High-Temperature Superconductivity: History and Outlook

HISTORY OF HIGHTEMPERATURE SUPERCONDUCTIVITY After superconductivity was discovered by Onnes in 1911, many superconductors were discovered and the critical temperature rose year by year. More than 2000 superconducting materials had been discovered by 1975, and the critical temperature had reached 22.3 K with the discovery of Nb3Ge in 1973. After that, however, no higher critical temperature was obtained for more than 10 years. When the BCS theory in 1957 provided an elegant explanation for superconducting phenomena, many scientists believed that higher critical temperatures could not be reached without finding new superconducting phenomena. Already in 1954 Fröhlich 1) had proposed a model of high-temperature superconductivity in charge density wave systems, and in 1964 Little 2) proposed a so-called excitonic superconductivity model. No such superconductors, however, have actually been found. Sleight 3) reported in 1975 that superconductivity was seen in BaPb(1-x)BixO3 and that the critical temperature of this material, which can be as high as 13 K, changes with the Bi/Pb ratio. This was just the beginning of research on the oxide superconductors. Tanaka and others 4) immediately studied this material and found that the carrier concentration in it is more than an order of magnitude smaller than that in ordinary metals. At that time there was a little bit of hope that the superconductivity seen in this material is non-BCS superconductivity, but today it is still thought to be BCS superconductivity. In the early 1980s investigators all over the world began looking for new types of superconductivity. In Switzerland a new superconductor, PbMo6S8 was found by Chevrell. It had high upper magnetic critical field, but its critical temperature was still only about 16 K. In Japan a government project called “New Superconducting Material” started in 1984, and in the United States a new conference on the “Material and Mechanism of Superconductivity” was held in 1985. Bednorz and Müller5) found early in 1986 that in Ba-doped LaCuO3 the temperature dependence of the conductivity in the transition region from the normal state to the superconducting state changes with the current density (Fig. 1) and they pointed out this might indicate the possibility of high-temperature superconductivity. Tanaka, Kitazawa, Uchida, and Takagi 6) at the University of Tokyo immediately started to explore this possibility and at the end of 1986 reported that measurements of resistivity and diamagnetism confirmed that the critical temperature for superconductivity in Badoped La2CuO4 was as high as 30 K (Fig. 2). Furthermore, in early 1987 Tanaka 7) pointed Shoji TANAKA Director General Superconductivity Research Laboratory 1-10-13 Shinonome Koto-ku, Tokyo 135-0062, JAPAN High-Temperature Superconductivity: History and Outlook