A
)
the relation
U
R
â
U
A
â
w
R
â
w
A
holds for short soft-core ranges, while
U
R
â
U
A
â
3
w
R
â
w
A
holds for large soft-core ranges
The zero-temperature phase diagram of soft-repulsive particle fluids
Effective pair interactions with a soft-repulsive component are a well-known
feature of polymer solutions and colloidal suspensions, but they also provide a
key to interpret the high-pressure behaviour of simple elements. We have
computed the zero-temperature phase diagram of four different model potentials
with various degrees of core softness. Among the reviewed crystal structures,
there are also a number of non-Bravais lattices, chosen among those observed in
real systems. Some of these crystals are indeed found to be stable for the
selected potentials. We recognize an apparently universal trend for unbounded
potentials, going from high- to low-coordinated crystal phases and back upon
increasing the pressure. Conversely, a bounded repulsion may lead to
intermittent appearance of compact structures with compression and no eventual
settling down in a specific phase. In both cases, the fluid phase repeatedly
reenters at intermediate pressures, as suggested by a cell-theory treatment of
the solids. These findings are of relevance for soft matter in general, but
they also offer fresh insight into the mechanisms subtended to solid
polymorphism in elemental substances.Comment: 16 pages, 5 figures, to be published on Soft Matte
Anomalous melting behavior under extreme conditions: hard matter turning "soft"
We show that a system of particles interacting through the exp-6 pair
potential, commonly used to describe effective interatomic forces under high
compression, exhibits anomalous melting features such as reentrant melting and
a rich solid polymorphism, including a stable BC8 crystal. We relate this
behavior to the crossover, with increasing pressure, between two different
regimes of local order that are associated with the two repulsive length scales
of the potential. Our results provide a unifying picture for the high-pressure
melting anomalies observed in many elements and point out that, under extreme
conditions, atomic systems may reveal surprising similarities with soft matter.Comment: 10 pages, 4 figure
Liquid-Liquid Phase Transition for an Attractive Isotropic Potential with Wide Repulsive Range
Recent experimental and theoretical results have shown the existence of a
liquid-liquid phase transition in isotropic systems, such as biological
solutions and colloids, whose interaction can be represented via an effective
potential with a repulsive soft-core and an attractive part. We investigate how
the phase diagram of a schematic general isotropic system, interacting via a
soft-core squared attractive potential, changes by varying the parameters of
the potential. It has been shown that this potential has a phase diagram with a
liquid-liquid phase transition in addition to the standard gas-liquid phase
transition and that, for a short-range soft-core, the phase diagram resulting
from molecular dynamics simulations can be interpreted through a modified van
der Waals equation. Here we consider the case of soft-core ranges comparable
with or larger than the hard-core diameter. Because an analysis using molecular
dynamics simulations of such systems or potentials is too time-demanding, we
adopt an integral equation approach in the hypernetted-chain approximation.
Thus we can estimate how the temperature and density of both critical points
depend on the potential's parameters for large soft-core ranges. The present
results confirm and extend our previous analysis, showing that this potential
has two fluid-fluid critical points that are well separated in temperature and
in density only if there is a balance between the attractive and repulsive part
of the potential. We find that for large soft-core ranges our results satisfy a
simple relation between the potential's parameters
Simple Fluids with Complex Phase Behavior
We find that a system of particles interacting through a simple isotropic
potential with a softened core is able to exhibit a rich phase behavior
including: a liquid-liquid phase transition in the supercooled phase, as has
been suggested for water; a gas-liquid-liquid triple point; a freezing line
with anomalous reentrant behavior. The essential ingredient leading to these
features resides in that the potential investigated gives origin to two
effective core radii.Comment: 7 pages including 3 eps figures + 1 jpeg figur
Generic mechanism for generating a liquid-liquid phase transition
Recent experimental results indicate that phosphorus, a single-component
system, can have two liquid phases: a high-density liquid (HDL) and a
low-density liquid (LDL) phase. A first-order transition between two liquids of
different densities is consistent with experimental data for a variety of
materials, including single-component systems such as water, silica and carbon.
Molecular dynamics simulations of very specific models for supercooled water,
liquid carbon and supercooled silica, predict a LDL-HDL critical point, but a
coherent and general interpretation of the LDL-HDL transition is lacking. Here
we show that the presence of a LDL and a HDL can be directly related to an
interaction potential with an attractive part and two characteristic
short-range repulsive distances. This kind of interaction is common to other
single-component materials in the liquid state (in particular liquid metals),
and such potentials are often used to decribe systems that exhibit a density
anomaly. However, our results show that the LDL and HDL phases can occur in
systems with no density anomaly. Our results therefore present an experimental
challenge to uncover a liquid-liquid transition in systems like liquid metals,
regardless of the presence of the density anomaly.Comment: 5 pages, 3 ps Fig