Cuprates high Tc superconductors

All the high Tc superconductors discovered so far, with one exception, contain weakly coupled copper oxide, Cu02, planes. The highest critical temperatures are found for cuprates containing a Group 2 metal (Ca, Ba, Sr) and a heavy metal such as Tl, Bi, or Hg. The structures of all the cuprate superconductors are based on, or related to, the perovskite structure. The one report (in 2000) of a non-cuprate high T superconductor is of surface superconductivity in Na WOs. The structure of NUxWOs is also based on the perovskite structure. 

Ab initio methods as well as looking at the structure can of course consider band structure. In the first rush of excitement following the discovery of high Tc superconductors, a number of studies on the band structure of cuprates were undertaken. The results of these up to 1989 are reviewed in detail by Pickett. These calculations established that the bands that crossed the Fermi level were antibonding Cu 3d /O 2p bands. Most of them used an LDA functional but these compounds are expected to be strongly correlated and there is experimental evidence for local Coulomb repulsion on Cu(ii) sites. Larbaoui et found that using a GGA functional... 

Concluding this subsection, we stress that there is no experimental evidence for a clear superconducting energy gap in any of the cuprates. Residual absorption is universally found in high-Tc superconductors. This behavior is different from the response of other... 

Electrical and Structural Data of High-Tc Superconductors. In Tables 4.2-13 and 4.2-16 electrical and structural data, respectively, of selected high- Tc cuprates are summarized. The data were compiled on the basis of review articles [2.13-16]. In many cases the real stoichiometric coefficients of oxygen are a few tenths higher or lower than the one-digit numbers indicated in the formulas of the tables. The value A7[ is the difference between the temperatures at which the resistance reached 90% and 10% of the normal state resistance, respectively, during cooling of the sample p Q) is the residual resistivity expressed by the formula p T> Tc) = p(P) + piT) with p = dp/dT. 

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,... 

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). 

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