Volume 96, Issue 6, 15 September 2004
Index of content:
- MAGNETISM AND SUPERCONDUCTIVITY (PACS 74-76)
Field solutions for the Swihart wave and surface plasmon in superconducting/dielectric film multilayer structures in the mixed state96(2004); http://dx.doi.org/10.1063/1.1781761View Description Hide Description
Microwave propagation characteristics in superconducting/dielectric multilayer structures in the mixed state are theoretically investigated. We use the electrodynamic field equations for a type-II superconductor in the mixed state to derive the field solutions for two model structures such as a superconducting planar waveguide and a superlattice. With these solutions, we present several quantitative studies of the propagation constant as a function of static magnetic field, superconducting film thickness, and radiation frequency for the Swihart wave, surface plasmon, and the dielectric image waveguide, respectively. In the waveguide structure, it is found that there exists a certain value of thickness of superconducting film such that the attenuation constant attains a minimum. This value is shown to increase with increasing static magnetic field. This behavior, however, is smeared out in the superconductingsuperlattice structure regardless of the applied static magnetic field. The roles played by the superconducting film thickness together with the vortices in the microwave propagation for a multilayer structure are thus elucidated.
THz Josephson properties of grain boundary YBaCuO junctions on symmetric, tilted bicrystal sapphire substrates96(2004); http://dx.doi.org/10.1063/1.1782273View Description Hide Description
Superconducting Josephson junctions with high characteristic voltages ( larger than 4 mV at 4.2 K) are fabricated by depositing on tilted sapphire bicrystal substrates, where the tilting axis is along the grain boundary. The good junction quality and low microwave losses in sapphire gave high frequency response well into the THz region. High quality epitaxialfilms were deposited on tilted sapphire substrates with buffer layers by pulsed laser deposition. YBaCuO films have smaller tilt angles, from 1.0° up to 10.3°, compared to inclination angles of the substrates from 1.5° to 13.6°. X-ray diffraction shows only a single orientation of the films in the plane, as well as an absence of -axis particles and outgrowths. Critical temperatures as high as and were obtained in all films. The grain boundary in a common high- superconducting junction is usually much less straight than in the in-plane rotated bicrystal substrate, depressing Josephson currents. The waviness of the artificial grain boundary in a tilted bicrystal film is three times less than in an in-plane (untilted) bicrystal. Tilted Josephson junctions of widths from 1.5 to 6 μm were tested at temperatures from 0.26 to 77 K. products as high as 4.5 mV were observed at . Shapiro steps were observed at voltages over 3 mV under 300 GHz irradiation. Josephson radiation from the tilted junction was measured at frequencies up to 1.7 THz by a cryogenic bolometer. Suppressing the critical current with a magnetic field can separate Josephson radiation and thermal radiation. A parabolic dependence of the response on bias voltage for thermal radiation corresponds to an increase of junction temperature from 260 mK at zero bias to 3 K at 1 mV bias.
96(2004); http://dx.doi.org/10.1063/1.1775303View Description Hide Description
Nickel microspheres were produced by the sparkerosion technique under both liquid nitrogen and water conditions. Density measurements and Scanning electron microscope analysis revealed that a significant portion of the particles produced under the liquid nitrogen conditions are hollow spheres with a density of while the particles produced in water are primarily solid spheres with a density of , close to the bulk nickel value of . Nickel∕polymer composites incorporating the hollow and solid nickel microspheres were manufactured with volume fractions of and , respectively. The hollow and solid nickelcomposites exhibited saturation magnetostrictions of and , respectively. In addition, small quantities of Terfenol-D were spark eroded under liquid argon conditions with by volume of the spark eroded particles being solid microspheres. Calculations indicate that aligned composites that incorporate these Terfenol-D microspheres could reach a strain value of .
96(2004); http://dx.doi.org/10.1063/1.1783612View Description Hide Description
We report on the fabrication of highly transparent superconductor∕normal metal∕two-dimensional electron gasjunctions formed by a superconducting NbN electrode, a thin Au interlayer, and a two-dimensional electron gas in an heterostructure. High junction transparency has been achieved by exploiting developed process of evaporation and rapid annealing at . This allowed us to observe a decrease in the differential resistance with pronounced double-dip structure within the superconducting energy gap in superconductor/two-dimensional electron gas(2DEG) proximity systems. The effect of a magnetic field perpendicular to the plane of the 2DEG on the differential resistance of the interface was studied. It has been found that the reduced subgap resistance remains in high magnetic fields. Zero-field data are analyzed within the previously established quasiclassical model for the proximity effect.
96(2004); http://dx.doi.org/10.1063/1.1778215View Description Hide Description
Melt-processed samples of (YBCO)doped with Li additions substituting at Cu(2) plane sites were used to study the effects of doping on the superconducting temperature, critical current density, irreversibility field, upper critical field, coherence length, and magnetic relaxation as a function of temperature and magnetic field. The intrinsic superconducting properties were found only slightly influenced by Li additions at some optimal concentration . At this Li concentration the volume pinning force of doped material is about five times that of undoped YBCO in intermediate magnetic fields at . No significant change of the maximum volume pinning force with oxygen defects after long-time oxygenation experiment has been observed. The normalized pinning force of the sample optimally doped with Li obeys the same scaling law as a function of magnetic field and temperature before and after additional heat treatment, indicative of a single type of pinning centers. The time dependence of the magnetization in the standard and in the Li-doped sample has been analyzed in the framework of collective pinning theory. The magnetic relaxation measurements combined with the phase diagram data provide evidence of an elastic-plastic transition. The pinning parameters estimated show a considerable enhancement of the critical current density in the optimally doped sample, but only an insignificant change in the effective activation energy, pointing to a weak pinning of pointlike pinning centers, created by Li additions.