The 0 and the pi phase Josephson coupling through an insulating barrier with magnetic impurities

We have studied temperature and field dependencies of the critical current $I_{C}$ in the Nb-Fe$_{0.1}$Si$_{0.9}$-Nb Josephson junction with tunneling barrier formed by paramagnetic insulator. We demonstrate that in these junctions the co-existence of both the 0 and the $\pi$ states within one tunnel junction takes place which leads to the appearance of a sharp cusp in the temperature dependence $I_{C}(T)$ similar to the $I_{C}(T)$ cusp found for the $0-\pi$ transition in metallic $\pi$ junctions. This cusp is not related to the $0-\pi$ temperature induced transition itself, but is caused by the different temperature dependencies of the opposing 0 and $\pi$ supercurrents through the barrier.Comment: Accepted in Physical Review

Orientation of Vortices in a Superconducting Thin-Film: Quantitative Comparison of Spin-Polarized Neutron Reflectivity and Magnetization

We present a quantitative comparison of the magnetization measured by spin-polarized neutron reflectivity (SPNR) and DC magnetometry on a 1370 \AA\ -thick Nb superconducting film. As a function of magnetic field applied in the film plane, SPNR exhibits reversible behavior whereas the DC magnetization shows substantial hysteresis. The difference between these measurements is attributed to a rotation of vortex magnetic field out of the film plane as the applied field is reduced. Since SPNR measures only the magnetization parallel to the film plane whereas DC magnetization is strongly influenced by the perpendicular component of magnetization when there is a slight sample tilt, combining the two techniques allows one to distinguish two components of magnetization in a thin film.Comment: 12 pages, 8 figures, It will be printed in PRB, Oct. 200

Flux-Induced Vortex in Mesoscopic Superconducting Loops

We predict the existence of a quantum vortex for an unusual situation. We study the order parameter in doubly connected superconducting samples embedded in a uniform magnetic field. For samples with perfect cylindrical symmetry, the order parameter has been known for long and no vortices are present in the linear regime. However, if the sample is not symmetric, there exist ranges of the field for which the order parameter vanishes along a line, parallel to the field. In many respects, the behavior of this line is qualitatively different from that of the vortices encountered in type II superconductivity. For samples with mirror symmetry, this flux-induced vortex appears at the thin side for small fluxes and at the opposite side for large fluxes. We propose direct and indirect experimental methods which could test our predictions.Comment: 6 pages, Latex, 4 figs., uses RevTex, extended to situations far from cylindrical symmetr

Vortex Pinning and the Non-Hermitian Mott Transition

The boson Hubbard model has been extensively studied as a model of the zero temperature superfluid/insulator transition in Helium-4 on periodic substrates. It can also serve as a model for vortex lines in superconductors with a magnetic field parallel to a periodic array of columnar pins, due to a formal analogy between the vortex lines and the statistical mechanics of quantum bosons. When the magnetic field has a component perpendicular to the pins, this analogy yields a non-Hermitian boson Hubbard model. At integer filling, we find that for small transverse fields, the insulating phase is preserved, and the transverse field is exponentially screened away from the boundaries of the superconductor. At larger transverse fields, a ``superfluid'' phase of tilted, entangled vortices appears. The universality class of the transition is found to be that of vortex lines entering the Meissner phase at H_{c1}, with the additional feature that the direction of the tilted vortices at the transition bears a non-trivial relationship to the direction of the applied magnetic field. The properties of the Mott Insulator and flux liquid phases with tilt are also discussed.Comment: 20 pages, 12 figures included in text; to appear in Physical Review

Matching and surface barrier effects of the flux-line lattice in superconducting films and multilayers.

The flux-line lattice dissipation and the pinning force of Bi2Sr2CaCu2O8 and YBa2Cu3O7 films and a Nb/Cu multilayer are investigated with the vibrating reed technique. In magnetic fields oriented under a small angle with respect to the film surfaces the Bi-2:2:1:2 film shows a series of pronounced dissipation maxima at matching fields BN in the irreversible region of the magnetic phase diagram. The Y-1:2:3 film shows tiny damping maxima, whereas no structure in the dissipation of the Nb/Cu multilayer is detected below the upper critical field. The comparison of the matching fields to an anisotropic London model shows that the dissipation maxima are caused by rearrangements of the flux-line lattice configuration due to interactions with the sample surface. The different behavior of the high-temperature superconductors and the Nb/Cu multilayer is understood by explicitly taking the surface barrier into account. Deviations from the surface induced commensurability of the flux-line lattice due to the intrinsic pinning are discussed. Our results indicate that pancake vortices in the Bi-2:2:1:2 film should be coupled below the irreversibility line and below magnetic fields B??0.5 T perpendicular to the film surface

Preparation of nickel, nickel-iron, and silver-copper nanoparticles in ionic liquids

Room temperature ionic liquids (RTILs) are unique in many respects. The preparation method reported here is based on their extremely low vapour pressure that allows for their usage in vacuum. By magnetron sputtering of nickel or by co-sputtering of Ni-Fe or Ag-Cu on a surface of several imidazolium-based RTILs, we prepared nanocolloids containing metallic nanoparticles ranging in size from 6 to 30 nm, depending on the kind of RTIL, with low size dispersion. The nanocolloids are stable in time, no sedimentation or agglomeration was observed after two years. The method can be used also for preparation of the nanoparticles from semiconducting or insulating materials. As the next step, nanocolloids prepared in this way can be used for decoration of thin-film gas sensors by proper nanoparticles aiming at enhancement of their sensitivity and selectivity. Two ways of immobilization are presented mild heating of a drop of colloid containing Ni-Fe nanoparticles placed on TEM mesh, and Ag-Cu nanoparticles that were immobilized on the surface of gold thin-film electrode by electrophoresis

Single-electron transport and magnetic properties of Fe-SiO2 nanocomposites prepared by ion implantation

The electric transport, magnetic, and magnetotransport properties of Fe-SiO2 nanocomposites prepared by Fe-ion implantation into silica were investigated. The structural studies revealed bcc Fe nanoparticles of an average size of 3 nm dispersed in a 100-nm-thick nanocomposite layer formed within the silica substrate. Using special thin-film electrodes that were only 100 nm apart, in-plane electrical measurements were performed in a temperature range of 4-300 K. Though no external gate electrode was used, single-electron transport phenomena (Coulomb blockade and Coulomb staircase) were observed at 4 K. The presence of Coulomb steps in I-V curves implies that the electric transport was realized by the tunneling of electrons via a random quasi-one-dimensional chain of a few isolated iron nanoparticles. The magnetic properties of the nanoparticles were determined by surface effects and by the superparamagnetic behavior. The nanoparticles exhibited enhanced anisotropy and were dipolarly interacting. However, the tunneling current was found to be independent of external magnetic field; i.e., no tunneling magnetoresistivity (TMR) was measured at 77 K. At this temperature the nanoparticles were superparamagnetic. Presumably, a low volumetric concentration of Fe nanoparticles (< 14%) and a spin-flip process due to residual single Fe atoms present in the silica barriers were responsible for the absence of the TMR effect