Superconductors cuprates, high

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

The structures of ternary oxides such as spinels, perovskites, pyrochlores, layered cuprates (high-7 c superconductors), and other lamellar oxides are fascinating subjects by themselves and are beyond the scope of the present discussion. 

All the high Tc superconductors discovered so far, with one exception, contain weakly coupled copper oxide, Cu02, planes. The highest critical temperatures are found for cuprates containing a Group 2 metal (Ca, Ba, Sr) and a heavy metal such as Tl, Bi, or Hg. The structures of all the cuprate superconductors are based on, or related to, the perovskite structure. The one report (in 2000) of a non-cuprate high T superconductor is of surface superconductivity in Na WOs. The structure of NUxWOs is also based on the perovskite structure. 

In conclusion, it was shown that the condensation energy in cuprate superconductors is highly sensitive to the doping level and is greatly reduced in underdoped region. 

Since the first observation of high superconductivity at 110 K (onset) in (Sri cCaJi, Cu02 which was first synthesized by Siegrist et al.. The synthesis and superconductivity of this compound have been widely studied because the cuprate represents the core part, known as the infinite layer , of cuprate high superconductors. The preparation of infinite layers in a single phase is difficult at ambient pressure and must be carried under high pressure up to 6 GPa. 

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. 

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 ". 

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