Infrared response of ordered polarons in layered perovskites
  We report on the infrared absorption spectra of three oxides where charged superlattices have been recently observed in diffraction experiments. In La$_{1.67}$Sr$_{0.33}$NiO$_4$, polaron localization is found to suppress the low-energy conductivity through the opening of a gap and to split the $E_{2u}$-$A_{2u}$ vibrational manifold of the oxygen octahedra. Similar effects are detected in Sr$_{1.5}$La$_{0.5}$MnO$_4$ and in La$_2$NiO$_{4+y}$, with peculiar differences related to the type of charge ordering. 
P. Calvani, A. Paolone, P. Dore, S. Lupi, P. Maselli, P. G. Medaglia   (INFM - Dipartimento di Fisica, Universita di Roma ``...