Electrostatic bonding of high-T c superconducting films to any substrate

Complete integration of high-T _c thin-film superconducting technology with other electronic technologies requires hybrid structures with both technologies on the same substrate. This is difficult to do with direct growth of the superconductor on certain substrates (GaAs, InP) because of the high temperatures required for formation of the superconductor. A method is proposed to circumvent this problem by using electrostatic forces and appropriate thin-film materials to bond superconducting films to any substrate at 300°C. The same principle can be applied to the bonding of other devices on other substrates.     

Thin-film processing of high-Tc superconductors

This review of high-T _c superconducting thin-film processing focuses on the developments in thin-film deposition technologies since 1987. The common deposition processes are described with reference to their effects on superconductor film performance. A comparative evaluation of the potential of the technologies is also given. The development of multilayers and heterostructures is an important requirement for future device applications and is also described. The latest results of the deposition of novel superconducting materials and deposition on uncommon substrates are discussed. The outlook on some imminent topics of future development in process technologies for high-T _c superconducting thin films is discussed.     

Observation of X-band SQUID signals in a High-Tc superconducting film device

A microwave superconducting magnetometer is described in which a microstrip resonator is coupled to a two-hole high-T _c thin-film SQUID device. Both the microstrip circuit and the thin-film SQUID were fabricated by photolithography techniques. The YBCO thin film was deposited on single-crystal substrate of yttria-stabilized zirconia [YSZ(100)] by an ion beam sputtering technique producing a superconducting transition measured at a critical temperature ofT _c =92 K to within T 3 K. Non linear oscillatory behavior was observed in the microstrip resonator when inductively coupled to the SQUID. This nonlinear behavior yielded a microwave device in which the reflected microwave power varied with applied DC magnetic flux.     

Direct Peritectic Growth of c-Axis Textured YBa2Cu3Ox on a Flexible Metallic Substrate

Previous work in the development of YBa₂Cu₃O _x (YBCO) superconducting wires and tapes has been focused on the deposition of YBCO on buffered metallic substrates. Although such an approach has proved successful in terms of achieving grain texturing and high transport current density, critical issues involving continuous processing of long-length conductors and stabilization of the superconductor have not yet been entirely settled. We have developed a novel process, the so-called direct peritectic growth (DPG), in which textured YBCO thick films have been successfully deposited directly onto a silver alloy substrate. No buffer layer is employed in the film deposition process. The textured YBCO grains have been obtained through peritectic solidification over a wide range of temperatures and times. The substrate materials have not demonstrated any observable reaction with the YBCO melt at the maximum processing temperature near 1010°C. The transport J _c has reached a respectable value of 10⁴ A/cm² at 77 K and zero magnetic field. Based on the experimental results in this work, we show that the DPG method offers an effective alternative for the fabrication of long-length YBCO conductors. Also reported is a physical explanation of the texturing mechanism on the metal substrate.     

Oxygen content determination of Bi2Sr2Ca2Cu4O11+x superconductor by thermogravimetric analysis

Thermogravimetric analysis of the individual, binary, ternary, and bismuth-based superconductor mixtures have been carried out to elucidate the excess oxygen content of the Bi₂Sr₂Ca₂Cu₄O_11+x . Our systematic approach eliminates the need to assume initial phase present, original oxygen content, and degree of reduction as in other TG studies. The excess oxygen content of the bismuth superconductor increases fromx=0.38 tox=0.64 after three heating cycles in oxygen atmosphere. Most of the excess oxygen is associated with the highly oxidized copper (Cu³⁺) in the superconducting phase/phases.     

Structural analysis of high-Tccuprate superconductors

We have shown in this paper that members of the Tl-Ba-Ca-Cu-O and Bi-Sr-Ca-Cu-O superconducting families fabricated so far are built up of superconductor-semiconductor arrays. Four general homologous series are written down for these two families. The basic fundamental block in each series is itself a superconductor. We suggest that the Y-Ba-Cu-O family has a similar characteristic if YBaCu₂O_5-d is a superconductor.     

High-temperature superconducting microwave devices: Fundamental issues in materials, physics, and engineering

High-T _c superconductivity has generated a great deal of interest because of the challenges it presents in the fields of material science, condensed matter physics, and electrical engineering, and because of the potential applications which may result from these research efforts. Thin-film passive microwave components may become the first high-temperature superconducting (HTS) devices available for widespread use and commercialization. In this article, we review aspects of material science, physics, and engineering which directly impact high-T _c superconducting microwave devices and discuss issues which determine the performance of these devices. Methods of growing HTS thin films on large-area substrates, techniques for fabricating single-level HTS passive microwave components, and the relevant properties of high-T _c superconducting films are discussed, with a focus on thin films of the HTS material YBa₂Cu₃O_7–. Several known mechanisms for microwave loss in both the superconductor and the dielectric substrate are presented. An overview of the general classes of superconducting passive microwave devices is given, and representative microwave devices which have been recently demonstrated are described in detail. Examples of a select few HTS active microwave components are also presented. Potential microwave applications are illustrated with comparisons to current technology.     

Low-field magnetic properties of high-TcGdxY1?xBa2Cu3Oy superconductor

The magnetization of Gd_0.85Y_0.15Ba₂Cu₃O _y high-T _c superconductor was measured at low fields (<100 Oe) and at a constant temperature of 77 K. The magnetization curves deviate from linearity at 25 Oe, causing the destruction of the weak couplings of the superconducting grains. The hysteresis due to granular superconductivity was also observed.     

Spin-Injection Quasiparticle Nonequilibrium in Cuprate/Manganite Heterostructures

Quasiparticle nonequilibrium due to spin-polarized current injection in perovskite superconductor/ferromagnet (S/F) thin-film heterostructures has been studied with the technique of cryogenic scanning tunneling spectroscopy. The spin-injection heterostructures consisted of epitaxial bilayers of the high-T _c superconductor YBa₂Cu₃O_{7 –} and the half-metallic ferromagnet La_0.7Ca_0.3MnO₃, with a spin-polarized quasiparticle current injected into the cuprate layer from the manganite layer. The tunneling conductance measured at 4.2 K on the cuprate layer showed a distinctly nonequilibrium quasiparticle spectrum as a result of the spin injection. Quantitative analysis of the tunneling spectral evolution versus the injection current yielded an estimate of the quasiparticle spin-diffusion depth and the spin-relaxation time in the superconducting cuprate.     

High-pressure dc sputtering of High-Tc BPSCCO thin films withTc(0) above 100 K

High-T _c Bi(Pb)-Sr-Ca-Cu-O thin films have been made on single-crystal MgO substrates using high-pressure dc sputtering technique. X-ray studies confirm the crystallinity and highly oriented structure withc-axis perpendicular to the substrate. By optimizing the annealing schedule the formation of the high-T _c phase is stabilized. The best film exhibited superconducting transition temperature with zero-resistance temperature,T _c(0), as high as 101 K. Temperature dependence ofJ _c indicates the presence of Josephson-type weak links.     

Thin-film processing of high-T c superconductors

This review of high-T _c superconducting thin-film processing focuses on the developments in thin-film deposition technologies since 1987. The common deposition processes are described with reference to their effects on superconductor film performance. A comparative evaluation of the potential of the technologies is also given. The development of multilayers and heterostructures is an important requirement for future device applications and is also described. The latest results of the deposition of novel superconducting materials and deposition on uncommon substrates are discussed. The outlook on some imminent topics of future development in process technologies for high-T _c superconducting thin films is discussed.     

Proximity Effect in Bulk LaBa2Cu3O7?y Samples with Ag Additions

Bulk LaBa ₂ Cu ₃ O _(7–y) samples with different Ag additions were investigated. It was shown that Ag does not enter the crystallographic structure of the superconductor and segregates on the grain boundary region. Current path in these samples occurs through the proximity-connected grains, and this was confirmed from the temperature dependence of the critical current density and mutual inductance. By using the theory developed for the thin-film structures, we conclude that growing of the Ag content increased the effective cross-section and the normal metal thickness. The effective cross-section prevails at low concentration, increasing the current. The normal metal thickness dominates at higher concentration, leading to saturation or even lowering of the critical current.     

Current controlled high Tc superconducting switch

When a current is applied above the critical current l_c of a superconductor, the material is in its normal state and has a finite resistance. Below l_c the material becomes a superconductor with zero resistance. Switching between these two states can be achieved by modulating a current through the sample. Various high T_c superconducting (HTS) line structures have been made. In the normal state these structures are ordinary resistors with resistances ranging from 10 μ to 100 kμ. The critical currents are in the range 10 μA–100 mA. Switching behaviour has been observed in a simple divider circuit using the HTS lines at 77 K. Applications of the current controlled HTS switch to digital and logic circuits are discussed.     

Preparation of YBa2Cu3O7–x high-Tcceramic superconductors from melt

Fabrication of high-T _c ceramic superconductor in the system Y₂O₃-BaO-CuO by melting a mixture of component oxides has been investigated. The compositions of the resulting specimens and the effects of heat treatment have been investigated. It was determined that molten material was composed of phases including BaCuO₂, CuO, Y₂O₃, and Y₂BaCuO₅. A subsequent heat treatment in air produced a nominal amount of the high-T _c phase, while heat treatment in an O₂ atmosphere resulted in a significantly large percentage of the superconducting phase.     

MOCVD-grown homoepitaxial YBa2Cu3Ox thin films and its multilayers

YBa₂Cu₃O _x (YBCO) films, Zn-doped YBCO (YBCO : Zn) films, and their bilayers have been epitaxially grown on SrTiO₃(100) and single-crystal YBCO(001) substrates by metalorganic chemical vapour deposition. The YBCO(001) films homoepitaxially grown on YBCO(001) substrates have flat surfaces on an atomic scale, and interfaces free from crystalline defects. We can systematically reduce the superconducting transition temperature (T _c) of YBCO : Zn films from 90 K to 37 K by increasing Zn concentration. The bilayers have a sharp distribution of Zn as evaluated fromT _c measurements of the upper YBCO films and depth profiles of secondary ion mass spectrometer, suggesting the possibility to form the homoepitaxial SNS (S, superconductor; N, normal metal) junction operatable between 40 K and 90 K.     

Room-Tc Superconductivity on Whispering Mode in Quasi-1D Composite of Superconducting Nanotubes: Is It Possible?

Combining Little's and Ginzburg's ideas with recent progress in nanotubes research, a novel type of material is advanced as a perspective high-T _c superconductor on a base of a close-packed lattice of quasi-1D superconducting nanotubes. Idea is offered that superconducting coaxial multilayer nanotubes of the correlation length in diameter is an ideal and natural trap for pinning of Abrikosov vortex. Nanotube should be layered superconductor, such as LuNiBC. Mechanism of superconductivity was proposed and substantiated quantitatively on a base of a whispering mode, which is shown to be responsible for a strong enhancement of electron–phonon interaction and for an increase of critical temperature. Nanocomposite built from such quasi-1D nanotubes when coinciding with vortex lattice provides ideal conditions for the pinning, resonance, distortion, ordering, and Little–Parks effects, the joint action of which is suggested to result in synergetic effect increasing the superconductivity. Such quasi-1D or 2D nanotubular crystal is proposed to synthesize by template approach using zeolite-like membrane.     

Magnetization and Susceptibility Studies on BaZrO3-Doped YBa2Cu3O7 ? x Bulk Superconductors

We studied the YBa₂Cu₃O_{7 – x} bulk superconductor doped with BaZrO₃ up to 50 wt.%, obtained by solid-state reaction powder technology. From DC magnetization loops and low frequency AC susceptibility measurements we determined the influence of the BaZrO₃ doping level on the critical temperature, critical current density, field for full penetration, and intergrain lower critical field. The results show that even high content of BaZrO₃ does not lead to degradation of the superconducting properties of bulk YBa₂Cu₃O_{7 – x} .     

Preparation of epitaxial YBa2Cu3O7−x films by magnetron sputtering

Thin films of highT _c superconductor YBa₂Cu₃O_7−x were obtained by magnetron sputtering. MgO, YSZ, YSH and Al₂O₃ single crystals were used as substrates. Epitaxial films with tetragonal structure havingT _c 55–60 K grow at substrate temperaturesT _s between 930 K and 980 K. Orientation of the films in thisT _s range was (100) and (001) for (100) MgO substrate, (111) and (001) for (1012) Al₂O₃ and (111) YSH and (113) or (103) on (110) YSZ and (111) YSH. Single crystalline films with orthorhombic structure and (001) orientation were grown on all the substrates whenT _s exceeded 980 K. They haveT _c>80 K.     

1/f Noise properties of swift heavy ion irradiated epitaxial thin films of YBCO

Effect of 250 MeV¹⁰⁷Ag ion irradiation induced columnar defects on the noise properties of the YBCO superconductor in the normal and superconducting state have been investigated. Magnitude of the spectral density of the noise is found to scale inversely with the frequency and exhibit a quadratic dependence on the bias current confirming that the noise arises due to the resistance fluctuations. The magnitude ofS _v has been found to decrease with decrease in temperature and shows a noise peak in the transition region. The noise performance of these materials in the vicinity of the superconducting transition as well as in the normal state is found to improve by an order of magnitude after irradiation with 250 MeV¹⁰⁷Ag ions. The decrease in the magnitude of 1/f noise peak is due the irradiation induced enhanced flux pinning of the material which suppresses the flux motion induced noise in the vicinity ofT _c.     

Microwave Properties of In Situ Grown MgB2 Superconducting Thin Films

The electrodynamic response at 20 GHz of c-axis oriented MgB₂ superconducting thin films is reported. Mg-rich Mg-B precursor samples were grown on MgO and Al₂O₃ single crystal substrates by a d.c. planar magnetron sputtering technique, and subsequently annealed in situ at 800°C for 10 min in a In-sealed Nb box in the presence of saturated Mg vapor. The films were characterized by a variety of structural and electronic techniques including XRD, EDS, STM-AFM analyses, and transport measurements. The dependence of the surface impedance from temperature and radiofrequency (r.f.) field amplitude was measured via a dielectric resonator technique. Temperature data clearly confirm the s-wave nature of the newly discovered superconductor, even if the value of the energy gap is smaller than BCS prediction. An effective two-band model can be applied to quantitatively explain the experimental results. In spite of previous reports claiming the absence of weak link behavior in MgB₂, the power dependence show that granularity governs the performance of these films in the microwave region.