Solids superconductors

One example, more extreme than most hopefully, occurred at a meeting of the Material Research Society a year after the initial explosion of interest in superconductivity. During a symposium mention was made of a recently published experiment reporting an increase in Tc by treatment of the solid superconductor with a gas. The chairman asked how many members of the audience had repeated this experiment. Fifty hands were counted out of a total of 450. It is likely that others, not in the audience, had also tried. Almost all of this effort is an unproductive use of the resources alloted to the field. Little appeared in the literature as negative results with such complex solids are rarely definitive and publishable. But remedies are not readily available. 

The appUcation of this method to HTc superconductors is based on the oxidizing power of the (Cu—0)+P complex, shown by its reaction with acid, water and other oxidizable species. Thus, when superconductors containing holes, i.e. (Cu-0)+p, come in contact with water or acid, oxygen is evolved. The important point is that this oxygen is not the result of the oxidation of the solution but its origin is the solid superconductor, as shown first by Shafer et al. (1988). Assuming an intermediate hexahydrate this process is schematically written for 123-0 as... 

Valence electron density for the diamond structures of carbon and silicon. (Figure redrawn from Cohen M L i. Predicting New Solids and Superconductors. Science 234 549-553.)... 

In the ceramics field many of the new advanced ceramic oxides have a specially prepared mixture of cations which determines the crystal structure, through the relative sizes of the cations and oxygen ions, and the physical properties through the choice of cations and tlreh oxidation states. These include, for example, solid electrolytes and electrodes for sensors and fuel cells, fenites and garnets for magnetic systems, zirconates and titanates for piezoelectric materials, as well as ceramic superconductors and a number of other substances... 

A unique application of the solid oxygen electrolytes is in dre preparation of mixed oxides from metal vapour deposits. For example, the ceramic superconductors described below, have been prepared from mixtures of the metal vapours in the appropriate proporhons which are deposited on the surface of a solid electrolyte. Oxygen is pumped tluough the electrolyte by the application of a polarizing potential across the electrolyte to provide the oxidant for the metallic layer which is formed. 

Current availability of individual lanthanides (plus Y and La) in a state of high purity and relatively low cost has stimulated research into potential new applications. These are mainly in the field of solid state chemistry and include solid oxide fuel cells, new phosphors and perhaps most significantly high temperature superconductors... 

Metals and semiconductors are electronic conductors in which an electric current is carried by delocalized electrons. A metallic conductor is an electronic conductor in which the electrical conductivity decreases as the temperature is raised. A semiconductor is an electronic conductor in which the electrical conductivity increases as the temperature is raised. In most cases, a metallic conductor has a much higher electrical conductivity than a semiconductor, but it is the temperature dependence of the conductivity that distinguishes the two types of conductors. An insulator does not conduct electricity. A superconductor is a solid that has zero resistance to an electric current. Some metals become superconductors at very low temperatures, at about 20 K or less, and some compounds also show superconductivity (see Box 5.2). High-temperature superconductors have enormous technological potential because they offer the prospect of more efficient power transmission and the generation of high magnetic fields for use in transport systems (Fig. 3.42). 

All metals conduct electricity on account of the mobility of the electrons that bind the atoms together. Ionic, molecular, and network solids are typically electrical insulators or semiconductors (see Sections 3.f3 and 3.14), but there are notable exceptions, such as high-temperature superconductors, which are ionic or ceramic solids (see Box 5.2), and there is currently considerable interest in the electrical conductivity ol some organic polymers (see Box 19.1). 

Because of the high values of conductivity which in individual cases are found at room temperature, such compounds are often called superionic conductors or ionic superconductors but these designations are unfounded, and a more correct designation is solid ionic conductors. Strictly unipolar conduction is typical for all solid ionic conductors in the silver double salts, conduction is due to silver ion migration, whereas in the sodium polyaluminates, conduction is due to sodium ion migration. 

Egami T (2005) Electron-Phonon Coupling in High-Tc Superconductors 114 267-286 Egami T (2007) Local Structure and Dynamics of Ferroelectric Solids. 124 69-88 Eisenstein O, see Clot E (2004) 113 1-36... 

Typical solid-state systems like alloys, polynary halides, oxides, and higher chalcogenides (e.g., semi- and superconductors), and so on, are not included in this context. 

For e > 0.1,there is a possibility to adjust e to the recent experimental data on k(T) (Brandstatter,1994) for high — Tc cuprate superconductor TI2CC12(7 — 2223). Our calculations show that,the best choice of e is found to be e = 0.21.The appropriate k(t) is presented in Fig.4 (solid line). The dashed line in this figure shows k(t) for D = 3. This fitting process allows us to get an estimation on the effective dimensionality of the high — Tc superconducting materials. 

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