Interlayer Josephson Coupling Research Articles

Effect of Cu-O layer spacing on the magnetic field induced resistive broadening of high-temperature superconductors

For H‖ c-axis, the magnetic field induced broadening of the resistive transitions of high- T c superconductors (HTS) is shown to depend strongly on the Cu-O layer spacing. For the highly anisotropic HTS, we show experimental evidence that flux motion results from a thermally activated crossover from three dimensional (3D) vortex lines to 2D independent pancake-like vortices in the Cu-O layers, which is intrinsic to the material and occurs when k B T exceeds the Josephson coupling energy of these layers. At low temperatures, however, thermally activated conventional depinning (which can be sample dependent) or melting in the uncoupled 2D Cu-O layers is also required for flux motion. For YBa 2Cu 3O 7, this dimensional crossover does not occur below H c2, presumably because the conducting Cu-O chains short-circuit the Josephson interlayer coupling, leading to better superconducting properties in a magnetic field. These results show that strong interlayer coupling is a key to finding good alternatives.

Synthesis and Characterization of Nb/Al2O3 Multilayer Superconductors

The metal/insulator multilayer superconductor, Nb/Al2O3, has been prepared by use of a dual sputtering technique. Sample characterization has been made by X-ray diffraction and electron transport measurements. The superconducting transition temperature Tc and the upper critical fields parallel and perpendicular to the multilayer plane have been examined as a function of Nb and Al2O3 sublayer thickness. Anisotropy of the critical fields is analyzed on the basis of the Josephson tunneling model for a quasi-two-dimensional superconductor and the strength of the interlayer Josephson coupling through the Al2O3 sublayer is evaluated.

Two-dimensional vortices in a stack of thin superconducting films: A model for high-temperature superconducting multilayers.

The structure of vortices within an infinite stack of thin superconducting layers is considered and examined in detail in the limit of zero interlayer Josephson coupling. The basic building block for the description of three-dimensional (3D) vortex lines is shown to be the 2D pancake vortex, which is a vortex located in only one of the layers; the other layers contain no vortices, but have an important effect in screening the magnetic field generated by currents in the first layer. It is shown that 3D vortex lines can be built up by superposing the contributions of stacks of 2D pancake vortices. Thermal excitation is shown to break up a single 3D vortex line at a temperature corresponding to the Kosterlitz-Thouless temperature of a single superconducting layer. The effect of thermally induced decoupling of the 2D vortex solids in different layers, corresponding to melting only in the direction perpendicular to the layers, is also considered. It is shown that Josephson coupling can be neglected in the high-temperature superconductors only under very stringent conditions. Although these conditions evidently are not met in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ and ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{1}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$, they should be satisfied in superconducting-insulating multilayer systems, such as ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$/${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$.

Interlayer coupling in high temperature superconductors

Abstract We consider the high temperature superconductors such as YBaCuO as a stack of planar superconductors. The interlayer Josephson coupling deduced from the perpendicular penetration depth is of order 10 8 A/cm 2 for YBaCuO, and some evidence supports a similar conclusion in BiSrCaCuO. Comparison with resistivities of YBaCuO and BiSrCaCuO crystals indicates clean limit behavior for in-plane supercurrents but dirty limit behavior for perpendicular supercurrents. The much lower values of transport critical current density J c reported so far are understood in terms of weak pinning for the interplanar vortices as they slide between the planes.

Superconducting properties of Mo/Si multilayer films

Multilayered Mo/Si thin films have been prepared by dual rf-magnetron sputtering technique, and the superconducting transition temperature Tc and the upper critical field Hc2 have been examined by resistance measurements. X-ray diffractometry shows that for the modulation wavelength λ shorter than 3 nm both Mo and Si sublayers are amorphous, while for λ longer than 3 nm, crystalline bcc structures appear in Mo sublayers. The λ dependence of Tc of the multilayer films with equal sublayer thickness shows that Tc takes a maximum value of 6.1 K for λ=2.3 nm and approaches the Tc value of amorphous Mo56Si44 in the small λ limit. The superconductivity of the multilayers has been confirmed to result from amorphous MoSi phase formed in the interfacial region of the sublayers. The thickness of the superconducting amorphous layer is estimated to be about 3 nm from the λ dependence of Tc and from the result of the anisotropy of Hc2 . Keeping the Mo sublayer thickness less than 2.4 nm to suppress the appearance of the crystalline Mo and increasing the Si sublayer thickness to reduce the interlayer Josephson coupling, an ideal quasi-two-dimensional superconductor with dHc2∥ /dT as high as 180 kOe/K has been realized.

Superconductivity of NbTiGe multilayers in weakly localized regime

The superconducting properties of NbTiGe multilayers have been studied in the weakly localized regime. By systematically varying the NbTi and Ge layer thicknesses, we studied the effect of disorder and interlayer Josephson coupling on T c. The suppression of T c with increasing Ge layer thickness suggests the importance of the barrier width in determining the superconducting properties in a multilayered system.

Effects of impurities on the nonuniform state in layered superconductors

The possibility of the existence of a nonuniform state is investigated, and the nature of the phase transition is determined in layered superconductors with Josephson-like interlayer coupling as a function of sample purity and temperature for a field parallel to the layers. The dependence of the upper critical field HK2(‖) on temperature and electron mean free path within the layers is obtained. At temperatures T < 0.55 Tc, it is shown that depending on the purity of the superconductor and the temperature, either a second-order phase transition into a nonuniform superconducting state or a first-order transition into a uniform superconducting state is observed in layered superconductors with Josephson interlayer coupling for a field parallel to the layers.