24,236 research outputs found
The role of singletons in compactifications
We derive the isometry irrep content of squashed seven-sphere
compactifications of eleven-dimensional supergravity, i.e., the left-squashed
() with and right-squashed () with supersymmetry, in a manner completely independent of the round sphere.
Then we compare this result with the spectrum obtained by Higgsing the round
sphere spectrum. This way we discover features of the spectra which makes it
possible to argue that the only way the round spectrum can be related by a
Higgs mechanism to the one of is if the singletons are included in the
round sphere spectrum. For this to work also in the case it seems that
the gravitino of the spectrum must be replaced by a fermionic singleton
present in the spectrum.Comment: 24 pages including appendix with 12 figure, v2 minor typos correcte
The field inside a random distribution of parallel dipoles
We determine the probability distribution for the field inside a random
uniform distribution of electric or magnetic dipoles.
For parallel dipoles, simulations and an analytical derivation show that
although the average contribution from any spherical shell around the probe
position vanishes, the Levy stable distribution of the field is symmetric
around a non-vanishing field amplitude.
In addition we show how omission of contributions from a small volume around
the probe leads to a field distribution with a vanishing mean, which, in the
limit of vanishing excluded volume, converges to the shifted distribution.Comment: RevTeX, 4 pages, 3 figures. Submitted to Phys. Rev. Let
Timelike self-similar spherically symmetric perfect-fluid models
Einstein's field equations for timelike self-similar spherically symmetric
perfect-fluid models are investigated. The field equations are rewritten as a
first-order system of autonomous differential equations. Dimensionless
variables are chosen in such a way that the number of equations in the coupled
system is reduced as far as possible and so that the reduced phase space
becomes compact and regular. The system is subsequently analysed qualitatively
using the theory of dynamical systems.Comment: 23 pages, 6 eps-figure
Bound states and magnetic field-induced valley splitting in gate-tunable graphene quantum dots
The magnetic field dependence of energy levels in gapped single- and bilayer
graphene quantum dots (QDs) defined by electrostatic gates is studied
analytically in terms of the Dirac equation. Due to the absence of sharp edges
in these types of QDs, the valley degree of freedom is a good quantum number.
We show that its degeneracy is efficiently and controllably broken by a
magnetic field applied perpendicular to the graphene plane. This opens up a
feasible route to create well-defined and well controlled spin- and
valley-qubits in graphene QDs. We also point out the similarities and
differences in the spectrum between single- and bilayer graphene quantum dots.
Striking in the case of bilayer graphene is the anomalous bulk Landau level
(LL) that crosses the gap which results in crossings of QD states with this
bulk LL at large magnetic fields in stark contrast to the single-layer case
where this LL is absent. The tunability of the gap in the bilayer case allows
us to observe different regimes of level spacings directly related to the
formation of a pronounced ``Mexican hat'' in the bulk bandstructure. We discuss
the applicability of such QDs to control and measure the valley isospin and
their potential use for hosting and controlling spin qubits.Comment: 12 pages, 10 figure
Clustering outdoor soundscapes using fuzzy ants
A classification algorithm for environmental sound recordings or "soundscapes" is outlined. An ant clustering approach is proposed, in which the behavior of the ants is governed by fuzzy rules. These rules are optimized by a genetic algorithm specially designed in order to achieve the optimal set of homogeneous clusters. Soundscape similarity is expressed as fuzzy resemblance of the shape of the sound pressure level histogram, the frequency spectrum and the spectrum of temporal fluctuations. These represent the loudness, the spectral and the temporal content of the soundscapes. Compared to traditional clustering methods, the advantages of this approach are that no a priori information is needed, such as the desired number of clusters, and that a flexible set of soundscape measures can be used. The clustering algorithm was applied to a set of 1116 acoustic measurements in 16 urban parks of Stockholm. The resulting clusters were validated against visitor's perceptual measurements of soundscape quality
Charge distribution and screening in layered graphene systems
The charge distribution induced by external fields in finite stacks of
graphene planes, or in semiinfinite graphite is considered. The interlayer
electronic hybridization is described by a nearest neighbor hopping term, and
the charge induced by the self consistent electrostatic potential is calculated
within linear response (RPA). The screening properties are determined by
contributions from inter- and intraband electronic transitions. In neutral
systems, only interband transitions contribute to the charge polarizability,
leading to insulating-like screening properties, and to oscillations in the
induced charge, with a period equal to the interlayer spacing. In doped
systems, we find a screening length equivalent to 2-3 graphene layers,
superimposed to significant charge oscillations.Comment: 8 page
Polarization Effects in Superdeformed Nuclei
A detailed theoretical investigation of polarization effects in superdeformed
nuclei is performed. In the pure harmonic oscillator potential it is shown that
when one particle (or hole) with the mass single-particle quadrupole moment
q_{nu} is added to a superdeformed core, the change of the electric quadrupole
moment can be parameterized as q_{eff}=e(bq_{nu}+a), and analytical expressions
are derived for the two parameters, and . Simple numerical expressions
for q_{eff}(q_\nu}) are obtained in the more realistic modified oscillator
model. It is also shown that quadrupole moments of nuclei with up to 10
particles removed from the superdeformed core of 152Dy can be well described by
simply subtracting effective quadrupole moments of the active single-particle
states from the quadrupole moment of the core. Tools are given for estimating
the quadrupole moment for possible configurations in the superdeformed A
150-region.Comment: 28 pages including 9 figure
Characterizing glucocorticoid levels in five species of sea ducks occurring in Alaska
Thesis (M.S.) University of Alaska Fairbanks, 2004Stress hormone analysis, used in conjunction with other physiological parameters, may help identify factors affecting sea duck populations in their natural environment. Corticosterone, the primary 'stress hormone' in birds, is secreted in response to a stressor and enhances an individual's chance of survival by inducing physiological and behavioral changes. Establishing a valid method for evaluating stress hormone levels in sea ducks and gaining basic information on baseline concentrations and stress response in these birds are important first steps to identify factors that may negatively affect sea duck populations. This study validated a radioimmunoassay (RIA) procedure to measure corticosterone concentrations in harlequin duck serum and feces and in Steller's, spectacled, common, and king eider serum. Other objectives included characterization of baseline corticosterone concentrations, investigation of stress response, and the relationship between corticosterone and other variables in captive and wild sea ducks. The results indicate that fecal samples can be used to non-invasively measure corticosterone in harlequin ducks. Captive birds exhibited overall lower baseline levels of corticosterone than wild birds. The stress response observed in harlequin ducks was similar to other avian species. Rapid post-capture blood sampling is critical for evaluation of baseline corticosterone levels
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