As I write, it is about six years since Bednorz and Müller sent off their amazing paper reporting superconductivity at about 30 K for a mixed phase sample in the La-Ba-Cu-O system. After an incubation period of a few months, during which only a handful of people paid any attention, the community suddenly woke up (literally overnight) to the realization that the discovery was genuine, when Kitazawa (Tokyo University) and Chu (University of Houston) confirmed the result at the 1986 Fall Meeting of the Materials Research Society. No need to repeat the stories of the next frantic couple of years: the Nobel Prize for the discovery, the tantalizing prospect of another prize for understanding the superconducting mechanism, the almost limitless prospects of new superconducting technologies which appeared in article after article, designed not just for scientists and engineers, but for the general public at large. Now, six years later, perhaps some perspective on the high-temperature superconductivity discoveries is possible.

At the fundamental science level, the discoveries have indeed been spectacular. Many layered structures based on the CuO2 sheets have been discovered. The first advance was the Tokyo group's discovery of the structure and composition of the superconducting phase La2-xBaxCuO4-δ, where x is optimally about 0.15. Then came the Alabama/Houston discovery of superconductivity at 92 K in YBa2Cu3O7-δ, followed rather quickly by the discoveries of 110 K superconductivity in the Bi-Sr-Ca-Cu-O (BSCCO) system at Tsukuba and then 125 K in the Tl-Ba-Ca-Cu-O (TBCCO) system by the Arkansas group.