17 research outputs found
Topological regularization and self-duality in four-dimensional anti-de Sitter gravity
It is shown that the addition of a topological invariant (Gauss-Bonnet term)
to the anti-de Sitter (AdS) gravity action in four dimensions recovers the
standard regularization given by holographic renormalization procedure. This
crucial step makes possible the inclusion of an odd parity invariant
(Pontryagin term) whose coupling is fixed by demanding an asymptotic (anti)
self-dual condition on the Weyl tensor. This argument allows to find the dual
point of the theory where the holographic stress tensor is related to the
boundary Cotton tensor as , which
has been observed in recent literature in solitonic solutions and hydrodynamic
models.
A general procedure to generate the counterterm series for AdS gravity in any
even dimension from the corresponding Euler term is also briefly discussed.Comment: 13 pages, no figures; enlarged discussion on self-duality condition
for AAdS spacetimes, references added, final version for PR
Thermodynamics of Einstein-Born-Infeld black holes with negative cosmological constant
We study the thermodynamics associated to topological black hole solutions of
AdS gravity coupled to nonlinear electrodynamics (Born-Infeld) in any
dimension, using a background-independent regularization prescription for the
Euclidean action given by boundary terms which explicitly depend on the
extrinsic curvature (Kounterterms series). A finite action principle leads to
the correct definition of thermodynamic variables as Noether charges, which
satisfy a Smarr-like relation. In particular, for the odd-dimensional case, a
consistent thermodynamic description is achieved if the internal energy of the
system includes the vacuum energy for AdS spacetime.Comment: 29 pages, no figures; additional comments on extreme black hole case,
a few references added; final version for PR
BPS defects in AdS supergravity
AdS supergravity admits supersymmetric solutions that describe BPS defects.
Here, we investigate such solutions in AdS supergravity, which is
formulated as a Chern-Simons theory on . We compute the Killing spinor equation on the BTZ geometry
in different ways, looking for BPS solutions on the entire space of parameters.
We focus our attention on defects that represent geometries with integer
angular excesses; these correspond to specific negative values of the BTZ mass.
We compare our solutions with other results in the literature, finding exact
agreement. We argue that, in the semiclassical limit, the BPS defects can be
associated to degenerate representations of the Virasoro symmetry at the
boundary. The case of non-diagonal representations, describing stationary,
non-static defects, is also discussed.Comment: 26 pages, 1 figur
Couplings between Chern-Simons gravities and 2p-branes
The interaction between Chern-Simons (CS) theories and localized external
sources (2p-branes) is analyzed. This interaction generalizes the minimal
coupling between a point charge (0-brane) and a gauge connection. The external
currents that define the 2p-branes are covariantly constant (D-2p-1)-forms
coupled to (2p-1) CS forms. The general expression for the sources --charged
with respect to the corresponding gauge algebra-- is presented, focusing on two
special cases: 0-branes and (D-3)-branes.
In any dimension, 0-branes are constructed as topological defects produced by
a surface deficit of (D-2)-sphere in AdS space, and they are not constant
curvature spaces for D>3. They correspond to dimensionally continued black
holes with negative mass.
On the other hand, in the case of CS (super) gravities, the (D-3)-branes are
naked conical singularities (topological defects) obtained by identification of
points with a Killing vector. In 2+1 dimensions, extremal spinning branes of
this type are BPS states. Stable (D-3)-branes are shown to exist also in higher
dimensions, as well.
Classical field equations are also discussed and in the presence of sources
there is a large number of inequivalent and disconnected sectors in solution
space.Comment: 29 pages, no figures; version accepted in PRD; extended introduction
and several references added; some sections have been reorganized and several
minor corrections mad
Stable p-branes in Chern-Simons AdS supergravities
We construct static codimension-two branes in any odd dimension D, with
negative cosmological constant, and show that they are exact solutions of
Chern-Simons (super)gravity theory for (super)AdS coupled to external sources.
The stability of these solutions is analyzed by counting the number of
preserved supersymmetries. It is shown that static massive (D-3)-branes are
unstable unless some suitable gauge fields are added and the brane is extremal.
In particular, in three dimensions, a 0-brane is recognized as the negative
mass counterpart of the BTZ black hole. For these 0-branes, we write explicitly
electromagnetically charged BPS states with various number of preserved
supersymmetries within the OSp(p|2) x OSp(q|2) supergroups. In five dimensions,
we prove that stable 2-branes with electromagnetic charge always exist for the
generic supergroup SU(2,2|N), where N is different than 4. For the special case
N=4, in which the CS supergravity requires the addition of a nontrivial gauge
field configuration in order to preserve maximal number of degrees of freedom,
we show for two different static 2-branes that they are BPS states (one of
which is the ground state), and from the corresponding algebra of charges we
show that the energy is bounded from below. In higher dimensions, our results
admit a straightforward generalization, although there are presumably more
solutions corresponding to different intersections of the elementary objects.Comment: 43 pages, revtex4.cls; v2: slight amendments and references added to
match published versio
Geometry and stability of spinning branes in AdS gravity
The geometry of spinning codimension-two branes in AdS spacetime is analyzed
in three and higher dimensions. The construction of non-extremal solutions is
based on identifications in the covering of AdS space by isometries that have
fixed points. The discussion focuses on the cases where the parameters of
spinning states can be related to the velocity of a boosted static
codimension-two brane. The resulting configuration describes a single spinning
brane, or a set of intersecting branes, each one produced by an independent
identification. The nature of the singularity is also examined, establishing
that the AdS curvature acquires one in the form of a Dirac delta distribution.
The stability of the branes is studied in the framework of Chern-Simons AdS
supergravity. A class of branes, characterized by one free parameter, are shown
to be stable when the BPS conditions are satisfied. In 3D, these stable branes
are extremal, while in higher dimensions, the BPS branes are not the extremal
ones.Comment: 40 pages, 6 figure
Social Cognition in Patients With Cerebellar Neurodegenerative Disorders
ObjectiveCerebellar neurodegenerative disorders (CDs) are a heterogeneous group of disorders. It is known that the cerebellum plays a role not only in motor, but also in cognitive and social cognitive functions. The aim of this study was to investigate social cognition in patients with different CDs.Materials and MethodsSocial cognition was examined in 34 patients, 12 with spinocerebellar ataxia type 1 (SCA1), 6 with spinocerebellar ataxia type 2 (SCA2), and 16 with idiopathic late onset cerebellar ataxia (ILOCA). All patients were clinically evaluated using the Scale for the Rating and Assessment of Ataxia. In addition, 34 age, sex, and education-matched healthy control (HC) subjects were similarly analyzed. Social cognition was studied using two tests: the Faux Pas Recognition Test and the Reading the Mind in the Eyes Test (RMET). An appropriate array of neuropsychological tests was used to assess the global cognitive status as well as the frontal functions and mood.ResultsCD patients achieved significantly worse results on both tests of social cognition compared to the HCs. The SCA1 + 2 group achieved the poorest results on the Faux Pas Recognition Test and exhibited poor performance on all cognitive tests, but was only significantly worse compared to the ILOCA group on the Free and Cued Selective Reminding Test (FCSRT) – recognition. The patients in the SCA1 + 2 and ILOCA groups obtained similar scores on RMET. In the SCA1 + 2 group the findings significantly correlated with clinical parameters of disease severity and duration and executive functions (EFs), and with mood and executive functions in the ILOCA group. In the SCA group EFs appeared as the only significant predictor of RMET achievement. The Boston Naming Test (BTN) was a significant predictor of the CD patients’ achievement on RMET, while the BTN, the Trail Making Test Part A and FCSRT – Delayed free recall predicted their performance on the Faux Pas Recognition Test.ConclusionPatients with CD have social cognitive impairments as demonstrated by the Faux Pas Test and the RMET test results. The SCA1 and 2 patients exhibited a more pronounced impairment compared with the ILOCA patients. The independent cognitive predictors of social cognition impairment were EFs and language
Microstructure and toughness of CuNiMo austempered ductile iron
The effect of austempering on the microstructure and toughness of nodular cast iron (designated as CuNiMoSG) alloyed with molybdenum, copper, nickel, and manganese has been studied. Light microscopy (LM), scanning electron microscopy (SEM), and X-ray diffraction technique were performed for microstructural characterization, whereas impact energy test was applied for toughness measurement. Specimens were austenitised at 860 degreesC, then austempered for various times at 320 and 400 degreesC, followed by ice-water quenching. Austempering at 320 degreesC produces a microstructure consisting of a mixture of acicular bainitic ferrite and the stable carbon-enriched austenite. In this microstructure epsilon-carbides are also identified after austempering up to 5 h. Fracture mode is changed from ductile to brittle with the prolonged time of austempering at 320 degreesC. The highest impact energy (115 kJ) corresponds not only to ductile fracture, but also to the maximum value of the volume fraction of retained austenite. Only martensitic structure was observed during austempering at 400 degreesC, inducing brittle fracture and significantly low-impact energy (10-12 kJ)