Polaron Conductions from Band to Hopping Types

Transport properties of polaron are investigated theoretically in the case of acoustic phonon coupling. Large polaron states with very little polarization cloud and small polaron states with large lattice distortion co-exist almost orthogonally to each other. Critical coupling strength at which the ground state changes from large to small polarons is smaller than the coupling strength at which the activated state for the hopping of small polaron becomes lower than the large polaron continuum. The polaron conduction changes successively among three types as the coupling strength increases. The first is the band conduction of large polaron with no activation energy, while the third is the thermally activated hopping of small polaron. In the intermediate type, thermally populated large polaron states play dominant role in the conduction while the small polaron is energetically stable. The drift mobility and the thermoelectric power are calculated for the three types. Optical absorption is also mentioned.