High T c Oxide Superconductors
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motors, generators, and transmission lines to small-scale applications such as SQUID magnetometers and digital electronic components based on Josephson junction devices. However, formidable technological obstacles remain to be surmounted, such as the fabrication of conductors with high critical current densities and thin films that are compatible with silicon and other electronic materials. The articles in this issue of the MRS BULLETIN give an overview of the developments that have taken place in high Tc superconductivity during the past two years; due to the vastness of the field, they are not intended to provide a comprehensive review. This issue also commemorates the second anniversary of the December 1986 Fall Meeting of the Materials Research Society in Boston, where initial research results were reported, touching off an explosion of research on the high Tc oxides in the United States and throughout the world! One of the most striking aspects of all presently known high Tc superconductors with Tc's greater than —30 K is that they are copper oxides with layered perovskite-like crystal structures, all possessing Cu0 2 planes. Like its predecessor Ba(Pb,.;tBix)03, which has a maxim u m Tc of —14 K, t h e s y s t e m (Ba1.IKI)Bi03, with a maximum TC near 30 K, has a cubic perovskite crystal structure. The series of Tc = 95 K RBa2Cu307.j superconductors, where R = rare earth element, has a crystal structure which, in addition to Cu0 2 planes, contains CuO chains which appear to play an important role in the superconductivity of these materials. I.K. Schullerand J.D. Jorgensen describe the crystal structures of the various families of high Tc oxide superconductors and the relationship between the crystal structure and the superconducting properties. The material responsible for the indi-
cations of superconductivity at —30 K in the groundbreaking experiments of J.G. Bednorz and K.A. Miiller1 is (La2.xBax)CuOM, where 8 is the oxygen vacancy concentration and x = 0.15. Superconductivity has since been found in several (La2.iMj)Cu04.8 systems, where M is one of the divalent alkaline earths Ca, Sr, and Ba, with respective maximum Tc's of —20 K, —40 K, and -30 K, or the alkali metal Na with Tc « 20 K. Surprisingly, the undoped parent compound La2Cu04.6 is an insulating antiferromagnet with a Neel temperature TN = 250 K and a magnetic moment JU, = 0.5 fiB per Cu2+ ion. The substitution of the monovalent or divalent M cations suppresses the antiferromagnetism and transforms the material into a superconducting metal. Superconductivity, magnetism, and the physical properties of the (La2.IM,)Cu04.4 (M = Ca, Sr, Ba, and Na) are described in the article by Z. Fisk, S-W. Cheong, and D.C. Johnston. The discovery of superconductivity at —90 K in material now known to have the chemical formula YBa 2 Cu 3 0 7 . 6 , where 8 = 0.1, by M.K. Wu, C.W. Chu, and their co-workers2 gave a great impetus to the field of high Tc superconductivity in oxides, since it broke the 77 K liquid nitrogen temperature barrier. Shortly thereafter, superc
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