25,169 research outputs found
Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells
We present an absorption study of the neutral and positively charged exciton
(trion) under the influence of a femtosecond, circularly polarized, resonant
pump pulse. Three populations are involved: free holes, excitons, and trions,
all exhibiting transient spin polarization. In particular, a polarization of
the hole gas is created by the formation of trions. The evolution of these
populations is studied, including the spin flip and trion formation processes.
The contributions of several mechanisms to intensity changes are evaluated,
including phase space filling and spin-dependent screening. We propose a new
explanation of the oscillator strength stealing phenomena observed in p-doped
quantum wells, based on the screening of neutral excitons by charge carriers.
We have also found that binding heavy holes into charged excitons excludes them
from the interaction with the rest of the system, so that oscillator strength
stealing is partially blockedComment: 4 pages, 4 figure
TPCI: The PLUTO-CLOUDY Interface
We present an interface between the (magneto-) hydrodynamics code PLUTO and
the plasma simulation and spectral synthesis code CLOUDY. By combining these
codes, we constructed a new photoionization hydrodynamics solver: The
PLUTO-CLOUDY Interface (TPCI), which is well suited to simulate
photoevaporative flows under strong irradiation. The code includes the
electromagnetic spectrum from X-rays to the radio range and solves the
photoionization and chemical network of the 30 lightest elements. TPCI follows
an iterative numerical scheme: First, the equilibrium state of the medium is
solved for a given radiation field by CLOUDY, resulting in a net radiative
heating or cooling. In the second step, the latter influences the (magneto-)
hydrodynamic evolution calculated by PLUTO. Here, we validated the
one-dimensional version of the code on the basis of four test problems:
Photoevaporation of a cool hydrogen cloud, cooling of coronal plasma, formation
of a Stroemgren sphere, and the evaporating atmosphere of a hot Jupiter. This
combination of an equilibrium photoionization solver with a general MHD code
provides an advanced simulation tool applicable to a variety of astrophysical
problems.Comment: 13 pages, 10 figures, accepted for publication in A&
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Characterization of high purity germanium point contact detectors with low net impurity concentration
High Purity germanium point-contact detectors have low energy thresholds and excellent energy resolution over a wide energy range, and are thus widely used in nuclear and particle physics. In rare event searches, such as neutrinoless double beta decay, the point-contact geometry is of particular importance since it allows for pulse-shape discrimination, and therefore for a significant background reduction. In this paper we investigate the pulse-shape discrimination performance of ultra-high purity germanium point contact detectors. It is demonstrated that a minimal net impurity concentration is required to meet the pulse-shape performance requirements
On different cascade-speeds for longitudinal and transverse velocity increments
We address the problem of differences between longitudinal and transverse
velocity increments in isotropic small scale turbulence. The relationship of
these two quantities is analyzed experimentally by means of stochastic
Markovian processes leading to a phenomenological Fokker- Planck equation from
which a generalization of the Karman equation is derived. From these results, a
simple relationship between longitudinal and transverse structure functions is
found which explains the difference in the scaling properties of these two
structure functions.Comment: 4 pages, 5 figures, now with corrected postscrip
Quantitative assessment of angiogenesis in murine antigen-induced arthritis by intravital fluorescence microscopy
Inhibition of angiogenesis might be a therapeutic approach to prevent joint destruction caused by the overgrowing synovial tissue during chronic joint inflammation. The aim of this study was to investigate angiogenesis in the knee joint of mice with antigen-induced arthritis (AIA) by means of intravital microscopy. In 14 mice (C57BL6/129Sv) intravital microscopic assessment was performed on day 8 after AIA induction in two groups (controls, AIA). Synovial tissue was investigated by intravital fluorescence microscopy using FITC-dextran (150 kD). Quantitative assessment of vessel density was performed according to the following categories: functional capillary density (FCD, vessels 10 mum) and FVD of vessels with angiogenic criteria (convoluted vessels, abrupt changes of diameter, vessels which are generated by sprouting and progressively pruned and remodelled). Microvessel count was performed using immunohistochemistry. There was no significant difference in FCD between the control group (337 +/- 9 cm/cm(2); mean +/-SEM) and the AIA group (359 +/- 13 cm/cm(2)). The density of vessels larger than 10 gm diameter was significantly increased in animals with AIA (135 +/- 10 vs. 61 +/- 5 cm/cm(2) in control). The density of blood vessels with angiogenic criteria was enhanced in arthritic animals (79 +/- 17 vs. 12 +/- 2 cm/cm(2) in control). There was a significant increase in the microvessel count in arthritic animals (297 +/- 25 vs. 133 +/- 16 mm(-2) in control). These findings demonstrate that angiogenesis in murine AIA can be assessed quantitatively using intravital microscopy. Further studies will address antiangiogenic strategies in AIA
Granulometria do milho na amostra natural x amostra seca.
Projeto/Plano de Ação: 03.10.01.030
Structural Transition Kinetics and Activated Behavior in the Superconducting Vortex Lattice
Using small-angle neutron scattering, we investigated the behavior of a
metastable vortex lattice state in MgB2 as it is driven towards equilibrium by
an AC magnetic field. This shows an activated behavior, where the AC field
amplitude and cycle count are equivalent to, respectively, an effective
"temperature" and "time". The activation barrier increases as the metastable
state is suppressed, corresponding to an aging of the vortex lattice.
Furthermore, we find a cross-over from a partial to a complete suppression of
metastable domains depending on the AC field amplitude, which may empirically
be described by a single free parameter. This represents a novel kind of
collective vortex behavior, most likely governed by the nucleation and growth
of equilibrium vortex lattice domains.Comment: 5 pages plus 3 pages of supplemental materia
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