Berlin 2015 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 96: Ultrafast Electron Dynamics at Surfaces and Interfaces

O 96.1: Hauptvortrag

Freitag, 20. März 2015, 10:30–11:00, MA 004

Ultrafast electron dynamics at oxide surfaces: How metallic is a semiconductor? — •Julia Stähler — Fritz-Haber-Institut der Max-Planck-Gesellschaft, Dept. of Physical Chemistry, Faradayweg 4-6, 14195 Berlin, Germany

Light absorption in a semiconductor creates non-equilibrium conditions that relax by a multitude of pathways involving electron dynamics on femto- and picosecond timescales. These can be monitored in real time using time-resolved one- and two-photon photoelectron spectroscopy of the occupied and unoccupied electronic structure, respectively. On ZnO(1010), hydrogen adsorption causes the formation of a charge accumulation layer through downward surface band bending. Despite this metallicity, highly stable sub surface-bound excitons form within only 200 fs after above band gap photoexcitation. Strong excitation close to the Mott limit enhances the screening of the Coulomb interaction (CIA) and reduces the exciton formation probability [1]. On the other hand, in the case of the strongly correlated electron material VO₂, strong photoexcitation even leads to an instantaneous collapse of the band gap, followed by hot carrier relaxation within 200 fs. In conjunction with many body perturbation theory, these results show that the photoinduced semiconductor-to-metal transition is caused by photohole doping at the top of the VO₂ valence band: The significantly enhanced screening of the CIA through low-energy intraband transitions causes the drastic band gap renormalization [2].

[1] J.-C. Deinert et al., Phys Rev Lett 113, 057602 (2014)

[2] D. Wegkamp, M. Herzog et al., Phys Rev Lett 113, 216401(2014)