Raman spectroscopy of the charge-orbital ordering in layered manganites

The lattice and electronic response in the course of concomitant charge and orbital-ordering transition has been investigated for single crystals of layered manganites, single-layered La0.5Sr1.5MnO4, and bilayered LaSr2Mn2O7, by Raman-scattering spectroscopy. In the charge-orbital-ordered state (T<217 K for La0.5Sr1.5MnO4 and similar to 100 K<T<210 K for LaSr2Mn2O7), four phonon modes are activated in the MnO2 plane, indicating the existence of both Jahn-Teller type and breathing mode type lattice distortions. The diffuse scattering response with B-1g symmetry is suppressed in the charge-orbital-ordered state, both for La0.5Sr1.5MnO4 and LaSr2Mn2O7, signaling that the fluctuation caused by the dynamical charge-orbital correlation is suppressed in the charge-orbital-ordered state. These features reveal the unique nature of the charge-orbital-ordering transition realized by the complex interplay of charge and orbital degrees of freedom.