  Coupling between electrons and phonons (lattice vibrations) drives the
formation of the electron pairs responsible for conventional superconductivity.
The lack of direct evidence for electron-phonon coupling in the electron
dynamics of the high transition temperature superconductors has driven an
intensive search for an alternative mechanism. A coupling of an electron with a
phonon would result in an abrupt change of its velocity and scattering rate
near the phonon energy. Here we use angle resolved photoemission spectroscopy
to probe electron dynamics -velocity and scattering rate- for three different
families of copper oxide superconductors. We see in all of these materials an
abrupt change of electron velocity at 50-80meV, which we cannot explain by any
known process other than to invoke coupling with the phonons associated with
the movement of the oxygen atoms. This suggests that electron-phonon coupling
strongly influences the electron dynamics in the high-temperature
superconductors, and must therefore be included in any microscopic theory of
superconductivity.
