899 research outputs found
Observing the intrinsic linewidth of a quantum-cascade laser: beyond the Schawlow-Townes limit
A comprehensive investigation of the frequency-noise spectral density of a
free-running mid-infrared quantum-cascade laser is presented for the first
time. It provides direct evidence of the leveling of this noise down to a white
noise plateau, corresponding to an intrinsic linewidth of a few hundred Hz. The
experiment is in agreement with the most recent theory on the fundamental
mechanism of line broadening in quantum-cascade lasers, which provides a new
insight into the Schawlow-Townes formula and predicts a narrowing beyond the
limit set by the radiative lifetime of the upper level.Comment: 4 pages, 4 figure
Surface effects on the Mott-Hubbard transition in archetypal VO
We present an experimental and theoretical study exploring surface effects on
the evolution of the metal-insulator transition in the model Mott-Hubbard
compound Cr-doped VO. We find a microscopic domain formation that is
clearly affected by the surface crystallographic orientation. Using scanning
photoelectron microscopy and X-ray diffraction, we find that surface defects
act as nucleation centers for the formation of domains at the
temperature-induced isostructural transition and favor the formation of
microscopic metallic regions. A density functional theory plus dynamical mean
field theory study of different surface terminations shows that the surface
reconstruction with excess vanadyl cations leads to doped, and hence more
metallic surface states, explaining our experimental observations.Comment: 5 pages, 4 figure
Canopy structure, topography, and weather are equally important drivers of small-scale snow cover dynamics in sub-alpine forests
In mountain regions, forests that overlap with seasonal
snow mostly reside in complex terrain. Due to persisting major observational
challenges in these environments, the combined impact of forest structure
and topography on seasonal snow cover dynamics is still poorly understood.
Recent advances in forest snow process representation and increasing
availability of detailed canopy structure datasets, however, now allow for
hyper-resolution (<5 m) snow model simulations capable of resolving
tree-scale processes. These can shed light on the complex process
interactions that govern forest snow dynamics. We present multi-year
simulations at 2 m resolution obtained with FSM2, a mass- and energy-balance-based forest snow model specifically developed and validated for metre-scale
applications. We simulate an ∼3 km2 model domain
encompassing forested slopes of a sub-alpine valley in the eastern Swiss
Alps and six snow seasons. Simulations thus span a wide range of canopy
structures, terrain characteristics, and meteorological conditions. We
analyse spatial and temporal variations in forest snow energy balance
partitioning, aiming to quantify and understand the contribution of
individual energy exchange processes at different locations and times. Our
results suggest that snow cover evolution is equally affected by canopy
structure, terrain characteristics, and meteorological conditions. We show
that the interaction of these three factors can lead to snow accumulation
and ablation patterns that vary between years. We further identify higher
snow distribution variability and complexity in slopes that receive solar
radiation early in winter. Our process-level insights corroborate and
complement existing empirical findings that are largely based on snow
distribution datasets only. Hyper-resolution simulations as presented here
thus help to better understand how snowpacks and ecohydrological regimes in
sub-alpine regions may evolve due to forest disturbances and a warming
climate. They could further support the development of process-based
sub-grid forest snow cover parameterizations or tiling approaches for
coarse-resolution modelling applications.</p
Dentin proteoglycans: an immunocytochemical FEISEM study.
Dentin proteoglycans are fundamental constituents of the dentin matrix and are distributed ubiquitously both in dentin and cement. They have several important functional properties; in particular, they have a fundamental role in the maintenance and the correct stabilization of collagen fibers. The use of phosphoric acid on dentin, as proposed in most common dental adhesive systems to establish a reliable bond, may affect the molecular structure of proteoglycans. The aim of this study was to evaluate, after the application of EDTA or phosphoric acid on dentin, the dentin proteoglycans with an immunocytochemical approach with high resolution SEM. For this purpose, dentin disks obtained from recently extracted human molars were etched with a 35% water solution of phosphoric acid for 15 s, 30 s, and 60 s. Control specimens were conditioned with EDTA. Specimens were immunolabeled with a monoclonal antibody antichondroitin sulfate and visualized with a gold-conjugated secondary antibody. Conditioning dentin with EDTA resulted in a distinct labeling of the proteoglycans, as visualized on branching fibrillar structures in the order of 10–20 nm. The use of 35% phosphoric acid on dentin revealed a coagulation of proteoglycans after etching for 15 s while a very low labeling signal was detectable after 30 s. No labeling was obtained after etching dentin with phosphoric acid for 60 s. These results suggest that the use of 35% phosphoric acid on dentin is able to produce significant structural modifications of the dentin proteoglycans even after short application times. Additionally, when applied on the dentin surface for more than 30 s, phosphoric acid produces a dramatic decrease in proteoglycans’ antigenicity, probably due to structural modifications of the three-dimensional conformation of these molecules
Indium selenide: An insight into electronic band structure and surface excitations
We have investigated the electronic response of single crystals of indium selenide by means of angle-resolved photoemission spectroscopy, electron energy loss spectroscopy and density functional theory. The loss spectrum of indium selenide shows the direct free exciton at similar to 1.3 eV and several other peaks, which do not exhibit dispersion with the momentum. The joint analysis of the experimental band structure and the density of states indicates that spectral features in the loss function are strictly related to single-particle transitions. These excitations cannot be considered as fully coherent plasmons and they are damped even in the optical limit, i.e. for small momenta. The comparison of the calculated symmetry-projected density of states with electron energy loss spectra enables the assignment of the spectral features to transitions between specific electronic states. Furthermore, the effects of ambient gases on the band structure and on the loss function have been probed
Ultrastructural Patterns of Cell Damage and Death Following Gamma Radiation Exposure of Murine Erythroleukemia Cells
Radiation causes damage to cell surface membranes, cytoplasmic organelles, and the nuclear process of DNA synthesis and repair, and this eventually results in different modes of cell death. In this study we examined murine erythroleukemia (MEL) cells, exposed to 15 and 60 Gy of 10 MeV photonic energy, and left in culture for up to 96 hours. Electron microscopical analysis was performed on conventionally embedded samples and freeze-fracture replicas, in order to detect ultrastructural patterns of cell damage and death. Of interest was the observation of chromatin condensates, nuclear membrane associations and nuclear pore redistribution during early apoptosis. Pronounced rearrangements of transmembrane particles during late stages of cellular necrosis were also found. The morphological damage induced by both doses of radiation as a function of time after exposure was only quantitatively but not qualitatively different
Reproductive Biology of Female Northern African Rock Pythons (Python sebae) in Southern Florida, USA
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