28,342 research outputs found
Suppression of hole-hole scattering in GaAs/AlGaAs heterostructures under uniaxial compression
Resistance, magnetoresistance and their temperature dependencies have been
investigated in the 2D hole gas at a [001] p-GaAs/AlGaAs
heterointerface under [110] uniaxial compression. Analysis performed in the
frame of hole-hole scattering between carriers in the two spin splitted
subbands of the ground heavy hole state indicates, that h-h scattering is
strongly suppressed by uniaxial compression. The decay time of the
relative momentum reveals 4.5 times increase at a uniaxial compression of 1.3
kbar.Comment: 5 pages, 3 figures. submitted to Phys.Rev.
Ammonia emissions from deciduous forest after leaf fall
The understanding of biochemical feedback mechanisms in the climate system is lacking knowledge in relation to bi-directional ammonia (NH3) exchange between natural ecosystems and the atmosphere. We therefore study the atmospheric NH3 fluxes during a 25-day period during autumn 2010 (21 October to 15 November) for the Danish beech forest Lille Bøgeskov to address the hypothesis that NH3 emissions occur from deciduous forests in relation to leaf fall. This is accomplished by using observations of vegetation status, NH3 fluxes and model calculations. Vegetation status was observed using plant area index (PAI) and leaf area index (LAI). NH3 fluxes were measured using the relaxed eddy accumulation (REA) method. The REA-based NH3 concentrations were compared to NH3 denuder measurements. Model calculations of the atmospheric NH3 concentration were obtained with the Danish Ammonia MOdelling System (DAMOS). The relative contribution from the forest components to the atmospheric NH3 flux was assessed using a simple two-layer bi-directional canopy compensation point model. A total of 57.7% of the fluxes measured showed emission and 19.5% showed deposition. A clear tendency of the flux going from deposition of −0.25 ± 0.30 μg NH3-N m−2 s−1 to emission of up to 0.67 ± 0.28 μg NH3-N m−2 s−1 throughout the measurement period was found. In the leaf fall period (23 October to 8 November), an increase in the atmospheric NH3 concentrations was related to the increasing forest NH3 flux. Following leaf fall, the magnitude and temporal structure of the measured NH3 emission fluxes could be adequately reproduced with the bi-directional resistance model; it suggested the forest ground layer (soil and litter) to be the main contributing component to the NH3 emissions. The modelled concentration from DAMOS fits well the measured concentrations before leaf fall, but during and after leaf fall, the modelled concentrations are too low. The results indicate that the missing contribution to atmospheric NH3 concentration from vegetative surfaces related to leaf fall are of a relatively large magnitude. We therefore conclude that emissions from deciduous forests are important to include in model calculations of atmospheric NH3 for forest ecosystems. Finally, diurnal variations in the measured NH3 concentrations were related to meteorological conditions, forest phenology and the spatial distribution of local anthropogenic NH3 sources. This suggests that an accurate description of ammonia fluxes over forest ecosystems requires a dynamic description of atmospheric and vegetation processes
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The rise and fall of early oil field technology: The torsion balance gradiometer
Today elementary physics students take for granted such quantities as "big G," the universal gravitational constant. In fact in the late 1700s the value of this quantity was unknown, and the quest to determine it led to some of the earliest geophysical instrumentation. Just after the Revolutionary War in the United States, Cavendish developed the first system to measure the universal gravitational constant, the familiar "big G." Unfortunately, for geologists (at this time still mostly "gentlemen scientists"), this apparatus produced data which were difficult to interpret geologically, and it was far too large and cumbersome for field use. The geologic limitation was that the system only measured the horizontal derivative of a horizontal component of the gravity field, a quantity which by itself is difficult to interpret. Thus no applications of this elegant yet laboratory-bound instrument emerged
Recommended from our members
The rise and fall of early oil field technology: The torsion balance gradiometer
Today elementary physics students take for granted such quantities as "big G," the universal gravitational constant. In fact in the late 1700s the value of this quantity was unknown, and the quest to determine it led to some of the earliest geophysical instrumentation. Just after the Revolutionary War in the United States, Cavendish developed the first system to measure the universal gravitational constant, the familiar "big G." Unfortunately, for geologists (at this time still mostly "gentlemen scientists"), this apparatus produced data which were difficult to interpret geologically, and it was far too large and cumbersome for field use. The geologic limitation was that the system only measured the horizontal derivative of a horizontal component of the gravity field, a quantity which by itself is difficult to interpret. Thus no applications of this elegant yet laboratory-bound instrument emerged
Udryddelse af endemisk ektromeli i en kommerciel museavl ved hjaelp af hygiejniske og organisatoriske foranstaltninger
The [Y/Mg] clock works for evolved solar metallicity stars
Previously [Y/Mg] has been proven to be an age indicator for solar twins.
Here, we investigate if this relation also holds for helium-core-burning stars
of solar metallicity. High resolution and high signal-to-noise ratio (S/N)
spectroscopic data of stars in the helium-core-burning phase have been obtained
with the FIES spectrograph on the NOT 2.56m telescope and the HIRES
spectrograph on the Keck I 10 m telescope. They have been analyzed to determine
the chemical abundances of four open clusters with close to solar metallicity;
NGC 6811, NGC 6819, M67 and NGC 188. The abundances are derived from equivalent
widths of spectral lines using ATLAS9 model atmospheres with parameters
determined from the excitation and ionization balance of Fe lines. Results from
asteroseismology and binary studies were used as priors on the atmospheric
parameters, where especially the is determined to much higher
precision than what is possible with spectroscopy. It is confirmed that the
four open clusters are close to solar metallicity and they follow the [Y/Mg]
vs. age trend previously found for solar twins. The [Y/Mg] vs. age clock also
works for giant stars in the helium-core burning phase, which vastly increases
the possibilities to estimate the age of stars not only in the solar
neighborhood, but in large parts of the Galaxy, due to the brighter nature of
evolved stars compared to dwarfs.Comment: 5 pages, 3 figures, accepted for publication as a Letter to A&
Practical thermodynamics of Yukawa systems at strong coupling
Simple practical approach to estimate thermodynamic properties of strongly
coupled Yukawa systems, in both fluid and solid phases, is presented. The
accuracy of the approach is tested by extensive comparison with direct computer
simulation results (for fluids and solids) and the recently proposed
shortest-graph method (for solids). Possible applications to other systems of
softly repulsive particles are briefly discussed.Comment: Published in J. Chem. Phy
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