Spin polarization versus lifetime effects at point contacts between superconducting niobium and normal metals

Point-contact Andreev reflection spectroscopy is used to measure the spin polarization of metals but analysis of the spectra has encountered a number of serious challenges, one of which is the difficulty to distinguish the effects of spin polarization from those of the finite lifetime of Cooper pairs. We have recently confirmed the polarization-lifetime ambiguity for Nb-Co and Nb-Cu contacts and suggested to use Fermi surface mismatch, the normal reflection due to the difference of Fermi wave vectors of the two electrodes, to solve this dilemma. Here we present further experiments on contacts between superconducting Nb and the ferromagnets Fe and Ni as well as the noble metals Ag and Pt that support our previous results. Our data indicate that the Nb - normal metal interfaces have a transparency of up to about 80 per cent and a small, if not negligible, spin polarization.Comment: 7 pages, 2 figures, submitted to Proceedings of the 26th Conference on Low Temperature Physic

Phonon-drag induced suppression of the Andreev hole current in superconducting niobium contacts

We have investigated how the Andreev-reflection hole current at ballistic point contacts responds to a large bias voltage. Its strong suppression could be explained by the drag excerted by the non-equilibrium phonon wind generated by high-energy electrons flowing through the contact. The hole - phonon interaction leads to scattering lengths of the low-energetic holes down to 100\,nm, thereby destroying the coherent retracing of the electron path by the Andreev-reflected holes.Comment: 7 pages, 4 figures, submitted to Proceedings 26th International Conference on Low Temperature Physic

Maternal pomegranate juice intake and brain structure and function in infants with intrauterine growth restriction: A randomized controlled pilot study.

Polyphenol-rich pomegranate juice has been shown to have benefit as a neuroprotectant in animal models of neonatal hypoxic-ischemia. No published studies have investigated maternal polyphenol administration as a potential neuroprotectant in at-risk newborns, such as those with intrauterine growth restriction (IUGR). This was a randomized, placebo-controlled, double-blind pilot study to investigate the impact of maternal pomegranate juice intake in pregnancies with IUGR, on newborn brain structure and function at term-equivalent age (TEA). Mothers with IUGR at 24-34 weeks\u27 gestation were recruited from Barnes-Jewish Hospital obstetrical clinic. Consented mothers were randomized to treatment (8 oz. pomegranate juice) or placebo (8 oz. polyphenol-free juice) and continued to take juice daily from enrollment until delivery (mean 20.1 and 27.1 days, respectively). Infants underwent brain MRI at TEA (36-41 weeks\u27 gestation). Brain measures were compared between groups including: brain injury score, brain metrics, brain volumes, diffusion tensor imaging and resting state functional connectivity. Statistical analyses were undertaken as modified intention-to-treat (including randomized participants who received their allocated intervention and whose infants received brain MRI) and per-protocol (including participants who strictly adhered to the protocol, based on metabolite status). Seventy-seven mothers were randomized to treatment (n = 40) or placebo (n = 37). Of these, 28 and 27 infants, respectively, underwent term-equivalent MRI. There were no group differences in brain injury, metrics or volumes. However, treatment subjects displayed reduced diffusivity within the anterior and posterior limbs of the internal capsule compared with placebo. Resting state functional connectivity demonstrated increased correlation and covariance within several networks in treatment subjects, with alterations most apparent in the visual network in per-protocol analyses. Direct effects on health were not found. In conclusion, maternal pomegranate juice intake in pregnancies with known IUGR was associated with altered white matter organization and functional connectivity in the infant brain, suggesting differences in brain structure and function following in utero pomegranate juice exposure, warranting continued investigation. Clinical trial registration. NCT00788866, registered November 11, 2008, initial participant enrollment August 21, 2012

Assessing the Variation in Rail Interoperability in 11 European Countries, and Barriers to its Improvement

Work conducted within REORIENT, a Sixth Framework project for the European Commission (EC), is described. One objective of REORIENT was to explain the status of transformation of the European railway sector into a functionally integrated, liberalized, interoperable system. The status of interoperability within and between eleven countries in a corridor stretching from Greece to the Nordic countries was assessed, and conditions in the countries that appear to be barriers to achieving the EC’s goals were identified. (Barriers were defined as shortcomings in conditions that would facilitate the implementation of requirements presumed by the EC to lead to seamless international freight transport (“implementation conditions”)). The primary data source for the analysis was a set of interviews with the major actors and stakeholders associated with each country’s rail freight system. The (qualitative) information from the interviews was translated into numeric scores, which were subjected to statistical analysis. The primary objective of the statistical analysis was to provide an assessment of the relationships between the requirements and the implementation conditions. The statistical analysis involved both the identification of relevant relationships and an assessment of the strength of these relationships. Overall, we found that there was considerable variation in interoperability status across the countries on practically all of the requirements. However, there was also considerable variation in the status of the implementation conditions across the countries. As a result, we found that most of the variability was able to be explained by relationships that were found to exist between the requirements and implementation conditions. A ‘Barrier Significance Score’ (BSS) was computed for each country and for each implementation condition. These scores were used to assess the relative importance of barriers across the countries, and to identify the most critical barriers to be removed in order to improve interoperability. Large differences in BSS’s were found among countries. In general there are fewer barriers in Nordic countries and more barriers in the south

Andreev-reflection spectroscopy with superconducting indium — a case study

We have investigated Andreev reflection at interfaces between superconducting indium (Tc = 3.4 K) and several normal conducting nonmagnetic metals (palladium, platinum, and silver) down to T = 0.1 K as well as zinc (Tc = 0.87 K) in its normal state at T = 2.5 K. We analyzed the point-contact spectra with the modified onedimensional BTK theory valid for ballistic transport. It includes Dynes’ quasiparticle lifetime as fitting parameter Γ in addition to superconducting energy gap 2Δ and strength Z of the interface barrier. For contact areas from less than 1 nm² to 10000 nm² the BTK Z-parameter was close to 0.5, corresponding to transmission coefficients of about 80%, independent of the normal metal. The very small variation of Z indicates that the interfaces have a negligible dielectric tunneling barrier. Also Fermi surface mismatch does not account for the observed Z. The extracted value Z ≈ 0.5 can be explained by assuming that practically all of our point contacts are in the diffusive regime

Mechanisms of normal reflection at metal interfaces studied by Andreev-reflection spectroscopy

Andreev-reflection spectroscopy of elemental superconductors in contact with nonmagnetic normal metals reveals that the strength of normal-reflection varies only slightly. This observation imposes strong constrictions on the three possible normal-reflection mechanisms: tunneling through a dielectric barrier, reflection due to the different electronic properties of the two electrodes, and diffusive transport caused by elastic scattering in the contact region. We discuss in detail the role played by Fermi-surface mismatch, represented by the different Fermi velocities on both sides of the contact interface. We find that it is at least not the dominant mechanism and possibly completely absent in the Andreev-reflection process

Andreev-reflection spectroscopy of ferromagnets: the impact of Fermi surface mismatch

We have investigated point contacts between a superconductor (Nb, AuIn2) and a normal metal (ferromagnetic Co, nonmagnetic Cu). The observed Andreev-reflection spectra were analyzed using the modified BTK theory including spin polarization effects. This resulted in a polarization of Co that agrees with observations by others, but lifetime effects describe the spectra equally well. On the other hand, the spectra with nonmagnetic Cu can be well described using the spin-polarization model. The ambiguity between polarization and lifetime interpretation poses a dilemma which can be resolved by considering the normal reflection at those interfaces due to Fermi surface mismatch. Our data suggest that Andreev reflection at Nb–Co contacts does deliver the true magnetic polarization of Co only when lifetime effects and the mentioned intrinsic normal reflection are included

Mechanisms of normal reflection at metal interfaces studied by Andreev-reflection spectroscopy

Andreev-reflection spectroscopy of elemental superconductors in contact with nonmagnetic normal metals reveals that the strength of normal-reflection varies only slightly. This observation imposes strong constrictions on the three possible normal-reflection mechanisms: tunneling through a dielectric barrier, reflection due to the different electronic properties of the two electrodes, and diffusive transport caused by elastic scattering in the contact region. We discuss in detail the role played by Fermi-surface mismatch, represented by the different Fermi velocities on both sides of the contact interface. We find that it is at least not the dominant mechanism and possibly completely absent in the Andreev-reflection process. (C) 2014 AIP Publishing LLC.</p