Abstract
Theoretical and experimental work concerning high-temperature superconductors in general and cuprates in particular is reviewed. A detailed analysis of the current knowledge of the subject suggests that when in the normal state, superconducting cuprates behave very much the same as 'conventional' metals. Experimental evidence is presented for the existence of strong relaxation processes in the normal state of HTSC systems at low energy. Ab initio calculations of the optical spectra and the electron–phonon interaction (EPI) show that the electron-phonon mechanism explains many features of low-energy relaxation processes in HTSC systems, including the high critical temperature. However, many properties of the superconducting state, for example, the anisotropic d pairing in cuprates, cannot be explained with the EPI mechanism alone. A number of models for cuprate superconductors with EPI and Coulomb repulsion are discussed in detail.