Electron-Phonon Mass Enhancement in Multi-Layers
  A strong electron-phonon interaction in a metal increases the electron density of states in the vicinity of the Fermi energy dramatically. This phenomenon is called electron-phonon mass enhancement. In this paper the question is investigated whether the mass enhancement can be manipulated in multi-layers of two metals with strong and weak electron-phonon interaction. A rich behavior is observed for different thickness ranges of the layers. For thin layers one observes a rather homogeneous averaged enhancement. However, for an intermediate thickness range the mass enhancement is highly anisotropic, i.e. direction dependent, as well as position dependent. For large layer thicknesses one obtains the bulk behavior for each metal. 
Gerd Bergmann