Weakly collisional Landau damping and three-dimensional Bernstein-Greene-Kruskal modes: New results on old problems

Landau damping and Bernstein-Greene-Kruskal (BGK) modes are among the most fundamental concepts in plasma physics. While the former describes the surprising damping of linear plasma waves in a collisionless plasma, the latter describes exact undamped nonlinear solutions of the Vlasov equation. There does exist a relationship between the two: Landau damping can be described as the phase-mixing of undamped eigenmodes, the so-called Case-Van Kampen modes, which can be viewed as BGK modes in the linear limit. While these concepts have been around for a long time, unexpected new results are still being discovered. For Landau damping, we show that the textbook picture of phase-mixing is altered profoundly in the presence of collision. In particular, the continuous spectrum of Case-Van Kampen modes is eliminated and replaced by a discrete spectrum, even in the limit of zero collision. Furthermore, we show that these discrete eigenmodes form a complete set of solutions. Landau-damped solutions are then recovered as true eigenmodes (which they are not in the collisionless theory). For BGK modes, our interest is motivated by recent discoveries of electrostatic solitary waves in magnetospheric plasmas. While one-dimensional BGK theory is quite mature, there appear to be no exact three-dimensional solutions in the literature (except for the limiting case when the magnetic field is sufficiently strong so that one can apply the guiding-center approximation). We show, in fact, that two- and three-dimensional solutions that depend only on energy do not exist. However, if solutions depend on both energy and angular momentum, we can construct exact three-dimensional solutions for the unmagnetized case, and two-dimensional solutions for the case with a finite magnetic field. The latter are shown to be exact, fully electromagnetic solutions of the steady-state Vlasov-Poisson-Amp\`ere system

Signal concentration and related concepts in time-frequency and on the unit sphere

Unit sphere signal processing is an increasingly active area of research with applications in computer vision, medical imaging, geophysics, cosmology and wireless communications. However, comparing with signal processing in time-frequency domain, characterization and processing of signals defined on the unit sphere is relatively unfamiliar for most of the engineering researchers. In order to better understand and analysis the current issues using the spherical model, such as analysis of brain neural electronic activities in medical imaging and neuroscience, target detection and tracking in radar systems, earthquake occurrence prediction and seismic origin detection in seismology, it is necessary to set up a systematic theory for unit sphere signal processing. How to efficiently analyze and represent functions defined on the unit sphere are central for the unit sphere signal processing, such as filtering, smoothing, detection and estimation in the presence of noise and interference. Slepian-Landau-Pollak time-frequency energy concentration theory and the essential dimensionality of time-frequency signals by the Fourier transform are the fundamental tools for signal processing in the time-frequency domain. Therefore, our research work starts from the analogies of signals between time-frequency and spatial-spectral. In this thesis, we first formulate the k-th moment time-duration weighting measure for a band-limited signal using a general constrained variational method, where a complete, orthonormal set of optimal band-limited functions with the minimum fourth moment time-duration measure is obtained and the prospective applications are discussed. Further, the formulation to an arbitrary signal with second and fourth moment weighting in both time and frequency domain is also developed and the corresponding optimal functions are obtained, which are helpful for practical waveform designs in communication systems. Next, we develop a k-th spatially global moment azimuthal measure (GMZM) and a k-th spatially local moment zenithal measure (LMZM) for real-valued spectral-limited signals. The corresponding sets of optimal functions are solved and compared with the spherical Slepian functions. In addition, a harmonic multiplication operation is developed on the unit sphere. Using this operation, a spectral moment weighting measure to a spatial-limited signal is formulated and the corresponding optimal functions are solved. However, the performance of these sets of functions and their perspective applications in real world, such as efficiently analysis and representation of spherical signals, is still in exploration. Some spherical quadratic functionals by spherical harmonic multiplication operation are formulated in this thesis. Next, a general quadratic variational framework for signal design on the unit sphere is developed. Using this framework and the quadratic functionals, the general concentration problem to an arbitrary signal defined on the unit sphere to simultaneously achieve maximum energy in the finite spatial region and finite spherical spectrum is solved. Finally, a novel spherical convolution by defining a linear operator is proposed, which not only specializes the isotropic convolution, but also has a well defined spherical harmonic characterization. Furthermore, using the harmonic multiplication operation on the unit sphere, a reconstruction strategy without consideration of noise using analysis-synthesis filters under three different sampling methods is discussed

Tournament Directed Graphs

Paired comparison is the process of comparing objects two at a time. A tournament in Graph Theory is a representation of such paired comparison data. Formally, an n-tournament is an oriented complete graph on n vertices; that is, it is the representation of a paired comparison, where the winner of the comparison between objects x and y (x and y are called vertices) is depicted with an arrow or arc from the winner to the other. In this thesis, we shall prove several results on tournaments. In Chapter 2, we will prove that the maximum number of vertices that can beat exactly m other vertices in an n-tournament is min{2m + 1,2n - 2m - 1}. The remainder of this thesis will deal with tournaments whose arcs have been colored. In Chapter 3 we will define what it means for a k-coloring of a tournament to be k-primitive. We will prove that the maximum k such that some strong n-tournament can be k-colored to be k-primitive lies in the interval [(n-12), (n2) - [n/4]). In Chapter 4, we shall prove special cases of the following 1982 conjecture of Sands, Sauer, and Woodrow from [14]: Let T be a 3-arc-colored tournament containing no 3-cycle C such that each arc in C is a different color. Then T contains a vertex v such that for any other vertex x, x has a monochromatic path to v

ON FOURIER COEFFICIENTS OF SIEGEL MODULAR FORMS OF DEGREE TWO WITH RESPECT TO CONGRUENCE SUBGROUPS

We prove a formula of Petersson’s type for Fourier coefficients of Siegel cusp forms of degree 2 with respect to congruence subgroups, and as a corollary, show upper bound estimates of individual Fourier coefficient. The method in this paper is essentially a generalization of Kitaoka’s previous work which studied the full modular case, but some modification is necessary to obtain estimates which are sharp with respect to the level aspect

Two dimensional P-wave superconductors with long range interactions

L' interesse crescente che circonda lo studio delle proprietà topologiche della materia è profondamente collegato all' effettiva possibilità di verifica in laboratorio. Negli ultimi decenni infatti la fisica sperimentale degli atomi ultrafreddi ha raggiunto livelli di precisione prima inimmaginabili. Attraverso reticoli ottici si possono riprodurre sistemi multicorpo fortemente interagenti di cui si possono controllare in maniera quasi esatta i parametri fisici, come i potenziali. In questo contesto si inserisce il modello bidimensionale P-wave con interazioni a lungo raggio. Le interazioni in questo modello avvengono tra tutte le componenti, quindi in tutte le direzioni. Questo sistema fisico topologico inoltre è caratterizzato da una Hamiltoniana con potenziale di interazione che decade con la distanza secondo una legge di potenza per cui, per quanto detto, la sua realizzazione sperimentale è possibile. In questo lavoro abbiamo iniziato studiando lo spettro di questo sistema partendo da un approccio analitico. Dopo aver compreso il comportamento dei vari termini energetici abbiamo selezionato dei casi di studio per diversi range di interazione. In questi casi successivamente abbiamo analizzato le varie fasi e transizioni di fase tramite simulazioni numeriche. All' aumentare del range di interazione abbiamo visto l' emergere di nuovi fenomeni assenti nei modelli con interazione a corto raggio

The Fermi liquid as a renormalization group fixed point

The renormalization-group (RG) method is applied to study interacting fermions at finite temperature. A model based on the [psi][superscript]4-Grassmann effective action with SU(N )-invariant short-range interaction and a rotationally invariant Fermi surface in spatial dimensions d = 2, 3 is studied. We show how the key results of the Landau Fermi liquid theory can be recovered by this finite-temperature RG technique. Applying the RG to response functions, we find the compressibility and the spin susceptibility as solutions of the RG flow equations. We discuss subtleties associated with the symmetry properties of the four-point vertex (the implications of the Pauli principle). We point out distinctions between three quantities: the bare interaction of the low-energy effective action, the Landau function and the forward scattering vertex.The bare interaction of the effective action is not a RG fixed point, but a common starting point of the flow trajectories of two limiting forms of the four-point vertex. We have derived RG equations for the Landau channel that take into account both contributions of the direct (ZS) and the exchange (ZS' ) particle-hole graphs at one-loop level.The basic quantities of Fermi Liquid theory, the Landau interaction function and the forward scattering vertex, are calculated as fixed points of these flows in terms of the effective action's interaction function. The classic derivations of Fermi Liquid theory applying the Bethe-Salpeter equation and other analogous approaches, tantamount to some sort of RPA-type (decoupled) approximation, neglect the zero-angle singularity in the ZS' graph. As a consequence, the antisymmetry of the forward scattering vertex is not guaranteed in the final result, and the RPA sum rule must be imposed by hand on the components of the Landau function to satisfy the Pauli principle. This sum rule, not indispensable in the original phenomenological formulation of the Landau FLT, is equivalent, from the RG point of view, to a fine tuning of the effective interaction. Our results show that the strong interference of the direct and exchange processes of the particle-hole scattering near zero angle invalidates the RPA (decoupled) approximation in this region, resulting in temperature-dependent narrow-angle anomalies in the Landau function and scattering vertex, revealed by the RG analysis. In the present RG approach the Pauli principle is automatically satisfied. As follows from the RG solution, the amplitude sum rule, being an artefact of the RPA approximation, is not needed to respect statistics and, moreover, is not valid

The nature of superfuidity and Bose-Einstein condensation: from liquid ⁴He to dilute ultracold atomic gases (Review Article)

We present a brief overview of crucial historical stages in creation of superfluidity theory and of the current state of the microscopic theory of superfluid ⁴He. We pay special attention to the role of Bose-Einstein condensates (BECs) in understanding of physical mechanisms of superfluidity and identification of quantum mechanical structure of ⁴He superfluid component below -point, in particular — the possibility that at least two types of condensates may appear and coexist simultaneously in superfluid ⁴He. In this context we discuss the properties of the binary mixtures of BECs and types of excitations, which may appear due to intercomponent interaction in such binary mixtures of condensates. We also discuss current status of investigations of persistent currents in toroidal optical traps and present an outlook of our recent findings on this subject

Singularly perturbed and non-local modulation equations for systems with interacting instability mechanisms

Two related systems of coupled modulation equations are studied and compared in this paper. The modulation equations are derived for a certain class of basic systems which are subject to two distinct, interacting, destabilizing mechanisms. We assume that, near criticality, the ratio of the widths of the unstable wavenumber-intervals of the two (weakly) unstable modes is small { as for instance can be the case in double-layer convection. Based on these assumptions we rst derive a singularly perturbed modulation equation and then a modulation equation with a non-local term. The reduction of the singularly perturbed system to the non-local system can be interpreted as a limit in which the width of the smallest unstable interval vanishes. We study and compare the behaviour of the stationary solutions of both systems. It is found that spatially periodic stationary solutions of the non-local system exist under the same conditions as spatially periodic stationary solutions of the singularly perturbed system. Moreover, these solutions can be interpreted as representing the same quasi-periodic patterns in the underlying basic system. Thus, the `Landau-reduction' to the non-local system has no signicant in uence on the stationary quasi-periodic patterns. However, a large variety of intricate heteroclinic and homoclinic connections is found for the singularly perturbed system. These orbits all correspond to so-called `localised structures' in the underlying system: they connect simple periodic patterns at x!1. None of these patterns can be described by the non-local system. So, one may conclude that the reduction to the non-local system destroys a rich and important set of patterns