Electron – phonon interaction in high-temperature superconductors

Results indicating an important role of the electron – phonon interaction in high-temperature superconductivity compounds are presented with emphasis on its implications for angle resolved photoemission and optical conductivity. The dimensionless phonon coupling constant $\lambda$ is determined by comparing the experimental and theoretical results. While undoped materials are in the strong coupling ($\lambda$-near-unity) regime, hole doping decreases $\lambda$, bringing compounds to the intermediate coupling regime at the optimum hole concentrations.