Local and Nonlocal Electron Dynamics of Au / Fe / MgO ( 001 ) Heterostructures Analyzed by Time-Resolved Two-Photon Photoemission Spectroscopy

Employing femtosecond laser pulses in front and back side pumping of Au / Fe / MgO ( 001 ) combined with detection in two-photon photoelectron emission spectroscopy, we analyze local relaxation dynamics of excited electrons in buried Fe, injection into Au across the Fe-Au interface, and electron transport across the Au layer at 0.6 to 2.0 eV above the Fermi energy. By analysis as a function of Au film thickness we obtain the electron lifetimes of bulk Au and Fe and distinguish the relaxation in the heterostructure's constituents. We also show that the excited electrons propagate through Au in a superdiffusive regime and conclude further that electron injection across the epitaxial interface proceeds ballistically by electron wave packet propagation.