Photon emission induced by elastic exciton--carrier scattering in semiconductor quantum wells

We present a study of the elastic exciton--electron ($X-e^-$) and exciton--hole ($X-h$) scattering processes in semiconductor quantum wells, including fermion exchange effects. The balance between the exciton and the free carrier populations within the electron-hole plasma is discussed in terms of ionization degree in the nondegenerate regime. Assuming a two-dimensional Coulomb potential statically screened by the free carrier gas, we apply the variable phase method to obtain the excitonic wavefunctions, which we use to calculate the 1$s$ exciton--free carrier matrix elements that describe the scattering of excitons into the light cone where they can radiatively recombine. The photon emission rates due to the carrier-assisted exciton recombination in semiconductor quantum-wells (QWs) at room temperature and in a low density regime are obtained from Fermi's golden rule, and studied for mid-gap and wide-gap materials. The quantitative comparison of the direct and exchange terms of the scattering matrix elements shows that fermion exchange is the dominant mechanism of the exciton--carrier scattering process. This is confirmed by our analysis of the rates of photon emission induced by electron-assisted and hole-assisted exciton recombinations.Comment: Thoroughly revised version of previous work. Weak and incorrect assumptions have been removed from the paper, and its scope has evolved: see abstract. This is the final version, i.e. as accepted for publication in the European Physical Journal

The Horn, Polkemmet: Experimental Assessment of the Aerodynamic Response Characteristics. G.U. Aero Report 9609

A qualitative assessment of the aerodynamic response characteristics of the Horn, planned for Polkemmet Country Park, West Lothian, is presented. The assessment is based on scale model experiments conducted in the smoke flow visualisation wind tunnel in the Department of Aerospace Engineering, University of Glasgow for a range of flow conditions compatible with the expected wind environment. A number of adverse unsteady aerodynamic phenomena are identified which are highly dependent on wind direction. These include a dominant transient vortex system emanating from the mouth of the Horn, periodic vortex shedding from the neck and support mast, conical vortices generated on the Horn surface, and a general bluff body wake in the lee of the Horn. It is anticipated that the aerodynamic behaviour identified in the model tests persists, at least qualitatively, under full scale conditions. As a result, the potential exists for both aerodynamically induced transient and periodic structural excitation of the Horn. Means of alleviating the adverse aerodynamic characteristics are available. However, any assessment of the effectiveness of such measures requires further experimental investigation

On the morphological deviation in additive manufacturing of porous Ti6Al4V scaffold: a design consideration

Additively manufactured Ti scaffolds have been used for bone replacement and orthopaedic applications. In these applications, both morphological and mechanical properties are important for their in vivo performance. Additively manufactured Ti6Al4V triply periodic minimal surface (TPMS) scaffolds with diamond and gyroid structures are known to have high stiffness and high osseointegration properties, respectively. However, morphological deviations between the as-designed and as-built types of these scaffolds have not been studied before. In this study, the morphological and mechanical properties of diamond and gyroid scaffolds at macro and microscales were examined. The results demonstrated that the mean printed strut thickness was greater than the designed target value. For diamond scaffolds, the deviation increased from 7.5 μm (2.5% excess) for vertical struts to 105.4 μm (35.1% excess) for horizontal struts. For the gyroid design, the corresponding deviations were larger, ranging from 12.6 μm (4.2% excess) to 198.6 μm (66.2% excess). The mean printed pore size was less than the designed target value. For diamonds, the deviation of the mean pore size from the designed value increased from 33.1 μm (-3.0% excess) for vertical struts to 92.8 μm (-8.4% excess) for horizontal struts. The corresponding deviation for gyroids was larger, ranging from 23.8 μm (-3.0% excess) to 168.7 μm (-21.1% excess). Compressive Young's modulus of the bulk sample, gyroid and diamond scaffolds was calculated to be 35.8 GPa, 6.81 GPa and 7.59 GPa, respectively, via the global compression method. The corresponding yield strength of the samples was measured to be 1012, 108 and 134 MPa. Average microhardness and Young's modulus from α and β phases of Ti6Al4V from scaffold struts were calculated to be 4.1 GPa and 131 GPa, respectively. The extracted morphology and mechanical properties in this study could help understand the deviation between the as-design and as-built matrices, which could help develop a design compensation strategy before the fabrication of the scaffolds

Biodiversity of Bear Seamount, New England Seamount chain: Results of exploratory trawling

Bear Seamount (39°55′N 67°30′W) is an extinct undersea volcano located inside the U.S. Exclusive Economic Zone south of Georges Bank. The fauna associated with the seamount was little known until twenty trawl stations were made 2-7 December 2000, by the NOAA ship R/V Delaware II. The objective of the survey was to begin to document the biodiversity on and over the seamount, particularly of fishes, cephalopods, and crustaceans. Representatives of most species were preserved as vouchers and for subsequent definitive identification. This report presents a description of the biodiversity. A total of at least 274 species were collected. Preliminary identifications indicate the capture of 115 fish species. Among these were a number of new fish records for the area or rare species, including Acromycter pertubator (Congridae), Diastobranchus capensis (Synaphobranchidae), Alepocephalus bairdii (Alepocephalidae), Mirognathus normani (Alepocephalidae), Bathygadus favosus (Bathygadidae), Nezumia longebarbata (Macrouridae), Gaidropsarus argentatus (Phycidae), Dibranchus tremendus (Ogcocephalidae) and Kali indica (Chiasmodontidae). Only two fish species of potential commercial importance were encountered: Coryphaenoides rupestris and Macrourus berglax. Cephalopods comprised 26 species from 15 families, including one new distributional record and several rarely-collected species. The crustacean fauna was diverse with at least 46 species. Totals for other invertebrate species are pending laboratory identification, but number at least 87 species from 10 Phyla. This includes a number of new distributional records and a new species of gorgonian

Taxonomic voucher specimens for study of post-wildfire forest habitat in Douglas County, Oregon

this publication provides data about voucher specimens deposited in the museum in conjunction with a research project on pollinators

Procedure for the rapid evaluation of carbon stripper foils

Evaluation of the average lifetime of carbon stripper foils is described in which a beam of 20 keV ions is used to simulate the beam from a high energy accelerator. The method is used to evaluate different methods of foil fabrication. (GHT

Lifetime measurements on carbon stripper foils

Fabrication of carbon stripper foils and their performance in electrostatic accelerators are described. (GHT

Analysis of model rotor blade pressures during parallel interaction with twin vortices

This paper presents and provides analysis of unsteady surface pressures measured on a model rotor blade as the blade experienced near parallel blade vortex interaction with a twin vortex system. To provide a basis for analysis, the vortex system was characterized by hot-wire measurements made in the interaction plane but in the absence of the rotor. The unsteady pressure response resulting from a single vortex interaction is then presented to provide a frame of reference for the twin vortex results. A series of twin vortex interaction cases are then presented and analyzed. It is shown that the unsteady blade pressures and forces are very sensitive to the inclination angle and separation distance of the vortex pair. When the vortex cores lie almost parallel to the blade chord, the interaction is characterized by a two-stage response associated with the sequential passage of the two cores. Conversely, when the cores lie on a plane that is almost perpendicular to the blade chord, the response is similar to that of a single vortex interaction. In all cases, the normal force response is consistent with the distribution of vertical velocity in the flow field of the vortex system. The pitching moment response, on the other hand, depends on the localized suction associated with the vortex cores as they traverse the blade chord

The Northwest Tropical Atlantic Station (NTAS) : NTAS-4 mooring turnaround cruise report

The Northwest Tropical Atlantic Station (NTAS) was established to address the need for accurate air-sea flux estimates and upper ocean measurements in a region with strong sea surface temperature anomalies and the likelihood of significant local air–sea interaction on interannual to decadal timescales. The approach is to maintain a surface mooring outfitted for meteorological and oceanographic measurements at a site near 15°N, 51°W by successive mooring turnarounds. These observations will be used to investigate air–sea interaction processes related to climate variability. Deployment of the first (NTAS-1), second (NTAS-2) and third (NTAS-3) moorings were documented in previous reports (Plueddemann et al., 2001; 2002; 2003). This report documents recovery of the NTAS-3 mooring and deployment of the NTAS-4 mooring at the same site. Both moorings used 3-meter discus buoys as the surface element. These buoys were outfitted with two Air–Sea Interaction Meteorology (ASIMET) systems. Each system measures, records, and transmits via Argos satellite the surface meteorological variables necessary to compute air–sea fluxes of heat, moisture and momentum. The upper 150 m of the mooring line were outfitted with oceanographic sensors for the measurement of temperature and velocity. The mooring turnaround was done on the NOAA ship Ronald H. Brown, Cruise RB-04-01, by the Upper Ocean Processes Group of the Woods Hole Oceanographic Institution. The cruise took place between 12 and 25 February 2004. The NTAS-3 buoy was found adrift and recovered on 19 February at 14°53.7’N, 51°22.8’W. Deployment of the NTAS-4 mooring was on 21 February at approximately 14°44.4’N, 50°56.0’W in 5038 m of water. A 30-hour intercomparison period followed, after which dragging operations to recover the lower portion of the NTAS-3 mooring commenced. This report describes these operations, as well as other work done on the cruise and some of the pre-cruise buoy preparations.Funding was provided by the National Oceanic and Atmospheric Administration under Grant No. NA17RJ1223 and the Cooperative Institute for Climate and Ocean Research (CICOR)

The Northwest Tropical Atlantic Station (NTAS) : NTAS-2 mooring turnaround cruise report

The Northwest Tropical Atlantic Station (NTAS) was established to address the need for accurate air-sea flux estimates and upper ocean measurements in a region with strong sea surface temperature anomalies and the likelihood of significant local air–sea interaction on interannual to decadal timescales. The approach is to maintain a surface mooring outfitted for meteorological and oceanographic measurements at a site near 15°N, 51°W by successive mooring turnarounds. These observations will be used to investigate air–sea interaction processes related to climate variability. Deployment of the first NTAS mooring (NTAS-1) at 14°50′ N, 51°00′ W on 30 March 2001 was documented in a previous report (Plueddemann et al., 2001). This report documents recovery of the NTAS-1 mooring and deployment of the NTAS-2 mooring at the same site. Both moorings used 3-meter discus buoys as the surface element. These buoys were outfitted with two Air–Sea Interaction Meteorology (ASIMET) systems. Each system measures, records, and transmits via Argos satellite the surface meteorological variables necessary to compute air–sea fluxes of heat, moisture and momentum. The upper 120 m of the NTAS-1 mooring line, and the upper 150 m of the NTAS-2 mooring line, were outfitted with oceanographic sensors for the measurement of temperature and velocity. The mooring turnaround was done on the NOAA Ship Ronald H. Brown, Cruise RB-02-02, by the Upper Ocean Processes Group of the Woods Hole Oceanographic Institution. The cruise took place between 2 and 8 March 2002. A SeaBeam bathymetry survey of the site was done first, followed by deployment of the NTAS-2 mooring on 4 March at approximately 14°44.3′ N, 50°56.8′ W in 5043 m of water. A 24-hour intercomparison period followed, after which the NTAS-1 mooring was recovered. This report describes these operations, as well as some of the pre-cruise buoy preparations.Funding was provided by the National Oceanic and Atmospheric Administration and the Cooperative Institute for Climate and Ocean Research (CICOR) under Grant No. NA17RJ1223