6,346 research outputs found
Vacuum type I spacetimes and aligned Papapetrou fields: symmetries
We analyze type I vacuum solutions admitting an isometry whose Killing
2--form is aligned with a principal bivector of the Weyl tensor, and we show
that these solutions belong to a family of type I metrics which admit a group
of isometries. We give a classification of this family and we study the
Bianchi type for each class. The classes compatible with an aligned Killing
2--form are also determined. The Szekeres-Brans theorem is extended to non
vacuum spacetimes with vanishing Cotton tensor.Comment: 19 pages; a reference adde
On the classification of type D spacetimes
We give a classification of the type D spacetimes based on the invariant
differential properties of the Weyl principal structure. Our classification is
established using tensorial invariants of the Weyl tensor and, consequently,
besides its intrinsic nature, it is valid for the whole set of the type D
metrics and it applies on both, vacuum and non-vacuum solutions. We consider
the Cotton-zero type D metrics and we study the classes that are compatible
with this condition. The subfamily of spacetimes with constant argument of the
Weyl eigenvalue is analyzed in more detail by offering a canonical expression
for the metric tensor and by giving a generalization of some results about the
non-existence of purely magnetic solutions. The usefulness of these results is
illustrated in characterizing and classifying a family of Einstein-Maxwell
solutions. Our approach permits us to give intrinsic and explicit conditions
that label every metric, obtaining in this way an operational algorithm to
detect them. In particular a characterization of the Reissner-Nordstr\"{o}m
metric is accomplished.Comment: 29 pages, 0 figure
Gap solitons in quasiperiodic optical lattices
Families of solitons in one- and two-dimensional (1D and 2D) Gross-Pitaevskii
equations with the repulsive nonlinearity and a potential of the
quasicrystallic type are constructed (in the 2D case, the potential corresponds
to a five-fold optical lattice). Stable 1D solitons in the weak potential are
explicitly found in three bandgaps. These solitons are mobile, and they collide
elastically. Many species of tightly bound 1D solitons are found in the strong
potential, both stable and unstable (unstable ones transform themselves into
asymmetric breathers). In the 2D model, families of both fundamental and
vortical solitons are found and are shown to be stable.Comment: 8 pages, 11 figure
Local thermal equilibrium and ideal gas Stephani universes
The Stephani universes that can be interpreted as an ideal gas evolving in
local thermal equilibrium are determined. Five classes of thermodynamic schemes
are admissible, which give rise to five classes of regular models and three
classes of singular models. No Stephani universes exist representing an exact
solution to a classical ideal gas (one for which the internal energy is
proportional to the temperature). But some Stephani universes may approximate a
classical ideal gas at first order in the temperature: all of them are
obtained. Finally, some features about the physical behavior of the models are
pointed out.Comment: 20 page
Simbol-X Background Minimization: Mirror Spacecraft Passive Shielding Trade-Off Study
The present work shows a quantitative trade-off analysis of the Simbol-X
Mirror Spacecraft (MSC) passive shielding, in the phase space of the various
parameters: mass budget, dimension, geometry, and composition. A simplified
physical (and geometrical) model of the sky screen, implemented by means of a
GEANT4 simulation, has been developed to perform a performance-driven mass
optimization and evaluate the residual background level on Simbol-X focal
plane.Comment: 3 pages, 6 figures, to appear in the proceedings of the second
Simbol-X International Symposium "Simbol-X - Focusing on the Hard X-ray
Universe", AIP Conf. Proc. Series, P. Ferrando and J. Rodriguez ed
Angular dependence of magnetoresistivity in c-oriented MgB2 thin film
The anisotropy of MgB2 is still under debate: its value, strongly dependent
on the sample and on the measuring method, ranges between 1.2 and 13. In this
work we present our results on a MgB2 c-oriented superconducting thin film. To
evaluate the anisotropy, we followed two different approaches. Firstly,
magnetoresistivity was measured as a function of temperature at selected
magnetic fields applied both parallel and perpendicular to the c-axis;
secondly, we measured magnetoresistivity at selected temperatures and magnetic
fields, varying the angle q between the magnetic field and the c-axis. The
anisotropy estimated from the ratio between the upper critical fields parallel
and perpendicular to the c-axis and the one obtained in the framework of the
scaling approach within the anisotropic Ginzburg-Landau theory are different
but show a similar trend in the temperature dependence. The obtained results
are compared and discussed in the light of the two-band nature of MgB2. A
comparison between critical fields in thin films and single crystal is also
performed.Comment: 13 pages, 4 figures, European Physical Journal B in pres
Dynamical confinement in bosonized QCD2
In the bosonized version of two dimensional theories non trivial boundary
conditions (topology) play a crucial role. They are inevitable if one wants to
describe non singlet states. In abelian bosonization, color is the charge of a
topological current in terms of a non-linear meson field. We show that
confinement appears as the dynamical collapse of the topology associated with
its non trivial boundary conditions.Comment: 11 pages, figures not included, ftuv/92-
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