Spatio-Temporal Electron Propagation Dynamics in Au/Fe/MgO(001) in nonequilibrium: Revealing Single Scattering Events and the Ballistic Limit
arxiv(2023)
摘要
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 [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 [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.
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