Optical conductivity of single crystals of La2-xSrxNiO4.

Abstract

Reflectance data (50--50 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) on single crystals of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4}$ (x=0,0.2) are used to determine the a-b plane optical conductivity (${\mathrm{\ensuremath{\sigma}}}_{1}$). Similar to the La cuprates, Sr doping introduces a strong mid-ir band in ${\mathrm{\ensuremath{\sigma}}}_{1}$ with a peak at 0.6 eV. The ${\mathrm{\ensuremath{\sigma}}}_{1}$ data are not in agreement with the marginal Fermi-liquid model proposed recently, but are rather well described by a modified Drude-Lorentz model. The hole plasma is overdamped and exhibits a large effective mass m\ensuremath{\sim}30${\mathit{m}}_{0}$. The mid-ir band is well fit by a broad Lorentz oscillator, suggesting a collective mode; its origin cannot be ascribed to interband transitions involving Sr states, or to charge-transfer absorption.