Abstract

The high-temperature superconductors now fall into several structural and compositional classes. However, they all are oxides of copper, show little or no isotope effect, are relatively poor metals in their normal state, have very low carrier densities, and are closely related in composition to compounds which exhibit magnetic order. All of these attributes, together of course with the very high values of T_c, strongly suggest that the heretofore very successful microscopic theory of superconductivity based on the electron-photon interaction is inadequate here. Alternative theories have now appeared which invoke excitons, plasmons, polarons, magnons, or other pairing mediators. Recently inelastic light scattering has provided direct observation of the spin and electronic excitation spectra in single crystals of the superconducting cuprates and their insulating magnetically ordered relatives. This paper reviews these observations. Particular emphasis is given to the spin fluctuations which typically are characterized by quite high (~1000-cm^-1) exchange energies and their behavior with temperature and composition. Correlations between the spin fluctuation spectra and T_c are described. In addition evidence for a superconducting energy gap is presented as is the influence of electron-phonon interactions on Raman line shapes. The relationships of all these results to candidate theories for high T_c superconductivity are discussed.

© 1988 Optical Society of America