Raman scattering in high T-c superconductors: phonons, electrons, and electron-phonon interaction

It was realized very soon after the discovery of high T-c superconductors t hat Raman spectroscopy is an excellent technique for the investigation of l ow energy elementary excitations in these materials and also for their char acterization. The primitive cell of high T-c superconductors is usually cen trosymmetric and contains a large number of atoms. This fact results in the existence of a large number of Raman-active (i.e., even) and also IR-activ e (odd) phonons. Although inelastic neutron scattering experiments have bee n performed, most of the experimental knowledge available for high T-c supe rconductors originates from their Raman as well as IR spectra. Raman scatte ring call also be used to investigate low frequency electronic excitations. It was one of the first spectroscopic techniques that revealed the existen ce of a gap for electronic excitations in high T-c superconductors. Soon af ter, it was realized that gaps observed in the electronic Raman spectra exh ibit different features depending on the symmetry of the configuration used for the scattering experiments. Of particular interest is the behavior of the scattering efficiency when the Raman shift tends to zero. The electroni c excitations responsible for the observed Raman gap couple noticeably to s ome of the Raman active phonons, a fact that results in changes in phonon s elf-energies and spectral strengths when crossing T-c. Particularly strong effects that reveal a remarkably large electron-phonon interaction have bee n recently observed for Hg1234 and the isomorphic material (CuC) 1234. In t his article these effects, and Raman scattering related to crystal field ex citations of the rare earth constituents, are reviewed. (C) 1999 Elsevier S cience B.V. All rights reserved.