Optical absorption from a non degenerate polaron gas
  We apply the Dynamical Mean Field Theory to calculate the low-density limit of the optical conductivity in the Holstein model. This non perturbative treatment allows to span continuously from the weak-coupling quasi-free electron behaviour to the strong-coupling small polaron picture. New features arise in the intermediate coupling regime: i) a noticeable transfer of spectral weight towards low frequencies occurs at low temperatures, when coherent processes acquire importance; ii) thermally activated Polaron Interband Transitions appear at non zero temperature, whose spectral weight has a non monotonic temperature behaviour. We propose an interpretation of the infrared active vibrations (IRAV) observed in the optical spectra of the underdoped superconducting cuprates in terms of small polarons in the intermediate coupling regime. 
S. Fratini F. de Pasquale S. Ciuchi