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

The charge and spin correlations at surfaces and interfaces of ferro-, antiferromagnetic and ferroelectric oxides will be studied by means of electron induced two-electron emission (e,2e). It is known that the measured spectra give access to the spin-resolved spectral function as well as to the screened Coulomb interaction in the system. In the theoretical part of our project these two functions will numerically be computed on the basis of the many-body perturbation theory. On the experimental side, we investigate the two-electron emission from the NiO @ Fe compound and at a later stage from of Fe @ BaTiO₃.

Schematically the (e,2e) process is shown below. Two experimental realizations, reflection and transmission geometry, are possible

The two possible experimental realizations

An electron with the energy E₀ and momentum k₀ is scattered at the surface and is detected with the energy E₁ and momentum k₁. Simultaneously, a second electron is emitted from the sample and is detected in coincidence with the first electron. From the conservation laws one infer the amount of energy (E) and momentum (q) that have been transferred to the system. At certain conditions the probability (I) of such a process as a function of q and E can be related to the product of the single-particle spectral density (obtained from the retarded Green function G) and the screened Coulomb interaction (W).

This constitutes the basic formula for the interpretation of the experimental results.

The use of spin-polarized electrons for probing the sample is an important aspect of the present study. We will measure the dependence of I (coincidence signal) on the angle between the spin-polarization vector of the incident electrons and the magnetization axis of the antiferromagnetic NiO. Here the Fe-substrate will be used in order to fix the quantization direction of the magnetic moments in NiO. It is planned to perform corresponding symmetry analysis of the (e,2e) response for antiferromagnetic system. This should facilitate further theoretical analysis based on the ab initio calculations.

Although (e,2e) spectroscopy even with spin-polarized electrons is well developed, preparation of layered samples, viz. NiO @ Fe and Fe @ BaTiO₃ will be one of the main challenges. Here, however, we expect a tight collaboration with other groups involved with extensive expertise in the field. On the theoretical side the calculations of the electronic structure of NiO is a complicated electron correlation problem due to its partially filled d-orbitals. Our approach will be based on version of the density functional theory beyond the local density approximation and the so-called GW method.