347 research outputs found
Dissipative Dynamics of Matter Wave Soliton in Nonlinear Optical Lattice
Dynamics and stability of solitons in two-dimensional (2D) Bose-Einstein
condensates (BEC), with low-dimensional (1D) conservative plus dissipative
nonlinear optical lattices are investigated. In the case of focusing media
(with attractive atomic systems) the collapse of the wave packet is arrested by
the dissipative periodic nonlinearity. The adiabatic variation of the
background scattering length leads to metastable matter-wave solitons.
When the atom feeding mechanism is used, a dissipative soliton can exist in
focusing 2D media with 1D periodic nonlinearity. In the defocusing media
(repulsive BEC case) with harmonic trap in one dimension and one dimensional
nonlinear optical lattice in other direction, the stable soliton can exist.
This prediction of variational approach is confirmed by the full numerical
simulation of 2D Gross-Pitaevskii equation.Comment: 9 pages, 8 figure
Adiabatic Compression of Soliton Matter Waves
The evolution of atomic solitary waves in Bose-Einstein condensate (BEC)
under adiabatic changes of the atomic scattering length is investigated. The
variations of amplitude, width, and velocity of soliton are found for both
spatial and time adiabatic variations. The possibility to use these variations
to compress solitons up to very high local matter densities is shown both in
absence and in presence of a parabolic confining potential.Comment: to appear in J.Phys.
Faraday waves in quasi-one-dimensional superfluid Fermi-Bose mixtures
Generation of Faraday waves in superfluid Fermi-Bose mixtures in elongated
traps is investigated. The generation of waves is achieved by periodically
changing a parameter of the system in time. Two types of modulations of
parameters are considered, first a variation of the fermion-bosons scattering
length, and secondly the boson-boson scattering length. We predict the
properties of the generated Faraday patterns and study the parameter regions
where they can be excited.Comment: Final published versio
Perturbation theory for localized solutions of sine-Gordon equation: decay of a breather and pinning by microresistor
We develop a perturbation theory that describes bound states of solitons
localized in a confined area. External forces and influence of inhomogeneities
are taken into account as perturbations to exact solutions of the sine-Gordon
equation. We have investigated two special cases of fluxon trapped by a
microresistor and decay of a breather under dissipation. Also, we have carried
out numerical simulations with dissipative sine-Gordon equation and made
comparison with the McLaughlin-Scott theory. Significant distinction between
the McLaughlin-Scott calculation for a breather decay and our numerical result
indicates that the history dependence of the breather evolution can not be
neglected even for small damping parameter
Nonlinear localized modes in dipolar Bose-Einstein condensates in optical lattices
The modulational instability and discrete matter wave solitons in dipolar
BEC, loaded into a deep optical lattice, are investigated analytically and
numerically. The process of modulational instability of nonlinear plane matter
waves in a dipolar nonlinear lattice is studied and the regions of instability
are established. The existence and stability of bulk discrete solitons are
analyzed analytically and confirmed by numerical simulations. In a marked
contrast with the usual DNLS behavior (no dipolar interactions), we found a
region where the two fundamental modes are simultaneously unstable allowing
enhanced mobility across the lattice for large norm values. To study the
existence and properties of surface discrete solitons, an analysis of the dimer
configuration is performed. The properties of symmetric and antisymmetric modes
including the stability diagrams and bifurcations are investigated in closed
form. For the case of a bulk medium, properties of fundamental on-site and
inter-site localized modes are analyzed. On-site and inter-site surface
localized modes are studied finding that they do not exist when nonlocal
interactions predominate with respect to local ones.Comment: 12 pages, 13 figure
Theory of Nonlinear Dispersive Waves and Selection of the Ground State
A theory of time dependent nonlinear dispersive equations of the Schroedinger
/ Gross-Pitaevskii and Hartree type is developed. The short, intermediate and
large time behavior is found, by deriving nonlinear Master equations (NLME),
governing the evolution of the mode powers, and by a novel multi-time scale
analysis of these equations. The scattering theory is developed and coherent
resonance phenomena and associated lifetimes are derived. Applications include
BEC large time dynamics and nonlinear optical systems. The theory reveals a
nonlinear transition phenomenon, ``selection of the ground state'', and NLME
predicts the decay of excited state, with half its energy transferred to the
ground state and half to radiation modes. Our results predict the recent
experimental observations of Mandelik et. al. in nonlinear optical waveguides
Travel time stability in weakly range-dependent sound channels
Travel time stability is investigated in environments consisting of a
range-independent background sound-speed profile on which a highly structured
range-dependent perturbation is superimposed. The stability of both
unconstrained and constrained (eigenray) travel times are considered. Both
general theoretical arguments and analytical estimates of time spreads suggest
that travel time stability is largely controlled by a property of the background sound speed profile. Here, is
the range of a ray double loop and is the ray action variable. Numerical
results for both volume scattering by internal waves in deep ocean environments
and rough surface scattering in upward refracting environments are shown to
confirm the expectation that travel time stability is largely controlled by
.Comment: Submitted to J. Acoust. Soc. Am., 30 June 200
Stabilization of bright solitons and vortex solitons in a trapless three-dimensional Bose-Einstein condensate by temporal modulation of the scattering length
Using variational and numerical solutions of the mean-field Gross-Pitaevskii
equation we show that a bright soliton can be stabilized in a trapless
three-dimensional attractive Bose-Einstein condensate (BEC) by a rapid periodic
temporal modulation of scattering length alone by using a Feshbach resonance.
This scheme also stabilizes a rotating vortex soliton in two dimensions. Apart
from possible experimental application in BEC, the present study suggests that
the spatiotemporal solitons of nonlinear optics in three dimensions can also be
stabilized in a layered Kerr medium with sign-changing nonlinearity along the
propagation direction.Comment: 6 pages, 7 PS figure
Dual role of DNA methylation inside and outside of CTCF-binding regions in the transcriptional regulation of the telomerase hTERT gene
Expression of hTERT is the major limiting factor for telomerase activity. We previously showed that methylation of the hTERT promoter is necessary for its transcription and that CTCF can repress hTERT transcription by binding to the first exon. In this study, we used electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) to show that CTCF does not bind the methylated first exon of hTERT. Treatment of telomerase-positive cells with 5-azadC led to a strong demethylation of hTERT 5âČ-regulatory region, reactivation of CTCF binding and downregulation of hTERT. Although complete hTERT promoter methylation was associated with full transcriptional repression, detailed mapping showed that, in telomerase-positive cells, not all the CpG sites were methylated, especially in the promoter region. Using a methylation cassette assay, selective demethylation of 110âbp within the core promoter significantly increased hTERT transcriptional activity. This study underlines the dual role of DNA methylation in hTERT transcriptional regulation. In our model, hTERT methylation prevents binding of the CTCF repressor, but partial hypomethylation of the core promoter is necessary for hTERT expression
Controlling collapse in Bose-Einstein condensates by temporal modulation of the scattering length
We consider, by means of the variational approximation (VA) and direct
numerical simulations of the Gross-Pitaevskii (GP) equation, the dynamics of 2D
and 3D condensates with a scattering length containing constant and
harmonically varying parts, which can be achieved with an ac magnetic field
tuned to the Feshbach resonance. For a rapid time modulation, we develop an
approach based on the direct averaging of the GP equation,without using the VA.
In the 2D case, both VA and direct simulations, as well as the averaging
method, reveal the existence of stable self-confined condensates without an
external trap, in agreement with qualitatively similar results recently
reported for spatial solitons in nonlinear optics. In the 3D case, the VA again
predicts the existence of a stable self-confined condensate without a trap. In
this case, direct simulations demonstrate that the stability is limited in
time, eventually switching into collapse, even though the constant part of the
scattering length is positive (but not too large). Thus a spatially uniform ac
magnetic field, resonantly tuned to control the scattering length, may play the
role of an effective trap confining the condensate, and sometimes causing its
collapse.Comment: 7 figure
- âŠ