Unsteady axisymmetric stagnation flow on a circular cylinder

Unsteady, axisymmetric stagnation flow about a circular cylinderis examined when the far-field flow is a periodic function oftime with a fixed time average and an oscillatory part of prescribedamplitude and frequency. Solutions are computed for arbitraryvalues of the Reynolds number, quantifying the effects of surfacecurvature, and a frequency parameter based on the period ofthe far-field flow. It is found that solutions remain regularand periodic provided that the far-field amplitude lies belowa critical value. Above this value, solutions terminate in afinite-time singularity. The blow-up time is delayed by increasingthe curvature of the surface. These results are corroboratedby asymptotic predictions valid in the limits of small and largeamplitude and frequency. For large Reynolds number, the problemreduces to the two-dimensional stagnation-point flow againsta plane wall studied by previous authors.     

High Curie temperature in B-site ordered Sr2CrWO6 epitaxial thin films

Epitaxial and c-axis oriented double perovskite Sr₂CrWO₆ thin films were prepared on SrTiO₃ (100) and LaAlO₃ (100) substrates by pulsed-laser deposition. Structural, magnetic and transport properties were found to be sensitive to the gas conditions employed during the deposition. A small amount of oxygen along with Ar during the deposition was found to be essential for B-site ordering; such films displayed lattice parameters close to the bulk value and display ferromagnetic metallic behavior. The Curie temperature observed above 500 K in these films is higher than bulk Sr₂CrWO₆ samples. Films grown without oxygen were observed to have long c-parameter and no B-site ordering; they were non-magnetic and semiconducting.     

Polyethylene glycol matrix reduces the rates of photochemical and thermal release of nitric oxide from S-nitroso-N-acetylcysteine

S -nitrosothiols have many biological activities and may act as nitric oxide (NO) carriers and donors, prolonging NO half-life in vivo. In spite of their great potential as therapeutic agents, most S -nitrosothiols are too unstable to isolate. We have shown that the S -nitroso adduct of N -acetylcysteine (SNAC) can be synthesized directly in aqueous and polyethylene glycol (PEG) 400 matrix by using a reactive gaseous (NO/O₂) mixture. Spectral monitoring of the S–N bond cleavage showed that SNAC, synthesized by this method, is relatively stable in nonbuf-fered aqueous solution at 25°C in the dark and that its stability is greatly increased in PEG matrix, resulting in a 28-fold decrease in its initial rate of thermal decomposition. Irradiation with UV light (λ= 333 nm) accelerated the rate of decomposition of SNAC to NO in both matrices, indicating that SNAC may find use for the photogeneration of NO. The quantum yield for SNAC decomposition decreased from 0.65 ± 0.15 in aqueous solution to 0.047 ± 0.005 in PEG 400 matrix. This increased stability in PEG matrix was assigned to a cage effect promoted by the PEG microenvironment that increases the rate of geminated radical pair recombination in the homolytic S–N bond cleavage process. This effect allowed for the storage of SNAC in PEG at −20°C in the dark for more than 10 weeks with negligible decomposition. Such stabilization may represent a viable option for the synthesis, storage and handling of S -nitrosothiol solutions for biomedical applications.     

Response to comment on “Controlled injection of spin triplet supercurrents into a strong ferromagnet”

The comment by Lin He proposes that at the interface between holmium and cobalt an interesting magnetic proximity effect occurs in which the spiral magnetic wavelength (λ) of holmium is enhanced to 5.4 nm. Consequently, the optimum holmium layer thickness for the superconductor spin triplet proximity effect to occur in Nb/Ho/Co/Ho/Nb Josephson junctions of ≈4.5 and ≈10 nm would then correspond to the presence of ≈λ/2 and ≈3λ/2 spiral wavelengths. Although intriguing, this idea seems to be at odds with the conventional micromagnetics of either thin film (polycrystalline) holmium or single-crystal Ho at low temperatures (10 K). Although an increase in λ in Ho/metal multilayer thin films with epitaxially matched interfaces has been reported, the possibility of such an effect in a polycrystalline system seems unlikely.     

Hysteretic vortex pinning in superconductor-ferromagnet nanocomposites

This Letter reports the observation of hysteresis in the vortex pinning in a superconductor-ferromagnetic epitaxial nanocomposite consisting of fcc Gd particles incorporated in a Nb matrix. We show that this hysteretic pinning is associated with magnetic reversal losses in the Gd particles and is fundamentally different in origin to pinning interactions previously observed for ferromagnetic particles or other microstructural features.     

Submm heterodyne mixing using NbCN/Nb sis tunnel junctions

We describe heterodyne mixing experiments with NbCN/Nb quasi-particle tunnel junctions at submillimeter wavelengths. In this wavelength range junctions with niobium nitride as superconducting material are promising because of the high gap voltage, about 5.7 mV, as compared to 3 mV for the more commonly used niobium. As a first step towards all-NbN junctions we investigate the development of junctions with a NbN base electrode and a Nb upper electrode. We present results from samples with two different insulating barriers: aluminum oxide and magnesium oxide. Measured noise temperatures range from 500 K at 345 GHz and 1600 K at 482 GHz to 8500 K at 644 GHz. These results are about one order of magnitude worse than the best results obtained with all-Nb junctions. The difference can be attributed partly to the relatively high radio-frequency (rf) losses in NbN films as compared to Nb. Also sensitivity is reduced because of the relatively high leakage current in the sub-gap region of the I-V curve of the junction.     

Effects of interlayer coupling on the irreversibility lines of NbN/AlN superconducting multilayers

We have studied the temperature dependence of the in-plane resistivity of NbN/AlN multilayer samples with varying insulating layer thickness in magnetic fields up to 7 T parallel and perpendicular to the films. The upper critical field shows a crossover from 2D to 3D behavior in parallel fields. The irreversibility lines have the form (1-T/T(c))(alpha), where alpha varies from 4 / 3 to 2 with increasing anisotropy. The results are consistent with simultaneous melting and decoupling transitions for the low anisotropy sample, and with melting of decoupled pancakes in the superconducting layers for higher anisotropy samples.     

Ferromagnetic spin ordering in disordered Sr2FeMoO6 films

Epitaxial films of Sr₂FeMoO₆ were grown on SrTiO₃ (100) substrates by pulsed laser deposition; optimised growth conditions give a saturation magnetization of 4.0μ_B implying a high degree of B-site ordering. The films show very high crystallinity and smooth surfaces. We have compared the B-site disorder in the films with saturation magnetization and show that the rate of suppression with disorder is lower than that suggested by Monte Carlo simulations, suggesting the ferromagnetic coupling in the Fe/Mo sublattices.