1,529 research outputs found
A simple ansatz for the study of velocity autocorrelation functions in fluids at different timescales
A simple ansatz for the study of velocity autocorrelation functions in fluids
at different timescales is proposed. The ansatz is based on an effective
summation of the infinite continued fraction at a reasonable assumption about
convergence of relaxation times of the higher order memory functions, which
have a purely kinetic origin. The VAFs obtained within our approach are
compared with the results of the Markovian approximation for memory kernels. It
is shown that although in the "overdamped" regime both approaches agree to a
large extent at the initial and intermediate times of the system evolution, our
formalism yields power law relaxation of the VAFs which is not observed at the
description with a finite number of the collective modes. Explicit expressions
for the transition times from kinetic to hydrodynamic regimes are obtained from
the analysis of the singularities of spectral functions in the complex
frequency plane.Comment: 14 pages, 2 figure
Depletion potentials near geometrically structured substrates
Using the recently developed so-called White Bear version of Rosenfeld's
Fundamental Measure Theory we calculate the depletion potentials between a
hard-sphere colloidal particle in a solvent of small hard spheres and simple
models of geometrically structured substrates: a right-angled wedge or edge. In
the wedge geometry, there is a strong attraction beyond the corresponding one
near a planar wall that significantly influences the structure of colloidal
suspensions in wedges. In accordance with an experimental study, for the edge
geometry we find a free energy barrier of the order of several which
repels a big colloidal particle from the edge.Comment: 7 pages, 7 figure
Microscopic dynamics in liquid metals: the experimental point of view
The experimental results relevant for the understanding of the microscopic
dynamics in liquid metals are reviewed, with special regards to the ones
achieved in the last two decades. Inelastic Neutron Scattering played a major
role since the development of neutron facilities in the sixties. The last ten
years, however, saw the development of third generation radiation sources,
which opened the possibility of performing Inelastic Scattering with X rays,
thus disclosing previously unaccessible energy-momentum regions. The purely
coherent response of X rays, moreover, combined with the mixed
coherent/incoherent response typical of neutron scattering, provides enormous
potentialities to disentangle aspects related to the collectivity of motion
from the single particle dynamics.
If the last twenty years saw major experimental developments, on the
theoretical side fresh ideas came up to the side of the most traditional and
established theories. Beside the raw experimental results, therefore, we review
models and theoretical approaches for the description of microscopic dynamics
over different length-scales, from the hydrodynamic region down to the single
particle regime, walking the perilous and sometimes uncharted path of the
generalized hydrodynamics extension. Approaches peculiar of conductive systems,
based on the ionic plasma theory, are also considered, as well as kinetic and
mode coupling theory applied to hard sphere systems, which turn out to mimic
with remarkable detail the atomic dynamics of liquid metals. Finally, cutting
edges issues and open problems, such as the ultimate origin of the anomalous
acoustic dispersion or the relevance of transport properties of a conductive
systems in ruling the ionic dynamic structure factor are discussed.Comment: 53 pages, 41 figures, to appear in "The Review of Modern Physics".
Tentatively scheduled for July issu
Magnetism of small V clusters embedded in a Cu fcc matrix: an ab initio study
We present extensive first principles density functional theory (DFT)
calculations dedicated to analyze the magnetic and electronic properties of
small V clusters (n=1,2,3,4,5,6) embedded in a Cu fcc matrix. We consider
different cluster structures such as: i) a single V impurity, ii) several
V dimers having different interatomic distance and varying local atomic
environment, iii) V and iv) V clusters for which we assume compact
as well as 2- and 1-dimensional atomic configurations and finally, in the case
of the v) V and vi) V structures we consider a square pyramid and a
square bipyramid together with linear arrays, respectively. In all cases, the V
atoms are embedded as substitutional impurities in the Cu network. In general,
and as in the free standing case, we have found that the V clusters tend to
form compact atomic arrays within the cooper matrix. Our calculated non
spin-polarized density of states at the V sites shows a complex peaked
structure around the Fermi level that strongly changes as a function of both
the interatomic distance and local atomic environment, a result that
anticipates a non trivial magnetic behavior. In fact, our DFT calculations
reveal, in each one of our clusters systems, the existence of different
magnetic solutions (ferromagnetic, ferrimagnetic, and antiferromagnetic) with
very small energy differences among them, a result that could lead to the
existence of complex finite-temperature magnetic properties. Finally, we
compare our results with recent experimental measurements.Comment: 7 pages and 4 figure
Candida albicans repetitive elements display epigenetic diversity and plasticity
Transcriptionally silent heterochromatin is associated with repetitive DNA. It is poorly understood whether and how heterochromatin differs between different organisms and whether its structure can be remodelled in response to environmental signals. Here, we address this question by analysing the chromatin state associated with DNA repeats in the human fungal pathogen Candida albicans. Our analyses indicate that, contrary to model systems, each type of repetitive element is assembled into a distinct chromatin state. Classical Sir2-dependent hypoacetylated and hypomethylated chromatin is associated with the rDNA locus while telomeric regions are assembled into a weak heterochromatin that is only mildly hypoacetylated and hypomethylated. Major Repeat Sequences, a class of tandem repeats, are assembled into an intermediate chromatin state bearing features of both euchromatin and heterochromatin. Marker gene silencing assays and genome-wide RNA sequencing reveals that C. albicans heterochromatin represses expression of repeat-associated coding and non-coding RNAs. We find that telomeric heterochromatin is dynamic and remodelled upon an environmental change. Weak heterochromatin is associated with telomeres at 30?°C, while robust heterochromatin is assembled over these regions at 39?°C, a temperature mimicking moderate fever in the host. Thus in C. albicans, differential chromatin states controls gene expression and epigenetic plasticity is linked to adaptation
Slogging and Stumbling Toward Social Justice in a Private Elementary School: The Complicated Case of St. Malachy
This case study examines St. Malachy, an urban Catholic elementary school primarily serving children traditionally marginalized by race, class, linguistic heritage, and disability. As a private school, St. Malachy serves the public good by recruiting and retaining such traditionally marginalized students. As empirical studies involving Catholic schools frequently juxtapose them with public schools, the author presents this examination from a different tack. Neither vilifying nor glorifying Catholic schooling, this study critically examines the pursuit of social justice in this school context. Data gathered through a 1-year study show that formal and informal leaders in St. Malachy adapted their governance, aggressively sought community resources, and focused their professional development to build the capacity to serve their increasingly pluralistic student population. The analysis confirms the deepening realization that striving toward social justice is a messy, contradictory, and complicated pursuit, and that schools in both public and private sectors are allies in this pursuit
Density functional formalism in the canonical ensemble
Density functional theory, when applied to systems with , is based
on the grand canonical extension of the Hohenberg-Kohn-Sham theorem due to
Mermin (HKSM theorem). While a straightforward canonical ensemble
generalization fails, work in nanopore systems could certainly benefit from
such extension. We show that, if the asymptotic behaviour of the canonical
distribution functions is taken into account, the HKSM theorem can be extended
to the canonical ensemble. We generate -modified correlation and
distribution functions hierarchies and prove that, if they are employed, either
a modified external field or the density profiles can be indistinctly used as
independent variables. We also write down the % -modified free energy
functional and prove that its minimum is reached when the equilibrium values of
the new hierarchy are used. This completes the extension of the HKSM theorem.Comment: revtex, to be submitted to Phys. Rev. Let
Integral equations for simple fluids in a general reference functional approach
The integral equations for the correlation functions of an inhomogeneous
fluid mixture are derived using a functional Taylor expansion of the free
energy around an inhomogeneous equilibrium distribution. The system of
equations is closed by the introduction of a reference functional for the
correlations beyond second order in the density difference from the equilibrium
distribution. Explicit expressions are obtained for energies required to insert
particles of the fluid mixture into the inhomogeneous system. The approach is
illustrated by the determination of the equation of state of a simple,
truncated Lennard--Jones fluid and the analysis of the behavior of this fluid
near a hard wall. The wall--fluid integral equation exhibits complete drying
and the corresponding coexisting densities are in good agreement with those
obtained from the standard (Maxwell) construction applied to the bulk fluid.
Self--consistency of the approach is examined by analyzing the
virial/compressibility routes to the equation of state and the Gibbs--Duhem
relation for the bulk fluid, and the contact density sum rule and the Gibbs
adsorption equation for the hard wall problem. For the bulk fluid, we find good
self--consistency for stable states outside the critical region. For the hard
wall problem, the Gibbs adsorption equation is fulfilled very well near phase
coexistence where the adsorption is large.For the contact density sum rule, we
find some deviationsnear coexistence due to a slight disagreement between the
coexisting density for the gas phase obtained from the Maxwell construction and
from complete drying at the hard wall.Comment: 29 page
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