17,005 research outputs found
The Isometries of Low-Energy Heterotic M-Theory
We study the effective D=4, N=1 supergravity description of five-dimensional
heterotic M-theory in the presence of an M5 brane, and derive the Killing
vectors and isometry group for the Kahler moduli-space metric. The group is
found to be a non-semisimple maximal parabolic subgroup of Sp(4,R), containing
a non-trivial SL(2,R) factor. The underlying moduli-space is then naturally
realised as the group space Sp(4,R)/U(2), but equipped with a nonhomogeneous
metric that is invariant only under that maximal parabolic group. This
nonhomogeneous metric space can also be derived via field truncations and
identifications performed on Sp(8,R)/U(4) with its standard homogeneous metric.
In a companion paper we use these symmetries to derive new cosmological
solutions from known ones.Comment: 11 pages, 1 table; two foonotes added, minor corrections to conten
Effectively Closed Infinite-Genus Surfaces and the String Coupling
The class of effectively closed infinite-genus surfaces, defining the
completion of the domain of string perturbation theory, can be included in the
category , which is characterized by the vanishing capacity of the ideal
boundary. The cardinality of the maximal set of endpoints is shown to be
2^{\mit N}. The product of the coefficient of the genus-g superstring
amplitude in four dimensions by in the limit is an
exponential function of the genus with a base comparable in magnitude to the
unified gauge coupling. The value of the string coupling is consistent with the
characteristics of configurations which provide a dominant contribution to a
finite vacuum amplitude.Comment: TeX, 33 page
Behavioural clusters and predictors of performance during recovery from stroke
We examined the patterns and variability of recovery post-stroke in multiple behavioral domains. A large cohort of first time stroke patients with heterogeneous lesions was studied prospectively and longitudinally at 1-2 weeks, 3 months and one year post-injury with structural MRI to measure lesion anatomy and in-depth neuropsychological assessment. Impairment was described at all timepoints by a few clusters of correlated deficits. The time course and magnitude of recovery was similar across domains, with change scores largely proportional to the initial deficit and most recovery occurring within the first three months. Damage to specific white matter tracts produced poorer recovery over several domains: attention and superior longitudinal fasciculus II/III, language and posterior arcuate fasciculus, motor and corticospinal tract. Finally, after accounting for the severity of the initial deficit, language and visual memory recovery/outcome was worse with lower education, while the occurrence of multiple deficits negatively impacted attention recovery
A Computational Procedure to Detect a New Type of High Dimensional Chaotic Saddle and its Application to the 3-D Hill's Problem
A computational procedure that allows the detection of a new type of
high-dimensional chaotic saddle in Hamiltonian systems with three degrees of
freedom is presented. The chaotic saddle is associated with a so-called
normally hyperbolic invariant manifold (NHIM). The procedure allows to compute
appropriate homoclinic orbits to the NHIM from which we can infer the existence
a chaotic saddle. NHIMs control the phase space transport across an equilibrium
point of saddle-centre-...-centre stability type, which is a fundamental
mechanism for chemical reactions, capture and escape, scattering, and, more
generally, ``transformation'' in many different areas of physics. Consequently,
the presented methods and results are of broad interest. The procedure is
illustrated for the spatial Hill's problem which is a well known model in
celestial mechanics and which gained much interest e.g. in the study of the
formation of binaries in the Kuiper belt.Comment: 12 pages, 6 figures, pdflatex, submitted to JPhys
Abelian link invariants and homology
We consider the link invariants defined by the quantum Chern-Simons field
theory with compact gauge group U(1) in a closed oriented 3-manifold M. The
relation of the abelian link invariants with the homology group of the
complement of the links is discussed. We prove that, when M is a homology
sphere or when a link -in a generic manifold M- is homologically trivial, the
associated observables coincide with the observables of the sphere S^3. Finally
we show that the U(1) Reshetikhin-Turaev surgery invariant of the manifold M is
not a function of the homology group only, nor a function of the homotopy type
of M alone.Comment: 18 pages, 3 figures; to be published in Journal of Mathematical
Physic
Integrability and strong normal forms for non-autonomous systems in a neighbourhood of an equilibrium
The paper deals with the problem of existence of a convergent "strong" normal
form in the neighbourhood of an equilibrium, for a finite dimensional system of
differential equations with analytic and time-dependent non-linear term. The
problem can be solved either under some non-resonance hypotheses on the
spectrum of the linear part or if the non-linear term is assumed to be (slowly)
decaying in time. This paper "completes" a pioneering work of Pustil'nikov in
which, despite under weaker non-resonance hypotheses, the nonlinearity is
required to be asymptotically autonomous. The result is obtained as a
consequence of the existence of a strong normal form for a suitable class of
real-analytic Hamiltonians with non-autonomous perturbations.Comment: 10 page
Spectroscopic evidence for strong correlations between local superconducting gap and local Altshuler-Aronov density-of-states suppression in ultrathin NbN films
Disorder has different profound effects on superconducting thin films. For a
large variety of materials, increasing disorder reduces electronic screening
which enhances electron-electron repulsion. These fermionic effects lead to a
mechanism described by Finkelstein: when disorder combined to electron-electron
interactions increases, there is a global decrease of the superconducting
energy gap and of the critical temperature , the ratio
/ remaining roughly constant. In addition, in most films an
emergent granularity develops with increasing disorder and results in the
formation of inhomogeneous superconducting puddles. These gap inhomogeneities
are usually accompanied by the development of bosonic features: a pseudogap
develops above the critical temperature and the energy gap
starts decoupling from . Thus the mechanism(s) driving the appearance of
these gap inhomogeneities could result from a complicated interplay between
fermionic and bosonic effects. By studying the local electronic properties of a
NbN film with scanning tunneling spectroscopy (STS) we show that the
inhomogeneous spatial distribution of is locally strongly correlated
to a large depletion in the local density of states (LDOS) around the Fermi
level, associated to the Altshuler-Aronov effect induced by strong electronic
interactions. By modelling quantitatively the measured LDOS suppression, we
show that the latter can be interpreted as local variations of the film
resistivity. This local change in resistivity leads to a local variation of
through a local Finkelstein mechanism. Our analysis furnishes a purely
fermionic scenario explaining quantitatively the emergent superconducting
inhomogeneities, while the precise origin of the latter remained unclear up to
now.Comment: 11 pages, 4 figure
Diffraction by a small aperture in conical geometry: Application to metal coated tips used in near-field scanning optical microscopy
Light diffraction through a subwavelength aperture located at the apex of a
metallic screen with conical geometry is investigated theoretically. A method
based on a multipole field expansion is developed to solve Maxwell's equations
analytically using boundary conditions adapted both for the conical geometry
and for the finite conductivity of a real metal. The topological properties of
the diffracted field are discussed in detail and compared to those of the field
diffracted through a small aperture in a flat screen, i. e. the Bethe problem.
The model is applied to coated, conically tapered optical fiber tips that are
used in Near-Field Scanning Optical Microscopy. It is demonstrated that such
tips behave over a large portion of space like a simple combination of two
effective dipoles located in the apex plane (an electric dipole and a magnetic
dipole parallel to the incident fields at the apex) whose exact expressions are
determined. However, the large "backward" emission in the P plane - a salient
experimental fact that remained unexplained so far - is recovered in our
analysis which goes beyond the two-dipole approximation.Comment: 21 pages, 6 figures, published in PRE in 200
Confinement of superconducting fluctuations due to emergent electronic inhomogeneities
The microscopic nature of an insulating state in the vicinity of a
superconducting state, in the presence of disorder, is a hotly debated
question. While the simplest scenario proposes that Coulomb interactions
destroy the Cooper pairs at the transition, leading to localization of single
electrons, an alternate possibility supported by experimental observations
suggests that Cooper pairs instead directly localize. The question of the
homogeneity, granularity, or possibly glassiness of the material on the verge
of this transition is intimately related to this fundamental issue. Here, by
combining macroscopic and nano-scale studies of superconducting ultrathin NbN
films, we reveal nanoscopic electronic inhomogeneities that emerge when the
film thickness is reduced. In addition, while thicker films display a purely
two-dimensional behaviour in the superconducting fluctuations, we demonstrate a
zero-dimensional regime for the thinner samples precisely on the scale of the
inhomogeneities. Such behavior is somehow intermediate between the Fermi and
Bose insulator paradigms and calls for further investigation to understand the
way Cooper pairs continuously evolve from a bound state of fermionic objects
into localized bosonic entities.Comment: 29 pages 9 figure
The response of a neutral atom to a strong laser field probed by transient absorption near the ionisation threshold
We present transient absorption spectra of an extreme ultraviolet attosecond pulse train in helium dressed by an 800 nm laser field with intensity ranging from W/cm to W/cm. The energy range probed spans 16-42 eV, straddling the first ionisation energy of helium (24.59 eV). By changing the relative polarisation of the dressing field with respect to the attosecond pulse train polarisation we observe a large change in the modulation of the absorption reflecting the vectorial response to the dressing field. With parallel polarized dressing and probing fields, we observe significant modulations with periods of one half and one quarter of the dressing field period. With perpendicularly polarized dressing and probing fields, the modulations of the harmonics above the ionisation threshold are significantly suppressed. A full-dimensionality solution of the single-atom time-dependent Schr odinger equation obtained using the recently developed ab-initio time-dependent B-spline ADC method reproduce some of our observations
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