Cuprates studies

Resistivity near T = 0 In the region above Tc(x) and below a temperature characterized by Tp(x), a change in the temperature dependence of the resistivity from linear to a higher power is observed. Similarly below a temperature 7> (x) for x > xc, a change in the power law tending towards the Fermi-liquid value of 2 is observed. This is observed in all the cuprates studied, Fig.(4) organizes the data in a number of compounds [8],... 

There have been several studies which report the temperature dependence of the two coefficients C and C2 for different cuprate superconductors. As discussed below in more detail, these results seem to indicate that C has the same behavior in most of the cuprates studied while the behavior of C2 is sample and/or system dependent i.e., the power-law temperature dependence of C is the same for all the specimens studied with an exponent a decreasing as the samples are more underdoped, while C2 displays temperature dependences which differ from system to system. 

After investigating the positron behavior, the next step is to solve the band structure and to calculate the momentum density p (p). It is rather controversial to describe the electronic structure of high-Tc superconductors by band structure. But it is almost the only way to obtain a calculated momentum density in order to interpret the positron measurements. We refer to the chapter by Pickett and Mazin in this volume (ch. 193) for a general discussion of band-structure calculations in superconducting cuprates. Studies in relation with positron annihilation have been made using different methods FLAPW (Massidda 1990, Massidda et al. 1991), LMTO (Bansil et al. 1988, Barbiellini et al. 1992,... 

The thermodynamics of the 123 compound have been extensively studied and writing dre formula as (YBa2Cu3)Oj. it has been shown that at 1000 K the value of z changes from 6 to 6.6 in dre oxygen pressure range 10 " to one atmosphere of oxygen. Above this temperature, the 123 compound decomposes to 211 plus barium cuprate and cuprous oxide according to the reaction... 

Asymmetric induction by sulfoxide is a very attractive feature. Enantiomerically pure cyclic a-sulfonimidoyl carbanions have been prepared (98S919) through base-catalyzed cyclization of the corresponding tosyloxyalkylsulfoximine 87 to 88 followed by deprotonation with BuLi. The alkylation with Mel or BuBr affords the diastereomerically pure sulfoximine 89, showing that the attack of the electrophile at the anionic C-atom occurs, preferentially, from the side of the sulfoximine O-atom independently from the substituent at Ca-carbon. The reaction of cuprates 90 with cyclic a,p-unsaturated ketones 91 was studied but very low asymmetric induction was observed in 92. 

Studies of die structures of cuprate species were initiated to elucidate die niedi-anisnis by wbidi tliey interact witli substrates and to understand dieit special reactivities. tn die early days tliese investigaiions were restricted to solution studies by spectroscopic tediniques. It was not until 1982 dial tlie dtst example of a cuprate species - [iCu Pbi-jiLiiTHFj))] - was stiuctutally diatacterlzed by X-tay crystal stiucture deterniination [ 100] ivide infra). It sbotild be noted tliat most of diese studies, reviewed previously [29, 45, 101], were limited to "simple" alkyl and aryl derivatives. 

On tlie basis of tliis Lotidusioti and on NMR studies of Lomplexes of 17b witli Lewis adds, a transition state model to explain tlie observed sdectivity was proposed. Tliis involved initial Lomplexation of a cuprate litliium ion to tlie tliree different betetoatoms in tlie substrate, followed by fotniation of a d-E complexation... 

Conjugate additions lo a,/ -unsalutaled kelones and eslets ate die most Impotlanl ctiptale reactions. Kinetic studies by Ktauss and Sniidi on MezCuIi and a variety of ketones teveaied die following kinetic cliatacterislics lEq. 10.5), fitsl otdet bodi in cuprate dimer and in die etione [60]. 

Rate enhancement and an improved stereoselectivity was also found for higher-order cuprates when chlorotrimethylsilane was addedl9. H- and 29Si-NMR studies revealed that higher-order... 

Among the high-temperature superconductors one finds various cuprates (i.e., ternary oxides of copper and barium) having a layered structure of the perovskite type, as well as more complicated oxides on the basis of copper oxide which also include oxides of yttrium, calcium, strontium, bismuth, thallium, and/or other metals. Today, all these oxide systems are studied closely by a variety of specialists, including physicists, chemists, physical chemists, and theoreticians attempting to elucidate the essence of this phenomenon. Studies of electrochemical aspects contribute markedly to progress in HTSCs. 

There has been a great deal of study concerning the effect of solvents and other reaction conditions on the stability and reactivity of organocuprate species.8 These studies have found, for example, that (CH3)2S-CuBr, a readily prepared and purified complex of CuBr, is an especially reliable source of Cu(I) for cuprate preparation.9 Copper(I) cyanide and iodide are also generally effective and, in some cases, preferable.10... 

The important and widely studied copper-oxide-derived high-temperature superconductors, known as cuprate superconductors, are basically insulators. Doping converts these into metallic materials, many of which are superconductors over rather more restricted composition ranges. Several of these materials have already been discussed La2Cu04 and Sr2Cu02F2 (Section 4.3.3), La2 A.SrxCu04 (Section 8.5.1), and Nd2, Ce,Cu04 (Section 8.5.2). 

The solution structures of cyano-Gilman cuprates and lower-order cyanocuprates have been studied by cryoscopic measurements in THF [141]. The results of this study have in several cases shown ways to obtain useful single crystals of several higher- and lower-order cyanocuprates and consequently to determine their structures in the solid state. It appears that a number of these cyanocuprates retain their observed solid-state structure when dissolved in THF. 

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