抄録
The system of two electrons coupling with phonon fields is studied with the use of the path integral method. In particular, the condition of the formation of a bipolaron and its physical properties are investigated in detail. The deformation potential interaction with acoustic phonons (the coupling constant Sac) and the Fröhlich interaction with optical phonons (Sop) act to help the bipolaron formation against the direct Coulomb repulsion which prevents it. Usually, Sac plays a dominant role. With ε∞⁄ε0<<1 and the optical phonon energy not so large compared with the electron band width, however, the bipolaron can be formed at large values of Sop even when Sac=0. The bipolaron always has an enormous effective mass; thus it is practically localized. The mean distance between two electrons in the bipolaron is usually of the order of the interatomic distance.
