We report measurements of an apparent magnetic-field-dependent absorption (imaginary part of the a.c. magnetic susceptibility) in superconducting Y1Ba2Cu3O7 ceramics and crystals. The absorption, which is observed over a wide range of frequencies but only when the material is below the superconducting transition temperature, is characterized by a narrow (∼ 30 Gauss FWHM at 6 MHz) peak and a wide (> 10 kG) feature, both of which are maximum at zero magnetic field. The absorption strength varies approximately as one over the square root of the frequency. The unusual magnetic-field-dependent peaks in the magnetic susceptibility are inherent in single grains and therefore do not originate from intergrain Josephson currents or multigrain (i.e., percolative) loops. The susceptibility peaks must be due to bulk behavior, interactons at grain surfaces, intragrain current loops, or intra-grain Josephson Junctions.

Microwave absorption measurements from 20 to 80 K in magnetic fields up to 12 kG are reported. Below a certain characteristic temperature T* = 80 ± 2 K < Tc the absorption in magnetic-field-cooled samples is smaller and broader in comparison to the zero-field-cooled samples. The incident microwave radiation induces a dc voltage across the sample which is also magnetic field dependent and peaks at zero magnetic field.

Giaever tunneling measurements have been applied to investigate the electronic structures of the high Tc superconductor YB2Cu3Oy near Tc. The superconducting energy gap of 17 meV was observed in V-dl/dv curve at 77 K. In the light of BCS model, 2Δ0Δ/kBTc was estimated as 3.3. Several dip structures were observed in the bias region up to 0.5 eV in V-dl/dV curve, using the 3rd derivative analysis. Through the temperature dependence, these structures were found to be associated with the superconductivity.

The discovery of high temperature superconductivity in the La-Ba-Cu-O system [1] has stimulated extensive study of these and related materials. A number of band structure calculations [2,3] have been performed for these materials to explain their electronic structure, and models invoking electron-phonon, electron-electron, or excitonic mechanisms have been proposed for the unusually high Tc [4–9]. Several optical studies were performed[10–12] on sintered pellets or single crystals in an attempt to reveal the electronic structure. It is important to understand the electronic structure of the parent La2CuO4 compound and the defect states that are introduced into the system upon addition or removal of electrons- accomplished by doping or photoexcitation-as they are critical in understanding the mechanism for superconductivity.

The temperature dependence of the electrical conductivity measured in the a-b basal plane of YB2Cu3O7 crystals displays a weak, nearly logarithmic divergence near Tc, which is weaker than the power-law functions predicted by superconducting fluctuations in 1, 2, or 3 dimensions. On the other hand, we find that quasi-particle fluctuations and pair-breaking in two-dimensions as given by Maki and Thompson, where the pair-breaking parameter δ is assumed to be proportional to temperature, gives by itself an excellent fit to the data.

The high-T oxide-superconductor YBa2Cu3O7-δ is expected to be a low carrier-density material having optical ana nonequilibrium properties. Light can penetrate the superconductor and can effectively excite the quasi-particles in it. To clarify the nonequilibrium phenomenon, we have studied this oxide superconductor exposed to light.

Current-voltage characteristics and dV/dI curves of a YBCO/Nb bulk junction at 4.2 K are reported. The presence of the Josephosn effect is clearly shown. The dV/dI spectrum shows sharp structures which can be related to the Niobium and the YBCO energy gaps Δnb and ΔNb and ΔYBCO. A value ΔYBCO = 26.2 ± 1.0 meV can be estimated, confirming the strong coupling character of such a compound.

Recent developments in superconductivity have taken 77 K superconducting electronics from a dream to a likelihood. Rather than following the conventional path by developing Josephson junction-based devices, this paper discusses the unique possibilities of hybrid superconductor/semiconductor devices. The two devices discussed are a true three-terminal hybrid resonant tunneling transistor and the semiconductor-coupled Josephson junction. Also, a list is given of as yet uninvestigated issues concerning the new superconductors and their proximity effects with semiconductors -issues that are critical to the operation of these hybrid superconductor/semiconductor devices.

The resistivity measurements of bulk super-conducting oxides La2−xSrxCuO4−x/2+ 〈0.15〈x〈0.2) irradiated by 2.5 Mev-electrons and YBa2Cu3O7-δ 〈0〈δ〈0.15) irradiated by 2.9 GeU-krypton ions have shown a regular and constant decrease of the critical temperature Tc as a function of the fluence.

On the contrary, an increase of Tc has been observed durino irradiation of super-conduct ino La2CuO4 compound by 2.9 GeV-krypton ions, which saturates at Tc/Tco = 0.1 for fluences hioher than 3.1012 Kr.cm−. The evolution of the R(T) curves during irradiation is very similar to the one observed under Dressure.

A comparison of the variations of Tc as a function of the number of di solacements oer atom for various particles (n. e−. He+. O. Kr) suggests that the hijjh electronic stoppino power of the heavy ions could be the main factor of damage.

Magnetisation curves have been measured on powders of various sizes. The hysteresis decreases with size for articles smaller than 20μ showing that barriers to current flow are at least this far apart. The current density is 6×106 amps/cm2 at 1.5T and this is consistent with theoretical calculations of Jc. Inductive transitions are consistent with a penetration depth of about 0.5μ at 78K. The results of Hot Isostatic Pressing are also discussed.

An YBa2Cu3Ox bar specimen was air annealed at 930°C followed by water quenching under protection of a quartz sheath, then annealed at a series of temperatures between 300° -500°C. Isothermal dilatation curves were measured at each annealing temperature. An Avrami type plot of these data gave rise to an activation energy H=1.2eV for the tetragonal-to-orthorhombic reaction. It is suggested that this is the activation enthalpy of oxygen diffusion in the tetragonal phase which remains continuous and therefore rate controlling throughout the whole course of transition. Based upon this transition process, we expect a fully annealed specimen to be composed of numerous superconducting domains separated by a continuous film of semiconducting tetragonal phase, one or two molecular layers thick. At the points of contact, this film is highly strained by the impinging domains, rendering the material superconductive by S/N/S junctions.

The La-Ba-Cu-O system contains several phases of special interest in the field of high Tc superconductivity. Specifically intriguing is the La3−xBa3+xCu6O4+y (“3–3–6”) phase [1,2], the La1·8 5Ba·1 5CuO4 (‘K2NiF4’) phase with a zero resistance temperature (Tc) of about 20 K [3,4], and the La1Ba2CU3O9–6 (“1–2–3”) phase with Tc's reported up to 75 K [5–7]. It has been suggested that the “3–3–6” phase may also be superconducting, especially in a Ba-doped version such that the stoichiometry is 2–4–6 [8–10]. In addition to defining a new high temperature superconducting phase, this latter suggestion would also rule out the possibility of Cu-0 “chains” being important to 90 K superconductivity since these chains are absent in the proposed configuration [1,2] of this phase.

Superconducting compounds of Y1B2C3−xFexOy, with x=0.125, 0.0625, 0.125 and 0.300, were studied using X-ray powder diffractometry. Two sets of samples were prepared. Samples quenched from 950°C to liquid nitrogen temperature showed a tetragonal non-superconducting phase, whose lattice parameters depend on the Fe content and differ from those of the yttrium 1:2:3 tetragonal phase. Samples oxygenated at 450°C showed an orthorhombic to tetragonal phase transition for Fe contents higher than x=0.0625. The lattice parameters of this tetragonal phase were different from those of both tetragonal phases mentioned above. All these observations allow us to conclude that C(l) sites are occupied at least partially by Fe impurities.

The reactivity of sintered samples of YBa2Cu3O7-y with moisture at elevated temperatures has recently been reported [1]. The reaction sequence proposed was:

Such instability of the 1–2–3 phase has serious implications regarding not only the use of the sintered material, but powder processing as well. While the need for fine particle 1–2–3 powder exhibiting good sinterability is recognized, it is also anticipated that the degradation of this material will accelerate with decreasing particle size because of the larger surface area. Thus, we have conducted a series of experiments to monitor the effect of ambient atmosphere on fine particle 1–2–3 powder. The powder was monitored as a function of time by x-ray diffraction, scanning electron microscopy, sintering characteristics and magnetic flux exclusion.

The oxygen-deficient orthorhombic oxide YBa2Cu3O7-δ has been the center of intense recent interest because of its high Tc superconducting proper-ties[l-3]. Recently, the structurally related La3Ba3Cu3O15-δ, has received increasing attention[4–9]. The pure Y analog[10,11] cannot be synthesized by usual solid state reaction routes probably because of its metastability near 880°C. Here we report on the successful synthesis of pure tetragonal Y3,Ba3CU6O15-δ (referred to as Y 3–3–6), via the firing of an atomically mixed citrate precursor at a relatively low temperature. X-ray diffraction data characterize Y 3–3–6 to be isostructural with the corresponding La compound[4]. The unit cell composition can then be written as: Y(Ba2−x Yx) CU3O7+δ with the parent compound at x = 0.50. Specimens of Y 3–3–6 at × = 0.50 and 0.375 annealed at 650°C under 1 atmosphere of O2 are non-superconducting. Higher pressure O2 annealing and fluorine-doping leads to a 1–5% superconducting volume fraction with onset at 85K. Annealing near 880°C creates a 10% superconducting fraction which shows near-zero resistivity at 62K and a reproducible, small resistive transition near 260K.

After the discovery of YBa2Cu3O7-y with an unusually high superconducting transition temperature (Tc), around 95 K [1], similar compounds were made by completely replacing Y by the other rare-earth elements. It was found that RBa2Cu3O7-y (RBCO; R = rare-earth) possessed the same orthorhombically distorted layered perovskite structure and virtually the same Tc for almost all R [2–8]. The exceptions are compounds with R = Ce, Pr and Tb. The stable tetravalent state of these three R elements, as contrasted with the stable trivalent state of the other R elements, may be the reason behind this problem [9]. But there are also problems in producing single-phase, high Tc samples with La, Yb and Lu substitutes [10–12]. One of the reasons for this may be the size of these R ions: the difference between their radii and that of Y3+ is comparably large. Here we present the results of our systematic study of the structure and transport properties of the series of compounds La1−xYbxBCO (0.05 ≤ × ≤ 0.95).

Thermal cycling for four cycles between 294 and 78°K has been observed to shift the superconducting transition temperature from 85 to 28°K. Samples cycled for 12 cycles between 373 and 78°K do not display superconductivity at temperatures down to 37°K. Guinier de Wolff X-ray diffraction observations of these samples reveal no crystallography changes during thermal cycling up to 12 cycles. Scanning electron microscopy studies of polished samples of the thermally cycled material revealed the presence of cracks in individual grains within each sample. The percentage of grains displaying cracks climbs rapidly with thermal cycling. We conclude that the loss of superconducting behavior is a consequence of removal of continuous paths but we are not, at this time, able to rationalize the shift of Tc with thermal cycling. We conclude that the thermal expansion anisotropy produces strains which lead to destructive fractures within the material.

Experimental high resolution electron micrographs and computer simulation experiments have been used to evaluate the visibility of the atomic constituents of YBa2Cu3O7−x. In practice, the detection of oxygen has not been possible in contradiction to that predicted by modelling of perfect crystalline material. Preliminary computer experiments of the electron diffraction patterns when oxygen vacancies are introduced on the Cu-O sheets separating Ba layers show the diffuse streaks characteristic of short range ordering.

We have studied the phase transition of the oxygen sublattice in the YBa2Cu3O7−x system by Differential Scanning Calorimetry (DSC), Guinier Camera, and high temperature x-ray diffraction (ht-xrd) techniques. The transition was examined under different thermodynamic constraints (constant oxygen partial pressure and constant oxygen concentration) and for a range of oxygen partial pressures. The variation of the endothermic peak temperature with DSC scanning rate was used to deduce an activation energy for the transition. The average activation energy for the process is between 38.7–75.0 kcal/mole(l:2:3) [mole], depending on the thermodynamic constraint imposed on the system.

Pressure versus concentration isotherms (P(C)) were used to determine the chemical potential of the oxygen in this system.

The microstructure of Y-Ba-Cu-O compound, sintered at 950°C for 16h, was examined by using transmission electron microscope(TEM). For the furnace cooling sample, two variants of–orthorhombic YBa2-Cu3O7−x (O-Y123) twins, grown on (110) and (110) planes, respectively, are crystallographically related to the matrix and produce a Widmanstatten morphology. The orientation relationship between the lath twin and the matrix is (001)T//(001) and [100]T//[010]M. The O-Y123 compound is unstable under tne electron irradition. Twin boundaries are identified as S-boundaries. The minor phases as Y2BaCuO5 (Y211) and BaCuO2 were also observed. For the air cooling sample, no superconductivity above 77K was found. The major phase is tetragonal Y123 (T-Y123), while small amount of 0-Y123 is also existent.