655 research outputs found
Core-Softened System With Attraction: Trajectory Dependence of Anomalous Behavior
In the present article we carry out a molecular dynamics study of the
core-softened system and show that the existence of the water-like anomalies in
this system depends on the trajectory in space along which the
behavior of the system is studied. For example, diffusion and structural
anomalies are visible along isotherms as a function of density, but disappears
along the isochores and isobars as a function of temperature. On the other
hand, the diffusion anomaly may be seen along adiabats as a function of
temperature, density and pressure. It should be noted that it may be no
signature of a particular anomaly along a particular trajectory, but the
anomalous region for that particular anomaly can be defined when all possible
trajectories in the same space are examined (for example, signature of
diffusion anomaly is evident through the crossing of different isochors.
However, there is no signature of diffusion anomaly along a particular
isochor). We also analyze the applicability of the Rosenfeld entropy scaling
relations to this system in the regions with the water-like anomalies. It is
shown that the validity of the Rosenfeld scaling relation for the diffusion
coefficient also depends on the trajectory in the space along which
the kinetic coefficients and the excess entropy are calculated.Comment: 16 pages, 21 figures. arXiv admin note: this contains much of the
content of arXiv:1010.416
Inversion of Sequence of Diffusion and Density Anomalies in Core-Softened Systems
In this paper we present a simulation study of water-like anomalies in
core-softened system introduced in our previous publications. We investigate
the anomalous regions for a system with the same functional form of the
potential but with different parameters and show that the order of the region
of anomalous diffusion and the region of density anomaly is inverted with
increasing the width of the repulsive shoulder.Comment: 8 pages, 10 figure
Quasi-binary amorphous phase in a 3D system of particles with repulsive-shoulder interactions
We report a computer-simulation study of the equilibrium phase diagram of a
three-dimensional system of particles with a repulsive step potential. Using
free-energy calculations, we have determined the equilibrium phase diagram of
this system. At low temperatures, we observe a number of distinct crystal
phases. However, under certain conditions the system undergoes a glass
transition in a regime where the liquid appears thermodynamically stable. We
argue that the appearance of this amorphous low-temperature phase can be
understood by viewing this one-component system as a pseudo-binary mixture.Comment: 4 pages, 4 figure
A central extension of \cD Y_{\hbar}(\gtgl_2) and its vertex representations
A central extension of \cD Y_{\hbar}(\gtgl_2) is proposed. The bosonization
of level module and vertex operators are also given.Comment: 10 pages, AmsLatex, to appear in Lett. in Math. Phy
Local orientational order in the Stockmayer liquid
Phase behaviour of the Stockmayer fluid is studied with a method similar to
the Monte-Carlo annealing scheme. We introduce a novel order parameter which is
sensitive to the local co-orientation of the dipoles of particles in the fluid.
We exhibit a phase diagram based on the behaviour of the order parameter in the
density region 0.1 \leq {\rho}\ast \leq 0.32. Specifically, we observe and
analyse a second order locally disordered fluid \rightarrow locally oriented
fluid phase transition.Comment: 13 pages, 7 figure
Lattice Model for water-solute mixtures
A lattice model for the study of mixtures of associating liquids is proposed.
Solvent and solute are modeled by adapting the associating lattice gas (ALG)
model. The nature of interaction solute/solvent is controlled by tuning the
energy interactions between the patches of ALG model. We have studied three set
of parameters, resulting on, hydrophilic, inert and hydrophobic interactions.
Extensive Monte Carlo simulations were carried out and the behavior of pure
components and the excess properties of the mixtures have been studied. The
pure components: water (solvent) and solute, have quite similar phase diagrams,
presenting: gas, low density liquid, and high density liquid phases. In the
case of solute, the regions of coexistence are substantially reduced when
compared with both the water and the standard ALG models. A numerical procedure
has been developed in order to attain series of results at constant pressure
from simulations of the lattice gas model in the grand canonical ensemble. The
excess properties of the mixtures: volume and enthalpy as the function of the
solute fraction have been studied for different interaction parameters of the
model. Our model is able to reproduce qualitatively well the excess volume and
enthalpy for different aqueous solutions. For the hydrophilic case, we show
that the model is able to reproduce the excess volume and enthalpy of mixtures
of small alcohols and amines. The inert case reproduces the behavior of large
alcohols such as, propanol, butanol and pentanol. For last case (hydrophobic),
the excess properties reproduce the behavior of ionic liquids in aqueous
solution.Comment: 28 pages, 13 figure
Free Energy Approach to the Formation of an Icosahedral Structure during the Freezing of Gold Nanoclusters
The freezing of metal nanoclusters such as gold, silver, and copper exhibits
a novel structural evolution. The formation of the icosahedral (Ih) structure
is dominant despite its energetic metastability. This important phenomenon,
hitherto not understood, is studied by calculating free energies of gold
nanoclusters. The structural transition barriers have been determined by using
the umbrella sampling technique combined with molecular dynamics simulations.
Our calculations show that the formation of Ih gold nanoclusters is attributed
to the lower free energy barrier from the liquid to the Ih phases compared to
the barrier from the liquid to the face-centered-cubic crystal phases
Two liquid states of matter: A new dynamic line on a phase diagram
It is generally agreed that the supercritical region of a liquid consists of
one single state (supercritical fluid). On the other hand, we show here that
liquids in this region exist in two qualitatively different states: "rigid" and
"non-rigid" liquid. Rigid to non-rigid transition corresponds to the condition
{\tau} ~ {\tau}0, where {\tau}is liquid relaxation time and {\tau}0 is the
minimal period of transverse quasi-harmonic waves. This condition defines a new
dynamic line on the phase diagram, and corresponds to the loss of shear
stiffness of a liquid at all available frequencies, and consequently to the
qualitative change of many important liquid properties. We analyze the dynamic
line theoretically as well as in real and model liquids, and show that the
transition corresponds to the disappearance of high-frequency sound,
qualitative changes of diffusion and viscous flow, increase of particle thermal
speed to half of the speed of sound and reduction of the constant volume
specific heat to 2kB per particle. In contrast to the Widom line that exists
near the critical point only, the new dynamic line is universal: it separates
two liquid states at arbitrarily high pressure and temperature, and exists in
systems where liquid - gas transition and the critical point are absent
overall.Comment: 21 pages, 8 figure
Waterlike thermodynamic anomalies in a repulsive-step potential system
We report a computer-simulation study of the equilibrium phase diagram of a
three-dimensional system of particles with a repulsive step potential. The
phase diagram is obtained using free-energy calculations. At low temperatures,
we observe a number of distinct crystal phases. We show that at certain values
of the potential parameters the system exhibits the water-like thermodynamic
anomalies: density anomaly and diffusion anomaly. The anomalies disappear with
increasing the repulsive step width: their locations move to the region inside
the crystalline phase.Comment: 6 pages, 5 figure
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