Anomalous density of states in a metallic film in proximity with a superconductor

We investigated the local electronic density of states in superconductor-normal metal (Nb-Au) bilayers using a very low temperature (60 mK) STM. High resolution tunneling spectra measured on the normal metal (Au) surface show a clear proximity effect with an energy gap of reduced amplitude compared to the bulk superconductor (Nb) gap. Within this mini-gap, the density of states does not reach zero and shows clear sub-gap features. We show that the experimental spectra cannot be described with the well-established Usadel equations from the quasi-classical theory.Comment: 4 pages, 5 figure

Strong-coupling perturbation theory for the two-dimensional Bose-Hubbard model in a magnetic field

The Bose-Hubbard model in an external magnetic field is investigated with strong-coupling perturbation theory. The lowest-order secular equation leads to the problem of a charged particle moving on a lattice in the presence of a magnetic field, which was first treated by Hofstadter. We present phase diagrams for the two-dimensional square and triangular lattices, showing a change in shape of the phase lobes away from the well-known power-law behavior in zero magnetic field. Some qualitative agreement with experimental work on Josephson-junction arrays is found for the insulating phase behavior at small fields.Comment: 7 pages, 5 figures include

Inter- and intragrain currents in bulk melt-grown YBaCuO rings

A simple contactless method suitable to discern between the intergrain (circular) current, which flows in the thin superconducting ring, and the intragrain current, which does not cross the weakest link, has been proposed. At first, we show that the intergrain current may directly be estimated from the magnetic flux density $B(\pm z_0)$ measured by the Hall sensor positioned in the special points $\pm z_0$ above/below the ring center. The experimental and the numerical techniques to determine the value $z_0$ are discussed. Being very promising for characterization of a current flowing across the joints in welded YBaCuO rings (its dependencies on the temperature and the external magnetic field as well as the time dissipation), the approach has been applied to study corresponding properties of the intra- and intergrain currents flowing across the $a$-twisted grain boundaries which are frequent in bulk melt-textured YBaCuO samples. We present experimental data related to the flux penetration inside a bore of MT YBaCuO rings both in the non-magnetized, virgin state and during the field reversal. The shielding properties and their dependence on external magnetic fields are also studied. Besides, we consider the flux creep effects and their influence on the current re-distribution during a dwell.Comment: 13 pages, 16 figures (EPS), RevTeX4. In the revised version, corrections to perturbing effects near the weak links are introduced, one more figure is added. lin

Quasiclassical description of transport through superconducting contacts

We present a theoretical study of transport properties through superconducting contacts based on a new formulation of boundary conditions that mimics interfaces for the quasiclassical theory of superconductivity. These boundary conditions are based on a description of an interface in terms of a simple Hamiltonian. We show how this Hamiltonian description is incorporated into quasiclassical theory via a T-matrix equation by integrating out irrelevant energy scales right at the onset. The resulting boundary conditions reproduce results obtained by conventional quasiclassical boundary conditions, or by boundary conditions based on the scattering approach. This formalism is well suited for the analysis of magnetically active interfaces as well as for calculating time-dependent properties such as the current-voltage characteristics or as current fluctuations in junctions with arbitrary transmission and bias voltage. This approach is illustrated with the calculation of Josephson currents through a variety of superconducting junctions ranging from conventional to d-wave superconductors, and to the analysis of supercurrent through a ferromagnetic nanoparticle. The calculation of the current-voltage characteristics and of noise is applied to the case of a contact between two d-wave superconductors. In particular, we discuss the use of shot noise for the measurement of charge transferred in a multiple Andreev reflection in d-wave superconductors

Évaluation de la contamination et des effets écotoxiques des microplastiques en estuaire de Seine ; les premiers enseignements du projet Plastic-Seine

International audienceLe bassin versant de la Seine peut être considéré comme représentatif des bassins hydrographiques fortement impactés par l'activité humaine et à ce titre présente une forte pression de pollution. La pollution chimique constitue une menace supplémentaire pour des écosystèmes aquatiques déjà trèsfragiles tels que les estuaires et les zones marines côtières. Peu d'études se sont concentrées sur la contamination et l'impact des microplastiques (MP, taille < 5 mm) dans les écosystèmes estuariens et aucune à ce jour en estuaire de Seine. Le projet Plastic-Seine (GIP Seine-Aval et CPIER Vallée de Seine) vise d’une part à quantifier la présence et le flux de MP dans l'estuaire de la Seine et d’autre part à évaluer leur devenir et leur impact sur différentes espèces du réseau trophique. Plusieurs campagnes de terrain ont été menées depuis mars 2017 sur le continuum eau douce-eau salée à différentes saisons pour échantillonner les eaux de surface, les sédiments et sept espèces du réseau trophique (poissons, crustacés, mollusques, annélides) représentatives de l'écosystème estuarienne de la Seine. Les premiers résultats indiquent une contamination modérée (1,8-5,1 fragments/m3) mais généralisée des différents compartiments abiotiques et biotiques par des MP comprenant principalement des fibres et des fragments. Des MP ont été identifiés dans toutes les espèces analysées, quel que soit le site ou la saison de prélèvement. Des expositions à des MP environnementaux en conditions contrôlées de laboratoire ont démontré l’ingestion mais aussi l’égestion très rapide de ces MP par les annélides et la sole commune (les analyses sont en cours sur les copépodes). Des premiers effets significatifs des MP sur la survie et la reproduction du copépode E. affinis et sur le comportement des soles ont été observés. Ce projet devrait fournir d’ici 2020 une première évaluation de la distributionet du flux de microplastiques dans l'estuaire de la Seine et de la faune aquatique associée