Quasiparticle 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 spectral functions. Although in a certain regime of the parameter space of the electron-lattice coupling and adiabaticity ratio, such polaronic quasiparticles can in principle exist as itinerant states, we show that their bandwidth is so small that their formation in real materials is most unlikely. Outside this parameter regime where such potentially itinerant states are theoretically possible, we show that their residual interaction with the lattice vibrations and/or with each other leads to a dynamical dephasing between the charge carriers and the local lattice deformations surrounding them and hence destroys any such itinerant quasiparticle features. The conjectures made here are meant to serve as a guideline to interpret experimental investigations using photoemission spectroscopy and are tested theoretically on the basis of small clusters using exact diagonalization studies.