In oxide superconductors

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. 

Sleight, A.W., Valency Considerations in Oxide Superconductors, Proc. Welch Conf. on Chem. Res., XXXEL Valency, p. 123, (1989). 

The chapter is organized as follows. The first section recalls that the germinal idea behind the discovery of SC in oxide superconductors was the JT effect. Afterwards we present some of the well established electronic properties of these materials (Sect. 3). It is important to recognize that the energy scale set by the electronic interactions led a substantial part of the scientific community to believe that HTS has its origin in purely electronic interactions. Therefore in Sect. 4 a brief review of models derived from these features is included. In Sect. 5 we illustrate the degree of controversy between e-ph and electron-based theories. 

Among various signals coming from the volume excited by the focused probe in STEM, we will introduce two forms of spectroscopy that have proved to be most useful in oxide superconductor research, as well as microdiffraction. 

The simplicity of the ratio method for thin films is to be contrasted with the complexity of microanalysis for bulk samples using EDX. Combined with the high spatial resolution achievable, it makes EDX a very attractive analytical technique. It has been extensively used in oxide superconductor research, for example, in phase identification of powdered samples, and in identifying the... 

Electronic Structure and High Critical Temperature in Oxide Superconductors... 

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... 

A further indication of the rapid advances that have occurred in the chemistry of the elements during the past 15 years can be gauged from the several completely new sections which have been added to review work in what were previously both nonexistent and unsuspected areas. These include (a) coordination compounds of dihapto-dihydrogen, (b) the fullerenes and their many derivatives, (c) the metcars, and (d) high-temperature oxide superconductors. 

Ceramic oxide superconductors have distinct atomic layers. The Cu-containing superconductors contain planes of copper and oxygen atoms, as the molecular view shows. These planes alternate with layers containing oxygen and the other metals that make up the superconductor. Superconductivity takes place in the Cu—O planes. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

Since this chapter is aimed at presenting much of the information leading to the discovery of the new high Tc oxide superconductors, we shall present here an overview of the oxide systems which have been studied for superconducting properties prior to 1985. In the next section of this chapter, we will give a more detailed and descriptive narration of the work performed on oxide systems, presented in terms of the crystal classes which have yielded the most important oxide superconductors. 

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