  In conventional superconductors, the electron pairing that allows
superconductivity is caused by exchange of virtual phonons, which are quanta of
lattice vibration. For high-transition-temperature (high-Tc) superconductors,
it is far from clear that phonons are involved in the pairing at all. For
example, the negligible change in Tc of optimally doped Bi2Sr2CaCu2O8 (Bi2212)
upon oxygen isotope substitution (16O to 18O leads to Tc decreasing from 92 to
91 K) has often been taken to mean that phonons play an insignificant role in
this material. Here we provide a detailed comparison of the electron dynamics
of Bi2212 samples containing different oxygen isotopes, using angle-resolved
photoemission spectroscopy. Our data show definite and strong isotope effects.
Surprisingly, the effects mainly appear in broad high-energy humps, commonly
referred to as "incoherent peaks". As a function of temperature and electron
momentum, the magnitude of the isotope effect closely correlates with the
superconducting gap - that is, the pair binding energy. We suggest that these
results can be explained in a dynamic spin-Peierls picture, where the singlet
pairing of electrons and the electron-lattice coupling mutually enhance each
other.
