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The review discusses extensively the phase-sensitive methods supporting d-wave pairing symmetry to explain and clarify the controversies surrounding pairing symmetry in the cuprates. The d-wave theory embodies phase-sensitive methods and half-integer flux-quantum effect studies.

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 [...]

Constraints on the pairing symmetry, a superconducting order parameter, is obtained by the use of the half-integer flux quantization in superconducting rings having three grain-boundary Josephson junctions. The constraints reduce the various ways in which the Cooper pairs are formed in the high temperature superconductor YBa2Cu3O(7-delta). This parameter depends on the direction of the crystal phase isotropy.

Knowledge of the symmentry of the order parameter to describe electron pairing in the superconducting state is important to the development of theoretical models for high temperature superconductivity. An experiment is described studying the single-layer tetragonal superconductor TI sub 2 Ba sub 2 Cu0 sub 6 + delta. The results showed that the material has pure dx squared-y squared pairing symmetry, providing further understanding of the more complicated mixed s + d state appearing in other high temperature superconductors.

Experimental studies reveal that high temperatures support large in-plane current densities in high-quality c-axis-oriented epitaxial films of the superconductor, HgBa2CaCu2O(6 + delta). The critical current density of this superconductor at 110 K is significantly higher than that of the Bi- or Tl-based superconductors. Intrinsic pinning in this superconductor accounts for the high critical current density.

The superconducting transport characteristics of [HgBA.sub.2][CaCu.sub.2][O.sub.6+δ (Hg-1212) grain-boundary junctions grown on (100)-oriented [SrTiO.sub.3] bicrystal substrates have been investigated. The films exhibit a zero-resistance temperature of 120 kelvin and sustain large critical current densities, with values as high as 106 amperes per square centimeter at around 1 00 kelvin. On the other hand, the grain boundaries behave as weak links, with substantially lower critical currents, as is observed for other cuprate superconductors. A reduction of three orders of magnitude in critical current was observed for transport across a 36.8° grain boundary. The current-voltage characteristics of bridges across such a grain boundary show weak-link behavior qualitatively resembling that of a resistively shunted junction. Single-level direct-current superconducting quantum interference devices SQUIDs) have been fabricated with such bicrystal junctions. These SQUIDs show clear periodic voltage modulations when subjected to applied magnetic fields. The SQUIDs operate at temperatures as high as 11 1.8 kelvin, which makes them attractive for operation in portable sensors and devices that utilize nonconventional cooling methods., Grain boundaries (GBs) of high-transition temperature ([T.sub.c]) cuprates have been extensively studied in recent years. High-angle GBs in these materials are known to form superconducting weak links. In particular, with [...]

The synthesis of high-quality films of the recently discovered mercury-based cuprate films with high transition temperatures has been plagued by problems such as the air sensitivity of the cuprate precursor and the volatility of Hg and HgO. These processing difficulties have been circumvented by a technique of atomic-scale mixing of the HgO and cuprate precursors, use of a protective cap layer, and annealing in an appropriate Hg and [O.sub.2] environment. With this procedure, a zero-resistance transition temperature as high as 124 kelvin in c axis-oriented epitaxial [HgBa.sub.2][CaCu.sub.2][O.sub.6+δfilms has been achieved., The recent discovery of superconductivity in mercury-based cuprates (1, 2) ([HgBa.sub.2][Ca.sub.n-1] [Cu.sub.n][O.sub.2n+2+δ]) has pushed the superconducting transition temperature ([T.sub.c]) to values higher than 130 K. More recently, the [T.sub.c] of [...]

Spatially Resolved Observation of Supercurrents Across Grain Boundaries in YBaCuO Films A DETAILED UNDERSTANDING OF THE electronic properties of the grain boundaries in superconducting [YBa.sub.2.Cu.sub.3.O.sub.7] is important both for fundamental [...]

Focuses on a study on the temperature dependence of the half-integer magnetic flux quantum effect in thin-film tricrystal samples of a high-critical-temperature cuprate super-conductor YBa2Cu3O7-delta. Methods; Results; Conclusions.

Demonstrates a design for a scanning superconducting quantum interference device (SQUID) microscope in which the sample temperature can be varied. Magnetic imaging of samples at varying temperatures with micron-scale spatial resolution.