Étude des transitions de phases quantiques supraconducteur -- isolant, métal -- isolant dans des matériaux amorphes désordonnés proches de la dimension 2

The understanding of the effect of disorder on the superconductivity remains a major issue in solid state physics. This problem involves the competition between the disordered-induced localization that leads to an insulator and the formation of the Cooper pairs that leads to an infinite conductivity. These effects are dramatic in dimension 2, the lower dimension for the existence of either a metal or a superconductor. The system is often described by a Superconductor -- Insulator Transition (SIT) and is interpreted as a Quantum Phase Transitions, that occur at and induced by a parameter crossing a critical value. Among these parameters, for example intrinsic to the system, we can cite the electronic density of state, the microscopical disorder and the thickness. The NbSi alloy is of great interest for the study of the SIT. The material is homogeneous and amorphous down to 2.5nm and up to 250 of annealing. We observed a SIT induced by the composition, the annealing and the thickness, that we compared to the fermionic theories (Finkel'stein) relying on the weakening of the superconductivity by disorder, and the bosonic theories (Dirty Boson Model, Fisher), relying on the Cooper pairs localization. Theses experiments undermine the idea that a unique parameter such as the sheet resistance or the product of the Fermi wave vector and the electron mean free path can describe all disorder. They especially outline the specific effect of the thickness. Moreover, we study the metallic state that appears at the lowest temperature between the superconductive and the insulating states, that contradicts the dogma according to which no metal can exist at 2D. The phase diagram we obtain with these samples is well interpreted with the Bose Metalconcept of Das Doniach. The NbSi is thus a promising system for an advance study of this new metallic state.La compréhension du rôle du désordre sur la supraconductivité reste un problème fondamental de la physique du solide. Ce sujet illustre la compétition entre les phénomènes de localisation par le désordre qui conduisent à des isolants et la formation de paires de Cooper qui conduit à une conductivité infinie. Ces effets prennent un caractère spectaculaire en dimension 2, dimension limite pour l'existence de l'état métallique ou de l'état supraconducteur. Généralement le système décrit une Transition directe Supraconducteur -- Isolant (TSI) qui a les caractéristiques d'une Transition de Phase Quantique, transitions définies à T=0 et provoquées par le franchissement d'une valeur critique par le paramètre moteur de la transition. Parmi ces paramètres (intrinsèques au système), on peut citer la densité d'états électroniques, le désordre microscopique et l'épaisseur. L'alliage Nb(x)Si(1-x) est un matériau particulièrement intéressant pour l'étude de cette TSI. Le matériau est amorphe et homogène jusqu'à des épaisseurs de 2,5 nm et des températures de recuit de 250°C et nous observons une TSI induite par la composition, le recuit et l'épaisseur, que nous avons étudiée par rapport aux théories fermionique (Finkel'stein) d'affaiblissement de la supraconductivité par le désordre et bosonique (Dirty Boson Model de Fisher) s'interprétant par la localisation des paires de Cooper. Ces expériences remettent en cause la possibilité de réduire la mesure du " désordre " par un unique paramètre tel que la résistance carrée ou le produit k_F l du vecteur d'onde de Fermi par le libre parcours moyen électronique. En particulier, elles soulignent l'effet spécifique de l'épaisseur. Par ailleurs, pour certaines valeurs des paramètres, nous observons une phase " métallique " qui apparaît à très basse température entre les phases supraconductrices et isolantes, contredisant les théories de la non-existence d'un métal à 2D. Nous avons montré que le diagramme de phase associé à ces échantillons pouvait s'interpréter en introduisant le concept de " métal de Bose", prédit par Das & Doniach. Le Nb(x)Si(1-x) est donc un système prometteur pour l'étude plus approfondie de ce nouvel état métallique

Model-Based Assessment of the Role of Uneven Partitioning of Molecular Content on Heterogeneity and Regulation of Differentiation in CD8 T-Cell Immune Responses

Activation of naive CD8 T-cells can lead to the generation of multiple effector and memory subsets. Multiple parameters associated with activation conditions are involved in generating this diversity that is associated with heterogeneous molecular contents of activated cells. Although naive cell polarisation upon antigenic stimulation and the resulting asymmetric division are known to be a major source of heterogeneity and cell fate regulation, the consequences of stochastic uneven partitioning of molecular content upon subsequent divisions remain unclear yet. Here we aim at studying the impact of uneven partitioning on molecular-content heterogeneity and then on the immune response dynamics at the cellular level. To do so, we introduce a multiscale mathematical model of the CD8 T-cell immune response in the lymph node. In the model, cells are described as agents evolving and interacting in a 2D environment while a set of differential equations, embedded in each cell, models the regulation of intra and extracellular proteins involved in cell differentiation. Based on the analysis of in silico data at the single cell level, we show that immune response dynamics can be explained by the molecular-content heterogeneity generated by uneven partitioning at cell division. In particular, uneven partitioning acts as a regulator of cell differentiation and induces the emergence of two coexisting sub-populations of cells exhibiting antagonistic fates. We show that the degree of unevenness of molecular partitioning, along all cell divisions, affects the outcome of the immune response and can promote the generation of memory cells

Topological surface states of strained Mercury-Telluride probed by ARPES

The topological surface states of strained HgTe have been measured using high-resolution ARPES measurements. The dispersion of surface states form a Dirac cone, which origin is close to the top of the \ghh band: the top half of the Dirac cone is inside the stress-gap while the bottom half lies within the heavy hole bands and keeps a linear dispersion all the way to the X-point. The circular dichroism of the photo-emitted electron intensity has also been measured for all the bands.Comment: with supplementary materia

Tunable Superconducting Properties of a-NbSi Thin Films and Application to Detection in Astrophysics

We report on the superconducting properties of amorphous NbxSi1-x thin films. The normal-state resistance and critical temperatures can be separately adjusted to suit the desired application. Notably, the relatively low electron-phonon coupling of these films makes them good candidates for an "all electron bolometer" for Cosmological Microwave Background radiation detection. Moreover, this device can be made to suit both high and low impedance readouts

Investigating the role of the experimental protocol in phenylhydrazine-induced anemia on mice recovery

International audienceProduction of red blood cells involves growth-factor mediated regulation of erythroid progenitor apoptosis and self-renewal. During severe anemia, characterized by a strong fall of the hematocrit followed by a recovery phase, these controls allow a fast recovery of the hematocrit and survival of the organism. Using a mathematical model of stress erythropoiesis and an ad hoc numerical method, we investigate the respective roles of anemia-inducing phenylhydrazine injections and physiological regulation on the organism's recovery. By explicitly modeling the experimental protocol, we show that it mostly characterizes the fall of the hematocrit following the anemia and its severeness, while physiological process regulation mainly controls the recovery. We confront our model and our conclusions to similar experiments inducing anemia and show the model's ability to reproduce several protocols of phenylhydrazine-induced anemia. In particular, we establish a link between phenylhydrazine effect and the severeness of the anemia

IL-2 sensitivity and exogenous IL-2 concentration gradient tune the productive contact duration of CD8+ T cell-APC: a multiscale modeling study

International audienceAbstractBackgroundThe CD8+ T cell immune response fights acute infections by intracellular pathogens and, by generating an immune memory, enables immune responses against secondary infections. Activation of the CD8+ T cell immune response involves a succession of molecular events leading to modifications of CD8+ T cell population. To understand the endogenous and exogenous mechanisms controlling the activation of CD8+ T cells and to investigate the influence of early molecular events on the long-term cell population behavior, we developed a multiscale computational model. It integrates three levels of description: a Cellular Potts model describing the individual behavior of CD8+ T cells, a system of ordinary differential equations describing a decision-making molecular regulatory network at the intracellular level, and a partial differential equation describing the diffusion of IL-2 in the extracellular environment.ResultsWe first calibrated the model parameters based on in vivo data and showed the model’s ability to reproduce early dynamics of CD8+ T cells in murine lymph nodes after influenza infection, both at the cell population and intracellular levels. We then showed the model’s ability to reproduce the proliferative responses of CD5hi and CD5lo CD8+ T cells to exogenous IL-2 under a weak TCR stimulation. This stressed the role of short-lasting molecular events and the relevance of explicitly describing both intracellular and cellular scale dynamics. Our results suggest that the productive contact duration of CD8+ T cell-APC is influenced by the sensitivity of individual CD8+ T cells to the activation signal and by the IL-2 concentration in the extracellular environment.ConclusionsThe multiscale nature of our model allows the reproduction and explanation of some acquired characteristics and functions of CD8+ T cells, and of their responses to multiple stimulation conditions, that would not be accessible in a classical description of cell population dynamics that would not consider intracellular dynamics

Calibration, Selection and Identifiability Analysis of a Mathematical Model of the in vitro Erythropoiesis in Normal and Perturbed Contexts

The in vivo erythropoiesis, which is the generation of mature red blood cells in the bone marrowof whole organisms,has been described by a variety of mathematical models in the past decades. However, the in vitro erythropoiesis, whichproduces red blood cells in cultures, has received much less attention from the modelling community. In this paper, wepropose the first mathematical model of in vitro erythropoiesis. We start by formulating different models and select the bestone at fitting experimental data of in vitro erythropoietic differentiation obtained from chicken erythroid progenitor cells. Itis based on a set of linear ODE, describing 3 hypothetical populations of cells at different stages of differentiation. We thencompute confidence intervals for all of its parameters estimates, and conclude that our model is fully identifiable. Finally,we use this model to compute the effect of a chemical drug called Rapamycin, which affects all states of differentiation inthe culture, and relate these effects to specific parameter variations. We provide the first model for the kinetics of in vitrocellular differentiation which is proven to be identifiable. It will serve as a basis for a model which will better account for thevariability which is inherent to the experimental protocol used for the model calibration

A multi-scale model of erythropoiesis

International audienceIn this paper, a multi-scale mathematical model of erythropoiesis is proposed in which erythroid progenitors are supposed to be able to self-renew. Three cellular processes control erythropoiesis: self-renewal, differentiation and apoptosis.We describe these processes and regulatory networks that govern them. Two proteins (ERK and Fas) are considered as the basic proteins participating in this regulation. All erythroid progenitors are divided into several sub-populations depending on their maturity level. Feedback regulations by erythropoietin, glucocorticoids and Fas ligand (FasL) are introduced in the model. The model consists of a system of ordinary differential equations describing intracellular protein concentration evolution and cell population dynamics. We study steady states and their stability. We carry out computer simulations of an anaemia situation and analyse the results

Practical identifiability in the frame of nonlinear mixed effects models: the example of the in vitro erythropoiesis

International audienceBackgroundNonlinear mixed effects models provide a way to mathematically describe experimental data involving a lot of inter-individual heterogeneity. In order to assess their practical identifiability and estimate confidence intervals for their parameters, most mixed effects modelling programs use the Fisher Information Matrix. However, in complex nonlinear models, this approach can mask practical unidentifiabilities.ResultsHerein we rather propose a multistart approach, and use it to simplify our model by reducing the number of its parameters, in order to make it identifiable. Our model describes several cell populations involved in the in vitro differentiation of chicken erythroid progenitors grown in the same environment. Inter-individual variability observed in cell population counts is explained by variations of the differentiation and proliferation rates between replicates of the experiment. Alternatively, we test a model with varying initial condition.ConclusionsWe conclude by relating experimental variability to precise and identifiable variations between the replicates of the experiment of some model parameters