Elasticity studies of aluminium-substituted highT c superconducting materials

Longitudinal and shear wave velocities of Al-substituted Bi-Sr-Ca-Cu-O superconducting materials prepared by the ceramic technique were measured by the pulse transmission technique. Using the data, several elasticity parameters like Young’s modulus, rigidity modulus, Poisson’s ratio, Debye temperature etc were evaluated. Variation of elasticity parameters with composition has been explained in terms of binding forces between various atoms of the superconducting materials. Temperature variation of longitudinal wave velocity has also been measured and attempts have been made to explain the anomalies at 200 K.     

Process parameters optimization for laser deposition of highT c superconducting thin films on Si and other substrate materials

Thin films of Y-Ba-Cu-O superconductor have been deposited on different substrates by pulsed excimer laser ablation from a superconducting pellet. The dependence of various process parameters such as substrate temperature, laser energy density, oxygen partial pressure, applied bias field and cooling rates on the quality of the films has been studied.     

Electrochemical synthesis of Tl-based highT c single-phase superconductor thin films

Co-deposition of the Tl-based superconductors using electrodeposition technique was successfully carried out. Different processing parameters such as deposition potential, current density, deposition period etc were studied. The Tl-based alloyed thin films were deposited at a constant potential of −1·25V with respect to SCE electrode onto silver substrates and oxidized at 850°C in oxygen atmosphere. The samples thus prepared showed superconducting behaviour below 122·5 K and the critical current density was 1·5×10³A/cm². Electrochemical synthesis of Tl-based highT _c single-phase superconductor thin films is also reported.     

Properties of Bi-Sr-Ca-Cu-O highT c superconductor

The effect of sintering in air and annealing in oxygen flow at various temperatures and times on the superconducting transition of Bi₂Sr₂Ca₁Cu₂O₈ ceramic was studied. Samples sintered at 1138 K for 24 h in air and slowly-cooled to room temperature are single-phase with the highest zero resistivity temperature of 88 K. XRD pattern showed that the single-phase was characteristic of tetragonal structure witha=0.5398 nm andc=3.0620 nm. Both a.c. susceptibility and resistivity measurements give similar values for the onset (100 K) of superconducting transition.     

Slip processing of highT c superconductors

Slip-processing technique has been used to fabricate tapes and alumina-supported films of superconducting YBa₂Cu₃O_7−x . Good densification and connectivity are revealed by scanning electron microscopy. Both the tape and film show superconductivity well above the liquid nitrogen temperature with a transition range of 3° and 5°K respectively.     

Photoemission from the highT c superconductors

The electronic structures of Y Ba₂Cu₃O_x and YBa₂Cu₄O₈ were studied by angleresolved photoemission with an energy resolution of better than 10 meV. A detailed study in the 1st and 2nd Brillouin zone of untwinned single crystals shows the presence of extended saddle point singularities in the band structure in the vicinity of the Fermi energy, which give rise to van Hove singularities in the density of states. The binding energy of the singularity depends on the doping. We show that this extended saddle point singularity leads to a density of states which diverges as ^–1/2, which in turn can lead to very high transition temperatures even in a BCS scheme with weak coupling.This work was supported by DOE contract W-31-109-ENG-38 and NSF grants DMR 8914120 and DMR8809854. The Synchrotron Radiation Center is supported by NSF grant DMR 8601349.     

Growth of highT c Bi-Sr-Ca-Cu-O thick films

Thick films of Bi-Sr-Ca-Cu-O were deposited on (100) MgO substrates by screen-printing technique with the starting composition 1112. To attain the superconducting state, the films were subjected to two-step heat-treatment. R-T and XRD have been studied for films annealed at different durations of the second step. InitiallyT _c (R=0) increased from 77 to 103 K as the annealing duration was increased after whichT _c decreased. Kinetics of the growth of highT _c phase is discussed in the light of our results.     

New effects in highT c superconductors

We show that a proper interpretation of the critical field measurements can distinguish spin fluctuation mechanism from others in the context of the pairing hypothesis. We can predict the existence of a new mode in the mixed state of a superconductor.     

Characteristics of laser-induced plasma from highT c superconductor

The spectroscopic analysis of the emission from the plasma produced by irradiating a highT _c superconducting GdBa₂Cu₃O₇ target with a high power Nd:YAG laser beam shows the existence of the bands from different oxides in addition to the lines from neutrals and ions of the constituent elements. The spectral emissions by oxide species in laser-induced plasma show considerable time delays as compared to those from neutral and ionic species. Recombination processes taking place during the cooling of the hot plasma, rather than the plasma expansion velocities, have been found to be responsible for the observed time delays in this case. The decays of emission intensities from various species are found to be non-exponential.     

A microscopic theory of layered highT c superconductors

A generalized Gorkov formalism of superconductivity in a layered system with non-primitive lattice structure is given. This is used to justify theoretically a model of highT _c oxide superconductors proposed recently by one of us (SSJ). A numerical estimation of the critical temperature as a function of the number of superconducting layers is mentioned.     

Dislocation configurations in highT c oxide BiSrCaCuO

A study of dislocation configurations in superconducting oxide BiSrCaCuO has been made by transmission electron microscopy. Evidence was found for the presence of dislocation pairs composed of two parallel single dislocations holding together with the same Burgers vectors. The pair can further dissociate into partials, giving rise to a four-fold ribbon. Networks consisting of dislocation pairs were also observed. These phenomena are attributed to the possible ordering of strontium and calcium ions and the existence of some oxygen sheet of the Aurivillius type in the oxide crystal. Schemes to illustrate the configurations are suggested and discussed.     

Cathodoluminescence microscopy of high T c superconducting YBCO and BSCCO materials

Spectroscopic and imaging cathodoluminescence (CL) microscopy has been used to study high T _c YBCO and BSCCO superconducting thin films and pellets in the scanning electron microscope. The effects of beam parameters, such as voltage, current, and diameter, have been investigated with a view to optimizing the CL signal intensity whilst preventing sample damage. Limiting the CL signal generation volume to within the thin film is important in eliminating any substrate contribution. Areas of strong luminescence have been observed in YBCO and BSCCO pellets as well as BSCCO thin films. At low beam energies, there is some correlation between the CL and secondary electron images. The CL spectra of the strongly luminescent spots differed from those of the poorly luminescencing superconducting phases. CL was able to identify a copper-rich impurity phase in the BSCCO material, and a barium cuprate phase in the YBCO material. It is proposed that the quality of a thin film, with respect to impurities, can be monitored using CL.     

High resolutionV-I characteristics measurements on highT c superconductors

V-I characteristics of sintered superconducting pellets of YBa₂Cu₃O_7−δ was measured with a resolution one order of magnitude better than the usual dc techniques employing nanovoltmeters. For this purpose software-based lock-in-amplifier technique was developed and used. A square-wave excitation current was used. The voltage signal was digitized into a time series and Fourier-analysed on a computer in this technique. Our results show a very small frequency-dependent resistance in some samples which are nominally superconducting. These results help in checking the material quality and defining the critical current densities better.     

Weakly coupled highT c superconductors—An overview

A short review of the main achievements in the development of high-T _c Josephson junction investigation is given. The six years of history of high-T _c superconductivity have produced in this field a variety of interesting results providing exciting perspectives and, at the same time, raising challenging questions. We shall attempt here to give a brief sketch of this topic, suitable for a nonspecialized audience, which, although oversimplified as well as incomplete, could supply some ideas about where we stand and where we are headed in this field.     

Possibility of superconductivity with highT c in La-Sr-Nb-O system

We prepared La-Sr-Nb-O system samples both in bulk form and as a film, with sharp resistive transitions at temperatures of 228–323 K, depending on the annealing conditions. The sharp resistive transitions might be attributable to superconductivity. Magnetic ordering was observed over a narrow temperature range around 40 K for some of the samples.     

Relative thermodynamic stabilities of constituent oxides of highT c superconductors

A thermodynamic scale of the relative stabilities of constituent oxides can help the choice of stoichiometric multi-component compositions and can provide guidance for judicious selection of heat-treatment conditions for high-T _c superconductors. A thermodynamic analysis was undertaken to study Y₂O₃-BaO-CuO, BaO-K₂O-Bi₂O₃ and La₂O₃-SrO-NiO systems. The relative stability of the oxides was expressed in terms of two ratios,S _v andS _p computed using free-energy of formation and vapour pressure data. CuO, Bi₂O₃ and NiO were taken as reference oxides for YBa₂Cu₃O_7\t-\gd, Ba_0·6K_0·4BiO₃ and La_1·9Sr_0·1NiO₄ respectively. Thermodynamically, the reference oxide was found to be the least stable amongst the constituent oxides in each of these systems.     

Bulk Josephson energies in multi-gap models of highT c superconductors

Multi-gap models of high temperature ceramic superconductors have been of considerable recent interest. For the case of a two-gap model (Δ₁, and Δ₂), there exist two separate electron pair condensate reservoirs, and a bulk Josephson free energy can be associated with electron pair transitions from one reservoir to the other. Thus, a “Josephson effect” can exist in two-gap models without the requirement of a weak link separating the two electron pair reservoirs in space. A single bulk sample Josephson electron pair free energy is here derived from the conventional BCS model of the superconducting state.