Dresden 2011 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 59: Poster Session III (Nanotribology; Polymeric biomolecular films; Organic electronics and photovoltaics, Covalent networks on surfaces; Phase transitions; Particles and clusters; Transparent conductive oxides)
O 59.13: Poster
Mittwoch, 16. März 2011, 17:30–21:00, P3
Coherent phonons: an ultrafast probe of the VO2 phase transition — •Laura Foglia1, Simon Wall1, Daniel Wegkamp1, Joyeeta Nag2, Richard F. Haglund2, Julia Stähler1, and Martin Wolf1 — 1Fritz-Haber Institut der MPG, Dep. of Physical Chemistry, Berlin, Germany — 2Dep. of Physics and Astronomy, Vandebilt University, Nashville, TN, USA
The nature of the insulator to metal transition in VO2 at 340 K has been a matter of controversy, since both lattice distortion and electronic correlations play a crucial role. Understanding the ultrafast dynamics of the photoinduced phase transition (PPT) may elucidate the contributions of electrons and lattice to the process. We present transient reflectivity data showing a strong change in the coherent phonon spectrum crossing the critical fluence Φc=6.2 mJ/cm2, which can be explained by two scenarios: (i) The PPT is driven by the increase of the lattice temperature due to energy transfer from the photoexcited electrons. (ii) The sudden electronic excitation is sufficient to alter the symmetry of the lattice potential, even before ionic motion occurs. In a pump-pump-probe experiment we excite coherent phonons to monitor the changes of the lattice symmetry on a femtosecond time scale. Below Φc, we are able to coherently control the amplitude of the oscillations by changing the delay between the two pump pulses. Above threshold, this is not possible; on the contrary, the dynamics after the 2nd pump pulse is comparable to the one of the thermally induced metallic phase. These results suggest that the PPT begins with a lattice potential symmetry change due to the strong electronic excitation.