Résumé :
Dans cet article, il est question d’évaluer la déforestation dans la périphérie Ouest de la réserve de Biosphère du Dja à travers les techniques de Télédétection et de Système d’Information ...Géographique. Pour cela, 08 images Landsat de date différentes (2011 à 2018) ont été utilisées pour produire les cartes d’occupation du sol, à travers la méthode de classification supervisée et l’algorithme « maximum likelihood ». Les classes d’occupation de sol retenues pour cette classification sont : forêt dense, forêt dégradée, zone de culture, zone marécageuse, zone d’habitation, sol nu et eau. L’analyse des changements a été faite avec la technique de « change detection ». Les résultats de cette étude ont montré que la déforestation a été importante pendant la période d’étude (2011 – 2018). Les surfaces forestières se sont principalement transformées en zone de culture, marécage, forêt dégradée, sol nu. Le taux de déforestation observé est de 6,8% et dénote une importante baisse du couvert forestier dense. L’étude a montré des tendances de déforestation dans cette périphérie. Elle a permis d’observer que les zones tendancieuses sont concentrées principalement autour de certaines activités anthropiques présentes dans cette zone (la plantation agricole SUDCAM, le barrage de Mekin, les lieux habités).
Mots-clés : Déforestation, Changement climatique, forêt, tendances de déforestation, images satellites
One of the greatest challenges in modern physics is to understand the behaviour of an ensemble of strongly interacting particles. A class of quantum many-body systems (such as neutron star matter and ...cold Fermi gases) share the same universal thermodynamic properties when interactions reach the maximum effective value allowed by quantum mechanics, the so-called unitary limit. This makes it possible in principle to simulate some astrophysical phenomena inside the highly controlled environment of an atomic physics laboratory. Previous work on the thermodynamics of a two-component Fermi gas led to thermodynamic quantities averaged over the trap, making comparisons with many-body theories developed for uniform gases difficult. Here we develop a general experimental method that yields the equation of state of a uniform gas, as well as enabling a detailed comparison with existing theories. The precision of our equation of state leads to new physical insights into the unitary gas. For the unpolarized gas, we show that the low-temperature thermodynamics of the strongly interacting normal phase is well described by Fermi liquid theory, and we localize the superfluid transition. For a spin-polarized system, our equation of state at zero temperature has a 2 per cent accuracy and extends work on the phase diagram to a new regime of precision. We show in particular that, despite strong interactions, the normal phase behaves as a mixture of two ideal gases: a Fermi gas of bare majority atoms and a non-interacting gas of dressed quasi-particles, the fermionic polarons.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A mixture of Bose and Fermi superfluids Ferrier-Barbut, I.; Delehaye, M.; Laurent, S. ...
Science (American Association for the Advancement of Science),
08/2014, Letnik:
345, Številka:
6200
Journal Article
Recenzirano
Superconductivity and superfluidity of fermionic and bosonic systems are remarkable many-body quantum phenomena. In liquid helium and dilute gases, Bose and Fermi superfluidity has been observed ...separately, but producing a mixture in which both the fermionic and the bosonic components are superfluid is challenging. Here we report on the observation of such a mixture with dilute gases of two lithium isotopes, lithium-6 and lithium-7. We probe the collective dynamics of this system by exciting center-of-mass oscillations that exhibit extremely low damping below a certain critical velocity. Using high-precision spectroscopy of these modes, we observe coherent energy exchange and measure the coupling between the two superfluids. Our observations can be captured theoretically using a sum-rule approach that we interpret in terms of two coupled oscillators.
Interacting fermions are ubiquitous in nature, and understanding their thermodynamics is an important problem. We measured the equation of state of a two-component ultracold Fermi gas for a wide ...range of interaction strengths at low temperature. A detailed comparison with theories including Monte-Carlo calculations and the Lee-Huang-Yang corrections for low-density bosonic and fermionic superfluids is presented. The low-temperature phase diagram of the spin-imbalanced gas reveals Fermi liquid behavior of the partially polarized normal phase for all but the weakest interactions. Our results provide a benchmark for many-body theories and are relevant to other fermionic systems such as the crust of neutron stars.
We investigate the low-lying compression modes of a unitary Fermi gas with imbalanced spin populations. For low polarization, the strong coupling between the two spin components leads to a ...hydrodynamic behavior of the cloud. For large population imbalance we observe a decoupling of the oscillations of the two spin components, giving access to the effective mass of the Fermi polaron, a quasiparticle composed of an impurity dressed by particle-hole pair excitations in a surrounding Fermi sea. We find m*/m = 1.17(10), in agreement with the most recent theoretical predictions.
The human placenta is a unique organ that performs the function of the majority of fetal organs across gestation. How the placenta communicates with maternal tissues to prepare them for pregnancy is ...not fully understood. Recently, it has been established that placental cells can communicate with maternal tissues to regulate their biological function via extracellular vesicles (EVs). EVs are subclassified into exosomes or microvesicles (MVs) according to their size, cell or tissue of origin, functions, and physical features. Exosomes are a specific type of EVs from an endocytic origin, while MVs are released via budding from the plasma membrane. With regards to pregnancy, the role of EVs has been described in several functions such as immune responses and maternal metabolic adaptation to gestation. Interestingly, EVs of placental origin can be detected in a variety of body fluids including urine and blood, and have been identified in the maternal circulation at as early as 6 weeks of gestation. Moreover, the number of exosomes across gestation is higher in complications of pregnancies such as preeclampsia and gestational diabetes mellitus compared to normal pregnancies. Circulating exosomes contains proteins and RNAs that are representative of the cell of origin, including surface and cytoplasmic protein, messenger RNA, and micro-RNAs. Finally, exosomes are capable of transferring their contents to other cells and regulating the biological function of the target cell. In this review, we will discuss the effect of the maternal microenvironment on secretion and content of placenta-derived EVs, and how this may lead to complications of pregnancies with a special emphasis on exosomes.
Quantum statistics and symmetrization dictate that identical fermions do not interact via s-wave collisions. However, in the presence of spin–orbit coupling (SOC), fermions prepared in identical ...internal states with distinct momenta become distinguishable. The resulting strongly interacting system can exhibit exotic topological and pairing behaviours, many of which are yet to be observed in condensed matter systems. Ultracold atomic gases offer a promising pathway for simulating these rich phenomena, but until recently have been hindered by heating and losses. Here we enter a new regime of many-body interacting SOC in a fermionic optical lattice clock (OLC), where the long-lived electronic clock states mitigate unwanted dissipation. Using clock spectroscopy, we observe the precession of the collective magnetization and the emergence of spin-locking effects arising from an interplay between p-wave and SOC-induced exchange interactions. The many-body dynamics are well captured by a collective XXZ spin model, which describes a broad class of condensed matter systems ranging from superconductors to quantum magnets. Furthermore, our work will aid in the design of next-generation OLCs by offering a route for avoiding the observed large density shifts caused by SOC-induced exchange interactions.
We measure the magnetic susceptibility of a Fermi gas with tunable interactions in the low-temperature limit and compare it to quantum Monte Carlo calculations. Experiment and theory are in excellent ...agreement and fully compatible with the Landau theory of Fermi liquids. We show that these measurements shed new light on the nature of the excitations of the normal phase of a strongly interacting Fermi gas.
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