The recent revolution in high temperature superconducting materials has generated a wave of intense excitement and activity that has swept through the scientific community, attracting the attention of the news media and general public as well. The reason for this is twofold: the unexpected occurrence of superconductivity at such high temperatures is of immense scientific interest, and the new high temperature oxide superconductors may have important technological applications.

Based on a large amount of experimental information and (presumed!) theoretical understanding, the prevailing view prior to 1986, when high temperature superconductivity in oxides was discovered, was that the maximum value of the superconducting transition temperature Tc of any material would not increase much above ˜23 K, the high Tc record held since 1973 by the A15 compound Nb3Ge. In fact, between 1911 (the year H. Kammerlingh Onnes discovered superconductivity) and 1986, Tc only increased at an average rate of ˜0.25 K per year. However, within the last two years the maximum Tc value of the new copper oxide super-conductors has risen at an average rate of ˜50 K per year to its present value of ˜125 K! Thus, superconductivity near or above room temperature no longer seems out of the question, as it did a few short years ago! Moreover, the oxides were generally regarded as the least likely candidates for high Tc superconductivity due to their low concentrations of charge carriers. An understanding of the origin and nature of high Tc superconductivity in the new oxide compounds constitutes one of the most important and challenging scientific problems that has emerged in recent years.