246 research outputs found
Polydispersity induced solid-solid transitions in model colloids
Specialized Monte Carlo simulation techniques and moment free energy method
calculations, capable of treating fractionation exactly, are deployed to study
the crystalline phase behaviour of an assembly of spherical particles described
by a top-hat "parent" distribution of particle sizes. An increase in either the
overall density or the degree of polydispersity is shown to generate a
succession of phase transitions in which the system demixes into an ever
greater number of face-centred cubic "daughter" phases. Each of these phases is
strongly fractionated: it contains a much narrower distribution of particle
sizes than is present in the system overall. Certain of the demixing
transitions are found to be nearly continuous, accompanied by fluctuations in
local particle size correlated over many lattice spacings. We explore possible
factors controlling the stability of the phases and the character of the
demixing transitions.Comment: 14 pages, 13 figure
Crystalline phases of polydisperse spheres
We use specialized Monte Carlo simulation methods and moment free energy
calculations to provide conclusive evidence that dense polydisperse spheres at
equilibrium demix into coexisting fcc phases, with more phases appearing as the
spread of diameters increases. We manage to track up to four coexisting phases.
Each of these is fractionated: it contains a narrower distribution of particle
sizes than is present in the system overall. We also demonstrate that,
surprisingly, demixing transitions can be nearly continuous, accompanied by
fluctuations in local particle size correlated over many lattice spacings.Comment: 4 pages, 4 figures. Shortened abstract, added clarification to
technical point
Demixing cascades in cluster crystals
In a cluster crystal, each lattice site is occupied by multiple soft-core
particles. As the number density is increased at zero temperature, a `cascade'
of isostructural phase transitions can occur between states whose site
occupancy differs by unity. For low but finite temperature, each of these
transitions terminates in a critical point. Using tailored Monte Carlo
simulation techniques we have studied such demixing cascades in systems of soft
particles interacting via potentials of the generalized exponential form
. We have estimated the critical parameters
of the first few transitions in the cascade as a function of the softness
parameter . The critical temperature and pressure exhibit non-monotonic
behaviour as is varied, although the critical chemical potential remains
monotonic. The trends for the pressure and chemical potential are confirmed by
cell model calculations at zero temperature. As , all the transitions
that we have observed are preempted by melting although we cannot rule out that
clustering transitions survive at high density.Comment: 8 page
A Monte Carlo method for chemical potential determination in single and multiple occupancy crystals
We describe a Monte Carlo scheme which, in a single simulation, yields a
measurement of the chemical potential of a crystalline solid. Within the
isobaric ensemble, this immediately provides an estimate of the system free
energy, with statistical uncertainties that are determined precisely and
transparently. An extension to multiple occupancy ("cluster") solids permits
the direct determination of the cluster chemical potential and hence the
equilibrium conditions. We apply the method to a model exhibiting cluster
crystalline phases, where we find evidence for an infinite cascade of critical
points terminating coexistence between crystals of differing site occupancies.Comment: 5 pages, 3 figure
Liquid-gas coexistence and critical point shifts in size-disperse fluids
Specialized Monte Carlo simulations and the moment free energy (MFE) method
are employed to study liquid-gas phase equilibria in size-disperse fluids. The
investigation is made subject to the constraint of fixed polydispersity, i.e.
the form of the `parent' density distribution of the particle
diameters , is prescribed. This is the experimentally realistic
scenario for e.g. colloidal dispersions. The simulations are used to obtain the
cloud and shadow curve properties of a Lennard-Jones fluid having diameters
distributed according to a Schulz form with a large (40%) degree of
polydispersity. Good qualitative accord is found with the results from a MFE
method study of a corresponding van der Waals model that incorporates
size-dispersity both in the hard core reference and the attractive parts of the
free energy. The results show that polydispersity engenders considerable
broadening of the coexistence region between the cloud curves. The principal
effect of fractionation in this region is a common overall scaling of the
particle sizes and typical inter-particle distances, and we discuss why this
effect is rather specific to systems with Schulz diameter distributions. Next,
by studying a family of such systems with distributions of various widths, we
estimate the dependence of the critical point parameters on . In
contrast to a previous theoretical prediction, size-dispersity is found to
raise the critical temperature above its monodisperse value. Unusually for a
polydisperse system, the critical point is found to lie at or very close to the
extremum of the coexistence region in all cases. We outline an argument showing
that such behaviour will occur whenever size polydispersity affects only the
range, rather than the strength of the inter-particle interactions.Comment: 14 pages, 12 figure
Wetting transitions in polydisperse fluids
The properties of the coexisting bulk gas and liquid phases of a polydisperse
fluid depend not only on the prevailing temperature, but also on the overall
parent density. As a result, a polydisperse fluid near a wall will exhibit
density-driven wetting transitions inside the coexistence region. We propose a
likely topology for the wetting phase diagram, which we test using Monte Carlo
simulations of a model polydisperse fluid at an attractive wall, tracing the
wetting line inside the cloud curve and identifying the relationship to
prewetting.Comment: 4 Pages, 4 figures. Accepted for publication in Physical Review
Letter
Accurate simulation estimates of cloud points of polydisperse fluids
We describe two distinct approaches to obtaining cloud point densities and
coexistence properties of polydisperse fluid mixtures by Monte Carlo simulation
within the grand canonical ensemble. The first method determines the chemical
potential distribution (with the polydisperse attribute)
under the constraint that the ensemble average of the particle density
distribution matches a prescribed parent form. Within the region
of phase coexistence (delineated by the cloud curve) this leads to a
distribution of the fluctuating overall particle density n, p(n), that
necessarily has unequal peak weights in order to satisfy a generalized lever
rule. A theoretical analysis shows that as a consequence, finite-size
corrections to estimates of coexistence properties are power laws in the system
size. The second method assigns such that an equal peak weight
criterion is satisfied for p(n)\mu(\sigma)$. We show how to ascertain the
requisite weight factor operationally. A theoretical analysis of the second
method suggests that it leads to finite-size corrections to estimates of
coexistence properties which are {\em exponentially small} in the system size.
The scaling predictions for both methods are tested via Monte Carlo simulations
of a novel polydisperse lattice gas model near its cloud curve, the results
showing excellent quantitative agreement with the theory.Comment: 8 pages, 6 figure
Phase behaviour and particle-size cutoff effects in polydisperse fluids
We report a joint simulation and theoretical study of the liquid-vapor phase
behaviour of a fluid in which polydispersity in the particle size couples to
the strength of the interparticle interactions. Attention is focussed on the
case in which the particles diameters are distributed according to a fixed
Schulz form with degree of polydispersity . The coexistence
properties of this model are studied using grand canonical ensemble Monte Carlo
simulations and moment free energy calculations. We obtain the cloud and shadow
curves as well as the daughter phase density distributions and fractional
volumes along selected isothermal dilution lines. In contrast to the case of
size-{\em independent} interaction strengths (N.B. Wilding, M. Fasolo and P.
Sollich, J. Chem. Phys. {\bf 121}, 6887 (2004)), the cloud and shadow curves
are found to be well separated, with the critical point lying significantly
below the cloud curve maximum. For densities below the critical value, we
observe that the phase behaviour is highly sensitive to the choice of upper
cutoff on the particle size distribution. We elucidate the origins of this
effect in terms of extremely pronounced fractionation effects and discuss the
likely appearance of new phases in the limit of very large values of the
cutoff.Comment: 12 pages, 15 figure
ISIS2: Pixel Sensor with Local Charge Storage for ILC Vertex Detector
ISIS (In-situ Storage Imaging Sensor) is a novel CMOS sensor with multiple
charge storage capability developed for the ILC vertex detector by the Linear
Collider Flavour Identification (LCFI) collaboration. This paper reports test
results for ISIS2, the second generation of ISIS sensors implemented in a 0.18
micron CMOS process. The local charge storage and charge transfer were
unambiguously demonstrated.Comment: 11 pages, 16 figures, to be included in the Proceedings of
International Linear Collider Workshop 201
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