Temperature dependence of the mid-infrared absorptions in undoped cuprate Sr₂CuO₂Cl₂

We have studied temperature dependence of three mid-infrared absorption peaks of Sr₂CuO₂Cl₂, observed in transmission measurement; a lower-energy sharp peak and two higher-energy broad peaks. As temperature increases, the first one shows position redshift and width broadening. We relate the behaviors to Lorenzana and Sawatzky's model [J. Lorenzana, G.A. Sawatzky, Phys. Rev. Lett. 74, 1995, p. 1867] and attribute the peak to the phonon-assisted bimagnon excitation. We examine the temperature dependence of the peak width and suggest magnon scattering mechanism coupled with magnetic disorder in CuO₂ plane, in order to explain the anomalous broadening of the width around the Neel temperature.