Sign of the crossed conductances at a FSF double interface

Crossed conductance in hybrid Ferromagnet / Superconductor / Ferromagnet (FSF) structures results from the competition between normal transmission and Andreev reflection channels. Crossed Andreev reflection (CAR) and elastic cotunneling (EC) between the ferromagnets are dressed by local Andreev reflections, which play an important role for transparent enough interfaces and intermediate spin polarizations. This modifies the simple result previously obtained at lowest order, and can explain the sign of the crossed resistances in a recent experiment [D. Beckmann {\sl et al.}, cond-mat/0404360]. This holds both in the multiterminal hybrid structure model (where phase averaging over the Fermi oscillations is introduced ``by hand'' within the approximation of a single non local process) and for infinite planar interfaces (where phase averaging naturally results in the microscopic solution with multiple non local processes).Comment: 9 pages, 7 figure

Giant Conductance Oscillations In Mesoscopic Andreev Interferometers

We analyze the electrical conductance $G(\phi)$ of a two-dimensional, phase coherent structure in contact with two superconductors, which is known to be an oscillatory function of the phase difference $\phi$ between the superconductors. It is predicted that for a metallic sample, the amplitude of oscillation is enhanced by placing a normal barrier at the interface and that, by tuning the strength of the barrier, can be orders of magnitude greater than values observed in recent experiments. Giant oscillations can also be obtained without a barrier, provided a crucial sum rule is broken. This can be achieved by disorder induced normal scattering. In the absence of zero phase inter-channel scattering, the conductance possesses a zero phase minimum.Comment: 4 pages of Revtex, 6 figures available on reques

Strain evolution in GaN Nanowires: from free-surface objects to coalesced templates

Top-down fabricated GaN nanowires, 250 nm in diameter and with various heights, have been used to experimentally determine the evolution of strain along the vertical direction of 1-dimensional objects. X-ray diffraction and photoluminescence techniques have been used to obtain the strain profile inside the nanowires from their base to their top facet for both initial compressive and tensile strains. The relaxation behaviors derived from optical and structural characterizations perfectly match the numerical results of calculations based on a continuous media approach. By monitoring the elastic relaxation enabled by the lateral free-surfaces, the height from which the nanowires can be considered strain-free has been estimated. Based on this result, NWs sufficiently high to be strain-free have been coalesced to form a continuous GaN layer. X-ray diffraction, photoluminescence and cathodoluminescence clearly show that despite the initial strain-free nanowires template, the final GaN layer is strained

Phase coherent transport in hybrid superconducting structures: the case of d-wave superconductors

We examine the effect of d-wave symmetry on zero bias anomalies in normal-superconducting tunnel junctions and phase-periodic conductances in Andreev interferometers. In the presence of d-wave pairing, zero-bias anomalies are suppressed compared with the s-wave case. For Andreev interferometers with aligned islands, the phase-periodic conductance is insensistive to the nature of the pairing, whereas for non-aligned islands, the nature of the zero-phase extremum is reversed.Comment: 10 Pages, Revtex. 11 postscript figures available on reques

Barriers to healthcare access and experiences of stigma: Findings from a coproduced Long Covid case-finding study

Background and Aim Long Covid is often stigmatised, particularly in people who are disadvantaged within society. This may prevent them from seeking help and could lead to widening health inequalities. This coproduced study with a Community Advisory Board (CAB) of people with Long Covid aimed to understand healthcare and wider barriers and stigma experienced by people with probable Long Covid. Methods An active case finding approach was employed to find adults with probable, but not yet clinically diagnosed, Long Covid in two localities in London (Camden and Merton) and Derbyshire, England. Interviews explored the barriers to care and the stigma faced by participants and were analysed thematically. This study forms part of the STIMULATE-ICP Collaboration. Findings Twenty-three interviews were completed. Participants reported limited awareness of what Long Covid is and the available pathways to management. There was considerable self-doubt among participants, sometimes reinforced by interactions with healthcare professionals (HCPs). Participants questioned their deservedness in seeking healthcare support for their symptoms. Hesitancy to engage with healthcare services was motivated by fear of needing more investigation and concerns regarding judgement about the ability to carry out caregiving responsibilities. It was also motivated by the complexity of the clinical presentation and fear of all symptoms being attributed to poor mental health. Participants also reported trying to avoid overburdening the health system. These difficulties were compounded by experiences of stigma and discrimination. The emerging themes reaffirmed a framework of epistemic injustice in relation to Long Covid, where creating, interpreting and conveying knowledge has varied credibility based on the teller's identity characteristics and/or the level of their interpretive resources. Conclusion We have codeveloped recommendations based on the findings. These include early signposting to services, dedicating protected time to listening to people with Long Covid, providing a holistic approach in care pathways, and working to mitigate stigma. Regardless of the diagnosis, people experiencing new symptoms must be encouraged to seek timely medical help. Clear public health messaging is needed among communities already disadvantaged by epistemic injustice to raise awareness of Long Covid, and to share stories that encourage seeking care and to illustrate the adverse effects of stigma

Reflectionless tunneling in ballistic normal-metal--superconductor junctions

We investigate the phenomenon of reflectionless tunneling in ballistic normal-metal--superconductor (NS) structures, using a semiclassical formalism. It is shown that applied magnetic field and superconducting phase difference both impair the constructive interference leading to this effect, but in a qualitatively different way. This is manifested both in the conductance and in the shot noise properties of the system considered. Unlike diffusive systems, the features of the conductance are sharp, and enable fine spatial control of the current, as well as single channel manipulations. We discuss the possibility of conducting experiments in ballistic semiconductor-superconductor structures with smooth interfaces and some of the phenomena, specific to such structures, that could be measured. A general criterion for the barrier at NS interfaces, though large, to be effectively transparent to pair current is obtained.Comment: published versio

The impact of trench defects in InGaN/GaN light emitting diodes and implications for the "green gap" problem

The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated. Two mechanisms responsible for the structural degradation of the multiple quantum well (MQW) active region were identified. It was found that during the growth of the p-type GaN capping layer, loss of part of the active region enclosed within a trench defect occurred, affecting the top-most QWs in the MQW stack. Indium platelets and voids were also found to form preferentially at the bottom of the MQW stack. The presence of high densities of trench defects in the LEDs was found to relate to a significant reduction in photoluminescence and electroluminescence emission efficiency, for a range of excitation power densities and drive currents. This reduction in emission efficiency was attributed to an increase in the density of non-radiative recombination centres within the MQW stack, believed to be associated with the stacking mismatch boundaries which form part of the sub-surface structure of the trench defects. Investigation of the surface of green-emitting QW structures found a two decade increase in the density of trench defects, compared to its blue-emitting counterpart, suggesting that the efficiency of green-emitting LEDs may be strongly affected by the presence of these defects. Our results are therefore consistent with a model that the “green gap” problem might relate to localized strain relaxation occurring through defects.This is the accepted manuscript version. The final version is available from AIP at http://scitation.aip.org/content/aip/journal/apl/105/11/10.1063/1.4896279?showFTTab=true&containerItemId=content/aip/journal/apl

Dissipative Electron Transport through Andreev Interferometers

We consider the conductance of an Andreev interferometer, i.e., a hybrid structure where a dissipative current flows through a mesoscopic normal (N) sample in contact with two superconducting (S) "mirrors". Giant conductance oscillations are predicted if the superconducting phase difference $\phi$ is varied. Conductance maxima appear when $\phi$ is on odd multiple of $\pi$ due to a bunching at the Fermi energy of quasiparticle energy levels formed by Andreev reflections at the N-S boundaries. For a ballistic normal sample the oscillation amplitude is giant and proportional to the number of open transverse modes. We estimate using both analytical and numerical methods how scattering and mode mixing --- which tend to lift the level degeneracy at the Fermi energy --- effect the giant oscillations. These are shown to survive in a diffusive sample at temperatures much smaller than the Thouless temperature provided there are potential barriers between the sample and the normal electron reservoirs. Our results are in good agreement with previous work on conductance oscillations of diffusive samples, which we propose can be understood in terms of a Feynman path integral description of quasiparticle trajectories.Comment: 24 pages, revtex, 12 figures in eps forma

Genetic origins of honey bees (Apis mellifera) on Kangaroo Island and Norfolk Island (Australia) and the Kingdom of Tonga

International audienceAbstractWe examine the origin of honey bee (Apis mellifera) populations in Kangaroo Island (Australia), Norfolk Island (Australia) and the Kingdom of Tonga using a highly polymorphic mitochondrial DNA region and a panel of 37 single nucleotide polymorphisms that assigns ancestry to three evolutionary lineages: Eastern Europe, Western Europe and Africa. We also examine inbreeding coefficients and genetic variation using microsatellites and mitochondrial sequencing. The honey bees of Kangaroo Island have a high proportion of Eastern European ancestry (90.2%), consistent with claims that they are of the subspecies A. m. ligustica. The honey bees of Norfolk Island also had a majority of ancestry from Eastern Europe (73.1%) with some contribution from Western Europe (21.2%). The honey bees of Tonga are mainly of Western European (70.3%) origin with some Eastern European ancestry (27.4%). Despite the suspected severe bottlenecks experienced by these island population, inbreeding coefficients were low

Worker remittances and the global preconditions of ‘smart development’

With the growing environmental crisis affecting our globe, ideas to weigh economic or social progress by the ‘energy input’ necessary to achieve it are increasingly gaining acceptance. This question is intriguing and is being dealt with by a growing number of studies, focusing on the environmental price of human progress. Even more intriguing, however, is the question of which factors of social organization contribute to a responsible use of the resources of our planet to achieve a given social result (‘smart development’). In this essay, we present the first systematic study on how migration – or rather, more concretely, received worker remittances per GDP – helps the nations of our globe to enjoy social and economic progress at a relatively small environmental price. We look at the effects of migration on the balance sheets of societal accounting, based on the ‘ecological price’ of the combined performance of democracy, economic growth, gender equality, human development, research and development, and social cohesion. Feminism in power, economic freedom, population density, the UNDP education index as well as the receipt of worker remittances all significantly contribute towards a ‘smart overall development’, while high military expenditures and a high world economic openness are a bottleneck for ‘smart overall development’