1,353 research outputs found
Variabilité des propriétés physico-chimiques et dénombrement de la flore fermentaire du tchapalo, une bière traditionnelle de sorgho en Côte d’Ivoire
Une étude portant sur la variabilité des propriétés physico-chimiques etmicrobiologiques, l’évolution de ces propriétés au cours de la fermentation alcoolique a été réalisée sur des échantillons de moût sucré (produit intermédiaire) et de tchapalo (bière de sorgho) prélevés dans une zone de production massive à Abobo, commune située au Nord-Est d’Abidjan. Il ressort de cette étude que la qualité des moûts et tchapalo produits était constante. Le pH (3,4), les teneurs en sucres (3,6 g/100 g), en protéines (2,9 mg/mL) et en vitamine C (1,5 mg/100 mL) des moûts sucrés étaientstatistiquement les mêmes chez toutes les brasseuses et ne variaient pas d’une production à l’autre. Le tchapalo obtenu après fermentation contenait en moyenne 5,2 % d’éthanol. Le méthanol et le propanol n’ont pas été détectés. Les levures avec 1.9×108 ufc/mL constituaient la microflore dominante du tchapalo. Les bactéries lactiques isolées appartenaient aux genres Lactobacillus et Leuconostoc.Mots-clés : Tchapalo, bière traditionnelle, propriétés physico-chimiques, fermentation
Electrically Tunable Excitonic Light Emitting Diodes based on Monolayer WSe2 p-n Junctions
Light-emitting diodes are of importance for lighting, displays, optical
interconnects, logic and sensors. Hence the development of new systems that
allow improvements in their efficiency, spectral properties, compactness and
integrability could have significant ramifications. Monolayer transition metal
dichalcogenides have recently emerged as interesting candidates for
optoelectronic applications due to their unique optical properties.
Electroluminescence has already been observed from monolayer MoS2 devices.
However, the electroluminescence efficiency was low and the linewidth broad due
both to the poor optical quality of MoS2 and to ineffective contacts. Here, we
report electroluminescence from lateral p-n junctions in monolayer WSe2 induced
electrostatically using a thin boron nitride support as a dielectric layer with
multiple metal gates beneath. This structure allows effective injection of
electrons and holes, and combined with the high optical quality of WSe2 it
yields bright electroluminescence with 1000 times smaller injection current and
10 times smaller linewidth than in MoS2. Furthermore, by increasing the
injection bias we can tune the electroluminescence between regimes of
impurity-bound, charged, and neutral excitons. This system has the required
ingredients for new kinds of optoelectronic devices such as spin- and
valley-polarized light-emitting diodes, on-chip lasers, and two-dimensional
electro-optic modulators.Comment: 13 pages main text with 4 figures + 4 pages upplemental material
Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides
Motivated by the triumph and limitation of graphene for electronic
applications, atomically thin layers of group VI transition metal
dichalcogenides are attracting extensive interest as a class of graphene-like
semiconductors with a desired band-gap in the visible frequency range. The
monolayers feature a valence band spin splitting with opposite sign in the two
valleys located at corners of 1st Brillouin zone. This spin-valley coupling,
particularly pronounced in tungsten dichalcogenides, can benefit potential
spintronics and valleytronics with the important consequences of spin-valley
interplay and the suppression of spin and valley relaxations. Here we report
the first optical studies of WS2 and WSe2 monolayers and multilayers. The
efficiency of second harmonic generation shows a dramatic even-odd oscillation
with the number of layers, consistent with the presence (absence) of inversion
symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show
the crossover from an indirect band gap semiconductor at mutilayers to a
direct-gap one at monolayers. The PL spectra and first-principle calculations
consistently reveal a spin-valley coupling of 0.4 eV which suppresses
interlayer hopping and manifests as a thickness independent splitting pattern
at valence band edge near K points. This giant spin-valley coupling, together
with the valley dependent physical properties, may lead to rich possibilities
for manipulating spin and valley degrees of freedom in these atomically thin 2D
materials
Comparaison de deux techniques de replantation cacaoyère sur antécédents culturaux non-forestiers en Côte d’Ivoire
Le verger de cacaoyers de la Côte d’Ivoire, premier producteur mondial de cacao, est vieillissant et doit être régénéré pour assurer une production durable de cacao. Mais, les réserves de forêt utilisées autrefois pour l’extension des plantations sont épuisées. Les terres cultivables disponibles sont les jachères et les cacaoyères dégradées. Malheureusement, la plupart des tentatives de plantation sur ces antécédents culturaux dégradés échouent. En vue de proposer aux producteurs des itinéraires techniques adaptés, deux techniques de replantation ont été comparées sur 40 parcelles replantées dont 20 après des vieux cacaoyers âgés de plus de 30 ans avec moins de 700 arbres ha-1 et 20 autres après des jachères de durée inférieure à 10 ans. Ces parcelles ont été installées entre 2004 et 2006 dans 10 régions productrices de cacao. Les résultats ont montré que la replantation sous vieux cacaoyers est plus efficace. L’efficacité de cette technique est liée à la fertilité des sols relativement meilleure et au niveau d’enherbement plus faible sous les vieux cacaoyers par rapport aux jachères. Il est cependant nécessaire de bien gérer l’ombrage des vieux cacaoyers, en effectuant des éclaircies avant 6 et 24 mois après la mise en place des jeunes cacaoyers.Mots Clés: Cacao, itinéraires techniques, jachère, régénérationEnglish AbstractCocoa farms of Côte d’Ivoire, the world’s largest cocoa producer, are getting older and must be regenerated for sustainable cocoa production. However, forest reserves formerly used for the expansion of plantations are finished. Available cultivable land parcels are fallows with poor soil and degraded cocoa farms. Unfortunately, most attempts of replanting on these poor lands fail. In order to provide producers with appropriate replanting techniques, two replanting techniques were compared on a 40 replanted plots, including 20 plots after more than 30 years-old cocoa farms with less than 700 trees ha-1 and 20 other plots after fallows of less than 10 years. These plots were installed between 2004 and 2006 in 10 cocoa producing areas. Replanting under the old cocoa trees is more effective. The effectiveness of this practice is related to the relatively better soil fertility and lower level of the weeds proliferation under the old cocoa trees as compared to the fallows. It is, however, necessary to reduce the shade of the old cocoa trees before 6 and 24 months after the establishment of the young cocoa trees.Keywords: Cocoa, fallow, previous land, regeneratio
Serotonin and corticosterone rhythms in mice exposed to cigarette smoke and in patients with COPD:implication for COPD-associated neuropathogenesis
The circadian timing system controls daily rhythms of physiology and behavior, and disruption of clock function can trigger stressful life events. Daily exposure to cigarette smoke (CS) can lead to alteration in diverse biological and physiological processes. Smoking is associated with mood disorders, including depression and anxiety. Patients with chronic obstructive pulmonary disease (COPD) have abnormal circadian rhythms, reflected by daily changes in respiratory symptoms and lung function. Corticosterone (CORT) is an adrenal steroid that plays a considerable role in stress and anti-inflammatory responses. Serotonin (5-hydroxytryptamine; 5HT) is a neurohormone, which plays a role in sleep/wake regulation and affective disorders. Secretion of stress hormones (CORT and 5HT) is under the control of the circadian clock in the suprachiasmatic nucleus. Since smoking is a contributing factor in the development of COPD, we hypothesize that CS can affect circadian rhythms of CORT and 5HT secretion leading to sleep and mood disorders in smokers and patients with COPD. We measured the daily rhythms of plasma CORT and 5HT in mice following acute (3 d), sub-chronic (10 d) or chronic (6 mo) CS exposure and in plasma from non-smokers, smokers and patients with COPD. Acute and chronic CS exposure affected both the timing (peak phase) and amplitude of the daily rhythm of plasma CORT and 5HT in mice. Acute CS appeared to have subtle time-dependent effects on CORT levels but more pronounced effects on 5HT. As compared with CORT, plasma 5HT was slightly elevated in smokers but was reduced in patients with COPD. Thus, the effects of CS on plasma 5HT were consistent between mice and patients with COPD. Together, these data reveal a significant impact of CS exposure on rhythms of stress hormone secretion and subsequent detrimental effects on cognitive function, depression-like behavior, mood/anxiety and sleep quality in smokers and patients with COPD
Electrical Tuning of Valley Magnetic Moment via Symmetry Control
Crystal symmetry governs the nature of electronic Bloch states. For example,
in the presence of time reversal symmetry, the orbital magnetic moment and
Berry curvature of the Bloch states must vanish unless inversion symmetry is
broken. In certain 2D electron systems such as bilayer graphene, the intrinsic
inversion symmetry can be broken simply by applying a perpendicular electric
field. In principle, this offers the remarkable possibility of switching on/off
and continuously tuning the magnetic moment and Berry curvature near the Dirac
valleys by reversible electrical control. Here we demonstrate this principle
for the first time using bilayer MoS2, which has the same symmetry as bilayer
graphene but has a bandgap in the visible that allows direct optical probing of
these Berry-phase related properties. We show that the optical circular
dichroism, which reflects the orbital magnetic moment in the valleys, can be
continuously tuned from -15% to 15% as a function of gate voltage in bilayer
MoS2 field-effect transistors. In contrast, the dichroism is gate-independent
in monolayer MoS2, which is structurally non-centrosymmetric. Our work
demonstrates the ability to continuously vary orbital magnetic moments between
positive and negative values via symmetry control. This represents a new
approach to manipulating Berry-phase effects for applications in quantum
electronics associated with 2D electronic materials.Comment: 13 pages main text + 4 pages supplementary material
A Cytochrome P450 Conserved in Insects Is Involved in Cuticle Formation
The sequencing of numerous insect genomes has revealed dynamic changes in the number and identity of cytochrome P450 genes in different insects. In the evolutionary sense, the rapid birth and death of many P450 genes is observed, with only a small number of P450 genes showing orthology between insects with sequenced genomes. It is likely that these conserved P450s function in conserved pathways. In this study, we demonstrate the P450 gene, Cyp301a1, present in all insect genomes sequenced to date, affects the formation of the adult cuticle in Drosophila melanogaster. A Cyp301a1 piggyBac insertion mutant and RNAi of Cyp301a1 both show a similar cuticle malformation phenotype, which can be reduced by 20-hydroxyecdysone, suggesting that Cyp301a1 is an important gene involved in the formation of the adult cuticle and may be involved in ecdysone regulation in this tissue
Valley polarization in MoS2 monolayers by optical pumping
We report experimental evidences on selective occupation of the degenerate
valleys in MoS2 monolayers by circularly polarized optical pumping. Over 30%
valley polarization has been observed at K and K' valley via the polarization
resolved luminescence spectra on pristine MoS2 monolayers. It demonstrates one
viable way to generate and detect valley polarization towards the conceptual
valleytronics applications with information carried by the valley index
Entanglement of single-photons and chiral phonons in atomically thin WSe
Quantum entanglement is a fundamental phenomenon which, on the one hand,
reveals deep connections between quantum mechanics, gravity and the space-time;
on the other hand, has practical applications as a key resource in quantum
information processing. While it is routinely achieved in photon-atom
ensembles, entanglement involving the solid-state or macroscopic objects
remains challenging albeit promising for both fundamental physics and
technological applications. Here, we report entanglement between collective,
chiral vibrations in two-dimensional (2D) WSe host --- chiral phonons (CPs)
--- and single-photons emitted from quantum dots (QDs) present in it. CPs which
carry angular momentum were recently observed in WSe and are a
distinguishing feature of the underlying honeycomb lattice. The entanglement
results from a "which-way" scattering process, involving an optical excitation
in a QD and doubly-degenerate CPs, which takes place via two indistinguishable
paths. Our unveiling of entanglement involving a macroscopic, collective
excitation together with strong interaction between CPs and QDs in 2D materials
opens up ways for phonon-driven entanglement of QDs and engineering chiral or
non-reciprocal interactions at the single-photon level
Magnetic Control of Valley Pseudospin in Monolayer WSe2
Local energy extrema of the bands in momentum space, or valleys, can endow
electrons in solids with pseudo-spin in addition to real spin. In transition
metal dichalcogenides this valley pseudo-spin, like real spin, is associated
with a magnetic moment which underlies the valley-dependent circular dichroism
that allows optical generation of valley polarization, intervalley quantum
coherence, and the valley Hall effect. However, magnetic manipulation of valley
pseudospin via this magnetic moment, analogous to what is possible with real
spin, has not been shown before. Here we report observation of the valley
Zeeman splitting and magnetic tuning of polarization and coherence of the
excitonic valley pseudospin, by performing polarization-resolved
magneto-photoluminescence on monolayer WSe2. Our measurements reveal both the
atomic orbital and lattice contributions to the valley orbital magnetic moment;
demonstrate the deviation of the band edges in the valleys from an exact
massive Dirac fermion model; and reveal a striking difference between the
magnetic responses of neutral and charged valley excitons which is explained by
renormalization of the excitonic spectrum due to strong exchange interactions
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