PHONONS AND MAGNONS UNDER THE SEQUENTIAL STRUCTURAL PHASE-TRANSITIONS IN LA2NIO4

Phonons and magnons in La2NiO4 were investigated by Raman scattering. This crystal shows sequential structural phase transitions, HTT-(580 K)-LTO1-(73 K)-LTO2. The observation of soft modes relating to the structural phase transitions revealed a mixing of LTO1 and LTO2 structures. This phase mixing increases a structural disturbance and might cause carrier localization. The decrease of residual carrier density is also observed in the increase of two-magnon peak intensity in LTO2. The observation of multi-phonon scattering up to the fifth order in spite of the off resonant condition with the charge transfer excitation indicates that this resonant scattering is induced through Frank-Condon Raman polarizability and resonantly excited electronic states are localized mid-gap states with anharmonic vibrational potentials - possibly phonon polaronic states.