High-Temperature Superconductors 1992: Bringing the Materials Under Control
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High-Temperature Superconductors 1992: Bringing the Materials Under Control David C. Larbalestier, Guest Editor As I write, it is about six years since Bednorz and Miiller sent off their amazing paper reporting superconductivity at about 30 K for a mixed phase sample in the La-Ba-Cu-O system.1 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_6, where x is optimally about 0.15.2 Then came the Alabama/Houston3 discovery of superconductivity at 92 K in YBa2Cu3O7_8, followed rather quickly by the discoveries of 110 K superconductivity in the Bi-Sr-CaCu-O (BSCCO) system at Tsukuba4 and
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MRS BULLETIN/AUGUST 1992
High-Temperature Superconductors 1992: Bringing the Materials Under Control
version of this SQUID is now ready for sale as a working device. Space applications of passive devices will be demonstrated in the forthcoming HTSSE satellite. All in all, the prospects for thin film applications ready for market are strong, and the U.S. presence in the field is robust. Bulk applications have been more difficult. Due to the grain boundary problems already mentioned, they have lagged the thin film ones. Thin film applications have been easier because it is easy to grow HTS materials epitaxially onto a wide range of oxide substrates in such a way that the HTS films are single crystalline or contain only low-angle grain boundaries. For longlength applications, single-crystal substrates are not feasible, and randomly grain-oriented metallic substrates have advantages of flexibility and toughness not possessed by ceramics. David Shaw's article describes the tough problems required to control thin film deposition onto metallic substrates while simultaneously maintaining high critical current densities. Buffer layers are critical but, unfortunately, do not yet permit full epitaxy. Nevertheless, progress in developing epitaxy is occurring, and Shaw provides a window into this important technology. Eric Hellstrom describes the key issues
David Larbalestier, Guest Edi
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