Speaker
Description
A localized optical excitation of a metal/insulator heterostructure induces
ultrafast dynamics in its individual compounds, which can involve
charge and spin transfer processes as well as coupling to low
energy excitations mediated by e.g. electron-electron and electron-phonon
scattering. Femtosecond soft x-ray spectroscopy facilitates to
separate and identify these electronic and lattice excitations directly
in the time domain and, furthermore, is sensitive to the dynamics of
the individual constituents itself due to its element-specific character.
We have measured time- and element-resolved x-ray absorption spectroscopy
of a [2nm Fe/2nm MgO]8 multilayer at the Fe 𝐿- and O 𝐾-
edges with a time resolution of 150 fs. After optically exciting locally
Fe with a UV laser pulse of 266 nm wavelength we see a clear pump-induced
effect at both edges in fs time resolution. The Fe-signal shows
an ultrafast 0.5% dropdown of the signal in 240 fs, followed by a recovery
on a 1 ps timescale, while O reaches its maximum not until 1 ps.
This slower response suggests that the energy transfer from the metal
to the insulator is mediated by phonons, or a combined electronic-phononic
process rather than by a direct charge transfer excitation.
