1,721,213 research outputs found
Proximity effects in superconducting hybrid structures with spin-dependent interactions
No full textWhen superconductors are contacted by other, non-superconducting materials, the latter may attain some of the properties of the former, and vice versa. This is the origin of a wealth of interesting phenomena. A striking example is the coexistence of superconductivity and ferromagnetism in superconductor-ferromagnet hybrid structures, which under certain conditions gives rise to dissipationless spin transport.
In this work a variety of superconducting hybrid structures are investigated theoretically using the quasiclassical Green function formalism. Focus is given to two- and three-dimensional geometries, the study of which has been made possible with the use of a numerical framework known as the finite element method.
Among several topics discussed are superconducting vortices nucleated in non-superconducting materials without the application of external magnetic fields. The generation of long ranged triplet superconducting correlations in superconductor-ferromagnet structures is also explored, either as generated by an inhomogeneity of the magnetisation, or by spin-orbit coupling
Manipulating superconductivity in magnetic nanostructures in and out of equilibrium
Full text linkWe consider nanostructures that are constructed from superconducting, ferromagnetic, and spin–orbit-coupled materials. These structures are analyzed both theoretically and numerically, under both equilibrium and nonequilibrium conditions. Special emphasis is placed on how one can exert control over the superconducting properties of these systems. For instance, this includes developing new ways to toggle superconductivity on and off via electric or magnetic input signals, and new ways to shape the charge and spin supercurrents flowing through these systems. The thesis itself provides an introduction to how we performed our calculations, as well as a summary of some interesting research results. The main body of research consists of 15 enclosed publications, where we go into more detail on each specific project
RKKY interaction and coexistence with magnetism in superconducting systems
Full text linkWhen cooled down, the electrical resistivity of certain materials suddenly drops to zero at some specific temperature. This phenomenon is called superconductivity. As we are living in a world steadily demanding more energy, being able to prevent energy loss by transferring electrical currents through superconducting wires would be a huge advantage. Unfortunately, superconductivity normally sets in at quite low temperatures. Applications are therefore limited to cases where the benefit of superconductivity outweighs the cost of cooling down the material. Still, superconductors are, for example, used to make the strong magnetic fields that are needed in MRI devices, helping patients in hospitals.
Superconductors can also be useful for spin-based electronics where information is stored and transported using the electron spin, rather than the electron charge. Spin-based electronics seeks to make electronic devices smaller and faster, while wasting less energy than normal charge-based electronics. Combining superconductivity and spin-based electronics does, however, require combining superconductivity with magnetism. Although interesting phenomena can arise when superconductivity and magnetism meet, this can also be problematic as superconductors and magnets are natural enemies: Superconductors normally expel any magnetic fields applied to them and lose their superconducting properties when the applied field becomes too large.
This thesis provides an introduction to four research papers exploring interplay between superconductivity and magnetism. The magnetism is introduced through application of magnetic fields to superconductors or by placing superconductors next to magnetic materials. The articles investigate both interesting phenomena that can arise in such system, with possible applications within spin-based electronics, and how superconductors can be designed to withstand larger magnetic fields
Proximity effects, spin transport, and phase transitions in superconducting hybrids
Full text linkThis thesis presents our work on the theory of spin-dependent effects and transport in heterostructures involving superconductors, magnets, and non-centrosymmetric materials. The results are roughly divided into those concerning i) the superconducting phase transition and its influence on adjacent materials, and ii) spin transport. Our study of the superconducting phase transition is focused around spin-valve effects where the superconducting critical temperature can be controlled by the magnetic configuration or inversion symmetry breaking in materials in proximity to the superconductor. We also consider the reciprocal effect where decreasing the temperature below the superconducting transition temperature alters the magnetic anisotropy of an adjacent magnet. Furthermore, we consider how the superconducting critical temperature of an unconventional p-wave superconductor can be enhanced by proximity-coupling it via a ferromagnetic interlayer to a conventional superconductor with a higher critical temperature. Finally, we study the magnetic field-driven superconducting transition in a highly disordered hole-overdoped d-wave cuprate superconductor where superconducting pairing remains above the critical field where phase coherence is lost. Our work on spin transport includes the study of both dissipationless Cooper pair and resistive quasi-particle transport. We predict that due to interfacial exchange coupling, a supercurrent of spin-polarized Cooper pairs can induce a non-reciprocity in the magnon dispersion of a ferromagnetic insulator giving rise to magnon spin currents. We also study how non-equilibrium quasi-particle transport is affected by the pairing symmetry of the superconductor, how the inverse spin-Hall effect and spin-swapping are renormalized by a spin-splitting field, and how the coupling between two ferromagnetic insulators can be mediated by a superconductor thus affecting spin-pumping
A study of proximity-induced and magnon-mediated superconductivity on the surface of topological insulators
Full text linkSummary:
This thesis presents a study of superconducting phases in different effectively two-dimensional (2D) systems with spin-orbit coupling, with particular emphasis on systems consisting topological insulators (TIs) in proximity to superconductors (S) and magnetic insulators. The research has led to five papers.
The first paper examines the possible superconducting phases in a 2D repulsive Hubbard model with Zeeman splitting and Rashba spin-orbit coupling, showing that the Kohn-Luttinger mechanism is responsible for the effective attractive pairing. The spin-orbit coupling, however, indirectly affects the symmetry of the order parameter, leading to a chiral p ± ip or p state depending on the orientation of the Zeeman field.
Two papers consider the proximity effect between a superconductor and topological insulator, and the interplay with exchange fields. When a spin valve is placed on top of a TI Josephson junction, we find that vortices can be induced on the surface of the TI depending on the spin valve configuration. We also study the possibility of a strong inverse proximity effect — a significant reduction in the superconducting gap — in an S-TI bilayer, finding that this is unlikely for a conventional s-wave superconductor, but might occur in unconventional superconductors with low Fermi energies.
The final two papers examine superconductivity mediated by magnons on the surface of a TI coupled to a magnetic insulator. When neglecting the frequency dependence of the magnons we find that, depending on the coupling between the magnons, both BCS type and Amperean p-wave pairing is possible. Including the magnon frequency dependence, we also find the possibility of odd-frequency s-wave Amperean pairing.Sammendrag — Superledning i topologiske isolatorer
En superleder er et materiale som kan lede strøm uten motstand, og som derfor ikke utvikler varme. Med tanke på hvor mye energitap som skyldes motstanden i ledninger og elektriske komponenter, høres dette helt fantastisk ut. Problemet er at de fleste superledere må kjøles ned til minst minus 140 grader Celsius, noe som naturligvis setter begrensninger for praktisk bruk.
De siste årene har det blitt forsket på om superledere kan brukes i store datasentre. Datasentrene bruker nemlig allerede enorme mengder energi på kjøling, og det kan derfor faktisk lønne seg å bruke superledende komponenter. Spørsmålet blir da hvordan man kan sende og lagre informasjon ved hjelp av superledere, og hvilke andre materialer som skal inngå i slike systemer.
Et foreslått materiale er topologiske isolatorer. Disse materialene leder strøm kun på overflatene – det indre av materialet er isolerende og leder dermed ikke strøm. De metalliske tilstandene på overflaten av topologiske isolatorer er også robuste mot urenheter, noe som kan gjøre dem godt egnet til teknologiske anvendelser. Ved å plassere en superleder inntil en topologiske isolator smitter noen av de superledende egenskapene over på den topologiske isolatoren, og teoretiske beregninger har vist at dette kan gi en helt spesiell type partikler som kan brukes i kvantedatamaskiner.
I mitt doktorgradsarbeid har jeg gjort teoretiske undersøkelser av ulike typer superledning på overflaten av topologiske isolatorer. Superledningen kan enten arves fra en nærliggende superleder, eller oppstå grunnet vekselvirkninger med en magnet som gjør selve den topologiske isolatoren superledende.
Håpet er at stadig ny kunnskap om superledning etterhvert skal føre til teknologiske fremskritt som både bidrar til å redusere energibruk, og å lage mer effektive datamaskiner
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Critical temperature of triplet superconductor-ferromagnet bilayers as a probe for pairing symmetry
No full textIdentifying superconducting materials with spin-polarized Cooper pairs is an important objective both for exploration of new fundamental physics and for cryogenic applications in spintronics and quantum sensing. We here compute the critical temperature T(c )of the superconducting transition in a bilayer composed of a superconductor with an intrinsic spin-triplet order parameter and a ferromagnet. We determine how T-c varies both with the thickness of the ferromagnet and its magnetization direction. We show that both the orbital and spin part of the triplet superconducting order parameter leave clear signatures in T-c which do not appear in a bilayer of a conventional s-wave superconductor and a ferromagnet. In particular, the dependence of T-c on these variables changes depending on whether or not the superconducting order parameter features Andreev bound states and also changes qualitatively when the magnetization is rotated in the plane of the ferromagnetic film. We also show that a single-component triplet order parameter, such as py-wave, can have a strong enhancement near a boundary due to the Friedel oscillations causing an energy-dependent modulation of the local density of states. Measurements of T-c in such bilayers are therefore useful to identify the pairing symmetry of intrinsic triplet superconductors
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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