ABSTRACT

Superconductivity was discovered by Kamerlingh Onnes in 1911 when he found that mercury loses its resistance below a certain temperature, known as transition temperature, Tc, which is 4.2 K for mercury. This generated a lot of excitement and there was an intense search for superconducting materials. Subsequently, many other superconductors were found such as Pb, Nb, NbC, Nb3Ge, and so on. The technological applications of these materials were quickly recognized as these materials have no power dissipation as the electrical resistance is zero. However, Tc of these materials was very low and for useful applications in various devices, one would like Tc to be around room temperature. Later, many more superconducting materials were found but their Tc was rather low. Until 1980, the maximum Tc was found for Nb3Ge, which was about 23 K. However, this changed in 1986, when Bednorz and Müller (1986) (Figure 17.1a and b) found superconductivity in the La-BaCu-O system with Tc around 35 K. Soon after this discovery, many other systems such as YaBaCuO were discovered with much higher Tc. All these systems have layered structures with Cu-O planes and are called cuprates. At present, the highest Tc obtained in cuprates is about 135 K in mercury barium calcium copper oxide at ambient pressure, which reaches about 160 K at high pressure (Gao et al. 1994).