98 research outputs found
Efficient optical pumping of Zeeman spin levels in Nd3+:YVO4
We demonstrate that Zeeman ground-state spin levels in Nd3+:YVO4 provides the
possibility to create an efficient lambda-system for optical pumping
experiments. The branching ratio R in the lambda-system is measured
experimentally via absorption spectroscopy and is compared to a theoretical
model. We show that R can be tuned by changing the orientation of the magnetic
field. These results are applied to optical pumping experiments, where
significant improvement is obtained compared to previous experiments in this
system. The tunability of the branching ratio in combination with its good
coherence properties and the high oscillator strength makes Nd3+:YVO4 an
interesting candidate for various quantum information protocols.Comment: 8 pages, 6 figure
Investigating tropospheric effects and seasonal position variations in GPS and DORIS time-series from the Nepal Himalaya
Geodetic time-series from continuous GPS (cGPS) and 1 DORIS stations across the Himalaya of central Nepal show strong seasonal fluctuations observed on the horizontal and vertical components. Because the fluctuations determined at the different stations have similar phase but different amplitudes, these observations would imply that the secular shortening across the range is modulated by a seasonal strain. Given the geographic and climatic setting, there is however a possibility that the GPS positions be biased by tropospheric effects. We process these data using two different software packages and two different analysis strategies. Our analysis shows evidence for 1-strong seasonal fluctuation of zenithal delays consistent with in situ meteorological data and two strong horizontal tropospheric gradients in particular in the EW direction, that is, parallel to the mountain front at Gumba, also detected in DORIS results. We show that the tropospheric effects cannot however be the source of the observed seasonality of horizontal strain. This study supports the view that the seasonal strain in the Himalaya is real and probably driven by seasonal surface load variations. Our study adds support to the view that seasonal variations of seismicity in the Himalaya reflects seasonal variations of geodetic strain
Déformation Intersismique de l'Himalaya du Népal à partir de données GPS
The velocity of interseismic convergence through the Nepal Himalaya - essential parameter in the study of the seismic cycle - is discussed since several years. To constrain this velocity, we have successfully combined geodetic data from campaign GPS surveys, cGPS stations and DORIS stations to determine the plate motion of India and contemporary crustal strain across Nepal Himalaya. The pattern of crustal deformation across the Eastern and Central Himalaya, impliesthat the MHT is locked over a distance of about 115km from the surface to a depth of about 20km beneath the front of the mountain range. Horizontal shortening due to ductile creep along the deeper portion of the MHT appears to fall in the range 19+/-2.5mm/yr along strike in Central and Eastern Nepal. Small (The analysis of GPS time series showed, in addition to the secular term, important seasonal variations, in particular on the horizontal component perpendicular to the Himalayan range. We could highlight that these seasonal variations did not come from a processing artefact, but of the flexural response of the continental crust to an aquifer loading located in the Ganges basin. The flexural response to this seasonal load induced variations of Coulomb stress localised in the Himalayan midcrustal seismic swarm. These variations make it possible to explain in first order the seasonal variations seismicityrate highlighted on the Nepalese seismicity catalogue.The study of the grain sizes of quartz enabled us to determine the amplitude of the differential stress present around the brittle-ductile transition. From the combination of thermometric, structural, geodetic and paleopiezometric data, an experimental rheological law of the Lesser Himalayan quarzites could be defined. The determination of this law brings an additional constrain to the knowledge of the rheological structure of the Himalayan continental lithosphere, an essential parameter in the mechanical models of the seismic cycle.La vitesse de convergence intersismique de l'Himalaya - paramètre essentiel dans l'étude du cycle sismique - est débattue depuis plusieurs années. Pour contraindre cette dernière, nous avons combiné avec succès des données géodésiques de campagne, de stations cGPS et de stations DORIS, pour déterminer le mouvement de la plaque indienne ainsi que la déformation crustale actuelle au travers de l'Himalaya du Népal. Il ressort que le mouvement de l'Inde par rapport à l'Eurasie, que nous estimons à 35mm.an-1, est significativement moins rapide que celui déduit des modèles géologiques globaux de tectonique des plaques. Le modèle de déformation crustale au travers de l'Himalya oriental et central indique que le MHT est bloqué sur une distance d'environ 115 km, de la surface jusqu`à une profondeur d'environ 20 km sous le front de la haute chaîne. Le raccourcissement horizontal, induit par un fluage ductile le long de la partie la plus profonde du MHT, correspond à 19 ± 2.5 mm.an-1 dans le Centre-Est Népal. Au travers du Népal occidental, le modèle de la déformation et le taux de raccourcissement évalué sont de 13.4 ± 5 mm.an-1, taux moins contraint, par manque de stations cGPS. L'analyse des séries temporelles GPS a montré, outre le terme séculaire, des variations saisonnières importantes, notamment sur la composante horizontale perpendiculaire à la chaîne himalayenne. Nous avons pu mettre en évidence que ces variations saisonnières ne provenaient pas d'un artéfact de traitement, mais de la réponse flexurale de la croûte continentale à un chargement des aquifères situés dans la plaine du Gange. La réponse élastique de la lithosphère à cette charge saisonnière induit des variations de contrainte de Coulomb au niveau des l'essaim microsismique mi-crustal himalayen. Ces variations permettent d'expliquer au premier ordre les variations de taux de sismicité saisonnières mises en évidence à l'échelle du catalogue de sismicité népalais. L'étude microstructurale de quartzites nous a, par ailleurs, permis d'estimer les contraintes différentielles présentes autour de la transition fragile-ductile. A partir de la combinaison de données thermométriques, structurales, géodésiques et paléopiézométriques, une loi rhéologique expérimentale des quartzites du Moyen-Pays himalayen a pu être définie. La détermination de cette loi de fluage apporte une contrainte supplémentaire à la connaissance de la structure rhéologique de la lithosphère continentale himalayenne, un paramètre essentiel dans les modèles mécaniques du cycle sismique
Déformation intersismique de l' Himalaya du Népal à partir de données GPS
PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF
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https://galileodiscovery.unipd.it/discovery/fulldisplay?context=L&vid=39UPD_INST:VU1&search_scope=MyInst_and_CI&tab=Everything&docid=alma99001769288020604
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