Cuprate high-temperature superconductors

Cuprate high-temperature superconductors are copper-containing perovskite-derived oxides. Many of these maintain the superconducting state to temperatures above that of liquid nitrogen (Table 8.1). The stmctures of the superconductors are built up of slices of perovskite type linked by slabs with stmctures (in the main) of the rock salt (NaCl) or fluorite (CaFj) type (Section 4.6). The copper valence in most compounds lies between the formal values of Cu and Cu . 

There are a number of metallic perovskite-related cuprates that do not show superconductivity, typified by La BaCUjOjj, containing apex-linked CuO square pyramids and CuOg octahedra. A comparison of these materials with the superconductors makes it apparent that superconductivity in the cuprates depends on two-dimensional sheets of CuO planes. In these cuprates, there is a considerable spin-charge correlation which appears to be important in the appearance of a superconducting region. 

The appearance of superconductivity and the magnitude of the superconducting transition temperature are also closely coimected with the composition of these non-stoichiometric solids, which all exhibit considerable variation in oxygen content. The superconducting transition temperature is also affected by external factors such as pressure and crystal elastic strain. 

The balance between hole and vacancy creation is delicately poised. The process can be illustrated with respect to the most studied phase, Laj Sr CuO. Initially 

The structure of Nd CuO is very similar to the principal difference lying 

The holes can be regarded as transferred to Ti3+ ions converting them to Ti4+ by adding in the requisite number of extra ions  

The conductivity changes in a discontinuous fashion at a composition of approximately x = 0.12 at which composition the metal becomes a poor semiconductor. The formula is now Li [LiJi2Ti044Tut4]O4. This transition is reversible. Donor doping of insulating Li4/3Ti5/304 will return the compound to the metallic state. 

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CUPRATE HIGH-TEMPERATURE SUPERCONDUCTORS 8.6.1 Perovskite-Related Structures and Series... 

Fig. 27.5 (a) A unit cell of perovskite, CaTi03, using a polyhedral representation for the coordination environments of the Ti centres an O atom (red) lies at each vertex of the octahedra, and the Ca ion is shown in grey, (b) Part of a layer of stoichiometry Cu02 which forms a building block in all cuprate high-temperature superconductors colour code Cu, brown O, red. 

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