Power-law Spatial Correlations Research Articles

Vibrational density of states of amorphous solids with long-ranged power-law-correlated disorder in elasticity

.A theory of vibrational excitations based on power-law spatial correlations in the elastic constants (or equivalently in the internal stress) is derived, in order to determine the vibrational density of states D(omega) of disordered solids. The results provide the first prediction of a boson peak in amorphous materials where spatial correlations in the internal stresses (or elastic constants) are of power-law form, as is often the case in experimental systems, leading to a logarithmic enhancement of (Rayleigh) phonon attenuation. A logarithmic correction of the form sim -omega^{2}lnomega is predicted to occur in the plot of the reduced excess DOS for frequencies around the boson peak in 3D. Moreover, the theory provides scaling laws of the density of states in the low-frequency region, including a simomega^{4} regime in 3D, and provides information about how the boson peak intensity depends on the strength of power-law decay of fluctuations in elastic constants or internal stress. Analytical expressions are also derived for the dynamic structure factor for longitudinal excitations, which include a logarithmic correction factor, and numerical calculations are presented supporting the assumptions used in the theory.Graphical abstract

Non-Gaussian polymers described by alpha-stable chain statistics: Model, effective interactions in binary mixtures, and application to on-surface separation.

The Gaussian chain model is the classical description of a polymeric chain, which provides analytical results regarding end-to-end distance, the distribution of segments around the mass center of a chain, coarse-grained interactions between two chains and effective interactions in binary mixtures. This hierarchy of results can be calculated thanks to the α stability of the Gaussian distribution. In this paper we show that it is possible to generalize the model of Gaussian chain to the entire class of α-stable distributions, obtaining the analogous hierarchy of results expressed by the analytical closed-form formulas in the Fourier space. This allows us to establish the α-stable chain model. We begin with reviewing the applications of Levy flights in the context of polymer sciences, which include: chains described by the heavy-tailed distributions of persistence length; polymers adsorbed to the surface; and the chains driven by a noise with power-law spatial correlations. Further, we derive the distribution of segments around the mass center of the α-stable chain and construct the coarse-grained interaction potential between two chains. These results are employed to discuss the model of binary mixture consisting of the α-stable chains. In what follows, we establish the spinodal decomposition condition generalized to the mixtures of the α-stable polymers. This condition is further applied to compare the on-surface phase separation of adsorbed polymers (which are known to be described with heavy-tailed statistics) with the phase separation condition in the bulk. Finally, we predict the four different scenarios of simultaneous mixing and demixing in the two- and three-dimensional systems.

Aging of orientation fluctuations in stripe phases.

Stripe patterns, observed in a large variety of physical systems, often exhibit a slow nonequilibrium dynamics because ordering is impeded by the presence of topological defects. Using computer simulations based on a well-established model for stripe formation, we show that a slow dynamics and aging occur also in stripe patterns free of topological defects. For a wide range of noise strengths, the two-time orientation correlation function follows a scaling form that is typical for systems exhibiting a growing length scale. In our case, the underlying mechanism is the coarsening of orientation fluctuations, ultimately leading to power-law spatial correlations perpendicular to the stripes. Our results show that even for the smallest amount of noise, stripe phases without topological defects do not reach equilibrium. This constitutes an important aspect of the dynamics of modulated phases.

Spectral analysis of finite-time correlation matrices near equilibrium phase transitions

We study spectral densities for systems on lattices, which, at a phase transition display, power-law spatial correlations. Constructing the spatial correlation matrix we prove that its eigenvalue density shows a power law that can be derived from the spatial correlations. In practice time series are short in the sense that they are either not stationary over long time intervals or not available over long time intervals. Also we usually do not have time series for all variables available. We shall make numerical simulations on a two-dimensional Ising model with the usual Metropolis algorithm as time evolution. Using all spins on a grid with periodic boundary conditions we find a power law, that is, for large grids, compatible with the analytic result. We still find a power law even if we choose a fairly small subset of grid points at random. The exponents of the power laws will be smaller under such circumstances. For very short time series leading to singular correlation matrices we use a recently developed technique to lift the degeneracy at zero in the spectrum and find a significant signature of critical behavior even in this case as compared to high temperature results which tend to those of random matrix models.

Flocking at a distance in active granular matter.

The self-organized motion of vast numbers of creatures in a single direction is a spectacular example of emergent order. Here, we recreate this phenomenon using actuated nonliving components. We report here that millimetre-sized tapered rods, rendered motile by contact with an underlying vibrated surface and interacting through a medium of spherical beads, undergo a phase transition to a state of spontaneous alignment of velocities and orientations above a threshold bead area fraction. Guided by a detailed simulation model, we construct an analytical theory of this flocking transition, with two ingredients: a moving rod drags beads; neighbouring rods reorient in the resulting flow like a weathercock in the wind. Theory and experiment agree on the structure of our phase diagram in the plane of rod and bead concentrations and power-law spatial correlations near the phase boundary. Our discovery suggests possible new mechanisms for the collective transport of particulate or cellular matter.

Fractality of eroded coastlines of correlated landscapes

Using numerical simulations of a simple sea-coast mechanical erosion model, we investigate the effect of spatial long-range correlations in the lithology of coastal landscapes on the fractal behavior of the corresponding coastlines. In the model, the resistance of a coast section to erosion depends on the local lithology configuration as well as on the number of neighboring sea sides. For weak sea forces, the sea is trapped by the coastline and the eroding process stops after some time. For strong sea forces erosion is perpetual. The transition between these two regimes takes place at a critical sea force, characterized by a fractal coastline front. For uncorrelated landscapes, we obtain, at the critical value, a fractal dimension D=1.33, which is consistent with the dimension of the accessible external perimeter of the spanning cluster in two-dimensional percolation. For sea forces above the critical value, our results indicate that the coastline is self-affine and belongs to the Kardar-Parisi-Zhang universality class. In the case of landscapes generated with power-law spatial long-range correlations, the coastline fractal dimension changes continuously with the Hurst exponent H, decreasing from D=1.34 to 1.04, for H=0 and 1, respectively. This nonuniversal behavior is compatible with the multitude of fractal dimensions found for real coastlines.

Heterogeneous long-range correlated deformation of semiflexible random fiber networks

The deformation of dense random fiber networks is important in a variety of applications including biological and nonliving systems. In this paper it is shown that semiflexible fiber networks exhibit long-range power-law spatial correlations of the density and elastic properties. Hence, the stress and strain fields measured over finite patches of the network are characterized by similar spatial correlations. The scaling is observed over a range of scales bounded by a lower limit proportional to the segment length and an upper limit on the order of the fiber length. If the fiber bending stiffness is reduced below a threshold, correlations are lost. The issue of solving boundary value problems defined on large domains of random fiber networks is also addressed. Since the direct simulation of such systems is impractical, the network is mapped into an equivalent continuum with long-range correlated elastic moduli. A technique based on the stochastic finite element method is used to solve the resulting stochastic continuum problem. The method provides the moments of the distribution function of the solution (e.g., of the displacement field). It performs a large dimensionality reduction which is based on the scaling properties of the underlying elasticity of the material. Two examples are discussed in closure.

Power-law spatial correlations in arrays of locally coupled lasers.

We investigate correlations of the intensity fluctuations of two-dimensional arrays of nonidentical, locally coupled lasers, numerically and experimentally. We find evidence of a power-law dependence of spatial correlations as a function of laser pair distance (or coupling strength) near the phase-locking threshold.

Persistence in higher dimensions: A finite size scaling study

We show that the persistence probability P(t,L), in a coarsening system of linear size L at a time t, has the finite-size scaling form P(t,L) approximately L(-zstraight theta)f(t/L(z)), where straight theta is the persistence exponent and z is the coarsening exponent. The scaling function f(x) approximately x(-straight theta) for x<<1 and is constant for large x. The scaling form implies a fractal distribution of persistent sites with power-law spatial correlations. We study the scaling numerically for the Glauber-Ising model at dimension d=1 to 4 and extend the study to the diffusion problem. Our finite-size scaling ansatz is satisfied in all these cases providing a good estimate of the exponent straight theta.

Anomalous spatio-temporal chaos in a two-dimensional system of nonlocally coupled oscillators.

A two-dimensional system of nonlocally coupled complex Ginzburg-Landau oscillators is investigated numerically for the first time. As previously shown for the one-dimensional case, this two-dimensional system exhibits anomalous spatio-temporal chaos characterized by power-law spatial correlations. In this chaotic regime, the amplitude difference between neighboring elements displays temporal noisy on-off intermittency. The system is also spatially intermittent in this regime, as revealed by multiscaling analysis: The amplitude field is multiaffine and the difference field is multifractal. Correspondingly, the probability distribution function of the measure defined for each field is strongly non-Gaussian, exhibiting scale-dependent deviations in the tail due to intermittency. (c) 1999 American Institute of Physics.

Scale Invariant Correlations in a Driven Dissipative Gas

We investigate the steady state properties of a randomly driven dissipative gas. As the number of particles is increased power-law spatial correlations in the density and velocity are seen to develop, and the exponents are found to be independent of the model details. We show how the critical properties of this system can be understood in terms of a one particle model which is amenable to exact analysis.

Power-law spatial correlations and the onset of individual motions in self-oscillatory media with non-local coupling

It is demonstrated numerically that power-law spatial correlations appear generically in self-oscillatory media with non-local coupling. This occurs in length scales smaller than the range of coupling when the turbulent fluctuations generated through the Benjamin-Feir instability spread deep into this regime. The associated exponent varies continuously with the coupling strength. However, the latter has a lower critical value below which the pattern can no longer sustain its spatial continuity giving way to individual motions of the oscillators. Our numerical analyses are carried out on a one-dimensional oscillator lattice where two types of model oscillators are examined. They are the complex Ginzburg-Landau type oscillators and the Brusselator. A non-trivial effect of breaking the special symmetry of the former model is also discussed.

Origin of power-law spatial correlations in distributed oscillators and maps with nonlocal coupling.

It is argued that power-law spatial correlation at short distances is a generic property of spatiotemporal chaos exhibited by active dynamical elements coupled nonlocally. While this fact was suggested earlier from some numerical analysis of coupled limit cycles, further evidence is provided here from an analysis for chaotic R\ossler oscillators and logistic maps. A theory is presented to explain why such short-range nonanalyticity of correlation with parameter-dependent exponent is so universal when the coupling is nonlocal.