Ref HAL: hal-01292660_v1
Ref Arxiv: 1603.06648v1
DOI: 10.1088/1475-7516/2016/05/048
WoS: 000378041500049
Ref. & Cit.: NASA ADS
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10 Citations
Résumé:

We consider the possibility to produce a bouncing universe in the framework of scalar-tensor gravity when the scalar field has a nonconformal coupling to the Ricci scalar. We prove that bouncing universes regular in the future with essentially the same dynamics as for the conformal coupling case do exist when the coupling deviates slightly from it. This is found numerically for more substantial deviations as well. In some cases however new features are found like the ability of the system to leave the effective phantom regime.

Ref HAL: hal-01289903_v1
DOI: 10.1103/PhysRevE.93.012118
WoS: WOS:000367901000011
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4 Citations
Résumé:

Ising spin glasses with bimodal and Gaussian near-neighbor interaction distributions are studied through numerical simulations. The non-self-averaging (normalized intersample variance) parameter $U_{22}(T,L)$ for the spin glass susceptibility [and for higher moments $U_{nn}(T,L)$] is reported for dimensions 2,3,4,5, and 7. In each dimension $d$ the non-self-averaging parameters in the paramagnetic regime vary with the sample size $L$ and the correlation length $ξ(T,L)$ as $U_{nn}(β,L)=[K_dξ(T,L)/L]^d$ and so follow a renormalization group law due to Aharony and Harris [Phys. Rev. Lett. 77, 3700 (1996)]. Empirically, it is found that the $K_d$ values are independent of $d$ to within the statistics. The maximum values $[U_{nn}(T,L)]_{max}$ are almost independent of $L$ in each dimension, and remarkably the estimated thermodynamic limit critical $[U_{nn}(T,L)]_{max}$ peak values are also practically dimension-independent to within the statistics and so are “hyperuniversal.” These results show that the form of the spin-spin correlation function distribution at criticality in the large $L$ limit is independent of dimension within the ISG family. Inspection of published non-self-averaging data for three-dimensional Heisenberg and $XY$ spin glasses the light of the Ising spin glass non-self-averaging results show behavior which appears to be compatible with that expected on a chiral-driven ordering interpretation but incompatible with a spin-driven ordering scenario.

Ref HAL: hal-01289849_v1
DOI: 10.1103/PhysRevE.93.022119
WoS: WOS:000370029400002
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8 Citations
Résumé:

An analysis is given of numerical simulation data to size $L = 128$ on the archetype square lattice Ising spin glasses (ISGs) with bimodal $(±J )$ and Gaussian interaction distributions. It is well established that the ordering temperature of both models is zero. The Gaussian model has a nondegenerate ground state and thus a criticalexponent $η ≡ 0$, and a continuous distribution of energy levels. For the bimodal model, above a size-dependent crossover temperature $T∗(L)$ there is a regime of effectively continuous energy levels; below $T∗(L)$ there is a distinct regime dominated by the highly degenerate ground state plus an energy gap to the excited states.$T∗(L)$ tends to zero at very large $L$, leaving only the effectively continuous regime in the thermodynamic limit. The simulation data on both models are analyzed with the conventional scaling variable $t = T$ and witha scaling variable $\tau_b = T^2/(1 + T^2)$ suitable for zero-temperature transition ISGs, together with appropriate scaling expressions. The data for the temperature dependence of the reduced susceptibility $χ(\tau_b,L)$ and second moment correlation length $ξ (\tau_b,L)$ in the thermodynamic limit regime are extrapolated to the $\tau_b = 0$ critical limit.The Gaussian critical exponent estimates from the simulations, $η = 0$ and $ν = 3.55(5)$, are in full agreement with the well-established values in the literature. The bimodal critical exponents, estimated from the thermodynamic limit regime analyses using the same extrapolation protocols as for the Gaussian model, are $η = 0.20(2)$ and$ν = 4.8(3)$, distinctly different from the Gaussian critical exponents.

Ref HAL: hal-01261246_v2
Ref Arxiv: 1510.06320
DOI: 10.1063/1.4939640
WoS: 000368618400033
Ref. & Cit.: NASA ADS
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17 Citations
Résumé:

Cavity point-to-set correlations are real-space tools to detect the roughening of the free-energy landscape that accompanies the dynamical slowdown of glass-forming liquids. Measuring these correlations in model glass formers remains, however, a major computational challenge. Here, we develop a general parallel-tempering method that provides orders-of-magnitude improvement for sampling and equilibrating configurations within cavities. We apply this improved scheme to the canonical Kob-Andersen binary Lennard-Jones model for temperatures down to the mode-coupling theory crossover. Most significant improvements are noted for small cavities, which have thus far been the most difficult to study. This methodological advance also enables us to study a broader range of physical observables associated with thermodynamic fluctuations. We measure the probability distribution of overlap fluctuations in cavities, which displays a non-trivial temperature evolution. The corresponding overlap susceptibility is found to provide a robust quantitative estimate of the point-to-set length scale requiring no fitting. By resolving spatial fluctuations of the overlap in the cavity, we also obtain quantitative information about the geometry of overlap fluctuations. We can thus examine in detail how the penetration length as well as its fluctuations evolve with temperature and cavity size.

Commentaires: 12 pages, 7 figures; v2: minor revisions made, figure 1b added, Sec.V extensively revised for clarification, references added. Réf Journal: J. Chem. Phys. 144, 024501 (2016)

Ref HAL: hal-01259346_v1
PMID 26592422
DOI: 10.1039/c5sm02265g
WoS: 000369747900028
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Résumé:

We develop a theory which predicts curvature-related structural peculiarities of soft spherical nanostructures with a dodecagonal local arrangement of subunits. Spherical templates coated with a thin film of a soft quasicrystal (QC)-forming material constitute the most promising direction to realize these nanostructures. Disordered and perfect spherical nanostructures are simulated using two approaches. The first of them models a random QC-like spherical nanostructure with extended curvature-induced topological defects similar to scars in colloidal spherical crystals. The second approach is inspired by the physics of viral capsids. It deals with the most regular spherical nanostructures with a local QC-like order derived from three well-known planar dodecagonal tilings. We explain how the additional QC-like degrees of freedom assist the nanostructure stabilization and determine the point defect number and location without extended scar formation. Unusual for nanoassemblies snub cube geometry is shown to be the most energetically favorable global organization of these spherical QC nanostructures.

Résumé:

The competition between order and disorder in dense states of matter (liquids, crystals, glasses) is one of the most fascinating aspects of statistical physics. The microscopic mechanisms that allow us to transform some liquids into glass with ease, while others crystallize quickly when cooled or compressed, are subtle and still remain poorly understood. To try to understand how the collective behavior of these systems emerges from the interactions between the atoms, physicists often build on simple theoretical models, whose prototype is a system of purely repulsive hard spheres. These models, which were introduced during the second half of the twentieth century, have recently found important applications in glass transition studies, soft matter modeling, as well as in packing problems. The generalization to different spatial dimensions - from the Euclidean plane to the infinite dimensional limit - brings additional insight into which microscopic features stabilize disordered phases of matter and into the interplay between structure and dynamics. In reviewing these recent advances, I will emphasize the crucial role of numerical simulations, which help interpreting the experiments and provide stringent tests of theoretical models. I will discuss how to exploit powerful yet cheap architectures, such as graphics cards, and efficient algorithms to dramatically accelerate the sampling of configurational space.

Ref HAL: hal-01254327_v1
Ref Arxiv: 1602.00551
DOI: 10.1021/acs.macromol.5b01481
WoS: 000368322000045
Ref. & Cit.: NASA ADS
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4 Citations
Résumé:

By using the blob theory and computer simulations, we investigate the properties of a linear polymer performing a stationary rotational motion around a long impenetrable rod. In particular, in the simulations the rotation is induced by a torque applied to the end of the polymer that is tethered to the rod. Three different regimes are found, in close analogy with the case of polymers pulled by a constant force at one end. For low torques the polymer rotates maintaining its equilibrium conformation. At intermediate torques the polymer assumes a trumpet shape, being composed by blobs of increasing size. At even larger torques the polymer is partially wrapped around the rod. We derive several scaling relations between various quantities as angular velocity, elongation and torque. The analytical predictions match the simulation data well. Interestingly, we find a "thinning" regime where the torque has a very weak (logarithmic) dependence on the angular velocity. We discuss the origin of this behavior, which has no counterpart in polymers pulled by an applied force.

Commentaires: 30 pages, 8 figures, 1 TOC figure; video abstract at https://youtu.be/LwicoSkh3mI. Réf Journal: Macromolecules, 2016, 49 (1), 405-414