308 research outputs found

### Topological Casimir effect in nanotubes and nanoloopes

The Casimir effect is investigated in cylindrical and toroidal carbon
nanotubes within the framework of the Dirac-like model for the electronic
states. The topological Casimir energy is positive for metallic cylindrical
nanotubes and is negative for semiconducting ones. The toroidal
compactification of a cylindrical nanotube along its axis increases the Casimir
energy for metallic-type (periodic) boundary conditions along its axis and
decreases the Casimir energy for the semiconducting-type compactifications. For
finite length metallic nanotubes the Casimir forces acting on the tube edges
are always attractive, whereas for semiconducting-type ones they are attractive
for small lengths of the nanotube and repulsive for large lengths.Comment: 5 pages, 1 figure, Contribution to Proceedings of QFEXT09, 21-25
September 2009, Oklahoma, US

### Spinor Casimir densities for a spherical shell in the global monopole spacetime

We investigate the vacuum expectation values of the energy-momentum tensor
and the fermionic condensate associated with a massive spinor field obeying the
MIT bag boundary condition on a spherical shell in the global monopole
spacetime. In order to do that it was used the generalized Abel-Plana summation
formula. As we shall see, this procedure allows to extract from the vacuum
expectation values the contribution coming from to the unbounded spacetime and
explicitly to present the boundary induced parts. As to the boundary induced
contribution, two distinct situations are examined: the vacuum average effect
inside and outside the spherical shell. The asymptotic behavior of the vacuum
densities is investigated near the sphere center and surface, and at large
distances from the sphere. In the limit of strong gravitational field
corresponding to small values of the parameter describing the solid angle
deficit in global monopole geometry, the sphere-induced expectation values are
exponentially suppressed. As a special case we discuss the fermionic vacuum
densities for the spherical shell on background of the Minkowski spacetime.
Previous approaches to this problem within the framework of the QCD bag models
have been global and our calculation is a local extension of these
contributions.Comment: 20 pages, 4 figure

### Wightman function and vacuum fluctuations in higher dimensional brane models

Wightman function and vacuum expectation value of the field square are
evaluated for a massive scalar field with general curvature coupling parameter
subject to Robin boundary conditions on two codimension one parallel branes
located on $(D+1)$-dimensional background spacetime $AdS_{D_1+1}\times \Sigma$
with a warped internal space $\Sigma$. The general case of different Robin
coefficients on separate branes is considered. The application of the
generalized Abel-Plana formula for the series over zeros of combinations of
cylinder functions allows us to extract manifestly the part due to the bulk
without boundaries. Unlike to the purely AdS bulk, the vacuum expectation value
of the field square induced by a single brane, in addition to the distance from
the brane, depends also on the position of the brane in the bulk. The brane
induced part in this expectation value vanishes when the brane position tends
to the AdS horizon or AdS boundary. The asymptotic behavior of the vacuum
densities near the branes and at large distances is investigated. The
contribution of Kaluza-Klein modes along $\Sigma$ is discussed in various
limiting cases. As an example the case $\Sigma =S^1$ is considered,
corresponding to the $AdS_{D+1}$ bulk with one compactified dimension. An
application to the higher dimensional generalization of the Randall-Sundrum
brane model with arbitrary mass terms on the branes is discussed.Comment: 25 pages, 2 figures, discussion added, accepted for publication in
Phys.Rev.

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