74 research outputs found
Polar Antiferromagnets Produced with Orbital-Order
Polar magnetic states are realized in pseudocubic manganite thin films
fabricated on high-index substrates, in which a Jahn-Teller (JT) distortion
remains an active variable. Several types of orbital-orders were found to
develop large optical second harmonic generation, signaling
broken-inversion-symmetry distinct from their bulk forms and films on (100)
substrates. The observed symmetry-lifting and first-principles calculation both
indicate that the modified JT q2 mode drives Mn-site off-centering upon orbital
order, leading to the possible cooperation of "Mn-site polarization" and
magnetism.Comment: 5 pages, 4 figure
Braneworld reheating in the bulk inflaton model
In the context of the braneworld inflation driven by a bulk scalar field, we
study the energy dissipation from the bulk scalar field into the matter on the
brane in order to understand the reheating after inflation. Deriving the
late-time behavior of the bulk field with dissipation by using the Green's
function method, we give a rigorous justification of the statement that the
standard reheating process is reproduced in this bulk inflaton model as long as
the Hubble parameter on the brane and the mass of the bulk scalar field are
much smaller than the 5-dimensional inverse curvature scale. Our result
supports the idea that the brane inflation model caused by a bulk scalar field
is expected to be a viable alternative scenario of the early universe.Comment: 5 pages, no figures, final version to be published in PR
Bulk scalar field in the braneworld can mimic the 4D inflaton dynamics
Based on the recently proposed scenario of inflation driven by a bulk scalar
field in the braneworld of the Randall-Sundrum (RS) type, we investigate the
dynamics of a bulk scalar field on the inflating braneworld. We derive the late
time behavior of the bulk scalar field by analyzing the property of the
retarded Green function. We find that the late time behavior is basically
dominated by a single (or a pair of) pole(s) in the Green function irrespective
of the initial condition and of the signature of , where
is the potential of the bulk scalar field. Including the lowest order
back-reaction to the geometry, this late time behavior can be well approximated
by an effective 4-dimensional scalar field with . The
mapping to the 4-dimensional effective theory is given by a simple scaling of
the potential with a redefinition of the field. Our result supports the picture
that the scenario of inflation driven by a bulk scalar field works in a quite
similar way to that in the standard 4-dimensional cosmology.Comment: 12 pages, no figures, final version to be published in PR
Quantum fluctuations in brane-world inflation without inflaton on the brane
A Randall-Sundrum type brane-cosmological model in which slow-roll inflation
on the brane is driven solely by a bulk scalar field was recently proposed by
Himemoto and Sasaki. We analyze their model in detail and calculate the quantum
fluctuations of the bulk scalar field with . We decompose
the bulk scalar field into the infinite mass spectrum of 4-dimensional fields;
the field with the smallest mass-square, called the zero-mode, and the
Kaluza-Klein modes above it with a mass gap. We find the zero-mode dominance of
the classical solution holds if , where is
the curvature radius of the effectively anti-de Sitter bulk, but it is violated
if , though the violation is very small. Then we evaluate
the vacuum expectation value on the brane. We find the
zero-mode contribution completely dominates if similar
to the case of classical background. In contrast, we find the Kaluza-Klein
contribution is small but non-negligible if the value of is
large.Comment: 12pages, 1 figure, typos corrected, a couple of paragraphs modified
but no major change in the text, final version to be published in PR
Brane-world inflation without inflaton on the brane
Inspired by the Randall-Sundrum brane-world scenario, we investigate the
possibility of brane-world inflation driven not by an inflaton field on the
brane, but by a bulk, dilaton-like gravitational field. As a toy model for the
dilaton-like gravitational field, we consider a minimally coupled massive
scalar field in the bulk 5-dimensional spacetime, and look for a perturbative
solution in the anti-de Sitter (AdS) background. For an adequate range of the
scalar field mass, we find a unique solution that has non-trivial dependence on
the 5th dimensional coordinate and that induces slow-roll inflation on the
brane.Comment: 10 pages, 2 figures, final version, to be published in Phys. Rev.
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