2,108 research outputs found
Why do ultrasoft repulsive particles cluster and crystallize? Analytical results from density functional theory
We demonstrate the accuracy of the hypernetted chain closure and of the
mean-field approximation for the calculation of the fluid-state properties of
systems interacting by means of bounded and positive-definite pair potentials
with oscillating Fourier transforms. Subsequently, we prove the validity of a
bilinear, random-phase density functional for arbitrary inhomogeneous phases of
the same systems. On the basis of this functional, we calculate analytically
the freezing parameters of the latter. We demonstrate explicitly that the
stable crystals feature a lattice constant that is independent of density and
whose value is dictated by the position of the negative minimum of the Fourier
transform of the pair potential. This property is equivalent with the existence
of clusters, whose population scales proportionally to the density. We
establish that regardless of the form of the interaction potential and of the
location on the freezing line, all cluster crystals have a universal Lindemann
ratio L = 0.189 at freezing. We further make an explicit link between the
aforementioned density functional and the harmonic theory of crystals. This
allows us to establish an equivalence between the emergence of clusters and the
existence of negative Fourier components of the interaction potential. Finally,
we make a connection between the class of models at hand and the system of
infinite-dimensional hard spheres, when the limits of interaction steepness and
space dimension are both taken to infinity in a particularly described fashion.Comment: 19 pages, 5 figures, submitted to J. Chem. Phys; new version: minor
changes in structure of pape
Mean Field Fluid Behavior of the Gaussian Core Model
We show that the Gaussian core model of particles interacting via a
penetrable repulsive Gaussian potential, first considered by Stillinger (J.
Chem. Phys. 65, 3968 (1976)), behaves like a weakly correlated ``mean field
fluid'' over a surprisingly wide density and temperature range. In the bulk the
structure of the fluid phase is accurately described by the random phase
approximation for the direct correlation function, and by the more
sophisticated HNC integral equation. The resulting pressure deviates very
little from a simple, mean-field like, quadratic form in the density, while the
low density virial expansion turns out to have an extremely small radius of
convergence. Density profiles near a hard wall are also very accurately
described by the corresponding mean-field free-energy functional. The binary
version of the model exhibits a spinodal instability against de-mixing at high
densities. Possible implications for semi-dilute polymer solutions are
discussed.Comment: 13 pages, 2 columns, ReVTeX epsfig,multicol,amssym, 15 figures;
submitted to Phys. Rev. E (change: important reference added
Fluid and solid phases of the Gaussian core model
We study the structural and thermodynamic properties of a model of point
particles interacting by means of a Gaussian pair potential first introduced by
Stillinger [Stillinger F H 1976 J. Chem. Phys. 65, 3968]. By employing integral
equation theories for the fluid state and comparing with Monte Carlo simulation
results, we establish the limits of applicability of various common closures
and examine the dependence of the correlation functions of the liquid on the
density and temperature. We employ a simple, mean-field theory for the high
density domain of the liquid and demonstrate that at infinite density the
mean-field theory is exact and that the system reduces to an `infinite density
ideal gas', where all correlations vanish and where the hypernetted chain (HNC)
closure becomes exact. By employing an Einstein model for the solid phases, we
subsequently calculate quantitatively the phase diagram of the model and find
that the system possesses two solid phases, face centered cubic and body
centered cubic, and also displays reentrant melting into a liquid at high
densities. Moreover, the system remains fluid at all densities when the
temperature exceeds 1% of the strength of the interactions.Comment: 22 pages, 10 figure
Can Polymer Coils be modeled as "Soft Colloids"?
We map dilute or semi-dilute solutions of non-intersecting polymer chains
onto a fluid of ``soft'' particles interacting via a concentration dependent
effective pair potential, by inverting the pair distribution function of the
centers of mass of the initial polymer chains. A similar inversion is used to
derive an effective wall-polymer potential; these potentials are combined to
successfully reproduce the calculated exact depletion interaction induced by
non-intersecting polymers between two walls. The mapping opens up the
possibility of large-scale simulations of polymer solutions in complex
geometries.Comment: 4 pages, 3 figures ReVTeX[epsfig,multicol,amssymb] references update
Exotic fluids and crystals of soft polymeric colloids
We discuss recent developments and present new findings in the colloidal
description of soft polymeric macromolecular aggregates. For various
macromolecular architectures, such as linear chains, star polymers, dendrimers
and polyelectrolyte stars, the effective interactions between suitably chosen
coordinates are shown to be ultrasoft, i.e., they either remain finite or
diverge very slowly at zero separation. As a consequence, the fluid phases have
unusual characteristics, including anomalous pair correlations and mean-field
like thermodynamic behaviour. The solid phases can exhibit exotic, strongly
anisotropic as well as open crystal structures. For example, the diamond and
the A15-phase are shown to be stable at sufficiently high concentrations.
Reentrant melting and clustering transitions are additional features displayed
by such systems, resulting in phase diagrams with a very rich topology. We
emphasise that many of these effects are fundamentally different from the usual
archetypal hard sphere paradigm. Instead, we propose that these fluids fall
into the class of mean-field fluids.Comment: 22 pages, uses iopart.cls and iopart10.clo; submitted to Journal of
Physics Condensed Matter, special issue in honour of professor Peter Puse
Highly site-specific H2 adsorption on vicinal Si(001) surfaces
Experimental and theoretical results for the dissociative adsorption of H_2
on vicinal Si(001) surfaces are presented. Using optical second-harmonic
generation, sticking probabilities at the step sites are found to exceed those
on the terraces by up to six orders of magnitude. Density functional theory
calculations indicate the presence of direct adsorption pathways for
monohydride formation but with a dramatically lowered barrier for step
adsorption due to an efficient rehybridization of dangling orbitals.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Lett. (1998). Other
related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Exact Criterion for Determining Clustering vs. Reentrant Melting Behavior for Bounded Interaction Potentials
We examine in full generality the phase behavior of systems whose constituent
particles interact by means of potentials which do not diverge at the origin,
are free of attractive parts and decay fast enough to zero as the interparticle
separation r goes to infinity. By employing a mean field-density functional
theory which is shown to become exact at high temperatures and/or densities, we
establish a criterion which determines whether a given system will freeze at
all temperatures or it will display reentrant melting and an upper freezing
temperature.Comment: 5 pages, 3 figures, submitted to PRL on March 29, 2000 New version:
10 pages, 9 figures, forwarded to PRE on October 16, 200
Observation and Characterization of a Cosmic Muon Neutrino Flux from the Northern Hemisphere using six years of IceCube data
The IceCube Collaboration has previously discovered a high-energy
astrophysical neutrino flux using neutrino events with interaction vertices
contained within the instrumented volume of the IceCube detector. We present a
complementary measurement using charged current muon neutrino events where the
interaction vertex can be outside this volume. As a consequence of the large
muon range the effective area is significantly larger but the field of view is
restricted to the Northern Hemisphere. IceCube data from 2009 through 2015 have
been analyzed using a likelihood approach based on the reconstructed muon
energy and zenith angle. At the highest neutrino energies between 191 TeV and
8.3 PeV a significant astrophysical contribution is observed, excluding a
purely atmospheric origin of these events at significance. The
data are well described by an isotropic, unbroken power law flux with a
normalization at 100 TeV neutrino energy of
and a hard spectral index of . The observed spectrum is
harder in comparison to previous IceCube analyses with lower energy thresholds
which may indicate a break in the astrophysical neutrino spectrum of unknown
origin. The highest energy event observed has a reconstructed muon energy of
which implies a probability of less than 0.005% for
this event to be of atmospheric origin. Analyzing the arrival directions of all
events with reconstructed muon energies above 200 TeV no correlation with known
-ray sources was found. Using the high statistics of atmospheric
neutrinos we report the currently best constraints on a prompt atmospheric muon
neutrino flux originating from charmed meson decays which is below in
units of the flux normalization of the model in Enberg et al. (2008).Comment: 20 pages, 21 figure
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