Critical temperature of superconductor/ferromagnet bilayers

Superconductor/ferromagnet bilayers are known to exhibit nontrivial dependence of the critical temperature T_c on the thickness d_f of the ferromagnetic layer. We develop a general method for investigation of T_c as a function of the bilayer's parameters. It is shown that interference of quasiparticles makes T_c(d_f) a nonmonotonic function. The results are in good agreement with experiment. Our method also applies to multilayered structures.Comment: 4 pages, 2 EPS figures; the style file jetpl.cls is included. Version 2: typos correcte

Critical current of 3He-A in narrow channels

The critical current Jc of superfluid 3He-A in 0.8-μm-diam channels has been measured by the observation of the pressure difference along the channels versus the mass current. During warming Jc was found to decrease by about 30% at TBA(cyl) and by another 30% at TBA; TBA(cyl) is the reduced B→A transition temperature in the narrow flow channels, with TBA(cyl)TBA=0.92 at 27.4 bars. Above TBA a second dissipative mechanism was observed at lower currents. These features are believed to be associated with the ends of the channels.Peer reviewe

Evidence for two-dimensional nucleation of superconductivity in MgB2_{2}

According to the crystal structure of MgB$_{2}$ and band structure calculations quasi-two-dimensional (2D) boron planes are responsible for the superconductivity. We report on critical fields and resistance measurements of 30 nm thick MgB$_{2}$ films grown on MgO single crystalline substrate. A linear temperature dependence of the parallel and perpendicular upper critical fields indicate a 3D-like penetration of magnetic field into the sample. Resistivity measurements, in contrast, yield a temperature dependence of fluctuation conductivity above T$_{c}$ which agrees with the Aslamazov-Larkin theory of fluctuations in 2D superconductors. We consider this finding as an experimental evidence of two-dimensional nucleation of superconductivity in MgB$_{2}$.Comment: 5 RevTex pages, 3 PostScript Figures ZIPed in archive Sidoren.zip. Submitted to EuroPhys. Lett. December 3, 200

On the scalar sector of the covariant graviton two-point function in de Sitter spacetime

We examine the scalar sector of the covariant graviton two-point function in de Sitter spacetime. This sector consists of the pure-trace part and another part described by a scalar field. We show that it does not contribute to two-point functions of gauge-invariant quantities. We also demonstrate that the long-distance growth present in some gauges is absent in this sector for a wide range of gauge parameters.Comment: 15 pages, no figures, LaTeX, considerably shortene

Search for the ac Josephson effect in superfluid 3He

Experiments testing for the existence of the ac Josephson effect in superfluid 3He, analogous to phenomena observed in superconducting microbridges, have been performed. Small holes were employed as the weak link between two reservoirs filled with 3He; several different orifice geometries were tried. Simple model calculations suggest that steps in the flow characteristics should be observable with our resolution when an ac pressure modulation is applied across the weak link. We found that such effects do not exist for the parameter values used in our experiments.Peer reviewe

Superconducting triplet spin valve

We study the critical temperature Tc of SFF trilayers (S is a singlet superconductor, F is a ferromagnetic metal), where the long-range triplet superconducting component is generated at noncollinear magnetizations of the F layers. We demonstrate that Tc can be a nonmonotonic function of the angle α between the magnetizations of the two F layers. The minimum is achieved at an intermediate α, lying between the parallel (P, α = 0) and antiparallel (AP, α = π) cases. This implies a possibility of a "triplet" spin-valve effect: at temperatures above the minimum Tc Tr but below Tc P and Tc AP, the system is superconducting only in the vicinity of the collinear orientations. At certain parameters, we predict a reentrant Tc(α) behavior. At the same time, considering only the P and AP orientations, we find that both the "standard" (Tc P Tc AP) switching effects are possible depending on parameters of the system. © 2010 Pleiades Publishing, Ltd

Role of interface transparency and exchange field in the superconducting triplet spin-valve effect

© (2015) Trans Tech Publications, Switzerland. We study the superconducting transition temperature Tc of F2/F1/S trilayers (Fi is a metallic ferromagnet, S is a s-superconductor), where the long-range triplet superconducting component is generated at canted magnetizations of the F layers. In this paper we show that it is possible to realize different spin-valve effect modes - the standard switching effect, the triplet spin-valve effect, reentrant Tc(α) dependence or reentrant Tc(α) dependence with the inverse switching effect - by variation of the F2/F1 interface transparency or the exchange splitting energy. In addition, we show that position of the Tc minimum can be changed by joint variation of the F2/F1 interface transparency and the layer thicknesses

Superconducting triplet spin valve

We study the critical temperature Tc of SFF trilayers (S is a singlet superconductor, F is a ferromagnetic metal), where the long-range triplet superconducting component is generated at noncollinear magnetizations of the F layers. We demonstrate that Tc can be a nonmonotonic function of the angle α between the magnetizations of the two F layers. The minimum is achieved at an intermediate α, lying between the parallel (P, α = 0) and antiparallel (AP, α = π) cases. This implies a possibility of a "triplet" spin-valve effect: at temperatures above the minimum Tc Tr but below Tc P and Tc AP, the system is superconducting only in the vicinity of the collinear orientations. At certain parameters, we predict a reentrant Tc(α) behavior. At the same time, considering only the P and AP orientations, we find that both the "standard" (Tc P Tc AP) switching effects are possible depending on parameters of the system. © 2010 Pleiades Publishing, Ltd