40 research outputs found
Renormalization Group Approach to the Infrared Behavior of a Zero-Temperature Bose System
We exploit the renormalization-group approach to establish the {\em exact}
infrared behavior of an interacting Bose system at zero temperature. The
local-gauge symmetry in the broken-symmetry phase is implemented through the
associated Ward identities, which reduce the number of independent running
couplings to a single one. For this coupling the -expansion can be
controlled to all orders in (). For spatial dimensions the Bogoliubov fixed point is unstable towards a different fixed point
characterized by the divergence of the longitudinal correlation function. The
Bogoliubov linear spectrum, however, is found to be independent from the
critical behavior of this correlation function, being exactly constrained by
Ward identities. The new fixed point properly gives a finite value of the
coupling among transverse fluctuations, but due to virtual intermediate
longitudinal fluctuations the effective coupling affecting the transverse
correlation function flows to zero. As a result, no transverse anomalous
dimension is present. This treatment allows us to recover known results for the
quantum Bose gas in the context of a unifying framework and also to reveal the
non-trivial skeleton structure of its perturbation theory.Comment: 21 page
Vortex -- Kink Interaction and Capillary Waves in a Vector Superfluid
Interaction of a vortex in a circularly polarized superfluid component of a
2d complex vector field with the phase boundary between superfluid phases with
opposite signs of polarization leads to a resonant excitation of a
``capillary'' wave on the boundary. This leads to energy losses by the
vortex--image pair that has to cause its eventual annihilation.Comment: LaTeX 7 pages, no figure
Weakly non-linear dynamics in reaction -- diffusion systems with L\'{e}vy flights
Reaction--diffusion equations with a fractional Laplacian are reduced near a
long wave Hopf bifurcation. The obtained amplitude equation is shown to be the
complex Ginzburg-Landau equation with a fractional Laplacian. Some of the
properties of the normal complex Ginzburg-Landau equation are generalised for
the fractional analogue. In particular, an analogue of Kuramoto-Sivashinsky
equation is derived
Non-homogeneous random walks, subdiffusive migration of cells and anomalous chemotaxis
This paper is concerned with a non-homogeneous in space and non-local in time
random walk model for anomalous subdiffusive transport of cells. Starting with
a Markov model involving a structured probability density function, we derive
the non-local in time master equation and fractional equation for the
probability of cell position. We show the structural instability of fractional
subdiffusive equation with respect to the partial variations of anomalous
exponent. We find the criteria under which the anomalous aggregation of cells
takes place in the semi-infinite domain.Comment: 18 pages, accepted for publicatio
Domain walls and vortices in linearly coupled systems
We investigate 1D and 2D radial domain-wall (DW) states in the system of two
nonlinear-Schr\"{o}dinger/Gross-Pitaevskii equations, which are coupled by the
linear mixing and by the nonlinear XPM (cross-phase-modulation). The system has
straightforward applications to two-component Bose-Einstein condensates, and to
the bimodal light propagation in nonlinear optics. In the former case, the two
components represent different hyperfine atomic states, while in the latter
setting they correspond to orthogonal polarizations of light. Conditions
guaranteeing the stability of flat continuous wave (CW) asymmetric bimodal
states are established, followed by the study of families of the corresponding
DW patterns. Approximate analytical solutions for the DWs are found near the
point of the symmetry-breaking bifurcation of the CW states. An exact DW
solution is produced for ratio 3:1 of the XPM and SPM coefficients. The DWs
between flat asymmetric states, which are mirror images to each other, are
completely stable, and all other species of the DWs, with zero crossings in one
or two components, are fully unstable. Interactions between two DWs are
considered too, and an effective potential accounting for the attraction
between them is derived analytically. Direct simulations demonstrate merger and
annihilation of the interacting DWs. The analysis is extended for the system
including single- and double-peak external potentials. Generic solutions for
trapped DWs are obtained in a numerical form, and their stability is
investigated. An exact stable solution is found for the DW trapped by a
single-peak potential. In the 2D geometry, stable two-component vortices are
found, with topological charges s=1,2,3. Radial oscillations of annular
DW-shaped pulsons, with s=0,1,2, are studied too. A linear relation between the
period of the oscillations and the mean radius of the DW ring is derived
analytically.Comment: Physical Review E, in pres
Oscillatory instability in super-diffusive reaction -- diffusion systems: fractional amplitude and phase diffusion equations
Nonlinear evolution of a reaction--super-diffusion system near a Hopf
bifurcation is studied. Fractional analogues of complex Ginzburg-Landau
equation and Kuramoto-Sivashinsky equation are derived, and some of their
analytical and numerical solutions are studied
Stable two-dimensional solitary pulses in linearly coupled dissipative Kadomtsev-Petviashvili equations
A two-dimensional (2D) generalization of the stabilized Kuramoto -
Sivashinsky (KS) system is presented. It is based on the Kadomtsev-Petviashvili
(KP) equation including dissipation of the generic (Newell -- Whitehead --
Segel, NWS) type and gain. The system directly applies to the description of
gravity-capillary waves on the surface of a liquid layer flowing down an
inclined plane, with a surfactant diffusing along the layer's surface.
Actually, the model is quite general, offering a simple way to stabilize
nonlinear waves in media combining the weakly-2D dispersion of the KP type with
gain and NWS dissipation. Parallel to this, another model is introduced, whose
dissipative terms are isotropic, rather than of the NWS type. Both models
include an additional linear equation of the advection-diffusion type, linearly
coupled to the main KP-NWS equation. The extra equation provides for stability
of the zero background in the system, opening a way to the existence of stable
localized pulses. The consideration is focused on the case when the dispersive
part of the system of the KP-I type, admitting the existence of 2D localized
pulses. Treating the dissipation and gain as small perturbations and making use
of the balance equation for the field momentum, we find that the equilibrium
between the gain and losses may select two 2D solitons, from their continuous
family existing in the conservative counterpart of the model (the latter family
is found in an exact analytical form). The selected soliton with the larger
amplitude is expected to be stable. Direct simulations completely corroborate
the analytical predictions.Comment: a latex text file and 16 eps files with figures; Physical Review E,
in pres
Nonadiabatic Dynamics of Atoms in Nonuniform Magnetic Fields
Dynamics of neutral atoms in nonuniform magnetic fields, typical of
quadrupole magnetic traps, is considered by applying an accurate method for
solving nonlinear systems of differential equations. This method is more
general than the adiabatic approximation and, thus, permits to check the limits
of the latter and also to analyze nonadiabatic regimes of motion. An unusual
nonadiabatic regime is found when atoms are confined from one side of the
z-axis but are not confined from another side. The lifetime of atoms in a trap
in this semi-confining regime can be sufficiently long for accomplishing
experiments with a cloud of such atoms. At low temperature, the cloud is
ellipsoidal being stretched in the axial direction and moving along the z-axis.
The possibility of employing the semi-confining regime for studying the
relative motion of one component through another, in a binary mixture of gases
is discussed.Comment: 1 file, 17 pages, RevTex, 2 table
Linear Stability of Fractional Reaction - Diffusion Systems
Theoretical framework for linear stability of an anomalous sub-diffusive
activator-inhibitor system is set. Generalized Turing instability conditions are found to depend on
anomaly exponents of various species. In addition to monotonous instability, known from
normal diffusion, in an anomalous system oscillatory modes emerge. For equal anomaly
exponents for both species the type of unstable modes is determined by the ratio of the reactants'
diffusion coefficients. When the ratio exceeds its normal critical value, the monotonous modes
become stable, whereas oscillatory instability persists until the anomalous critical value is
also exceeded. An exact formula for the anomalous critical value is obtained. It is shown
that in the short wave limit the growth rate is a power law of the wave number. When
the anomaly exponents differ, disturbance modes are governed by power laws of the distinct
exponents. If the difference between the diffusion anomaly exponents is small, the splitting
of the power law exponents is absent at the leading order and emerges only as a next-order
effect. In the short wave limit the onset of instability is governed by the anomaly exponents,
whereas the ratio of diffusion coefficients influences the complex growth rates. When the
exponent of the inhibitor is greater than that of the activator, the system is always unstable
due to the inhibitor enhanced diffusion relatively to the activator. If the exponent of the
activator is greater, the system is always stable. Existence of oscillatory unstable modes is
also possible for waves of moderate length
Low-Dimensional Description of Pulses under the Action of Global Feedback Control
The influence of a global delayed feedback control which acts on a system governed by a
subcritical complex Ginzburg-Landau equation is considered. The method based on a
variational principle is applied for the derivation of low-dimensional evolution models.
In the framework of those models, one-pulse and two-pulse solutions are found, and their
linear stability analysis is carried out. The application of the finite-dimensional model
allows to reveal the existence of chaotic oscillatory regimes and regimes with
double-period and quadruple-period oscillations. The diagram of regimes resembles those
found in the damped-driven nonlinear Schrödinger equation. The obtained results are
compared with the results of direct numerical simulations of the original problem