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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 3822-3825 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electroabsorption has been investigated in semi-insulating asymmetric GaAs/AlGaAs double quantum wells presenting high linear Stark responses, adequate for photorefractive applications. We have used the envelope function approximation to calculate the linear Stark shifts of the energy levels and select a suitable structure for the experimental study. The experimental data indicate that the response to the applied field critically depends on a complicated interplay of effects that compete or cooperate to suppress or enhance the electroabsorption. For positive field polarity, the competing contributions of the overlapping e1–hh1 and e1–hh2 transitions partially cancel the electroabsorption despite large linear Stark shifts. On the other hand, small negative fields induce large electroabsorption because the Stark shifts of the two transitions have opposite signs. © 1999 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 1268-1274 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A two-dimensional treatment suitable both for perpendicular (Pockels readout optical modulators, etc.) and for parallel (image amplifiers, etc.) photorefractive devices is presented. A partial differential equation of the third order, valid for both families of devices, is set up and solved analytically for the relevant initial and boundary conditions in the full time domain. The results are illustrated in a few examples showing the relative significance of bulk and surface charges, and some new features of the solution for the perpendicular configuration. © 1996 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 2070-2075 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A two-dimensional time-dependent analysis is presented to describe photorefractive recording in a film with anisotropic transport parameters. The solution of the rate equations together with suitable boundary conditions on the film faces has been obtained by numerical methods for a parallel recording geometry. The recording curves very markedly depend on the anisotropy ratio ai=μz/μx (z being the axis perpendicular to the film faces), the grating period, and the ratio between film and buffer permittivities. For ai=1, the solution coincides with that previously obtained using an analytical approach. Approximate analytical solutions for ai≠1 have also been worked out and compared to the numerical results. © 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 78 (1995), S. 4840-4844 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The initial holographic recording rate has been analytically studied for a photorefractive thin slab. The finite thickness is explicitly taken into account by using a two-dimensional formulation (coordinates x and z parallel and perpendicular to the slab faces, respectively). It is shown that for a thickness comparable to the grating period, the solution appreciably departs from the usual one-dimensional result, i.e., edge effects are remarkable. In particular, significant space-charge fields perpendicular to the slab faces are generated. These effects may substantially modify the diffraction properties of the recorded photorefractive gratings, as exemplified for a GaAs film. © 1995 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 69 (1996), S. 1349-1351 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown that the photorefractive grating produced by a pair of plane waves in periodically poled lithium niobate includes an additional set of spatial harmonics related to the periodic domain structure. This results in new schemes for photorefractive wave coupling. Using the modified phase matching conditions and the concept of optical oscillation we accurately describe the position of the diffraction peaks and explain the main characteristics of self-organized photoinduced scattering reported recently. © 1996 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 68 (1996), S. 517-519 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Output diffraction efficiencies approaching 40% and input diffraction efficiencies approaching 3% have been achieved in photorefractive p-i-n quantum well diodes operating in the longitudinal Stark geometry. The device structure consists of a low-temperature-grown multiple quantum well isolated from the doped contacts by an intrinsic standoff layer of Al0.5Ga0.5As. All charge trapping and screening occurs within the quantum wells without the need for trapping in specialized buffer layers used in previous designs. This new design operates at lower voltages (18 V) and lower fields (6 V/μm) than previously demonstrated. © 1996 American Institute of Physics.
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  • 7
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: A bioactivity-guided chemical study of Iostephane heterophylla (Asteraceae) led to the isolation of xanthorrhizol (1) as the compound that causes inhibition of ATP synthesis, H+-uptake and electron flow from water to methylviologen (basal, phosphorylating and uncoupled) in freshly lysed spinach chloroplasts, thus acting as an inhibitor of the Hill reaction. Acetyl (2), dihydro (3) and acetyl-dihydro (4) derivatives were synthesized. It was found that 4 was less active than 1 and 2 in ATP synthesis, whereas 3 was the most potent inhibitor of the Hill reaction and was also an inhibitor of H+-ATPase. Studies of the photosynthetic partial redox reactions from PQ to MV indicated that 1 partially inhibited the PQ pool, but that 3 did not. However, both inhibited the uncoupled electron transport in PSII from water to DCBQ. Uncoupled electron flow from water to silicomolybdate was completely inhibited by 3 and partially by 1. The reaction from DPC to DCPIP was inhibited by both 1 and 3. These results indicate that the inhibition site is located within PSII for 1 and 3 as was corroborated by fluorescence decay data.
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  • 8
    ISSN: 1572-9753
    Keywords: Apis mellifera ; non-Apis bee conservation ; pollination efficiency ; Yucatán ; México
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Nature of Science, Research, Systems of Higher Education, Museum Science
    Notes: Abstract Although the honey bee, Apis mellifera, has been considered the best pollinator for crops needing insect pollination, the current pandemic of varroatosis among honeybees highlights the need to find additional or alternative species as managed crop pollinators. Moreover, there is evidence that A. mellifera may not always be the most efficient pollinator. Introduction of A. mellifera into crops may be unnecessary, and even detrimental to non-Apis bee populations, which should be considered as an alternative for crop production improvement. Evaluating the pollination efficiency of non-Apis bees is one of the first steps in planning successful strategies for their conservation. In this study, we evaluated the pollination efficiency of Peponapis limitaris and A. mellifera in plots of Cucurbita moschata: pollen removal and deposition; pollinator visit frequency; and the pollinator visit–nectar production relationship. The results show P. limitaris to be the most efficient pollinator as: (1) both males and females remove and deposit almost four times as much pollen as A. mellifera; (2) they make significantly more floral visits than A. mellifera; and (3) their visit frequency shows a strong relationship to C. moschata nectar production during anthesis. Recommendations arising from this study are: (1) the introduction of A. mellifera be avoided in C. moschata crops; and (2) basic research be done on the biology of P. limitaris that contribute to its conservation and greater exploitation.
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  • 9
    ISSN: 1617-6278
    Keywords: Charcoal analysis ; Prehispanic period ; Canary Islands ; Tenerife ; Palaeoenvironment
    Source: Springer Online Journal Archives 1860-2000
    Topics: Archaeology , Biology
    Notes: Abstract Changes in wood taxa used as fuel on the island of Tenerife from 250 B.C. to 1500 A.D. were studied, using charcoal analyses from three archaeological sites. These sites were occupied by “guanche” prehistoric peoples up to the conquest of the island by the king of Castille in 1496. It appears that the collection of wood by prehistoric people had not significantly altered the woody vegetation around the vicinity of the sites. The arrival of European settlers, however, brought new economic and cultural practices which were radically different from those of the prehistoric society.
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  • 10
    ISSN: 1573-5060
    Keywords: AP-PCR ; cultivar identification ; Oleaeuropaea ; olive tree ; RAPD
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract Málaga is a province of Spain where olive-trees are cultivated in a large range of environments, climates and soils. We have developed a reliable and reproducible method to detect RAPD and AP-PCR polymorphisms, using DNA from olive-tree (Olea europaea L.) leaves. Starting from their natural orchards, fifty-six olive-tree cultivars throughout Málaga province, including oil and table olive cultivars, were screened and grouped into 22 varieties. A total of 62 informative polymorphic loci that provide 601 conspicuous bands were enough to differentiate the varieties. Clustering analyses managing 3 different pairwise distances, as well as phylogenetic analyses, led to the same result: olive-trees in Málaga can be divided into three main groups. Group I (90% of certainty) contains wild type and two introduced varieties, group II (83% of certainty) covers some native olive-trees, and group III (58% of certainty) is an heterogeneous cluster that includes varieties originating and cultivated in a number of Andalusian locations. Geographic location seems to be the first responsible of this classification, and morphological traits are needed to justify the group III subclustering. These results are consistent with the hypothesis of autochthonic origin of most olive-tree cultivars, and have been used to support a Label of Origin for the olive oil produced by the varieties included in group II.
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