Possible (bi)polaron effects in the high-Tc superconductors

In the present paper, the theory of the large bipolaron is reviewed and the possibility of bipolaron formation in the high-Tc superconductors is indicated. Operator and path-integral formulations of the large bipolaron problem are compared. In the strong-coupling limit, the effect of non-optimal upper-bounds to the single-polaron groundstate energy is emphasized. The fact that the interaction with multiple phonon branches enhances the electron-phonon interaction and might result in a larger stability region for bipolaron formation is indicated. Experimental values for the static and high-frequency dielectric constants are used to discuss the relevance of bipolaron formation as a pair-forming mechanism in the high-Tc superconductors. (orig.)