9,893 research outputs found
Exciton-phonon scattering and photo-excitation dynamics in J-aggregate microcavities
We have developed a model accounting for the photo-excitation dynamics and
the photoluminescence of strongly coupled J-aggregate microcavities. Our model
is based on a description of the J-aggregate film as a disordered Frenkel
exciton system in which relaxation occurs due to the presence of a thermal bath
of molecular vibrations. In a strongly coupled microcavity exciton-polaritons
are formed, mixing superradiant excitons and cavity photons. The calculation of
the microcavity steady-state photoluminescence, following a CW non resonant
pumping, is carried out. The experimental photoluminescence intensity ratio
between upper and lower polariton branches is accurately reproduced. In
particular both thermal activation of the photoluminescence intensity ratio and
its Rabi splitting dependence are a consequence of the bottleneck in the
relaxation, occurring at the bottom of the excitonic reservoir. The effects due
to radiative channels of decay of excitons and to the presence of a
paritticular set of discrete optical molecular vibrations active in relaxation
processes are investigared.Comment: 8 pages, 6 figure
The Nature and Nurture of Star Clusters
Star clusters have hierarchical patterns in space and time, suggesting
formation processes in the densest regions of a turbulent interstellar medium.
Clusters also have hierarchical substructure when they are young, which makes
them all look like the inner mixed parts of a pervasive stellar hierarchy.
Young field stars share this distribution, presumably because some of them came
from dissolved clusters and others formed in a dispersed fashion in the same
gas. The fraction of star formation that ends up in clusters is apparently not
constant, but may increase with interstellar pressure. Hierarchical structure
explains why stars form in clusters and why many of these clusters are
self-bound. It also explains the cluster mass function. Halo globular clusters
share many properties of disk clusters, including what appears to be an upper
cluster cutoff mass. However, halo globulars are self-enriched and often
connected with dwarf galaxy streams. The mass function of halo globulars could
have initially been like the power law mass function of disk clusters, but the
halo globulars have lost their low mass members. The reasons for this loss are
not understood. It could have happened slowly over time as a result of cluster
evaporation, or it could have happened early after cluster formation as a
result of gas loss. The latter model explains best the observation that the
globular cluster mass function has no radial gradient in galaxies.Comment: to be published in IAUS266: Star Clusters Basic Galactic Building
Blocks Throughout Time And Space, eds. Richard de Grijs and Jacques Lepine,
Cambridge University Press, 11 page
Radiative corrections to the excitonic molecule state in GaAs microcavities
The optical properties of excitonic molecules (XXs) in GaAs-based quantum
well microcavities (MCs) are studied, both theoretically and experimentally. We
show that the radiative corrections to the XX state, the Lamb shift
and radiative width , are
large, about of the molecule binding energy , and
definitely cannot be neglected. The optics of excitonic molecules is dominated
by the in-plane resonant dissociation of the molecules into outgoing
1-mode and 0-mode cavity polaritons. The later decay channel,
``excitonic molecule 0-mode polariton + 0-mode
polariton'', deals with the short-wavelength MC polaritons invisible in
standard optical experiments, i.e., refers to ``hidden'' optics of
microcavities. By using transient four-wave mixing and pump-probe
spectroscopies, we infer that the radiative width, associated with excitonic
molecules of the binding energy meV, is
meV in the microcavities and
meV in a reference GaAs single quantum
well (QW). We show that for our high-quality quasi-two-dimensional
nanostructures the limit, relevant to the XX states, holds at
temperatures below 10 K, and that the bipolariton model of excitonic molecules
explains quantitatively and self-consistently the measured XX radiative widths.
We also find and characterize two critical points in the dependence of the
radiative corrections against the microcavity detuning, and propose to use the
critical points for high-precision measurements of the molecule bindingenergy
and microcavity Rabi splitting.Comment: 16 pages, 11 figures, accepted for publication in Phys. Rev.
Minimal entropy and geometric decompositions in dimension four
We show vanishing results about the infimum of the topological entropy of the
geodesic flow of homogeneous smooth four manifolds. We prove that any closed
oriented geometric four manifold has zero minimal entropy if and only if it has
zero simplicial volume. We also show that if a four manifold M admits a
geometric decomposition, in the sense of Thurston, and does not have geometric
pieces modelled on hyperbolic four-space, the complex hyperbolic plane or the
product of two hyperbolic planes, then M admits an F-structure. It follows that
M has zero minimal entropy and collapses with curvature bounded from below. We
then analyse whether or not M admits a metric whose topological entropy
coincides with the minimal entropy of M and provide new examples of manifolds
for which the minimal entropy problem cannot be solved.Comment: 38 pages, corrected flat case; Theorem A is now in terms of
F-structures, improved exposition; new final section collects all results
needed for the proofs of the main theorem
Autophagy activation and enhanced mitophagy characterize the Purkinje cells of pcd mice prior to neuronal death
Purkinje cells are a class of specialized neurons in the cerebellum, and are among the most metabolically active of all neurons, as they receive immense synaptic stimulation, and provide the only efferent output from the cerebellum. Degeneration of Purkinje cells is a common feature of inherited ataxias in humans and mice. To understand Purkinje neuron degeneration, investigators have turned to naturally occurring Purkinje cell degeneration phenotypes in mice to identify key regulatory proteins and cellular pathways. The Purkinje cell degeneration (pcd) mouse is a recessive mutant characterized by complete and dramatic post-natal, cell autonomous Purkinje neuron degeneration and death. As the basis of Purkinje cell death in pcd is unresolved, and contradictory data has emerged for the role of autophagy in Purkinje cell degeneration, we studied the mechanism of Purkinje cell death in pcd mice. BAX null status did not suppress Purkinje neuron death in pcd mice, indicating that classic apoptosis is not responsible for Purkinje cell loss. Interestingly, LC3 Western blot analysis and GFP-LC3 immunostaining of degenerating pcd cerebellum revealed activation of the autophagy pathway. Ultrastructural studies confirmed increased autophagy pathway activity in Purkinje cells, and yielded evidence for mitophagy, in agreement with LC3 immunoblotting of cerebellar fractions. As p62 levels were decreased in pcd cerebellum, our findings suggest that pcd Purkinje cell neurons can execute effective autophagy. However, our results support a role for dysregulated autophagy activation in pcd, and suggest that increased or aberrant mitophagy contributes to the Purkinje cell degeneration in pcd mice
Implementing intelligent asset management systems (IAMS) within an industry 4.0 manufacturing environment
9th IFAC Conference on Manufacturing Modelling, Management and Control, MIM 2019; Berlin; Germany; 28 August 2019 through 30 August 2019. Publicado en IFAC-PapersOnLine 52(13), p. 2488-2493This paper aims to define the different considerations and results obtained in the implementation in an Intelligent Maintenance System of a laboratory designed based on basic concepts of Industry 4.0. The Intelligent Maintenance System uses asset monitoring techniques that allow, on-line digital modelling and automatic decision making. The three fundamental premises used for the development of the management system are the structuring of information, value identification and risk management
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Polyglutamine-expanded androgen receptor interferes with TFEB to elicit autophagy defects in SBMA.
Macroautophagy (hereafter autophagy) is a key pathway in neurodegeneration. Despite protective actions, autophagy may contribute to neuron demise when dysregulated. Here we consider X-linked spinal and bulbar muscular atrophy (SBMA), a repeat disorder caused by polyglutamine-expanded androgen receptor (polyQ-AR). We found that polyQ-AR reduced long-term protein turnover and impaired autophagic flux in motor neuron-like cells. Ultrastructural analysis of SBMA mice revealed a block in autophagy pathway progression. We examined the transcriptional regulation of autophagy and observed a functionally significant physical interaction between transcription factor EB (TFEB) and AR. Normal AR promoted, but polyQ-AR interfered with, TFEB transactivation. To evaluate physiological relevance, we reprogrammed patient fibroblasts to induced pluripotent stem cells and then to neuronal precursor cells (NPCs). We compared multiple SBMA NPC lines and documented the metabolic and autophagic flux defects that could be rescued by TFEB. Our results indicate that polyQ-AR diminishes TFEB function to impair autophagy and promote SBMA pathogenesis
Observational constraints on early dark energy
We review and update constraints on the Early Dark Energy (EDE) model from
cosmological data sets, in particular Planck PR3 and PR4 cosmic microwave
background (CMB) data and large-scale structure (LSS) data sets including
galaxy clustering and weak lensing data from the Dark Energy Survey, Subaru
Hyper Suprime-Cam, and KiDS+VIKING-450, as well as BOSS/eBOSS galaxy clustering
and Lyman- forest data. We detail the fit to CMB data, and perform the
first analyses of EDE using the CAMSPEC and Hillipop likelihoods for Planck CMB
data, rather than Plik, both of which yield a tighter upper bound on the
allowed EDE fraction than that found with Plik. We then supplement CMB data
with large-scale structure data in a series of new analyses. All these analyses
are concordant in their Bayesian preference for CDM over EDE, as
indicated by marginalized posterior distributions. We perform a series of tests
of the impact of priors in these results, and compare with frequentist analyses
based on the profile likelihood, finding qualitative agreement with the
Bayesian results. All these tests suggest prior volume effects are not a
determining factor in analyses of EDE. This work provides both a review of
existing constraints and several new analyses.Comment: 59 pages, 23 figures, 11 tables, Invited review for International
Journal of Modern Physics
An alternative differential method of femtosecond pump-probe examination of materials
We describe an alternative method for femtosecond pumpprobe beam examination of energy transport properties of materials. All already reported techniques have several drawbacks which limit precise measurements of reflection coefficient as function of time. A typical problem is present when rough samples are being studied. In this case the pump-beam polarization changes randomly which may produce a spurious signal, drastically reducing the signal to noise ratio. Some proposals to alleviate such problema have been reported, however, they have not been totally satisfactory. The method presented here consists on measuring the difference between the two delays’ signals of the probe-beam. As will be explained, our proposal is free of typical drawbacks. We also propose a numerical method to recover the DR(t)/R curve from the measured data. Numerical simulations show that our proposal is a viable alternative
HD 69686: A Mysterious High Velocity B Star
We report on the discovery of a high velocity B star, HD 69686. We estimate
its space velocity, distance, surface temperature, gravity, and age. With these
data, we are able to reconstruct the trajectory of the star and to trace it
back to its birthplace. We use evolutionary tracks for single stars to estimate
that HD 69686 was born 73 Myr ago in the outer part of our Galaxy (
kpc) at a position well below the Galactic plane ( kpc), a very
unusual birthplace for a B star. Along the star's projected path in the sky, we
also find about 12 other stars having similar proper motions, and their
photometry data suggest that they are located at the same distance as HD 69686
and probably have the same age. We speculate on the origin of this group by
star formation in a high velocity cloud or as a Galactic merger fragment.Comment: 28 pages, 6 figures, accepted for publication in Ap
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