Superconducting layered cuprates

Most of the superconducting layered cuprates can be described by the general formula [ACu03.x]m [AO]n, where m and n represent the number of copper layers and [AOjoo layers in the perovskite and rock salt slabs respectively. The different members of this... 

In these and the other cuprate superconductors, the part of the structure that leads to superconductivity is the slab of Cu02 sheets. When more than one sheet is present, they are separated by cation layers, Q (usually Ca or Y) to give a sequence Cu02-(Q-Cu02) i, which forms the superconducting layer in the material. The index n is the total number of Cu02 layers in the phase, which is equal to the formula number of Cu atoms present (Fig. 8.5). 

Figure 8.5 Superconducting planes found in cuprate superconductors (a) a single Cu02 sheet and (b) a Cu02 (Q — Cu02) i superconducting layer.

Figure 8.6 Schematic depiction of a cuprate superconductor. Doping into the charge reservoir layers results in the transfer of holes to the superconducting layers.

More than twenty different cuprates have been found to exhibit superconducting properties. All of them are characterized by a bidimensional character of the copper oxygen framework, i.e. their structure is formed of copper oxygen superconductive layers separated... 

Lone pair cations exhibit external pairs of electrons which do not participate in the bonds but can influence dramatically the geometry of the structures (52). This is the case of cations like Bi(III), Pb(II) or T1(I) whose 6s2 lone pairs have been shown to present an important stereochemical activity. Such cations which can be found in the rock salt type layers are capable of influencing the oxygen framework and may consequently affect the superconducting properties of the layered cuprates. 

M.B. Maple, High-temperature superconductivity in layered cuprates overview 1... 

More than 15 years after the discovery of high-Tc superconductivity in layered cuprates its mechanism is still under debate. This has to do with the asymmetry of physical properties between the electron-doped and hole-doped side of the complex phase diagram, temperature vs. doping, T(x), and with the fact that no consensus has been reached about the question what are the key experiments a theory of high-Tc superconductivity must be able to explain. In this paper we argue that the elementary excitations and their interdependence with spin excitations in the cuprates are of central interest in order to learn more about the correlations in general and, in particular, about the mechanism for Cooper-pairing in these systems. 

Since the discovery of high temperature superconductivity in cuprates, there has been intense interest in transition metal oxides with strongly layered, (quasi) two-dimensional (2D) crystal structures and electronic properties. For several years now alkali-metal intercalated layered cobaltates, particularly Na CoCL (NxCO)withx 0.50 — 0.75, have been pursued for their thermoelectric properties [1] IAX C0O2 is of course of great interest and importance due to its battery applications. The recent discovery[2] and confirmation[3-5] of superconductivity in this system, for x 0.3 when intercalated with H20, has heightened interest in the NxCO system. 

Several other novel strategies have been employed for the synthesis of superconducting cuprates some of them were mentioned earlier while discussing the various methods. Especially noteworthy are the use of the combustion method and the alkali-flux method for cuprate synthesis. Superconducting infinite-layered cuprates seem to be possible only when prepared under high pressures because of bonding (structural) considerations [87, 88]. In Table 7 we list the various cuprate superconductors along with their properties and the preferred methods of synthesis. 

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