Berlin 2015 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 70: Structural Dynamics in Nanoscale Materials Probed by Ultrashort Electron Pulses

O 70.5: Poster

Mittwoch, 18. März 2015, 18:15–21:00, Poster A

Delayed surface phonon excitation in Bi(111) films — Verena N. Tinnemann, Tim Frigge, •Boris Krenzer, Bernd Hafke, Annika Kalus, Carla Streubühr, Ping Zhou, Manuel Ligges, Dietrich von der Linde, Uwe Bovensiepen, and Michael Horn-von Hoegen — Universität Duisburg-Essen, Lotharstr. 1, 47057 Duisburg

We used ultra-fast reflection high energy electron diffraction to follow the excitation of surface phonons on an epitaxial Bi(111) film on Si(111) upon impulsive excitation with a femtosecond laserpulse. The thermal motion of the atoms is determined from the transient intensity drop described through the Debye-Waller effect. The excitation of vibrational motion of the surface atoms occurs with a time constant of 12 ps. For strained Bi(111) films grown on Si(001) we observe a bi-exponential behaviour with a fast (< 6 ps) and slow, temperature dependent (12 to 60 ps) component for the vibrational excitation of the surface. We attribute the different response to the presence of an electronic surface state for relaxed 4.5 nm thick Bi films on Si(111) while this state is absent on the strained 4.5 nm thick Bi films grown on Si(001). In the first case the excited carriers populate the surface state without heating the bulk and weak electron phonon coupling excite surface phonons on a timescale of 12 ps. On Si(001) the excited carriers remain in the bulk of the film, couple to phonons on a much faster time scale < 6 ps, and surface phonons are then excited only by anharmonic coupling between bulk and surface phonon modes.