240 research outputs found
Entropy/IP: Uncovering Structure in IPv6 Addresses
In this paper, we introduce Entropy/IP: a system that discovers Internet
address structure based on analyses of a subset of IPv6 addresses known to be
active, i.e., training data, gleaned by readily available passive and active
means. The system is completely automated and employs a combination of
information-theoretic and machine learning techniques to probabilistically
model IPv6 addresses. We present results showing that our system is effective
in exposing structural characteristics of portions of the IPv6 Internet address
space populated by active client, service, and router addresses.
In addition to visualizing the address structure for exploration, the system
uses its models to generate candidate target addresses for scanning. For each
of 15 evaluated datasets, we train on 1K addresses and generate 1M candidates
for scanning. We achieve some success in 14 datasets, finding up to 40% of the
generated addresses to be active. In 11 of these datasets, we find active
network identifiers (e.g., /64 prefixes or `subnets') not seen in training.
Thus, we provide the first evidence that it is practical to discover subnets
and hosts by scanning probabilistically selected areas of the IPv6 address
space not known to contain active hosts a priori.Comment: Paper presented at the ACM IMC 2016 in Santa Monica, USA
(https://dl.acm.org/citation.cfm?id=2987445). Live Demo site available at
http://www.entropy-ip.com
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
Surface Induced Order in Liquid Metals and Binary Alloys
Measurements of the surface x-ray scattering from several pure liquid metals
(Hg, Ga, and In) and from three alloys (Ga-Bi, Bi-In, and K-Na) with different
heteroatomic chemical interactions in the bulk phase are reviewed.
Surface-induced layering is found for each elemental liquid metal. The surface
structure of the K-Na alloy resembles that of an elemental liquid metal. Bi-In
displays pair formation at the surface. Surface segregation and a wetting film
are found for Ga-Bi.Comment: 10 pages, 3 fig, published in Journal of Physics: Condensed Matte
Optimized random phase approximations for arbitrary reference systems: extremum conditions and thermodynamic consistence
The optimized random phase approximation (ORPA) for classical liquids is
re-examined in the framework of the generating functional approach to the
integral equations. We show that the two main variants of the approximation
correspond to the addition of the same correction to two different first order
approximations of the homogeneous liquid free energy. Furthermore, we show that
it is possible to consistently use the ORPA with arbitrary reference systems
described by continuous potentials and that the same approximation is
equivalent to a particular extremum condition for the corresponding generating
functional. Finally, it is possible to enforce the thermodynamic consistence
between the thermal and the virial route to the equation of state by requiring
the global extremum condition on the generating functional.Comment: 8 pages, RevTe
Surface Structure of Liquid Metals and the Effect of Capillary Waves: X-ray Studies on Liquid Indium
We report x-ray reflectivity (XR) and small angle off-specular diffuse
scattering (DS) measurements from the surface of liquid Indium close to its
melting point of C. From the XR measurements we extract the surface
structure factor convolved with fluctuations in the height of the liquid
surface. We present a model to describe DS that takes into account the surface
structure factor, thermally excited capillary waves and the experimental
resolution. The experimentally determined DS follows this model with no
adjustable parameters, allowing the surface structure factor to be deconvolved
from the thermally excited height fluctuations. The resulting local electron
density profile displays exponentially decaying surface induced layering
similar to that previously reported for Ga and Hg. We compare the details of
the local electron density profiles of liquid In, which is a nearly free
electron metal, and liquid Ga, which is considerably more covalent and shows
directional bonding in the melt. The oscillatory density profiles have
comparable amplitudes in both metals, but surface layering decays over a length
scale of \AA for In and \AA for Ga. Upon controlled
exposure to oxygen, no oxide monolayer is formed on the liquid In surface,
unlike the passivating film formed on liquid Gallium.Comment: 9 pages, 5 figures; submitted to Phys. Rev.
Thermodynamics and structure of self-assembled networks
We study a generic model of self-assembling chains which can branch and form
networks with branching points (junctions) of arbitrary functionality. The
physical realizations include physical gels, wormlike micells, dipolar fluids
and microemulsions. The model maps the partition function of a solution of
branched, self-assembling, mutually avoiding clusters onto that of a Heisenberg
magnet in the mathematical limit of zero spin components. The model is solved
in the mean field approximation. It is found that despite the absence of any
specific interaction between the chains, the entropy of the junctions induces
an effective attraction between the monomers, which in the case of three-fold
junctions leads to a first order reentrant phase separation between a dilute
phase consisting mainly of single chains, and a dense network, or two network
phases. Independent of the phase separation, we predict the percolation
(connectivity) transition at which an infinite network is formed that partially
overlaps with the first-order transition. The percolation transition is a
continuous, non thermodynamic transition that describes a change in the
topology of the system. Our treatment which predicts both the thermodynamic
phase equilibria as well as the spatial correlations in the system allows us to
treat both the phase separation and the percolation threshold within the same
framework. The density-density correlation correlation has a usual
Ornstein-Zernicke form at low monomer densities. At higher densities, a peak
emerges in the structure factor, signifying an onset of medium-range order in
the system. Implications of the results for different physical systems are
discussed.Comment: Submitted to Phys. Rev.
Transitions between Inherent Structures in Water
The energy landscape approach has been useful to help understand the dynamic
properties of supercooled liquids and the connection between these properties
and thermodynamics. The analysis in numerical models of the inherent structure
(IS) trajectories -- the set of local minima visited by the liquid -- offers
the possibility of filtering out the vibrational component of the motion of the
system on the potential energy surface and thereby resolving the slow
structural component more efficiently. Here we report an analysis of an IS
trajectory for a widely-studied water model, focusing on the changes in
hydrogen bond connectivity that give rise to many IS separated by relatively
small energy barriers. We find that while the system \emph{travels} through
these IS, the structure of the bond network continuously modifies, exchanging
linear bonds for bifurcated bonds and usually reversing the exchange to return
to nearly the same initial configuration. For the 216 molecule system we
investigate, the time scale of these transitions is as small as the simulation
time scale ( fs). Hence for water, the transitions between each of
these IS is relatively small and eventual relaxation of the system occurs only
by many of these transitions. We find that during IS changes, the molecules
with the greatest displacements move in small ``clusters'' of 1-10 molecules
with displacements of nm, not unlike simpler liquids.
However, for water these clusters appear to be somewhat more branched than the
linear ``string-like'' clusters formed in a supercooled Lennar d-Jones system
found by Glotzer and her collaborators.Comment: accepted in PR
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
Peeling the astronomical onion
Water ice is the most abundant solid in the Universe. Understanding the formation, structure and multiplicity of physicochemical roles for water ice in the cold, dense interstellar environments in which it is predominantly observed is a crucial quest for astrochemistry as these are regions active in star and planet formation. Intuitively, we would expect the mobility of water molecules deposited or synthesised on dust grain surfaces at temperatures below 50 K to be very limited. This work delves into the thermally-activated mobility of H2O molecules on model interstellar grain surfaces. The energy required to initiate this process is studied by reflection-absorption infrared spectroscopy of small quantities of water on amorphous silica and highly oriented pyrolytic graphite surfaces as the surface is annealed. Strongly non-Arrhenius behaviour is observed with an activation energy of 2 kJ mol-1 on the silica surface below 25 K and 0 kJ mol-1 on both surfaces between 25 and 100 K. The astrophysical implication of these results is that on timescales shorter than that estimated for the formation of a complete monolayer of water ice on a grain, aggregation of water ice will result in a non-uniform coating of water, hence leaving bare grain surface exposed. Other molecules can thus be formed or adsorbed on this bare surface
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