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O: Fachverband Oberflächenphysik
O 13: Ultrafast Electron Dynamics at Surfaces and Interfaces II
O 13.7: Talk
Monday, March 18, 2024, 16:45–17:00, MA 041
Non-Equilibrium Pathways for Excitation of Bulk and Surface Phonons through Anharmonic Coupling — C. Brand1, V. Tinnemann1, A. Hanisch-Blicharski11,3, M. Tajik1, •J. D. Fortmann1, A. Kaßen1, F. Thiemann1, and M. Horn-von Hoegen1,2 — 1Department of Physics, University of Duisburg-Essen, 47048 Duisburg, Germany — 2Center for Nanointegration (CENIDE), 47048 Duisburg, Germany — 3present address: Institut für IT-Management & Digitalisierung, FOM Hochschule, Germany
Upon impulsive optical excitation of solid-state materials, the non-equilibrium flow of energy from the excited electronic system to the lattice degrees of freedom typically happens in a few picoseconds. Here we identified the surface of thin Bi films grown on Si(001) as an additional subsystem which is excited much slower on a 100 ps timescale that is caused by decoupling due to mismatched phonon dispersions relations of bulk and surface. Anharmonic coupling among the phonon systems provides pathways for excitations which exhibits a 1/T-dependence causing a speed-up of surface excitation at higher temperatures. A quantitative justification is provided by phonon Umklapp processes from lattice thermal conductivity of the Bi bulk. Three-temperature model simulations reveal a pronounced non-equilibrium situation up to nanoseconds: initially, the surface is colder than the bulk, that situation is then inverted during cooling and the surface feeds energy back into the bulk phonon system.
Keywords: Non-Equilibrium Dynamics; phonon Umklapp processes; electron-phonon coupling; surface state