Generation of pure spin currents by superconducting proximity effect in quantum dots

We investigate electronic transport in a three-terminal hybrid system, composed by an interacting quantum dot tunnel coupled to one superconducting, one ferromagnetic, and one normal lead. Despite the tendency of the charging energy to suppress the superconducting proximity effect when the quantum dot is in equilibrium, the non-equilibrium proximity effect can give rise to a large Andreev current. The presence of the ferromagnet can lead to a finite spin accumulation on the dot. We find that the interplay of the Andreev current and spin accumulation can generate a pure spin current, with no associated charge transport, in the normal lead. This situation is realised by tuning the quantum-dot spectrum by means of a gate voltage.Comment: 6 pages, 5 figure

Transport through Hybrid Superconducting/Normal Nanostructures

We mainly investigate transport through interacting quantum dots proximized by superconductors. For this purpose we extend an existing theory to describe transport through proximized quantum dots coupled to normal and superconducting leads. It allows us to study the influence of a strong Coulomb interaction on Andreev currents and Josephson currents. This is a particularly interesting topic because it combines two competing properties: in superconductors Cooper pairs are formed by two electrons which experience an attractive interaction while two electrons located on a quantum dot repel each other due to the Coulomb interaction. It seems at first glance that transport processes involving Cooper pairs should be suppressed because of the two competing interactions. However, it is possible to proximize the dot in nonequilibrium situations. At first, we study a setup composed of a quantum dot coupled to one normal, one ferromagnetic, and one superconducting lead in the limit of an infinitely-large superconducting gap. Within this limit the coupling between dot and superconductor is described exactly by the presented theory. It leads to the formation of Andreev-bound states (ABS) and an additional bias scheme opens in which a pure spin current, i.e. a spin current with a vanishing associated charge current, can be generated. In a second work, starting from the infinite-gap limit, we perform a systematic expansion of the superconducting gap around infinity and investigate Andreev currents and Josephson currents. This allows us to estimate the validity of infinite-gap calculations for real systems in which the superconducting gap is usually a rather small quantity. We find indications that a finite gap renormalizes the ABS and propose a resummation approach to explore the finite-gap ABS. Despite the renormalization effects the modifications of transport by finite gaps are rather small. This result lets us conclude that the infinite-gap calculation is a valuable tool to study transport through proximized interacting quantum dots. Not only does a finite superconducting gap give rise to renormalization effects but also the coupling to the normal lead can evoke renormalizations. To explore these we calculate the correction terms arising from the coupling to the normal lead and identify renormalizations of the 0-pi~transitions of the Josephson current, the extrema of the Andreev current, and the average dot charge. In the previous works the occurring normal conducting leads are assumed to be metallic so that the shape of the band structure can be neglected. If the normal conducting region is a semiconductor, the shape of the band structure plays an important role. In our last calculation we consider a p-type semiconductor--superconductor interface and study oblique injections of light holes and heavy holes. Solving the Bogoliubov-de Gennes equations for a 6 by 6 Kane model we find that light holes and heavy holes can be Andreev reflected and in this process converted into each other. Moreover, in perpendicular incidence heavy holes cannot be Andreev reflected. Two types of critical angles occur. First, a critical angle above which conversion-less Andreev reflection is no longer possible and, second, a critical angle above which heavy holes cannot be converted into light holes anymore.Transport durch hybride Supraleiter/Normalleiter Nanostrukturen In dieser Arbeit untersuchen wir hauptsächlich den Transport durch wechselwirkende Quantenpunkte. Dafür erweitern wir eine bereits existierende Theorie, um mit dieser den elektrischen Transport durch proximisierte Quantenpunkte mit normalen- und supraleitenden Zuleitungen zu beschreiben. Dadurch sind wir in der Lage, den Einfluss einer starken Coulombwechselwirkung auf Josephson- und Andreevströme zu studieren. Dies ist eine besonders interessante Themenstellung, da hier zwei gegensätzliche Mechanismen aufeinandertreffen: In Supraleitern erfahren Elektronen paarweise eine anziehende Wechselwirkungskraft, wodurch sie sich zu Cooper-Paaren zusammenschließen, wohingegen sich zwei Elektronen auf dem Quantenpunkt durch die Coulomb-Kraft gegenseitig abstoßen. Das suggeriert, dass elektrische Transportprozesse, die Cooper-Paare involvieren, unterdrückt sein müssten. Allerdings ist es möglich, den supraleitenden Proximityeffekt in Nichtgleichgewichtssituationen auf dem Dot zu induzieren. Zunächst untersuchen wir ein System, das aus einem Quantenpunkt besteht, der an eine normale-, eine ferromagnetische- und eine supraleitende Zuleitung gekoppelt ist. Im Limes einer unendlich großen Energielücke des Supraleiters wird die Kopplung zum Supraleiter durch die vorgestellte Theorie exakt beschrieben. In diesem Limes formieren sich Andreev-gebundene Zustände (ABS), die es erlauben, einen Spinstrom, der durch keinen Ladungsstrom begleitet wird, zu generieren. Als nächstes führen wir, ausgehend vom Limes unendlich großer Energielücken, eine Störungsentwicklung des Gaps um unendlich durch. Anhand der Josephson- und Andreevströme überprüfen wir, wie gut die Vorhersagen der Rechnungen im Limes unendlich großer Energielücken für reale Systeme mit kleinen Energielücken sind. Wir finden Hinweise auf eine Renormierung der ABS und stellen einen Resummationsansatz vor, der es erlaubt, die ABS für eine endliche Energielücke abzuschätzen. Da abgesehen von der Renormierung die Änderungen der Ströme relativ klein sind, folgern wir, dass mit den Rechnungen im Limes unendlich großer Gaps auch für Systeme mit endlichen Gaps verlässliche Vorhersagen getroffen werden können. Neben den endlichen Gaps ist es auch möglich, Renormierungen durch die Kopplung des Quantenpunkts an eine normale Zuleitung herbeizuführen. Um diese zu untersuchen, berechnen wir die Korrekturen, die sich durch die Kopplung an die normale Zuleitung ergeben und identifizieren Renormierungen der 0-Pi-Übergänge des Josephsonstroms, der Extrema des Andreevstroms und der gemittelten Ladung auf dem Quantenpunkt. In unserer letzten Arbeit betrachten wir ein hybrides System, bestehend aus einem p-dotierten Halbleiter, der sich an einer Grenzfläche zu einem Supraleiter befindet. Dieses System untersuchen wir auf mögliche Andreevreflexionen von leichten- und schweren Löchern für beliebige Injektionswinkel. Dazu lösen wir die Bogoliubov-de Gennes-Gleichung für ein 6 mal 6-Kane-Modell und entdecken, dass sowohl leichte Löcher, wie auch schwere Löcher Andreev reflektiert werden und dabei auch ineinander umgewandelt werden können. Es stellt sich heraus, dass senkrecht injizierte schwere Löcher nicht Andreev reflektiert werden können, und dass es zweierlei kritische Winkel gibt. Einen kritischen Winkel für umwandlungsfreie Andreevreflexionen und einen kritischen Winkel, oberhalb dem schwere Löcher nicht mehr in leichte Löcher umgewandelt werden können

Non-local Andreev transport through an interacting quantum dot

We investigate sub-gap transport through a single-level quantum dot tunnel coupled to one superconducting and two normal-conducting leads. Despite the tendency of a large charging energy to suppress the equilibrium proximity effect, a finite Andreev current through the dot can be achieved in non-equilibrium situations. We propose two schemes to identify non-local Andreev transport. In one of them, the presence of strong Coulomb interaction leads to negative values of the non-local conductance as a clear signal of non-local Andreev transport.Comment: 5 pages, 4 figure

Probing the exchange field of a quantum-dot spin valve by a superconducting lead

Electrons in a quantum-dot spin valve, consisting of a single-level quantum dot coupled to two ferromagnetic leads with magnetizations pointing in arbitrary directions, experience an exchange field that is induced on the dot by the interplay of Coulomb interaction and quantum fluctuations. We show that a third, superconducting lead with large superconducting gap attached to the dot probes this exchange field very sensitively. In particular, we find striking signatures of the exchange field in the symmetric component of the supercurrent with respect to the bias voltage applied between the ferromagnets already for small values of the ferromagnets' spin polarization.Comment: published version, 10 pages, 7 figure

Prostate cancer, treatment modalities and complications: an evaluation of the scientific literature

Prostate (PR) cancer (CA) is one of the most common malignant neoplasms in men all over the world. In general, if prostate cancer (PC) is detected early, treatment usually involves either surgical removal of the prostate or radiotherapy (RT). Hormone Therapy (HT) or chemotherapy (CH) is the preferred treatment for more advanced cases of PC or if CA spreads beyond the PT. A number of complications, such as urinary incontinence (IU) or erectile dysfunction (ED), can be associated with some modalities of treatment of the PC. The aim of this work is to evaluate, in PubMed, the number of publications related with prostate cancer and the main modalities of treatment, as well as some clinical complications. The searches were performed in PubMed (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi) in the period 1950 to 2008 using the words: (i) CA, (ii) CA and PR or penis or testis, (iii) CA and PR and RT, CA and PR and surgery (SU), CA and PR and CH and, CA and PR and HT and (iv) CA and PR and RT and IU or ED, CA and PR and SU and IU or ED, CA and PR and CH and IU or ED and, CA and PR and HT and CH and IU or ED, and (V) PC and the same modalities of treatment. The data was obtained on July 20th, 2008. PC, as expected has been cited extensively and surgery has been identified as the most widely referenced modality of treatment. Furthermore, urinary incontinence and erectile dysfunction are important complications that have attracted significant scientific interest. In conclusion, these findings have shown the relevance of the PubMed to analyze quantitatively the publications in cancer and this information could be worthwhile in aiding the comprehension of some clinical aspects related with PC, as well as the development of preventative actions. The analysis of the scientific interest, considering the number of publications in the PubMed, reveals research trends in the field and demonstrates the importance of the surgical procedures in the treatment of the prostate cancer. Moreover, this finding is relevant due to the fact that surgery is the treatment of choice when early detection of PC is achieved. However, it is important to consider clinical complications related to such procedures, such as urinary incontinence and erectile dysfunctions that can reduce the quality of life of the patient

Tyrosine Kinase Syk Non-Enzymatic Inhibitors and Potential Anti-Allergic Drug-Like Compounds Discovered by Virtual and In Vitro Screening

In the past decade, the spleen tyrosine kinase (Syk) has shown a high potential for the discovery of new treatments for inflammatory and autoimmune disorders. Pharmacological inhibitors of Syk catalytic site bearing therapeutic potential have been developed, with however limited specificity towards Syk. To address this topic, we opted for the design of drug-like compounds that could impede the interaction of Syk with its cellular partners while maintaining an active kinase protein. To achieve this challenging task, we used the powerful potential of intracellular antibodies for the modulation of cellular functions in vivo, combined to structure-based in silico screening. In our previous studies, we reported the anti-allergic properties of the intracellular antibody G4G11. With the aim of finding functional mimics of G4G11, we developed an Antibody Displacement Assay and we isolated the drug-like compound C-13, with promising in vivo anti-allergic activity. The likely binding cavity of this compound is located at the close vicinity of G4G11 epitope, far away from the catalytic site of Syk. Here we report the virtual screen of a collection of 500,000 molecules against this new cavity, which led to the isolation of 1000 compounds subsequently evaluated for their in vitro inhibitory effects using the Antibody Displacement Assay. Eighty five compounds were selected and evaluated for their ability to inhibit the liberation of allergic mediators from mast cells. Among them, 10 compounds inhibited degranulation with IC50 values ≤10 µM. The most bioactive compounds combine biological activity, significant inhibition of antibody binding and strong affinity for Syk. Moreover, these molecules show a good potential for oral bioavailability and are not kinase catalytic site inhibitors. These bioactive compounds could be used as starting points for the development of new classes of non-enzymatic inhibitors of Syk and for drug discovery endeavour in the field of inflammation related disorders

Implementing nudges for suicide prevention in real-world environments: project INSPIRE study protocol

Background: Suicide is a global health issue. There are a number of evidence-based practices for suicide screening, assessment, and intervention that are not routinely deployed in usual care settings. The goal of this study is to develop and test implementation strategies to facilitate evidence-based suicide screening, assessment, and intervention in two settings where individuals at risk for suicide are especially likely to present: primary care and specialty mental health care. We will leverage methods from behavioral economics, which involves understanding the many factors that influence human decision making, to inform strategy development. Methods: We will identify key mechanisms that limit implementation of evidence-based suicide screening, assessment, and intervention practices in primary care and specialty mental health through contextual inquiry involving behavioral health and primary care clinicians. Second, we will use contextual inquiry results to systematically design a menu of behavioral economics-informed implementation strategies that cut across settings, in collaboration with an advisory board composed of key stakeholders (i.e., behavioral economists, clinicians, implementation scientists, and suicide prevention experts). Finally, we will conduct rapid-cycle trials to test and refine the menu of implementation strategies. Primary outcomes include clinician-reported feasibility and acceptability of the implementation strategies. Discussion: Findings will elucidate ways to address common and unique barriers to evidence-based suicide screening, assessment, and intervention practices in primary care and specialty mental health care. Results will yield refined, pragmatically tested strategies that can inform larger confirmatory trials to combat the growing public health crisis of suicide

Cloud droplet number closure for tropical convective clouds during the ACRIDICON CHUVA campaign

The main objective of the ACRIDICON-CHUVA campaign in September 2014 was the investigation of aerosol-cloud-interactions in the Amazon Basin. Cloud properties near cloud base of growing convective cumuli were characterized by cloud droplet size distribution measurements using a cloud combination probe and a cloud and aerosol spectrometer. In the current study, an adiabatic parcel model was used to perform cloud droplet number closure studies for several flights in differently polluted air masses