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 $\pi$ 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

Controls on Open‐Ocean North Atlantic ΔpCO2 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 sea‐air CO2 difference (ΔpCO2) on seasonal and interannual time scales. We demonstrate that on seasonal time scales, the subpolar North Atlantic ΔpCO2 signal is predominantly correlated with biological processes, whereas seawater temperature dominates in the subtropics. However, the same factors do not necessarily control ΔpCO2 on interannual time scales. Our results imply that the mechanisms driving seasonal variability in ΔpCO2 cannot necessarily be extrapolated to predict how ΔpCO2, 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

Nuclear moments of neon isotopes in the range from 17^17Ne at the proton drip line to the neutron-rich 25^25Ne

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