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B7: Imaging of short-range correlations at oxide surfaces and interfaces

The electron pair emission from oxide surfaces is studied. The excitation can be mediated by primary electron impact (e,2e experiment) or a photon (double photoemission-DPE). Theoretically we studied the DPE-process from a strongly-correlated system (B. D. Napitu and J. Berakdar, Phys. Rev. B 81, 195108 (2010)). The description was based on the Hubbard model in which the parameter U describes the strength of the electron correlation. An important prediction is the U dependence of the DPE intensity. A typical strongly correlated material is NiO for which one usually uses U=6-8 eV. A experimental (e,2e) study on NiO/Ag(100) films revealed an order of magnitude higher coincidence intensity compared to metals. The result is a first confirmation of the DPE calculation, despite the differences of the pair emission in a (e,2e) and DPE experiment. This is further supported by theoretical considerations which yield similar U dependences for the (e,2e) and DPE  process.

Theory and experiment strongly suggest that under suitable conditions the strength and the dynamical properties of the electron-electron interaction can be accessed via the electron pair emission. DPE experiments on selected oxides like NiO, CoO and ferroelectric materials like BaTiO₃ are planned. These studies will be conducted in the lab by utilizing a set-up with electro-static analyzers. We also plan to perform (e,2e) studies on these materials. Theory will determine the electronic properties and the pair emission spectra beyond the model calculation. In particular, we will develop a theory and implement it numerically which allows a material dependent determination of the non-local and dynamically screened electron-electron interaction. A further theoretical development concerns the dynamical properties of the scattering process. In particular, we will investigate the influence of the hole dynamics on the measured energy spectra.

Principal Investigators

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