arXiv subjects

A. M. Kamchatnov

Publications and source records attributed to A. M. Kamchatnov.

At least 55 records · Page 3Linked to original sources

arXiv2018

Long-time evolution of pulses in the Korteweg-de Vries equation in the absence of solitons revisited: Whitham method

We consider the long-time evolution of pulses in the Korteweg-de Vries equation theory for initial distributions which produce no soliton, but instead lead to the formation of a dispersive shock wave and of a rarefaction wave. An approach based on Whitham modulation theory makes it possible to obtain an analytic description of the structure and to describe its self-similar behavior near the soliton edge of the shock. The results are compared with numerical simulations.

nlin.PS↗

arXiv2018

Self-similar wave breaking in dispersive Korteweg-de Vries hydrodynamics

We discuss the problem of breaking of a nonlinear wave in the process of its propagation into a medium at rest. It is supposed that the profile of the wave is described at the breaking moment by the function $(-x)^{1/n}$ ($x<0$, positive pulse) or $-x^{1/n}$ ($x>0$, negative pulse) of the coordinate $x$. Evolution of the wave is governed by the Korteweg-de Vries equation resulting in formation of a dispersive shock wave. In the positive pulse case, the dispersive shock wave forms at the leading edge of the wave structure, and in the negative pulse case at its rear edge. The dynamics of dispersive shock waves is described by the Whitham modulation equations. For power law initial profiles, this dynamics is self-similar and the solution of the Whitham equations is obtained in a closed form for arbitrary $n>1$.

nlin.PS↗

arXiv2018

Riemann problem for the photon fluid: self-steepening effects

We consider the Riemann problem of evolution of initial discontinuities for the photon fluid propagating in a normal dispersion fiber with account of self-steepening effects. The dynamics of light field is described by the nonlinear Schroedinger (NLS) equation with self-steepening term appearing due to retardation of the fiber material response to variations of the electromagnetic signal. It is shown that evolution dynamics in this case is much richer than that for the NLS equation. Complete classification of possible wave structures is given for all possible jump conditions at the discontinuity.

nlin.PS↗

arXiv2018

Simple waves in a two-component Bose-Einstein condensate

We consider dynamics of simple waves in a two-component Bose-Einstein condensates. The evolution of the condensate is described by the Gross-Pitaevskii equations which can be reduced for simple wave solutions to a system of ordinary differential equations which coincide with those derived by Ovsyannikov for the two-layer fuid dynamics. We solve the Ovsyannikov system for two typical situations of large and small difference between inter-species and intra-species nonlinear interaction constants. Our analytic results are confirmed by numerical simulations.

cond-mat.quant-gas↗

arXiv2017

Evolution of initial discontinuities in the DNLS equation theory

We present the full classification of wave patterns evolving from an initial step-like discontinuity for arbitrary choice of boundary conditions at the discontinuity location in the DNLS equation theory. In this non-convex dispersive hydrodynamics problem, solutions of the Whitham modulation equations are mapped to parameters of a modulated wave by two-valued functions what makes situation much richer than that for a convex case of the NLS equation type. In particular, new types of simple-wave-like structures appear as building elements of the whole wave pattern. The developed here theory can find applications to propagation of light pulses in fibers and to the theory of Alfvén dispersive shock waves.

nlin.PS↗

arXiv2017

Solution of the Riemann problem for polarization waves in a two-component Bose-Einstein condensate

We provide a classification of the possible flow of two-component Bose-Einstein condensates evolving from initially discontinuous profiles. We consider the situation where the dynamics can be reduced to the consideration of a single polarization mode (also denoted as "magnetic excitation") obeying a system of equations equivalent to the Landau-Lifshitz equation for an easy-plane ferro-magnet. We present the full set of one-phase periodic solutions. The corresponding Whitham modulation equations are obtained together with formulas connecting their solutions with the Riemann invariants of the modulation equations. The problem is not genuinely nonlinear, and this results in a non-single-valued mapping of the solutions of the Whitham equations with physical wave patterns as well as to the appearance of new elements --- contact dispersive shock waves --- that are absent in more standard, genuinely nonlinear situations. Our analytic results are confirmed by numerical simulations.

cond-mat.quant-gas↗

arXiv2017

Evolution of initial discontinuities in the Riemann problem for the Kaup-Boussinesq equation with positive dispersion

We consider the space-time evolution of initial discontinuities of depth and flow velocity for an integrable version of the shallow water Boussinesq system introduced by Kaup. We focus on a specific version of this "Kaup-Boussinesq model" for which a flat water surface is modulationally stable, we speak below of "positive dispersion" model. This model also appears as an approximation to the equations governing the dynamics of polarisation waves in two-component Bose-Einstein condensates. We describe its periodic solutions and the corresponding Whitham modulation equations. The self-similar, one-phase wave structures are composed of different building blocks which are studied in detail. This makes it possible to establish a classification of all the possible wave configurations evolving from initial discontinuities. The analytic results are confirmed by numerical simulations.

nlin.PS↗

arXiv2017

Expansion Dynamics of a Two-Component Quasi-One-Dimensional Bose-Einstein Condensate: Phase Diagram, Self-Similar Solutions, and Dispersive Shock Waves

We investigate the expansion dynamics of a Bose-Einstein condensate that consists of two components and is initially confined in a quasi-one-dimensional trap. We classify the possible initial states of the two-component condensate by taking into account the non-uniformity of the distributions of its components and construct the corresponding phase diagram in the plane of nonlinear interaction constants. The differential equations that describe the condensate evolution are derived by assuming that the condensate density and velocity depend on the spatial coordinate quadratically and linearly, respectively, what reproduces the initial equilibrium distribution of the condensate in the trap in the Thomas-Fermi approximation. We obtained self-similar solutions of these differential equations for several important special cases and wrote out asymptotic formulas describing the condensate motion on long time scales, when the condensate density becomes so low that the interaction between atoms can be neglected. The problem on the dynamics of immiscible components with the formation of dispersive shock waves was also considered. We compare the numerical solutions of the Gross-Pitaevskii equations with their approximate analytical solutions and study numerically the situations when the analytical method admits no exact solutions.

cond-mat.quant-gas↗

arXiv2016

Dispersive hydrodynamics of nonlinear polarization waves in two-component Bose-Einstein condensates

We study one dimensional mixtures of two-component Bose-Einstein condensates in the limit where the intra-species and inter-species interaction constants are very close. Near the mixing-demixing transition the polarization and the density dynamics decouple. We study the nonlinear polarization waves, show that they obey a universal (i.e., parameter free) dynamical description, identify a new type of algebraic soliton, explicitly write simple wave solutions, and study the Gurevich-Pitaevskii problem in this context.

cond-mat.quant-gas↗

arXiv2016

Oblique spatial dispersive shock waves in nonlinear Schrödinger flows

In dispersive media, hydrodynamic singularities are resolved by coherent wavetrains known as dispersive shock waves (DSWs). Only dynamically expanding, temporal DSWs are possible in one-dimensional media. The additional degree of freedom inherent in two-dimensional media allows for the generation of time-independent DSWs that exhibit spatial expansion. Spatial oblique DSWs, dispersive analogs of oblique shocks in classical media, are constructed utilizing Whitham modulation theory for a class of nonlinear Schrödinger boundary value problems. Self-similar, simple wave solutions of the modulation equations yield relations between the DSW's orientation and the upstream/downstream flow fields. Time dependent numerical simulations demonstrate a convective or absolute instability of oblique DSWs in supersonic flow over obstacles. The convective instability results in an effective stabilization of the DSW.

nlin.PS↗

arXiv2016

On exact solutions of nonlinear acoustic equations

Solutions of nonlinear acoustic equations describing propagation of strong sound pulses with account of curvature of wave fronts in multi-dimensional geometry are obtained from simple physical considerations. The form of these solutions suggests ansatz suitable for finding solutions of much more general equations of Khokhlov-Zabolotskaya type. General method is illustrated by an example of nonlinear sound pulse focused in one transverse direction and defocused in the other direction.

nlin.PS↗

arXiv2015

Whitham theory for perturbed Korteweg-de Vries equation

Original Whitham's method of derivation of modulation equations is applied to systems whose dynamics is described by a perturbed Korteweg-de Vries equation. Two situations are distinguished: (i) the perturbation leads to appearance of right-hand sides in the modulation equations so that they become non-uniform; (ii) the perturbation leads to modification of the matrix of Whitham velocities. General form of Whitham modulation equations is obtained for each case. The essential difference between them is illustrated by an example of so-called `generalized Korteweg-de Vries equation'. Method of finding steady-state solutions of perturbed Whitham equations in the case of dissipative perturbations is considered.

nlin.PS↗

arXiv2015

Interference effects in the two-dimensional scattering of microcavity polaritons by an obstacle: phase dislocations and resonances

We consider interference effects within the linear description of the scattering of two-dimensional microcavity polaritons by an obstacle. The polariton wave may exhibit phase dislocations created by the interference of the incident and the scattered fields. We describe these structures within the general framework of singular optics. We also discuss another type of interference effects appearing due to the formation of (quasi)resonances in the potential of a repulsive obstacle with sharp boundaries. We discuss the relevance of our approach for the description of recent experimental results and propose a criterion for evaluating the importance of nonlinear effects.

cond-mat.quant-gas↗

arXiv2015

Nonlinear waves in two-component Bose-Einstein condensates: Manakov system and Kowalevski equations

Traveling waves in two-component Bose-Einstein condensates whose dynamics is described by the Manakov limit of the Gross-Pitaevskii equations are considered in general situation with relative motion of the components when their chemical potentials are not equal to each other. It is shown that in this case the solution is reduced to the form known in the theory of motion of S.~Kowalevski top. Typical situations are illustrated by the particular cases when the general solution can be represented in terms of elliptic functions and their limits. Depending on the parameters of the wave, both density waves (with in-phase motions of the components) and polarization waves (with counter-phase their motions) are considered.

cond-mat.quant-gas↗

arXiv2014

On periodic solutions and their modulations for the Manakov system

Periodic solutions of the Manakov system are studied with the aim to establish links between recent results of Wright (Physica D 2013 {\bf 264} 1) and Kamchatnov (EPL 2013 \textbf{103} 60003), where the one-phase solutions have been obtained by different methods and with different parameterizations. Two types of waves, which in the context of dynamics of two-component Bose-Einstein condensates can be called {\it density} and {\it polarization} waves, are determined as particular solutions of the Manakov system. Whitham equations describing modulations of these two types of waves are obtained.

nlin.SI↗

arXiv2013

Wave patterns generated by a flow of two-component Bose-Einstein condensate with spin-orbit interaction past a localized obstacle

It is shown that spin-orbit interaction leads to drastic changes in wave patterns generated by a flow of two-component Bose-Einstein condensate (BEC) past an obstacle. The combined Rashba and Dresselhaus spin-orbit interaction affects in different ways two types of excitations---density and polarization waves---which can propagate in a two-component BEC. We show that the density and polarization "ship wave" patterns rotate in opposite directions around the axis located at the obstacle position and the angle of rotation depends on the strength of spin-orbit interaction. This rotation is accompanied by narrowing of the Mach cone. The influence of spin-orbit coupling on density solitons and polarization breathers is studied numerically.

cond-mat.quant-gas↗

arXiv2013

Periodic waves in two-component Bose-Einstein condensates with repulsive interactions between atoms

We consider periodic waves in miscible two-component Bose-Einstein condensates with repulsive nonlinear interactions constants. Exact one-phase solution is found for the case when all these constants are equal to each other (i.e., for Manakov limit). New types of nonlinear polarization waves are considered in detail. The connection of the solutions found with experimentally observed periodic structures in two-component condensates is discussed.

cond-mat.quant-gas↗

arXiv2013

Polarization hydrodynamics in a one-dimensional polariton condensate

We study the hydrodynamics of a nonresonantly-pumped polariton condensate in a quasi-one-dimensional quantum wire taking into account the spin degree of freedom. We clarify the relevance of the Landau criterion for superfluidity in this dissipative two-component system. Two Cherenkov-like critical velocities are identified corresponding to the opening of different channels of radiation: one of (damped) density fluctuations and another of (weakly damped) polarization fluctuations. We determine the drag force exerted onto an external obstacle and propose experimentally measurable consequences of the specific features of the fluctuations of polarization.

cond-mat.mes-hall↗