Berlin 2012 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 8: Electron and spin dynamics

O 8.9: Vortrag

Montag, 26. März 2012, 12:30–12:45, A 060

Time-resolved 2PPE study on NiO(001) thin films — •Mario Kiel, Stephan Grosser, and Wolf Widdra — Martin-Luther-Universität Halle-Wittenberg, 06120 Halle, Germany

The electronic structure of unoccupied states within ultra-thin films of the transition metal nickel oxide has been investigated by time-resolved two-photon photoemission (2PPE) in combination with scanning tunneling spectroscopy (STS). The thin films have been prepared under UHV conditions by molecular beam epitaxy in an oxygen atmosphere on a Ag(001) substrate at room temperature.

For the NiO bilayer a series of image-potential states has been identified. From time-resolved data lifetimes of 30, 50, and 120 fs have been determined for the n=1–3 image state, respectively. The observed binding energies can be well described on the basis of the dielectric properties of NiO. For the bilayer the permittivity is already close to the bulk like. Besides the image-potential states unoccupied Ni 3d states can be identified for the NiO bilayer as well as for thicker films in the 2PPE spectra. These states exhibit layer-dependent energy shifts which will be compared with layer-resolved STS data in the range from 2 to 8 monlolayers NiO. Additionally, 2PPE features from occupied NiO states have been identified due to their characteristic energy shift upon photon energy variation.