High-temperature superconductors

Abstract

The discovery by J. G. Bednorz and K. A. Müller in 1986 that the superconducting state can exist in oxides at temperatures above 30 K stimulated research in the field of superconductivity and opened a new field of research. Within a few years a large number of cuprate superconductors with transition temperatures well above the boiling point of liquid nitrogen have been found. In this chapter, an overview of the major families of high-temperature superconductors and their physical properties is presented. Starting from the well-known characteristics of conventional superconductors, described in Sect. 50.1, the new phenomena observed in high-temperature superconductors are considered. The complexity of the physical properties of the cuprate superconductors is closely related to the fact that these materials are close to a metal–insulator transition. In Sects. 50.2 and 50.3, the crystal structures, the general trends for the critical temperatures, the anisotropy of the physical properties, and the factors limiting the transport critical current density are discussed. Because of their importance in the field of electronics some features of thin films are presented in Sect. 50.4. Strictly speaking, the binary compound MgB2 is not a high-temperature superconductor. Nevertheless, an overview of the physical properties of this interesting metallic superconductor, characterized by a transition temperature as high as 39 K, has been included in the present chapter (Sect. 50.5). In 2008, H. Hosono et al. discovered that the layered superconductor La (O1−xFx ) FeAs (x = 0.11) superconducts at a temperature as high as 26 K. A maximum critical temperature of 58 K has been reported for the iron-pnictide superconductor Sm (O0.74F0.26 ) FeAs. The iron-based superconductors are therefore considered as a second class of high-temperature superconductors. Their properties are described in Sect. 50.6.