Long-range spatial correlations in a simple diffusion model

A simple anisotropic diffusion model, according to semiphenomenological arguments, exhibits long-ranged spatial correlations in uniform stationary states.     

On the definition of a diffusion constant in a nonequilibrium system

A diffusion constant for electrons in a current-carrying semiconductor can be unambiguously defined in nearly uniform systems. For frequency-dependent density gradients it is $$D_{\alpha \beta } (\omega ) \equiv \int\limits_0^\infty {dt e^{i\omega t} \overline {\Delta \upsilon _\alpha (t)\Delta \upsilon _\beta (0),} } $$ where \(\overline {\Delta \upsilon _\alpha (t)\Delta \upsilon _\beta (0)} \) is the velocity correlation function with respect to the steady state in a bias field. This result has been elucidated in the relaxation approximation by different approaches to the diffusion problem. Essential for its derivation is a statistical independence assumption of space and velocities, and in order to get a classical diffusion law of Fick's type certain velocities have to be distributed according to the steady state in a bias field. Diffusion constant and noise temperature are discussed for a few band structures in the relaxation approximation.     

Remarks on nonequilibrium correlations in a simple dynamic model

We study a deterministic conservative dynamic model inspired by the Kac ring model. We show that some initial probability distributions with long range correlation, which go to equilibrium asymptotically, can be transformed, through a transition to new dissipative and stochastic dynamics, into states with damped correlations.     

Spatial correlations in nonequilibrium systems: The effect of diffusion

We show how the ideas of the fluctuation-dissipation theory can be used to calculate spatial correlations in interacting systems away from equilibrium. The only spatially dependent dissipative process considered is diffusion, and spatial correlations are generated by the nonlocal spatial dependence of the chemical potential. The results are the lowest order in a hierarchy of generalized hydrodynamic type calculations applicable to nonequilibrium systems. We derive equations for the density correlation functions at stationary state supported by diffusive fluxes and show that they have the local equilibrium form. The static correlation function is obtained from the fluctuation-dissipation theorem, which we show to be equivalent to the Ornstein-Zernike integral equation. At equilibrium we demonstrate that the dynamical structure factor obtained by these methods includes the expected wave-vector dependent diffusion constant. Finally we derive a necessary and sufficient condition for local equilibrium to obtain at a stationary state and show by explicit calculation that chemical processes can give rise to significant nonequilibrium correlations.     

On the positivity of correlations in nonequilibrium spin systems

We consider Ising spin systems, equivalently lattice gases evolving under discrete- or continuous-time Markov processes, i.e., stochastic cellular automata or interacting particle systems. We show that for certain spin-flip probabilities or rates and suitable initial states the expectation values of products of spin variables taken at equal or different times are nonnegative; they satisfy the same inequalities as the equal-time correlations of ferromagnetic systems in equilibrium (first Griffiths inequality). Extensions of FKG inequalities to time-displaced correlations are also discussed.     

On the fluctuation-dissipation theorem in a disordered nonequilibrium system

We present a modified perturbational calculation for a model with quenched disorder in nonequilibrium. A systematic way is proposed to obtain corrections to the usual fluctuation-dissipation theorem.     

Long-range correlations in the nonequilibrium quantum relaxation of a spin chain

We consider the nonstationary quantum relaxation of the Ising spin chain in a transverse field of strength h. Starting from a homogeneously magnetized initial state the system approaches a stationary state by a process possessing quasi-long-range correlations in time and space, independent of the value of h. In particular, the system exhibits aging (or lack of time-translational invariance on intermediate time scales) although no indications of coarsening are present.     

Temporal and spatial correlations in a viscoelastic model of heterogeneous faults

We study the temporal and spatial correlations in a one-dimensional model of a heterogeneous fault zone, in the presence of viscoelastic effects. As a function of dynamical weakening and of dissipation, the system exhibits three different “phases": one in which there are no time correlations between the events, a second, in which there are “Omori's law” type temporal correlations, and a third, runaway phase with quasiperiodic system size events. Received 30 May 2000 and Received in final form 7 September 2000     

Pattern formation in a fractional reaction–diffusion system

We investigate pattern formation in a fractional reaction–diffusion system. By the method of computer simulation of the model of excitable media with cubic nonlinearity we are able to show structure formation in the system with time and space fractional derivatives. We further compare the patterns obtained by computer simulation with those obtained by simulation of the similar system without fractional derivatives. As a result, we are able to show that nonlinearity plays the main role in structure formation and fractional derivative terms change the transient dynamics. So, when the order of time derivative increases and approaches the value of 1.5, the special structure formation switches to homogeneous oscillations. In the case of space fractional derivatives, the decrease of the order of these derivatives leads to more contrast dissipative structures. The variational principle is used to find the approximate solution of such fractional reaction–diffusion model. In addition, we provide a detailed analysis of the characteristic dissipative structures in the system under consideration.     

Spatial correlations in bounded nonequilibrium fluid systems

We study the influence of boundaries on the equal-time thermal correlations in a three-dimensional fluid maintained under a constant temperature gradient. Within the confines of the model for an idealized fluid bounded by two infinite, parallel walls, we show that it is crucial to retain the unbounded spatial components in the problem so that the solutions approach meaningful results as we move the walls infinitely far apart. In addition, we consider a composite system by including the dynamics of the walls, and we investigate the conditions for the relevant physical parameters under which the details of wall dynamics may be neglected by employing the simple boundary condition T=0.     

Evolution of large-scale correlations in a strongly nonequilibrium Bose condensation process

The evolution of off-diagonal correlation functions (for the example of a single-particle density matrix) in the process of Bose condensation of an initially nonequilibrium interacting gas is discussed. Special attention is given to the character of the decay of the density matrix at distances much greater than the size of the quasicondensate region. Specifically, it is shown that the exponential decay of the density matrix necessarily presupposes the presence of a chaotic vortex structure — a tangle of vortex lines — in the system. When topological order is established but there is no off-diagonal long-range order, the density matrix decays with distance according to a power law. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 495–501 (10 April 1998)     

Pattern formation in reaction diffusion systems: a Galerkin model

Reaction diffusion systems are extremely useful for studying pattern formation in biological systems. We carry out a Lorenz like few mode truncation of a reaction diffusion system and show that it not only gives the same qualitative behaviour as the more complicated systems but also indicates of the existence of a Hopf-bifurcation in the turing region. Received 10 May 2000 and Received in final form 14 March 2001     

A stochastic approach to nonequilibrium thermodynamics of chemical reaction system

Nonequilibrium thermodynamics is formulated by combining the nonlinear Fokker-Planck equation with the so-called Gibbs entropy postulate. The entropy production thus derived consists of two parts: one is of the same form as the usual entropy production and the other is the fluctuating part attendant on it. The evolution criterion can easily be verified in the stochastic framework. For illustration the system governed by the linear Fokker-Planck equation is in detail discussed.     

Velocity correlations and the structure of nonequilibrium hard-core fluids

A model for the pair-distribution function of nonequilibrium hard-core fluids is proposed based on a model for the effect of velocity correlations on the structure. Good agreement is found with molecular dynamics simulations of granular fluids and of sheared elastic hard spheres. It is argued that the incorporation of velocity correlations are crucial to correctly modeling atomic scale structure in nonequilibrium fluids.     

On the absence of diffusion in a semiinfinite one-dimensional system

For obvious reasons, the self-diffusion coefficient in bounded many-body systems must be strictly zero, provided that it is defined as the limit of [R(t)–R(0)]²/(2td) whent grows indefinitely [d is the dimensionality,R() is the position of a given particle at time]. Thus, the time integral of the velocity correlation function is strictly zero. A system of hard points on a half-infinite line with a reflective wall at the origin does exhibit this property of absence of diffusion, since each particle has an average position. We study in detail the difference between the velocity correlation functions of the infinite and of the half-infinite systems.     

Structural correlations and the “structural diffusion” model for liquids

The local order in liquids is characterized by a set of spatially varying parameters defining a certain local lattice structure at every point in the liquid. The correlations between these parameters at different spatial positions are obtained, in the spirit of the “structural diffusion” model of Prins, from a Fokker-Planck-like equation concerning the spatial change of the probability distribution of the local structure parameters. From the structural correlations qualitatively correct molecular pair distributions are obtained. They depend on chosen lattice constants and “structural” diffusion constants. Possible systematic improvements and refinements of this model are suggested.     

On correlations between charged and neutral particles in a cluster model

Correlations between charged and neutral pions at high energies are investigated in the framework of a cluster model in which neutral clusters have a decay distribution determined by isospin conservation and statistical independence. For 〈n₀〉_n? an asymptotic expansion around the mean 〈n_?〉 leading asymptotically to a quadratic form in n_? is derived and compared with data.