Spatio-Temporal Electron Propagation Dynamics in Au/Fe/MgO(001) in nonequilibrium: Revealing Single Scattering Events and the Ballistic Limit

TL;DR

This study investigates the microscopic spatio-temporal dynamics of hot electron transport in Au/Fe/MgO heterostructures, revealing the transition from super-diffusive to ballistic regimes and how layer thickness influences electron velocity and transport behavior.

Contribution

It combines femtosecond time-resolved spectroscopy with real-time density functional theory and microscopic simulations to elucidate energy-dependent electron propagation regimes in heterostructures.

Findings

Identification of ballistic transport of minority electrons at certain energies.

Observation of super-diffusive transport with 1 to 4 scattering events at lower energies.

Electron velocity increases from 0.3 to 1 nm/fs with Au layer thickness.

Abstract

Understanding the microscopic spatio-temporal dynamics of nonequilibrium charge carriers in heterosystems promises optimization of process and device design towards desired energy transfer. Hot electron transport is governed by scattering with other electrons, defects, and bosonic excitations. Analysis of the energy dependence of scattering pathways and identification of diffusive, super-diffusive, and ballistic transport regimes are current challenges. We determine in femtosecond time-resolved two-photon photoelectron emission spectroscopy the energy-dependent change of the electron propagation time through epitaxial Au/Fe(001) heteostructures as a function of Au layer thickness for energies of 0.5 to \unit[2.0]{eV} above the Fermi energy. We describe the laser-induced nonequilibrium electron excitation and injection across the Fe/Au interface using real-time time-dependent density functional theory and analyze the electron propagation through the Au layer by microscopic electron transport simulations. We identify ballistic transport of minority electrons at energies with a nascent, optically excited electron population which is determined by the combination of photon energy and the specific electronic structure of the material. At lower energy, super-diffusive transport with 1 to 4 scattering events dominates. The effective electron velocity accelerates from 0.3 to \unit[1]{nm/fs} with an increase in the Au layer thickness from 10 to 100~nm. This phenomenon is explained by electron transport that becomes preferentially aligned with the interface normal for thicker Au layers, which facilitates electron momentum / energy selection by choice of the propagation layer thickness.