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  • Springer  (69)
  • Institute of Physics  (10)
  • American Institute of Physics (AIP)  (7)
  • 1995-1999  (86)
  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 1895-1897 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The extent of interdiffusion in Cd1−xMnxTe/CdTe quantum-well structures grown by molecular-beam epitaxy was monitored by photoluminescence and photoluminescence excitation spectra. Thermal annealing of as-grown and ion-implanted structures in over pressures of cadmium (or tellurium) provide clear evidence that diffusion is controlled by cation vacancies and are consistent with a strong dependence of the interdiffusion coefficient on the vacancy concentration. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 72 (1998), S. 2232-2234 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Optical and electron spin resonance spectroscopies have been used to investigate the thermal stability of lasers based on Cr3+:LiSr0.8Ca0.2AlF6, a material in which the onset of critically inhibiting nonradiative decay at temperatures below 300 K might be expected based on results from other Cr3+-activated disordered gain media. The influence of disorder on the Cr3+ environment is shown to be very small in LiSr0.8Ca0.2AlF6 so that the 4T2→4A2 fluorescence transition is not broadened relative to that in LiSrAlF6 and the onset of significant nonradiative decay is also held above room temperature. © 1998 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 82 (1997), S. 4378-4383 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effects on the carrier wave functions of introducing monolayer disorder at the heterojunction interfaces of an undoped superlattice are elucidated. In particular the rate and extent of the resulting localization is quantified by means of an entity C, which is the joint probability of finding two carriers in the same region of space. Evaluating the latter for all states in the miniband provides clear evidence for the existence of a mobility edge in finite two-dimensional structures. Using the entity C as the basis for a semiclassical evaluation of carrier transport, it is shown that the experimentally observed activated transport can be accounted for in terms of the excitation of carriers from the localized states at the bottom of the miniband into the more extended midband miniband states. Comparison of the theoretical results with the experimental results given [Chen et al., J. Cryst. Growth 159, 1066 (1996)], enables conclusions to be drawn concerning the mechanism of activated carrier transport in doped superlattice systems. © 1997 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 84 (1998), S. 5621-5625 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The role of biexcitons has been studied in two optically pumped quantum well (QW) structures of Zn1−xCdxS/ZnS, one with 18% Cd concentration and the other with 3% in the wells. For the x=18% QW structure, high excitation photoluminescence and stimulated emission indicate that the laser gain mechanism involves biexcitons. For the x=3% QWs, even though biexcitons are clearly observed in the spontaneous emission, they are not responsible for laser gain in this structure. Instead exciton–exciton scattering may be the more likely mechanism responsible for laser gain close to threshold, while at higher densities an estimate of the carrier density indicates an electron–hole plasma as the likely source of optical gain. The different mechanisms in the two cases can, very likely, be attributed to one of differing degrees of localization both within the QW and at alloy fluctuations. © 1998 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 67 (1995), S. 109-111 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We show that small structures can be defined in high mobility two-dimensional electron gases formed at a depth of 2770 A(ring) below the surface in GaAs/Al0.33Ga0.67As heterostructures. The differential conductance of one-dimensional constrictions defined by split gates in such deep electron gases showed more than 20 quantised plateaus. The absence of resonant structures on the plateaus demonstrates the absence of potential fluctuations in the constrictions. By applying a dc source-drain bias we have measured the energy spacings of the first 18 subbands, and the effect of a small perpendicular magnetic field on the energy spacings has been investigated. © 1995 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 69 (1996), S. 1506-1508 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Nanocrystalline Ge particles embedded in a SiO2 layer, directly formed by an excision process from a SiGe strained layer during low temperature ultraviolet-assisted oxidation are shown to exhibit visible photoluminescence. The emission maxima of the photoluminescence spectra are situated at around 2.18 eV, a value that corresponds, according to recent data to an average particle size of 5 nm, in excellent agreement with our previous Raman and transmission electron microscopy measurements of particle size. It is proposed that stress effects associated with the oxidation of small spherical particles allow the formed nanocrystalline Ge to survive during prolonged ultraviolet oxidation of the SiGe layer. © 1996 American Institute of Physics.
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  • 7
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We demonstrate optically pumped lasing at room temperature from ZnS/ZnCdS quantum-well structures grown on (100) GaP substrates by molecular beam epitaxy. In the structures with the lowest Cd composition, optically pumped lasing at wavelengths as low as 333 nm (at 8 K) is observed. We present spectroscopic evidence which suggests that the stimulated emission involves states in the low-energy tail of an inhomogeneously broadened excitonic resonance. While lasing is excitonic at low thresholds, a transition to an electron-hole plasma mechanism may occur if the pump power approaches 100 kW cm−2. © 1996 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Agriculture and human values 14 (1997), S. 303-310 
    ISSN: 1572-8366
    Keywords: Biological control ; Environmental policy ; IPM programs
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract An emergent contradiction in the contemporary development of biological control is that of the prevalence of the substitution of periodic releases of natural enemies for chemical insecticides and the dominance of biotechnologically developed transgenic crops. Input substitution leaves in place the monoculture nature of agroecosystems, which in itself is a key factor in encouraging pest problems. Biotechnology, now under corporate control, creates more dependency and can potentially lead to Bt resistance, thus excluding from the market a key biopesticide. Approaches for putting back biological control into the hands of farmers (from artesanal biotechnology for grassroots biopesticide production Cuban style to farmer-to-farmer IPM networks, etc.) have been developed as a way to create a farmer centered approach to biological control
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  • 9
    ISSN: 1572-8366
    Keywords: Urban agriculture ; Cuba ; Agroecology ; Sustainable agriculture ; Food security ; Biological
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract Urban agriculture in Cuba has rapidly become a significant source of fresh produce for the urban and suburban populations. A large number of urban gardens in Havana and other major cities have emerged as a grassroots movement in response to the crisis brought about by the loss of trade, with the collapse of the socialist bloc in 1989. These gardens are helping to stabilize the supply of fresh produce to Cuba's urban centers. During 1996, Havana's urban farms provided the city's urban population with 8,500 tons of agricultural produce, 4 million dozens of flowers, 7.5 million eggs, and 3,650 tons of meat. This system of urban agriculture, composed of about 8,000 gardens nationwide has been developed and managed along agroecological principles, which eliminate the use of synthetic chemical pesticides and fertilizers, emphasizing diversification, recycling, and the use of local resources. This article explores the systems utilized by Cuba's urban farmers, and the impact that this movement has had on Cuban food security.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Journal of heuristics 5 (1999), S. 199-214 
    ISSN: 1572-9397
    Keywords: heuristics ; grid search algorithm ; vertex enumeration ; cobweb search
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract In this paper, several new developments are suggested in order to increase the efficiency of grid search procedures used within vertex enumeration solution algorithms based on both the classical cobweb technique (a two constraint approach) and a single constraint approach. The gains in efficiency (and robustness) are demonstrated by incorporating the developments into a grid search procedure (which is a part of a heuristic solution algorithm used to obtain solutions to a non-linear bi-level multi-period model of an aluminium smelter) and applying it to a test suite of problems. The classical cobweb technique is shown to have some serious problems when used in this application but with step size improvements suggested in the paper, is shown to be very competitive. The developments are also incorporated into a single constraint based approach which does not suffer from the cobweb specific difficulties. The improvements considered include the use of a step mechanism initially with a step size calculated {a priori} and then finally with a tailored (best) one. The latter development when applied to the cobweb approach, requires the approximation of the two constraints (and hence the intersection of their graphs) using second degree polynomials.
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