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

Abstract

The dynamics of collective excitations of electrons, phonons, and spins are offundamental interest to develop a microscopic understanding of interactions amongelementary excitations and of the respective relaxation mechanisms. The present workemploys pump–probe investigations on the femto- and picosecond timescale tostudy the ultrafast dynamics of electrons, spin-waves, and phonons after intenseoptical excitation. The Gd(0001) surface, which serves as a model system for aferromagnetic metal, is investigated by complementary time-resolved techniques,photoelectron spectroscopy and linear/nonlinear optical spectroscopy. The energyrelaxation of hot electrons is analysed by transient changes of the electron distributionfunction and of the complex self-energy of the occupied component of the5dz2 surface state. In combination with a simplified description by the two-temperature modelthis analysis characterizes the optically excited state quantitatively. We analyse themechanism that leads to a drop in spin polarization upon optical excitation, which isobserved in nonlinear magneto-optics. Since the exchange splitting, analysed byphotoemission, is not affected under these non-equilibrium conditions, we propose spin-flipscattering of hot electrons to be responsible. The Gd(0001) surface presents a previouslyunknown coupled phonon–magnon mode, which can be excited by femtosecond laser pulses.Time-resolved detection of the optical second harmonic yield separates spin dynamicsfrom electron and lattice contributions. A coherent phonon–magnon mode at3 THz, which is driven by electronic excitations of surface and bulk states, isobserved. Time-resolved photoemission provides information on the interactionmechanism. We find that the binding energy of the surface state oscillates at the samefrequency. In combination with calculations of the surface state binding energyupon lattice contraction this suggests a phonon–magnon coupling due to spin-flipscattering, in contrast to the conventional type mediated by spin–orbit interaction.