Cuprate superconductors, importance

The important and widely studied copper-oxide-derived high-temperature superconductors, known as cuprate superconductors, are basically insulators. Doping converts these into metallic materials, many of which are superconductors over rather more restricted composition ranges. Several of these materials have already been discussed La2Cu04 and Sr2Cu02F2 (Section 4.3.3), La2 A.SrxCu04 (Section 8.5.1), and Nd2, Ce,Cu04 (Section 8.5.2). 

The entire area has experienced a rebirth in the last few years, for several reasons. The most important perhaps is the realization that cuprate superconductors are not far from a largely correlation driven metal-insulator transition. Another is the appreciation that the transition is of wide occurrence, some recent examples being the superconductor-insulator, the quantum Hall fluid-insulator, and the stable quasicrystal metal-insulator transitions. Hopefully, the next few years should see considerable progress in the experimental and theoretical description of this basic electronic transition. 

Gross stoichiometry variations in these modular systems are often acconunodated by the formation of other series members or the incorporation of planar faults, often equivalent to isolated lamellae of other series members, into the structure. However, many individual phases also show composition variation which is sometimes accommodated by random populations of point defects and in other phases by ordering and the generation of new structures. In both cases composition flexibility is important in influencing the physical and chemical properties of these materials. In particular this effect has been well studied in the cuprate superconductors, where changes in composition have repercussions for the superconducting transition temperature, r, of the phase. 

Table 9.1 General formulas of important homologous series of cuprate superconductors and derived hole concentration, p.

Table 9.3 Crystallographic data of important orthorhombic cuprate superconductors. The incommensurate modulation of the Bi compounds has been neglected.

Although several other types of exotic superconductors (e.g., organic superconductors, heavy-fermion f-electron superconductors, magnetically ordered superconductors, multinary rare-earth, actinide, and transition-metal superconductors) have been investigated intensely since 1986, the cuprate superconductors have received by far the most attention because the highest values of the superconducting critical temperature are found in this class of materials. Rare-earth and actinide elements are key constituents of many of the high-temperature cuprate superconductors and have played a prominent role in the development of the first and some of the more important cuprate superconductors. 

In view of the importance of high-temperature superconductivity in the layered cuprates and the role played by the rare earths, it seemed appropriate to prepare these volumes of the Handbook on the Physics and Chemistry of Rare Earths on High-Temperature Superconductivity in Layered Cuprates . We believe that researchers already working in this field, as well as those intending to enter this field, will find valuable information in the review articles contained in these volumes. Since many of the cuprate superconductors do not contain rare-earth or actinide elements, yet have characteristics and properties similar to those that do, the range of materials considered in these volumes has been broadened to a limited extent to include all high-temperature cuprate superconductors, irrespective of whether they contain rare-earth or actinide elements. 

Some important classes of cuprate superconductors and the maximum value of observed in each class. 

Having identified methods to deposit conductive polymer and molecular metal systems onto cuprate superconductor structures without damage to either material, it becomes important now to consider the electronic interactions that occur when the two conductors are in contact with one another. Of particular importance is the interaction that occurs between the polymer-derived charge carriers and the superconducting Cooper pairs. Important background information related to this area can be obtained from the well-documented behavior of the more classical metal/superconductor and semicon-ductor/superconductor systems. Thus, prior to considering experimental data and theoretical treatments for organic conductor proximity effects, we review previous studies of proximity effects in the more classical systems. 

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