Quantum tunneling in a metal and the heavy electron problem

If the spectrum of single-electron states in a metal contains a comparatively narrow peak in the density of states associated with the presence of a narrow band, hybridization, or a phase region with a small group velocity, then the electron polaron effect leads to a sharp decrease in the peak's width. This narrowing is due to the Coulomb interaction between electrons in the density peak and those in the remaining part of phase space. If the renormalized width is smaller than the Debye temperature, additional narrowing of a purely phonon nature occurs which is nonlinearly amplified due to the electron polaron effect.