Violation of the Migdal theorem for a strongly coupled electron-phonon system
Aleksandrov A. S. , Grebenev V. N. , Mazur E. A.
The classical theory of electron-phonon interaction in metals shows that in the case of strong coupling, λ > 1, allowance for the finite width of the electron band leads to its polaron collapse. The band width renormalized by the electron-phonon interaction is smaller than the characteristic phonon frequencies and the interaction becomes essentially nonadiabatic.