662 research outputs found
Analytic model for a frictional shallow-water undular bore
We use the integrable Kaup-Boussinesq shallow water system, modified by a
small viscous term, to model the formation of an undular bore with a steady
profile. The description is made in terms of the corresponding integrable
Whitham system, also appropriately modified by friction. This is derived in
Riemann variables using a modified finite-gap integration technique for the
AKNS scheme. The Whitham system is then reduced to a simple first-order
differential equation which is integrated numerically to obtain an asymptotic
profile of the undular bore, with the local oscillatory structure described by
the periodic solution of the unperturbed Kaup-Boussinesq system. This solution
of the Whitham equations is shown to be consistent with certain jump conditions
following directly from conservation laws for the original system. A comparison
is made with the recently studied dissipationless case for the same system,
where the undular bore is unsteady.Comment: 24 page
Formation of shock waves in a Bose-Einstein condensate
We consider propagation of density wave packets in a Bose-Einstein
condensate. We show that the shape of initially broad, laser-induced, density
perturbation changes in the course of free time evolution so that a shock wave
front finally forms. Our results are well beyond predictions of commonly used
zero-amplitude approach, so they can be useful in extraction of a speed of
sound from experimental data. We discuss a simple experimental setup for shock
propagation and point out possible limitations of the mean-field approach for
description of shock phenomena in a BEC.Comment: 8 pages & 6 figures, minor changes, more references, to appear in
Phys. Rev.
The theory of optical dispersive shock waves in photorefractive media
The theory of optical dispersive shocks generated in propagation of light
beams through photorefractive media is developed. Full one-dimensional
analytical theory based on the Whitham modulation approach is given for the
simplest case of sharp step-like initial discontinuity in a beam with
one-dimensional strip-like geometry. This approach is confirmed by numerical
simulations which are extended also to beams with cylindrical symmetry. The
theory explains recent experiments where such dispersive shock waves have been
observed.Comment: 26 page
On dissipationless shock waves in a discrete nonlinear Schr\"odinger equation
It is shown that the generalized discrete nonlinear Schr\"odinger equation
can be reduced in a small amplitude approximation to the KdV, mKdV, KdV(2) or
the fifth-order KdV equations, depending on values of the parameters. In
dispersionless limit these equations lead to wave breaking phenomenon for
general enough initial conditions, and, after taking into account small
dispersion effects, result in formation of dissipationless shock waves. The
Whitham theory of modulations of nonlinear waves is used for analytical
description of such waves.Comment: 15 pages, 9 figure
A sol-gel method for growing superconducting MgB2 films
In this paper we report a new sol-gel method for the fabrication of MgB2
films. Polycrystalline MgB2 films were prepared by spin-coating a precursor
solution of Mg(BH_4)_2 diethyl ether on (001)Al2O3 substrates followed with
annealing in Mg vapor. In comparison with the MgB2 films grown by other
techniques, our films show medium qualities including a superconducting
transition temperature of Tc ~ 37 K, a critical current density of Jc(5 K, 0 T)
~ 5 {\times} 10^6 A cm^{-2}, and a critical field of H_{c2}(0) ~ 19 T. Such a
sol-gel technique shows potential in the commercial fabrication of practically
used MgB2 films as well as MgB2 wires and tapes.Comment: 8 pages, 5 figure
Differential-Algebraic Integrability Analysis of the Generalized Riemann Type and Korteweg-de Vries Hydrodynamical Equations
A differential-algebraic approach to studying the Lax type integrability of
the generalized Riemann type hydrodynamic equations at N = 3; 4 is devised. The
approach is also applied to studying the Lax type integrability of the well
known Korteweg-de Vries dynamical system.Comment: 11 page
Transient heat generation in a quantum dot under a step-like pulse bias
We study the transient heat generation in a quantum dot system driven by a
step-like or a square-shaped pulse bias. We find that a periodically
oscillating heat generation arises after adding the sudden bias. One
particularly surprising result is that there exists a heat absorption from the
zero-temperature phonon subsystem. Thus the phonon population in
non-equilibrium can be less than that of the equilibrium electron-phonon
system. In addition, we also ascertain the optimal conditions for the operation
of a quantum dot with the minimum heat generation.Comment: 6 pages, 4 figure
Influence of vortex-vortex interaction on critical currents across low-angle grain boundaries in YBa2Cu3O7-delta thin films
Low-angle grain boundaries with misorientation angles theta < 5 degrees in
optimally doped thin films of YBCO are investigated by magnetooptical imaging.
By using a numerical inversion scheme of Biot-Savart's law the critical current
density across the grain boundary can be determined with a spatial resolution
of about 5 micrometers. Detailed investigation of the spatially resolved flux
density and current density data shows that the current density across the
boundary varies with varying local flux density. Combining the corresponding
flux and current pattern it is found that there exists a universal dependency
of the grain boundary current on the local flux density. A change in the local
flux density means a variation in the flux line-flux line distance. With this
knowledge a model is developped that explains the flux-current relation by
means of magnetic vortex-vortex interaction.Comment: 7 pages, 14 figure
Evidence of spin-density-wave order in RFeAsO from measurements of thermoelectric power
Data on the magneto-thermopower and specific heat of three compounds
belonging to '1111' oxypnictides family are reported. One specimen
(SmAsFeO0.8F0.2) is a superconductor with Tc = 53 K, while two others (SmAsFeO
and NdAsFeO) are nonsuperconducting parent compounds. Our results confirm that
spin density wave (SDW) order is present in SmAsFeO and NdAsFeO. In these two
samples a strict connection between the thermoelectric power and electronic
specific heat is found in the vicinity of SDW transition, what indicates that
the chemical potential of charge carriers strongly depends on temperature in
this region. Low temperature data suggest presence of significant contribution
magnon-drag to the thermoelectric power.Comment: 14 pages, 5 figures; adjusted to referees' suggestions; to appear in
Phys. Rev.
Nonlinear diffraction of light beams propagating in photorefractive media with embedded reflecting wire
The theory of nonlinear diffraction of intensive light beams propagating
through photorefractive media is developed. Diffraction occurs on a reflecting
wire embedded in the nonlinear medium at relatively small angle with respect to
the direction of the beam propagation. It is shown that this process is
analogous to the generation of waves by a flow of a superfluid past an
obstacle. The ``equation of state'' of such a superfluid is determined by the
nonlinear properties of the medium. On the basis of this hydrodynamic analogy,
the notion of the ``Mach number'' is introduced where the transverse component
of the wave vector plays the role of the fluid velocity. It is found that the
Mach cone separates two regions of the diffraction pattern: inside the Mach
cone oblique dark solitons are generated and outside the Mach cone the region
of ``ship waves'' is situated. Analytical theory of ``ship waves'' is developed
and two-dimensional dark soliton solutions of the equation describing the beam
propagation are found. Stability of dark solitons with respect to their decay
into vortices is studied and it is shown that they are stable for large enough
values of the Mach number.Comment: 18 page
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