20,872 research outputs found
Electrodynamic phenomena induced by a dark fluid: Analogs of pyromagnetic, piezoelectric, and striction effects
We establish a new model of coupling between a cosmic dark fluid and
electrodynamic systems, based on an analogy with effects of electric and
magnetic striction, piezo-electricity and piezo-magnetism, pyro-electricity and
pyro-magnetism, which appear in classical electrodynamics of continuous media.
Extended master equations for electromagnetic and gravitational fields are
derived using Lagrange formalism. A cosmological application of the model is
considered, and it is shown that a striction-type interaction between the dark
energy (the main constituent of the dark fluid) and electrodynamic system
provides the universe history to include the so-called unlighted epochs, during
which electromagnetic waves can not propagate and thus can not scan the
universe interior.Comment: 14 pages, 1 Figure, accepted for publication in Phys.Rev.
Elastic waves and transition to elastic turbulence in a two-dimensional viscoelastic Kolmogorov flow
We investigate the dynamics of the two-dimensional periodic Kolmogorov flow
of a viscoelastic fluid, described by the Oldroyd-B model, by means of direct
numerical simulations. Above a critical Weissenberg number the flow displays a
transition from stationary to randomly fluctuating states, via periodic ones.
The increasing complexity of the flow in both time and space at progressively
higher values of elasticity accompanies the establishment of mixing features.
The peculiar dynamical behavior observed in the simulations is found to be
related to the appearance of filamental propagating patterns, which develop
even in the limit of very small inertial non-linearities, thanks to the
feedback of elastic forces on the flow.Comment: 10 pages, 14 figure
Strong charge-transfer excitonic effects and Bose-Einstein exciton-condensate in graphane
Using first principles many-body theory methods (GW+BSE) we demonstrate that
optical properties of graphane are dominated by localized charge-transfer
excitations governed by enhanced electron correlations in a two-dimensional
dielectric medium. Strong electron-hole interaction leads to the appearance of
small radius bound excitons with spatially separated electron and hole, which
are localized out-of-plane and in-plane, respectively. The presence of such
bound excitons opens the path on excitonic Bose-Einstein condensate in graphane
that can be observed experimentally.Comment: 8 pages, 6 figure
Electromagnetic form factors in the light-front formalism and the Feynman triangle diagram: spin-0 and spin-1 two-fermion systems
The connection between the Feynman triangle diagram and the light-front
formalism for spin-0 and spin-1 two-fermion systems is analyzed. It is shown
that in the limit q+ = 0 the form factors for both spin-0 and spin-1 systems
can be uniquely determined using only the good amplitudes, which are not
affected by spurious effects related to the loss of rotational covariance
present in the light-front formalism. At the same time, the unique feature of
the suppression of the pair creation process is maintained. Therefore, a
physically meaningful one-body approximation, in which all the constituents are
on their mass-shells, can be consistently formulated in the limit q+ = 0.
Moreover, it is shown that the effects of the contact term arising from the
instantaneous propagation of the active constituent can be canceled out from
the triangle diagram by means of an appropriate choice of the off-shell
behavior of the bound state vertexes; this implies that in case of good
amplitudes the Feynman triangle diagram and the one-body light-front result
match exactly. The application of our covariant light-front approach to the
evaluation of the rho-meson elastic form factors is presented.Comment: corrected typos in the reference
Active nematic gels as active relaxing solids
I put forward a continuum theory for active nematic gels, defined as fluids
or suspensions of orientable rodlike objects endowed with active dynamics, that
is based on symmetry arguments and compatibility with thermodynamics. The
starting point is our recent theory that models (passive) nematic liquid
crystals as relaxing nematic elastomers. The interplay between viscoelastic
response and active dynamics of the microscopic constituents is naturally taken
into account. By contrast with standard theories, activity is not introduced as
an additional term of the stress tensor, but it is added as an external
remodeling force that competes with the passive relaxation dynamics and drags
the system out of equilibrium. In a simple one-dimensional channel geometry, we
show that the interaction between non-uniform nematic order and activity
results in either a spontaneous flow of particles or a self-organization into
sub-channels flowing in opposite directions
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