750 research outputs found
Long-range interacting many-body systems with alkaline-earth-metal atoms
Alkaline-earth-metal atoms exhibit long-range dipolar interactions, which are
generated via the coherent exchange of photons on the 3P_0-3D_1-transition of
the triplet manifold. In case of bosonic strontium, which we discuss here, this
transition has a wavelength of 2.7 \mu m and a dipole moment of 2.46 Debye, and
there exists a magic wavelength permitting the creation of optical lattices
that are identical for the states 3P_0 and 3D_1. This interaction enables the
realization and study of mixtures of hard-core lattice bosons featuring
long-range hopping, with tuneable disorder and anisotropy. We derive the
many-body Master equation, investigate the dynamics of excitation transport and
analyze spectroscopic signatures stemming from coherent long-range interactions
and collective dissipation. Our results show that lattice gases of
alkaline-earth-metal atoms permit the creation of long-lived collective atomic
states and constitute a simple and versatile platform for the exploration of
many-body systems with long-range interactions. As such, they represent an
alternative to current related efforts employing Rydberg gases, atoms with
large magnetic moment, or polar molecules
Interactive feature space extension for multidimensional data projection
Projecting multi-dimensional data to a lower-dimensional visual display is a commonly used approach for identifying and analyzing patterns in data. Many dimensionality reduction techniques exist for generating visual embeddings, but it is often hard to avoid cluttered projections when the data is large in size and noisy. For many application users who are not machine learning experts, it is difficult to control the process in order to improve the “readability” of the projection and at the same time to understand their quality. In this paper, we propose a simple interactive feature transformation approach that allows the analyst to de-clutter the visualization by gradually transforming the original feature space based on existing class knowledge. By changing a single parameter, the user can easily decide the desired trade-off between structural preservation and the visual quality during the transforming process. The proposed approach integrates semi-interactive feature transformation techniques as well as a variety of quality measures to help analysts generate uncluttered projections and understand their quality
Attributing Tropical Cyclogenesis to Equatorial Waves in the Western North Pacific
The direct influences of equatorial waves on the genesis of tropical cyclones are evaluated. Tropical cyclogenesis is attributed to an equatorial wave when the filtered rainfall anomaly exceeds a threshold value at the genesis location. For an attribution threshold of 3 mm/day, 51% of warm season western North Pacific tropical cyclones are attributed to tropical depression (TD)-type disturbances, 29% to equatorial Rossby waves, 26% to mixed Rossby-Gravity waves, 23% to Kelvin waves, 13% to the Madden-Julian oscillation (MJO), and 19% are not attributed to any equatorial wave. The fraction of tropical cyclones attributed to TD-type disturbances is consistent with previous findings. Past studies have also demonstrated that the MJO significantly modulates tropical cyclogenesis, but fewer storms are attributed to the MJO than any other wave type. This disparity arises from the difference between attribution and modulation. The MJO produces broad regions of favorable conditions for cyclogenesis, but the MJO alone might not determine when and where a storm will develop within these regions. Tropical cyclones contribute less than 17% of the power in any portion of the equatorial wave spectrum because tropical cyclones are relatively uncommon equatorward of 15deg latitude. In regions where they are active, however, tropical cyclones can contribute more than 20% of the warm season rainfall and up to 50% of the total variance. Tropical cyclone-related anomalies can significantly contaminate wave-filtered precipitation at the location of genesis. To mitigate this effect, the tropical cyclone-related rainfall anomalies were removed before filtering in this study
Re-entrant superconductivity in Nb/Cu(1-x)Ni(x) bilayers
We report on the first observation of a pronounced re-entrant
superconductivity phenomenon in superconductor/ferromagnetic layered systems.
The results were obtained using a superconductor/ferromagnetic-alloy bilayer of
Nb/Cu(1-x)Ni(x). The superconducting transition temperature T_{c} drops sharply
with increasing thickness d_{CuNi} of the ferromagnetic layer, until complete
suppression of superconductivity is observed at d_{CuNi}= 4 nm. Increasing the
Cu(1-x)Ni(x) layer thickness further, superconductivity reappears at
d_{CuNi}=13 nm. Our experiments give evidence for the pairing function
oscillations associated with a realization of the quasi-one dimensional
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state in the ferromagnetic layer.Comment: 3 pages, 3 figures, REVTEX4/twocolum
How does an interacting many-body system tunnel through a potential barrier to open space?
The tunneling process in a many-body system is a phenomenon which lies at the
very heart of quantum mechanics. It appears in nature in the form of
alpha-decay, fusion and fission in nuclear physics, photoassociation and
photodissociation in biology and chemistry. A detailed theoretical description
of the decay process in these systems is a very cumbersome problem, either
because of very complicated or even unknown interparticle interactions or due
to a large number of constitutent particles. In this work, we theoretically
study the phenomenon of quantum many-body tunneling in a more transparent and
controllable physical system, in an ultracold atomic gas. We analyze a full,
numerically exact many-body solution of the Schr\"odinger equation of a
one-dimensional system with repulsive interactions tunneling to open space. We
show how the emitted particles dissociate or fragment from the trapped and
coherent source of bosons: the overall many-particle decay process is a quantum
interference of single-particle tunneling processes emerging from sources with
different particle numbers taking place simultaneously. The close relation to
atom lasers and ionization processes allows us to unveil the great relevance of
many-body correlations between the emitted and trapped fractions of the
wavefunction in the respective processes.Comment: 18 pages, 4 figures (7 pages, 2 figures supplementary information
Differential p38-dependent signalling in response to cellular stress and mitogenic stimulation in fibroblasts
p38 MAP kinase is known to be activated by cellular stress finally leading to cell cycle arrest or apoptosis. Furthermore, a tumour suppressor role of p38 MAPK has been proposed. In contrast, a requirement of p38 for proliferation has also been described. To clarify this paradox, we investigated stress- and mitogen-induced p38 signalling in the same cell type using fibroblasts. We demonstrate that - in the same cell line - p38 is activated by mitogens or cellular stress, but p38-dependent signalling is different. Exposure to cellular stress, such as anisomycin, leads to a strong and persistent p38 activation independent of GTPases. As a result, MK2 and downstream the transcription factor CREB are phosphorylated. In contrast, mitogenic stimulation results in a weaker and transient p38 activation, which upstream involves small GTPases and is required for cyclin D1 induction. Consequently, the retinoblastoma protein is phosphorylated and allows G1/S transition. Our data suggest a dual role of p38 and indicate that the level and/or duration of p38 activation determines the cellular response, i.e either proliferation or cell cycle arrest
Superfluidity in the interior-gap states
We investigate superfluidity in the interior-gap states proposed by Liu and
Wilczek. At weak coupling, we find the {\em gapless} interior-gap state
unstable in physically accessible regimes of the parameter space, where the
superfluid density is shown to be always negative. We therefore conclude that
the spatially-uniform interior-gap phase is extremely unstable unless it is
fully gapped; in this case, however, the state is rather similar to
conventional BCS states.Comment: To appear in Physical Review
Localization of Narrowband Single Photon Emitters in Nanodiamonds
© 2016 American Chemical Society. Diamond nanocrystals that host room temperature narrowband single photon emitters are highly sought after for applications in nanophotonics and bioimaging. However, current understanding of the origin of these emitters is extremely limited. In this work, we demonstrate that the narrowband emitters are point defects localized at extended morphological defects in individual nanodiamonds. In particular, we show that nanocrystals with defects such as twin boundaries and secondary nucleation sites exhibit narrowband emission that is absent from pristine individual nanocrystals grown under the same conditions. Critically, we prove that the narrowband emission lines vanish when extended defects are removed deterministically using highly localized electron beam induced etching. Our results enhance the current understanding of single photon emitters in diamond and are directly relevant to fabrication of novel quantum optics devices and sensors
One-dimensional non-interacting fermions in harmonic confinement: equilibrium and dynamical properties
We consider a system of one-dimensional non-interacting fermions in external
harmonic confinement. Using an efficient Green's function method we evaluate
the exact profiles and the pair correlation function, showing a direct
signature of the Fermi statistics and of the single quantum-level occupancy. We
also study the dynamical properties of the gas, obtaining the spectrum both in
the collisionless and in the collisional regime. Our results apply as well to
describe a one-dimensional Bose gas with point-like hard-core interactions.Comment: 11 pages, 5 figure
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