Non-local Quantities Research Articles

Non-local energy transport in tunneling and plasmonic structures

Various definitions of the velocity of propagation of the electromagnetic field have been adopted in experimental and theoretical studies of tunneling and plasmonic systems. Tunneling problems are often analyzed by invoking the group delay (or dwell time) velocities. On the other hand, slow light and plasmonic systems are considered by using the wave packet group velocity. This paper discusses various definitions for the velocity of the electromagnetic wave propagation and compares them in applications to the problems of slow light and superluminality in resonant and tunneling structures. Energy propagation is, in general, a nonlocal quantity and depends on the global properties of the system, rather than being simply a local quantity. The energy propagation velocity takes into account the non-local characteristics of the wave propagation and offers a natural generalization for those situations when the group velocity is ill defined or gives unphysical results. It is shown that the group delay velocity, which may be superluminal away from the resonance, becomes equal to the energy velocity at the resonant point.

Viscosity, Shear Waves, and Atomic-Level Stress-Stress Correlations

The Green-Kubo equation relates the macroscopic stress-stress correlation function to a liquid's viscosity. The concept of the atomic-level stresses allows the macroscopic stress-stress correlation function in the equation to be expressed in terms of the space-time correlations among the atomic-level stresses. Molecular dynamics studies show surprisingly long spatial extension of stress-stress correlations and also longitudinal and transverse waves propagating in liquids over ranges which could exceed the system size. The results reveal that the range of propagation of shear waves corresponds to the range of distances relevant for viscosity. Thus our results show that viscosity is a fundamentally nonlocal quantity. We also show that the periodic boundary conditions play a nontrivial role in molecular dynamics simulations, effectively masking the long-range nature of viscosity.

Modelling of localization and propagation of phase transformation in superelastic SMA by a gradient nonlocal approach

In this work, a nonlocal phenomenological behavior model is proposed in order to describe the localization and propagation of stress-induced martensite transformation in shape memory alloy (SMA) wires and thin films. It is a nonlocal extension of an existing local model that was derived from a micromechanical-inspired Gibbs free energy expression. The proposed model uses, besides the local field of the internal variable, namely the martensite volume fraction, a nonlocal counterpart. This latter acts as an additional degree of freedom, which is determined by solving an additional partial differential equation (PDE), derived so as to be equivalent to the integral definition of a nonlocal quantity. This PDE involves an internal length parameter, dictating the global scale at which the nonlocal interactions of the underlying micromechanisms are manifested during phase transformation. Moreover, to account for the unstable softening behavior, the transformation yield force parameter is considered as a gradually decreasing function of the martensite fraction. Possible material and geometric imperfections that are responsible for localization initiation are also considered in this analysis. The obtained constitutive equations are implemented in the Abaqus® finite element code in one and two dimensions. This requires the development of specific finite elements having the nonlocal volume fraction variable as an additional degree of freedom. This implementation is achieved through the UEL user’s subroutine. The effect of martensitic localization on the superelastic global behavior of SMA wire and holed thin plate, subjected to tension loading, is analyzed. Numerical results show that the developed tool correctly captures the commonly observed unstable superelastic behavior characterized by nucleation and propagation of martensitic phase. In particular, they show the influence of the internal length parameter, appearing in the nonlocal model, on the size of the localization area and the stress nucleation peak.

Very singular diffusion equations: second and fourth order problems

This paper studies singular diffusion equations whose diffusion effect is so strong that the speed of evolution becomes a nonlocal quantity. Typical examples include the total variation flow as well as crystalline flow which are formally of second order. This paper includes fourth order models which are less studied compared with second order models. A typical example of this model is an H −1 gradient flow of total variation. It turns out that such a flow is quite different from the second order total variation flow. For example, we prove by giving an explicit example that the solution may instantaneously develop a jump discontinuity for the fourth order total variation flow.

The Relationship between Flux Coefficient and Entrainment Ratio in Density Currents

Abstract The authors explore the theoretical and empirical relationship between the nonlocal quantities of the entrainment ratio E, the appropriately depth- and time-averaged flux coefficient Γ, and the bulk Froude number Fro in density currents. The main theoretical result is that E = 0.125 Γ Fro2(CU3/CL)/cosθ, where θ is the angle of the slope over which the density current flows, CL is the ratio the turbulent length scale to the depth of the density current, and CU is the ratio of the turbulent velocity scale to the mean velocity of the density current. In the case of high bulk Froude numbers Γ ∼ Fro−2 and (CU3/CL) = Cε ∼ 1, so E ∼ 0.1, consistent with observations of a constant entrainment ratio in unstratified jets and weakly stratified plumes. For bulk Froude numbers close to one, Γ is constant and has a value in the range of 0.1–0.3, which means that E ∼ Fro2, again in agreement with observations and previous experiments. For bulk Froude numbers less than one, Γ decreases rapidly with bulk Froude number, explaining the sudden decrease in entrainment ratios that has been observed in all field and experimental observations.

Fractional Poincaré inequalities for general measures

We prove a fractional version of Poincaré inequalities in the context of R n endowed with a fairly general measure. Namely we prove a control of an L 2 norm by a non-local quantity, which plays the role of the gradient in the standard Poincaré inequality. The assumption on the measure is the fact that it satisfies the classical Poincaré inequality, so that our result is an improvement of the latter inequality. Moreover we also quantify the tightness at infinity provided by the control on the fractional derivative in terms of a weight growing at infinity. The proof goes through the introduction of the generator of the Ornstein–Uhlenbeck semigroup and some careful estimates of its powers. To our knowledge this is the first proof of fractional Poincaré inequality for measures more general than Lévy measures.

Very fine near-wall structures in turbulent scalar mixing

Passive scalar dynamics in wall-bounded turbulence is studied via Direct Numerical Simulations of plane channel flow, for a friction Reynolds number Re ∗ = 160 and a Schmidt number Sc = 1. Peculiar to the present research is that the spatial resolution reaches beyond what has been employed in similar past studies. Our aim is to examine the statistics of the most dissipative events across the various layers of the channel flow, and to compare them to the homogeneous isotropic case, where the recent studies by Schumacher et al. (2005) and Watanabe and Gotoh (2007) have described a range of scalar micro-scales that require extremely high spatial resolution to be properly resolved. Resolution effects are observed on integral-scale (non-local) quantities such as the mean profiles of the scalar dissipation and its variance. By examining probability distributions, it is found that the finest resolution is essential for correctly computing small-scale statistics in the near-wall region of the channel. As expected, this high-resolution requirement extends outwards to the channel centerline, where the behaviour of isotropic turbulence is recovered. However, marginal resolution, that underemphasizes high-intensity scalar dissipation events in the nearly-isotropic central region, is found to overemphasize them significantly near the wall.

The influence of the non-resolved scales of thermal radiation in large eddy simulation of turbulent flows: A fundamental study

The filtered radiative transfer equation has been solved without subgrid-scale model and compared to a-priori estimations from direct numerical simulation of statistically stationary homogeneous isotropic turbulence. The importance of the large eddy simulation non-resolved scales on the thermal radiation is analysed in this academic case. The influence of the mean and variance of temperature of the system on the subgrid-scale correlations is studied. It is shown that the turbulence–radiation interaction (TRI) is greater for low-temperature flows, where the radiative transfer is lower. Moreover, the strongest subgrid-scale correlations, which are the temperature self-correlation and the absorption coefficient-temperature correlation, have opposite effects, which suggests that it is better to neglect both correlations, instead of modelling only one. Therefore, the assumption of neglecting the TRI in LES leads to good predictions. Three-dimensional filtering effects, as well as the grid influence on non-local quantities, such as the radiation intensity, are also analysed. These effects have a local influence, but are negligible in the tested cases.

Autocorrelations and thermal fragility of anyonic loops in topologically quantum ordered systems

Are systems that display topological quantum order (TQO), and have a gap to excitations, hardware fault-tolerant at finite temperatures? We show that in models that display low $d$-dimensional gaugelike symmetries, such as Kitaev's and its generalizations, the expectation value of topological symmetry operators vanishes at any nonzero temperature, a phenomenon that we coined thermal fragility. The autocorrelation time for the nonlocal topological quantities in these systems may remain finite even in the thermodynamic limit. We provide explicit expressions for the autocorrelation functions in Kitaev's toric code model. If temperatures far below the gap may be achieved then these autocorrelation times, albeit finite, can be made large. The physical engine behind the loss of correlations at large spatial and/or temporal distance is the proliferation of topological defects at any finite temperature as a result of a dimensional reduction. This raises an important question: How may we best quantify the degree of protection of quantum information in a topologically ordered system at finite temperature?

Method to derive Lagrangian and Hamiltonian for a nonlinear dynamical system with variable coefficients

A general method is developed to derive a Lagrangian and Hamiltonian for a nonlinear system with a quadratic first-order time derivative term and coefficients varying in the space coordinates. The method is based on variable transformations that allow removing the quadratic term and writing the equation of motion in standard form. Based on this form, an auxiliary Lagrangian for the transformed variables is derived and used to obtain the Lagrangian and Hamiltonian for the original variables. An interesting result is that the obtained Lagrangian and Hamiltonian can be non-local quantities, which do not diverge as the system evolves in time. Applications of the method to several systems with different coefficients shows that the method may become an important tool in studying nonlinear dynamical systems with a quadratic velocity term.

Globally Maximizing, Locally Minimizing: Unsupervised Discriminant Projection with Applications to Face and Palm Biometrics

This paper develops an unsupervised discriminant projection (UDP) technique for dimensionality reduction of high-dimensional data in small sample size cases. UDP can be seen as a linear approximation of a multimanifolds-based learning framework which takes into account both the local and nonlocal quantities. UDP characterizes the local scatter as well as the nonlocal scatter, seeking to find a projection that simultaneously maximizes the nonlocal scatter and minimizes the local scatter. This characteristic makes UDP more intuitive and more powerful than the most up-to-date method, Locality Preserving Projection (LPP), which considers only the local scatter for clustering or classification tasks. The proposed method is applied to face and palm biometrics and is examined using the Yale, FERET, and AR face image databases and the PolyU palmprint database. The experimental results show that UDP consistently outperforms LPP and PCA and outperforms LDA when the training sample size per class is small. This demonstrates that UDP is a good choice for real-world biometrics applications.

ON SOME NONLOCAL VARIATIONAL PROBLEMS

We study uniqueness and non uniqueness of minimizers of functionals involving nonlocal quantities. We give also conditions which lead to a lack of minimizers and we show how minimization on an infinite dimensional space reduces here to a minimization on ℝ. Among other things, we prove that uniqueness of minimizers of functionals of the form ∫Ω a(∫Ω gu dx)|∇u|2 dx - 2 ∫Ω fu dx is ensured if a > 0 and 1/a is strictly concave in the sense that (1/a)″ < 0 on (0, ∞).

Dislocation Dynamics: Short-time Existence and Uniqueness of the Solution

We study a mathematical model describing dislocation dynamics in crystals. We consider a single dislocation line moving in its slip plane. The normal velocity is given by the Peach-Koehler force created by the dislocation line itself. The mathematical model is an eikonal equation with a velocity which is a non-local quantity depending on the whole shape of the dislocation line. We study the special case where the dislocation line is assumed to be a graph or a closed loop. In the framework of discontinuous viscosity solutions for Hamilton–Jacobi equations, we prove the existence and uniqueness of a solution for small time. We also give physical explanations and a formal derivation of the mathematical model. Finally, we present numerical results based on a level-sets formulation of the problem. These results illustrate in particular the fact that there is no general inclusion principle for this model.

Modeling of the Electric-Field Dynamics in the Atmosphere Using the Test-Structure Method

We propose the test-structure method for modeling of electric-field pulsations in the atmosphere. Numerical calculations necessary for interpretation of the behavior of experimental spectra and structure functions of the electric field are performed. Analysis of experimental data shows that the aeroelectric-field strength, being a nonlocal quantity, is formed by an inhomogeneous distribution of space charges surrounding the observation point. Quantitative assessments of the state of the atmospheric boundary layer, electro-gas-dynamic turbulence and convection parameters are discussed on the basis of spectral and structure functions of the electric field.

Dislocation dynamics described by non-local Hamilton–Jacobi equations

We study a mathematical model describing dislocation dynamics in crystals. This phase-field model is based on the introduction of a core tensor which mollifies the singular field on the core of the dislocation. We present this model in the case of the motion of a single dislocation, without cross-slip. The dynamics of a single dislocation line, moving in its slip plane, is described by an Hamilton–Jacobi equation whose velocity is a non-local quantity depending on the whole shape of the dislocation line. Introducing a level sets formulation of this equation, we prove the existence and uniqueness of a continuous viscosity solution when the dislocation stays a graph in one direction. We also propose a numerical scheme for which we prove that the numerical solution converges to the continuous solution.

Large-scale perturbations on the brane and the isotropy of the cosmological singularity

We present the complete set of propagation and constraint equations for the kinematic and non-local first order quantities which describe general linear inhomogeneous and anisotropic perturbations of a flat FRW braneworld with vanishing cosmological constant and decompose them in the standard way into their scalar, vector and tensor contributions. A detailed analysis of the perturbation dynamics is performed using dimensionless variables that are specially tailored for the different regimes of interest; namely, the low energy GR regime, the high energy regime and the dark energy regime. Tables are presented for the evolution of all the physical quantities, making it easy to do a detailed comparison of the past asymptotic behaviour of the perturbations of these models. We find results that exactly match those obtained in the analysis of the spatially inhomogeneous $G_{2}$ braneworld cosmologies presented recently; i.e., that isotropization towards the ${\cal F}_b$ model occurs for $\gamma > 4/3$.

Defining 〈A2〉 in the finite volume Hamiltonian formalism

It is shown how in principle for non-Abelian gauge theories it is possible in the finite volume Hamiltonian framework to make sense of calculating the expectation value of ‖A‖2=∫d3x(Aai(x→))2. Gauge invariance requires one to replace ‖A‖2 by its minimum over the gauge orbit, which makes it a highly non-local quantity. We comment on the difficulty of finding an expression for ‖A‖2min analogous to that found for the Abelian case, and the relation of this question to Gribov copies. We deal with these issues by implementing the Hamiltonian on the so-called fundamental domain, with appropriate boundary conditions in field space, essential to correctly represent the physics of the problem.

Functionals of the square kinetic energy density

We construct exchange energy functionals that depend on the non-interacting square kinetic energy density τ2(r)=14∑k|Δφk(r)|2 rather than the electron density. Since τ2 is a non-local functional of the electron density, it may describe non-local effects beyond standard generalized gradient approximation models. These effects may be essential in dealing with such a non-local quantity as the exchange energy. The problem of v-representability of τ2 for a slightly perturbed electron gas and Coulomb systems is discussed. A gradient expansion technique that permits construction of gradient-corrected functionals of τ2 is developed, and the performance of the new exchange functionals is analyzed.

Asymptotic behaviour of some nonlocal diffusion problems

We deal with a parabolic equation having a diffusion coefficient depending on a nonlocal quantity. We investigate the convergence of the solution towards a steady state, extending previous results obtained by [M. Chipot and B. Lovat (1999). On the asympotic behaviour of some nonlocal problems. Positivity, 3, 65-81]. Using the dynamical systems point of view, we are able to treat the case of a continuum of steady states

Geodesic propagators and black hole holography

One of the most challenging technical aspects of the dualities between string theory on anti--de Sitter spaces and conformal field theories is understanding how location in the interior of spacetime is represented in the field theory. It has recently been argued that the interior of the spacetime can be directly probed by using intrinsically non-local quantities in the field theory. In addition, Balasubramanian and Ross [Phys. Rev. D 61, 044009 (2000)] argued that when the spacetime described the formation of an ${\mathrm{AdS}}_{3}$ black hole, the propagator in the field theory probed the whole spacetime, including the region behind the horizon. We use the same approach to study the propagator for the BTZ black hole and a black hole solution with a single exterior region, and show that it reproduces the propagator associated with the natural vacuum states on these spacetimes. We compare our result with a toy model of the CFT for the single-exterior black hole, finding remarkable agreement. The spacetimes studied in this work are analytic, which makes them quite special. We also discuss the interpretation of this propagator in more general spacetimes, shedding light on certain issues involving causality, black hole horizons, and products of local operators on the boundary.