Anharmonicity due to Electron-Phonon Coupling in Magnetite

TL;DR

This study investigates phonon behavior in magnetite near the Verwey transition, revealing strong anharmonicity due to electron-phonon interactions and suggesting a cooperative transition involving many phonons and charge fluctuations.

Contribution

It provides the first combined experimental and ab initio analysis linking phonon anharmonicity to electron-phonon coupling in magnetite's Verwey transition.

Findings

Phonon widths peak away from high-symmetry points indicating incommensurate fluctuations.

Strong phonon anharmonicity explained by ab initio calculations including Coulomb interactions.

Verwey transition involves a broad spectrum of coupled phonons and charge fluctuations.

Abstract

We present the results of inelastic x-ray scattering for magnetite and analyze the energies and spectral widths of the phonon modes with different symmetries in a broad range of temperature 125<T<293 K. The phonon modes with X_4 and Delta_5 symmetries broaden in a nonlinear way with decreasing temperature when the Verwey transition is approached. It is found that the maxima of phonon widths occur away from high-symmetry points which indicates the incommensurate character of critical fluctuations. Strong phonon anharmonicity induced by electron-phonon coupling is discovered within ab initio calculations which take into account local Coulomb interactions at Fe ions. It (i) explains observed anomalous phonon broadening, and (ii) demonstrates that the Verwey transition is a cooperative phenomenon which involves a wide spectrum of phonons coupled to charge fluctuations condensing in the low-symmetry phase.