High-temperature superconductors, notable

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). 

Many people in the field believe that some high-temperature superconductors, most notably of the Nd jCe CuCU class, are n-type. But since most high-temperature superconductors rely on oxygen to superconduct, and since at least some oxygen in these materials is already in the O 2 state, to dope them n-type requires forming O 3, which does not form. 

The high-temperature superconductors based on copper oxides (see Table 8.2, p. 284) conform to some but not all of these points. Points (a) and (b) apparently apply to those systems as well, with one notable exception there seems to exist ground for doubting that electron-phonon interactions alone can be responsible for the pairing of electrons. Magnetic order plays an important role in the physics of these materials, and the presence of strong antiferromagnetic interactions may be... 

The layer-type structures and chemical nature of the constituents of the bismuth and thallium-based cuprate superconductors - notably the lone-pair stereochemistry of Bis+, variable valence of copper, and considerable exchange among some of the cation sites - combine to make structural non-ideality, nonstoichiometry, and phase intergrowth the rule rather that the exception in these families of materials. These features, as well as the probable metastability of the phases (and possibly all high-temperature oxide superconductors), also contribute to the difficulties typically encountered in preparing single-phase samples with reproducible properties and compositions. 

Ceramic superconducting films are divided into three classes, Bl-type compounds, ternary compounds, and high-temperature oxide superconductors. The Bl-type (NaCl-type structure) compound superconductors consist of nitrides and carbides with 5A, 6A, and 7A transition metals, such as TiN, ZrN, HfN, VN, NbN TaN, MoN, WN, TiC, ZrC, HfC, VC, NbC, TaC, MoC, WC, NbNi tC t, hex-MoN, and hex-MoC. Regarding the thin-film material, it is notable that NbN and NbN] (C ( (x = 0.08 and 0.15) have superconducting critical temperature, T, values of 17.3 and 17.8 K, respectively. The deposition method used is almost always sputtering or CVD. The properties of films deposited by the former method are superior. A highly reliable Josephson device was realized with an NbN film. 

Many of the properties of superconductors have been successfully accounted for by the Bardeen-Cooper-Schrieffer (BCS) microscopic theory of superconductivity. However, the theory cannot predict the occurrence of superconductivity in materials, i.e., it does not tell us what atomic constituents should be put together and what crystal system is necessary in order to obtain materials exhibiting high critical temperatures 7. However, prior to the advent of the BCS theory a large body of information regarding the occurrence of the superconductive state in elements, alloys, and intermetallic compounds was accummulated, notably by Matthias and his... 

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