Berlin 2012 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 43: Correlated Electrons: Metal-Insulator Transition 2
TT 43.8: Vortrag
Donnerstag, 29. März 2012, 17:00–17:15, H 3010
Coherent phonons: an all-optic probe of ultrafast structural phase transitions — •Laura Foglia1, Simon Wall1, Daniel Wegkamp1, Kannatassen Appavoo2, Joyeeta Nag2, Richard F. Haglund2, Julia Stähler1, and Martin Wolf1 — 1Fritz-Haber-Institut der MPG, Dep. of Phys. Chem., Berlin, Germany — 2Dep. of Physics and Astronomy, Vanderbilt University, Nashville, USA
The ionic equilibrium position in a crystal and its vibrational response to an external perturbation are determined by the lattice potential, resulting from the interaction of cores and electrons. During a structural phase transition, the lattice potential symmetry changes, modifying both ionic positions and phonon spectrum. If the transition is driven out of equilibrium, the potential may change on a different time scale than the ionic positions. We exploit the generation of coherent phonons by a laser pulse as an all-optical probe of the lattice potential symmetry with fs time resolution, and apply this technique to the study of the photoinduced phase transition in VO2. Coherent phonons are observed as a modulation of the broadband transient reflectivity for fluences below the transition threshold Φc=6.2mJ/cm2. This modulation, due to the four lowest Raman active modes of the monoclinic VO2 phase, disappears above Φc. By measuring the coherent response of the excited state in a three pulse experiment, we show that the change of the potential symmetry occurs on a sub-phonon-period time scale, much faster than the lattice rearrangement. We conclude that, in this non-equilibrium regime, the photoinduced phase transition is directly driven by electron-induced changes of the lattice potential.