Understanding the Oxide Superconductors

Abstract

Here you will find descriptions of the various classes of compounds in which bonding between metal atoms and oxygen is a prominent feature and which become superconducting at relatively high temperatures (often greater than 77K). Compounds of this kind, much investigated since the epochal work of Bednorz and Müller in 1986, are high-temperature or high-transition-temperature superconductors — usually referred to as high-T _c superconductors. It is argued that the bonds referred to, which are affected by doping, give rise to resonances (somewhat narrow absorptions) in the spectral density function associated with the dielectric permittivity DP (as defined in one of the usual ways), and that these resonances (excitations) can lead to Cooper-pairing of the electrons and hence superconductivity. The resonances are excitations of many (coupled) small sub-systems, in each of which the distances between the metal and oxygen atoms are strongly sensitive to the state of excitation of the bonding electrons. It is argued further that analysis of such a system is greatly facilitated by including at an early stage consideration of its DP. DP’s for simple model systems are exhibited, special attention being paid to the way in which the resonances are incorporated in them and how these resonances enter into the determination of T _c as a functional of the DP. Finally, it is conjectured that Bose condensation of bipolarons formed form the self-trapped holes which give rise to the resonances — as well as Cooper-pairing of the electrons — can contribute to the superconductivity of these oxides.