Raman scattering in high Tc superconductors: phonons, electrons, and electron–phonon interaction

Abstract

It was realized very soon after the discovery of high T c superconductors that Raman spectroscopy is an excellent technique for the investigation of low energy elementary excitations in these materials and also for their characterization. The primitive cell of high T c superconductors is usually centrosymmetric 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-active (odd) phonons. Although inelastic neutron scattering experiments have been performed, most of the experimental knowledge available for high T c superconductors originates from their Raman as well as IR spectra. Raman scattering can also be used to investigate low frequency electronic excitations. It was one of the first spectroscopic techniques that revealed the existence of a gap for electronic excitations in high T c superconductors. Soon after, it was realized that gaps observed in the electronic Raman spectra exhibit 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 electronic excitations responsible for the observed Raman gap couple noticeably to some of the Raman active phonons, a fact that results in changes in phonon self-energies and spectral strengths when crossing T c. Particularly strong effects that reveal a remarkably large electron–phonon interaction have been recently observed for Hg1234 and the isomorphic material (CuC) 1234. In this article these effects, and Raman scattering related to crystal field excitations of the rare earth constituents, are reviewed.