Regensburg 2016 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 30: 1D Metal Wires on Semiconductors I

O 30.9: Vortrag

Dienstag, 8. März 2016, 12:45–13:00, S052

Beyond thermal equilibrium: ultrafast non-thermal melting of a surface CDW in the In/Si(111) atomic-wire system — •Tim Frigge, Bernd Hafke, Tobias Witte, Boris Krenzer, and Michael 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 groundstate. We used time-resolved electron diffraction in surface-sensitive geometry to investigate the transient non-equilibrium dynamics of the impulsively driven (8x2)→(4x1) phase transition. Optical excitation of the (8x2) groundstate with fs-laser pulses revealed the existence of a metastable, supercooled (4x1) excited state at 30 K. This phase survives for hundreds of picoseconds because the recovery of the (8x2) phase is hindered due to an energy barrier of 40 meV. At fluences of 3-7 mJ/cm² the CDW groundstate is lifted and the structure changes within 350 fs. This photoinduced transition can not be explained by a simple thermal excitation scenario because laser induced heating takes place on timescales 6 times longer. Instead, we explain the observed dynamics through an accelerated displacive excitation scenario upon changes of the potential energy landscape. This also explains the observation of an increase of the excitation time constant towards lower fluences below 3 mJ/cm².