Josephson current in a superconductor -- ferromagnet -- superconductor junction with in-plane ferromagnetic domains

We study a diffusive superconductor--ferromagnet--superconductor (SFS) junction with in-plane ferromagnetic domains. Close to the superconducting transition temperature, we describe the proximity effect in the junction with the linearized Usadel equations. We find that properties of such a junction depend on the size of the domains relative to the magnetic coherence length. In the case of large domains, the junction exhibits transitions to the $\pi$ state, similarly to a single-domain SFS junction. In the case of small domains, the magnetization effectively averages out, and the junction is always in the zero state, similarly to a superconductor--normal metal--superconductor (SNS) junction. In both those regimes, the influence of domain walls may be approximately described as an effective spin-flip scattering. We also study the inhomogeneous distribution of the local current density in the junction. Close to the 0--$\pi$ transitions, the directions of the critical current may be opposite in the vicinity of the domain wall and in the middle of the domains.Comment: 9 pages, 6 figure

How spin-flip scattering influences the minigap in a S-N junction

A superconductor in contact with a normal metal modifies the behaviour of the electrons in the normal part (”proximity effect”). The electrons in the adjacent normal metal will exhibit superconducting properties like: • apparition of an energy gap in the density of states • modification of the conductance (strongly dependent on energy) • screening of a magnetic field The scale at which these properties can be observed is a so called ”mesoscopic scale”, characterised by the presence of a sufficient number of particles to use statistical methods but still showing a non negligible phase coherence effect. In this work, we will study the influence of inelastic and spin-flip scattering on the electron density of states in diffusive S-N junctions

Nanotechnologie : conception à l'échelle atomique par Stéphane Redon. Dynamique moléculaire interactive, entretien avec Serge Crouzy, propos recueillis par Dominique Chouchan.

National audienceL'exploration et la manipulation de l'infiniment petit passeront sans doute par la conception assistée par ordinateur : le prototypage virtuel d'objets nanoscopiques concerne un grand nombre d'applications industrielles, pharmacologiques..

Effects of magnetism in superconducting hybrid structures

In this thesis we study various effects of magnetism in proximity structures, composed of superconducting electrodes in contact with a normal metal. Magnetism can be present in the system through the Zeeman and the orbital coupling. Proximity structures offer in particular a unique opportunity to study the interplay between ferromagnetism and conventional superconductivity, which can hardly coexist in bulk samples. The orbital effect of an external magnetic field applied to a Josephson junction results in interference effects between local currents. In Chapter 1, we give an introduction to the main features of the proximity effect and to the theoretical formalism used throughout the thesis. In Chapter 2 we study the Josephson effect in a superconductor–ferromagnet–superconductor (SFS) junction with ferromagnetic domains of noncollinear magnetization. It is well known [1] that as a consequence of the exchange splitting of the Fermi level [2] the Cooper pair wave function shows damped oscillations in a ferromagnet, leading to the appearance of the so-called "π state" in SFS junctions [3]. In the π state, the superconducting order parameter is of opposite sign in the two S electrodes of the Josephson junction, and a spontaneous non-dissipative current can appear in a ring containing such a junction. As a model for our study of the influence of magnetic domains on the π state formation, we consider a diffusive junction with two ferromagnetic domains along the junction. We find analytically the critical current as a function of domain lengths and of the angle between the orientations of their magnetizations. Varying those parameters, the junction may undergo transitions between 0 and π phases. We find that the presence of domains reduces the range of junction lengths at which the π phase is observed. For the junction with two domains of the same length, the π phase totally disappears as soon as the misorientation angle exceeds π/2. We further comment on possible implications of our results for experimentally observable 0–π transitions in SFS junctions. Experimentally, π junctions are realized as thin films deposited in layers. In Chapter 3, we study therefore the influence of in-plane magnetic domains on the Josephson current. We find that the properties of the junction depend on the size of the domains relative to the magnetic coherence length. In the case of large domains, the junction exhibits transitions to the π state, similarly to a single-domain SFS junction. In the case of small domains, the magnetization effectively averages out, and the junction is always in the zero state, similarly to a superconductor–normal metal–superconductor (SNS) junction. In both those regimes, the influence of domain walls may be approximately described as an effective spin-flip scattering. We also study the inhomogeneous distribution of the local current density in the junction. Close to the 0–π transitions, the directions of the critical current may be opposite in the vicinity of the domain wall and in the middle of the domains. In Chapter 4, we discuss the orbital effects of an external magnetic field in a SNS junction. In the limit of a long junction, we find that the properties of such a system depend on the width of the junction relative to the length associated with the magnetic field. We compute the critical width separating the regime of pure decay (narrow junction) and the regime of damped oscillations (wide junction) of the critical current as a function of the magnetic flux through the junction. We find an exponential damping of the current, different from the well known Fraunhofer limit which corresponds to the limit of a tunnel junction. In the limit of a wide junction, the superconducting pair correlations and the critical current become localized near the border of the junction

Cd2+- or Hg2+-binding proteins can replace the Cu+-chaperone Atx1 in delivering Cu+ to the secretory pathway in yeast

AbstractCopper delivery to Ccc2 – the Golgi Cu+-ATPase – was investigated in vivo, replacing the Cu+-chaperone Atx1 by various structural homologues in an atx1-Δ yeast strain. Various proteins, displaying the same ferredoxin-like fold and (M/L)(T/S)CXXC metal-binding motif as Atx1 and known as Cu+-, Cd2+- or Hg2+-binding proteins were able to replace Atx1. Therefore, regardless of their original function, these proteins could all bind copper and transfer it to Ccc2, suggesting that Ccc2 is opportunistic and can interact with many different proteins to gain Cu+. The possible role of electrostatic potential surfaces in the docking of Ccc2 with these Atx1-homologues is discussed

Josephson current in a superconductor-ferromagnet junction with two non-collinear magnetic domains

We study the Josephson effect in a superconductor--ferromagnet--superconductor (SFS) junction with ferromagnetic domains of non-collinear magnetization. As a model for our study we consider a diffusive junction with two ferromagnetic domains along the junction. The superconductor is assumed to be close to the critical temperature $T_c$, and the linearized Usadel equations predict a sinusoidal current-phase relation. We find analytically the critical current as a function of domain lengths and of the angle between the orientations of their magnetizations. As a function of those parameters, the junction may undergo transitions between 0 and $\pi$ phases. We find that the presence of domains reduces the range of junction lengths at which the $\pi$ phase is observed. For the junction with two domains of the same length, the $\pi$ phase totally disappears as soon as the misorientation angle exceeds $\pi/2$. We further comment on possible implication of our results for experimentally observable 0--$\pi$ transitions in SFS junctions.Comment: 9 pages, 4 figures, minor changes, references adde

Tyrosine metabolism: identification of a key residue in the acquisition of prephenate aminotransferase activity by 1β aspartate aminotransferase

International audienceAlternative routes for the post-chorismate branch of the biosynthetic pathway leading to tyrosine exist, the 4-hydroxyphenylpyruvate or the arogenate route. The arogenate route involves the transamination of prephenate into arogenate. In a previous study, we found that, depending on the microorganisms possessing the arogenate route, three different aminotransferases evolved to perform prephenate transamination, that is, 1β aspartate aminotransferase (1β AAT), N-succinyl-l,l-diaminopimelate aminotransferase, and branched-chain aminotransferase. The present work aimed at identifying molecular determinant(s) of 1β AAT prephenate aminotransferase (PAT) activity. To that purpose, we conducted X-ray crystal structure analysis of two PAT competent 1β AAT from Arabidopsis thaliana and Rhizobium meliloti and one PAT incompetent 1β AAT from R. meliloti. This structural analysis supported by site-directed mutagenesis, modeling, and molecular dynamics simulations allowed us to identify a molecular determinant of PAT activity in the flexible N-terminal loop of 1β AAT. Our data reveal that a Lys/Arg/Gln residue in position 12 in the sequence (numbering according to Thermus thermophilus 1β AAT), present only in PAT competent enzymes, could interact with the 4-hydroxyl group of the prephenate substrate, and thus may have a central role in the acquisition of PAT activity by 1β AAT

Real-time pollen monitoring using digital holography

We present the first validation of the SwisensPoleno, currently the only operational automatic pollen mon-itoring system based on digital holography. The device pro-vides in-flight images of all coarse aerosols, and here wedevelop a two-step classification algorithm that uses theseimages to identify a range of pollen taxa. Deterministiccriteria based on the shape of the particle are applied toinitially distinguish between intact pollen grains and othercoarse particulate matter. This first level of discriminationidentifies pollen with an accuracy of 96 %. Thereafter, in-dividual pollen taxa are recognized using supervised learn-ing techniques. The algorithm is trained using data obtainedby inserting known pollen types into the device, and out ofeight pollen taxa six can be identified with an accuracy ofabove 90 %. In addition to the ability to correctly identifyaerosols, an automatic pollen monitoring system needs to beable to correctly determine particle concentrations. To fur-ther verify the device, controlled chamber experiments us-ing polystyrene latex beads were performed. This providedreference aerosols with traceable particle size and numberconcentrations in order to ensure particle size and samplingvolume were correctly characterized

Magnetic interference patterns in long disordered Josephson junctions

We study a diffusive superconductor - normal metal - superconductor (SNS) junction in an external magnetic field. In the limit of a long junction, we find that the form of the dependence of the Josephson current on the field and on the length of the junction depends on the ratio between the junction width and the length associated with the magnetic field. A certain critical ratio between these two length scales separates two different regimes. In narrow junctions, the critical current exhibits a pure decay as a function of the junction length or of the magnetic field. In wide junctions, the critical current exhibits damped oscillations as a function of the same parameters. This damped oscillating behavior differs from the Fraunhofer pattern typical for short or tunnel junctions. In wide and long junctions, superconducting pair correlations and supercurrent are localized along the edges of the junction.Comment: 9 pages, 4 figures, minor modifications corresponding to the published versio