Ultrafast dynamics of a coherent phonon-magnon mode at the Gd(0001) surface (Invited Paper)

Abstract

The Gd(0001) surface is investigated by femtosecond pump-probe experiments using laser pulses at 740-860 nm wave length. By non-linear optical second harmonic generation a coherent phonon-magnon mode at a frequency of 3 THz is observed which is excited through the exchange-split surface state. In parallel, electron-electron and electron-phonon interaction and their magnetic counterparts lead to incoherent dynamics of the electron, lattice, and spin subsystems. Variation of the optical wave length shows that for longer wave lengths up to 860 nm the coherent mode is excited predominantly while for shorter ones (less than or equal to 740 nm) incoherent contributions are favored. This presents a strong indication that depopulation of the occupied surface state component drives the coherent excitation. We find identical time scales for damping of the coherent mode and for electron-lattice equilibration which identifies electron-phonon scattering as an important relaxation channel for the coherent contribution. Increasing the temperature results in faster damping indicating that scattering of coherent phonons with thermal ones is an active relaxation channel as well.