Inversion de population

Résumé

vignette|Association Warungi. En physique, notamment en physique statistique, une inversion de population se produit lorsqu'un système (à trois niveaux d'énergie minimum) comme un groupe d'atomes ou molécules, se trouve dans un état dans lequel la majorité des éléments sont dans un état excité plutôt que dans leur état fondamental, i.e. : N_{2} > N_{1}. Le phénomène d’inversion de population est une étape nécessaire dans le fonctionnement d'un laser. Avoir une population plus élevée dans le niveau de plus haute énergie N_{2} que dans le niveau N_{1} n'est pas une situation d'équilibre. En effet, à l'équilibre thermodynamique, la répartition des populations sur les niveaux est donnée par la loi de Boltzmann : N_{2}=N_{1}\exp\left(-\frac{E_{2}-E_{1}}{k_{B}T}\right). Dans ce cas, N_{2} est toujours inférieur à N_{1}. Il faut donc créer une situation hors équilibre en apportant de l'énergie au

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https://fr.wikipedia.org/wiki/Inversion_de_population

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Emergence of Nontrivial Low-Energy Dirac Fermions in Antiferromagnetic EuCd2As2

Hang Li, Joël Mesot, Pierre Richard, Vladimir N. Strocov, Han Wang

Parity-time symmetry plays an essential role for the formation of Dirac states in Dirac semimetals. So far, all of the experimentally identified topologically nontrivial Dirac semimetals (DSMs) possess both parity and time reversal symmetry. The realization of magnetic topological DSMs remains a major issue in topological material research. Here, combining angle-resolved photoemission spectroscopy with density functional theory calculations, it is ascertained that band inversion induces a topologically nontrivial ground state in EuCd2As2. As a result, ideal magnetic Dirac fermions with simplest double cone structure near the Fermi level emerge in the antiferromagnetic (AFM) phase. The magnetic order breaks time reversal symmetry, but preserves inversion symmetry. The double degeneracy of the Dirac bands is protected by a combination of inversion, time-reversal, and an additional translation operation. Moreover, the calculations show that a deviation of the magnetic moments from the c-axis leads to the breaking of C3 rotation symmetry, and thus, a small bandgap opens at the Dirac point in the bulk. In this case, the system hosts a novel state containing three different types of topological insulator: axion insulator, AFM topological crystalline insulator (TCI), and higher order topological insulator. The results provide an enlarged platform for the quest of topological Dirac fermions in a magnetic system.

WILEY-V C H VERLAG GMBH2020

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Cellular memory enhances bacterial chemotactic navigation in rugged environments

Adám Gosztolai

The response of microbes to external signals is mediated by biochemical networks with intrinsic time scales. These time scales give rise to a memory that impacts cellular behaviour. Here we study theoretically the role of cellular memory in Escherichia coli chemotaxis. Using an agent-based model, we show that cells with memory navigating rugged chemoattractant landscapes can enhance their drift speed by extracting information from environmental correlations. Maximal advantage is achieved when the memory is comparable to the time scale of fluctuations as perceived during swimming. We derive an analytical approximation for the drift velocity in rugged landscapes that explains the enhanced velocity, and recovers standard Keller-Segel gradient-sensing results in the limits when memory and fluctuation time scales are well separated. Our numerics also show that cellular memory can induce bet-hedging at the population level resulting in long-lived, multi-modal distributions in heterogeneous landscapes. Microbes respond to biochemical signals from the environment, a process known as chemotaxis. Using mathematical models, the authors explore the role of cellular memory in cell motion and show that, when swimming across a rugged chemoattractant landscape, E. coli extract spatio-temporal information to improve their navigation.

NATURE PUBLISHING GROUP2020

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, ,

Organogenesis relies on the spatiotemporal balancing of differentiation and proliferation driven by an expanding pool of progenitor cells. In the mouse pancreas, lineage tracing at the population level has shown that the expanding pancreas progenitors can initially give rise to all endocrine, ductal, and acinar cells but become bipotent by embryonic day 13.5, giving rise to endocrine cells and ductal cells. However, the dynamics of individual progenitors balancing self-renewal and lineage-specific differentiation has never been described. Using three-dimensional live imaging and in vivo clonal analysis, we reveal the contribution of individual cells to the global behaviour and demonstrate three modes of progenitor divisions: symmetric renewing, symmetric endocrinogenic, and asymmetric generating a progenitor and an endocrine progenitor. Quantitative analysis shows that the endocrine differentiation process is consistent with a simple model of cell cycle-dependent stochastic priming of progenitors to endocrine fate. The findings provide insights to define control parameters to optimize the generation of beta-cells in vitro.

Public Library Science2015

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