125 research outputs found
Density scaling in viscous liquids: From relaxation times to four-point susceptibilities
We present numerical calculations of a four-point dynamic susceptibility,
chi_4(t), for the Kob-Andersen Lennard-Jones mixture as a function of
temperature T and density rho. Over a relevant range of T and rho, the full
t-dependence of chi_4(t) and thus the maximum in chi_4(t), which is
proportional to the dynamic correlation volume, are invariant for state points
for which the scaling variable rho^gamma/T is constant. The value of the
material constant gamma is the same as that which superposes the relaxation
time, tau, of the system versus rho^gamma/T. Thus, the dynamic correlation
volume is directly related to tau for any thermodynamic condition in the regime
where density scaling holds. Finally, we examine the conditions under which the
density scaling properties are related to the existence of strong correlations
between pressure and energy fluctuations.Comment: 5 pages, 4 figures, updated reference
Understanding fragility in supercooled Lennard-Jones mixtures. I. Locally preferred structures
We reveal the existence of systematic variations of isobaric fragility in
different supercooled Lennard-Jones binary mixtures by performing molecular
dynamics simulations. The connection between fragility and local structures in
the bulk is analyzed by means of a Voronoi construction. We find that clusters
of particles belonging to locally preferred structures form slow, long-lived
domains, whose spatial extension increases by decreasing temperature. As a
general rule, a more rapid growth, upon supercooling, of such domains is
associated to a more pronounced super-Arrhenius behavior, hence to a larger
fragility.Comment: 14 pages, 14 figures, minor revisions, one figure adde
Understanding fragility in supercooled Lennard-Jones mixtures. II. Potential energy surface
We numerically investigated the connection between isobaric fragility and the
properties of high-order stationary points of the potential energy surface in
different supercooled Lennard-Jones mixtures. The increase of effective
activation energies upon supercooling appears to be driven by the increase of
average potential energy barriers measured by the energy dependence of the
fraction of unstable modes. Such an increase is sharper, the more fragile is
the mixture. Correlations between fragility and other properties of high-order
stationary points, including the vibrational density of states and the
localization features of unstable modes, are also discussed.Comment: 13 pages, 13 figures, minor revisions, one figure adde
Histamine H3 Receptors Expressed in Ventral Horns Modulate Spinal Motor Output
Motoneuron activity is modulated by histamine receptors. While H1 and H2 receptors have been widely explored, H3 histamine receptors (H3Rs) have not been sufficiently characterized. This paper targets the effects of the selective activation of H3Rs and their expression on the membranes of large ventral horn cells. The application of selective pharmacological agents to spinal cords isolated from neonatal rats was used to identify the presence of functional H3Rs on the membrane of physiologically identified lumbar motoneurons. Intra and extracellular recordings revealed that H3R agonist, \u3b1-methylhistamine, depolarized both single motoneurons and ventral roots, even in the presence of tetrodotoxin, an effect prevented by H3R antagonist, thioperamide. Finally, immunohistochemistry located the expression of H3Rs on a subpopulation of large cells in lamina IX. This study identifies H3Rs as a new exploitable pharmacological target against motor disturbances
Dynamics and energy landscape in a tetrahedral network glass-former: Direct comparison with models of fragile liquids
We report Molecular Dynamics simulations for a new model of tetrahedral
network glass-former, based on short-range, spherical potentials. Despite the
simplicity of the forcefield employed, our model reproduces some essential
physical properties of silica, an archetypal network-forming material.
Structural and dynamical properties, including dynamic heterogeneities and the
nature of local rearrangements, are investigated in detail and a direct
comparison with models of close-packed, fragile glass-formers is performed. The
outcome of this comparison is rationalized in terms of the properties of the
Potential Energy Surface, focusing on the unstable modes of the stationary
points. Our results indicate that the weak degree of dynamic heterogeneity
observed in network glass-formers may be attributed to an excess of localized
unstable modes, associated to elementary dynamical events such as bond breaking
and reformation. On the contrary, the more fragile Lennard-Jones mixtures are
characterized by a larger fraction of extended unstable modes, which lead to a
more cooperative and heterogeneous dynamics.Comment: 26 pages, 18 figures, added links to animations, corrected typos in
sec.
Are there localized saddles behind the heterogeneous dynamics of supercooled liquids?
We numerically study the interplay between heterogeneous dynamics and
properties of negatively curved regions of the potential energy surface in a
model glassy system. We find that the unstable modes of saddles and
quasi-saddles undergo a localization transition close to the Mode-Coupling
critical temperature. We also find evidence of a positive spatial correlation
between clusters of particles having large displacements in the unstable modes
and dynamical heterogeneities.Comment: 7 pages, 3 figures, submitted to Europhys. Let
Strong enhancement of d-wave superconducting state in the three-band Hubbard model coupled to an apical oxygen phonon
We study the hole binding energy and pairing correlations in the three-band
Hubbard model coupled to an apical oxygen phonon, by exact diagonalization and
constrained-path Monte Carlo simulations. In the physically relevant
charge-transfer regime, we find that the hole binding energy is strongly
enhanced by the electron-phonon interaction, which is due to a novel
potential-energy-driven pairing mechanism involving reduction of both
electronic potential energy and phonon related energy. The enhancement of hole
binding energy, in combination with a phonon-induced increase of quasiparticle
weight, leads to a dramatic enhancement of the long-range part of d-wave
pairing correlations. Our results indicate that the apical oxygen phonon plays
a significant role in the superconductivity of high- cuprates.Comment: 5 pages, 5 figure
Heterogeneous Dynamics, Marginal Stability and Soft Modes in Hard Sphere Glasses
In a recent publication we established an analogy between the free energy of
a hard sphere system and the energy of an elastic network [1]. This result
enables one to study the free energy landscape of hard spheres, in particular
to define normal modes. In this Letter we use these tools to analyze the
activated transitions between meta-bassins, both in the aging regime deep in
the glass phase and near the glass transition. We observe numerically that
structural relaxation occurs mostly along a very small number of
nearly-unstable extended modes. This number decays for denser packing and is
significantly lowered as the system undergoes the glass transition. This
observation supports that structural relaxation and marginal modes share common
properties. In particular theoretical results [2, 3] show that these modes
extend at least on some length scale where
corresponds to the maximum packing fraction, i.e. the jamming
transition. This prediction is consistent with very recent numerical
observations of sheared systems near the jamming threshold [4], where a similar
exponent is found, and with the commonly observed growth of the rearranging
regions with compression near the glass transition.Comment: 6 pages, improved versio
Dynamics in binary cluster crystals
As a result of the application of coarse-graining procedures to describe
complex fluids, the study of systems consisting of particles interacting
through bounded, repulsive pair potentials has become of increasing interest in
the last years. A well known example is the so-called Generalized Exponential
Model (GEM-), for which the interaction between particles is described by
the potential . Interactions with
lead to the formation of a novel phase of soft matter consisting of cluster
crystals. Recent studies on the phase behavior of binary mixtures of GEM-
particles have provided evidence for the formation of novel kinds of alloys,
depending on the cross interactions between the two species. This work aims to
study the dynamic behavior of such binary mixtures by means of extensive
molecular dynamics simulations, and in particular to investigate the effect of
the addition of non-clustering particles on the dynamic scenario of
one-component cluster crystals. Analogies and differences with the
one-component case are revealed and discussed by analyzing self- and collective
dynamic correlators.Comment: 17 pages, 8 figures, submitted to JSTA
Disentangling the electronic and phononic glue in a high-Tc superconductor
Unveiling the nature of the bosonic excitations that mediate the formation of
Cooper pairs is a key issue for understanding unconventional superconductivity.
A fundamen- tal step toward this goal would be to identify the relative weight
of the electronic and phononic contributions to the overall frequency (\Omega)
dependent bosonic function, \Pi(\Omega). We perform optical spectroscopy on
Bi2212 crystals with simultaneous time- and frequency-resolution; this
technique allows us to disentangle the electronic and phononic contributions by
their different temporal evolution. The strength of the interaction
({\lambda}~1.1) with the electronic excitations and their spectral distribution
fully account for the high critical temperature of the superconducting phase
transition.Comment: 9 pages, 4 figure
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