Understanding the interplay between lattice dynamics and phase transitions in strongly correlated oxides remains a central challenge in condensed matter physics. Even magnetite (Fe₃O₄), despite being studied for decades, continues to be the focus of active research due to its puzzling Verwey transition, a metal-insulator transition accompanied by a complex structural distortion and charge/orbital ordering [1].
In this framework, the use of ultrafast techniques allowed for novel approaches to the detection of transient states associated to the phase transitions, in particular highlighting the importance of the coupling of nonthermal electronic perturbations with symmetry-specific phonons in driving or stabilizing the Verwey transition [2,3].
Here, we present the results of a time-resolved spontaneous Raman spectroscopy (TRRS) study on Fe₃O₄, aimed at probing the mode-resolved phonon dynamics following impulsive 633 nm excitation. Measuring both the antiStokes and Stokes sides of the Raman spectrum of a magnetite sample poised below the Verwey transition temperature, we were able to simultaneously monitor the evolution of the crystal symmetry and the transient phonon population subsequent to the laser pump pulse, in the picosecond timescale. The comparison of these distinct dynamics can point out the role of specific phonons in accompanying or driving the phase transition.
The observed spectral changes, consistent with previously reported signatures of the Verwey transition [4], reveal the disruptive effect of the pump pulse on the low-temperature charge/orbital ordered phase and the photoinduced transition to the high-symmetry state. The antiStokes spectra show the prompt enhancement of the phonon population induced by the optical pumping and reveal distinct behavior of two Raman-active optical phonons. The A1g mode exhibits “hot phonon” characteristics, indicating a strong coupling with the photoexcited electrons, while the Eg mode behaves as a “colder”, less coupled, phonon. This points to the presence of mode-selective relaxation pathways, with energy transfer from excited carriers preferentially directed to lattice vibrations of specific energy or symmetry.
Overall, our findings provide new insights into phonon-specific energy flow and phase evolution in magnetite and underscore the capability of TRRS to unravel ultrafast lattice dynamics, making it a valuable tool to explore the physics of phase transitions in complex oxides and strongly correlated materials.

References
[1]  J. García and G. Subías, J. Phys.: Condens. Matter 16 R145 (2004).
[2]  W. Wang et al., Sci. Adv. 9 eadf8220 (2023).
[3]  B. Truc et al., Proc. Natl. Acad. Sci. U.S.A. 121 (26) e2316438121 (2024).
[4]  S. Borroni et al., Phys Rev B 98, 184301 (2018).