Magnetic correlations in high temperature superconductivity

Abstract

Many experimental findings in the materials family of cuprate superconductors are converging towards an electronic picture which involves strong local Coulomb correlations. Characteristic normal state properties appear to be beyond the conventional framework of Landau's Fermi liquid theory. Spin correlations among the charge carriers are discussed as a possible source for the intricate electronic properties. The low energy physics is determined by the charge and spin dynamics in CuO 2 layers which all cuprate superconductors have in common despite their otherwise complex composition. Strong short range Coulomb repulsion between the charge carriers in these planar units imply magnetic ordering phenomena among the Cu spins. Depending on the doping controlled carrier concentration the cuprate compounds are either antiferromagnetic (AF) insulators or correlated metals with short range spin correlations extending also into the superconducting state. The purpose of this article is twofold: In the first part we review results of basic experiments which probe the magnetic correlations in the CuO 2 planes. Particular emphasis is given on neutron and Raman scattering, nuclear magnetic relaxation, and a selected set of experiments which contain information about the nature of the superconducting state. Based on these experiments the evolution of the spin dynamics from the AF insulator into the doped metallic regime will be discussed. On the other hand we give an overview of current phenomenological and microscopic approaches in the context of purely electronic Hubbard type models which focus on the role of AF spin fluctuations. This includes the results for single particle properties as well as for the dynamic spin susceptibility. Special attention is given to the discussion of spin fluctuation exchange as the possible mechanism underlying high temperature superconductivity.