Phase separation in the nonequilibrium Verwey transition in magnetite

Abstract

We present equilibrium and out-of-equilibrium studies of the Verwey transition in magnetite. In the equilibrium optical conductivity, we find a step-like change at the phase transition for photon energies below about 2 eV. The possibility of triggering a non-equilibrium transient metallic state in insulating magnetite by photo excitation was recently demonstrated by an x-ray study. Here we report a full characterization of the optical properties in the visible frequency range across the nonequilibrium phase transition. Our analysis of the spectral features is based on a detailed description of the equilibrium properties. The out-of-equilibrium optical data bear the initial electronic response associated to localized photo-excitation, the occurrence of phase separation, and the transition to a transient metallic phase for excitation density larger than a critical value. This allows us to identify the electronic nature of the transient state, to unveil the phase transition dynamics, and to study the consequences of phase separation on the reflectivity, suggesting a spectroscopic feature that may be generally linked to out-of-equilibrium phase separation.

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

“…Typically, ultrashort laser pulses are used to melt the order parameter of the lowsymmetry phase and thus establish the electronic and structural properties of the disordered phase [3,4]. Here, we discussed a more subtle effect, with implications on the coherent control of transition-metal oxides.…”

“…Optical probes, such as reflectivity [20], ellipsometry [4], infrared spectroscopy and spontaneous Raman scattering [21,22] have disclosed important information on the electronic and structural properties of magnetite relevant to the mechanism of the Verwey transition. Transfers of spectral weight among different energy regions of the optical spectrum are signatures of the strongly correlated nature of magnetite [23], which manifest themselves both in the discontinuous change of the electronic structure at T V and in the precursor variations which progressively preempt the Verwey transition.…”

“…Transfers of spectral weight among different energy regions of the optical spectrum are signatures of the strongly correlated nature of magnetite [23], which manifest themselves both in the discontinuous change of the electronic structure at T V and in the precursor variations which progressively preempt the Verwey transition. The absorption structures below 2.5 eV were generally attributed to interband transitions between Fe 3d states, although alternative interpretations were proposed, at variance on whether only B-or also A-sites are involved [4,20,24,25]. The appearance of a rich spectrum of additional infrared-and Raman-active modes below T V reflects the symmetry lowering of the crystal structure which characterizes the Verwey transition [21,22].…”

Discovery of the soft electronic modes of the trimeron order in magnetite

“…We further note that the dynamics of the damped oscillations can be accurately tracked due to the lack of any relaxation background in the temporal trace. The latter is typically expected from the excitation and subsequent relaxation of charge carriers that are photodoped above the optical gap (E G ∼ 200 meV) [17,24]. Its absence sig- Fig.…”

“…To overcome this experimental difficulty, we illuminate our magnetite crystal with an ultrashort near-infrared laser pulse and drive its collective modes coherently [26]. We vary the laser fluence absorbed by our sample, exploring a regime that allows us to transiently increase the lattice temperature but not completely melt the trimeron order [23,24] (see Supplementary Note 5). We then mon-a b 15…”