Infinite layer cuprate

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. 

NOTE Infinitely layered cuprates are since found to be superconducting... 

NOTE Infinitely layered cuprates are since found to be superconducting Phil. Trans. S. Soc. Land. A (1991)... 

A blocking layer is necessary the infinite layer cuprate system is not superconducting (unless doped by cation substitution, in which case the cation layer would itself be the blocking layer). However, the blocking layer can be either insulating or metallic. 

Charge localization at the interface between La,, Sr Mn03 and the infinite layers cuprate CaCu02. /. Appl Phys., 112,123901. 

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. 

The first excitation in Figure 8.4 along the ah-plane is associated with the CUO2. In all of the undoped cuprates, there is a sharp excitation below the broad 3 eV excitation. In the present case, due to the energy resolution (0.5 eV) of the spectrometer, the 2 spectrum for the ah-plane represents a mixture of the sharp excitation and the broad excitation. The anisotropic dielectric function indicates that the Hubbard band lies mostly in the ah-plane. This is consistent with the very anisotropic O K-pre-edge data reported from XAS measurements, which indicate that only 1 1% of the upper Hubbard band is out of the ah-plane for the infinite-layer compound [8.27]. The reason for this significant anisotropy in the Hubbard band for the infinite-layer compound is clearly associated with the lack of apical oxygen in the structure. 

Figure 93 Basic structural types of cuprate superconductors, (a) Perovskite structure (cubic) (b) Infinite- layered structure (tetragonal) (c) Rocksalt prototype structure (reduced cell) (d) Composite layer between infinite layer and rocksalt building blocks.

The crystal chemistry of the rare-earth cuprates is discussed in chapter 188 by B. Raveau, C. Michel and H. Hervieu. These authors noted that all of the hiTc superconductors are derived from the perovskite structure. This is done by disconnecting the CuOe octahedra of the perovskite structure along one direction so then an infinite number of CUO2 layers are formed. These layers are responsible for the superconductivity. Raveau et al. note that the rare-earth cations are not directly responsible for the appearance of superconductivity, but by virtue of their large size and trivalent character they help stabilize these layered structures. 

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