High Temperature Superconductors

Abstract

This chapter is dedicated to the study of the superconductor–insulator transition in high temperature superconductors. We saw, in Sect.1.3.3, that metal superconductors were not so interesting from the point of view of critical phenomena. This is because their coherence length at T=0 is of the order of a micron, so the critical region is around 10−14K, which is obviously impossible to observe. However this is not true for high temperature superconductors, which exhibit a critical temperature of the order of 100 K. The most important high temperature superconductor family is that of the superconducting cuprates discovered in 1986 by Bednorz and Muller. Since then other families have been discovered for example C60, MgB2 and AsFe. However the most important family is still the one that was discovered first, the cuprate family, which is the only one with superconducting properties above 100K. This is also the most documented family both experimentally and theoretically. For these reasons, in this chapter we will focus only on superconducting cuprates. Their coherence length at T=0 being of the order of \(15\r{A}\), the critical region can reach a few tens of kelvins! So these materials represent, in principle, an ideal case in which to study critical phenomena, even more so given that the transition can be induced by a magnetic field (as for all superconductors) or by doping (a unique property of high temperature superconductors). In principle, these two parameters, can induce quantum transitions at T=0. This physical system certainly seems interesting to test new descriptions of critical phenomena.