Non-Equilibrium Pathways for Excitation of Bulk and Surface Phonons through Anharmonic Coupling
Abstract
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
Cite
@article{arxiv.2310.15740,
title = {Non-Equilibrium Pathways for Excitation of Bulk and Surface Phonons through Anharmonic Coupling},
author = {C. Brand and V. Tinnemann and A. Hanisch-Blicharski and M. Tajik and J. D. Fortmann and A. Kaßen and F. Thiemann and M. Horn-von Hoegen},
journal= {arXiv preprint arXiv:2310.15740},
year = {2024}
}