Optical absorption from a nondegenerate polaron gas - art. no. 153101

We apply the dynamical mean field theory to calculate the low-density limit of the optical conductivity in the Holstein model. This nonperturbative tr eatment allows continuous spanning from the weak-coupling quasifree-electro n behavior to the strong-coupling small polaron picture. Different features arise in the intermediate coupling regime: (i) a noticeable transfer of sp ectral weight toward low frequencies occurs at low temperatures, when coher ent processes acquire importance; (ii) thermally activated polaron interban d transitions appear at nonzero temperature, whose spectral weight has a no nmonotonic temperature dependence. We propose an interpretation of the infr ared active vibrations observed in the optical spectra of underdoped superc onducting cuprates in terms of small polarons in the intermediate coupling regime.