Fig. 6: Schematic of the energy potential surface after the CT excitation.
a Schematic energy surface on the QAFD and QFE plane before (blue) and just after (red) photoexcitation. Photoexcited itinerant electron (photocarrier) is coupled with the localized electrons on the ground state surface. b Time evolution of the quartic potential surface cutting along the direction of QPPCC mode shown in (a). The circles and the arrows in the local minima of the potential curves indicate the electrons and spins, respectively. The CT excitation by laser induces the excitation into the different potential surface as the Fe2+ itinerant electrons which is distinct from that of the localized electrons on the ground state energy surface. The excited itinerant and localized electrons are tightly bound with each other and relaxed into to a metastable state within a few ps through the combined energy potential surfaces. Due to coupling between phonon modes and localized photocarrier (PPCC effect) similar to the phonon-dressed-exciton, phonon modes are strongly softened.
