Light-induced structural hidden phases in Magnetite*

Magnetite (Fe₃O₄) is probably one of the most studied transition metal oxides due to its atypical phase transition. Near 125K, the system undergoes a metal-insulator transition (MIT) accompanied by a structural transition from a cubic to a monoclinic phase, as well as a magnetic rearrangement, known as the Verwey transition. This peculiar transition originates from the complex interplay of degrees of freedom (charge ordering, orbital ordering, and trimeron formation). Ultrafast photon pulses offer the appealing capability to manipulate such degrees of freedom and their respective coupling giving rise to the emergence of metastable (hidden) states. 

Here, using two different photon energies, we directly visualize the photo-induced structural dynamics by means of ultrafast electron diffraction (UED) technique providing sub pm/ps spatio-temporal resolution. Our data reveal opposite behaviors when photoexciting the system in the visible (400nm) or with near-infrared (800nm) light. The difference is associated with the triggering of two distinct electronic excitations. The 800nm photoexcitation destroys the long-range trimeron ordering consistent with previous studies and induces the recovery towards the cubic phase, while the 400nm acts as photodoping and strengthens the network, leading to a stronger monoclinic distortion. These results demonstrate the ability to establish novel hidden phases in quantum materials via specific electronic excitations in a strongly correlated environment.