Ultrafast carrier and coherent phonon dynamics in van der Waals metallic ferromagnet Fe4GeTe2

Abstract

Two-dimensional van der Waals ferromagnets are attractive for their potential in novel technological applications. In this work, femtosecond transient optical spectroscopy is used to study the dynamics of photoexcited carriers and coherent optical phonons in Fe₄GeTe₂ single crystals at 10-300 K. The optical response consists of exponential decay due to the relaxation of carriers and damped oscillations due to the A_1g phonon vibration. Our measurements reveal an anomalous temperature dependence of electron-phonon thermalization time around a critical temperature of spin-reorientation transition, which can be attributed to the abrupt change of electronic structure near the Fermi surface. We also discover a spin-lattice relaxation process showing an anomaly below the Curie temperature. Such behavior originates from the temperature dependence of the magnetic specific heat. The damped oscillations of the coherent optical phonons can be well described with the anharmonicity model including lattice thermal expansion and phonon-phonon coupling. Our findings provide valuable insight into the nonequilibrium carrier and lattice properties in Fe₄GeTe₂.