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The confirmation of high-temperature superconductivity in a copper-oxide perovskites discovered by Georg Bednorz and Alex Muller was announced to the public last January 1997. The discovery was presented to the annual American Physical Society's March meeting wherein the session devoted to superconductivity was called the Woodstock of physics. The emergence of practical applications is due in the next two years yet the mechanism of high-temperature superconductivity is still unresolved.

A high-performance antenna beam forming network (BFN) for communications systems was developed using high-temperature superconductor (HTS) technology. A cascade of 3dB branch-line couplers was used for multiple power divider Ku-band design and measurements. The resulting HTS BFN features low power loss and is small and lighter as compared to conventional BFNs.

The complex phase diagram of high-critical temperature ([T.sub.c]) superconductors can be deduced from an SO(5) symmetry principle that unifies antiferromagnetism and d-wave superconductivity. The approximate SO(5) symmetry has been derived from the microscopic Hamiltonian, and it becomes exact under renormalization group flow toward a bicritical point. This symmetry enables the construction of a SO(5) quantum nonlinear model that describes the phase diagram and the effective low-energy dynamics of the system. This model naturally explains the basic phenomenology of the high-[T.sub.c] superconductors from the insulating to the underdoped and the optimally doped region., The high-[T.sub.c] superconductors are among the most complex systems studied in condensed-matter physics. Anderson (1) and Zhang and Rice (2) argued rather successfully that a good starting point for modeling [...]

The results of magnetization and thermal expansion measurements on samples of the copper oxide superconductor La(sub 2-x)Sr(sub x)CuO(sub 4) are discussed. These characterize the oxygen-isotope effects on the carrier density and on the in-plane penetration depth. Results also showed that polaronic charge carriers exist and condense into Cooper pairs in high-temperature superconductors. It was also confirmed that a strong Jahn-Teller effect can result in the formation of these polarons.

A termination for 77kV-class high Tc superconducting power cables was designed and fabricated. The termination is insulated with SF6 gas, nitrogen gas and liquid nitrogen and is cooled with liquid nitrogen. The current lead has a cross sectional area altered along its axis in order to decrease the heat inlet and to prevent SF6 gas from liquefying. The current lead is insulated with Ethylene-propylene Rubber (EPR) because it passes through nitrogen gas which has low dielectric strength in comparison with liquid nitrogen and SF6 gas. 200A AC current and 70kV AC voltage to ground were successfully applied simultaneously for 40 hours in total. The measured heat loss of the termination, 19W, is in good agreement with the calculated value. As the next step, the applied voltage was increased and the termination was confirmed to withstand the voltage up to 120kV AC and 400kV Imp to ground.

In this study we examine magnetic hysteresis losses in Bi-2223 bulk superconductors. Two kinds of hysteresis losses were found in the samples: losses in superconducting grains and losses due to the intergranular weak link structure. Total losses at 77 K were about an order of magnitude higher than losses in grains. This indicates the domination of intergranular hysteresis losses with respect to intragrain losses. To understand the experimental results, we numerically simulated the behavior with model specimens, where different kinds of Josephson junction arrays worked as the models for the intergrain system. A special attention was paid to the influence of porosity, second phase particles, and magnetically induced granularity on the hysteresis losses. If at least one of the inhomogeneity sources was present, the magnetization of the array could exhibit remarkable hysteresis and losses.

As an extension of the Melt Cast Process (MCP) the centrifugal casting technique is a flexible method for manufacturing [Bi.sub.2][Sr.sub.2]Ca[Cu.sub.2][O.sub.x] bulk parts. It provides large scale rings and tubes, needed for high current electrotechnical applications as current leads and inductive fault current limiters. Low resistance contacts are integrated right in the fabrication process. Intrinsic (self field free) current densities up to 4 kA/[cm.sup.2] (T = 77 K) could be determined on small samples. Tubes with a diameter up to 70mm and a length of 200mm were investigated with respect to their DC and AC current carrying capability (I = 7500 A, DC, 4[[micro]volts]/cm and [L.sub.eff] = 8300 A, AC, T = 77 K). This current essentially is limited by selffield effects as is shown in a numerical simulation. The quenching behaviour is related to the shape and temperature dependence of the U(I)-characteristics. First applications, i.e. hybrid current leads with integrated MCP bulk parts, are presented.

When a levitated superconductor vibrates, the levitation force has nonlinear relationship among an air gap, amplitude, and frequency, so the usual static analysis for levitation forces is invalid. There are two phenomena of a flux creep and flux flow when the conductor vibrates in a magnetic field. The present article provides an analytical result for the levitation forces of a thick superconducting disc with consideration of the phenomena and effects of flux variations on the critical current. It is significantly difficult to have the analytical result of levitation force by using the exact boundary conditions. This paper presents approximate boundary conditions which give an appropriate result. To validate the proposed boundary conditions, eddy currents in a conductor are first discussed, then the superconductor is discussed. Numerical results for the levitation forces were obtained and compared with the previously published experimental data. The levitation force becomes a restoring force having the nonlinear relationship, so it is difficult to solve vibrations. The present article gives a simplified method for solving nonlinear vibration problems for the levitated conductor. Numerical calculations were carried out for some typical examples.

Using a new proposed model, the electromagnetic behaviour of high-temperature superconductors (HTSs) is discussed. It indicates that the real part of the complex conductivity of HTSs is dependent on the driving frequency, and the simulated results agree well with measured data in literature. Nonlinear surface resistances dependence on an external magnetic field are also simulated and compared with experimental results.

Magnetization of a bulk high [T.sub.c] superconductor (HTSC) is evaluated with the numerical simulation codes. The field-cooled magnetization of the HTSC is analyzed by using the critical state model. The pulse magnetization process is also analyzed by using the flux creep-flow model. The numerical solutions are discussed to clarify the time depending effects in the reported experimental results on the pulse magnetization.

We present a method to calculate the current density distribution, the magnetic field distribution and the inductance matrix of three dimensional superconducting structures by minimizing the free energy. In the case of high-[T.sub.c] superconductors, the kinetic inductance becomes substantial and an accurate calculation of inductances is required. The London penetration depth [Lambda] and the kinetic inductance are included. The results of a program which realizes this method agree well with measurements of high-[T.sub.c] dc-SQUIDs (superconducting quantum interference devices).

To make the transition from the quasi-long-range order in a chain of antiferromagnetically coupled S = 1/2 spins to the true long-range order that occurs in a plane, one can assemble chains to make ladders of increasing width. Surprisingly, this crossover between one and two dimensions is not at all smooth. Ladders with an even number of legs have purely short-range magnetic order and a finite energy gap to all magnetic excitations. Predictions of this ground state have now been verified experimentally. Holes doped into these ladders are predicted to pair and possibly superconduct., The unexpected discovery of high-temperature superconductivity (1) in lightly doped antiferromagnets has sparked renewed interest in low-dimensional quantum magnets. The parent cuprate insulators are now considered the best examples of [...]

A high-resolution scanning superconducting quantum interference device microscopy study of tetragonal single-layer [TI.sub.2][Ba.sub.2][CuO.sub.6+δ] films, deposited on tricrystal [SrTiO.sub.3] substrates, demonstrates the effect of spontaneously generated half flux quanta. This observation shows that in addition to [YBa.sub.2][Cu.sub.3][O.sub.7], the order parameter symmetry in [TI.sub.2][Ba.sub.2][CuO.sub.6+δ] is consistent with that of a [d.sub.[x.sup.2]-[y.sup.2]] pair state. This result also rules out any bilayer or twinning effects and any pairing that is incompatible with the fourfold rotational symmetry as in the [TI.sub.2][Ba.sub.2][CuO.sub.6+δ], superconducting system., An unambiguous determination of the pairing symmetry in copper oxide superconductors is necessary to understand the origin of high-temperature superconductivity. A test of order parameter symmetry requires a definitive measurement [...]

The recently discovered high-temperature superconductors have a number of very unusual properties. Not only is the superconducting transition temperature [T.sub.c] much higher than ever achieved before, but evidence is now growing that they may also be unconventional superconductors. An unconventional superconductor is one which has fundamentally different symmetries from those of other superconductors. Previously unconventional superconductivity was known to occur only in the heavy-fermion systems, such as the compounds [UPt.sub.3] and [UBe.sub.13]. This review examines the basic concepts of symmetry applied to superconductors. I show how conventional (s-wave) and unconventional (p-wave and d-wave) superconductors arise naturally from the spontaneous symmetry breaking which leads to superconductivity. I then discuss the key experiments which can test for unconventional superconductivity in the high [T.sub.c] and heavy-fermion systems. These experiments include measurements of the energy gap, phase-sensitive Josephson junction experiments, and observations of multiple superconducting phases.

Stability and reproducibility in electron beam irradiated high-temperature junctions made of YBa2Cu3O7-pi films are studied. The films are fabricated by laser ablation and are 200 nm thick. Results indicate that junctions can be stabled with a suitable overdamage-anneal protocol. Chip-to-chip reproducibility is not better than 20%, although chip uniformity is very good.

A simple, small, single-layer high-temperature superconductivity superconducting quantum interference device (SQUID) is subjected to non-contacting alternating current electromagnetic non-destructive evaluation (EM NDE) using the induction, or eddy current, technique. EM NDE is analyzed using the finite element method. Results indicate the feasibility of using EM NDE to assess SQUIDs.

A high temperature superconductor BST/YBCO/LaAlO3 multilayer structure is developed using a pulsed laser deposition technique that yields varying film thicknesses for the YBCO and BST thin films. The electrical characteristics of the multilayer structure are presented. The YBCO film shows excellent superconducting qualities while the BST layer shows strong epitaxy with the YBCO film and strained in tension as a result of x-ray diffraction. Results indicate the films' potential for use in tunable microwave components.

BSCCO interplane conduction devices at frequencies of up to 35 GHz are fabricated through iron beam milling and silver epoxy bonding. Radio frequency (RF) coupling to BSCCO single crystals with c-axis bias currents are investigated. Results indicate that RF signals resulted in RF induced steps at low frequencies and suppression of critical current and modified current-voltage curve structure at higher frequencies.

The feasibility of using YBa2Cu3O7-pi superconductor-normal-superconductor Josephson junctions as mixers of 30 THz radiation is investigated. CO2 laser difference frequencies of up to 27 GHz are observed when a third microwave signal is added to the junctions. Results indicate that the direct current (DC) bias dependence of the difference frequency signal is caused by either hot-electron mixing in the junction banks at high DC biases and bolometric Josephson mixing at low DC biases.

Superconductor-normal metal-superconductor edge junctions with excellent current-voltage characteristics, high critical current-resistance and low noise are the basis for much of the recent research on high-transition-temperature (high-Tc) Josephson junctions for electronic applications. However, existing research provides little evidence to support the contention that proximity effect theory can describe the behavior of high-Tc Josephson junctions.

A fabrication process for high-temperature superconductor-normal metal-superconductor edge junctions is developed by using a Josephson junction that can be fabricated and reproduced with a set of target characteristics. The process uses YBa2Cu2.8Co0.2O7-pi as its normal barrier and has YBa2Cu3O7-pi as its semiconductor. Barrier thickness and/or morphology variations appear to significantly contribute to the process' critical current and resistance spreads.

The electrical transport characteristics of superconductor-normal metal-superconductor step-edge YBCO/Au junctions were studied using the resistively shunted junction model. The Au is deposited at 100 degrees and 600 degrees Celsius. Results show that critical currents are similar for the Au at the two deposit sites while the normal resistance at 100 degrees is consistently higher by 25% than the resistance at 600 degrees. The critical current's low magnetic field modulation indicates either a Fraunhofer-like pattern or a double-junction modulation for the devices.

The optical response of superconducting NbN and YBa2Cu3O7-delta thin films to femtosecond visible light pulses at temperatures below the critical level is measured by means of a sampling oscilloscope. The voltage traces are averaged 512 times to improve the signal resolution and obtain reliable values. The measured data show that the photoresponse of the thin films i generally bipolar. This can be attributed to a nonthermal change in the films' kinetic inductance.

The feasibility of using silicon ion implantation techniques in shifting the critical temperature of superconductive YBaCuO films continuously to lower temperatures is evaluated. A new bolometer structure with a locally implanted sensor area and original critical temperature contact pads is used in the experiments. Results show that silicon ion implantation leads to a continuous decrease in the films' critical temperature.

The characterization of high-temperature superconducting wire under conditions that simulate the operating environment of a superconducting magnet was carried out. Specifically, DC transport current measurements were performed on powder-in-tube (PIT) and open-tape samples of Bi-2212 in pulsed magnetic fields of up to 41 T. The results indicated negligible degradation in the open tape in contrast to the PIT samples which performed poorly.

A four-point sliding contact was employed to quantify the longitudinal and transverse components of the surface potential along several centimeters of a silver-clad Bi-2223 tape for 'fixed values of current and external fields.' The instrument, which may be used to construct voltage-current curves, enabled a correlation between microstructure and superconducting performance to be drawn from the presence of 'deliberately included defects.'

The impact of axial strain on the superconducting performance of Bi2Sr2CaCu2O(8+delta) (Bi-2212) wires with a silver matrix were investigated. Specifically, Bi-2212 wires were soldered to brass substrates and bent to induce compressive and tensile strains. Consequently, measurements at magnetic fields of up to 16 T and at 4.2 K were carried out. These demonstrated an irreversible drop in critical currents 'due to fractures in the filaments.'

The production of kilometer lengths of Bi-2223 high-temperature superconducting wires for use in motors, generators, current limiters, transformers and power lines is now possible. As a demonstration, multifilamentary tapes were fabricated using the oxide powder-in-tube process. The wires were also tested with respect to critical current, mechanical strain and environmental durability.

The design, fabrication and performance of monolithic Bi2Sr2CaCu2Ox (Bi-2212) conductors for operation with conduction cooling at zero field and a temperature range between four kelvin and 77 K are described. The designs, which were implemented by way of elastomer processing and composite reaction texturing, include current leads with lengths of 10-20 cm., rated currents of 200-1,000 A, thermal conduction losses below 0.05 W and flexural load capacity of over 10 N.

A generalized approach to the design of gas-cooled current leads made of high-temperature superconductors is introduced. The method, which is derived from modifications to the one-dimensional heat balance equations for copper leads, integrates the parameters of the conductor and cooling channel into a single cooling parameter, the laminar flow of the cooling gas. This is combined with other parameters such as heat flow from the cold parts of the leads, the conductor's geometrical structure and the pressure drop of the cooling gas in a design chart for reference purposes.

The development of high-temperature superconducting (HTS) current leads that will supply power to an HTS coil in a high gradient magnetic separation system is discussed. The current leads consist of a nitrogen heat pipe made of conducting and insulating materials for simultaneous thermal diffusion and electrical isolation between the lead and a thermal heat intercept. It also features yttrium-barium-copper-oxide thick films to minimize heat dissipation at the lower stage.

The operation of high-temperature superconducting magnets in a controlled mode such as magnetic suspension applications is examined via simulations and experiments. Specifically, a block diagram model of the correlations between current, airgap and flux density is derived while the magnitude and phase of impedance, current, flux and voltage variations of a magnet consisting of four BSCCO coils and an iron core were measured.

The low rotational drag and energy losses of several magnetic bearings for flywheel energy storage applications were investigated. Specifically, coefficient of friction of different bearing designs was measured via spin-down experiments in a vacuum chamber. The results showed that a coefficient of friction of 0.0000003 may be obtained at low velocities for a bearing made of an 89-mm.-diameter ring permanent magnet levitated over an array of yttrium-barium-copper-oxide cylinders.

The magnetic shielding of high-temperature superconducting (HTS) current leads is proposed as a means of achieving high permeability and low thermal conductivity between superconducting magnets and HTS current leads. To this end, a structure consisting of Bi-Sr-Ca-Cu pipes and ferrite powder padding was constructed. Upon insertion of HTS leads, the critical current increase and magnetic flux densities of the innermost HTS pipe were measured.

A two-dimensional analytical model and a finite element method for characterizing the thermal stability of high-temperature superconducting tapes designed for use at 4.2 K are presented. The former is limited to certain operating modes only because it was adapted from a stability model for low-temperature superconductors. The latter, however, solves the heat conduction equation in each region of an Ag/BSCCO superconducting tape.

The thermal stability of high-temperature superconductors was studied. Specifically, two experiments to measure the normal zone propagation in a three-layer pancake coil and temperature distributions in a conductor undergoing quenching were carried out, respectively. The experiments, which used silver-sheathed BiPbSrCaCuO (2223) tape superconductors cooled by a cryocooler, were conducted at zero magnetic field at temperatures between 20 K and 50 K.

The stability of a high-temperature superconducting (HTS) magnet was investigated. Specifically, the coil, which was built by American Superconductor Corp., was tested for stability and normal zone propagation in background fields up to 2.5 T at temperatures between 4.2 K to 77 K. According to results, the Bi-2223 solenoidal magnet generates a central field of 1.1 T at 4.2 K. Its HTS superconductors' thermal stability margin over the entire range is more than 150 J/cm.

The status of high-temperature superconductivity research is examined. At present, the field faces several challenges to the development of new material and practical applications. These pertain to the s- vs. d-wave pairing controversy as well as superconductors' anisotropic and limited ampacity due to the irreversibility line. Other challenges involve the fabrication of Josephson junctions and superconductor-metal-superconductor proximity effect junctions.

The US Defense Dept.'s High Temperature Superconductivity Space Experiment (HTSSE) program is committed to developing feasible and cost-effective space applications for high temperature superconductors. Although HTSSE I failed, plans are afoot for the launch of HTSSE II aboard the Advanced Research and Global Observation Satellite in late 1995. Like HTSSE I, the subsystems contained in HTSSE II will exploit the reduced weight, size and increased power benefits of HTS elements.

Electromagnetic calculation of propagation parameters for high Tc superconductive coplanar and microstrip transmission lines is presented. Calculations are performed by a nodal circuit simulator (SPICE), with possibility of direct interfacing with CAE standards. The results are in good agreement with data obtained with a full-wave analysis and CPU time is much smaller than in standard MW applications. Then, current density distributions are shown.

Detailed data on the momentum-resolved temperature dependence of the superconducting gap of [Bi.sub.2][Sr.sub.2][CaCu.sub.2][O.sub.8+X] are presented, complemented by similar data on the intensity of the photoemission superconducting condensate spectral area. The gap anisotropy between the γ-M and γ-X directions increases markedly with increasing temperature, contrary to what happens for conventional anisotropic-gap superconductors, such as lead. Specifically, the size of the superconducting gap along the γ-X direction decreases to values indistinguishable from zero at temperatures for which the gap retains virtually full value along the γ-M direction. These data rule out the simplest type of d-wave order parameter., An 'order parameter' describes the type of phase transition that occurs in a material system. The order parameter of high-temperature superconductors is of extreme current interest and has been investigated [...]

Over the last several years there have been great improvements in the energy resolution and detection efficiency of angle-resolved photoemission spectroscopy. These improvements have made it possible to discover a number of fascinating features in the electronic structure of the high transition temperature ([T.sub.c]) superconductors: apparently bandlike Fermi surfaces, flat-band saddle points, and nested Fermi surface sections. Recent work suggests that these features, previously thought explainable only by one-electron band theory, may be better understood with a many-body approach. Furthermore, other properties of the high-[T.sub.c] superconductors, which are difficult to understand with band theory, are well described using a many-body picture. Angle-resolved photoemission spectroscopy has also been used to investigate the nature of the superconducting pairing state, revealing an anisotropic gap consistent with a d-wave order parameter and fueling the current debate over s-wave versus d-wave superconductivity., The excitement following the discovery of high-temperature superconductivity (1, 2) was shared between those who saw an opportunity to finally take advantage of the unique properties of a superconductor at [...]

The fundamentals of high-temperature superconductivity (HTS), as they apply to planar microwave filters, are briefly outlined. It is seen that some problems which are not usually prominent in the design of planar microwave filters come to the fore in the HTS case, and some of these problems are discussed. These problems include: preventing fields from impinging on normal conductor housings or radiating, reducing forward-coupling effects in order to permit the use of smaller substrates, coupling between the desired circuit mode and undesired housing waveguide-type modes, and estimating the maximum power that an HTS filter can handle without significant nonlinear effects.