Coherent generation of symmetry-forbidden phonons by light-induced electron-phonon interactions in magnetite

TL;DR

This study demonstrates that light can coherently generate symmetry-forbidden phonons in magnetite by coupling to critical charge fluctuations above the Verwey transition, revealing new insights into phase transition dynamics.

Contribution

The paper introduces a novel ultrafast spectroscopy approach combined with theoretical analysis to study light-induced phonon generation related to critical fluctuations in magnetite.

Findings

Coherent phonons are generated above the Verwey transition temperature.

Light couples to charge order fluctuations, affecting structural modes.

Methodology enables real-time study of critical fluctuations in phase transitions.

Abstract

Symmetry breaking across phase transitions often causes changes in selection rules and emergence of optical modes which can be detected via spectroscopic techniques or generated coherently in pump-probe experiments. In second-order or weakly first-order transitions, fluctuations of the order parameter are present above the ordering temperature, giving rise to intriguing precursor phenomena, such as critical opalescence. Here, we demonstrate that in magnetite (Fe$_3$O$_4$) light excitation couples to the critical fluctuations of the charge order and coherently generates structural modes of the ordered phase above the critical temperature of the Verwey transition. Our findings are obtained by detecting coherent oscillations of the optical constants through ultrafast broadband spectroscopy and analyzing their dependence on temperature. To unveil the coupling between the structural modes and the electronic excitations, at the origin of the Verwey transition, we combine our results from pump-probe experiments with spontaneous Raman scattering data and theoretical calculations of both the phonon dispersion curves and the optical constants. Our methodology represents an effective tool to study the real-time dynamics of critical fluctuations across phase transitions.