81,293 research outputs found
Stochastic group selection model for the evolution of altruism
We study numerically and analytically a stochastic group selection model in
which a population of asexually reproducing individuals, each of which can be
either altruist or non-altruist, is subdivided into reproductively isolated
groups (demes) of size . The cost associated with being altruistic is
modelled by assigning the fitness , with , to the
altruists and the fitness 1 to the non-altruists. In the case that the
altruistic disadvantage is not too large, we show that the finite
fluctuations are small and practically do not alter the deterministic results
obtained for . However, for large these fluctuations
greatly increase the instability of the altruistic demes to mutations. These
results may be relevant to the dynamics of parasite-host systems and, in
particular, to explain the importance of mutation in the evolution of parasite
virulence.Comment: 12 pages, 7 figure
Dynamic radiation force of acoustic waves on solid elastic spheres
The present study concerns the dynamic radiation force on solid elastic
spheres exerted by a plane wave with two frequencies (bichromatic wave)
considering the nonlinearity of the fluid. Our approach is based on solving the
wave scattering for the sphere in the quasilinear approximation within the
preshock wave range. The dynamic radiation force is then obtained by
integrating the component of the momentum flux tensor at the difference of the
primary frequencies over the boundary of the sphere. Results reveal that
effects of the nonlinearity of the fluid plays a major role in dynamic
radiation force leading it to a parametric amplification regime. The developed
theory is used to calculate the dynamic radiation force on three different
solid spheres (aluminium, silver, and tungsten). Resonances are observed in the
spectrum of the force on the spheres. They have larger amplitude and better
shape than resonances present in static radiation force.Comment: 9 pages, 4 figures, to appear in Physical Review
Symmetry-broken dissipative exchange flows in thin-film ferromagnets with in-plane anisotropy
Planar ferromagnetic channels have been shown to theoretically support a
long-range ordered and coherently precessing state where the balance between
local spin injection at one edge and damping along the channel establishes a
dissipative exchange flow, sometimes referred to as a spin superfluid. However,
realistic materials exhibit in-plane anisotropy, which breaks the axial
symmetry assumed in current theoretical models. Here, we study dissipative
exchange flows in a ferromagnet with in-plane anisotropy from a dispersive
hydrodynamic perspective. Through the analysis of a boundary value problem for
a damped sine-Gordon equation, dissipative exchange flows in a ferromagnetic
channel can be excited above a spin current threshold that depends on material
parameters and the length of the channel. Symmetry-broken dissipative exchange
flows display harmonic overtones that redshift the fundamental precessional
frequency and lead to a reduced spin pumping efficiency when compared to their
symmetric counterpart. Micromagnetic simulations are used to verify that the
analytical results are qualitatively accurate, even in the presence of nonlocal
dipole fields. Simulations also confirm that dissipative exchange flows can be
driven by spin transfer torque in a finite-sized region. These results
delineate the important material parameters that must be optimized for the
excitation of dissipative exchange flows in realistic systems.Comment: 20 pages, 5 figure
Flag-Dipole Spinor Fields in ESK Gravities
We consider the Riemann-Cartan geometry as a basis for the
Einstein-Sciama-Kibble theory coupled to spinor fields: we focus on and
conformal gravities, regarding the flag-dipole spinor fields, type-(4) spinor
fields under the Lounesto classification. We study such theories in specific
cases given for instance by cosmological scenarios: we find that in such
background the Dirac equation admits solutions that are not Dirac spinor
fields, but in fact the aforementioned flag-dipoles ones. These solutions are
important from a theoretical perspective, as they evince that spinor fields are
not necessarily determined by their dynamics, but also a discussion on their
structural (algebraic) properties must be carried off. Furthermore, the
phenomenological point of view is shown to be also relevant, since for
isotropic Universes they circumvent the question whether spinor fields do
undergo the Cosmological Principle.Comment: 18 pages, improved versio
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