23 research outputs found
Two-dimensional loosely and tightly bound solitons in optical lattices and inverted traps
We study the dynamics of nonlinear localized excitations (solitons) in
two-dimensional (2D) Bose-Einstein condensates (BECs) with repulsive
interactions, loaded into an optical lattice (OL), which is combined with an
external parabolic potential. First, we demonstrate analytically that a broad
(loosely bound, LB) soliton state, based on a 2D Bloch function near the edge
of the Brillouin zone (BZ), has a negative effective mass (while the mass of a
localized state is positive near the BZ center). The negative-mass soliton
cannot be held by the usual trap, but it is safely confined by an inverted
parabolic potential (anti-trap). Direct simulations demonstrate that the LB
solitons (including the ones with intrinsic vorticity) are stable and can
freely move on top of the OL. The frequency of elliptic motion of the
LB-soliton's center in the anti-trapping potential is very close to the
analytical prediction which treats the solition as a quasi-particle. In
addition, the LB soliton of the vortex type features real rotation around its
center. We also find an abrupt transition, which occurs with the increase of
the number of atoms, from the negative-mass LB states to tightly bound (TB)
solitons. An estimate demonstrates that, for the zero-vorticity states, the
transition occurs when the number of atoms attains a critical number N=10^3,
while for the vortex the transition takes place at N=5x10^3 atoms. The
positive-mass LB states constructed near the BZ center (including vortices) can
move freely too. The effects predicted for BECs also apply to optical spatial
solitons in bulk photonic crystals.Comment: 17 pages, 12 figure
Dark-in-Bright Solitons in Bose-Einstein Condensates with Attractive Interactions
We demonstrate a possibility to generate localized states in effectively
one-dimensional Bose-Einstein condensates with a negative scattering length in
the form of a dark soliton in the presence of an optical lattice (OL) and/or a
parabolic magnetic trap. We connect such structures with twisted localized
modes (TLMs) that were previously found in the discrete nonlinear
Schr{\"o}dinger equation. Families of these structures are found as functions
of the OL strength, tightness of the magnetic trap, and chemical potential, and
their stability regions are identified. Stable bound states of two TLMs are
also found. In the case when the TLMs are unstable, their evolution is
investigated by means of direct simulations, demonstrating that they transform
into large-amplitude fundamental solitons. An analytical approach is also
developed, showing that two or several fundamental solitons, with the phase
shift between adjacent ones, may form stable bound states, with
parameters quite close to those of the TLMs revealed by simulations. TLM
structures are found numerically and explained analytically also in the case
when the OL is absent, the condensate being confined only by the magnetic trap.Comment: 13 pages, 7 figures, New Journal of Physics (in press
Dynamics of positive- and negative-mass solitons in optical lattices and inverted traps
We study the dynamics of one-dimensional solitons in the attractive and
repulsive Bose-Einstein condensates (BECs) loaded into an optical lattice (OL),
which is combined with an external parabolic potential. First, we demonstrate
analytically that, in the repulsive BEC, where the soliton is of the gap type,
its effective mass is \emph{negative}. This gives rise to a prediction for the
experiment: such a soliton cannot be not held by the usual parabolic trap, but
it can be captured (performing harmonic oscillations) by an anti-trapping
inverted parabolic potential. We also study the motion of the soliton a in long
system, concluding that, in the cases of both the positive and negative mass,
it moves freely, provided that its amplitude is below a certain critical value;
above it, the soliton's velocity decreases due to the interaction with the OL.
At a late stage, the damped motion becomes chaotic. We also investigate the
evolution of a two-soliton pulse in the attractive model. The pulse generates a
persistent breather, if its amplitude is not too large; otherwise, fusion into
a single fundamental soliton takes place. Collisions between two solitons
captured in the parabolic trap or anti-trap are considered too. Depending on
their amplitudes and phase difference, the solitons either perform stable
oscillations, colliding indefinitely many times, or merge into a single
soliton. Effects reported in this work for BECs can also be formulated for
optical solitons in nonlinear photonic crystals. In particular, the capture of
the negative-mass soliton in the anti-trap implies that a bright optical
soliton in a self-defocusing medium with a periodic structure of the refractive
index may be stable in an anti-waveguide.Comment: 22pages, 9 figures, submitted to Journal of Physics
Community- and population-level changes in diatom size structure in a subarctic lake over the last two centuries
Controls on OpenâOcean North Atlantic ÎpCO2 at Seasonal and Interannual Time Scales Are Different
The North Atlantic is a substantial sink for anthropogenic CO2. Understanding the mechanisms driving the sink's variability is key to assessing its current state and predicting its potential response to global climate change. Here we apply a time series decomposition technique to satellite and in situ data to examine separately the factors (both biological and nonbiological) that affect the seaâair CO2 difference (ÎpCO2) on seasonal and interannual time scales. We demonstrate that on seasonal time scales, the subpolar North Atlantic ÎpCO2 signal is predominantly correlated with biological processes, whereas seawater temperature dominates in the subtropics. However, the same factors do not necessarily control ÎpCO2 on interannual time scales. Our results imply that the mechanisms driving seasonal variability in ÎpCO2 cannot necessarily be extrapolated to predict how ÎpCO2, and thus the North Atlantic CO2 sink, may respond to increases in anthropogenic CO2 over longer time scales
Methane fluxes in marine sediments quantified through core analyses and seismo-acoustic mapping (Bornholm Basin, Baltic Sea)
Methane is a terminal product of anaerobic degradation of organic matter in subsurface marine sediments depleted of reactive oxidants. The depth and age of the sediment where sulfate is depleted determines the extent of methane production relative to the burial of organic carbon. We aimed to understand how this methane production is controlled and distributed in an apparently uniform sediment basin. We combined seismo-acoustic surveys and geochemical analyses of sediment cores to explore the distribution of methane fluxes in brackish-marine mud deposits of the Bornholm Basin, southern Baltic Sea. Geophysical mapping revealed the depth distribution of (a) the thickness of the Holocene organic-rich mud layer overlaying organic-poor Postglacial clay, and (b) the upper boundary of methane gas bubbles trapped in the Holocene sediment. By correlating these two parameters with the methane distributions in sediment cores from 44 stations we developed algorithms to estimate and map, at high spatial resolution, the diffusive methane fluxes in the 75-95-m deep and >4000 km(2) large Bornholm Basin. The two approaches, termed the FGD (Free Gas Depth) model and the HML (Holocene Mud Layer) model, yielded similar budgets for the total upwards methane flux through the sediment column in the Bornholm Basin, about 17 ton methane C day(-1) . Complete sulfate depletion at depth, and thus onset of methane production, required a minimum threshold thickness of the Holocene mud layer of 4 m. Although the Bornholm Basin has an even depth contour and uniform surface sediments, methane production was strongly focused in hotspots where the HML has greatest thickness. This heterogeneity could not be predicted from bathymetry or from properties of the surface sediment but was related to the topography of the glacial landscape buried underneath the Holocene mud blanket. This demonstrates the importance of including seismo-acoustic mapping of subsurface stratigraphy for the interpretation and geographic extrapolation of sediment core data for biogeochemical processes such as methane production and methane flux. The two approaches, which were here combined for the first time, may thereby be applied to map methane production also in other coastal and shelf sediments with shallow gas and distinct Holocene deposits. (C) 2018 Elsevier Ltd. All rights reserved
MultiSimplex optimisation and comparison of different purge-and-trap extractions of volatile organic compounds in soil samples
Nuclear moments of neon isotopes in the range from Ne at the proton drip line to the neutron-rich Ne
Nuclear moments of odd-A neon isotopes in the range 17 < A < 25 have been determined from optical hyperfine structures measured by collinear fast-beam laser spectroscopy. The magnetic dipole moments of 17Ne, 23Ne and 25Ne, as well as the electric quadrupole moment of 23Ne are either reported for the first time or improved considerably. The measurements also decide for a 1/2+ ground state of 25Ne. The behavior of the magnetic moments of the proton drip-line nucleus 17Ne and its mirror partner 17N suggests isospin symmetry. Thus, no clear indication of an anomalous nuclear structure is found for 17Ne. The magnetic moments of the investigated nuclei are discussed in a shell-model approach