104 research outputs found
Dynamics of apparent horizons in quantum gravitational collapse
We study the gravitational collapse of a massless scalar field within the
effective scenario of loop quantum gravity. Classical singularity is avoided
and replaced by a quantum bounce in this model. It is shown that, quantum
gravity effects predict a threshold scale below which no horizon can form as
the collapse evolves towards the bounce.Comment: Contribution to the Spanish Relativity Meeting in Portugal 2012
(ERE2012), Guimaraes, Portuga
Localization of a matter wave packet in a disordered potential
We theoretically study the Anderson localization of a matter wave packet in a
one-dimensional disordered potential. We develop an analytical model which
includes the initial phase-space density of the matter wave and the spectral
broadening induced by the disorder. Our approach predicts a behavior of the
localized density profile significantly more complex than a simple exponential
decay. These results are confirmed by large-scale and long-time numerical
calculations. They shed new light on recent experiments with ultracold atoms
and may impact their analysis
Anderson localization of matter waves in tailored disordered potentials
We show that, in contrast to immediate intuition, Anderson localization of
noninteracting particles induced by a disordered potential in free space can
increase (i.e., the localization length can decrease) when the particle energy
increases, for appropriately tailored disorder correlations. We predict the
effect in one, two, and three dimensions, and propose a simple method to
observe it using ultracold atoms placed in optical disorder. The increase of
localization with the particle energy can serve to discriminate quantum versus
classical localization
Tailoring Anderson localization by disorder correlations in 1D speckle potentials
We study Anderson localization of single particles in continuous, correlated,
one-dimensional disordered potentials. We show that tailored correlations can
completely change the energy-dependence of the localization length. By
considering two suitable models of disorder, we explicitly show that disorder
correlations can lead to a nonmonotonic behavior of the localization length
versus energy. Numerical calculations performed within the transfer-matrix
approach and analytical calculations performed within the phase formalism up to
order three show excellent agreement and demonstrate the effect. We finally
show how the nonmonotonic behavior of the localization length with energy can
be observed using expanding ultracold-atom gases
Three-dimensional localization of ultracold atoms in an optical disordered potential
We report a study of three-dimensional (3D) localization of ultracold atoms
suspended against gravity, and released in a 3D optical disordered potential
with short correlation lengths in all directions. We observe density profiles
composed of a steady localized part and a diffusive part. Our observations are
compatible with the self-consistent theory of Anderson localization, taking
into account the specific features of the experiment, and in particular the
broad energy distribution of the atoms placed in the disordered potential. The
localization we observe cannot be interpreted as trapping of particles with
energy below the classical percolation threshold.Comment: published in Nature Physics; The present version is the initial
manuscript (unchanged compared to version 1); The published version is
available online at
http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2256.htm
Matter Wave Transport and Anderson Localization in Anisotropic 3D Disorder
We study quantum transport in anisotropic 3D disorder and show that non
rotation invariant correlations can induce rich diffusion and localization
properties. For instance, structured finite-range correlations can lead to the
inversion of the transport anisotropy. Moreover, working beyond the
self-consistent theory of localization, we include the disorder-induced shift
of the energy states and show that it strongly affects the mobility edge.
Implications to recent experiments are discussed.Comment: Text unchanged, Reference added: EPL 99 (2012) 5000
Bose-Einstein Condensate in Weak 3d Isotropic Speckle Disorder
The effect of a weak three-dimensional (3d) isotropic laser speckle disorder
on various thermodynamic properties of a dilute Bose gas is considered at zero
temperature. First, we summarize the derivation of the autocorrelation function
of laser speckles in 1d and 2d following the seminal work of Goodman. The goal
of this discussion is to show that a Gaussian approximation of this function,
proposed in some recent papers, is inconsistent with the general background of
laser speckle theory. Then we propose a possible experimental realization for
an isotropic 3d laser speckle potential and derive its corresponding
autocorrelation function. Using a Fourier transform of that function, we
calculate both condensate depletion and sound velocity of a Bose-Einstein
condensate as disorder ensemble averages of such a weak laser speckle potential
within a perturbative solution of the Gross-Pitaevskii equation. By doing so,
we reproduce the expression of the normalfluid density obtained earlier within
the treatment of Landau. This physically transparent derivation shows that
condensate particles, which are scattered by disorder, form a gas of
quasiparticles which is responsible for the normalfluid component
Framing image registration as a landmark detection problem for better representation of clinical relevance
Nowadays, registration methods are typically evaluated based on
sub-resolution tracking error differences. In an effort to reinfuse this
evaluation process with clinical relevance, we propose to reframe image
registration as a landmark detection problem. Ideally, landmark-specific
detection thresholds are derived from an inter-rater analysis. To approximate
this costly process, we propose to compute hit rate curves based on the
distribution of errors of a sub-sample inter-rater analysis. Therefore, we
suggest deriving thresholds from the error distribution using the formula:
median + delta * median absolute deviation. The method promises differentiation
of previously indistinguishable registration algorithms and further enables
assessing the clinical significance in algorithm development
Cerebral Changes Occurring in Arginase and Dimethylarginine Dimethylaminohydrolase (DDAH) in a Rat Model of Sleeping Sickness
Involvement of nitric oxide (NO) in the pathophysiology of human African trypanosomiasis (HAT) was analyzed in a HAT animal model (rat infected with Trypanosoma brucei brucei). With this model, it was previously reported that trypanosomes were capable of limiting trypanocidal properties carried by NO by decreasing its blood concentration. It was also observed that brain NO concentration, contrary to blood, increases throughout the infection process. The present approach analyses the brain impairments occurring in the regulations exerted by arginase and N(G), N(G)-dimethylarginine dimethylaminohydrolase (DDAH) on NO Synthases (NOS). In this respect: (i) cerebral enzymatic activities, mRNA and protein expression of arginase and DDAH were determined; (ii) immunohistochemical distribution and morphometric parameters of cells expressing DDAH-1 and DDAH-2 isoforms were examined within the diencephalon; (iii) amino acid profiles relating to NOS/arginase/DDAH pathways were established.Arginase and DDAH activities together with mRNA (RT-PCR) and protein (western-blot) expressions were determined in diencephalic brain structures of healthy or infected rats at various days post-infection (D5, D10, D16, D22). While arginase activity remained constant, that of DDAH increased at D10 (+65%) and D16 (+51%) in agreement with western-blot and amino acids data (liquid chromatography tandem-mass spectrometry). Only DDAH-2 isoform appeared to be up-regulated at the transcriptional level throughout the infection process. Immunohistochemical staining further revealed that DDAH-1 and DDAH-2 are contained within interneurons and neurons, respectively.In the brain of infected animals, the lack of change observed in arginase activity indicates that polyamine production is not enhanced. Increases in DDAH-2 isoform may contribute to the overproduction of NO. These changes are at variance with those reported in the periphery. As a whole, the above processes may ensure additive protection against trypanosome entry into the brain, i.e., maintenance of NO trypanocidal pressure and limitation of polyamine production, necessary for trypanosome growth
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