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O: Fachverband Oberflächenphysik

O 77: Focus Session: Structural Dynamics in Nanoscale Materials Probed by Ultrashort Electron Pulses

O 77.7: Vortrag

Donnerstag, 19. März 2015, 12:45–13:00, MA 005

Fluence-Dependence of the Photo Induced (8×2)↔(4×1) Phase Transition in the In/Si(111) System — •B. Hafke, T. Frigge, B. Krenzer, C. Streubühr, P. Zhou, M. Ligges, D. von der Linde, U. Bovensiepen, and M. Horn-von Hoegen — Universität Duisburg-Essen, Lotharstr. 1, 47057 Duisburg, Deutschland

High-energy electron diffraction at grazing incidence (RHEED) in a pump-probe setup is used to study ultrafast surface phenomena on a femtosecond timescale. We employed this technique to follow the initial dynamics of a driven phase transition of Indium on a Silicon(111) substrate. Optical pumping of the Charge Density Wave (8×2) ground state at a base temperature of 30 K triggers an electronically driven structural phase transition to an undercooled metastable (4×1) phase. For fluences above 1.3 mJ/cm² the entire surface is excited into the (4×1) phase. At lower fluences the surface is excited only partially. We observe fluence-dependent changes of diffraction intensities: For low fluences the transition into the (4×1) phase occurs within 11 ps, at higher fluences of 1.8 mJ/cm² within 5 ps. We explain this by excitation of free electrons upon laser excitation, which accumulate at specific surface areas, e.g. defects, domain walls or steps. These areas act as nucleation seeds for the (8×2)→(4×1) transition which propagates linear in time. At higher fluences more nucleation seeds contribute, resulting in a shorter conversion time. For fluences below 1.3 mJ/cm² the excited carrier density is not sufficient to induce the conversion of the entire surface.