2,634 research outputs found
Orbital Magnetism in Two-dimensional Integrable Systems
We study orbital magnetism of a degenerate electron gas in a number of
two-dimensional integrable systems, within linear response theory. There are
three relevant energy scales: typical level spacing, the energy related to the
inverse time of flight across the system, and the Fermi energy.
Correspondingly, there are three distinct temperature regimes: microscopic,
mesoscopic, and macroscopic. In the first two regimes there are large
finite-size effects in the magnetic susceptibility, whereas in the third regime
the susceptibility approaches its macroscopic value. In some cases, such as a
quasi-one-dimensional strip or a harmonic confining potential, it is possible
to obtain analytic expressions for the susceptibility in the entire temperature
range.Comment: 28 pages, Latex, 4 Postscript figure
Effect of Multiple Scattering on the Critical Electric Field for Runaway Electrons in the Atmosphere
A simple method for taking into account the multiple Coulomb scattering in
construction of a separatrix (the line separating the regions of runaway and
decelerating electrons in an electric field) is described. The desired line is
obtained by solving a simple transcendental equation.Comment: 3 pages, 2 figure
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
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
On the temperature dependence of ballistic Coulomb drag in nanowires
We have investigated within the theory of Fermi liquid dependence of Coulomb
drag current in a passive quantum wire on the applied voltage across an
active wire and on the temperature for any values of . We assume
that the bottoms of the 1D minibands in both wires almost coincide with the
Fermi level. We come to conclusions that 1) within a certain temperature
interval the drag current can be a descending function of the temperature ;
2) the experimentally observed temperature dependence of the drag
current can be interpreted within the framework of Fermi liquid theory; 3) at
relatively high applied voltages the drag current as a function of the applied
voltage saturates; 4) the screening of the electron potential by metallic gate
electrodes can be of importance.Comment: 7 pages, 1 figur
Numerical study of a multiscale expansion of the Korteweg de Vries equation and Painlev\'e-II equation
The Cauchy problem for the Korteweg de Vries (KdV) equation with small
dispersion of order \e^2, \e\ll 1, is characterized by the appearance of a
zone of rapid modulated oscillations. These oscillations are approximately
described by the elliptic solution of KdV where the amplitude, wave-number and
frequency are not constant but evolve according to the Whitham equations.
Whereas the difference between the KdV and the asymptotic solution decreases as
in the interior of the Whitham oscillatory zone, it is known to be
only of order near the leading edge of this zone. To obtain a
more accurate description near the leading edge of the oscillatory zone we
present a multiscale expansion of the solution of KdV in terms of the
Hastings-McLeod solution of the Painlev\'e-II equation. We show numerically
that the resulting multiscale solution approximates the KdV solution, in the
small dispersion limit, to the order .Comment: 20 pages, 14 figure
Whitham systems and deformations
We consider the deformations of Whitham systems including the "dispersion
terms" and having the form of Dubrovin-Zhang deformations of Frobenius
manifolds. The procedure is connected with B.A. Dubrovin problem of
deformations of Frobenius manifolds corresponding to the Whitham systems of
integrable hierarchies. Under some non-degeneracy requirements we suggest a
general scheme of the deformation of the hyperbolic Whitham systems using the
initial non-linear system. The general form of the deformed Whitham system
coincides with the form of the "low-dispersion" asymptotic expansions used by
B.A. Dubrovin and Y. Zhang in the theory of deformations of Frobenius
manifolds.Comment: 27 pages, Late
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