Polarization waves and superconducting instabilities in electron systems

Electron-electron attractions originating with dynamical correlations are considered for inhomogeneous systems where the electrons can be explicitly partitioned into two classes: itinerant (valence) and quasilocalized (core). Dynamical correlations in the near homogeneous itinerant class are well captured by analytical forms of local-field factors when constrained to obey sum rules. For the nonoverlapping quasilocalized electrons a reduced description of their collective dynamical behavior can be given by appealing to bosonic excitations of polarization wave character. It is argued that in addition to the Kohn-Luttinger mechanism of intrinsic superconductivity in the itinerant class, there is another attractive pairing channel (and hence a possible further enhancement of Tc) arising from the exchange of these polarization waves, again of wholly electronic origin. Numerical estimates, via solution of the Eliashberg equation, suggest that the polarization wave channel can be quite significant in mediating electron pairing.