  The spectra of the second derivative of tunneling current d^{2}I/dV^{2} in
the high-temperature superconductors YBa_{2}Cu_{3}O_{7-\delta} and
Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} show clear dip and peak features due to strong
coupling to the bosonic modes mediating electron pairing. The energies of all
the peaks in -d^{2}I/dV^{2}-like spectra match precisely with the energies of
the peaks in the phonon density of states obtained by inelastic neutron
scattering. The results clearly demonstrate that the bosonic modes mediating
the electron pairing are phonons and that high-temperature superconductivity
arises primarily from strong coupling to multiple phonon modes.
