On the Verwey transition in magnetite: the soft modes of the metal-insulator transition

Fe$_3$O$_4$ is the earliest discovered magnetic material. It is widely used in recording devices or as a catalyst [1, 2], and some of its electronic and magnetic properties hold promise for future applications [3]. Magnetite undergoes a structural phase transition (PT) accompanied by a Metal-Insulator Transition (MIT) below 120 K, which was originally proposed by Verwey to be driven by the onset of a periodic modulation of the charge on the iron sites [4]. While the pattern of the charge ordering has been shown to be far more complex than the original Verwey hypothesis [5, 6], the mechanism behind its formation is still a subject of debate. As an alternative to a purely electronic model, the interplay between structural and electronic degrees of freedom has been proposed to drive the MIT [7, 8]. However, direct evidence for the involvement of the structure has never been obtained, since no clear anomalies of any phonon mode have been observed across the critical region. Here, we reveal by means of ultrafast broadband optical spectroscopy and spontaneous Raman scattering that the primary order parameters of the structural PT are strongly coupled to the charge density wave, and undergo a sizable softening in the proximity of the critical temperature. Furthermore, we show that signatures of the low-temperature phase can be induced by impulsive charge transfer excitation even at room temperature, where fluctuations of the charge order have recently been shown to persist [9]. These results clarify the mechanism for the Verwey transition and open novel scenarios in the coherent control of MITs.