Discovery of the soft electronic modes involved in magnetite’s Verwey transition*

The Verwey transition in magnetite (Fe₃O₄), the first metal-insulator transition ever observed, is accompanied by charge ordering, orbital ordering, and a structural rearrangement. Due to the intricate interplay of these degrees of freedom, a complete understanding of the microscopic mechanism of this transition to the exotic low-temperature phase has not yet been reached. Recently, the charge-ordered structure was established as a network of small polarons extending across three sites, termed trimerons. However, the dynamics of the Verwey transition from an electronic point of view remains not understood, as no collective excitations of this trimeron order have been observed to date. In this talk, I will present our discovery of spectroscopic signatures of the low-energy electronic modes of the trimeron network using terahertz light. By exciting these collective modes coherently with an ultrashort laser pulse, we unveil their softening towards the transition temperature. These findings represent the first observation of collective modes of any sort displaying a critical softening in magnetite and thus shed light on the long-sought cooperative mechanism responsible for the Verwey transition.