High Tc superconducting oxides

Related Topics II Thallium-Based High-Tc Superconducting Oxides... 

Our detailed review on the summary of various site substitutions in thallium-based high-Tc superconducting oxides has led us to the following conclusions. 

High-Tc Superconducting Oxides Chemical Composition. Bednorz and Muller carried out a series of investigations aiming to enhance the electron-phonon interaction in superconducting oxides. As a result, they found superconductivity at around 30 K intheBa—La—Cu—O system [2.12]. These compounds... 

In terms of the electronic age, which includes the invention of the radio, television, calculator, and computer, it has been claimed that the discovery of high Tc superconductors has resulted in a "third electronics revolution" preceded by the transistor (1947), and the vacuum tube (1904). It now appears that we have shifted from "silicon valley" to a "copper-oxide valley" with the new discoveries in the field of high Tc superconductivity. 

Paul" Chu, and others at University of Houston, also reproduced Zurich s I.B.M. research results (156). Bell Lab s confirmation of Bednorz and Muller s discovery of high Tc superconductivity in copper oxide compounds was published (157) in the Jan. 1987 issue of Physical Review Letters. The electrical resistivity data from their work showing an onset of superconductivity at 36.5 K for the composition Lax gSr0 2Cu04 is plotted as Figure 28. This product also showed a 60-70% Meissner effect. 

With the discovery and disclosure of these events in the area of "High Tc Superconductivity", hundreds, if not thousands, of scientists actively became involved in research on these new materials. Newer materials and higher Tc s soon followed. The competition was fierce and the progress through 1987 and 1988 was moving at a rapid pace with numerous important discoveries. To date, the highest Tc is in the range of 110-125 K, some five times that obtained in 1973 on the revolutionary (A-15) intermetallic materials. These new copper -oxide systems, many of which will be described in detail by other contributors to this book, are presented in Table 10. 

The lack of homogeneity pervades most of the systems where high Tc superconductivity has been observed. The n-type copper oxides, e.g., Nd2 xCexCu04 and Nd2Cu04 xFx are particularly plagued with this problem because superconductivity exists over such a small range of x. The inevitable variations of x on a microscopic level will inevitably lead to materials which are not electronically homogeneous. 

One of the simple models of high-Tc superconducting copper oxides is a two-band Hubbard Hamiltonian (so called Emery model) [46]... 

B. Raveau, C. Michel, M. Hervieu and D. Groult, Crystal Chemistry of High Tc Superconducting Copper Oxides, Springer, Berlin, 1991. 

RE2CUO4 perovskites exhibit important and varying magnetic and electrical characteristics, and they are broadly studied as potential high-Tc superconductive materials. At room temperature, they show p-type semiconducting behaviors, and are used as electrode materials in fuel batteries. The catalytic properties of the perovskite oxides also make them effective in various oxidation and reduction reaction, hence they are considered as promising substitutes to the classical Pt/Rh-based catalysts applied to automotive pollution control. 

Bismuth oxide forms a number of complex mixed-metal phases with the divalent metal oxides of calcium, strontium, barium, lead, and cadmium, and these show a wide variety in composition. With transition metal oxides, mixed-metal oxide phases have been observed which are based upon a Perovskite-type lattice (10) containing layers of Bi202. It is notable that the high Tc superconducting materials which include bismuth also have this Perovskite-type of lattice with layers of copper oxide interleaved with bismuth oxide layers. 

One deals with the ab initio description of electronic excited states. These include the attachment or removal of electrons, the account of direct or inverse photo-emission spectra, and the electron-hole excitations of the d -> d or charge transfer type. Advanced methods are presently under development to account for them the GW method, the SIC method, the LDA-I-U method, etc. However, they imply an increased computation cost, which is not routinely accessible for complex systems, such as most oxide surfaces. These methods are also expected to open the field of strongly correlated materials, among which transition metal oxides, which have important technological applications high-Tc superconductivity, giant magneto-resistance, etc. 

Since the discovery of high temperature superconductivity (T > 30 K) in the La-Ba-Cu-O system hundreds of high temperature superconducting oxides have been found, with the record Tc of 164 K in Hg-1223 under high pressure. According to their major chemical difference, these superconductors can be classified into six major categories K2Nip4-type cuprates, rare-earth-based cuprates, bismuth-based cuprates, thallium-based cuprates, mercury-based cuprates, and other cuprates. Details of every superconductor are not presented, but at least one typical composition of each major type will be discussed in detail. Interested readers can read the relevant literature to get specific information on other systems. 

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