Vibronic excitation dynamics in orbitally degenerate correlated electron system

The orbital-lattice coupled excitation dynamics in orbitally degenerate correlated systems is examined. We present a theoretical framework where both local vibronic excitations and superexchange-type intersite interactions are treated on an equal footing. We generalize the spin-wave approximation so as to take local vibronic states into account. The present method is valid from weak to strong Jahn-Teller coupling magnitudes. Two characteristic excitation modes coexist, a low-energy dispersive mode and a high-energy multipeak mode; these are identified as a collective vibronic mode and Flanck-Condon excitations in a single Jahn-Teller center modified by the intersite interactions, respectively. The present formalism covers vibronic dynamics in several orbital-lattice coupled systems.