322 research outputs found
Photoluminescence upconversion at interfaces driven by a sequential two-photon absorption mechanism
This paper reports on the results of an investigation into the nature of photoluminescence upconversion at
GaAs/InGaP2 interfaces. Using a dual-beam excitation experiment, we demonstrate that the upconversion in our
sample proceeds via a sequential two-photon optical absorption mechanism. Measurements of photoluminescence
and upconversion photoluminescence revealed evidence of the spatial localization of carriers in the InGaP2
material, arising from partial ordering of the InGaP2. We also observed the excitation of a two-dimensional electron
gas at the GaAs/InGaP2 heterojunction that manifests as a high-energy shoulder in the GaAs photoluminescence
spectrum. Furthermore, the results of upconversion photoluminescence excitation spectroscopy demonstrate that
the photon energy onset of upconversion luminescence coincides with the energy of the two-dimensional electron
gas at the GaAs/InGaP2 interface, suggesting that charge accumulation at the interface can play a crucial role in
the upconversion process
Isotope Effect for the Penetration Depth in Superconductors
We show that various factors can lead to an isotopic dependence of the
penetration depth . Non-adiabaticity (Jahn-Teller crossing) leads to
the isotope effect of the charge carrier concentration and, consequently,
of in doped superconductors such as the cuprates. A general equation
relating the isotope coefficients of and of is presented for
London superconductors. We further show that the presence of magnetic
impurities or a proximity contact also lead to an isotopic dependence of
; the isotope coefficient turns out to be temperature dependent,
, in these cases. The existence of the isotope effect for the
penetration depth is predicted for conventional as well as for high-temperature
superconductors. Various experiments are proposed and/or discussed.Comment: 11 pages, 8 figures, accepted for publication in Phys. Rev.
Edaphic, structural and physiological contrasts across Amazon Basin forest-savanna ecotones suggest a role for potassium as a key modulator of tropical woody vegetation structure and function
Sampling along a precipitation gradient in tropical South America extending from ca. 0.8 to 2.0 m ag-1, savanna soils had consistently lower exchangeable cation concentrations and higher C/N ratios than nearby forest plots. These soil differences were also reflected in canopy averaged leaf traits with savanna trees typically having higher leaf mass per unit area but lower mass-based nitrogen (Nm) and potassium (Km). Both Nm and Km also increased with declining mean annual precipitation (PA), but most area-based leaf traits such as leaf photosynthetic capacity showed no systematic variation with PA or vegetation type. Despite this invariance, when taken in conjunction with other measures such as mean canopy height, area-based soil exchangeable potassium content, [K]sa , proved to be an excellent predictor of several photosynthetic properties (including 13C isotope discrimination). Moreover, when considered in a multivariate context with PA and soil plant available water storage capacity (θP) as covariates, [K]sa also proved to be an excellent predictor of stand-level canopy area, providing drastically improved fits as compared to models considering just PA and/or θP. Neither calcium, nor magnesium, nor soil pH could substitute for potassium when tested as alternative model predictors (ΔAIC > 10). Nor for any model could simple soil texture metrics such as sand or clay content substitute for either [K]sa or θP. Taken in conjunction with recent work in Africa and the forests of the Amazon Basin, this suggests-in combination with some newly conceptualised interacting effects of PA and θP also presented here-a critical role for potassium as a modulator of tropical vegetation structure and function.Natural Environment Research Council (NERC) TROBIT Consortium projectCNPqRoyal Society of London - Wolfson Research Merit Awar
Herschel observations of EXtra-Ordinary Sources (HEXOS): Observations of H2O and its isotopologues towards Orion KL
We report the detection of more than 48 velocity-resolved ground rotational state transitions of H 16
2 O, H 18
2 O, and H 17
2 O – most for the first time
– in both emission and absorption toward Orion KL using Herschel/HIFI. We show that a simple fit, constrained to match the known emission
and absorption components along the line of sight, is in excellent agreement with the spectral profiles of all the water lines. Using the measured
H 18
2 O line fluxes, which are less affected by line opacity than their H 16
2 O counterparts, and an escape probability method, the column densities
of H 18
2 O associated with each emission component are derived. We infer total water abundances of 7.4 × 10−5, 1.0× 10−5, and 1.6 × 10−5 for the
plateau, hot core, and extended warm gas, respectively. In the case of the plateau, this value is consistent with previous measures of the Orion-KL
water abundance as well as those of other molecular outflows. In the case of the hot core and extended warm gas, these values are somewhat higher
than water abundances derived for other quiescent clouds, suggesting that these regions are likely experiencing enhanced water-ice sublimation
from (and reduced freeze-out onto) grain surfaces due to the warmer dust in these sources
Height-diameter allometry of tropical forest trees
Copyright © 2011 European Geosciences Union. This is the published version available at http://www.biogeosciences.net/8/1081/2011/bg-8-1081-2011.html doi:10.5194/bg-8-1081-2011Tropical tree height-diameter (H:D) relationships may vary by forest type and region making large-scale estimates of above-ground biomass subject to bias if they ignore these differences in stem allometry. We have therefore developed a new global tropical forest database consisting of 39 955 concurrent H and D measurements encompassing 283 sites in 22 tropical countries. Utilising this database, our objectives were:
1. to determine if H:D relationships differ by geographic region and forest type (wet to dry forests, including zones of tension where forest and savanna overlap).
2. to ascertain if the H:D relationship is modulated by climate and/or forest structural characteristics (e.g. stand-level basal area, A).
3. to develop H:D allometric equations and evaluate biases to reduce error in future local-to-global estimates of tropical forest biomass.
Annual precipitation coefficient of variation (PV), dry season length (SD), and mean annual air temperature (TA) emerged as key drivers of variation in H:D relationships at the pantropical and region scales. Vegetation structure also played a role with trees in forests of a high A being, on average, taller at any given D. After the effects of environment and forest structure are taken into account, two main regional groups can be identified. Forests in Asia, Africa and the Guyana Shield all have, on average, similar H:D relationships, but with trees in the forests of much of the Amazon Basin and tropical Australia typically being shorter at any given D than their counterparts elsewhere. The region-environment-structure model with the lowest Akaike's information criterion and lowest deviation estimated stand-level H across all plots to within amedian −2.7 to 0.9% of the true value. Some of the plot-to-plot variability in H:D relationships not accounted for by this model could be attributed to variations in soil physical conditions. Other things being equal, trees tend to be more slender in the absence of soil physical constraints, especially at smaller D. Pantropical and continental-level models provided less robust estimates of H, especially when the roles of climate and stand structure in modulating H:D allometry were not simultaneously taken into account
Herschel observations of deuterated water towards Sgr B2(M)
Observations of HDO are an important complement for studies of water, because they give strong constraints on the formation processes – grain
surfaces versus energetic process in the gas phase, e.g. in shocks. The HIFI observations of multiple transitions of HDO in Sgr B2(M) presented
here allow the determination of the HDO abundance throughout the envelope, which has not been possible before with ground-based observations
only. The abundance structure has been modeled with the spherical Monte Carlo radiative transfer code RATRAN, which also takes radiative
pumping by continuum emission from dust into account. The modeling reveals that the abundance of HDO rises steeply with temperature from
a low abundance (2.5 × 10−11) in the outer envelope at temperatures below 100 K through a medium abundance (1.5 × 10−9) in the inner
envelope/outer core at temperatures between 100 and 200 K, and finally a high abundance ( 3.5 × 10−9) at temperatures above 200 K in the hot
core
Herschel observations of EXtra-Ordinary Sources (HEXOS): Detection of hydrogen fluoride in absorption towards Orion KL
We report a detection of the fundamental rotational transition of hydrogen fluoride in absorption towards Orion KL using Herschel/HIFI. After the
removal of contaminating features associated with common molecules (“weeds”), the HF spectrum shows a P-Cygni profile, with weak redshifted
emission and strong blue-shifted absorption, associated with the low-velocity molecular outflow. We derive an estimate of 2.9 × 1013 cm−2 for the
HF column density responsible for the broad absorption component. Using our best estimate of the H2 column density within the low-velocity
molecular outflow, we obtain a lower limit of ∼1.6 × 10−10 for the HF abundance relative to hydrogen nuclei, corresponding to ∼0.6% of the solar
abundance of fluorine. This value is close to that inferred from previous ISO observations of HF J = 2−1 absorption towards Sgr B2, but is in
sharp contrast to the lower limit of 6 × 10−9 derived by Neufeld et al. for cold, foreground clouds on the line of sight towards G10.6-0.4
Reversal of infall in SgrB2(M) revealed by Herschel/HIFI observations of HCN lines at THz frequencies
Aims. To investigate the accretion and feedback processes in massive star formation, we analyze the shapes of emission lines from hot molecular
cores, whose asymmetries trace infall and expansion motions.
Methods. The high-mass star forming region SgrB2(M) was observed with Herschel/HIFI (HEXOS key project) in various lines of HCN and
its isotopologues, complemented by APEX data. The observations are compared to spherically symmetric, centrally heated models with density
power-law gradient and different velocity fields (infall or infall+expansion), using the radiative transfer code RATRAN.
Results. The HCN line profiles are asymmetric, with the emission peak shifting from blue to red with increasing J and decreasing line opacity
(HCN to H13CN). This is most evident in the HCN 12–11 line at 1062 GHz. These line shapes are reproduced by a model whose velocity field
changes from infall in the outer part to expansion in the inner part.
Conclusions. The qualitative reproduction of the HCN lines suggests that infall dominates in the colder, outer regions, but expansion dominates
in the warmer, inner regions. We are thus witnessing the onset of feedback in massive star formation, starting to reverse the infall and finally
disrupting the whole molecular cloud. To obtain our result, the THz lines uniquely covered by HIFI were critically important
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