Narrow-band noise due to the moving vortex lattice in superconducting niobium

We report measurements of voltage noise due to vortex motion in Niobium, a conventional low-Tc superconductor. A coherent oscillation leading to narrow band noise (NBN) is evidenced. Its characteristic frequency is a linear function of the overcritical transport current in the flux-flow regime, and hence scales as the main velocity of the vortex flow. The associated length scale is not the intervortex distance but the width of the sample, indicating temporal coherence at a large scale. NBN is also observed in the non linear part of the V(I) at the onset of depinning, in apparent disagreement with a stochastic creep motion of flux bundles. NBN exists in the peak effect region, showing that long range temporal correlations are preserved in this regime.Comment: accepted for publication in Phys Rev

Ground Plane and Near-Surface Thermal Analysis for NASA's Constellation Program

Most spacecraft thermal analysis tools assume that the spacecraft is in orbit around a planet and are designed to calculate solar and planetary fluxes, as well as radiation to space. On NASA Constellation projects, thermal analysts are also building models of vehicles in their pre-launch condition on the surface of a planet. This process entails making some modifications in the building and execution of a thermal model such that the radiation from the planet, both reflected albedo and infrared, is calculated correctly. Also important in the calculation of pre-launch vehicle temperatures are the natural environments at the vehicle site, including air and ground temperatures, sky radiative background temperature, solar flux, and optical properties of the ground around the vehicle. A group of Constellation projects have collaborated on developing a cohesive, integrated set of natural environments that accurately capture worst-case thermal scenarios for the pre-launch and launch phases of these vehicles. The paper will discuss the standardization of methods for local planet modeling across Constellation projects, as well as the collection and consolidation of natural environments for launch sites. Methods for Earth as well as lunar sites will be discussed

Why pinning by surface irregularities can explain the peak effect in transport properties and neutron diffraction results in NbSe2 and Bi-2212 crystals?

The existence of a peak effect in transport properties (a maximum of the critical current as function of magnetic field) is a well-known but still intriguing feature of type II superconductors such as NbSe2 and Bi-2212. Using a model of pinning by surface irregularities in anisotropic superconductors, we have developed a calculation of the critical current which allows estimating quantitatively the critical current in both the high critical current phase and in the low critical current phase. The only adjustable parameter of this model is the angle of the vortices at the surface. The agreement between the measurements and the model is really very impressive. In this framework, the anomalous dynamical properties close to the peak effect is due to co-existence of two different vortex states with different critical currents. Recent neutron diffraction data in NbSe2 crystals in presence of transport current support this point of view

Essential versus accessory aspects of cell death: recommendations of the NCCD 2015

Cells exposed to extreme physicochemical or mechanical stimuli die in an uncontrollable manner, as a result of their immediate structural breakdown. Such an unavoidable variant of cellular demise is generally referred to as ‘accidental cell death’ (ACD). In most settings, however, cell death is initiated by a genetically encoded apparatus, correlating with the fact that its course can be altered by pharmacologic or genetic interventions. ‘Regulated cell death’ (RCD) can occur as part of physiologic programs or can be activated once adaptive responses to perturbations of the extracellular or intracellular microenvironment fail. The biochemical phenomena that accompany RCD may be harnessed to classify it into a few subtypes, which often (but not always) exhibit stereotyped morphologic features. Nonetheless, efficiently inhibiting the processes that are commonly thought to cause RCD, such as the activation of executioner caspases in the course of apoptosis, does not exert true cytoprotective effects in the mammalian system, but simply alters the kinetics of cellular demise as it shifts its morphologic and biochemical correlates. Conversely, bona fide cytoprotection can be achieved by inhibiting the transduction of lethal signals in the early phases of the process, when adaptive responses are still operational. Thus, the mechanisms that truly execute RCD may be less understood, less inhibitable and perhaps more homogeneous than previously thought. Here, the Nomenclature Committee on Cell Death formulates a set of recommendations to help scientists and researchers to discriminate between essential and accessory aspects of cell death

Etude de la dynamique d'un réseau de vortex dans un supraconducteur par mesures de bruit.

This work aims at investigating the vortex lattice dynamics in a superconductor by means of noise measurements. This technique enable an accurate information on how the moving vortex lattice interacts with its pinning centers. A low-noise experimental set-up has be specially achieved for this purpose.First, the noise regime is studied in bulk Niobium foils under different conditions: the sample sizes have been carefully tuned, and on the other hand, surface have been roughened using low energy ion irradiation, and monitored by atomic force microscopy. Low-frequency noise, as well as the critical current, are found to be mainly due to a surface pinning mechanism. Furthermore, the fluctuation range turns out to be correlated with the surface roughness spatial distribution.In surface sheath superconductivity, with no vortex in the bulk, it is experimentally shown that pure surface current fluctuations yields the same noise regime as in the mixed state, with bulk vortices. This result is confirmed by complementary experiments in Pb-In alloy bulk samples, where the voltage fluctuations have been collected in different directions.The noise process statistics have been investigated in micro-bridges made on a Niobium thick film: both surface roughness and edges state turn out to affect the depinning noise regime, during which non-Gaussian and non-stationary effects appear, similarly to Lévy flights.Le propos de cette thèse est l'étude de la dynamique du réseau de vortex dans un supraconducteur. La technique expérimentale utilisée, la mesure de bruit, offre une analyse fine de la nature des interactions du réseau de vortex en mouvement avec ses centres d'ancrage. Un dispositif de mesures de bruits de faible puissance a spécialement été conçu pour la réalisation des expériences.En premier lieu, le bruit du système est observé dans des lames de Niobium dans différentes conditions : différentes tailles d'échantillon ont été ajustées de manière contrôlée, et des défauts de surface ont été introduits par irradiation ionique basse énergie, puis caractérisés par imagerie à force atomique. Il apparaît que le bruit basse fréquence, de même que le courant critique sont principalement dus à l'ancrage de surface. De plus, la portée des fluctuations se trouve être corrélée à la distribution spatiale de la rugosité.Le régime bruyant en supraconductivité de surface, sans vortex dans le volume, montre clairement que de pures fluctuations de courant de surface conduisent au même bruit qu'en présence de vortex dans le volume. Ce résultat est confirmé par des mesures complémentaires de fluctuations de tension dans plusieurs directions, réalisée dans des échantillons massifs d'un alliage Pb-In.La statistique du processus bruyant a été étudiée dans des micro-ponts sur couche mince de Niobium: les résultats indiquent que l'état de la surface, ainsi que celui des bords du système influent sur le régime bruyant du dépiégeage, au cours duquel des effets non-gaussiens et non-stationnaires, typiques d'un vol de Lévy, apparaissent

Etude de la dynamique d'un réseau de vortex dans un supraconducteur par mesures de bruit.

This work aims at investigating the vortex lattice dynamics in a superconductor by means of noise measurements. This technique enable an accurate information on how the moving vortex lattice interacts with its pinning centers. A low-noise experimental set-up has be specially achieved for this purpose.First, the noise regime is studied in bulk Niobium foils under different conditions: the sample sizes have been carefully tuned, and on the other hand, surface have been roughened using low energy ion irradiation, and monitored by atomic force microscopy. Low-frequency noise, as well as the critical current, are found to be mainly due to a surface pinning mechanism. Furthermore, the fluctuation range turns out to be correlated with the surface roughness spatial distribution.In surface sheath superconductivity, with no vortex in the bulk, it is experimentally shown that pure surface current fluctuations yields the same noise regime as in the mixed state, with bulk vortices. This result is confirmed by complementary experiments in Pb-In alloy bulk samples, where the voltage fluctuations have been collected in different directions.The noise process statistics have been investigated in micro-bridges made on a Niobium thick film: both surface roughness and edges state turn out to affect the depinning noise regime, during which non-Gaussian and non-stationary effects appear, similarly to Lévy flights.Le propos de cette thèse est l'étude de la dynamique du réseau de vortex dans un supraconducteur. La technique expérimentale utilisée, la mesure de bruit, offre une analyse fine de la nature des interactions du réseau de vortex en mouvement avec ses centres d'ancrage. Un dispositif de mesures de bruits de faible puissance a spécialement été conçu pour la réalisation des expériences.En premier lieu, le bruit du système est observé dans des lames de Niobium dans différentes conditions : différentes tailles d'échantillon ont été ajustées de manière contrôlée, et des défauts de surface ont été introduits par irradiation ionique basse énergie, puis caractérisés par imagerie à force atomique. Il apparaît que le bruit basse fréquence, de même que le courant critique sont principalement dus à l'ancrage de surface. De plus, la portée des fluctuations se trouve être corrélée à la distribution spatiale de la rugosité.Le régime bruyant en supraconductivité de surface, sans vortex dans le volume, montre clairement que de pures fluctuations de courant de surface conduisent au même bruit qu'en présence de vortex dans le volume. Ce résultat est confirmé par des mesures complémentaires de fluctuations de tension dans plusieurs directions, réalisée dans des échantillons massifs d'un alliage Pb-In.La statistique du processus bruyant a été étudiée dans des micro-ponts sur couche mince de Niobium: les résultats indiquent que l'état de la surface, ainsi que celui des bords du système influent sur le régime bruyant du dépiégeage, au cours duquel des effets non-gaussiens et non-stationnaires, typiques d'un vol de Lévy, apparaissent

Etude de la dynamique d'un réseau de vortex dans un supraconducteur par mesures de bruit

CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

Pulsed laser deposition and optical characterizations of the magnetic samarium orthoferrite

International audienc