Berlin 2015 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 46: Focus Session: THz meets X-ray

O 46.2: Topical Talk

Mittwoch, 18. März 2015, 11:00–11:30, MA 001

Understanding the Ultrafast Insulator-Metal Tranisition in Vanadium Dioxide: An Ultrabroadband Terahertz Perspective — •Alfred Leitenstorfer, Bernhard Mayer, and Alexej Pashkin — Department of Physics and Center for Applied Photonics, University of Konstanz, 78465 Konstanz, Germany

VO₂ is a prototypical oxide with strong electronic correlations. A transition from a dielectric phase to a metal of higher lattice symmetry occurs at 340 K. Interband excitation non-thermally triggers this process on a fs time scale. The multi-THz electronic conductivity sets in instantly after pumping with 12-fs near-infrared pulses. Below a temperature-dependent threshold fluence, the insulating character recovers within 200 fs. Above threshold, switching into the metallic phase occurs. A coherent wave packet motion in the excited state is detected via the anharmonic coupling of V-V stretch motion to optical phonons. The sub-ps damping of the lattice coherent signature is consistent with a retarded structural transition. Surprisingly, a transient metallic phase is reached also when exciting with few-cycle multi-THz pulses at similar fluences. A threshold electric field of 14 MV/cm fits excellently to a model of tunneling breakdown of a Mott insulator with a pair correlation length of 2 Å. The fast decay of conductivity below threshold under NIR excitation is absent: interband tunneling creates electrons and holes in different elementary cells. This fact prevents exciton selftrapping and explains the efficiency of non-resonant multi-THz switching. Resonant pumping of optical phonons is of minor importance as compared to direct tunneling breakdown of the elctronic energy gap.