J. Talvacchio

EFFECT OF CU-O LAYER SPACING ON THE MAGNETIC FIELD INDUCED RESISTIVE BROADENING OF HIGH-TEMPERATURE SUPERCONDUCTORS

Abstract For H‖c-axis, the magnetic field induced broadening of the resistive transitions of high-Tc 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 kBT 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 YBa2Cu3O7, this dimensional crossover does not occur belowHc2, 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.

Low- and high-temperature superconducting microwave filters

Stripline and microstrip filters at X-band were designed and fabricated using low- and high-temperature superconductors in quarter-wave, parallel-coupled section configurations. Low-temperature superconducting niobium thin films, deposited on single-crystal sapphire, were used to build to six-pole stripline filters with adjacent passbands and approximately 3 dB crossovers and 1.2% bandwidth. Four- and six-pole microstrip filters were made with in situ epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films on LaAlO/sub 3/ substrates. All the YBCO filters showed 77 K passbands with clean skirts and high out-of-band rejection. The six-pole filters had adjacent passbands with -28 dB crossovers and 1.5% bandwidth. >

Surface stability of NbN single-crystal films

The surfaces of NbN(100) and NbN(111) single‐crystal films were analyzed as a function of annealing temperature in ultrahigh vacuum by low‐energy electron diffraction and x‐ray photoelectron spectroscopy. The NbN(100) surface changed from a (1×1) to a ((2)1/2×(2)1/2)45° structure at 900 °C accompanied by a loss of nitrogen. The NbN(111) surface, initially richer in nitrogen, transformed from a (1×1) to a (2×2) structure at 800 °C with an even greater loss of nitrogen from the surface. Increasing the carbon content of the films increased the temperatures at which reconstructions were observed. Structural models of the surfaces are presented that are consistent with these observations. At higher temperatures (up to 1100 °C), additional nitrogen was lost from films of both orientations. Measurements of superconducting transition temperatures and energy gaps indicated that the entire film was nitrogen deficient after annealing, with the greatest nitrogen loss at the surface.

Critical parameters in the single‐target sputtering of YBa2Cu3O7

The critical parameters in the single‐target magnetron sputtering of YBa2Cu3O7 have been identified and sufficiently optimized to allow the reproducible deposition of films with Tc’s of ≳90 K and Jc’s of ≫ 106 A/cm2 at 77 K. It was found that during film growth the bombardment of the YBa2Cu3O7 by energetic particles must be minimized and also a stronger oxidizing agent than molecular oxygen must be present to obtain films with these properties. Otherwise, films are deposited that, by x‐ray diffraction and energy dispersive x‐ray spectroscopy analyses, are indistinguishable from the highest‐Tc 1:2:3 stoichiometric material but which have critical temperatures of ≪90 K. Films need not have 1:2:3 overall stoichiometry to have optimum superconducting properties. In such cases the excess elements are present as second‐phase particles.The critical parameters in the single‐target magnetron sputtering of YBa2Cu3O7 have been identified and sufficiently optimized to allow the reproducible deposition of films with Tc’s of ≳90 K and Jc’s of ≫ 106 A/cm2 at 77 K. It was found that during film growth the bombardment of the YBa2Cu3O7 by energetic particles must be minimized and also a stronger oxidizing agent than molecular oxygen must be present to obtain films with these properties. Otherwise, films are deposited that, by x‐ray diffraction and energy dispersive x‐ray spectroscopy analyses, are indistinguishable from the highest‐Tc 1:2:3 stoichiometric material but which have critical temperatures of ≪90 K. Films need not have 1:2:3 overall stoichiometry to have optimum superconducting properties. In such cases the excess elements are present as second‐phase particles.

High‐Tc superconductor/normal‐metal/superconductor edge junctions and SQUIDs with integrated groundplanes

Epitaxial, high‐Tc superconductor/normal‐metal/superconductor (SNS) edge‐geometry weak links and superconducting quantum interference devices (SQUIDs) have been fabricated with integrated YBa2Cu3O7 (YBCO) groundplanes and SrTiO3 insulators, using a process which incorporates six epitaxial layers. The SNS edge junctions were produced using off‐axis sputtered films and Co‐doped‐YBCO normal metal interlayers. These devices show excellent performance with typical critical current‐resistance (IcRn) products of 500–800 μV for 100–150 A thick normal metal layers at 65 K, and 1‐σ critical current density ( Jc) spreads as small as 12%. SNS SQUIDs incorporating groundplanes exhibit voltage modulation of up to 130 μV at 65 K and 40 μV at 77 K. SQUID inductance measurements indicate microstrip inductance values of 1 pH per square at 65 K.

Sputter deposition of YBa2Cu3O7−y thin films

Thin films of YBa/sub 2/Cu/sub 3/O/sub 7-//sub y/ were prepared by magnetron sputtering. The films were characterized by x-ray diffraction, in situ x-ray photoelectron spectroscopy to determine the as-deposited oxygen content for various substrate temperatures, and scanning electron microscopy to analyze the microstructure. The orthorhombic phase was observed after an ex situ 700 /sup 0/C anneal. We have obtained films which were completely superconducting at 82 K with 6 K transition widths.Thin films of YBa2Cu3O7−y were prepared by magnetron sputtering. The films were characterized by x‐ray diffraction, in situ x‐ray photoelectron spectroscopy to determine the as‐deposited oxygen content for various substrate temperatures, and scanning electron microscopy to analyze the microstructure. The orthorhombic phase was observed after an ex situ 700 °C anneal. We have obtained films which were completely superconducting at 82 K with 6 K transition widths.

Meandering grain boundaries in YBa2Cu3Oy bi‐crystal thin films

Artificially induced [001] tilt grain boundaries in epitaxial YBa2Cu3Oy (YBCO) thin films were prepared by deposition onto SrTiO3 bi‐crystal substrates and subsequently examined by transmission electron microscopy and atomic force microscopy (AFM). It was found that the YBCO grain boundary deviated from the path defined by the underlying substrate boundary, with the ‘‘meandering’’ YBCO boundary only generally following the path defined by the boundary in the underlying substrate. The AFM studies suggest this ‘‘meandering’’ behavior is related to the nucleation and growth mechanisms of the film, and based on this, we were able to vary the magnitude of the meandering by changing the growth conditions. The implications of this meandering behavior are significant, suggesting potential variations in electrical behavior from point to point along these boundaries. This effect is likely to be exacerbated by reduced junction linewidths and may lead to inconsistent behavior in devices which utilize this type of bou...

High temperature superconducting space-qualified multiplexers and delay lines

A high temperature superconducting (HTS) four-channel multiplexer and a delay line were fabricated, space qualified and tested as part of the U.S. Navys High Temperature Superconductivity Space Experiment II (HTSSE-II). The multiplexer had an architecture that included two branch-line hybrids and two identical parallel-coupled line filters per channel. Its operation was centered at 4 GHz, with 50-MHz-wide channels. It was fully integrated, with microstrip interconnections between channels and thin-film load terminations in the out-of-phase port of the output hybrid. The delay line was made up of two cascaded modules for a total delay of 45 ns between 2 and 6 GHz. Both devices were made using 5-cm-diameter LaAlO/sub 3/ wafers coated with epitaxial thin film YBa/sub 2/ Cu/sub 3/O/sub 7/, on both sides in the case of the delay line. Both devices operated at 77 K.

Electrical contact to superconductors

Electrical contacts to conventional superconductors and high-critical-temperature oxide superconductors are reviewed. The technologically important conventional superconductors considered are Nb and its alloys and compounds, the oxide superconductors are typified by YBa/sub 2/Cu/sub 3/O/sub 7/. Three distinct forms in which the superconductor can be fabricated are considered: thin films for electronics applications, multifilamentary wires for magnets and large-scale applications, and monofilaments or tapes for measurements of the current-carrying capability of a superconducting material. It is concluded that electrical contact to conventional superconductors is relatively straightforward. However, contacting the high-temperature oxide superconductors is difficult not only because the characteristic lengths are short, 0.1 to 10 nm, but because they are strongly dependent on crystal orientation and the oxides tend to be chemically unstable within that distance of the surface.<<ETX>>

Tunnel junctions fabricated from coherent NbN/MgO/NbN and NbN/Al 2 O 3 /NbN structures

Trilayer structures consisting of two layers of epitaxial, single-crystal NbN and an intervening 2-nm-thick layer of epitaxial MgO or Al 2 O 3 , have been formed in four or two, respectively, crystallographic orientations. The NbN films were deposited by dc magnetron sputtering and oxide films were grown by dc sputtering or evaporation. Single-crystal NbN films did not have significantly higher superconducting transition temperatures or significantly lower normal-state resistivities than films grown by polycrystalline epitaxy. However, the use of single-crystal epitaxy to deposit tunnel barriers provided new experimental parameters for control of thickness uniformity that affect the tendency of an epitaxial layer to grow as a 2-D or 3-D film. One of these experimental parameters, increasing the barrier deposition temperature, was shown to decrease the junction subgap conductance and to increase the effective barrier height for epitaxial barriers, but was detrimental to polycrystalline junctions.