821 research outputs found
Unitarity in three-dimensional flat space higher spin theories
We investigate generic flat-space higher spin theories in three dimensions
and find a no-go result, given certain assumptions that we spell out. Namely,
it is only possible to have at most two out of the following three properties:
unitarity, flat space, non-trivial higher spin states. Interestingly, unitarity
provides an (algebra-dependent) upper bound on the central charge, like c=42
for the Galilean algebra. We extend this no-go result to rule
out unitary "multi-graviton" theories in flat space. We also provide an example
circumventing the no-go result: Vasiliev-type flat space higher spin theory
based on hs(1) can be unitary and simultaneously allow for non-trivial
higher-spin states in the dual field theory.Comment: 34 pp, v2: added two paragraphs in section 5.3 + minor change
Flux Jumps Driven by a Pulsed Magnetic Field
The understanding of flux jumps in the high temperature superconductors is of
importance since the occurrence of these jumps may limit the perspectives of
the practical use of these materials. In this work we present the experimental
study of the role of heavy ion irradiation in stabilizing the HTSC against flux
jumps by comparing un-irradiated and 7.5 10^10 Kr-ion/cm2 irradiated
(YxTm1-x)Ba2Cu3O7 single crystals. Using pulsed field magnetization
measurements, we have applied a broad range of field sweep rates from 0.1T/s up
to 1800 T/s to investigate the behavior of the flux jumps. The observed flux
jumps, which may be attributed to thermal instabilities, are incomplete and
have different amplitudes. The flux jumps strongly depend on the magnetic
field, on the magneto-thermal history of the sample, on the magnetic field
sweep rate, on the critical current density jc, on the temperature and on the
thermal contact with the bath in which the sample is immersed.Comment: 5 pages, PDF-fil
Axion-Dilaton Domain Walls and Fake Supergravity
Dynamical systems methods are used to investigate domain-wall solutions of a
two-parameter family of models in which gravity is coupled to an axion, and to
a dilaton with an exponential potential of either sign. A complete global
analysis is presented for (i) constant axion and (ii) flat walls, including a
study of bifurcations and a new exact domain-wall solution with non-constant
axion. We reconsider `fake supergravity' issues in light of these results. We
show, by example, how domain walls determine multi-valued superpotentials that
branch at stationary points that are not stationary points of the potential,
and we apply this result to potentials with anti-de Sitter vacua. We also show
by example that `adapted' truncation to a single-scalar model is sometimes
inconsistent, and we propose a `generalized' fake supergravity formalism that
applies in some such cases.Comment: 43pp, 19 figures; minor corrections and extensions; one additional
figur
Lagged and instantaneous dynamical influences related to brain structural connectivity
Contemporary neuroimaging methods can shed light on the basis of human neural
and cognitive specializations, with important implications for neuroscience and
medicine. Different MRI acquisitions provide different brain networks at the
macroscale; whilst diffusion-weighted MRI (dMRI) provides a structural
connectivity (SC) coincident with the bundles of parallel fibers between brain
areas, functional MRI (fMRI) accounts for the variations in the
blood-oxygenation-level-dependent T2* signal, providing functional connectivity
(FC).Understanding the precise relation between FC and SC, that is, between
brain dynamics and structure, is still a challenge for neuroscience. To
investigate this problem, we acquired data at rest and built the corresponding
SC (with matrix elements corresponding to the fiber number between brain areas)
to be compared with FC connectivity matrices obtained by 3 different methods:
directed dependencies by an exploratory version of structural equation modeling
(eSEM), linear correlations (C) and partial correlations (PC). We also
considered the possibility of using lagged correlations in time series; so, we
compared a lagged version of eSEM and Granger causality (GC). Our results were
two-fold: firstly, eSEM performance in correlating with SC was comparable to
those obtained from C and PC, but eSEM (not C nor PC) provides information
about directionality of the functional interactions. Second, interactions on a
time scale much smaller than the sampling time, captured by instantaneous
connectivity methods, are much more related to SC than slow directed influences
captured by the lagged analysis. Indeed the performance in correlating with SC
was much worse for GC and for the lagged version of eSEM. We expect these
results to supply further insights to the interplay between SC and functional
patterns, an important issue in the study of brain physiology and function.Comment: Accepted and published in Frontiers in Psychology in its current
form. 27 pages, 1 table, 5 figures, 2 suppl. figure
Shapiro steps in a superconducting film with an antidot lattice
Shapiro voltage steps at voltages V_n=nV_0 (n integer) have been observed in
the voltage-current characteristics of a superconducting film with a square
lattice of perforating microholes (antidots)in the presence of radiofrequent
radiation. These equidistant steps appear at the second matching field H_2 when
the flow of the interstitial vortex lattice in the periodic potential created
by the antidots and the vortices trapped by them, is in phase with the applied
rf frequency. Therefore, the observation of Shapiro steps clearly reveals the
presence of mobile intersitial vortices in superconducting films with regular
pinning arrays. The interstitial vortices, moved by the driving current,
coexist with immobile vortices strongly pinned at the antidots.Comment: 6 pages text, 3 EPS figures, RevTeX, accepted for publication in PRB
Rapid Communication
Non-relativistic ten-dimensional minimal supergravity
We construct a non-relativistic limit of ten-dimensional N=1 supergravity
from the point of view of the symmetries, the action, and the equations of
motion. This limit can only be realized in a supersymmetric way provided we
impose by hand a set of geometric constraints, invariant under all the
symmetries of the non-relativistic theory, that define a so-called `self-dual'
Dilatation-invariant String Newton-Cartan geometry. The non-relativistic action
exhibits three emerging symmetries: one local scale symmetry and two local
conformal supersymmetries. Due to these emerging symmetries the Poisson
equation for the Newton potential and two partner fermionic equations do not
follow from a variation of the non-relativistic action but, instead, are
obtained by a supersymmetry variation of the other equations of motion that do
follow from a variation of the non-relativistic action. We shortly discuss the
inclusion of the Yang-Mills sector that would lead to a non-relativistic
heterotic supergravity action.Comment: 40 pages, minor change
Confinement and Quantization Effects in Mesoscopic Superconducting Structures
We have studied quantization and confinement effects in nanostructured
superconductors. Three different types of nanostructured samples were
investigated: individual structures (line, loop, dot), 1-dimensional (1D)
clusters of loops and 2D clusters of antidots, and finally large lattices of
antidots. Hereby, a crossover from individual elementary "plaquettes", via
clusters, to huge arrays of these elements, is realized. The main idea of our
study was to vary the boundary conditions for confinement of the
superconducting condensate by taking samples of different topology and, through
that, modifying the lowest Landau level E_LLL(H). Since the critical
temperature versus applied magnetic field T_c(H) is, in fact, E_LLL(H) measured
in temperature units, it is varied as well when the sample topology is changed
through nanostructuring. We demonstrate that in all studied nanostructured
superconductors the shape of the T_c(H) phase boundary is determined by the
confinement topology in a unique way.Comment: 28 pages, 19 EPS figures, uses LaTeX's aipproc.sty, contribution to
Euroschool on "Superconductivity in Networks and Mesoscopic Systems", held in
Siena, Italy (8-20 september 1997
The Dutch version of the Child Posttraumatic Cognitions Inventory:validation in a clinical sample and a school sample
With the inclusion of trauma-related cognitions in the DSM-5 criteria for posttraumatic stress disorder (PTSD), the assessment of these cognitions has become essential. Therefore, valid tools for the assessment of these cognitions are warranted
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