QUASI-PARTICLE PROPERTIES OF SMALL POLARONS AND BIPOLARONS

The question to what extent small polarons and bipolarons can exist as well-defined itinerant quasiparticles is examined in terms of their s pectral functions. Although in a certain regime of the parameter space of the electron-lattice coupling and adiabaticity ratio, such polaron ic quasiparticles can in principle exist as itinerant states, we show that their bandwidth is so small that their formation in real material s is most unlikely. Outside this parameter regime where such potential ly itinerant states are theoretically possible, we show that their res idual interaction with the lattice vibrations and/or with each other l eads to a dynamical dephasing between the charge carriers and the loca l lattice deformations surrounding them and hence destroys any such it inerant quasiparticle features. The conjectures made here are meant to serve as a guideline to interpret experimental investigations using p hotoemission spectroscopy and are tested theoretically on the basis of small clusters using exact diagonalization studies. [S0163-1829(98)05 038-3].