Superconductors discovered

All the high Tc superconductors discovered so far, with one exception, contain weakly coupled copper oxide, Cu02, planes. The highest critical temperatures are found for cuprates containing a Group 2 metal (Ca, Ba, Sr) and a heavy metal such as Tl, Bi, or Hg. The structures of all the cuprate superconductors are based on, or related to, the perovskite structure. The one report (in 2000) of a non-cuprate high T superconductor is of surface superconductivity in Na WOs. The structure of NUxWOs is also based on the perovskite structure. 

Challenges to Established Theories, It is interesting to note that some theoreticians struggle with describing how superconductivity occurs at high temperatures in the newer, ceramic superconductors. This is understandable because the classic theory of superconductivity is tied to metals. Most ceranuc superconductors discovered to date incorporate distinctive layers of copper and oxygen atoms, One question posed by some researchers, Is the mechanism of high-temperature superconductivity the same in hole superconductors as it is in electron superconductors ... 

The structural feature common to all the high temperature cuprate superconductors discovered to date is the presence of two-dimensional sheets of Cu02. While much has been learned... 

The compound YBa2Cu307 has been widely studied because it is relatively easy to prepare and it was the first superconductor discovered with a above the boiling point of liquid nitrogen. The crystal... 

The compound YBajCUjO, has been widely studied because it was the first superconductor discovered with a T above the boiling point of liquid nitrogen. The crystal stmcture of YBajCUjO, consists of three perovskite-like unit cells stacked one on top of the other (Figure 4.9a). If the ions are allocated the formal charges of Y ", Ba " and 0 , the Cu must take an average charge of 2.33, which can be considered to arise from the nominal presence of two Cu ions and one Cu ion per unit cell. 

The La2-xMj Cu04 and RBa2Cu307 5 compounds are hole-doped superconductors, as are most of the high-Tc cuprates. The first system of electron-doped cuprate superconductors discovered was Ln2-.TMxCu04 y (Ln=Pr, Nd, Sm, Eu M = Ce, Th 2i 0.1-0.18 y 0.02) (Tokura et al. 1989, Markert and Maple 1989, Markert et al. 1989b, Early et al. 1989). The highest values of Tc found among the electron-doped superconductors are 25 K. 

Oxide superconductors have been known since the 1960s. Compounds such as niobium oxide [12034-57-0] NbO, TiO, SrTi02, and AWO, where A is an alkah or alkaline earth cation, were found to be superconducting at 6 K or below. The highest T observed in oxides before 1986 was 13 Kin the perovskite compound BaPb Bi O, x = 0.27. Then in 1986 possible superconductivity at 35 K in the La—Ba—Cu—O compound was discovered (21). The compound composition was later determined to be La 85 A the Y—Ba—Cu—O system was pubUshed in 1987 and reported a transition... 

Early work on superconductors concentrated on metals or metal mixtures (alloys). Niobium alloys are particularly good superconductors, and in 1973 a niobium alloy, Nb3Ge, was found to have Tc — 23 K, the highest known value for a metal superconductor. In 1986, a ceramic oxide with formula La2- Ba CuOq was found to show superconductivity at 30 K. Through intense research efforts on ceramic oxides, YBa2 C U3 Oj-, with Tc — 93 K, was discovered in 1987. 

Superconductivity has also been discovered in rather exotic materials, including the following Buckminsterfullerene (Cgo) doped with ICI Carbon nanotubes (superconductivity in just one direction) Nickel borocarbides, which contain Ni2 B2 layers alternating with R C sheets, where R is a rare earth element such as Er and organic superconductors that contain planar organic cations and oxoanions. Chemists and physicists continue to study these and other families of superconductors. 

Twenty years later, in 1933, the German physicist Walter Meissner (together with his co-worker Robert Ochsenfeld) discovered that superconductors cannot be crossed by magnetic field lines. This property is today defined as the Meissner effect. 

Superconductors which follow this behaviour completely are classified as Type I . The first superconductors to be discovered belong to this category. 

Oshima K, Urayama H, Yamochi H, Saito G (1988) Peculiar critical field behaviour in the recently discovered ambient pressure organic superconductor (BEDT-TTF)2Cu(NCS)2 (Pc = 10.4 K). J Phys Soc Jpn 57 730-733... 

At this point, it may be informative to present a chronological listing of the different discoveries in oxide superconductors reported prior to 1975. In this listing, Table 5, we present the year that the oxide compound was first reported, then the year in which superconductivity was first observed in the system and the group credited for the discovery. Of particular interest is the compound Ba(Pb1 xBix)Os discovered by Sleight at du Pont in 1975. This oxide material adopts the perovskite-type structure and contains no transition metals. 

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