Coherent and incoherent excitations of the Gd(0001) surface on ultrafast timescales

The dynamics of collective excitations of electrons, phonons, and spins are of fundamental interest to develop a microscopic understanding of interactions among elementary excitations and of the respective relaxation mechanisms. The present work employs pump-probe investigations on the femto- and picosecond timescale to study the ultrafast dynamics of electrons, spin-waves, and phonons after intense optical excitation. The Gd(0001) surface, which serves as a model system for a ferromagnetic metal, is investigated by complementary time-resolved techniques, photoelectron spectroscopy and linear/nonlinear optical spectroscopy. The energy relaxation of hot electrons is analysed by transient changes of the electron distribution function and of the complex self-energy of the occupied component of the 5d(z2) surface state. In combination with a simplified description by the two-temperature model this analysis characterizes the optically excited state quantitatively. We analyse the mechanism that leads to a drop in spin polarization upon optical excitation, which is observed in nonlinear magneto-optics. Since the exchange splitting, analysed by photoemission, is not affected under these non-equilibrium conditions, we propose spin-flip scattering of hot electrons to be responsible. The Gd(0001) surface presents a previously unknown coupled phonon-magnon mode, which can be excited by femtosecond laser pulses. Time-resolved detection of the optical second harmonic yield separates spin dynamics from electron and lattice contributions. A coherent phonon-magnon mode at 3 THz, which is driven by electronic excitations of surface and bulk states, is observed. Time-resolved photoemission provides information on the interaction mechanism. We find that the binding energy of the surface state oscillates at the same frequency. In combination with calculations of the surface state binding energy upon lattice contraction this suggests a phonon-magnon coupling due to spin-flip scattering, in contrast to the conventional type mediated by spin-orbit interaction.