Observation of a new collective mode involved in the Verwey transition in magnetite

Magnetite (Fe₃O₄), the oldest known magnetic material, is a strongly correlated transition metal oxide that exhibits complex electronic and structural properties. Below the Verwey transition, the low-temperature insulating phase displays rich features, including the coexistence of charge and orbital orders whose detailed nature is still under debate. Using pump-probe spectroscopy, we photoexcite the system out of equilibrium with a tailored femtosecond laser pulse and examine the evolution of the coherent oscillations due to the system’s collective modes in the time domain. We observe signatures of a new mode that undergoes a dramatic softening as a function of laser fluence. We discuss the implication of this collective mode on the understanding of the Verwey transition.