Proximity Effect, Andreev Reflections, and Charge Transport in Mesoscopic Superconducting-Semiconducting Heterostructures

In the quasi-twodimensional (Q2D) electron gas of an InAs channel between an AlSb substrate and superconducting Niobium layers the proximity effect induces a pair potential so that a Q2D mesoscopic superconducting-normal-superconducting (SNS) junction forms in the channel. The pair potential is calculated with quasiclassical Green's functions in the clean limit. For such a junction alternating Josephson currents and current-voltage characteristics (CVCs) are computed, using the non-equilibrium quasiparticle wavefunctions which solve the time-dependent Bogoliubov-de Gennes Equations. The CVCs exhibit features found experimentally by the Kroemer group: A steep rise of the current at small voltages ("foot") changes at a "corner current" to a much slower increase of current with higher voltages, and the zero-bias differential resistance increases with temperature. Phase-coherent multiple Andreev reflections and the associated Cooper pair transfers are the physical mechanisms responsible for the oscillating Josephson currents and the CVCs. Additional experimental findings not reproduced by the theory require model improvements, especially a consideration of the external current leads which should give rise to hybrid quasiparticle/collective mode excitations.Comment: 8 pages, 4 figures (consisting of 5 .ps-files), added referenc

Erwerb der L2-Artikulation im Erwachsenenalter: einige offene Fragen.

Es ist ein bekanntes Phänomen, dass sich Erwachsene im Gegensatz zu Kindern mit dem akzentfreien Lernen einer Fremdsprache schwer tun. In manchen Ansätzen zum Fremdsprachenlernen gilt dies sogar als unmöglich. Einzeluntersuchungen der letzten Jahrzehnte haben jedoch immer wieder Fälle gezeigt, in denen späte Lerner ein Ausspracheniveau erreichten, das von Muttersprachlern als authentisch eingestuft wurde. Dieser Beitrag formuliert die Frage, wie grundsätzlich ausgeschlossen das akzentfreie Lernen einer Fremdsprache im Erwachsenenalter wirklich ist und nennt einige Faktoren, die in Forschung und Unterrichtspraxis näher berücksichtigt werden müssen.It is a well-known phenomenon that adult learners face greater difficulties in acquiring an accent-free command of a foreign language than children. Some approaches even claim that this is impossible. However, occasional researches within the last decades have shown that there are cases in which late learners achieve a level in pronunciation that is judged as authentic by native speakers. This contribution focuses on the question, how fundamental the restrictedness of adult learners in achieving a native accent of the foreign language really is and specifies some of the factors which ought to receive further consideration in both research and teaching

Mesoscopic superconductors under irradiation: Microwave spectroscopy of Andreev states

We show that irradiation of a voltage-biased superconducting quantum point contact at frequencies of the order of the gap energy can remove the suppression of subgap dc transport through Andreev levels. Quantum interference among resonant scattering events involving photon absorption is furthermore shown to make microwave spectroscopy of the Andreev levels feasible. We also discuss how the same interference effect can be applied for detecting weak electromagnetic signals up to the gap frequency, and how it is affected by dephasing and relaxation.Comment: Submitted to a special volume of "Superlattices and Microstructures" on Mesoscopic Superconductivit

Dynamics of conversion of supercurrents into normal currents, and vice versa

The generation and destruction of the supercurrent in a superconductor (S) between two resistive normal (N) current leads connected to a current source is computed from the source equation for the supercurrent density. This equation relates the gradient of the pair potential's phase to electron and hole wavepackets that create and destroy Cooper pairs in the N/S interfaces. Total Andreev reflection and supercurrent transmission of electrons and holes are coupled together by the phase rigidity of the non-bosonic Cooper-pair condensate. The calculations are illustrated by snapshots from a computer film.Comment: 8 pages, 1 figure, accepted by Phys. Rev.

Barriers to using HIV pre-exposure prophylaxis (PrEP) and sexual behaviour after stopping PrEP: a cross-sectional study in Germany

Background Persistence of individuals at risk of HIV with Pre-Exposure Prophylaxis (PrEP) is critical for its impact on the HIV epidemic. We analysed factors associated with stopping PrEP, barriers that may deter people from continuing PrEP and investigated sexual behaviour after stopping PrEP. Methods Current and former PrEP users in Germany were recruited to complete an anonymous online survey on PrEP use and sexual behaviour. Participants were recruited through dating apps, a PrEP community website, anonymous testing sites and peers. The results were analysed using descriptive methods and logistic regression. Results We recruited 4848 current and 609 former PrEP users in two study waves (July–October 2018, April–June 2019). Former PrEP users were more likely 18–29 years old than current users (adjusted OR = 1.6, 95% confidence interval (CI) 1.1–2.3). Moreover, they were more often unhappy with their sex life, which was more pronounced in former daily PrEP users (aOR = 4.5, 95% CI 2.9–7.1) compared to former on-demand users (aOR = 1.8, 95% CI 1.1–2.9, pinteraction = 0.005). The most common reason for stopping PrEP was a reduced need for PrEP (49.1%). However, 31.4% of former users identified logistic reasons and 17.5% stopped due to side effects. Former PrEP users using PrEP < 3 months were more likely to stop PrEP due to concerns over long-term side effects (32.0% vs. 22.5%, p = 0.015) and not wanting to take a chemical substance (33.2% vs. 24.0%, p = 0.020) compared to former PrEP users who used PrEP for longer. After stopping PrEP, 18.7% of former PrEP users indicated inconsistent condom use while having ≥4 sex partners within the previous 6 months. Former PrEP users with many partners and inconsistent condom use more often indicated logistic reasons for stopping (46.5% vs. 27.9%, p < 0.001) than did other former PrEP users. Conclusions To maximise persistence with PrEP we need to develop strategies for younger PrEP users, reduce logistic barriers to access PrEP, and to develop effective communication on side-effect management. Moreover, prevention strategies for people stopping PrEP are required, since some remain at high risk for HIV.Peer Reviewe

HIV, STI and renal function testing frequency and STI history among current users of self-funded HIV pre-exposure prophylaxis, a cross-sectional study, Germany, 2018 and 2019

Introduction: Users of pre-exposure prophylaxis (PrEP) require periodic testing for HIV, sexually transmitted infections (STI) and renal function. Before PrEP was made free of charge through statutory health insurance in late 2019, PrEP users in Germany had to pay for testing themselves. Aim: We investigated self-reported HIV, STI and renal function testing frequencies among self-funded PrEP users in Germany, factors associated with infrequent testing, and STI diagnoses. Methods: A cross-sectional anonymous online survey in 2018 and 2019 recruited current PrEP users via dating apps for men who have sex with men (MSM), a PrEP community website, anonymous testing sites and friends. We used descriptive methods and logistic regression for analysis. Results: We recruited 4,848 current PrEP users. Median age was 37 years (interquartile range (IQR): 30–45), 88.7% identified as male, and respectively 26.3%, 20.9% and 29.2% were tested less frequently for HIV, STI and renal function than recommended. Participants with lower STI testing frequency were significantly less likely to report STI diagnoses during PrEP use, especially among those with many partners and inconsistent condom use. Factors most strongly associated with infrequent testing included not getting tested before starting PrEP, using PrEP from informal sources and on-demand/intermittent PrEP use. Discussion: In a setting of self-funded PrEP, many users obtained medical tests less frequently than recommended, which can lead to missed diagnoses. Barriers to testing should be addressed to enable proper medical supervision. The suitability of testing frequencies to PrEP users with less frequent risk exposures needs to be evaluated.Peer Reviewe

Multiple Andreev reflections as a transport problem in energy space

We present an approach for analyzing the dc current in voltage biased quantum superconducting junctions. By separating terms from different $n$-particle processes, we find that the $n$-particle current can be mapped on the problem of wave transport through a potential structure with $n$ barriers. We discuss the relation between resonances in such structures and the subgap structures in the current-voltage characteristics. At zero temperature we find, exactly, that only processes creating real excitations contribute to the current. Our results are valid for a general SXS-junction, where the X-region is an arbitrary non-superconducting region described by an energy-dependent transfer matrix.Comment: 11 pages, 4 figures, submitted to Superlattices and Microstructure

Nonequilibrium Josephson-like effects in wide mesoscopic S-N-S junctions

Mesoscopic superconducting-normal-metal-superconducting (S-N-S) junctions with a large separation between the superconducting electrodes (i.e. wide junctions) exhibit nonequilibrium supercurrents, even at temperatures for which the equilibrium Josephson effect is exponentially small. The second harmonic of the Josephson frequency dominates these currents, as observed in recent experiments. A simple description of these effects, in the spirit of the Resistively-Shunted-Junction model, is suggested here. It is used to calculate dc I-V characteristics, and to examine the effects of various types of noise and of external microwave radiation (Shapiro steps). It is found that the nonequilibrium supercurrents are excited when the junction is driven by a dc bias or an ac bias, or even by external noise. In the case of junctions which are also long in the direction perpendicular to the current flow, thermodynamic phase fluctuations (thermal noise) alone can drive the quasiparticles out of local equilibrium. Magnetic flux is then predicted to be trapped in units of Phi_0 /2 = hc/4e.Comment: 10 pages, to appear in a special issue of Superlattices & Microstructure