The modern age of the so-called high T_c superconductors bagan in 1986 with the initial discovery by J.G. Bednorz and A.K. Muller of IBM-Zurich on 35 K superconductivity in La-Ba-Cu-O system. The superconducting phase was identified to be (La,Ba)₂CuO_4-x with a perovskite structure. The new development prompted a flurry of activity in the area searching for new materials with even higher T_c's. The first important breakthrough came in January 1987, when Wu et al. at the University of Alabama and Chu et al. at the University of Houston reported 90 K superconductivity in the Y-Ba-Cu-O system, reaching a T_c above the boiling point of liquid nitrogen for the first time. A few days later, Zhao et al. in Beijing, China announced independently the discovery of 90 K superconductivity in the Y-Ba-Cu-O system, and first gave the chemical components of the material. The phase responsible for the superconductivity was rapidly identified to be YBa₂Cu₃O_7-x, again with a perovskite-type structure. Furthermore, analogous compounds with rare earths substituted for yttrium exhibited the same remarkable behavior. While the previous 30 K La-Ba-Cu-O had created interest in the advancement of superconductor applications, the 90 K Y-Ba-Cu-O galvanized the attention of the technological and commercial sectors. The "old" (classical) superconductors require expensive liquid helium for cooling, whereas the new rare earth-Ba-Cu-O materials could be cooled with relatively inexpensive liquid nitrogen at 77 K. The potential advantage in the simplicity of the refrigeration and the lower costs are enormous.
In spite of many reports of higher T_c in the rare earth-based oxides, further breakthroughs of T_c were made in non-rare earth copper oxides. In January of 1988, Sheng et al. at the University of Arkansas reported 90 K Tl-Ba-Cu-O superconducting system, the first non-rare earth system with T_c above liquid nitrogen temperature. Almost at the same time, Maeda et al. of Tsukuba Laboratories in Japan announced the discovery of the Bi-Sr-Ca-Cu-O superconducting system with T_c up to 110 K, and raised T_c to above 100 K for the first time. A new superconducting phase Bi₂Sr₂CaCu₂O₈ with T_c of about 80 K was identified. Three weeks later, in early February, 1988, Sheng et al. at the University of Arkansas again reported that above 120 K superconductivity had been achieved in the Tl-Ba-Ca-Cu-O system. Two superconducting phases Tl₂Ba₂CaCu₂O₈(2212 phase) and Tl₂Ba₂Ca₂Cu₃O₁₀(2223 phase) were rapidly identified.
In May 1993, scientists in Switzerland discovered superconducting materials with critical temperatures in the range of 130 to 133 K. These superconducting materials are HgBa₂Ca₂Cu₃O_1+x (with three CuO₂ layers per unit cell) and HgBa₂CaCuO₆ (with two CuO2 layers). The superconducting structure comprises a defined sequence of the unit cells of these phases. The magnetic and resistivity measurements of the Swiss scientists confirm a maximum transition or critical temperature of 133 K.
All the important superconducting materials can be seen in figure1 in chronological order of discovery, with particular emphasis of transition temperature (T_c).