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-$T_c$ 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 $l^*\sim (\phi_c-\phi)^{-1/2}$ where $\phi_c$ 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-$m$), for which the interaction between particles is described by the potential $v(r)=\epsilon\exp[-(r/\sigma)^m]$. Interactions with $m > 2$ 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-$m$ 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