92,869 research outputs found
Anisotropic stresses in inhomogeneous universes
Anisotropic stress contributions to the gravitational field can arise from
magnetic fields, collisionless relativistic particles, hydrodynamic shear
viscosity, gravitational waves, skew axion fields in low-energy string
cosmologies, or topological defects. We investigate the effects of such
stresses on cosmological evolution, and in particular on the dissipation of
shear anisotropy. We generalize some previous results that were given for
homogeneous anisotropic universes, by including small inhomogeneity in the
universe. This generalization is facilitated by a covariant approach. We find
that anisotropic stress dominates the evolution of shear, slowing its decay.
The effect is strongest in radiation-dominated universes, where there is slow
logarithmic decay of shear.Comment: 7 pages Revte
Slow shocks and conduction fronts from Petschek reconnection of skewed magnetic fields: two-fluid effects
In models of fast magnetic reconnection, flux transfer occurs within a small
portion of a current sheet triggering stored magnetic energy to be thermalized
by shocks. When the initial current sheet separates magnetic fields which are
not perfectly anti-parallel, i.e. they are skewed, magnetic energy is first
converted to bulk kinetic energy and then thermalized in slow magnetosonic
shocks. We show that the latter resemble parallel shocks or hydrodynamic shocks
for all skew angles except those very near the anti-parallel limit. As for
parallel shocks, the structures of reconnection-driven slow shocks are best
studied using two-fluid equations in which ions and electrons have independent
temperature. Time-dependent solutions of these equations can be used to predict
and understand the shocks from reconnection of skewed magnetic fields. The
results differ from those found using a single-fluid model such as
magnetohydrodynamics. In the two-fluid model electrons are heated indirectly
and thus carry a heat flux always well below the free-streaming limit. The
viscous stress of the ions is, however, typically near the fluid-treatable
limit. We find that for a wide range of skew angles and small plasma beta an
electron conduction front extends ahead of the slow shock but remains within
the outflow jet. In such cases conduction will play a more limited role in
driving chromospheric evaporation than has been predicted based on
single-fluid, anti-parallel models
In-plane magnetoelectric response in bilayer graphene
A graphene bilayer shows an unusual magnetoelectric response whose magnitude
is controlled by the valley-isospin density, making it possible to link
magnetoelectric behavior to valleytronics. Complementary to previous studies,
we consider the effect of static homogeneous electric and magnetic fields that
are oriented parallel to the bilayer's plane. Starting from a tight-binding
description and using quasi-degenerate perturbation theory, the low-energy
Hamiltonian is derived including all relevant magnetoelectric terms whose
prefactors are expressed in terms of tight-binding parameters. We confirm the
existence of an expected axion-type pseudoscalar term, which turns out to have
the same sign and about twice the magnitude of the previously obtained
out-of-plane counterpart. Additionally, small anisotropic corrections to the
magnetoelectric tensor are found that are fundamentally related to the skew
interlayer hopping parameter . We discuss possible ways to identify
magnetoelectric effects by distinctive features in the optical conductivity.Comment: 14 pages, 7 figure
Anomalous f-electron Hall Effect in the Heavy-Fermion System CeTIn (T = Co, Ir, or Rh)
The in-plane Hall coefficient of CeRhIn, CeIrIn, and
CeCoIn and their respective non-magnetic lanthanum analogs are reported
in fields to 90 kOe and at temperatures from 2 K to 325 K. is
negative, field-independent, and dominated by skew-scattering above 50 K
in the Ce compounds. becomes increasingly negative below 50 K
and varies with temperature in a manner that is inconsistent with skew
scattering. Field-dependent measurements show that the low-T anomaly is
strongly suppressed when the applied field is increased to 90 kOe. Measurements
on LaRhIn, LaIrIn, and LaCoIn indicate that the same
anomalous temperature dependence is present in the Hall coefficient of these
non-magnetic analogs, albeit with a reduced amplitude and no field dependence.
Hall angle () measurements find that the ratio
varies as below 20 K for all
three Ce-115 compounds. The Hall angle of the La-115 compounds follow this
T-dependence as well. These data suggest that the electronic-structure
contribution dominates the Hall effect in the 115 compounds, with -electron
and Kondo interactions acting to magnify the influence of the underlying
complex band structure. This is in stark contrast to the situation in most
and heavy-fermion compounds where the normal carrier contribution to the
Hall effect provides only a small, T-independent background to Comment: 23 pages and 8 figure
Effects of characteristic material lengths on ductile crack propagation.
The asymptotic fields near the tip of a crack steadily propagating in a ductile material under Mode III loading conditions areinvestigated by adopting an incremental version of the indeterminate theory of couple stress plasticity displaying linear strainhardening. The adopted constitutive model is able to account for the microstructure of the material by incorporating two distinctmaterial characteristic lengths. It can also capture the strong size effects arising at small scales, which results from theunderlying microstructures. The effects of microstructure on Mode III crack tip fields mainly consist in a substantial increase inthe singularities of the skew-symmetric stress and couple stress fields, which occurs also for small values of the strainhardening coefficient, whereas the symmetric stress field turns out to be non-singular according to the asymptotic crack tipfields for a stationary crack provided by the indeterminate theory of couple stress elasticity. The performed asymptotic analysisthus predicts a significant increase of the tractions level ahead of the crack-tip, due to the contribution of the rotation gradient
Comment on the formation of black holes in nonsymmetric gravity
We critically examine the claim made by Burko and Ori that black holes are
expected to form in nonsymmetric gravity and find their analysis to be
inconclusive. Their conclusion is a result of the approximations they make, and
not a consequence of the true dynamics of the theory. The approximation they
use fails to capture the crucial equivalence principle violations which enable
the full nonsymmetric field equations to detect and tame would-be horizons. An
examination of the dynamics of the full theory reveals no indication that black
holes should form. For these reasons, one cannot conclude from their analysis
that nonsymmetric gravity has black holes. A definitive answers awaits a
comprehensive study of gravitational collapse, using the full field equations.Comment: 6 pages, RevTe
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