214 research outputs found
Geant4 Simulation of a filtered X-ray Source for Radiation Damage Studies
Geant4 low energy extensions have been used to simulate the X-ray spectra of
industrial X-ray tubes with filters for removing the uncertain low energy part
of the spectrum in a controlled way. The results are compared with precisely
measured X-ray spectra using a silicon drift detector. Furthermore, this paper
shows how the different dose rates in silicon and silicon dioxide layers of an
electronic device can be deduced from the simulations
Double dynamical regime of confined water
The Van Hove self correlation function of water confined in a silica pore is
calculated from Molecular Dynamics trajectories upon supercooling. At long time
in the relaxation region we found that the behaviour of the real space
time dependent correlators can be decomposed in a very slow, almost frozen,
dynamics due to the bound water close to the substrate and a faster dynamics of
the free water which resides far from the confining surface. For free water we
confirm the evidences of an approach to a crossover mode coupling transition,
previously found in Q space. In the short time region we found that the two
dynamical regimes are overimposed and cannot be distinguished. This shows that
the interplay between the slower and the faster dynamics emerges in going from
early times to the relaxation region, where a layer analysis of the
dynamical properties can be performed.Comment: 6 pages with 9 figures. RevTeX. Accepted for pulbication in J. Phys.
Cond. Mat
On the surface critical behaviour in Ising strips: density-matrix renormalization-group study
Using the density-matrix renormalization-group method we study the surface
critical behaviour of the magnetization in Ising strips in the subcritical
region. Our results support the prediction that the surface magnetization in
the two phases along the pseudo-coexistence curve also behaves as for the
ordinary transition below the wetting temperature for the finite value of the
surface field.Comment: 15 pages, 9 figure
Simulation of fluid-solid coexistence in finite volumes: A method to study the properties of wall-attached crystalline nuclei
The Asakura-Oosawa model for colloid-polymer mixtures is studied by Monte
Carlo simulations at densities inside the two-phase coexistence region of fluid
and solid. Choosing a geometry where the system is confined between two flat
walls, and a wall-colloid potential that leads to incomplete wetting of the
crystal at the wall, conditions can be created where a single nanoscopic
wall-attached crystalline cluster coexists with fluid in the remainder of the
simulation box. Following related ideas that have been useful to study
heterogeneous nucleation of liquid droplets at the vapor-liquid coexistence, we
estimate the contact angles from observations of the crystalline clusters in
thermal equilibrium. We find fair agreement with a prediction based on Young's
equation, using estimates of interface and wall tension from the study of flat
surfaces. It is shown that the pressure versus density curve of the finite
system exhibits a loop, but the pressure maximum signifies the "droplet
evaporation-condensation" transition and thus has nothing in common with a van
der Waals-like loop. Preparing systems where the packing fraction is deep
inside the two-phase coexistence region, the system spontaneously forms a "slab
state", with two wall-attached crystalline domains separated by (flat)
interfaces from liquid in full equilibrium with the crystal in between;
analysis of such states allows a precise estimation of the bulk equilibrium
properties at phase coexistence
Does Young's equation hold on the nanoscale? A Monte Carlo test for the binary Lennard-Jones fluid
When a phase-separated binary () mixture is exposed to a wall, that
preferentially attracts one of the components, interfaces between A-rich and
B-rich domains in general meet the wall making a contact angle .
Young's equation describes this angle in terms of a balance between the
interfacial tension and the surface tensions ,
between, respectively, the - and -rich phases and the wall,
. By Monte Carlo simulations
of bridges, formed by one of the components in a binary Lennard-Jones liquid,
connecting the two walls of a nanoscopic slit pore, is estimated from
the inclination of the interfaces, as a function of the wall-fluid interaction
strength. The information on the surface tensions ,
are obtained independently from a new thermodynamic integration method, while
is found from the finite-size scaling analysis of the
concentration distribution function. We show that Young's equation describes
the contact angles of the actual nanoscale interfaces for this model rather
accurately and location of the (first order) wetting transition is estimated.Comment: 6 pages, 6 figure
Excess free energy and Casimir forces in systems with long-range interactions of van-der-Waals type: General considerations and exact spherical-model results
We consider systems confined to a -dimensional slab of macroscopic lateral
extension and finite thickness that undergo a continuous bulk phase
transition in the limit and are describable by an O(n) symmetrical
Hamiltonian. Periodic boundary conditions are applied across the slab. We study
the effects of long-range pair interactions whose potential decays as as , with and , on
the Casimir effect at and near the bulk critical temperature ,
for . For the scaled reduced Casimir force per unit cross-sectional
area, we obtain the form L^{d} {\mathcal F}_C/k_BT \approx \Xi_0(L/\xi_\infty)
+ g_\omega L^{-\omega}\Xi\omega(L/\xi_\infty) + g_\sigma L^{-\omega_\sigm a}
\Xi_\sigma(L \xi_\infty). The contribution decays for
algebraically in rather than exponentially, and hence
becomes dominant in an appropriate regime of temperatures and . We derive
exact results for spherical and Gaussian models which confirm these findings.
In the case , which includes that of nonretarded van-der-Waals
interactions in dimensions, the power laws of the corrections to scaling
of the spherical model are found to get modified by logarithms.
Using general RG ideas, we show that these logarithmic singularities originate
from the degeneracy that occurs for the spherical
model when , in conjunction with the dependence of .Comment: 28 RevTeX pages, 12 eps figures, submitted to PR
Vibrational Features of Water at the Low-Density/High-Density Liquid Structural Transformations
A structural transformation in water upon compression was recently observed
at the temperature ~K in the vicinity of the pressure ~Atm [R.M. Khusnutdinoff, A.V. Mokshin, J. Non-Cryst. Solids
\textbf{357}, 1677 (2011)]. It was found that the transformations are related
with the principal structural changes within the first two coordination shells
as well as the deformation of the hydrogen-bond network. In this work we study
in details the influence of these structural transformations on the vibrational
molecular dynamics of water by means of molecular dynamics simulations on the
basis of the model Amoeba potential (~K, ~Atm). The
equation of state and the isothermal compressibility are found for the
considered (,)-range. The vibrational density of states extracted for
-frequency range manifests the two distinct modes, where the
high-frequency mode is independent on pressure whereas the low-frequency one
has the strong, non-monotonic pressure-dependence and exhibits a step-like
behavior at the pressure ~Atm. The extended analysis of the
local structural and vibrational properties discovers that there is a strong
correlation between the primary structural and vibrational aspects of the
liquid-liquid structural transformation related with the molecular
rearrangement within the range of the second coordination shell.Comment: Accepted to Physica A: Statistical Mechanics and its Application
X-ray Diffraction and Molecular Dynamics Study of Medium-range Order in Ambient and Hot Water
We have developed x-ray diffraction measurements with high energy-resolution
and accuracy to study water structure at three different temperatures (7, 25
and 66 C) under normal pressure. Using a spherically curved Ge crystal an
energy resolution better than 15 eV has been achieved which eliminates
influence from Compton scattering. The high quality of the data allows a
precise oxygen-oxygen pair correlation function (PCF) to be directly derived
from the Fourier transform of the experimental data resolving shell structure
out to ~12 {\AA}, i.e. 5 hydration shells. Large-scale molecular dynamics (MD)
simulations using the TIP4P/2005 force-field reproduce excellently the
experimental shell-structure in the range 4-12 {\AA} although less agreement is
seen for the first peak in the PCF. The Local Structure Index [J. Chem. Phys.
104, 7671 (1996)] identifies a tetrahedral minority giving the
intermediate-range oscillations in the PCF and a disordered majority providing
a more featureless background in this range. The current study supports the
proposal that the structure of liquid water, even at high temperatures, can be
described in terms of a two-state fluctuation model involving local structures
related to the high-density and low-density forms of liquid water postulated in
the liquid-liquid phase transition hypothesis.Comment: Submitted to Phys. Chem. Chem. Phy
Supercooled confined water and the Mode Coupling crossover temperature
We present a Molecular Dynamics study of the single particle dynamics of
supercooled water confined in a silica pore. Two dynamical regimes are found:
close to the hydrophilic substrate molecules are below the Mode Coupling
crossover temperature, , already at ambient temperature. The water closer
to the center of the pore (free water) approaches upon supercooling as
predicted by Mode Coupling Theories. For free water the crossover temperature
and crossover exponent are extracted from power-law fits to both the
diffusion coefficient and the relaxation time of the late region.Comment: To be published, Phys. Rev. Lett., 4 pages, 3 figures, revTeX, minor
changes in the figures, references added, changes in the tex
Ab initio van der Waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like
The structure of liquid water at ambient conditions is studied in ab initio
molecular dynamics simulations using van der Waals (vdW) density-functional
theory, i.e. using the new exchange-correlation functionals optPBE-vdW and
vdW-DF2. Inclusion of the more isotropic vdW interactions counteracts highly
directional hydrogen-bonds, which are enhanced by standard functionals. This
brings about a softening of the microscopic structure of water, as seen from
the broadening of angular distribution functions and, in particular, from the
much lower and broader first peak in the oxygen-oxygen pair-correlation
function (PCF), indicating loss of structure in the outer solvation shells. In
combination with softer non-local correlation terms, as in the new
parameterization of vdW-DF, inclusion of vdW interactions is shown to shift the
balance of resulting structures from open tetrahedral to more close-packed. The
resulting O-O PCF shows some resemblance with experiment for high-density water
(A. K. Soper and M. A. Ricci, Phys. Rev. Lett., 84:2881, 2000), but not
directly with experiment for ambient water. However, an O-O PCF consisting of a
linear combination of 70% from vdW-DF2 and 30% from experiment on low-density
liquid water reproduces near-quantitatively the experimental O-O PCF for
ambient water, indicating consistency with a two-liquid model with fluctuations
between high- and low-density regions
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