Control of the Bright-Dark Exciton Splitting Using the Lamb Shift in a Two-Dimensional Semiconductor.

We investigate the exciton fine structure in atomically thin WSe_{2}-based van der Waals heterostructures where the density of optical modes at the location of the semiconductor monolayer can be tuned. The energy splitting Δ between the bright and dark exciton is measured by photoluminescence spectroscopy. We demonstrate that Δ can be tuned by a few meV as a result of a significant Lamb shift of the optically active exciton that arises from emission and absorption of virtual photons triggered by the vacuum fluctuations of the electromagnetic field. We also measure strong variations of the bright exciton radiative linewidth as a result of the Purcell effect. All these experimental results illustrate the strong sensitivity of the excitons to local vacuum field fluctuations. We find a very good agreement with a model that demonstrates the equivalence, for our system, of a classical electrodynamical transfer matrix formalism and quantum-electrodynamical approach. The bright-dark splitting control we demonstrate here in the weak light-matter coupling regime should apply to any semiconductor structures.

Optical Detection of Long Electron Spin Transport Lengths in a Monolayer Semiconductor.

Using a spatially resolved optical pump-probe experiment, we measure the lateral transport of spin-valley polarized electrons over very long distances (tens of micrometers) in a single WSe_{2} monolayer. By locally pumping the Fermi sea of 2D electrons to a high degree of spin-valley polarization (up to 75%) using circularly polarized light, the lateral diffusion of the electron polarization can be mapped out via the photoluminescence induced by a spatially separated and linearly polarized probe laser. Up to 25% spin-valley polarization is observed at pump-probe separations up to 20 µm. Characteristic spin-valley diffusion lengths of 18±3 µm are revealed at low temperatures. The dependence on temperature, pump helicity, pump intensity, and electron density highlight the key roles played by spin relaxation time and pumping efficiency on polarized electron transport in monolayer semiconductors possessing spin-valley locking.

Measurement of Conduction and Valence Bands g-Factors in a Transition Metal Dichalcogenide Monolayer.

The electron valley and spin degree of freedom in monolayer transition-metal dichalcogenides can be manipulated in optical and transport measurements performed in magnetic fields. The key parameter for determining the Zeeman splitting, namely, the separate contribution of the electron and hole g factor, is inaccessible in most measurements. Here we present an original method that gives access to the respective contribution of the conduction and valence band to the measured Zeeman splitting. It exploits the optical selection rules of exciton complexes, in particular the ones involving intervalley phonons, avoiding strong renormalization effects that compromise single particle g-factor determination in transport experiments. These studies yield a direct determination of single band g factors. We measure g_{c1}=0.86±0.1, g_{c2}=3.84±0.1 for the bottom (top) conduction bands and g_{v}=6.1±0.1 for the valence band of monolayer WSe_{2}. These measurements are helpful for quantitative interpretation of optical and transport measurements performed in magnetic fields. In addition, the measured g factors are valuable input parameters for optimizing band structure calculations of these 2D materials.

Unraveling the role of zinc complexes on indium phosphide nanocrystal chemistry.

The addition of zinc complexes to the syntheses of indium phosphide nanocrystals (InP NCs) has become commonplace, due to their ability to alter and significantly improve observed optical properties. In this paper, the role of zinc complexes on the synthesis and observed properties of InP is carefully examined. Produced InP and InP:Zn2+ NCs are thoroughly characterized from both structural (core and surface) and optical perspectives over a wide range of Zn2+ compositions (0%-43% atomic content). We find no differences in the physical (NC size and polydispersity) and structural properties (crystallographic phase) of InP and InP:Zn2+ NCs. Optically, significant changes are observed when zinc is added to InP syntheses, including blueshifted absorption edges and maxima, increased quantum yields, and the near elimination of surface state emission. These improved optical properties result from surface passivation by zinc carboxylate moieties. Changes to the optical properties begin at zinc concentrations as low as 5%, demonstrating the high sensitivity of InP optical properties to exogenous species.

Control of the Exciton Radiative Lifetime in van der Waals Heterostructures.

Optical properties of atomically thin transition metal dichalcogenides are controlled by robust excitons characterized by a very large oscillator strengths. Encapsulation of monolayers such as MoSe_{2} in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogenous exciton linewidth. We demonstrate that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect. The time-resolved photoluminescence measurements show that the neutral exciton spontaneous emission time can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. The inhibition of the radiative recombination can yield spontaneous emission time up to 10 ps. These results are in very good agreement with the calculated recombination rate in the weak exciton-photon coupling regime. The analysis shows that we are also able to observe a sizable enhancement of the exciton radiative decay rate. Understanding the role of these electrodynamical effects allows us to elucidate the complex dynamics of relaxation and recombination for both neutral and charged excitons.

In-Plane Propagation of Light in Transition Metal Dichalcogenide Monolayers: Optical Selection Rules.

The optical selection rules for interband transitions in WSe_{2}, WS_{2}, and MoSe_{2} transition metal dichalcogenide monolayers are investigated by polarization-resolved photoluminescence experiments with a signal collection from the sample edge. These measurements reveal a strong polarization dependence of the emission lines. We see clear signatures of the emitted light with the electric field oriented perpendicular to the monolayer plane, corresponding to an interband optical transition forbidden at normal incidence used in standard optical spectroscopy measurements. The experimental results are in agreement with the optical selection rules deduced from group theory analysis, highlighting the key role played by the different symmetries of the conduction and valence bands split by the spin-orbit interaction. These studies yield a direct determination of the bright-dark exciton splitting, for which we measure 40±1 meV and 55±2 meV in WSe_{2} and WS_{2} monolayer, respectively.

Giant enhancement of the optical second-harmonic emission of WSe(2) monolayers by laser excitation at exciton resonances.

We show that the light-matter interaction in monolayer WSe_{2} is strongly enhanced when the incoming electromagnetic wave is in resonance with the energy of the exciton states of strongly Coulomb bound electron-hole pairs below the electronic band gap. We perform second harmonic generation (SHG) spectroscopy as a function of laser energy and polarization at T=4 K. At the exciton resonance energies we record an enhancement by up to 3 orders of magnitude of the SHG efficiency, due to the unusual combination of electric dipole and magnetic dipole transitions. The energy and parity of the exciton states showing the strong resonance effects are identified in 1- and 2-photon photoluminescence excitation experiments, corroborated by first principles calculations. Targeting the identified exciton states in resonant 2-photon excitation allows us to maximize k-valley coherence and polarization.

Carrier and polarization dynamics in monolayer MoS2.

In monolayer MoS2, optical transitions across the direct band gap are governed by chiral selection rules, allowing optical valley initialization. In time-resolved photoluminescence (PL) experiments, we find that both the polarization and emission dynamics do not change from 4 to 300 K within our time resolution. We measure a high polarization and show that under pulsed excitation the emission polarization significantly decreases with increasing laser power. We find a fast exciton emission decay time on the order of 4 ps. The absence of a clear PL polarization decay within our time resolution suggests that the initially injected polarization dominates the steady-state PL polarization. The observed decrease of the initial polarization with increasing pump photon energy hints at a possible ultrafast intervalley relaxation beyond the experimental ps time resolution. By compensating the temperature-induced change in band gap energy with the excitation laser energy, an emission polarization of 40% is recovered at 300 K, close to the maximum emission polarization for this sample at 4 K.

The ethics of a clinical trial when the protocol clashes with international guidelines.

Due to their nature and complexity, clinical trials often take some time to launch after the protocol has been designed and ethics approval obtained. During this time, there may be changes in international treatment guidelines and recommendations that result in a conflict between study protocol and recommended international best practice. Here, we describe the situation that arose in a pharmacokinetic study on the use of two different doses of rifabutin in patients with human immunodeficiency virus-associated tuberculosis who initiated antiretroviral therapy (ART) with a lopinavir-ritonavir-based regimen in South Africa and Viet Nam. The study protocol specified that ART should be started 10 weeks after the start of anti-tuberculosis treatment. The study in South Africa was approved in June 2008, went ahead as scheduled and was completed in August 2010. The study in Viet Nam was approved in October 2008 and was started in June 2010. A few weeks later, the World Health Organization released their 2010 guidelines for adult ART; one of its strong recommendations (with moderate quality of evidence) was that ART should be started 2-8 weeks after the start of anti-tuberculosis treatment. Emerging scientific evidence also supported this recommendation. The investigators felt that the Viet Nam study protocol was in conflict with recommended international best practice, and the trial was stopped in October 2010. An amended study protocol in which ART was started at 2 weeks was developed and implemented. The ethics issues around this decision and the need to change the study protocol are discussed in this article.

Du fait de la nature et de la complexité des études cliniques, leur mise en œuvre est souvent longue après l'élaboration du protocole et son approbation éthique. Pendant cette période, il peut y avoir un changement des lignes directrices internationales de traitement et des recommandations qui provoquent un conflit entre le protocole et les meilleures pratiques recommandées internationalement. Nous décrivons ici la situation apparue dans une étude pharmacocinétique portant sur l'utilisation de deux doses différentes de rifabutin chez des patients atteints de tuberculose (TB) associée au virus de l'immunodéficience humaine et commençant un traitement antirétroviral (ART) à base de lopinavir-ritonavir en Afrique du Sud et au Viet Nam. Le protocole de l'étude spécifiait de commencer l'ART 10 semaines après le début de la thérapie antituberculeuse. En Afrique du Sud, l'étude a été approuvée en juin 2008, s'est déroulée comme prévu et a été achevée en juin 2010. Au Viet Nam, l'étude a été approuvée en octobre 2008 et a démarré en juin 2010. Quelques semaines après, l'Organisation Mondiale de la Santé a publié ses lignes directrices de 2010 pour l'utilisation de l'ART chez les adultes, dont l'une des vives recommandations (basée sur des données de qualité modérée) était de commencer l'ART entre 2 et 8 semaines après le début du traitement de la TB. L'arrivée de nouvelles preuves scientifiques est aussi venue à l'appui de cette recommandation. Les investigateurs ont eu le sentiment que le protocole d'étude au Viet Nam était en conflit avec les meilleures pratiques internationales et l'étude a été arrêtée en octobre 2010. Un protocole d'étude amendé a été développé et mis en œuvre. Les problèmes éthiques entourant cette décision et la nécessité de changer le protocole sont discutés dans ce papier.

Los ensayos clínicos, dadas sus características y su complejidad, suelen exigir mucho tiempo desde la elaboración del protocolo y la aprobación por parte del comité de ética hasta su realización. Durante este lapso, pueden surgir modificaciones en las recomendaciones y las directrices terapéuticas internacionales, lo cual genera un conflicto entre el protocolo del estudio y las prácticas óptimas recomendadas. A continuación se describe la situación que se presentó en Suráfrica y Viet Nam durante un estudio de farmacocinética sobre el uso de dos dosificaciones diferentes de rifabutina, en pacientes aquejados de tuberculosis (TB) asociada con la infección por el virus de la inmunodeficiencia humana (HIV), quienes habían comenzado el tratamiento antirretrovírico (ART) con un régimen basado en la asociación lopinavir y ritonavir. El protocolo del estudio precisaba que el ART se debía comenzar 10 semanas después de haber iniciado el tratamiento antituberculoso. En Suráfrica, el estudio recibió la aprobación en junio del 2008, comenzó en el tiempo previsto y se completó en agosto del 2010. En Viet Nam, se obtuvo la aprobación del estudio en octubre del 2008 y se comenzó en junio del 2010. A las pocas semanas, la Organización Mundial de Salud publicó sus directrices del ART en los adultos del 2010, una de cuyas recomendaciones más firmes consistía en que el ART se debía iniciar entre 2 y 8 semanas después de haber comenzado el tratamiento antituberculoso (con una calidad probatoria moderada). Algunos resultados científicos de aparición reciente respaldaban igualmente esta recomendación. Los investigadores consideraron que el protocolo del estudio en Viet Nam entraba en conflicto con las prácticas óptimas internacionales recomendadas e interrumpieron su realización en octubre del 2010. Se introdujeron modificaciones al protocolo, según las cuales el ART se comenzaría a las 2 semanas y se puso en práctica el estudio. En el presente artículo se analizan los aspectos éticos en torno a esta decisión y a la necesidad de modificar el protocolo del estudio.

Surfactant-free CZTS nanoparticles as building blocks for low-cost solar cell absorbers.

A process route for the fabrication of solvent-redispersible, surfactant-free Cu2ZnSnS4 (CZTS) nanoparticles has been designed with the objective to have the benefit of a simple sulfide source which advantageously acts as (i) a complexing agent inhibiting crystallite growth, (ii) a surface additive providing redispersion in low ionic strength polar solvents and (iii) a transient ligand easily replaced by an carbon-free surface additive. This multifunctional use of the sulfide source has been achieved through a fine tuning of ((Cu²âº)(a)(Zn²âº)(b)(Sn4⁺)(c)(Tu)(d)(OH⁻)(e))(t⁺), Tu = thiourea) oligomers, leading after temperature polycondensation and S²â» exchange to highly concentrated (c > 100 g l⁻¹), stable, ethanolic CZTS dispersions. The good electronic properties and low-defect concentration of the sintered, crack-free CZTSe films resulting from these building blocks was shown by photoluminescence investigation, making these building blocks interesting for low-cost, high-performance CZTSe solar cells.

Dark-bright mixing of interband transitions in symmetric semiconductor quantum dots.

In photoluminescence spectra of symmetric [111] grown GaAs/AlGaAs quantum dots in longitudinal magnetic fields applied along the growth axis, we observe in addition to the expected bright states also nominally dark transitions for both charged and neutral excitons. We uncover a strongly nonmonotonic, sign-changing field dependence of the bright neutral exciton splitting resulting from the interplay between exchange and Zeeman effects. Our theory shows quantitatively that these surprising experimental results are due to magnetic-field-induced ±3/2 heavy-hole mixing, an inherent property of systems with C(3v) point-group symmetry.

Full electrical control of the electron spin relaxation in GaAs quantum wells.

The electron spin dynamics in (111)-oriented GaAs/AlGaAs quantum wells is studied by time-resolved photoluminescence spectroscopy. By applying an external electric field of 50 kV/cm a two-order of magnitude increase of the spin relaxation time can be observed reaching values larger than 30 ns; this is a consequence of the electric field tuning of the spin-orbit conduction band splitting which can almost vanish when the Rashba term compensates exactly the Dresselhaus one. The measurements under a transverse magnetic field demonstrate that the electron spin relaxation time for the three space directions can be tuned simultaneously with the applied electric field.

Room-temperature defect-engineered spin filter based on a non-magnetic semiconductor.

Generating, manipulating and detecting electron spin polarization and coherence at room temperature is at the heart of future spintronics and spin-based quantum information technology. Spin filtering, which is a key issue for spintronic applications, has been demonstrated by using ferromagnetic metals, diluted magnetic semiconductors, quantum point contacts, quantum dots, carbon nanotubes, multiferroics and so on. This filtering effect was so far restricted to a limited efficiency and primarily at low temperatures or under a magnetic field. Here, we provide direct and unambiguous experimental proof that an electron-spin-polarized defect, such as a Ga(i) self-interstitial in dilute nitride GaNAs, can effectively deplete conduction electrons with an opposite spin orientation and can thus turn the non-magnetic semiconductor into an efficient spin filter operating at room temperature and zero magnetic field. This work shows the potential of such defect-engineered, switchable spin filters as an attractive alternative to generate, amplify and detect electron spin polarization at room temperature without a magnetic material or external magnetic fields.

[Pharmacological approach to desychronization of the sleep-wakefulness cycle in the military and sport environment]. / Approche pharmacologique de la désynchronisation du rythme veille-sommeil en milieu militaire et sportif.

Sports and military environments have many common features - intense physical activity, rigorous physical environment (heat, cold, high or low pressure, hypoxia, acceleration...), specific psychosocial atmosphere, team spirit. If combined with jet lag syndrome, these specific conditions can favor altered physical and mental performance. There is always the temptation to use drugs as a simple way to reduce the penalizing effects. The available compounds known to affect sleep and wakefulness include hypnotics, benzodiazepines and non benzodiazepines such as temazepam, zolopidem, and zopiclone, stimulants such as amphetamine and amphetamine-like agents, adrafinil, modafinil, caffeine and chronobiotics substances such as melatonin and, more recently, slow release caffeine. In the sports area, all of these substances except caffeine are on the list of forbidden products, although special authorizations linked to known disease conditions are allowed. In the military setting, the environment may be similar, but the context of use is very different. In the context of a rescue mission, the current practice in the French military organization is to place modafinil pills in the ejection seat of fight planes and in rescue boats. A second context is the use of anti-sleep agents under orders; the debate continues on this and the appropriate recommendations in this context. Self-medication is a third condition, in which case no rules have been defined.

Lost in translation: translational interference from a recurrent mutation in exon 1 of MECP2.

BACKGROUND: Rett syndrome (RTT) is an X linked neuro-developmental disorder affecting mostly girls. Mutations in the coding region of MECP2 are found in 80% of classic RTT patients. Until recently, the region encoding MECP2 was believed to comprise exons 2, 3, and 4 with the ATG start site located at the end of exon 2 (MeCP2_e2). METHODS: Recent reports of another mRNA transcript transcribed from exon 1 (MeCP2_e1) prompted us to screen exon 1 among RNA samples from 20 females with classic or atypical RTT. RESULTS: A previously reported 11 base pair deletion in exon 1 was detected in one subject with a milder phenotype. Although RNA expression for both protein isoforms was detected from the mutant allele, evaluation of MeCP2 protein in uncultured patient lymphocytes by immunocytochemistry revealed that MeCP2 protein production was restricted to only 74-76% of lymphocytes. X chromosome inactivation studies of genomic DNA revealed similar XCI ratios at the HUMARA locus (73:27 with HpaII and 74:26 with McrBC). We have demonstrated that translation but not transcription of the MeCP2_e2 isoform is ablated by the 11 nucleotide deletion, 103 nucleotides upstream of the e2 translation start site. CONCLUSIONS: These findings reveal that nucleotides within the deleted sequence in the 5'-UTR of the MeCP2_e2 transcript, while not required for transcription, are essential for translation.

Langerhans cell histiocytosis: report of a single organ involvement in a child.

Langerhans cell histiocytosis is a rare disorder characterized by abnormal proliferation of Langerhans cells that can affect various organ systems. The disease usually presents as a unifocal lytic bone lesion and can affect any age group. Less frequently it presents as a disseminated disease with multisystem involvement. Hepatic manifestation in Langerhans cell histiocytosis is relatively rare and usually presents as a part of a disseminated process. We report a case of Langerhans cell histiocytosis involving only the liver in a 9-years-old child.

Caffeine or melatonin effects on sleep and sleepiness after rapid eastward transmeridian travel.

We measured the effects of slow-release caffeine (SRC) and melatonin (Mlt) on sleep and daytime sleepiness after a seven-time zone eastbound flight. In a double-blind, randomized, placebo-controlled study, each of three groups of nine subjects was given either 300 mg SRC on recovery day 1 (D1) to D5 (0800) or 5 mg Mlt on preflight D-1 (1700), flight day D0 (1600), and from D1 to D3 (2300), or placebo (Pbo) at the same times. Nighttime sleep was evaluated by polysomnography and daytime sleepiness from measurements of sleep latencies and continuous wrist actigraphy. Compared with baseline, we found a significant rebound of slow-wave sleep on night 1 (N1) to N2 under Pbo and Mlt and a significant decrease in rapid eye movement sleep on N1 (Pbo) and N1-N3 (Mlt). Sleepiness was objectively increased under Pbo (D1-D6) and Mlt (D1-D3). SRC reduced sleepiness but also tended to affect sleep quality until the last drug day. In conclusion, both drugs have positive effects on some jet lag symptoms after an eastbound flight: SRC on daytime sleepiness, and Mlt on sleep.

Nycthemeral variations in core temperature and heart rate: continuous cycling exercise versus continuous rest.

Circadian rhythms have formed the subject of many researches in man during bed rest or usual routine, but have been little studied during continuous and sustained physical exercise. This study deals with the influence of time of day on biological markers in competitive cyclists during continuous physical exercise versus continuous rest. Ultra-distance cyclists were studied over a 24 h period (13:00 to 13:00 h the next day) in the laboratory. The subjects were requested to maintain a constant speed (set at 65% - 70% of their maximal aerobic speed obtained during a preliminary test) on their own bicycles which were equipped with home trainers. Workload, core temperature and heart rate were monitored continuously. The same measures were also recorded while the athletes were resting awake until 13:00 h the next day. Results show that in both situations, core temperature and heart rate exhibited significant circadian variations (p < 0.001). Furthermore, during exercise, an accentuation of amplitude and mean of every rhythm (p < 0.05) with a phase lag (p < 0.05) were observed. Despite a strenuous and continuous physical exercise requiring special physiological adaptations, the rhythmic variations observed at rest persisted, which highlighted the influence of biological clocks.

Resynchronization of hormonal rhythms after an eastbound flight in humans: effects of slow-release caffeine and melatonin.

The aim of this work was to investigate the potential chronobiotic properties of slow-release caffeine, in comparison with melatonin, on resynchronization of endogenous melatonin and cortisol secretions after an eastbound flight by jet incurring a time loss of 7 h. A group of 27 reservists of the US Air Force received either slow-release caffeine (300 mg), melatonin (5 mg) or placebo before, during and/or after the transmeridian flight. Saliva and urine were sampled before the flight in the United States (from day -2 to day 0) and after the flight in France (from day 1 to day 10). Saliva was collected once a day on waking to determine saliva melatonin and cortisol concentrations. In addition, concentrations of caffeine in saliva were determined three times a day and of 6-sulphatoxymelatonin in urine collected overnight to check that the treatment regimes had been complied with. From day 3 to day 5, post-flight saliva melatonin concentrations were significantly different from control values in the placebo group only. During treatment with melatonin, the mean urinary 6-sulphatoxymelatonin concentration in the melatonin group was more than twice as high as in the two other groups. In the slow-release caffeine group and the melatonin group, mean saliva cortisol concentrations were significantly lower than control from day 2 to day 5, whereas the placebo group had a mean saliva cortisol concentration significantly lower than the control value from day 2 to day 9. In conclusion, these results indicate that administration of slow-release caffeine, as well as of melatonin, allows a faster resynchronization of hormone rhythms during the 4 days following an eastbound flight incurring the loss of 7 h.

Cerebral cortical blood flow during loss of consciousness induced by gravitational stress in rhesus monkeys.

We investigated the influence of +Gz acceleration (head-to-foot inertial forces) onset on cerebral cortical blood flow (CBF) of rhesus monkeys to study the mechanisms underlying +Gz-induced loss of consciousness (G-LOC). CBF was measured through a chronic cranial window by laser-Doppler flowmetry and electrodes in contact with the dura mater were used to detect G-LOC (suppression of the cortical electrical activity). The animals were centrifuged up to +12 Gz with different G-onset rate until G-LOC occurred. G-LOC was preceded by a 2-5 s decrease in CBF. At G-LOC detection, CBF was not related to G-onset rate (mean CBF change: -76 +/- 9% of control value). We conclude that G-LOC results from cerebral ischemia rather than from mechanical stresses applied to the brain.