1,404 research outputs found
Higher Dimensional Gravity, Propagating Torsion and AdS Gauge Invariance
The most general theory of gravity in d-dimensions which leads to second
order field equations for the metric has [(d-1)/2] free parameters. It is shown
that requiring the theory to have the maximum possible number of degrees of
freedom, fixes these parameters in terms of the gravitational and the
cosmological constants. In odd dimensions, the Lagrangian is a Chern-Simons
form for the (A)dS or Poincare groups. In even dimensions, the action has a
Born-Infeld-like form. Torsion may occur explicitly in the Lagrangian in the
parity-odd sector and the torsional pieces respect local (A)dS symmetry for
d=4k-1 only. These torsional Lagrangians are related to the Chern-Pontryagin
characters for the (A)dS group. The additional coefficients in front of these
new terms in the Lagrangian are shown to be quantized.Comment: 10 pages, two columns, no figures, title changed in journal, final
version to appear in Class. Quant. Gra
Simple compactifications and Black p-branes in Gauss-Bonnet and Lovelock Theories
We look for the existence of asymptotically flat simple compactifications of
the form in -dimensional gravity theories with higher
powers of the curvature. Assuming the manifold to be spherically
symmetric, it is shown that the Einstein-Gauss-Bonnet theory admits this class
of solutions only for the pure Einstein-Hilbert or Gauss-Bonnet Lagrangians,
but not for an arbitrary linear combination of them. Once these special cases
have been selected, the requirement of spherical symmetry is no longer relevant
since actually any solution of the pure Einstein or pure Gauss-Bonnet theories
can then be toroidally extended to higher dimensions. Depending on and the
spacetime dimension, the metric on may describe a black hole or a
spacetime with a conical singularity, so that the whole spacetime describes a
black or a cosmic -brane, respectively. For the purely Gauss-Bonnet theory
it is shown that, if is four-dimensional, a new exotic class of black
hole solutions exists, for which spherical symmetry can be relaxed.
Under the same assumptions, it is also shown that simple compactifications
acquire a similar structure for a wide class of theories among the Lovelock
family which accepts this toroidal extension.
The thermodynamics of black -branes is also discussed, and it is shown
that a thermodynamical analogue of the Gregory-Laflamme transition always
occurs regardless the spacetime dimension or the theory considered, hence not
only for General Relativity.
Relaxing the asymptotically flat behavior, it is also shown that exact black
brane solutions exist within a very special class of Lovelock theories.Comment: 30 pages, no figures, few typos fixed, references added, final
version for JHE
Junction conditions in General Relativity with spin sources
The junction conditions for General Relativity in the presence of domain
walls with intrinsic spin are derived in three and higher dimensions. A stress
tensor and a spin current can be defined just by requiring the existence of a
well defined volume element instead of an induced metric, so as to allow for
generic torsion sources. In general, when the torsion is localized on the
domain wall, it is necessary to relax the continuity of the tangential
components of the vielbein. In fact it is found that the spin current is
proportional to the jump in the vielbein and the stress-energy tensor is
proportional to the jump in the spin connection. The consistency of the
junction conditions implies a constraint between the direction of flow of
energy and the orientation of the spin. As an application, we derive the
circularly symmetric solutions for both the rotating string with tension and
the spinning dust string in three dimensions. The rotating string with tension
generates a rotating truncated cone outside and a flat space-time with
inevitable frame dragging inside. In the case of a string made of spinning
dust, in opposition to the previous case no frame dragging is present inside,
so that in this sense, the dragging effect can be "shielded" by considering
spinning instead of rotating sources. Both solutions are consistently lifted as
cylinders in the four-dimensional case.Comment: 24 pages, no figures, CECS style. References added and misprints
corrected. Published Versio
Green's function formalism for spin transport in metal-insulator-metal heterostructures
We develop a Green's function formalism for spin transport through
heterostructures that contain metallic leads and insulating ferromagnets. While
this formalism in principle allows for the inclusion of various magnonic
interactions, we focus on Gilbert damping. As an application, we consider
ballistic spin transport by exchange magnons in a metal-insulator-metal
heterostructure with and without disorder. For the former case, we show that
the interplay between disorder and Gilbert damping leads to spin current
fluctuations. For the case without disorder, we obtain the dependence of the
transmitted spin current on the thickness of the ferromagnet. Moreover, we show
that the results of the Green's function formalism agree in the clean and
continuum limit with those obtained from the linearized stochastic
Landau-Lifshitz-Gilbert equation. The developed Green's function formalism is a
natural starting point for numerical studies of magnon transport in
heterostructures that contain normal metals and magnetic insulators.Comment: 13 pages, 8 figure
Transgression forms and extensions of Chern-Simons gauge theories
A gauge invariant action principle, based on the idea of transgression forms,
is proposed. The action extends the Chern-Simons form by the addition of a
boundary term that makes the action gauge invariant (and not just
quasi-invariant). Interpreting the spacetime manifold as cobordant to another
one, the duplication of gauge fields in spacetime is avoided. The advantages of
this approach are particularly noticeable for the gravitation theory described
by a Chern-Simons lagrangian for the AdS group, in which case the action is
regularized and finite for black hole geometries in diverse situations. Black
hole thermodynamics is correctly reproduced using either a background field
approach or a background-independent setting, even in cases with asymptotically
nontrivial topologies. It is shown that the energy found from the thermodynamic
analysis agrees with the surface integral obtained by direct application of
Noether's theorem.Comment: 28 pages, no figures. Minor changes in the introduction, final
comments and reference
- …