259 research outputs found
Constraint and competition in assemblages: a cross continental and modeling approach for ants
The mechanisms leading to structure in local assemblages are controversial. On the one hand, assemblage structure is thought to be the outcome of local interactions determined by the properties of species and their responses to the local environment. Alternatively, this structure has been shown to be an emergent property of assemblages of identical individuals or of random sampling of a regional assemblage.
In ants at baits, a combination of environmental stress and interspecific competition is widely held to lead to a unimodal relationship between the abundance of dominant ants and species richness. It is thought that in comparatively adverse environments, both abundance and richness are low. As habitats become more favorable, abundance increases until the abundance of dominant ants is so high that they exclude those that are subordinate and so depress richness. Here we demonstrate empirically that this relationship is remarkably similar across three continents. Using a null model approach, we then show that the ascending part of the relationship is largely constrained to take this form not simply as a consequence of stress but also as a result of the shape of abundance frequency distributions. While the form of the species-abundance frequency distribution can also produce the descending part of the relationship, interspecific competition might lead to it too. Scatter about the relationship, which is generally not discussed in the literature, may well be a consequence of resource availability and environmental patchiness. Our results draw attention to the significance of regional processes in structuring ant assemblages
An extended pair tunneling model: studies on bilayer splitting and some superconducting state properties
We consider an extended version of the pair tunneling model including
interlayer single particle hopping (ISPH) as a complementary process to pair
tunneling. The normal state gap, as found in cuprates, is taken to suppress the
effective ISPH in conformity with the experimental observations, and this in
turn enhances the pair tunneling process. The effective ISPH involves a
probability factor P for which we consider two choices and provide
phenomenological arguments in favour of them. We address the issue of bilayer
splitting by calculating the spectral density function and corresponding
photoemission intensity curves and show that our calculations conform with the
absence of bilayer splitting observed in ARPES experiments on Bi2212. We have
also studied the temperature variation of the superconducting gap and ratio of
the superconducting gap to T_c. Our results, obtained for both the choices of
P, are reasonably in good agreement with those from experiments on cuprate
superconductors. A linear T-dependent choice of P, however, yields a precise
match to the experimantal data of the temperature varying superconducting gap.Comment: Latex file, 17 pages, 5 figures (postscript files) include
Two-loop HTL Thermodynamics with Quarks
We calculate the quark contribution to the free energy of a hot quark-gluon
plasma to two-loop order using hard-thermal-loop (HTL) perturbation theory. All
ultraviolet divergences can be absorbed into renormalizations of the vacuum
energy and the HTL quark and gluon mass parameters. The quark and gluon HTL
mass parameters are determined self-consistently by a variational prescription.
Combining the quark contribution with the two-loop HTL perturbation theory free
energy for pure-glue we obtain the total two-loop QCD free energy. Comparisons
are made with lattice estimates of the free energy for N_f=2 and with exact
numerical results obtained in the large-N_f limit.Comment: 33 pages, 6 figure
Thermodynamic and transport properties of underdoped cuprates from ARPES data
he relationship between photoemission spectra of high-
cuprates and their thermodynamic and transport properties are discussed. The
doping dependence of the expected quasi-particle density at the Fermi level
() are compared with the electronic specific heat coefficient
and that of the spectral weight at with the in-plane
and out-of-plane superfluid density. We have estimated the electrical
resistivity of underdoped cuprates from the momentum distribution curve (MDC)
at in the nodal direction. The temperature dependence of the MDC
width is also consistent with that of the electrical resistivity.Comment: 14 pages, 4 figures, proceeding of International Symposium on
Synchrotron Radiatin Research for Spin and Electronic States in d and f
Electron Systems(SRSES2003
Onset of magnetism in B2 transition metals aluminides
Ab initio calculation results for the electronic structure of disordered bcc
Fe(x)Al(1-x) (0.4<x<0.75), Co(x)Al(1-x) and Ni(x)Al(1-x) (x=0.4; 0.5; 0.6)
alloys near the 1:1 stoichiometry, as well as of the ordered B2 (FeAl, CoAl,
NiAl) phases with point defects are presented. The calculations were performed
using the coherent potential approximation within the Korringa-Kohn-Rostoker
method (KKR-CPA) for the disordered case and the tight-binding linear
muffin-tin orbital (TB-LMTO) method for the intermetallic compounds. We studied
in particular the onset of magnetism in Fe-Al and Co-Al systems as a function
of the defect structure. We found the appearance of large local magnetic
moments associated with the transition metal (TM) antisite defect in FeAl and
CoAl compounds, in agreement with the experimental findings. Moreover, we found
that any vacancies on both sublattices enhance the magnetic moments via
reducing the charge transfer to a TM atom. Disordered Fe-Al alloys are
ferromagnetically ordered for the whole range of composition studied, whereas
Co-Al becomes magnetic only for Co concentration >0.5.Comment: 11 pages with 9 embedded postscript figures, to be published in
Phys.Rev.
From Coherent Modes to Turbulence and Granulation of Trapped Gases
The process of exciting the gas of trapped bosons from an equilibrium initial
state to strongly nonequilibrium states is described as a procedure of symmetry
restoration caused by external perturbations. Initially, the trapped gas is
cooled down to such low temperatures, when practically all atoms are in
Bose-Einstein condensed state, which implies the broken global gauge symmetry.
Excitations are realized either by imposing external alternating fields,
modulating the trapping potential and shaking the cloud of trapped atoms, or it
can be done by varying atomic interactions by means of Feshbach resonance
techniques. Gradually increasing the amount of energy pumped into the system,
which is realized either by strengthening the modulation amplitude or by
increasing the excitation time, produces a series of nonequilibrium states,
with the growing fraction of atoms for which the gauge symmetry is restored. In
this way, the initial equilibrium system, with the broken gauge symmetry and
all atoms condensed, can be excited to the state, where all atoms are in the
normal state, with completely restored gauge symmetry. In this process, the
system, starting from the regular superfluid state, passes through the states
of vortex superfluid, turbulent superfluid, heterophase granular fluid, to the
state of normal chaotic fluid in turbulent regime. Both theoretical and
experimental studies are presented.Comment: Latex file, 25 pages, 4 figure
Optical properties of dust
http://arxiv.org/abs/0808.4123Except in a few cases cosmic dust can be studied in situ or in terrestrial laboratories, essentially all of our information concerning the nature of cosmic dust depends upon its interaction with electromagnetic radiation. This chapter presents the theoretical basis for describing the optical properties of dust -- how it absorbs and scatters starlight and reradiates the absorbed energy at longer wavelengths.Partial support by a Chandra Theory program
and HST Theory Programs is gratefully acknowledged
Kaon Production and Kaon to Pion Ratio in Au+Au Collisions at \snn=130 GeV
Mid-rapidity transverse mass spectra and multiplicity densities of charged
and neutral kaons are reported for Au+Au collisions at \snn=130 GeV at RHIC.
The spectra are exponential in transverse mass, with an inverse slope of about
280 MeV in central collisions. The multiplicity densities for these particles
scale with the negative hadron pseudo-rapidity density. The charged kaon to
pion ratios are and
for the most central collisions. The ratio is lower than the same
ratio observed at the SPS while the is higher than the SPS result.
Both ratios are enhanced by about 50% relative to p+p and +p
collision data at similar energies.Comment: 6 pages, 3 figures, 1 tabl
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