Signature of stripes in the optical conductivity ofLa2NiO4.11

The temperature dependence of the infrared reflectivity spectra of a single crystal of ${\mathrm{La}}_{2}{\mathrm{NiO}}_{4+\ensuremath{\delta}},$ with an oxygen excess $\ensuremath{\delta}=0.11,$ is analyzed by both Kramers-Kronig analysis and an attempt to fit with optical conductivity models for the polarization within the basal plane. This oxygen excess corresponds to a composition of the phase diagram where stripe formation has been reported by other methods. In particular, highly anomalous low-frequency tails of incompletely screened phonon responses are suspected to be the signature of static charge stripes, treated here as multipolarons. By comparison with the positions of extra peaks and with the dispersion curve of phonons, a charge-order wave vector Q has been determined. More generally, the analysis allows us to discriminate trapped and mobile charge carriers. A change of regime from mobile to trapped polarons upon cooling is evidenced. Two thresholds in the temperature dependence of relevant parameters and/or data are evidenced for this sample. One is assigned to charge ordering and the other confirms the onset of combined charge and spin order reported at low temperature by other methods.