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

Abstract

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 timeresolved 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 z 2 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.

Exchange scattering as the driving force for ultrafast all-optical and bias-controlled reversal in ferrimagnetic metallic structures

“…Electron-electron inelastic scattering, responsible for the electron thermalization is, typically, in the range of 50-300 fs. [30][31][32][33] In the non-equilibrium state the A and B subsystems can be also considered as partly decoupled from each other and, therefore, we can discriminate between partial values of the critical temperatures T A;B C of these subsystems. We will consider the Curie temperatures for uncoupled A (pure TM metal) and B (pure TM or RE metals) systems as the partial critical temperatures T A C and T B C , respectively.…”

Femtosecond Spin Current Pulses Generated by the Nonthermal Spin-Dependent Seebeck Effect and Interacting with Ferromagnets in Spin Valves

“…The discovery of the spin-dependent Seebeck effect (SdSE), where thermal gradients over a bulk FM [2] or across an interface to a normal metal [3] generate SCs, opened a path towards overcoming such limitations. Indeed, shortlived thermal gradients can produce short (∼100 ps) SC pulses at densities exceeding the static Joule limit, as recently demonstrated upon laser excitation of spin-valve structures [4].Creating highly energetic electrons [5][6][7][8][9], femtosecond laser excitation is promising for SC pulse generation on subpicosecond time scales, before the electron-electron [9] and electron-lattice [8] equilibration is reached. …”

“…Creating highly energetic electrons [5][6][7][8][9], femtosecond laser excitation is promising for SC pulse generation on subpicosecond time scales, before the electron-electron [9] and electron-lattice [8] equilibration is reached.…”

“…Creating highly energetic electrons [5][6][7][8][9], femtosecond laser excitation is promising for SC pulse generation on subpicosecond time scales, before the electron-electron [9] and electron-lattice [8] equilibration is reached. Superdiffusive transport of laser-excited spin-polarized hot carriers on a femtosecond time scale [10,11] was evidenced in a plethora of experiments [12][13][14][15][16][17][18].…”

Extracting the temperature of hot carriers in time- and angle-resolved photoemission

“…Electron-electron scattering processes on the femtosecond time scale [8,9] lead to an ultrafast thermalization of the photoexcited electrons to a Fermi-Dirac distribution with a temperature that can be significantly higher than that of the lattice. This population of hot electrons then subsequently cools by transferring its energy to the lattice through electron-phonon scattering events [10,11].…”