858 research outputs found
Plant species roles in pollination networks: an experimental approach
Pollination is an important ecosystem service threatened by current pollinator declines, making flower planting schemes an important strategy to recover pollination function. However, ecologists rarely test the attractiveness of chosen plants to pollinators in the field. Here, we experimentally test whether plant species roles in pollination networks can be used to identify species with the most potential to recover plant–pollinator communities. Using published pollination networks, we calculated each plant's centrality and chose five central and five peripheral plant species for introduction into replicate experimental plots. Flower visitation by pollinators was recorded in each plot and we tested the impact of introduced central and peripheral plant species on the pollinator and resident plant communities and on network structure. We found that the introduction of central plant species attracted a higher richness and abundance of pollinators than the introduction of peripheral species, and that the introduced central plant species occupied the most important network roles. The high attractiveness of central species to pollinators, however, did not negatively affect visitation to resident plant species by pollinators. We also found that the introduction of central plant species did not affect network structure, while networks with introduced peripheral species had lower centralisation and interaction evenness than networks with introduced central species. To our knowledge, this is the first time species network roles have been tested in a field experiment. Given that most restoration projects start at the plant community, being able to identify the plants with the highest potential to restore community structure and functioning should be a key goal for ecological restoration
Mesoscopic Fermi gas in a harmonic trap
We study the thermodynamical properties of a mesoscopic Fermi gas in view of
recent possibilities to trap ultracold atoms in a harmonic potential. We focus
on the effects of shell closure for finite small atom numbers. The dependence
of the chemical potential, the specific heat and the density distribution on
particle number and temperature is obtained. Isotropic and anisotropic traps
are compared. Possibilities of experimental observations are discussed.Comment: 8 pages, 9 eps-figures included, Revtex, submitted to Phys. Rev. A,
minor changes to figures and captions, corrected typo
Multiple-membership multiple-classification models for social network and group dependences
The social network literature on network dependences has largely ignored other sources of dependence, such as the school that a student attends, or the area in which an individual lives. The multilevel modelling literature on school and area dependences has, in turn, largely ignored social networks. To bridge this divide, a multiple-membership multiple-classification modelling approach for jointly investigating social network and group dependences is presented. This allows social network and group dependences on individual responses to be investigated and compared. The approach is used to analyse a subsample of the Adolescent Health Study data set from the USA, where the response variable of interest is individual level educational attainment, and the three individual level covariates are sex, ethnic group and age. Individual, network, school and area dependences are accounted for in the analysis. The network dependences can be accounted for by including the network as a classification in the model, using various network configurations, such as ego-nets and cliques. The results suggest that ignoring the network affects the estimates of variation for the classifications that are included in the random part of the model (school, area and individual), as well as having some influence on the point estimates and standard errors of the estimates of regression coefficients for covariates in the fixed part of the model. From a substantive perspective, this approach provides a flexible and practical way of investigating variation in an individual level response due to social network dependences, and estimating the share of variation of an individual response for network, school and area classifications
Single-particle excitations and the order parameter for a trapped superfluid Fermi gas
We reveal a strong influence of a superfluid phase transition on the
character of single-particle excitations of a trapped neutral-atom Fermi gas.
Below the transition temperature the presence of a spatially inhomogeneous
order parameter (gap) shifts up the excitation eigenenergies and leads to the
appearance of in-gap excitations localized in the outer part of the gas sample.
The eigenenergies become sensitive to the gas temperature and are no longer
multiples of the trap frequencies. These features should manifest themselves in
a strong change of the density oscillations induced by modulations of the trap
frequencies and can be used for identifying the superfluid phase transition.Comment: 5 pages, RevTeX, 2 eps figure
Subtle temperature-induced changes in small molecule conformer dynamics-observed and quantified by NOE spectroscopy
NOE-distance relationships are shown to be sufficiently accurate to monitor very small changes in conformer populations in response to temperature (<0.5%/10 degrees C) - in good agreement with Boltzmann-predictions, illustrating the effectiveness of accurate NOE-distance measurements in obtaining high quality dynamics as well as structural information for small molecules
Airports at Risk: The Impact of Information Sources on Security Decisions
Security decisions in high risk organizations such as airports involve obtaining ongoing and frequent information about potential threats. Utilizing questionnaire survey data from a sample of airport
employees in European Airports across the continent, we analyzed
how both formal and informal sources of security information affect employee's decisions to comply with the security rules and
directives. This led us to trace information network flows to assess its impact on the degree employees making security decisions comply or deviate with the prescribed security rules. The results of the multivariate analysis showed that security information obtained through formal and informal networks differentially determine if employee will comply or not with the rules. Information sources emanating from the informal network tends to encourage employees to be more flexible in their security decisions
while formal sources lead to be more rigid with complying with rules and protocols. These results suggest that alongside the formal administrative structure of airports, there exists a diverse and pervasiveness set of informal communications networks that are a potent factor in determining airport security levels
Optical linewidth of a low density Fermi-Dirac gas
We study propagation of light in a Fermi-Dirac gas at zero temperature. We
analytically obtain the leading density correction to the optical linewidth.
This correction is a direct consequence of the quantum statistical correlations
of atomic positions that modify the optical interactions between the atoms at
small interatomic separations. The gas exhibits a dramatic line narrowing
already at very low densities.Comment: 4 pages, 2 figure
Stability of a vortex in a small trapped Bose-Einstein condensate
A second-order expansion of the Gross-Pitaevskii equation in the interaction
parameter determines the thermodynamic critical angular velocity Omega_c for
the creation of a vortex in a small axisymmetric condensate. Similarly, a
second-order expansion of the Bogoliubov equations determines the (negative)
frequency omega_a of the anomalous mode. Although Omega_c = -omega_a through
first order, the second-order contributions ensure that the absolute value
|omega_a| is always smaller than the critical angular velocity Omega_c. With
increasing external rotation Omega, the dynamical instability of the condensate
with a vortex disappears at Omega*=|omega_a|, whereas the vortex state becomes
energetically stable at the larger value Omega_c. Both second-order
contributions depend explicitly on the axial anisotropy of the trap. The
appearance of a local minimum of the free energy for a vortex at the center
determines the metastable angular velocity Omega_m. A variational calculation
yields Omega_m=|\omega_a| to first order (hence Omega_m also coincides with the
critical angular velocity Omega_c to this order). Qualitatively, the scenario
for the onset of stability in the weak-coupling limit is the same as that found
in the strong-coupling (Thomas-Fermi) limit.Comment: 8 pages, RevTe
Rotating Bose gas with hard-core repulsion in a quasi-2D harmonic trap: vortices in BEC
We consider a gas of N(=6, 10, 15) Bose particles with hard-core repulsion,
contained in a quasi-2D harmonic trap and subjected to an overall angular
velocity about the z-axis. Exact diagonalization of the
many-body Hamiltonian matrix in given subspaces of the total (quantized)
angular momentum L, with (e.g. for L=N=15, n =240782)
was carried out using Davidson's algorithm. The many-body variational ground
state wavefunction, as also the corresponding energy and the reduced
one-particle density-matrix were calculated. With the usual identification of
as the Lagrange multiplier associated with L for a rotating
system, the phase diagram (or the stability line) was determined
that gave a number of critical angular velocities at which the ground state angular momentum and the associated
condensate fraction undergo abrupt jumps.
A number of (total) angular momentum states were found to be stable at
successively higher critical angular velocities $\Omega_{{\bf c}i}, \
i=1,2,3,...L_{z}>N\Omega_{{\bf c}i}_{z}(\sim 4)$ orders of magnitude in the moderately to the weakly
interacting regime.Comment: Revtex, 11 pages, 1 table as ps file, 4 figures as ps file
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