Doping dependence of the a-b-plane optical conductivity of single-crystal La2-xSrxNiO4+ delta.

Abstract

Results of room-temperature a-b-plane optical-reflection studies of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4+\mathrm{\ensuremath{\delta}}}$ (\ensuremath{\delta}=0.0; x=0, 0.05, 0.10, 0.20, 0.21, and \ensuremath{\delta}=0.085; x=0) are presented and cover the photon energy range 0.0056 eV. A Kramers-Kronig analysis is used to compute the optical conductivity \ensuremath{\sigma}(\ensuremath{\omega}) and energy-loss function Im(-1/\ensuremath{\varepsilon}(\ensuremath{\omega})). The doping dependence of the TO and LO a-b-plane phonon modes, anomalous midinfrared absorption (mid-IR band), and interband transitions are discussed. The mid-IR band (band maximum ${\mathrm{\ensuremath{\omega}}}_{\mathit{m}}$\ensuremath{\sim}0.5 eV) is found to be similar to that observed in the isostructural materials ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$, and is identified with photon-assisted hopping of small polarons. A microscopic theory developed by Reik is used to obtain polaron parameters whose values are compared to those obtained similarly for other small-polaron materials.