5,385 research outputs found
Canonical Quantization of Spherically Symmetric Dust Collapse
Quantum gravity effects are likely to play a crucial role in determining the
outcome of gravitational collapse during its final stages. In this contribution
we will outline a canonical quantization of the LeMaitre-Tolman-Bondi models,
which describe the collapse of spherical, inhomogeneous, non-rotating dust.
Although there are many models of gravitational collapse, this particular class
of models stands out for its simplicity and the fact that both black holes and
naked singularity end states may be realized on the classical level, depending
on the initial conditions. We will obtain the appropriate Wheeler-DeWitt
equation and then solve it exactly, after regularization on a spatial lattice.
The solutions describe Hawking radiation and provide an elegant microcanonical
description of black hole entropy, but they raise other questions, most
importantly concerning the nature of gravity's fundamental degrees of freedom.Comment: 19 pages no figures. Contribution to a festschrift in honor of Joshua
N. Goldber
Role of hexagonal boron nitride in protecting ferromagnetic nanostructures from oxidation
Ferromagnetic contacts are widely used to inject spin polarized currents into
non-magnetic materials such as semiconductors or 2-dimensional materials like
graphene. In these systems, oxidation of the ferromagnetic materials poses an
intrinsic limitation on device performance. Here we investigate the role of
ex-situ transferred chemical vapour deposited hexagonal boron nitride (hBN) as
an oxidation barrier for nanostructured cobalt and permalloy electrodes. The
chemical state of the ferromagnets was investigated using X-ray photoemission
electron microscopy owing to its high sensitivity and lateral resolution. We
have compared the oxide thickness formed on ferromagnetic nanostructures
covered by hBN to uncovered reference structures. Our results show that hBN
reduces the oxidation rate of ferromagnetic nanostructures suggesting that it
could be used as an ultra-thin protection layer in future spintronic devices.Comment: 7 pages, 6 figure
Magnetic anisotropy modulation of magnetite in Fe3O4/BaTiO3(100) epitaxial structures
Temperature dependent magnetometry and transport measurements on epitaxial
Fe3O4 films grown on BaTiO3(100) single crystals by molecular beam epitaxy show
a series of discontinuities, that are due to changes in the magnetic anisotropy
induced by strain in the different crystal phases of BaTiO3. The magnetite film
is under tensile strain at room temperature, which is ascribed to the lattice
expansion of BaTiO3 at the cubic to tetragonal transition, indicating that the
magnetite film is relaxed at the growth temperature. From the magnetization
versus temperature curves, the variation in the magnetic anisotropy is
determined and compared with the magnetoelastic anisotropies. These results
demonstrate the possibility of using the piezoelectric response of BaTiO3 to
modulate the magnetic anisotropy of magnetite films.Comment: 4 pages, 4 figure
The Quantum Stress-Tensor in Self-Similar Spherical Dust Collapse
We calculate the quantum stress tensor for a massless scalar field in the 2-d
self-similar spherical dust collapse model which admits a naked singularity. We
find that the outgoing radiation flux diverges on the Cauchy horizon. This may
have two consequences. The resultant back reaction may prevent the naked
singularity from forming, thus preserving cosmic censorship through quantum
effects. The divergent flux may lead to an observable signature differentiating
naked singularities from black holes in astrophysical observations.Comment: Latex File, 19 page
The Eastward Enlargement of the Eurozone: Trade and FDI
Trade and FDI, Economic Integration
NEONATAL RESPIRATORY DISTRESS ... A CASE FOR OPHTHALMOLOGY?
Introdução: O dacriocistocelo congĂ©nito Ă© uma obstrução rara do canal lacrimo-nasal no recĂ©m-nascido, que pode dar sintomatologia precoce. A fisiologia deve-se Ă obstrução proximal e distal do canal lacrimo-nasal. A apresentação clĂnica Ă© variada e inclui formações quĂsticas, infetadas ou nĂŁo, dificuldade
na amamentação ou respiratĂłria. A apresentação clássica Ă© uma massa quĂstica de coloração azulada, abaixo do epicanto interno. O diagnĂłstico correto implica exames complementares. A intervenção terapĂŞutica está dependente da evolução clĂnica. O prognĂłstico Ă© bom e geralmente nĂŁo tem sequelas futuras.
Caso clĂnico: As autoras apresentam o caso de um recĂ©m nascido, internado por dificuldade respiratĂłria de inĂcio no segundo dia de vida. Ao exame objetivo tinha sinais de dificuldade respiratĂłria e lesĂŁo circinada de aspeto semelhante a hematoma no canto interno do olho esquerdo. Nesta localização houve desenvolvimento posterior de formação quĂstica com sinais inflamatĂłrios, com diagnĂłstico inicial de dacriocistite. ApĂłs realização de tomografia computorizada das Ăłrbitas mostrou tratar-se de um dacriocistocelo.
ConclusĂŁo: A dificuldade respiratĂłria pode mesmo ser um caso oftalmolĂłgico
Field-dependent anisotropic magnetoresistance and planar Hall effect in epitaxial magnetite thin films
A systematic study of the temperature and magnetic field dependence of the
longitudinal and transverse resistivities of epitaxial thin films of magnetite
(Fe3O4) is reported. The anisotropic magnetoresistance (AMR) and the planar
Hall effect (PHE) are sensitive to the in-plane orientation of current and
magnetization with respect to crystal axes in a way consistent with the cubic
symmetry of the system. We also show that the AMR exhibit sign reversal as a
function of temperature, and that it shows significant field dependence without
saturation up to 9 T. Our results provide a unified description of the
anisotropic magnetoresistance effects in epitaxial magnetite films and
illustrate the need for a full determination of the resistivity tensor in
crystalline systems
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