Dresden 2014 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 100: Surface Dynamics II
O 100.2: Vortrag
Freitag, 4. April 2014, 10:45–11:00, PHY C 213
Time-resolved two-photon photoemission of NiO(100) ultrathin films — •Konrad Gillmeister1, Mario Kiel1, and Wolf Widdra1,2 — 1Martin-Luther-Universität Halle-Wittenberg, Halle, Germany — 2Max-Planck-Institut für Mikrostrukturphysik, Halle, Germany
Although NiO is one of the most important models for a correlated electron system, not much is known about the electron dynamics across its charge-transfer band gap. Here we focus on the dynamics of unoccupied states for well-ordered NiO ultrathin films. The films with thicknesses up to nine monolayers (ML) have been grown via molecular beam epitaxy on Ag(001).
Based on two-photon photoemission spectroscopy, we find for all films unoccupied Ni 3d states in the range of 1.5 to 2.8 eV above EF. The lifetimes of these states are well below 20 fs. This ultrafast dynamics in the presence of the large NiO bandgap points towards the importance of electron correlation for the decay mechanism. At higher energies a series of image potential states is present starting from one NiO monolayer up to nine ML. The states are identified from their parabolic dispersion. With increasing film thickness, the lifetime of the n=1 image state decreases non-monotonically from 30 to 9 fs.