30,173 research outputs found
Pion production in nonequilibrium Chiral perturbation Theory
We apply the formalism of Chiral Perturbation Theory out of thermal
equilibrium to describe explosive production of pions via the parametric
resonance mechanism. To lowest order the lagrangian is that of the Nonlinear
Sigma Model where the pion decay constant becomes a time-dependent function.
This model alows for a consistent nonequilibrium formulation within the
framework of the closed time path method, where one-loop effects can be
systematically accounted for and renormalized. We work in the narrow resonance
regime where there is only one resonant band. The present approach is limited
to remain below the back-reaction time, although it accounts for nearly all the
pion production during the typical plasma lifetime. The space and time
components of the pion decay constant are also analyzed.Comment: 23 pages, 8 figures, RevTe
Homogenization of cohesive fracture in masonry structures
We derive a homogenized mechanical model of a masonry-type structure
constituted by a periodic assemblage of blocks with interposed mortar joints.
The energy functionals in the model under investigation consist in (i) a linear
elastic contribution within the blocks, (ii) a Barenblatt's cohesive
contribution at contact surfaces between blocks and (iii) a suitable unilateral
condition on the strain across contact surfaces, and are governed by a small
parameter representing the typical ratio between the length of the blocks and
the dimension of the structure. Using the terminology of Gamma-convergence and
within the functional setting supplied by the functions of bounded deformation,
we analyze the asymptotic behavior of such energy functionals when the
parameter tends to zero, and derive a simple homogenization formula for the
limit energy. Furthermore, we highlight the main mathematical and mechanical
properties of the homogenized energy, including its non-standard growth
conditions under tension or compression. The key point in the limit process is
the definition of macroscopic tensile and compressive stresses, which are
determined by the unilateral conditions on contact surfaces and the geometry of
the blocks
Interplay between strain, defect charge state and functionality in complex oxides
We use first-principles calculations to investigate the interplay between
strain and the charge state of point defect impurities in complex oxides. Our
work is motivated by recent interest in using defects as active elements to
provide novel functionality in coherent epitaxial films. Using oxygen vacancies
as model point defects, and CaMnO and MnO as model materials, we calculate
the changes in internal strain caused by changing the charge state of the
vacancies, and conversely the effect of strain on charge-state stability. Our
results show that the charge state is a degree of freedom that can be used to
control the interaction of defects with strain and hence the concentration and
location of defects in epitaxial films. We propose the use of field-effect
gating to reversibly change the charge state of defects and hence the internal
strain and corresponding strain-induced functionalities.Comment: 4 pages, 4 figure
BaNiF4: an electric field-switchable weak antiferromagnet
We show that in the antiferromagnetic ferroelectric BaNiF4 the
Dzyaloshinskii-Moriya interaction leads to a small canting of the magnetic
moments, away from the easy axis, resulting in a noncollinear magnetic
structure. The canting corresponds to an additional "weak" antiferromagnetic
order parameter whose orientation is determined by the polar structural
distortion and can be reversed by switching the ferroelectric polarization with
an electric field. Our results point the way to a more general coupling
mechanism between structural distortions and magnetic order parameters in
magnetoelectric multiferroics which can be exploited in the design of electric
field-switchable magnets.Comment: 4 pages, 4 figure
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