Strong electron-phonon interaction in the high-Tcsuperconductors: Evidence from the infrared

We show that low-frequency structure in the infrared reflectance of the high-temperature superconductor ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ results from the electron-phonon interaction. Characteristic antiresonant line shapes are seen in the phonon region of the spectrum and the frequency-dependent scattering rate of the mid-infrared electronic continuum has peaks at 150 ${\mathrm{cm}}^{\mathrm{-}1}$ (19 meV) and at 360 ${\mathrm{cm}}^{\mathrm{-}1}$ (45 meV) in good agreement with phonon density-of-states peaks in neutron time-of-flight spectra that develop in superconducting samples. The interaction between the phonons and the charge carriers can be understood in terms of a charged-phonon model.