161 research outputs found
Selective Transport and Mobility Edges in Quasi-1D Systems with a Stratified Correlated Disorder
We present analytical results on transport properties of many-mode waveguides
with randomly stratified disorder having long-range correlations. To describe
such systems, the theory of 1D transport recently developed for a correlated
disorder is generalized. The propagation of waves through such waveguides may
reveal a quite unexpected phenomena of a complete transparency for a subset of
propagating modes. We found that with a proper choice of long-range
correlations one can arrange a perfect transparency of waveguides inside a
given frequency window of incoming waves. Thus, mobility edges are shown to be
possible in quasi-1D geometry with correlated disorder. The results may be
important for experimental realizations of a selective transport in application
to both waveguides and electron/optic nanodevices.Comment: RevTex, 4 pages, no figure
Spectrum of an open disordered quasi-two-dimensional electron system: strong orbital effect of the weak in-plane magnetic field
The effect of an in-plane magnetic field upon open quasi-two-dimensional
electron and hole systems is investigated in terms of the carrier ground-state
spectrum. The magnetic field, classified as weak from the viewpoint of
correlation between size parameters of classical electron motion and the gate
potential spatial profile is shown to efficiently cut off extended modes from
the spectrum and to change singularly the mode density of states (MDOS). The
reduction in the number of current-carrying modes, right up to zero in magnetic
fields of moderate strength, can be viewed as the cause of
magnetic-field-driven metal-to-insulator transition widely observed in
two-dimensional systems. Both the mode number reduction and the MDOS
singularity appear to be most pronounced in the mode states dephasing
associated with their scattering by quenched-disorder potential. This sort of
dephasing is proven to dominate the dephasing which involves solely the
magnetic field whatever level of the disorder.Comment: RevTeX-4 class, 12 pages, 5 eps figure
Vortices in condensate mixtures
In a condensate made of two different atomic molecular species, Onsager's
quantization condition implies that around a vortex the velocity field cannot
be the same for the two species. We explore some simple consequences of this
observation. Thus if the two condensates are in slow relative translation one
over the other, the composite vortices are carried at a velocity that is a
fraction of the single species velocity. This property is valid for attractive
interaction and below a critical velocity which corresponds to a saddle-node
bifurcation.Comment: 4 pages, 3 figure
Lagrangian tracers on a surface flow: the role of time correlations
Finite time correlations of the velocity in a surface flow are found to be
important for the formation of clusters of Lagrangian tracers. The degree of
clustering characterized by the Lyapunov spectrum of the flow is numerically
shown to be in qualitative agreement with the predictions for the white-in-time
compressible Kraichnan flow, but to deviate quantitatively. For intermediate
values of compressibility the clustering is surprisingly weakened by time
correlations.Comment: 4 pages, 5 figures, to be published in PR
Spatial Pattern Formation in External Noise: Theory and Simulation
Spatial pattern formation in excitable fluctuating media was researched
analytically from the point of view of the order parameters concept. The
reaction-diffusion system in external noise is considered as a model of such
medium. Stochastic equations for the unstable mode amplitudes (order
parameters), dispersion equations for the unstable mode averaged amplitudes,
and the Fokker-Planck equation for the order parameters have been obtained. The
developed theory makes it possible to analyze different noise-induced effects,
including the variation of boundaries of ordering and disordering phase
transitions depending on the parameters of external noiseComment: 22 pages, 10 figure
Reflection coefficient and localization length of waves in one-dimensional random media
We develop a novel and powerful method of exactly calculating various
transport characteristics of waves in one-dimensional random media with (or
without) coherent absorption or amplification. Using the method, we compute the
probability densities of the reflectance and of the phase of the reflection
coefficient, together with the localization length, of electromagnetic waves in
sufficiently long random dielectric media. We find substantial differences
between our exact results and the previous results obtained using the random
phase approximation (RPA). The probabilty density of the phase of the
reflection coefficient is highly nonuniform when either disorder or absorption
(or amplification) is strong. The probability density of the reflectance when
the absorption or amplification parameter is large is also quite different from
the RPA result. We prove that the probability densities in the amplifying case
are related to those in the absorbing case with the same magnitude of the
imaginary part of the dielectric permeability by exact dual relationships. From
the analysis of the average reflectance that shows a nonmonotonic dependence on
the absorption or amplification parameter, we obtain a useful criterion for the
applicability of the RPA. In the parameter regime where the RPA is invalid, we
find the exact localization length is substantially larger than the RPA
localization length.Comment: 16 pages, 9 figure
Condensation and vortex formation in Bose-gas upon cooling
The mechanism for the transition of a Bose gas to the superfluid state via
thermal fluctuations is considered. It is shown that in the process of external
cooling some critical fluctuations (instantons) are formed above the critical
temperature. The probability of the instanton formation is calculated in the
three and two-dimensional cases. It is found that this probability increases as
the system approaches the transition temperature. It is shown that the
evolution of an individual instanton is impossible without the formation of
vortices in its superfluid part
High-frequency dynamics of wave localisation
We study the effect of localisation on the propagation of a pulse through a
multi-mode disordered waveguide. The correlator of the
transmitted wave amplitude u at two frequencies differing by delta_omega has
for large delta_omega the stretched exponential tail ~exp(-sqrt{tau_D
delta_omega/2}). The time constant tau_D=L^2/D is given by the diffusion
coefficient D, even if the length L of the waveguide is much greater than the
localisation length xi. Localisation has the effect of multiplying the
correlator by a frequency-independent factor exp(-L/2xi), which disappears upon
breaking time-reversal symmetry.Comment: 3 pages, 1 figur
Generation of small-scale structures in the developed turbulence
The Navier-Stokes equation for incompressible liquid is considered in the
limit of infinitely large Reynolds number. It is assumed that the flow
instability leads to generation of steady-state large-scale pulsations. The
excitation and evolution of the small-scale turbulence is investigated. It is
shown that the developed small-scale pulsations are intermittent. The maximal
amplitude of the vorticity fluctuations is reached along the vortex filaments.
Basing on the obtained solution, the pair correlation function in the limit
is calculated. It is shown that the function obeys the Kolmogorov law
.Comment: 18 page
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