1,970 research outputs found
Casimir densities for a plate in de Sitter spacetime
Wightman function, the vacuum expectation values of the field squared and the
energy-momentum tensor are investigated for a scalar field with general
curvature coupling parameter in the geometry of a plate in the de Sitter
spacetime. Robin boundary condition for the field operator is assumed on the
plate. The vacuum expectation values are presented as the sum of two terms. The
first one corresponds to the geometry of de Sitter bulk without boundaries and
the second one is induced by the presence of the plate. We show that for
non-conformal fields the vacuum energy-momentum tensor is non-diagonal with the
off-diagonal component corresponding to the energy flux along the direction
perpendicular to the plate. In dependence of the parameters, this flux can be
either positive or negative. The asymptotic behavior of the field squared,
vacuum energy density and stresses near the plate and at large distances is
investigated.Comment: 15 pages, 3 figures, figure 1 changed, figure 3 and references added,
to appear in Class. Quantum Gra
Fermionic currents in AdS spacetime with compact dimensions
We derive a closed expression for the vacuum expectation value (VEV) of the
fermionic current density in a (D+1)-dimensional locally AdS spacetime with an
arbitrary number of toroidally compactified Poincare spatial dimensions and in
the presence of a constant gauge field. The latter can be formally interpreted
in terms of a magnetic flux treading the compact dimensions. In the compact
subspace, the field operator obeys quasiperiodicity conditions with arbitrary
phases. The VEV of the charge density is zero and the current density has
nonzero components along the compact dimensions only. They are periodic
functions of the magnetic flux with the period equal to the flux quantum and
tend to zero on the AdS boundary. Near the horizon, the effect of the
background gravitational field is small and the leading term in the
corresponding asymptotic expansion coincides with the VEV for a massless field
in the locally Minkowski bulk. Unlike the Minkowskian case, in the system
consisting an equal number of fermionic and scalar degrees of freedom, with
same masses, charges and phases in the periodicity conditions, the total
current density does not vanish. In these systems, the leading divergences in
the scalar and fermionic contributions on the horizon are canceled and, as a
consequence of that, the charge flux, integrated over the coordinate
perpendicular to the AdS boundary, becomes finite. We show that in odd
spacetime dimensions the fermionic fields realizing two inequivalent
representations of the Clifford algebra and having equal phases in the
periodicity conditions give the same contribution to the VEV of the current
density. Combining the contributions from these fields, the current density in
odd-dimensional C-,P- and T -symmetric models are obtained. As an application,
we consider the ground state current density in curved carbon nanotubes.Comment: 22 pages, 6 figures, PACS numbers: 04.62.+v, 03.70.+k, 98.80.-k,
61.46.F
Casimir effect for parallel plates in de Sitter spacetime
The Wightman function and the vacuum expectation values of the field squared
and of the energy-momentum tensor are obtained, for a massive scalar field with
an arbitrary curvature coupling parameter, in the region between two infinite
parallel plates, on the background of de Sitter spacetime. The field is
prepared in the Bunch-Davies vacuum state and is constrained to satisfy Robin
boundary conditions on the plates. For the calculation, a mode-summation method
is used, supplemented with a variant of the generalized Abel-Plana formula.
This allows to explicitly extract the contributions to the expectation values
which come from each single boundary, and to expand the second-plate-induced
part in terms of exponentially convergent integrals. Several limiting cases of
interest are then studied. Moreover, the Casimir forces acting on the plates
are evaluated, and it is shown that the curvature of the background spacetime
decisively influences the behavior of these forces at separations larger than
the curvature scale of de Sitter spacetime. In terms of the curvature coupling
parameter and the mass of the field, two very different regimes are realized,
which exhibit monotonic and oscillatory behavior of the vacuum expectation
values, respectively. The decay of the Casimir force at large plate separation
is shown to be power-law (monotonic or oscillating), with independence of the
value of the field mass.Comment: 22 pages, 4 figures, added figures for a massless field, added
reference, added discussions and comments on thermal effect
Peculiarities of a Colloidal Polysaccharide of Newly Isolated Iron Oxidizing Bacteria in Armenia
Microorganisms belonging to different systematic and physiological groups produce various intra- and extracellular polysaccharides, which both play an important role in the life of microorganisms and have great practical application. Iron and sulfur oxidizing bacteria produce capsular (EPS) and colloidal polysaccharides. At present the properties and functional role of EPS are well studied. However, the properties of the colloidal polysaccharides produced by iron oxidizing bacteria have not been sufficiently explored. A new iron oxidizing bacteria Leptospirillum ferriphilium CC was isolated from sulfide ores of Armenia. Its morphological and physiological features have been studied. A colloidal polysaccharide has been isolated with the use of an original method developed by the authors, and its physical and chemical properties have been studied. It has been shown that the colloidal polysaccharide consists of three different monomers- glucose, fructose, mannose.. Investigations with a complex method of optical polarization microscopy and analytical programs allowed determining the size, shape change, perimeter, degree of hydratation and crystallization at 0.07% and 0.04% of polysaccharide concentration. It was shown that the size of a polysaccharide colloidal particle does not much depend on polysaccharide concentration, however, the number of identical colloidal formations is dependent on the concentration of polysaccharide
Massive scalar field in de Sitter spacetime: a two-loop calculation and a comparison with the stochastic approach
We examine long-wavelength correlation functions
of massive scalar fields in de Sitter spacetime. For the
theory with a quartic self-interaction, the two-point function
is calculated up to two loops. Comparing our results
with theHartree–Fock approximation and with the stochastic
approach shows that the former resums only the cactus type
diagrams, whereas the latter contains the sunset diagram as
well and produces the correct result. We also demonstrate
that the long-wavelength expectation value of the commutator
of two fields is equal to zero both for spacelike and
timelike separated points
Giant vortices, vortex rings and reentrant behavior in type-1.5 superconductors
We predict that in a bulk type-1.5 superconductor the competing magnetic
responses of the two components of the order parameter can result in a vortex
interaction that generates group-stabilized giant vortices and unusual vortex
rings in the absence of any extrinsic pinning or confinement mechanism. We also
find within the Ginzburg-Landau theory a rich phase diagram with successions of
behaviors like type-1 -> type-1.5 -> type-2 -> type-1.5 as temperature
decreases.Comment: 5 pages, 4 figure
Current-biased Transition-edge Sensors Based on Re-entrant Superconductors
AbstractTransition-edge sensors are widely recognized as one of the most sensitive tools for the photon and particles detection in many areas, from astrophysics to quantum computing. Their application became practical after understanding that rather than being biased in a constant current mode, they should be biased in a constant voltage mode. Despite the methods of voltage biasing of these sensors are well developed since then, generally the current biasing is more convenient for superconducting circuits. Thus transition-edge sensors designed inherently to operate in the current-biased mode are desirable. We developed a design for such detectors based on re-entrant superconductivity. In this case constant current biasing takes place in the normal state, below the superconducting transition, so that following the absorption of a photon it does not yield a latching. Rather, the sensor gains energy and shifts towards the lower resistant (e.g., superconducting) state, and then cools down faster (since Joule heating is now reduced), and resets in a natural way to be able to detect the next photon. We prototyped this kind of transition edge sensors and tested them operational in accordance with the outlined physics. The samples used in experiments were modified compositions of YBCO-superconductors in a ceramic form (which, as we discovered, reproducibly demonstrates re-entrant superconductivity). In this presentation we report their composition, methods of preparation, and the detection results. This approach, in some areas, may have practical advantage over the traditional voltage-biased devices
Massive scalar field in de Sitter spacetime: a two-loop calculation and a comparison with the stochastic approach
We examine long-wavelength correlation functions of massive scalar fields in
de Sitter spacetime. For the theory with a quartic self-interaction, the
two-point function is calculated up to two loops. Comparing our results with
the Hartree-Fock approximation and with the stochastic approach shows that the
former resums only the cactus type diagrams, whereas the latter contains the
sunset diagram as well and produces the correct result. We also demonstrate
that the long-wavelength expectation value of the commutator of two fields is
equal to zero both for spacelike and timelike separated points.Comment: Presentation expanded, matches published versio
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