8,528 research outputs found
Untwisting of a cholesteric elastomer by a mechanical field
A mechanical strain field applied to a monodomain cholesteric elastomer will
unwind the helical director distribution. There is an analogy with the
classical problem of an electric field applied to a cholesteric liquid crystal,
but with important differences. Frank elasticity is of minor importance unless
the gel is very weak. The interplay is between director anchoring to the rubber
elastic matrix and the external mechanical field. Stretching perpendicular to
the helix axis induces the uniform unwound state via the elimination of sharp,
pinned twist walls above a critical strain. Unwinding through conical director
states occurs when the elastomer is stretched along the helical axis.Comment: 4 pages, RevTeX 3 style, 3 EPS figure
Photoresponse of YBa2Cu3O(7-delta) granular and epitaxial superconducting thin films
The response is reported of thin films of YBa2Cu3O(7-delta) with either a very grainy or a smooth epitaxial morphology to visible radiation. SrTiO3 substrates were employed for both types of films. The grainy films were formed by sequential multi-layer electron beam evaporation while the epitaxial films were formed by laser ablation. Both films were patterned into H shaped detectors via a negative photolithographic process employing a Br/ethanol etchant. The bridge region of the H was 50 microns wide. The patterned films formed by laser ablation and sequential evaporation had critical temperatures of 74 K and 72 K respectively. The bridge was current biased and illuminated with chopped He-Ne laser radiation and the voltage developed in response to the illumination was measured. A signal was detected only above the critical temperature and the peak of the response coincided with the resistive transition for both types of films although the correspondence was less exact for the grainy film. The details of the responses and their analysis are presented
Electromagnetic Casimir piston in higher dimensional spacetimes
We consider the Casimir effect of the electromagnetic field in a higher
dimensional spacetime of the form , where is the
4-dimensional Minkowski spacetime and is an -dimensional
compact manifold. The Casimir force acting on a planar piston that can move
freely inside a closed cylinder with the same cross section is investigated.
Different combinations of perfectly conducting boundary conditions and
infinitely permeable boundary conditions are imposed on the cylinder and the
piston. It is verified that if the piston and the cylinder have the same
boundary conditions, the piston is always going to be pulled towards the closer
end of the cylinder. However, if the piston and the cylinder have different
boundary conditions, the piston is always going to be pushed to the middle of
the cylinder. By taking the limit where one end of the cylinder tends to
infinity, one obtains the Casimir force acting between two parallel plates
inside an infinitely long cylinder. The asymptotic behavior of this Casimir
force in the high temperature regime and the low temperature regime are
investigated for the case where the cross section of the cylinder in is
large. It is found that if the separation between the plates is much smaller
than the size of , the leading term of the Casimir force is the
same as the Casimir force on a pair of large parallel plates in the
-dimensional Minkowski spacetime. However, if the size of
is much smaller than the separation between the plates, the leading term of the
Casimir force is times the Casimir force on a pair of large parallel
plates in the 4-dimensional Minkowski spacetime, where is the first Betti
number of . In the limit the manifold vanishes, one
does not obtain the Casimir force in the 4-dimensional Minkowski spacetime if
is nonzero.Comment: 22 pages, 4 figure
The white dwarf in dwarf nova SDSS J080434.20+510349.2: Entering the instability strip?
SDSS J080434.20+510349.2 is the WZ type binary that displayed rare outburst
in 2006 (Pavlenko et al., 2007). During the long-lasting tail of the late stage
of the outburst binary shown the two-humped or four-humped profile of the
orbital light modulation. The amplitude of orbital light curve decreased while
the mean brightness decreased, more over that occurred 10 times faster
during the fast outburst decline in respect to the late quiet state of slow
outburst fading. There were no white dwarf pulsations detected neither 1 - 1.5
months prior to the outburst nor in 1.5 - 2 months after the 2006 outburst in
this system. However the strong non-radial pulsations with period 12.6 minutes
and mean amplitude of 0.05^m were first detected in V band with 2.6-m Shajn
mirror telescope of the Crimean astrophysical observatory in ~ 8 months after
the outburst. The evolution of pulsations over two years in 2006 - 2008 is
considered. It is supposed that pulsations first appeared when the cooling
white dwarf (after the outburst) entered the instability strip although the
possibility of temporary lack of pulsations at some occasions also could not be
excluded.Comment: Submitted to Proceedings of 16th European White Dwarf Workshop
(EUROWD08
Semi-soft Nematic Elastomers and Nematics in Crossed Electric and Magnetic Fields
Nematic elastomers with a locked-in anisotropy direction exhibit semi-soft
elastic response characterized by a plateau in the stress-strain curve in which
stress does not change with strain. We calculate the global phase diagram for a
minimal model, which is equivalent to one describing a nematic in crossed
electric and magnetic fields, and show that semi-soft behavior is associated
with a broken symmetry biaxial phase and that it persists well into the
supercritical regime. We also consider generalizations beyond the minimal model
and find similar results.Comment: 4 pages, 3 figure
Mapping the G-structures and supersymmetric vacua of five-dimensional N=4 supergravity
We classify the supersymmetric vacua of N=4, d=5 supergravity in terms of
G-structures. We identify three classes of solutions: with R^3, SU(2) and
generic SO(4) structure. Using the Killing spinor equations, we fully
characterize the first two classes and partially solve the latter. With the N=4
graviton multiplet decomposed in terms of N=2 multiplets: the graviton, vector
and gravitino multiplets, we obtain new supersymmetric solutions corresponding
to turning on fields in the gravitino multiplet. These vacua are described in
terms of an SO(5) vector sigma-model coupled with gravity, in three or four
dimensions. A new feature of these N=4 vacua, which is not seen from an N=2
point of view, is the possibility for preserving more exotic fractions of
supersymmetry. We give a few concrete examples of these new supersymmetric
(albeit singular) solutions. Additionally, we show how by truncating the N=4,
d=5 set of fields to minimal supergravity coupled with one vector multiplet we
recover the known two-charge solutions.Comment: 31 pages, late
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