Spatio-Temporal Electron Propagation Dynamics in Au/Fe/MgO(001) in Nonequilibrium

日時　2025年10月16日　（水）　14：00-15：00
場所　放射光科学研究所　2階　セミナー室
講師　Florian Denizer
(Faculty of Physics, University of Duisburg-Essen and CENIDE)

Optically excited charge carriers relax in metals and semiconductors on femtosecond to picosecond timescales. These electrons travel in metals ballistically on a femtosecond timescale until they scatter due to electron-electron or electron-phonon interactions. Here I will present an energy-resolved analysis of relaxation and transport processes of excited electrons in the time domain. We perform time-resolved two-photon photoelectron spectroscopy (tr-2PPE) with a particular excitation scheme; back-side pumping and front-side probing on epitaxially grown Au/Fe/MgO(001). The hot electrons are excited in the buried Fe layer 2 eV above the Fermi level ????F and experience subsequently spatiotemporal transport through the Fe-Au interface towards the Au surface and local inelastic relaxation. By analysis of the relaxation dynamics as a function of Au film thickness, we determine the electron lifetimes of bulk Au and Fe and distinguish the relaxation in the heterostructure constituents [1]. Additional measurements and comparison to front-side pumping allow us to discuss contributions of laser absorption profiles [2]. To gain a deeper understanding of the ballistic and few scattered electrons under non-equilibrium conditions, we combine time-resolved two-photon photoelectron emission spectroscopy with real-time time-dependent density functional theory(RT-TDDFT) as well as a random-walk-like transport simulation [3]. We find that the electron trajectories’ angular dependence is key to explaining the apparent thickness-dependent acceleration.

Funding by the DFG through Project No. 278162697 – SFB1242 is gratefully acknowledged.
[1] Y. Beyazit et al., “Local and nonlocal electron dynamics of Au/Fe/MgO(001) heterostructures analyzed by time-resolved two-photon photoemission spectroscopy” PRL, 125, 076803 (2020).
[2] Y. Beyazit et al., “Ultrafast electron dynamics in Au/Fe/MgO(001) analyzed by Au- and Fe-selective pumping in time-resolved two-photon photoemission spectroscopy: Separation of excitations in adjacent metallic layers” PRB 107, 085412 (2023).
[3] M. Heckschen et al., “Spatio-temporal electron propagation dynamics in Au/Fe/MgO(001) in nonequilibrium: Revealing single scattering events and the ballistic limit” PRX Energy 2, 043009 (2023).

問合せ先　黒田健太(先進理工系科学研究科)、出田真一郎(放射光科学研究所）