1,333 research outputs found
Food Webs: Experts Consuming Families of Experts
The question what determines the structure of natural food webs has been
listed among the nine most important unanswered questions in ecology. It arises
naturally from many problems related to ecosystem stability and resilience. The
traditional view is that population-dynamical stability is crucial for
understanding the observed structures. But phylogeny (evolutionary history) has
also been suggested as the dominant mechanism. Here we show that observed
topological features of predatory food webs can be reproduced to unprecedented
accuracy by a mechanism taking into account only phylogeny, size constraints,
and the heredity of the trophically relevant traits of prey and predators. The
analysis reveals a tendency to avoid resource competition rather than apparent
competition. In food webs with many parasites this pattern is reversed.Comment: 16 pages, 3 figures, 1 table + Appendix of 36 pages, 18 figures.
movie available from http://ag.rossberg.net/matching.mp
Quark Confinement Physics from Quantum Chromodynamics
We show the construction of the dual superconducting theory for the
confinement mechanism from QCD in the maximally abelian (MA) gauge using the
lattice QCD Monte Carlo simulation. We find that essence of infrared abelian
dominance is naturally understood with the off-diagonal gluon mass induced by the MA gauge fixing. In the MA gauge, the
off-diagonal gluon amplitude is forced to be small, and the off-diagonal gluon
phase tends to be random. As the mathematical origin of abelian dominance for
confinement, we demonstrate that the strong randomness of the off-diagonal
gluon phase leads to abelian dominance for the string tension. In the MA gauge,
there appears the macroscopic network of the monopole world-line covering the
whole system. We investigate the monopole-current system in the MA gauge by
analyzing the dual gluon field . We evaluate the dual gluon mass as 0.5GeV in the infrared region, which is the lattice-QCD evidence of
the dual Higgs mechanism by monopole condensation. Owing to infrared abelian
dominance and infrared monopole condensation, QCD in the MA gauge is
describable with the dual Ginzburg-Landau theory.Comment: Invited talk given at KEK-Tanashi International Symposium on Physics
of Hadrons and Nuclei, Tokyo, Japan, 14-17 Dec 199
Direct observation of oxygen polarization in SrIrO by O -edge x-ray magnetic circular dichroism
X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD)
measurements at the oxygen (O) -edge were performed to investigate the
magnetic polarization of ligand O atoms in the weak ferromagnetic (WFM) phase
of the Ir perovskite compound SrIrO. With the onset of the WFM phase
below K, XMCD signals corresponding to XAS peaks
respectively identified as originating from the magnetic moments of apical and
planar oxygen (O and O) in the IrO octahedra were
observed. The observation of magnetic moments at O sites is
consistent (except for the relative orientation) with that suggested by prior
muon spin rotation (SR) experiment in the non-collinear antiferromagnetic
(NC-AFM) phase below K. Assuming that the O
magnetic moment observed by SR is also responsible for the corresponding
XMCD signal, the magnetic moment of O is estimated to be consistent
with the previous SR result. Since the O XMCD signal is mainly
contributed by Ir 5 and orbitals which also hybridize with O, it is inferred that the relatively large O magnetic moment is
induced by Ir 5 orbitals. Moreover, the inversion of O
moments relative to Ir moments between the two magnetic phases revealed by XMCD
suggests the presence of competing magnetic interactions for O, with
which the ordering of O moments in the NC-AFM phase may be suppressed
to .Comment: 6 pages, 6 figure
Modulational instability of ion-acoustic wave packets in quantum pair-ion plasmas
Amplitude modulation of quantum ion-acoustic waves (QIAWs) in a quantum
electron-pair-ion plasma is studied. It is shown that the quantum coupling
parameter (being the ratio of the plasmonic energy density to the Fermi
energy) is ultimate responsible for the modulational stability of QIAW packets,
without which the wave becomes modulational unstable. New regimes for the
modulational stability (MS) and instability (MI) are obtained in terms of
and the positive to negative ion density ratio . The growth rate of MI
is obtained, the maximum value of which increases with and decreases
with . The results could be important for understanding the origin of
modulated QIAW packets in the environments of dense astrophysical objects,
laboratory negative ion plasmas as well as for the next generation laser solid
density plasma experiments.Comment: 4 pages, 2 figures (to appear in Astrophysics and Space Science
Magnetic-field dependence of the critical currents in a periodic coplanar array of narrow superconducting strip
We calculate the magnetic-field dependence of the critical current due to
both geometrical edge barriers and bulk pinning in a periodic coplanar array of
narrow superconducting strips. We find that in zero or low applied magnetic
fields the critical current can be considerably enhanced by the edge barriers,
but in modest applied magnetic fields the critical current reduces to that due
to bulk pinning alone.Comment: 23 pages, 7 figure
Off-diagonal Gluon Mass Generation and Infrared Abelian Dominance in the Maximally Abelian Gauge in Lattice QCD
We study effective mass generation of off-diagonal gluons and infrared
abelian dominance in the maximally abelian (MA) gauge. Using the SU(2) lattice
QCD, we investigate the propagator and the effective mass of the gluon field in
the MA gauge with the U(1) Landau gauge fixing. The Monte Carlo simulation
is performed on the lattice with , and
also on the and lattices with . In the MA
gauge, the diagonal gluon component shows long-range propagation, and
infrared abelian dominance is found for the gluon propagator. In the MA gauge,
the off-diagonal gluon component behaves as a massive vector boson
with the effective mass GeV in the region of r \gsim
0.2 fm, and its propagation is limited within short range. We conjecture that
infrared abelian dominance can be interpreted as infrared inactivity of the
off-diagonal gluon due to its large mass generation induced by the MA gauge
fixing.Comment: 31 pages, 7 figures and 2 tables included, changed title, corrected
typos and updated reference, accepted for publication in Physical Review
Dynamical gluon mass generation from <A^2> in linear covariant gauges
We construct the multiplicatively renormalizable effective potential for the
mass dimension two local composite operator A^2 in linear covariant gauges. We
show that the formation of is energetically favoured and that the gluons
acquire a dynamical mass due to this gluon condensate. We also discuss the
gauge parameter independence of the resultant vacuum energy.Comment: 21 pages. 14 .eps figures. v2: minor modifications. v3: version
accepted for publication in JHE
Quantitative localized proton-promoted dissolution kinetics of calcite using scanning electrochemical microscopy (SECM)
Scanning electrochemical microscopy (SECM) has been used to determine quantitatively the kinetics of proton-promoted dissolution of the calcite (101Ì
4) cleavage surface (from natural âIceland Sparâ) at the microscopic scale. By working under conditions where the probe size is much less than the characteristic dislocation spacing (as revealed from etching), it has been possible to measure kinetics mainly in regions of the surface which are free from dislocations, for the first time. To clearly reveal the locations of measurements, studies focused on cleaved âmirrorâ surfaces, where one of the two faces produced by cleavage was etched freely to reveal defects intersecting the surface, while the other (mirror) face was etched locally (and quantitatively) using SECM to generate high proton fluxes with a 25 ÎŒm diameter Pt disk ultramicroelectrode (UME) positioned at a defined (known) distance from a crystal surface. The etch pits formed at various etch times were measured using white light interferometry to ascertain pit dimensions. To determine quantitative dissolution kinetics, a moving boundary finite element model was formulated in which experimental time-dependent pit expansion data formed the input for simulations, from which solution and interfacial concentrations of key chemical species, and interfacial fluxes, could then be determined and visualized. This novel analysis allowed the rate constant for proton attack on calcite, and the order of the reaction with respect to the interfacial proton concentration, to be determined unambiguously. The process was found to be first order in terms of interfacial proton concentration with a rate constant k = 6.3 (± 1.3) Ă 10â4 m sâ1. Significantly, this value is similar to previous macroscopic rate measurements of calcite dissolution which averaged over large areas and many dislocation sites, and where such sites provided a continuous source of steps for dissolution. Since the local measurements reported herein are mainly made in regions without dislocations, this study demonstrates that dislocations and steps that arise from such sites are not needed for fast proton-promoted calcite dissolution. Other sites, such as point defects, which are naturally abundant in calcite, are likely to be key reaction sites
Pseudo-single crystal electrochemistry on polycrystalline electrodes : visualizing activity at grains and grain boundaries on platinum for the Fe2+/Fe3+ redox reaction
The influence of electrode surface structure on electrochemical reaction rates and mechanisms is a major theme in electrochemical research, especially as electrodes with inherent structural heterogeneities are used ubiquitously. Yet, probing local electrochemistry and surface structure at complex surfaces is challenging. In this paper, high spatial resolution scanning electrochemical cell microscopy (SECCM) complemented with electron backscatter diffraction (EBSD) is demonstrated as a means of performing âpseudo-single-crystalâ electrochemical measurements at individual grains of a polycrystalline platinum electrode, while also allowing grain boundaries to be probed. Using the Fe2+/3+ couple as an illustrative case, a strong correlation is found between local surface structure and electrochemical activity. Variations in electrochemical activity for individual high index grains, visualized in a weakly adsorbing perchlorate medium, show that there is higher activity on grains with a significant (101) orientation contribution, compared to those with (001) and (111) contribution, consistent with findings on single-crystal electrodes. Interestingly, for Fe2+ oxidation in a sulfate medium a different pattern of activity emerges. Here, SECCM reveals only minor variations in activity between individual grains, again consistent with single-crystal studies, with a greatly enhanced activity at grain boundaries. This suggests that these sites may contribute significantly to the overall electrochemical behavior measured on the macroscale
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