Influence of band occupation on electron-phonon coupling in gold

Electron-phonon coupling is a fundamental process that governs the energy relaxation dynamics of solids excited by ultrafast laser pulses. It has been found to strongly depend on electron temperature as well as on nonequilibrium effects. Recently, the effect of occupational nonequilibrium in noble metals, which outlasts the fully kinetic stage, has come into increased focus. In this work, we investigate the influence of nonequilibrium density distributions in gold on the electron-phonon coupling. We find a large effect on the coupling parameter which describes the energy exchange between the two subsystems. Our results challenge the conventional view that electron temperature alone is a sufficient predictor of electron-phonon coupling

Interplay of electron-magnon scattering and spin-orbit induced electronic spin-flip fcattering in a two-band Stoner model

Magnons are one of the carriers of angular momentum that are involved in the ultrafast magnetization dynamics in ferromagnets, but their contribution to the electronic dynamics and their interplay with other scattering process that occur during ultrafast demagnetization has not yet been studied in the framework of a microscopic dynamical model. The present paper presents such an investigation of electronic scattering dynamics in itinerant ferromagnets at the level of Boltzmann scattering integrals for the magnon distributions and spin-dependent electron distributions. In addition to electron-magnon scattering, we include spin-conserving and effective Elliott-Yafet like spin-flip electron-electron scattering processes and the influence of phonons. In our model system, the creation or annihilation of magnons leads to transitions between two spin-split electronic bands with energy and momentum conservation. Due to the presence of spin-orbit coupling, Coulomb scattering transitions between these bands are also possible, and we describe them on an equal footing in terms of Boltzmann scattering integrals. For an instantaneous carrier excitation process we analyze the influence of both interaction processes on the magnon and spin-dependent electron dynamics, and show that their interplay gives rise to an efficient creation of magnons at higher energies and wave vectors accompanied by only a small increase of the electronic spin polarization. These results present a microscopic dynamical scenario that shows how non-equilibrium magnons may dominate the magnetic response of a ferromagnet on ultrafast timescales

An adaptive model for the optical properties of excited gold

We study the temperature-dependent optical properties of gold over a broad energy spectrum covering photon energies below and above the interband threshold. We apply a semi-analytical Drude-Lorentz model with temperature-dependent oscillator parameters. Our approximations are based on the distribution of electrons over the active bands with a density of states provided by density functional theory. This model can be easily adapted to other materials with similar band structures and can also be applied to the case of occupational nonequilibrium. Our calculations show a strong enhancement of the intraband response with increasing electron temperature while the interband component decreases. Moreover, our model compares well with density functional theory-based calculations for the reflectivity of highly excited gold and reproduces many of its key features. Applying our methods to thin films shows a sensitive nonlinear dependence of the reflection and absorption on the electron temperature. These features are more prominent at small photon energies and can be highlighted with polarized light. Our findings offer valuable insights for modeling ultrafast processes, in particular, the pathways of energy deposition in laser-excited samples

Effect of iron thicknesses on spin transport in a Fe/Au bilayer system

This paper is concerned with a theoretical analysis of the behavior of optically excited spin currents in bilayer and multilayer systems of ferromagnetic and normal metals. As the propagation, control and manipulation of the spin currents created in ferromagnets by femtosecond optical pulses is of particular interest, we examine the influence of different thicknesses of the constituent layers for the case of electrons excited several electronvolts above the Fermi level. Using a Monte-Carlo simulation framework for such highly excited electrons, we first examine the spatio-temporal characteristics of the spin current density driven in a Fe layer, where the absorption profile of the light pulses plays an important role. Further, we examine how the combination of light absorption profiles, spin-dependent transmission probabilities, and iron layer thicknesses affect spin current density in a Fe/Au bilayer system. For high-energy electrons studied here, the interface and secondary electron generation have a small influence on spin transport in the bilayer system. However, we find that spin injection from one layer to another is most effective within a certain range of iron layer thicknesses