406 research outputs found
Magnetophotoluminescence of negatively charged excitons in narrow quantum wells
We present the results of photoluminescence experiments on the negatively charged exciton X- in GaAs/AlxGa1-xAs quantum wells (QW) in high magnetic fields (≤50 T). Three different QW widths are used here: 100, 120, and 150 Å. All optically allowed transitions of X- are observed, enabling us to experimentally verify its energy-level diagram. All samples behave consistently with this diagram. We have determined the binding energy Eb of the singlet and triplet state of X- between 23 and 50 T for the 120 and 150 Å QW, while only the triplet Eb is observed for the 100 Å QW. A detailed comparison with recent theoretical calculations shows an agreement for all samples across this entire field range
An Experimental Comparison of Vole Control Methods
This is a description of a new relatively long-term study of pine vole control under the North Carolina Agricultural Experiment Station. The objective is to evaluate on an experimental basis, the principal methods used to control vole populations and damage in apple orchards
Probing the potential landscape inside a two-dimensional electron-gas
We report direct observations of the scattering potentials in a
two-dimensional electron-gas using electron-beam diffaction-experiments. The
diffracting objects are local density-fluctuations caused by the spatial and
charge-state distribution of the donors in the GaAs-(Al,Ga)As heterostructures.
The scatterers can be manipulated externally by sample illumination, or by
cooling the sample down under depleted conditions.Comment: 4 pages, 4 figure
Lunar Flashlight: Illuminating the Lunar South Pole
Recent reflectance data from LRO instruments suggest water ice and other volatiles may be present on the surface in lunar permanentlyshadowed regions, though the detection is not yet definitive. Understanding the composition, quantity, distribution, and form of water and other volatiles associated with lunar permanently shadowed regions (PSRs) is identified as a NASA Strategic Knowledge Gap (SKG) for Human Exploration. These polar volatile deposits are also scientifically interesting, having the potential to reveal important information about the delivery of water to the Earth- Moon system
Lunar Flashlight: Illuminating the Moon's South Pole
Recent reflectance data from LRO instruments suggest water ice and other volatiles may be present on the surface in lunar permanently shadowed regions, though the detection is not yet definitive. Understanding the composition, quantity, distribution, and form of water and other volatiles associated with lunar permanently shadowed regions (PSRs) is identified as a NASA Strategic Knowledge Gap (SKG) for Human Exploration. These polar volatile deposits are also scientifically interesting, having the potential to reveal important information about the delivery of water to the Earth-Moon system
Observations of Lunar Swirls by the Diviner Lunar Radiometer Experiment
The presence of anomalous, high albedo markings on the lunar surface has been known since the Apollo era. These features, collectively known as lunar swirls, occur on both the mare and highlands. Some swirls are associated with the antipodes of major impact basins, while all are associated with magnetic field anomalies of varying strength. Three mechanisms have been proposed for the formation of the swirls: (1) solar wind standoff due to the presence of magnetic fields, (2) micrometeoroid or comet swarms impacting and disturbing the lunar surface, revealing unweathered regolith, and (3) transport and deposition of fine-grained feldspathic material. Diviner s unique capabilities to determine silicate composition and degree of space weathering of the lunar surface, in addition to its capabilities to determine thermophysical properties from night-time temperature measurements, make it an ideal instrument to examine the swirls and help differentiate among the three proposed formation mechanisms
Stability of trions in strongly spin-polarized two-dimensional electron gases
Low-temperature magneto-photoluminescence studies of negatively charged
excitons (X- trions) are reported for n-type modulation-doped ZnSe/Zn(Cd,Mn)Se
quantum wells over a wide range of Fermi energy and spin-splitting. The
magnetic composition is chosen such that these magnetic two-dimensional
electron gases (2DEGs) are highly spin-polarized even at low magnetic fields,
throughout the entire range of electron densities studied (5e10 to 6.5e11
cm^-2). This spin polarization has a pronounced effect on the formation and
energy of X-, with the striking result that the trion ionization energy (the
energy separating X- from the neutral exciton) follows the temperature- and
magnetic field-tunable Fermi energy. The large Zeeman energy destabilizes X- at
the nu=1 quantum limit, beyond which a new PL peak appears and persists to 60
Tesla, suggesting the formation of spin-triplet charged excitons.Comment: 5 pages (RevTex), 4 embedded EPS figs. Submitted to PRB-R
Photoluminescence studies of individual and few GaSb/GaAs quantum rings
We present optical studies of individual and few GaSb quantum rings embedded in a GaAs matrix. Contrary to expectation for type-II confinement, we measure rich spectra containing sharp lines. These lines originate from excitonic recombination and are observed to have resolution-limited full-width at half maximum of 200 µeV. The detail provided by these measurements allows the characteristic type-II blueshift, observed with increasing excitation power, to be studied at the level of individual nanostructures. These findings are in agreement with hole-charging being the origin of the observed blueshif
Evidence for Surface Water Ice in the Lunar Polar Regions Using Reflectance Measurements from the Lunar Orbiter Laser Altimeter and Temperature Measurements from the Diviner Lunar Radiometer Experiment
We find that the reflectance of the lunar surface within 5 deg of latitude of theSouth Pole increases rapidly with decreasing temperature, near approximately 110K, behavior consistent with the presence of surface water ice. The North polar region does not show this behavior, nor do South polar surfaces at latitudes more than 5 deg from the pole. This South pole reflectance anomaly persists when analysis is limited to surfaces with slopes less than 10 deg to eliminate false detection due to the brightening effect of mass wasting, and also when the very bright south polar crater Shackleton is excluded from the analysis. We also find that south polar regions of permanent shadow that have been reported to be generally brighter at 1064 nm do not show anomalous reflectance when their annual maximum surface temperatures are too high to preserve water ice. This distinction is not observed at the North Pole. The reflectance excursion on surfaces with maximum temperatures below 110K is superimposed on a general trend of increasing reflectance with decreasing maximum temperature that is present throughout the polar regions in the north and south; we attribute this trend to a temperature or illumination-dependent space weathering effect (e.g. Hemingway et al. 2015). We also find a sudden increase in reflectance with decreasing temperature superimposed on the general trend at 200K and possibly at 300K. This may indicate the presence of other volatiles such as sulfur or organics. We identified and mapped surfaces with reflectances so high as to be unlikely to be part of an ice-free population. In this south we find a similar distribution found by Hayne et al. 2015 based on UV properties. In the north a cluster of pixels near that pole may represent a limited frost exposure
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