163 research outputs found
Bright gap solitons of atoms with repulsive interaction
We report on the first experimental observation of bright matter-wave
solitons for 87Rb atoms with repulsive atom-atom interaction. This counter
intuitive situation arises inside a weak periodic potential, where anomalous
dispersion can be realized at the Brillouin zone boundary. If the coherent
atomic wavepacket is prepared at the corresponding band edge a bright soliton
is formed inside the gap. The strength of our system is the precise control of
preparation and real time manipulation, allowing the systematic investigation
of gap solitons.Comment: 4 pages, 4 figure
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
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
Observation of bright polariton solitons in a semiconductor microcavity
Microcavity polaritons are composite half-light half-matter quasi-particles,
which have recently been demonstrated to exhibit rich physical properties, such
as non-equilibrium Bose-Einstein condensation, parametric scattering and
superfluidity. At the same time, polaritons have some important advantages over
photons for information processing applications, since their excitonic
component leads to weaker diffraction and stronger inter-particle interactions,
implying, respectively, tighter localization and lower powers for nonlinear
functionality. Here we present the first experimental observations of bright
polariton solitons in a strongly coupled semiconductor microcavity. The
polariton solitons are shown to be non-diffracting high density wavepackets,
that are strongly localised in real space with a corresponding broad spectrum
in momentum space. Unlike solitons known in other matter-wave systems such as
Bose condensed ultracold atomic gases, they are non-equilibrium and rely on a
balance between losses and external pumping. Microcavity polariton solitons are
excited on picosecond timescales, and thus have significant benefits for
ultrafast switching and transfer of information over their light only
counterparts, semiconductor cavity lasers (VCSELs), which have only nanosecond
response time
Collisionally inhomogeneous Bose-Einstein condensates in double-well potentials
In this work, we consider quasi-one-dimensional Bose-Einstein condensates
(BECs), with spatially varying collisional interactions, trapped in double well
potentials. In particular, we study a setup in which such a 'collisionally
inhomogeneous' BEC has the same (attractive-attractive or repulsive-repulsive)
or different (attractive-repulsive) type of interparticle interactions. Our
analysis is based on the continuation of the symmetric ground state and
anti-symmetric first excited state of the noninteracting (linear) limit into
their nonlinear counterparts. The collisional inhomogeneity produces a
saddle-node bifurcation scenario between two additional solution branches; as
the inhomogeneity becomes stronger, the turning point of the saddle-node tends
to infinity and eventually only the two original branches remain present, which
is completely different from the standard double-well phenomenology. Finally,
one of these branches changes its monotonicity as a function of the chemical
potential, a feature especially prominent, when the sign of the nonlinearity
changes between the two wells. Our theoretical predictions, are in excellent
agreement with the numerical results.Comment: 14 pages, 12 figures, Physica D, in pres
Relationship between effects on time-to-disease progression and overall survival in studies of metastatic breast cancer
The relationship between overall survival (OS) and disease progression end points has been demonstrated in colorectal, colon, and non-small cell lung cancers. We assessed the association between OS and time-to-progression (TTP) or progression-free survival (PFS) in metastatic breast cancer (MBC) studies. A literature search retrieved all randomised controlled trials since 1994 in patients with MBC in which OS and either TTP or PFS were reported. Summary data on trial and patient characteristics were abstracted. Study effect sizes were derived as the ratio of median progression (or survival) times, which approximates the hazard ratio. Effects were centred at zero for regression analyses weighted by sample size. Numerous treatments were represented in 67 studies (17 081 patients). Modeling showed a positive association between outcomes for progression and survival (R2=0.30) with a slope of 0.32 (P<0.001) and a non-significant intercept. Thus, a treatment effect on TTP/PFS translated into a concordant effect on OS, but with attenuated effect size. Similar results were found in models of subsets and sensitivity analyses. These results demonstrate that treatment effects on progression end points in MBC trials are expected to result in treatment differences on OS that are smaller yet consistently in the same direction
Selective Small-Molecule Agonists of G Protein–Coupled Receptor 40 Promote Glucose-Dependent Insulin Secretion and Reduce Blood Glucose in Mice
OBJECTIVE— Acute activation of G protein–coupled receptor 40 (GPR40) by free fatty acids (FFAs) or synthetic GPR40 agonists enhances insulin secretion. However, it is still a matter of debate whether activation of GPR40 would be beneficial for the treatment of type 2 diabetes, since chronic exposure to FFAs impairs islet function. We sought to evaluate the specific role of GPR40 in islets and its potential as a therapeutic target using compounds that specifically activate GPR40
Evaluating the impact of handling and logger attachment on foraging parameters and physiology in southern rockhopper penguins
Logger technology has revolutionised our knowledge of the behaviour and physiology of free-living animals but handling and logger attachments may have negative effects on the behaviour of the animals and their welfare. We studied southern rockhopper penguin ( Eudyptes chrysocome ) females during the guard stage in three consecutive breeding seasons (2008/09−2010/11) to evaluate the effects of handling and logger attachment on foraging trip duration, dive behaviour and physiological parameters. Smaller dive loggers (TDRs) were used in 2010/11 for comparison to larger GPS data loggers used in all three seasons and we included two categories of control birds: handled controls and PIT control birds that were previously marked with passive integrative transponders (PITs), but which had not been handled during this study. Increased foraging trip duration was only observed in GPS birds during 2010/11, the breeding season in which we also found GPS birds foraging further away from the colony and travelling longer distances. Compared to previous breeding seasons, 2010/11 may have been a period with less favourable environmental conditions, which would enhance the impact of logger attachments. A comparison between GPS and TDR birds showed a significant difference in dive depth frequencies with birds carrying larger GPS data loggers diving shallower. Mean and maximum dive depths were similar between GPS and TDR birds. We measured little impact of logger attachments on physiological parameters (corticosterone, protein, triglyceride levels and leucocyte counts). Overall, handling and short-term logger attachments (1-3 days) showed limited impact on the behaviour and physiology of the birds but care must be taken with the size of data loggers on diving seabirds. Increased drag may alter their diving behaviour substantially, thus constraining them in their ability to catch prey. Results obtained in this study indicate that data recorded may also not represent their normal dive behaviour
Unmanned aircraft systems as a new source of disturbance for wildlife: A systematic review.
The use of small Unmanned Aircraft Systems (UAS; also known as "drones") for professional and personal-leisure use is increasing enormously. UAS operate at low altitudes (<500 m) and in any terrain, thus they are susceptible to interact with local fauna, generating a new type of anthropogenic disturbance that has not been systematically evaluated. To address this gap, we performed a review of the existent literature about animals' responses to UAS flights and conducted a pooled analysis of the data to determine the probability and intensity of the disturbance, and to identify the factors influencing animals' reactions towards the small aircraft. We found that wildlife reactions depended on both the UAS attributes (flight pattern, engine type and size of aircraft) and the characteristics of animals themselves (type of animal, life-history stage and level of aggregation). Target-oriented flight patterns, larger UAS sizes, and fuel-powered (noisier) engines evoked the strongest reactions in wildlife. Animals during the non-breeding period and in large groups were more likely to show behavioral reactions to UAS, and birds are more prone to react than other taxa. We discuss the implications of these results in the context of wildlife disturbance and suggest guidelines for conservationists, users and manufacturers to minimize the impact of UAS. In addition, we propose that the legal framework needs to be adapted so that appropriate actions can be undertaken when wildlife is negatively affected by these emergent practices
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