Electronic Phase Separation and Electron–Phonon Coupling in Cuprate Superconductors

Abstract

PrologueImmediately after the discovery of high-temperature superconductivity by Bednorz and Müller [1] late Ernst Sigmund and his group at the University of Stuttgart started to work on the problem how doping of charge carriers alters the electronic properties of the perovskite material. These efforts were supported by Vladimir Hizhnyakov from the University of Tartu, who together with Ernst Sigmund proposed an inhomogeneous doping mechanism for the cuprates in 1988 [2][3][4][5]. According to their picture the individual charge carriers form localized magnetic polarons in a fluctuating antiferromagnetic environment which then cluster together to form metallic regions (cf. Fig. 1.1). Above some critical doping a percolative network is realized, which then becomes superconducting. During these early days, such a scenario was viewed with scepticism by the high-T c community, in particular because only few experiments supported some kind of electronic phase separation in the cuprates. By a lucky coincidence, Reinhard Kremer from the nearby Max-Planck institute in Stuttgart started thermal quenching experiments on lanthanum cuprates which strongly supported the formation of an electronically inhomogeneous state in these materials. Alex Müller, who had an established, close contact