Berlin 2015 – scientific programme

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O: Fachverband Oberflächenphysik

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

O 77.6: Talk

Thursday, March 19, 2015, 12:30–12:45, MA 005

Initial Dynamics of the Photo Induced (8×2)↔(4×1) Peierls-like Phase Transition of the In/Si(111) System — •T. Frigge, B. Hafke, B. Krenzer, C. Streubühr, P. Zhou, M. Ligges, D. von der Linde, U. Bovensiepen, and M. Horn-von Hoegen — Department of Physics, University of Duisburg-Essen, Lotharstr. 1, 47057 Duisburg, Germany

The wire-type arrangement of Indium atoms on a Silicon(111) surface serves as a famous prototype for the formation of a quasi one-dimensional charge density wave in the groundstate. We used ultra-fast time-resolved surface-sensitive electron diffraction to investigate the transient non-equilibrium dynamics of the impulsively driven (8×2)→(4×1) phase transition. Excitation of the (8×2) groundstate with fs-laser pulses revealed the formation of a metastable, supercooled (4×1) excited state at 30 K. Utilizing a tilted pulse front scheme together with a new 30 keV electron gun design we improved the temporal resolution to less than less 400 fs. The high transversal coherence length of 40 nm allows to follow both the initial dynamics in the unit-cell and the kinetics of the phase front on the nano-scale. At fluences of 4 mJ/cm² the CDW groundstate is lifted across the entire surface on timescales of less than 400 fs. A transient temperature rise of the Indium layer was determined through the intensity of the thermal diffuse background intensity utilizing the Debye-Waller effect. The laser induced heating by less than 40 K takes place on timescales 5 times slower than the metal-to-insulator transition which clearly rules out a simple thermal excitation scenario of the phase transition.