39,421 research outputs found
Hemiparasitic plant impacts animal and plant communities across four trophic levels
1.Understanding the impact of species on community structure is a fundamental question in ecology. There is a growing body of evidence that suggests that both sub-dominant species and parasites can have a disproportionately large impact.
2.Here we report the impacts of an organism that is both subdominant and parasitic, the hemiparasite Rhinanthus minor. Whilst the impact of parasitic angiosperms on their hosts and, to a lesser degree, co-existing plant species, have been well characterized, much less is known about their impacts on higher trophic levels.
3.We experimentally manipulated field densities of the hemiparasite Rhinanthus minor in a species rich grassland, comparing the plant and invertebrate communities in plots where it was removed, at natural densities or at enhanced densities.
4.Plots with natural and enhanced densities of R. minor had lower plant biomass than plots without the hemiparasite, but enhanced densities almost doubled the abundance of invertebrates within the plots across all trophic levels, with effects evident in herbivores, predators and detritivores.
5.The hemiparasite R. minor, despite being a sub-dominant and transient component within plant communities that it inhabits, has profound effects on four different trophic levels. These effects persist beyond the life of the hemiparasite,
emphasizing its role as a keystone species in grassland communitie
On asymptotically flat solutions of Einstein's equations periodic in time II. Spacetimes with scalar-field sources
We extend the work in our earlier article [4] to show that time-periodic,
asymptotically-flat solutions of the Einstein equations analytic at scri, whose
source is one of a range of scalar-field models, are necessarily stationary. We
also show that, for some of these scalar-field sources, in stationary,
asymptotically-flat solutions analytic at scri, the scalar field necessarily
inherits the symmetry. To prove these results we investigate miscellaneous
properties of massless and conformal scalar fields coupled to gravity, in
particular Bondi mass and its loss.Comment: 29 pages, published in Class. Quant. Grav. Replaced. Typos corrected,
version which appeared in Class. Quant.Gra
The Measure Problem in Cosmology
The Hamiltonian structure of general relativity provides a natural canonical
measure on the space of all classical universes, i.e., the multiverse. We
review this construction and show how one can visualize the measure in terms of
a "magnetic flux" of solutions through phase space. Previous studies identified
a divergence in the measure, which we observe to be due to the dilatation
invariance of flat FRW universes. We show that the divergence is removed if we
identify universes which are so flat they cannot be observationally
distinguished. The resulting measure is independent of time and of the choice
of coordinates on the space of fields. We further show that, for some
quantities of interest, the measure is very insensitive to the details of how
the identification is made. One such quantity is the probability of inflation
in simple scalar field models. We find that, according to our implementation of
the canonical measure, the probability for N e-folds of inflation in
single-field, slow-roll models is suppressed by of order exp(-3N) and we
discuss the implications of this result.Comment: 22 pages, 6 figures. Revised version with clarifying remarks on
meaning of adopted measure, extra references and minor typographical
correction
Resonance-continuum interference in the di-photon Higgs signal at the LHC
A low mass Standard Model Higgs boson should be visible at the Large Hadron
Collider through its production via gluon-gluon fusion and its decay to two
photons. We compute the interference of this resonant process, gg -> H -> gamma
gamma, with the continuum QCD background, gg -> gamma gamma induced by quark
loops. Helicity selection rules suppress the effect, which is dominantly due to
the imaginary part of the two-loop gg -> gamma gamma scattering amplitude. The
interference is destructive, but only of order 5% in the Standard Model, which
is still below the 10-20% present accuracy of the total cross section
prediction. We comment on the potential size of such effects in other Higgs
models.Comment: 10 pages, 2 figure
Pair Correlations, Short Range Order and Dispersive Excitations in the Quasi-Kagome Quantum Magnet Volborthite
We present spatial and dynamic information on the s=1/2 distorted kagome
antiferromagnet volborthite, Cu3V2O7(OD)2.2D2O, obtained by polarized and
inelastic neutron scattering. The instantaneous structure factor, S(Q), is
dominated by nearest neighbor pair correlations, with short range order at wave
vectors Q1=0.65(3) {\AA}^-1 and Q2=1.15(5) {\AA}^-1 emerging below 5 K. The
excitation spectrum, S(Q,{\omega}), reveals two steep branches dispersing from
Q1 and Q2, and a flat mode at {\omega}=5.0(2) meV. The results allow us to
identify the cross-over at T*=1 K in 51V NMR and specific heat measurements as
the build-up of correlations at Q_1. We compare our data to theoretical models
proposed for volborthite, and demonstrate that the excitation spectrum can be
explained by spin-wave-like excitations with anisotropic exchange parameters,
as also suggested by recent local density calculations.Comment: Rewritten article resubmitted to Phys. Rev. Lett. 021
Stabilising the Blue Phases
We present an investigation of the phase diagram of cholesteric liquid
crystals within the framework of Landau - de Gennes theory. The free energy is
modified to incorporate all three Frank elastic constants and to allow for a
temperature dependent pitch in the cholesteric phase. It is found that the
region of stability of the cubic blue phases depends significantly on the value
of the elastic constants, being reduced when the bend elastic constant is
larger than splay and when twist is smaller than the other two. Most
dramatically we find a large increase in the region of stability of blue phase
I, and a qualitative change in the phase diagram, in a system where the
cholesteric phase displays helix inversion.Comment: 15 pages, 6 figure
Topology and Bistability in liquid crystal devices
We study nematic liquid crystal configurations in a prototype bistable device
- the Post Aligned Bistable Nematic (PABN) cell. Working within the Oseen-Frank
continuum model, we describe the liquid crystal configuration by a unit-vector
field, in a model version of the PABN cell. Firstly, we identify four distinct
topologies in this geometry. We explicitly construct trial configurations with
these topologies which are used as initial conditions for a numerical solver,
based on the finite-element method. The morphologies and energetics of the
corresponding numerical solutions qualitatively agree with experimental
observations and suggest a topological mechanism for bistability in the PABN
cell geometry
The influence of magazines on men: normalizing and challenging young men’s prejudice with “lads’ mags”
Social psychologists have argued that popular UK and USA men’s magazines known as lads’ mags have normalized hostile sexism among young men. Three studies develop this argument. First, a survey of 423 young UK men found that ambivalent sexism predicted attitudes toward the consumption of lads’ mags, but not other forms of direct sexual consumption (paying for sex or patronizing strip clubs). Second, Study 2 (N = 81) found that young men low in sexism rated sexist jokes as less hostile towards women, but not as either funnier nor more ironic, when those jokes were presented within a lads’ mags context. These findings refute the idea that young men readily read lads’ mags’ sexism as ironic or ‘harmless fun.’ They show instead that placing sexist jokes in lads’ mags contexts makes them appear less hostile. The third study (N = 275) demonstrated that young men perceived lads’ mags as less legitimate after attempting to distinguish the contents of lads’ mags from rapists’ legitimations of their crimes. Implications for contemporary studies of masculinities and consumption are discussed
Properties of short channel ballistic carbon nanotube transistors with ohmic contacts
We present self-consistent, non-equilibrium Green's function calculations of
the characteristics of short channel carbon nanotube transistors, focusing on
the regime of ballistic transport with ohmic contacts. We first establish that
the band lineup at the contacts is renormalized by charge transfer, leading to
Schottky contacts for small diameter nanotubes and ohmic contacts for large
diameter nanotubes, in agreement with recent experiments. For short channel
ohmic contact devices, source-drain tunneling and drain-induced barrier
lowering significantly impact the current-voltage characteristics. Furthermore,
the ON state conductance shows a temperature dependence, even in the absence of
phonon scattering or Schottky barriers. This last result also agrees with
recently reported experimental measurements.Comment: Nanotechnology, in pres
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