  The Verwey transition in magnetite (Fe$_3$O$_4$) is the first metal-insulator
transition ever observed and involves a concomitant structural rearrangement
and charge-orbital ordering. Due to the complex interplay of these intertwined
degrees of freedom, a complete characterization of the low-temperature phase of
magnetite and the mechanism driving the transition have long remained elusive.
It was demonstrated in recent years that the fundamental building blocks of the
charge-ordered structure are three-site small polarons called trimerons.
However, electronic collective modes of this trimeron order have not been
detected to date, and thus an understanding of the dynamics of the Verwey
transition from an electronic point of view is still lacking. Here, we discover
spectroscopic signatures of the low-energy electronic excitations of the
trimeron network using terahertz light. By driving these modes coherently with
an ultrashort laser pulse, we reveal their critical softening and hence
demonstrate their direct involvement in the Verwey transition. These findings
represent the first observation of soft modes in magnetite and shed new light
on the cooperative mechanism at the origin of its exotic ground state.

Strongly Correlated Electrons