Agissilaos Athanassoulis

University of Dundee · Division of Mathematics

We consider the Wigner equation corresponding to a nonlinear Schroedinger evolution of the Hartree type in the semiclassical limit $\hbar\to 0$. Under appropriate assumptions on the initial data and the interaction potential, we show that the Wigner function is close in $L^2$ to its weak limit, the solution of the corresponding Vlasov equation. The...

Smoothed Wigner transforms have been used in signal processing, as a regularized version of the Wigner transform, and have been proposed as an alternative to it in the homogenization and / or semiclassical limits of wave equations. We derive explicit, closed formulations for the coarse-scale representation of the action of pseudodifferential operat...

We analyse a nonlinear Schr\"odinger equation for the time-evolution of the wave function of an electron beam, interacting selfconsistently through a Hartree-Fock nonlinearity and through the repulsive Coulomb interaction of an atomic nucleus. The electrons are supposed to move under the action of a time dependent, rapidly periodically oscillating...

American Institute of Mathematical Sciences. The Alber equation is a moment equation for the nonlinear Schrodinger equation, formally used in ocean engineering to investigate the stability of stationary and homogeneous sea states in terms of their power spectra. In this work we present the first well-posedness theory for the Alber equation with the...

The nonlinear Schrödinger equation is widely used as an approximate model for the evolution in time of the water wave envelope. In the context of simulating ocean waves, initial conditions are typically generated from a measured power spectrum using the random-phase approximation, and periodized on an interval of length L. It is known that most rea...

We consider a nonconservative nonlinear Schrödinger equation (NCNLS) with time‐dependent coefficients, inspired by a water waves problem. This problem does not have mass or energy conservation, but instead mass and energy change in time under explicit balance laws. In this paper, we extend to the particular NCNLS two numerical schemes which are kno...

We consider a non-conservative nonlinear Schrödinger equation (NCNLS) with time-dependent coefficients, inspired by a water waves problem. This problem does not have mass or energy conservation, but instead mass and energy change in time under explicit balance laws. In this paper we extend to the particular NCNLS two numerical schemes which are kno...

The modulation instability (MI) is a well known feature of the focusing nonlinear Schr\"odinger equation (NLS), namely that plane wave solutions on the real line are linearly unstable. Simulations of the MI typically use a large computational domain of length $L$ equipped with periodic boundary conditions. We show for the first time that for $L$ sm...

It has long been known that plane wave solutions of the cubic nonlinear Schrödinger Equation (NLS) are linearly unstable. This fact is widely known as modulation instability (MI), and sometimes referred to as Benjamin-Feir instability in the context of water waves. In 1978, I.E. Alber introduced a methodology to perform an analogous linear stabilit...

We introduce a new structure preserving, second order in time relaxation-type scheme for approximating solutions of the Schrödinger-Poisson system. More specifically, we use the Crank-Nicolson scheme as a time stepping mechanism, whilst the nonlinearity is handled by means of a relaxation approach in the spirit of [10, 34] for the nonlinear Schrödi...

The Alber equation is a phase-averaged second-moment model used to study the statistics of a sea state, which has recently been attracting renewed attention. We extend it in two ways: firstly, we derive a generalized Alber system starting from a system of nonlinear Schrödinger equations, which contains the classical Alber equation as a special case...

We study the Alber equation, a phase-averaged second-moment model for the statistics of a wavefield, derived under a narrowbandedness assumption. More specifically, we use the Alber equation and its associated instability condition to quantify how close a given non-parametric spectrum is to being modulationally unstable, and apply this to a dataset...

We introduce a new second order in time Besse-type relaxation scheme for approximating solutions of the Schr\"odinger-Poisson system. More specifically, we use the Crank-Nicolson scheme as a time stepping mechanism, the standard conforming finite element method for the spatial discretization whilst the nonlinearity is handled by means of a relaxati...

The Wigner transform can be used to derive equations directly for the evolution of the autocorrelation of the sea elevation. This has been known in the literature as the derivation of the Alber equation, and applies to envelope equations. Wigner-Alber equations have been used to characterise spectra as either stable or unstable, and to predict Ferm...

We consider the semiclassical limit of nonlinear Schrödinger equations with initial data that are well localized in both position and momentum (non-parametric wavepackets). We recover the Wigner measure (WM) of the problem, a macroscopic phase-space density which controls the propagation of the physical observables such as mass, energy and momentum...

In this paper, we model Rogue Waves as localized instabilities emerging from homogeneous and stationary background wavefields, under NLS dynamics. This is achieved in two steps: given any background Fourier spectrum P(k), we use the Wigner transform and Penrose’s method to recover spatially periodic unstable modes, which we call unstable Penrose mo...

We study a nonlinear Schr\"odinger equation which arises as an effective single particle model in X-ray Free Electron Lasers (XFEL). This equation appears as a first-principles model for the beam-matter interactions that would take place in an XFEL molecular imaging experiment in \cite{frat1}. Since XFEL is more powerful by several orders of magnit...

Semiclassical asymptotics for linear Schr\"odinger equations with non-smooth potentials give rise to ill-posed formal semiclassical limits. These problems have attracted a lot of attention in the last few years, as a proxy for the treatment of eigenvalue crossings, i.e. general systems. It has recently been shown that the semiclassical limit for co...

We study the semiclassical behaviour of solutions of a Schrödinger equation with a scalar potential displaying a conical singularity. When a pure state interacts strongly with the singularity of the flow, there are several possible classical evolutions, and it is not known whether the semiclassical limit corresponds to one of them. Based on recent...

We consider the classical limit of the quantum evolution, with some rough potential, of wave packets concentrated near singular trajectories of the underlying dynamics. We prove that under appropriate conditions, even in the case of BV vector fields, the correct classical limit can be selected.

We present several results concerning the semiclassical limit of the time dependent Schr\"odinger equation with potentials whose regularity doesn't guarantee the uniqueness of the underlying classical flow. Different topologies for the limit are considered and the situation where two bicharateristics can be obtained out of the same initial point is...

In this paper we consider the nonlinear Hartree equation in presence of a given external potential, for an initial coherent state. Under suitable smoothness assumptions, we approximate the solution in terms of a time dependent coherent state, whose phase and amplitude can be determined by a classical flow. The error can be estimated in $L^2$ by $C...

Wigner and Husimi transforms have long been used for the phase-space reformulation of Schr\"odinger-type equations, and the study of the corresponding semiclassical limits. Most of the existing results provide approximations in appropriate weak topologies. In this work we are concerned with semiclassical limits in the strong topology, i.e. approxim...

We study a generalization of Husimi function in the context of wavelets. This leads to a nonnegative density on phase-space for which we compute the evolution equation corresponding to a Schr\"odinger equation.

The Wigner Transform (WT) has been extensively used in the formulation of phase-space models for a variety of wave propagation problems including high-frequency limits, nonlinear and random waves. It is well known that the WT features counterintuitive 'interference terms', which often make computation impractical. In this connection, we propose the...

The Wigner transform (WT) is a quadratic transform that takes an oscillatory function u (x): ℝn ↦ ℂd to a phase-space density W (x, k) = W [u ](x, k): ℝ2n ↦ ℂd ×d, resolving it over an additional set of 'wavenumber' variables. The WT and its variations have been heavily used in quantum mechanics, semiconductors, homogenization of wave equations, ti...

The numerical simulation of wave propagation in semiclassical (high-frequency) problems is well known to pose a formidable challenge. In this work, a new phase-space approach for the numerical simulation of semiclassical wave propagation, making use of the smoothed Wigner Transform (SWT), is proposed. There are numerous works which use the Wigner T...

The Wigner transform (WT) is a well known quadratic transform, mapping a wavefunction to a position-wavenumber quasi-density. WTs are used in the asymptotic treatment of semiclassical and other wave problems. The smoothed WT (SWT) method is a regularization and extension of WT-based approaches for the homogenization of wave propagation, which decou...

The Wigner transform (WT) is a quadratic nonparametric phase-space density, introduced by Eugene Wigner for the phase-space formulation of quantum mechanics. It has since been used extensively in the asymptotic study of many wave propagation problems; it is well known however that it is not amenable to practical computations. The smoothed Wigner tr...

Wigner measures have been used successfully in the description of semiclassical wave propagation in various contexts (4, 3, 5). This work concerns a modification of the Wigner transform (WT), and how it can be used (after the necessary functional analytic framework is worked out) for the homogenization of wave propagation. The main application pres...