countries, demonstrating the importance of a balanced national and international research effort.

There have been many reports of higher transition temperatures. So far these results have been difficult to reproduce, and there is no consensus at present about their validity. Nevertheless, there is widespread expectation that new phases will be discovered, and a frenetic search for them continues in many laboratories worldwide.

Synthesizing and characterizing these new materials have been multidisciplinary enterprises involving contributions from many fields. Unlike most earlier superconductors, the new materials are ceramics, so that many techniques of ceramic processing have been applied to their synthesis. The new superconductors have already been prepared in the form of wires, thin films, and single crystals, in addition to sintered granular material. Yet many challenges remain: the ceramic or polycrystalline material suffers from low critical current densities (the highest current the material can carry before losing its superconductivity), and the single crystals are still generally too small for decisive neutron-scattering studies, which would allow their magnetic correlations to be determined. Electron pairing also has yet to be controlled in single-crystal synthesis.

The problems in synthesis have also pointed to a variety of chemistry issues, including the interactions of the new high-temperature superconductors with environmental factors such as water and carbon dioxide. The large number of elemental constituents and the critical role of oxygen in the structures make their chemistry both complex and interesting. Formation of the superconducting phase seems to require high-temperature annealing, but proper control of the oxygen, at least in the “123” compounds, requires a lower temperature. Whether these temperatures can be lowered further will be vital to many applications that cannot tolerate high-temperature anneals. This is particularly true of thin-film applications, and studies have already revealed significant interdiffusion of the superconducting films with most substrates.

The nature of superconductivity in these materials and the nature of the materials in their nonsuperconducting states have rapidly become central problems in modern condensed-matter physics. There is growing evidence that new pairing mechanisms are required that go beyond the conventional electron-phonon coupling believed to account for most earlier superconductors. There is also growing evidence that earlier theories must be generalized to include effects arising from both large anisotropy and fluctuation phenomena enhanced by the short coherence length in these materials. The effects of different oxygen defect structures, of twin boundaries, and of copper oxide planes and chains on the superconductivity are all yet to be understood. The structure of a superconducting material with one of the highest transition temperatures yet achieved is shown in Figure 3.10.

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