Cuprate superconducting materials

For e > 0.1,there is a possibility to adjust e to the recent experimental data on k(T) (Brandstatter,1994) for high — Tc cuprate superconductor TI2CC12(7 — 2223). Our calculations show that,the best choice of e is found to be e = 0.21.The appropriate k(t) is presented in Fig.4 (solid line). The dashed line in this figure shows k(t) for D = 3. This fitting process allows us to get an estimation on the effective dimensionality of the high — Tc superconducting materials. 

This raises two questions (1) are the SrO layers (or the cuprate planes) of other superconducting materials the hosts of superconductivity and (2) do Sr2YRuOe s sister compounds, GdS C RuOs andGd2- Ce Sr2Cu2RuOio, superconduct in their cuprate planes or in their SrO layers ... 

The biggest explosion in materials chemistry and physics occurred in late 1986 when high-temperature superconductivity was discovered in a lanthanum cuprate, a material which was a ceramic and on which a few chemists had worked earlier. As stated in a report of the US National Academy of Sciences, this discovery changed the role of chemistry in the study of materials, and materials chemistry became a more significant part of materials science. It is around this time that even chemists started to consider solid state chemistry as an integral and important part of main-stream chemistry. 

The discovery of high-temperature superconductivity in a lanthanum-based cuprate perovskite material with a transition temperature of Tc = 35 K by Bednorz and Muller... 

We have seen in the previous section that the single-layer materials exhibit substantial R-Cu exchange coupling, and they also have modest (for cuprates) superconducting transition temperatures. For the materials that contain midtiple Cu-O layers the Tc values... 

Localisation of electron den ty over a small region in a crystal lattice occupied by 02 leads to the increased importance of electron correlation which cannot be tackled by traditional one-electron or HF theories[61]. Here we build upon our previous studies in which we used ab initio, semi-empirical and semi-classical approaches to study 0) ionic crystalline peroxides (e.g. Si and Ba02 [70,71]), (ii) point defects in the bulk and on the sui % of ionic and semi-ionic materials (e.g. corundum, silica and aluminium silicates [72,73]), and (iii) bipolaron formation in lanthanum cuprate (a superconducting material [74]). 

In 1986, it was discovered that some cuprate-perovskite materials have critical temperatures above 90 K. These high-Tc superconductors renewed interest in the topic because of the prospects for improvement and potential room-temperature superconductivity. From a practical perspective, even 90 K is relatively easy to reach with liquid N2 (boiling point = 77.4 K), resulting in more experiments and applications (see also Section 1.2.4). 

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). 

Although the high-temperature superconducting phases are formed from insulating materials by the introduction of defects, the precise relationship between dopant, structure, and properties is not fully understood yet. For example, in most of the cuprate phases it is extremely difficult to be exactly sure of the charges on the individual ions, and because of this the real defect structures are still uncertain. 

The electron density in transition metal complexes is of unusual interest. The chemistry of transition metal compounds is of relevance for catalysis, for solid-state properties, and for a large number of key biological processes. The importance of transition-metal-based materials needs no further mention after the discovery of the high-Tc superconducting cuprates, the properties of which depend critically on the electronic structure in the CuOz planes. 

Those readers not familiar with superconductivity in organic materials may find the Tc values rather low. However, they are comparable to values for inorgaiuc metallic elements. Here is a list of some selected examples FcCNb) = 9.25 K, rc(Pb) = 7.20 K, rc(a-Hg) = 4.15 K, rc(Sn) = 3.72 K, Tc(Al) = 1.17 K, Tc(ri) = 0.40 K, etc. It is interesting to note that copper does not exhibit a superconducting transition. The highest known Tc values of any material correspond to the copper-oxide series with Tc 138 K as the absolute record for the thallium-doped mercury-cuprate compound. 

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