Signature of odd-frequency pairing correlations induced by a magnetic interface

We investigate the mutual proximity effect in a normal metal contacted to a superconductor through a magnetic interface. Analytical and self-consistent numerical results are presented, and we consider both the diffusive and ballistic regimes. We focus on the density of states in both the normal and superconducting region, and find that the presence of spin-dependent phase-shifts occurring at the interface qualitatively modifies the density of states. In particular, we find that the proximity-induced pairing amplitudes in the normal metal region undergo a conversion at the Fermi level from pure even-frequency to odd-frequency. Above a critical value of the interface spin-polarization (or, equivalently, for fixed interface spin-polarization, above a critical interface resistance), only odd frequency correlations remain. This is accompanied by the replacement of the familiar proximity minigap or pseudogap in the normal layer by an enhancement of the density of states above its normal state value for energies near the chemical potential. The robustness of this effect towards inelastic scattering, impurity scattering, and the depletion of the superconducting order parameter close to the interface is investigated. We also study the inverse proximity effect in the diffusive limit. We find that the above-mentioned conversion persists also for thin superconducting layers comparable in size to the superconducting coherence length $\xi_\text{S}$, as long as the inverse proximity effect is relatively weak. Concomitantly, we find a shift in the critical interface resistance where the pairing conversion occurs. Our findings suggest a robust and simple method for producing purely odd-frequency superconducting correlations, that can be tested experimentally.Comment: 14 pages, 12 figures. Submitted to Physical Review. Chosen as Editors' Suggestio

The Leaky Pipeline Revisited : Using Sociocultural Fit and Intersectionality to Analytically and Empirically Tackle Unequal Educational Transitions on the Example of Gender and the Subject Selection of French

The leaky pipeline metaphor is used to describe the dropout rate of certain populations from certain careers like women from academia or people of color from teaching jobs. While the phenomena are acknowledged, the term leaky pipeline has been criticized for lacking systematicity in describing social inequalities and for its inherent functionalism. Based on this criticism, this paper discusses ways to systematically account for a) the social context, via sociocultural fit; b) the leaking populations and their interwoven social belongings, via intersectionality; and c) the theoretical integration and empirical application of these concepts, via a sociology of knowledge approach. I demonstrate these procedures on an example from the field of institutional language learning, the de/selection decision concerning French, to be made at the transition to high school

Die Fallkonstruktion in der rekonstruktiven Fremdsprachenforschung. Forschungspraxis in der rekonstruktiven Fremdsprachenforschung, empirische Zugänge, methodologische Grundlagen und theoretische Konsequenzen

In dem Beitrag thematisieren wir die Rolle der Fallkonstruktion in der rekonstruktiven Fremdsprachenforschung, insbesondere mit der dokumentarischen Methode. Die Fallkonstruktion wird anhand der Diskussion fremdsprachendidaktischer Studien anderer und eigener empirischer Arbeiten als wenig expliziertes, zugleich aber zentrales Scharnier rekonstruktiver Forschung sichtbar, das auf doppelte Weise theoretisch geprägt ist: gegenstandstheoretisch durch die Fragestellung und grundlagentheoretisch durch die verwendete Methodologie. Es zeigen sich die drei Herausforderungen der Fallkonstellation, der Annahme von Veränderung des Falls über Zeit und der Abstraktionsweise. Besondere Bedeutung kommt daher im Rahmen dokumentarischer Forschung dem zentralen Begriff des Orientierungsrahmens zu, der sich bezüglich dieser Herausforderung als offen erweist, aber in seinen grundlegenden Verankerungen in Kollektivität und Konjunktivität als streng. (DIPF/Orig.)In this article we discuss case construction in reconstructive research in foreign language didactics, focussing on the documentary method. Looking at empirical studies in others’ and own projects, we see case construction rarely as explicit topic in spite of its important role, which is twice driven by theory: object theory influencing the research question and social theory leading the methodology. We identified three challenges of case construction, concerning the case constellation, the assumption of change over time and ways of abstraction. For documentary research the key concept is the frame of orientation as it is malleable concerning these challenges but strict concerning its grounding in collectivity and conjunctivity. (DIPF/Orig.

A Numerical Study of the Superconducting Proximity Effect in Topological Surface States

We study the superconducting proximity effect induced in the surface states of the 3-d topological insulator Bi$_2$Se$_3$ by a singlet, s-wave superconductor deposited on its surface. To this effect, the $\mathbf{k}\cdot\mathbf{p}$-Hamiltonian of Bi$_2$Se$_3$ and the BCS-Hamiltonian are mapped onto tight-binding chains which we couple through a transfer-Hamiltonian at the interface. We then employ the Recursive Green's Function technique to obtain the local spectral function and infer the dispersion of the interface-states from it. In agreement with earlier microscopic studies of this problem, we find that the Fu-Kane model is a reasonable approximation at energies $\epsilon\ll \Delta_{\rm SC}$. However, for energies close to the SC bulk gap, the Fu-Kane model is expected to break down. Indeed, our numerical calculations show strong modifications of the interface-state dispersion for $\epsilon \gtrsim \Delta_{\rm SC}$. We find that the proximity effect can be strong enough to induce a gap in the surface state that is comparable to the superconducting gap. An analysis of the spatial profile of the states shows that their weight shifts towards the SC as the coupling strength increases. We conclude that an intermediate coupling is ideal for realising the Fu-Kane scenario.Comment: JOP: Conference Series (Proceedings of SCES 2011), accepte

Theory of superconductor-ferromagnet point-contact spectra: The case of strong spin polarization

We study the impact of spin-active scattering on Andreev spectra of point contacts between superconductors (SC) and strongly spin-polarized ferromagnets (FM) using recently derived boundary conditions for the quasiclassical theory of superconductivity. We describe the interface region by a microscopic model for the interface scattering matrix. Our model includes both spin filtering and spin mixing and is nonperturbative in both transmission and spin polarization. We emphasize the importance of spin-mixing caused by interface scattering, which has been shown to be crucial for the creation of exotic pairing correlations in such structures. We provide estimates for the magnitude of this effect in different scenarios and discuss its dependence on various physical parameters. Our main finding is that the shape of the interface potential has a tremendous impact on the magnitude of the spin-mixing effect. Thus, all previous calculations, being based on delta-function or box-shaped interface potentials, underestimate this effect gravely. As a consequence, we find that with realistic interface potentials the spin-mixing effect can easily be large enough to cause spin-polarized subgap Andreev bound states in SC/FM point contacts. In addition, we show that our theory generalizes earlier models based on the Blonder-Tinkham-Klapwijk approach

Uneindeutige Differenzkonstruktionen und Fachlichkeit im alltäglichen Spanisch- und Französischunterricht sowie der fachdidaktischen Unterrichtsforschung

In dem Beitrag thematisieren die Verfasser*innen, dass sie in einer breit angelegten videographischen Studie zum alltäglichen Spanisch- und Französischunterricht systematische Differenzkonstruktionen nur entlang der Kategorie Leistung rekonstruieren können. Dafür nutzen sie die dokumentarische Methode und stellen die in diesem Rahmen entwickelte Heuristik der Erst- und Zweitcodierungen vor, bevor sie Differenzierungsprozesse der Forschenden und der Beforschten thematisieren. Im empirischen Teil des Beitrags diskutieren die Autor*innen verschiedene Weisen der Differenzkonstruktionen und fokussieren hier schwer zu systematisierende, uneindeutige und unerwartete Befunde. Auf dieser Grundlage thematisieren sie abschließend, wie es dazu kommen kann, dass nur Leistung sich als systematisches Differenzierungskriterium erwiesen hat und gehen dabei auf die Spezifika der Fächer und ihr gymnasiales Sample ein. (DIPF/Orig.

Inverse proximity effect and influence of disorder on triplet supercurrents in strongly spin-polarized ferromagnets

We discuss the Josephson effect in strongly spin-polarized ferromagnets where triplet correlations are induced by means of spin-active interface scattering, extending our earlier work [Phys. Rev. Lett. 102, 227005 (2009)] by including impurity scattering in the ferromagnetic bulk and the inverse proximity effect in a fully self-consistent way. Our quasiclassical approach accounts for the differences of Fermi momenta and Fermi velocities between the two spin bands of the ferromagnet, and thereby overcomes an important short-coming of previous work within the framework of Usadel theory. We show that non-magnetic disorder in conjunction with spin-dependent Fermi velocities may induce a reversal of the spin-current as a function of temperature.Comment: 12 pages, 9 figure

Risk Factors for Marburg Hemorrhagic Fever, Democratic Republic of the Congo

We conducted two antibody surveys to assess risk factors for Marburg hemorrhagic fever in an area of confirmed Marburg virus transmission in the Democratic Republic of the Congo. Questionnaires were administered and serum samples tested for Marburg-specific antibodies by enzyme-linked immunosorbent assay. Fifteen (2%) of 912 participants in a general village cross-sectional antibody survey were positive for Marburg immunoglobulin G antibody. Thirteen (87%) of these 15 were men who worked in the local gold mines. Working as a miner (odds ratio [OR] 13.9, 95% confidence interval [CI] 3.1 to 62.1) and receiving injections (OR 7.4, 95% CI 1.6 to 33.2) were associated with a positive antibody result. All 103 participants in a targeted antibody survey of healthcare workers were antibody negative. Primary transmission of Marburg virus to humans likely occurred via exposure to a still unidentified reservoir in the local mines. Secondary transmission appears to be less common with Marburg virus than with Ebola virus, the other known filovirus

Coulomb dissociation of O-16 into He-4 and C-12

We measured the Coulomb dissociation of O-16 into He-4 and C-12 within the FAIR Phase-0 program at GSI Helmholtzzentrum fur Schwerionenforschung Darmstadt, Germany. From this we will extract the photon dissociation cross section O-16(alpha,gamma)C-12, which is the time reversed reaction to C-12(alpha,gamma)O-16. With this indirect method, we aim to improve on the accuracy of the experimental data at lower energies than measured so far. The expected low cross section for the Coulomb dissociation reaction and close magnetic rigidity of beam and fragments demand a high precision measurement. Hence, new detector systems were built and radical changes to the (RB)-B-3 setup were necessary to cope with the high-intensity O-16 beam. All tracking detectors were designed to let the unreacted O-16 ions pass, while detecting the C-12 and He-4