Table of contents

Measurements of the temperature dependence of the surface resistance at 3 GHz of 100 micron size fragments of MgB₂ separated powder are presented and discussed. The microwave surface resistance data are compared to experimental results of Nb, Bi₂Sr₂CaCu₂O_8+δ (BSCCO) and theoretical predictions of s-wave weak coupling electron-phonon theory (BCS).

Fine grained polycrystalline samples of MgB₂ superconductor were synthesized from the elements to contain < 5 % of impurity phases, according to x-ray powder diffractometry. The superconductive transition was sharp with a midpoint T_c = 38.5 K. The magnetization in the vortex state was studied as a function of applied field H, temperature T and time t. From the equilibrium magnetization, the London penetration depth was obtained. The supercurrent density J(T,H,t) in the vortex state (derived from the irreversible magnetization) decreases approximately linearly with T, in contrast to the quasi-exponential fall-off in high-T_c superconductors. The current is highly stable in time, with normalized decay rates S = - d ln(J)/d ln(t) well below those in high-T_c materials. These results are compared with those of other superconductors.

The formation of bound states at surfaces of materials with an energy gap in the bulk electron spectrum is a well known physical phenomenon. At superconductor surfaces, quasiparticles with energies inside the superconducting gap Δ may be trapped in bound states in quantum wells, formed by total reflection against the vacuum and total Andreev reflection against the superconductor. Since an electron reflects as a hole and sends a Cooper pair into the superconductor, the surface states give rise to resonant transport of quasiparticle and Cooper pair currents, and may be observed in tunnelling spectra. In superconducting junctions these surface states may hybridize and form bound Andreev states, trapped between the superconducting electrodes. In d-wave superconductors, the order parameter changes sign under 90° rotation and, as a consequence, Andreev reflection may lead to the formation of zero energy quasiparticle bound states, midgap states (MGS). The formation of MGS is a robust feature of d-wave superconductivity and provides a unified framework for many important effects which will be reviewed: large Josephson current, low-temperature anomaly of the critical Josephson current, π-junction behaviour, 0→π junction crossover with temperature, zero-bias conductance peaks, paramagnetic currents, time reversal symmetry breaking, spontaneous interface currents, and resonance features in subgap currents. Taken together these effects, when observed in experiments, provide proof for d-wave superconductivity in the cuprates.

We have prepared high-quality a-axis-oriented superconducting YBa₂Cu₃O_7-δ (YBCO) films using a self-template method on (100) SrTiO₃ (STO) substrates by dc sputtering. X-ray diffraction and transmission electron microscopy analysis showed that the films were grown epitaxially on the STO substrates with the a-axis perpendicular to the surface. The full-width at half-maximum value of 0.081 was measured through the rocking curve of the (200) diffraction peak. The effect of the deposition temperature and the thickness of the template layer and the upper layer on the quality of the films was investigated. We found evidence suggesting that structural strain was present at the interface between the template layer and the upper layer. For the film deposited under optimal conditions, the zero-resistance T_c was 85 K and the transition width was less than 2 K. The anisotropy of normal state resistance and critical current density was also studied.

Gd_1-x-zPr_xCa_zBa₂Cu₃O_7-δ (GdPrCa-123) high-temperature superconductor cuprate (HTSC) samples with 0.0⩽x⩽0.3 and 0.0⩽z⩽0.35 were prepared by standard solid-state reaction and characterized by XRD technique. The magnetoresistance of the samples were measured and analysed. The superconducting transition region broadened by the application of a magnetic field. The resistive transition showed two distinct parts. (1) A steep part near the onset of superconductivity, where the onset transition temperature remained unchanged. For the Gd_0.85Ca_0.15-123 sample, the steep part was more sensitive to the application of magnetic fields. (2) A transition tail part. The experimental data in this region were fitted with an Ambegakor and Halperin (AH) phase-slip model. The AH parameter, γ(H), was used to estimate the critical current density at zero temperature of samples at various magnetic fields. It was observed that the critical current density decreased with Pr substitution and increased with Ca substitution in Gd-123. An optimal value of the Ca doping concentration for increasing the critical current density was determined. We suggest that the Pr ion probably acts to enhance the weak link and the Ca ion acts as a flux pinning centre in the GdPrCa-123 system. Moreover, we observed that the AH parameter, γ, and activation energy, U₀, depend not only on temperature and magnetic field, but also on the Pr and Ca ion concentrations.

Part of this paper has been reported at the MSM-99 conference [26].

The origin of the second peak effect in melt-textured YBa₂Cu₃O_7-δ was investigated using the sweeping rate dependence of magnetic hysteresis loops. An anomalous change of the vortex-line shape was found around the second peak position. This phenomenon was ascribed to the competition between elastic energy E_el and pinning energy E_pin of the flux system. By comparing it with the results of crystal samples such as YBCO, NCCO and Pb-doped BSCCO, we concluded that this characteristic is uniform for these high-T_c superconductors and should be responsible for the widely observed second peak effect in magnetization measurements.

Short samples of BiSCCO-2223/Ag-alloy 55-filament tapes have been studied by transport techniques at 77 K. The nominal critical current I_c of the tapes was 40 A (77 K, self-field). By using multiple current and voltage taps on the sample, it was possible to assess the conductor inhomogeneity on a scale of a few millimetres. A scatter of up to 50% in the values of both the critical current I_c and the voltage-current characteristic power law, α, was observed, but with significant correlation between the two. This suggests that the inhomogeneity is caused mainly by cracks in the filaments, which the current has to bypass via the normal metal of the matrix. The effect on the transport properties of superconducting connections between filaments was studied using a simple numerical model. We have developed an experimental protocol for detection of these interconnections and their assessment. The estimated critical current density of the interconnections is an order of magnitude lower than that of the filaments J_c, and the power law of their V(I) characteristic is low.

The behaviour of the order parameter and its effects in intrinsically-layered superconductors are studied within a continuous Ginzburg-Landau model. A simple model of interlayer coupling is proposed in which we suggest that the coupling strength between superconducting and insulating layers may be qualitatively described by a reciprocal of a difference between the order parameters at the respective layers. The feasibility of such an approach is demonstrated by considering Hussey's magnetization experiments, the case of T_c depression observed in iodine-intercalated and Pb-substituted Bi₂Sr₂CaCu₂O_8+δ, and some other properties and experiments in layered superconductors.

The local current density distribution of grain boundaries in differently processed melt-textured YBa₂Cu₃O_7-δ (YBCO) is investigated by a magneto-optical technique and inversion of Biot-Savart's law. By correlating the local current density with the microstructure revealed by a scanning electron microscope, we are able to distinguish the depression of the critical current density j_c at clean grain boundaries (GBs) from that occurring at normal phases and cracks at and in the vicinity of GBs. The GBs are formed by different multi-seeding melt-growth processes using a non-stoichiometric precursor which is optimized with respect to the bulk critical current. In order to obtain GBs with extended regions free of normal phases and with large critical current densities the amount of liquid phase and the density of Y₂BaCuO₅ particles has to be controlled. The properties of GBs can be tailored by both controlling the amount of liquid phase and the seed distance. The related changes in the growth and microstructure are analysed and we show how to grow extended GBs almost free of normal phases with optimized critical current densities in multiple-seeded melt-growth YBCO.

Double-barrier Josephson junctions, with outer electrodes of superconducting Nb and a central thin layer of non-superconducting Al, are being developed as self-shunted junctions for high-frequency superconducting integrated circuits. This paper presents a simplified phenomenological model of the behaviour of such junctions (often called SINIS, for superconductor-insulator-normal-insulator-superconductor) based on the resistively shunted junction model, using Josephson coupling between the layers and the time-dependent Ginzburg-Landau equation to describe the dynamics of the weak induced energy gap in the Al layer. This analysis shows how non-hysteretic I-V characteristics can be obtained but suggests some limitations in their high-frequency performance. Predictions for critical current generally agree with prior experiments. The technological implications of this model are discussed.

YBa₂Cu₃O_x (YBCO) thin films on single crystal LaAlO₃ substrates were prepared from a trifluoroacetate (TFA) precursor heat treated in a low oxygen partial pressure argon atmosphere. The purpose of the study was to explore the possibility of forming high-purity, epitaxial YBCO films at heat treatment temperatures and oxygen partial pressures low enough to allow the TFA process to be adapted to metallic substrates. The formation of a superconducting YBCO phase was confirmed by measuring the critical temperature (T_c) inductively. In-plane texture and c-axis alignment of the films were determined from x-ray phi scans and omega scans, respectively. Raman microspectroscopy was used to interrogate grain connectivity, oxygen stoichiometry, c-axis verticality with respect to the substrate, and second phase formation. The optimum heat treatment temperature was found to be near 750 °C in an argon atmosphere, the equivalent oxygen partial pressure, which was about 10^-5 atm. The full-width half-maxima (FWHMs) of the YBCO (113) phi and (005) omega scans for a sample prepared at 750 °C were about 0.6 and 0.5°, respectively, which indicates that the film was epitaxial. This film showed a sharp superconducting transition at 91 K and an inductive critical current density of 1.3 MA cm^-2 at 77 K. The ability to form high-quality YBCO films under the conditions used is consistent with the thermodynamic predictions that the optimal heat treatment temperature decreases as oxygen partial pressure decreases. The results of this study define YBCO formation conditions for TFA-based precursors that may be suitable for application on metallic substrates.

Using a three square well model, we investigate the possibility that in low energy plasmons there could be an additional boson responsible for the attractive force binding the Cooper pairs that lead to superconductivity in layered electron-doped cuprates. The three square well model is, in principle, characterized by three coupling strengths, the electron-phonon (λ_ph), the electron-plasmon (λ_pl) and Coulomb screening parameter (µ*), that estimate the transition temperature (T_c) and oxygen isotope effect coefficient (α). Starting with the three square well model within the framework of the Eliashberg theory, the energy gap kernels allow us to visualize the relative interplay of the Coulomb, electron-phonon and electron-plasmon interactions and we correlate the T_c with these three coupling strengths. For a set of parameters (λ_ph≈1.0, λ_pl≈0.7 and µ*≈0.18), a T_c of 28 K is estimated for optimally doped Nd_1.85Ce_0.15CuO₄. The present approach also explains the reported sizeable oxygen isotope effect in electron-doped cuprates. We suggest that the extended attractive force in the proposed three square well scheme consistently explains the superconductivity in electron-doped cuprates.