179 research outputs found
Vibrational origin of the fast relaxation processes in molecular glass-formers
We study the interaction of the relaxation processes with the density
fluctuations by molecular dynamics simulation of a flexible molecule model for
o-terphenyl (oTP) in the liquid and supercooled phases. We find evidence,
besides the structural relaxation, of a secondary vibrational relaxation whose
characteristic time, few ps, is slightly temperature dependent. This i)
confirms the result by Monaco et al. [Phys. Rev, E 62, 7595 (2000)] of the
vibrational nature of the fast relaxation observed in Brillouin Light
Scattering (BLS) experiments in oTP; and ii) poses a caveat on the
interpretation of the BLS spectra of molecular systems in terms of a purely
center of mass dynamics.Comment: RevTeX, 5 pages, 4 eps figure
Equilibrium and out of equilibrium thermodynamics in supercooled liquids and glasses
We review the inherent structure thermodynamical formalism and the
formulation of an equation of state for liquids in equilibrium based on the
(volume) derivatives of the statistical properties of the potential energy
surface. We also show that, under the hypothesis that during aging the system
explores states associated to equilibrium configurations, it is possible to
generalize the proposed equation of state to out-of-equilibrium conditions. The
proposed formulation is based on the introduction of one additional parameter
which, in the chosen thermodynamic formalism, can be chosen as the local minima
where the slowly relaxing out-of-equilibrium liquid is trapped.Comment: 7 pages, 4 eps figure
Kovacs effects in an aging molecular liquid
We study by means of molecular dynamics simulations the aging behavior of a
molecular model of ortho-terphenyl. We find evidence of a a non-monotonic
evolution of the volume during an isothermal-isobaric equilibration process, a
phenomenon known in polymeric systems as Kovacs effect. We characterize this
phenomenology in terms of landscape properties, providing evidence that, far
from equilibrium, the system explores region of the potential energy landscape
distinct from the one explored in thermal equilibrium. We discuss the relevance
of our findings for the present understanding of the thermodynamics of the
glass state.Comment: RevTeX 4, 4 pages, 5 eps figure
On the Yang-Lee and Langer singularities in the O(n) loop model
We use the method of `coupling to 2d QG' to study the analytic properties of
the universal specific free energy of the O(n) loop model in complex magnetic
field. We compute the specific free energy on a dynamical lattice using the
correspondence with a matrix model. The free energy has a pair of Yang-Lee
edges on the high-temperature sheet and a Langer type branch cut on the
low-temperature sheet. Our result confirms a conjecture by A. and Al.
Zamolodchikov about the decay rate of the metastable vacuum in presence of
Liouville gravity and gives strong evidence about the existence of a weakly
metastable state and a Langer branch cut in the O(n) loop model on a flat
lattice. Our results are compatible with the Fonseca-Zamolodchikov conjecture
that the Yang-Lee edge appears as the nearest singularity under the Langer cut.Comment: 38 pages, 16 figure
Sub-sampling a large physical soil archive for additional analyses to support spatial mapping; a pre-registered experiment in the Southern Nations, Nationalities, and Peoples Region (SNNPR) of Ethiopia
The value of physical archives of soil material from field sampling activities has been widely recognized. If we want to use archive material for new destructive analyses to support a task, such as spatial mapping, then an efficient sub-sampling strategy is needed, both to manage analytical costs and to conserve the archive material. In this paper we present an approach to this problem when the objective is spatial mapping by ordinary kriging. Our objective was to subsample the physical archive from the Ethiopia Soil Information System (EthioSIS) survey of the Southern Nations, Nationalities and Peoples Region (SNNPR) for spatial mapping of two variables, concentrations of particular fractions of selenium and iodine in the soil, which had not been measured there. We used data from cognate parts of surrounding regions of Ethiopia to estimate variograms of these properties, and then computed prediction error variances for maps in SNNPR based on proposed subsets of the archive of different size, selected to optimize a spatial coverage criterion (with some close sample pairs included). On this basis a subsample was selected. This is a preregistered experiment in that we have proposed criteria for evaluating the success of our approach, and are publishing that in advance of receiving analytical data on the subsampled material from the laboratories where they are being processed. A subsequent short report will publish the outcome. The use of preregistered trials is widely recommended and used in areas of science including public health, and we believe that it is a sound strategy to promote reproducible research in soil science
A Potential Energy Landscape Study of the Amorphous-Amorphous Transformation in HO
We study the potential energy landscape explored during a
compression-decompression cycle for the SPC/E (extended simple point charge)
model of water. During the cycle, the system changes from low density amorphous
ice (LDA) to high density amorphous ice (HDA). After the cycle, the system does
not return to the same region of the landscape, supporting the interesting
possibility that more than one significantly different configuration
corresponds to LDA. We find that the regions of the landscape explored during
this transition have properties remarkably different from those explored in
thermal equilibrium in the liquid phase
Finite-Size Effects in a Supercooled Liquid
We study the influence of the system size on various static and dynamic
properties of a supercooled binary Lennard-Jones liquid via computer
simulations. In this way, we demonstrate that the treatment of systems as small
as N=65 particles yields relevant results for the understanding of bulk
properties. Especially, we find that a system of N=130 particles behaves
basically as two non-interacting systems of half the size.Comment: Proceedings of the III Workshop on Non Equilibrium Phenomena in
Supercooled Fluids, Glasses and Amorphous Materials, Sep 2002, Pis
Molecular dynamics simulation of the fragile glass former ortho-terphenyl: a flexible molecule model
We present a realistic model of the fragile glass former orthoterphenyl and
the results of extensive molecular dynamics simulations in which we
investigated its basic static and dynamic properties. In this model the
internal molecular interactions between the three rigid phenyl rings are
described by a set of force constants, including harmonic and anharmonic terms;
the interactions among different molecules are described by Lennard-Jones
site-site potentials. Self-diffusion properties are discussed in detail
together with the temperature and momentum dependencies of the
self-intermediate scattering function. The simulation data are compared with
existing experimental results and with the main predictions of the Mode
Coupling Theory.Comment: 20 pages and 28 postscript figure
Large Scale Cross-Correlations in Internet Traffic
The Internet is a complex network of interconnected routers and the existence
of collective behavior such as congestion suggests that the correlations
between different connections play a crucial role. It is thus critical to
measure and quantify these correlations. We use methods of random matrix theory
(RMT) to analyze the cross-correlation matrix C of information flow changes of
650 connections between 26 routers of the French scientific network `Renater'.
We find that C has the universal properties of the Gaussian orthogonal ensemble
of random matrices: The distribution of eigenvalues--up to a rescaling which
exhibits a typical correlation time of the order 10 minutes--and the spacing
distribution follow the predictions of RMT. There are some deviations for large
eigenvalues which contain network-specific information and which identify
genuine correlations between connections. The study of the most correlated
connections reveals the existence of `active centers' which are exchanging
information with a large number of routers thereby inducing correlations
between the corresponding connections. These strong correlations could be a
reason for the observed self-similarity in the WWW traffic.Comment: 7 pages, 6 figures, final versio
Interplay Between Time-Temperature-Transformation and the Liquid-Liquid Phase Transition in Water
We study the TIP5P water model proposed by Mahoney and Jorgensen, which is
closer to real water than previously-proposed classical pairwise additive
potentials. We simulate the model in a wide range of deeply supercooled states
and find (i) the existence of a non-monotonic ``nose-shaped'' temperature of
maximum density line and a non-reentrant spinodal, (ii) the presence of a low
temperature phase transition, (iii) the free evolution of bulk water to ice,
and (iv) the time-temperature-transformation curves at different densities.Comment: RevTeX4, 4 pages, 4 eps figure
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