Berlin 2018 – wissenschaftliches Programm
O 101: Focus Session: Structural Dynamics in Nanoscale Materials, Probed by Ultrafast Electron Pulses II
O 101.3: Vortrag
Donnerstag, 15. März 2018, 16:15–16:30, HE 101
Optically excited structural transition in atomic wires on surfaces at the quantum limit: a femtosecond ultrafast surface electron diffraction study — Tim Frigge1, Bernd Hafke1, Tobias Witte1, Boris Krenzer1, Manuel Ligges1, Uwe Bovensiepen1, Stefan Wippermann2, Wolf Gero Schmidt2, and •Michael Horn-von Hoegen1 — 1Fakultät für Physik und CENIDE, Uni Duisburg-Essen, Germany — 2Theoretische Materialphysik, Uni Paderborn, Germany
The Indium induced (4x1) reconstruction on Si(111) is the prototype for 1D atomic wires at surfaces. At 130 K a metal-insulator transition to the (8x2) ground state takes place. A Peierls-like distortion causes periodicity doubling, opening of a band gap, and formation of a CDW. The non-equilibrium structural dynamics is studied by ultra-fast RHEED with a fs-laser system in pump probe setup at temporal resolution better than 300 fs. Upon photo excitation the (8x2) ground state is driven in 350 fs to the (4x1) excited state as observed through the transient spot intensity changes. The transition is described in an accelerated displacive excitation scenario which relies on transient changes in the potential energy surface. The strong coupling between substrate and adsorbate is responsible for the sub-picosecond structural response by dephasing and damping the characteristic phonons in 1/4th of their oscillation period. Transient heating of the In atoms from 30 K to 80 K occurs delayed on a time scale of 2.2 ps. Thus the phase transition is driven by electronic entropy and not thermally.