HAD a mischievous deity set out to devise a phenomenon guaranteed to frustrate and torment condensed-matter physicists -and particularly the theorists among them - he could hardly have come up with a better candidate than high-temperature superconductivity in the cuprates. The compounds in question are> almost without exception chemically complicated and metallurgically delicate. Their experimental properties, including their transition temperature (ro) to the superconducting state, are often almost pathologically sensitive to the details of the preparation technique, and the "optimum." data therefore tend to be alarmingly time dependent. And to cap it all, yttrium barium copper oxide (YBCO), which was the first compound to break the 90 K superconducting barrier and has served as the E. coli of the whole business thanks to its desirable metallurgical and other properties, turns out In retrospect to be atypical of the class of compounds as a whole in just about every possible way.
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