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O: Fachverband Oberflächenphysik

O 93: Focus Session: Structural Dynamics in Nanoscale Materials, Probed by Ultrafast Electron Pulses I

O 93.5: Vortrag

Donnerstag, 15. März 2018, 12:00–12:15, HE 101

Electron-Phonon coupling in thin Pb films on Si(111) from the lattice’s perspective — •Tobias Witte, Tim Frigge, Bernd Hafke, Boris Krenzer, and Michael Horn-von Hoegen — Universität Duisburg-Essen, 47048 Duisburg, Germany

Employing time resolved reflection high energy electron diffraction (tr-RHEED) the non-equilibrium dynamics of the phonon system after optical excitation of ultrathin Pb films and islands on Si(111) has been investigated. The sample is pumped by fs-IR-laserpulses, while the transient response is probed by 30 keV electron pulses. Surface sensitivity is achieved through gracing incidence of the high energy electrons. fs-laserpulses cause impulsive excitation of the electron system of the Pb films and islands, respectively. The hot electron system thermalizes and heats the lattice system by electron-phonon coupling. The thermal motion of the (surface-) atoms is observed through the transient Debye-Waller effect. As Pb is known to exhibit strong electron-phonon coupling in the bulk the time constant for the energy transfer to the lattice system is - in the framework of the two temperature model - expected to be in the sub-ps regime. In contrast, a rather slow time constant of τ = 3.2 ps was observed at the surface. Moreover, incoherent lattice excitation of the islands occurs even slower at a time constant of τ = 4.6 ps. These results demonstrate how low-dimensionality and quantum-size effects reduce the energy flow mediated by electron-phonon coupling.