ASPECTS OF ELECTRON CORRELATIONS IN THE CUPRATE SUPERCONDUCTORS

We review concepts and effects of electron correlations in the copper- oxide superconductors. The purpose of this article is twofold. First, we provide an overview of results of various electron spectroscopies, Raman scattering and optical conductivity studies with a particular em phasis on experiments which identify the charge and spin correlations relevant to the cuprates. Second, we focus on microscopic theories of the single-particle excitations, and the charge and spin dynamics in t he normal state of cuprates considering those models which incorporate strong electron correlations. The single-particle spectrum of the thr ee-band Hubbard model is reviewed and related to results of electron s pectroscopy. The carrier dynamics in the t-J model and the one-band Hu bbard model at low doping is discussed in detail. We examine approache s which describe the single-particle excitations of correlated electro n systems at finite doping. Theories of the static and dynamic magneti c correlations are considered and we speculate on the consequences of the spin dynamics for Raman scattering and the optical conductivity. F inally, selected phenomenological ideas are reviewed.