Electron - phonon coupling in underdoped high-temperature superconductors

Results indicating the important role of the electron - phonon coupling in high-temperature superconductivity comp ounds 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. Although undoped materials are in the strong-coupling (lambda similar to 1) regime, hole doping decreases lambda, bringing compounds to the intermediate-coupling regime at optimum hole concentrations.