Strong coupling to multiple phonon modes in high-temperature superconductors

The spectra of the second derivative of tunneling current d²I∕dV² in the high-temperature superconductors YBa₂Cu₃O_7-δ and Bi₂Sr₂CaCu₂O_8+δ show clear dip and peak features due to strong coupling to the bosonic modes mediating electron pairing. The energy positions of nearly all the peaks in -d²I∕dV²-like spectra match precisely with those in the phonon density of states obtained by inelastic neutron scattering. The results 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.