Superconductivity high-temperature layered cuprates

In view of the importance of high-temperature superconductivity in the layered cuprates and the role played by the rare earths, it seemed appropriate to prepare these volumes of the Handbook on the Physics and Chemistry of Rare Earths on High-Temperature Superconductivity in Layered Cuprates . We believe that researchers already working in this field, as well as those intending to enter this field, will find valuable information in the review articles contained in these volumes. Since many of the cuprate superconductors do not contain rare-earth or actinide elements, yet have characteristics and properties similar to those that do, the range of materials considered in these volumes has been broadened to a limited extent to include all high-temperature cuprate superconductors, irrespective of whether they contain rare-earth or actinide elements. 

CUO2 layers appear in all cuprate superconductors and appear to be a necessary but not sufficient condition for high temperature superconduction. The La2SrCu20g 2 compound has CUO2 layers but does not superconduct. Experiments also indicate that T is proportional to the carrier density in the CUO2 layer but not to the volume carrier density, which is further evidence that the YBa2Cu202 is a two-dimensional superconductor. 

All told, the theory makes predictions for weakly doped cuprates for temperatures up to Tc which are in remarkable agreement with experimentation. Our end result is that high temperature superconductivity is primarily an electron correlation effect possibly supplemented by longer range polaronic attraction of the type discussed by Mott and Alexandrov (see [8] for other references). Indeed, it can be argued that this is a theory of unbound bipolarons on a cuprate layer where the Fermion statistics are strictly maintained. 

M.B. Maple, High-temperature superconductivity in layered cuprates overview 1... 

More than 15 years after the discovery of high-Tc superconductivity in layered cuprates its mechanism is still under debate. This has to do with the asymmetry of physical properties between the electron-doped and hole-doped side of the complex phase diagram, temperature vs. doping, T(x), and with the fact that no consensus has been reached about the question what are the key experiments a theory of high-Tc superconductivity must be able to explain. In this paper we argue that the elementary excitations and their interdependence with spin excitations in the cuprates are of central interest in order to learn more about the correlations in general and, in particular, about the mechanism for Cooper-pairing in these systems. 

Since the discovery of high temperature superconductivity in cuprates, there has been intense interest in transition metal oxides with strongly layered, (quasi) two-dimensional (2D) crystal structures and electronic properties. For several years now alkali-metal intercalated layered cobaltates, particularly Na CoCL (NxCO)withx 0.50 — 0.75, have been pursued for their thermoelectric properties [1] IAX C0O2 is of course of great interest and importance due to its battery applications. The recent discovery[2] and confirmation[3-5] of superconductivity in this system, for x 0.3 when intercalated with H20, has heightened interest in the NxCO system. 

Studies on other high-temperature superconductors Positron annihilation measurements across Tc, coupled with the calculations of PDD have been carried out in a variety of hole-doped superconductors that include YBa2Cu40g [48], Bi-Sr-Ca-Cu-0 [49], and Tl-Ba-Ca-Cu-0 [50, 51] systems. We will not labor with the details here, except to state that a variety of temperature dependencies are seen and these can be rationalized when the results are analysed in terms of positron density distribution and the electron-positron overlap function [39]. These calculations show that the positron s sensitivity to the superconducting transition arises primarily from the ability to probe the Cu-O network in the Cu-0 layer. The different temperature dependencies of lifetime, i.e., both the increase and decrease, can be understood in terms of a model of local electron transfer from the planar oxygen atom to the apical oxygen atom, after taking into account the correct positron density distribution within the unit cell of the cuprate superconductor. 

In the meantime some new routes towards high temperature and possibly exotic superconductivity have been investigated. This is the case for the heavy fermion compounds in which the close interplay between local magnetic moments and the spin of delocalized electrons has led to the possibility of a nonphonon mediated mechanism for electron pairing [4]. A very successful route towards high-T, s has been followed with conducting layered cuprates after the discovery of superconductivity in (La, Sr)2Cu04 [5]. 

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