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S. Löffler, S. Sack, T. Schachinger:
"Electron Vortices in Solids: From Crystalline to Amorphous Materials";
Talk: 13th Multinational Congress on Microscopy, Rovinj, Kroatien; 2017-09-24 - 2017-09-29; in: "13th Multinational Congress on Microscopy", (2017), 58 - 59.

Electron vortex beams (EVBs) are fundamental solutions to the free-space Schrödinger equation in cylindrical coordinates. Their peculiar properties, such as their orbital angular momentum (OAM) and the associated magnetic moment, make them a very interesting novel tool for nanoanalysis. Actual and predicted applications include nanomanipulation, energy-loss magnetic chiral dichroism (EMCD) measurements, and spectroscopy with ultimate energy resolution. Here, we investigate how vortices behave in a non-isotropic environment such as inside a crystalline or amorphous sample. The breaking of the rotational symmetry leads to superpositions of and scattering between different vortex orders, thus leading to a broadening of the OAM distribution. In some cases, such as nanomanipulation, the associated exchange of OAM between the beam and the sample is vital. In other cases, such as EMCD, this is an artifact that can severely distort the outcome of measurements or even prohibit their analysis. To elucidate the influence of elastic scattering, we use the theoretical framework of the multislice formalism. With it, dynamic scattering effects can be described in a general manner and can also be compared to numerical simulations. For crystalline materials, it is well-known that depending on the experimental geometry, the average OAM can oscillate drastically throughout the sample, but can also be protected by channeling along atomic columns (1). In amorphous materials, we find that the average OAM remains approximately unaffected over a significant thickness range (see Figure 1). Here, an incident m=1 beam was used. Of more practical relevance than the average OAM is the intensity of the m=1 component itself. This is found to decrease due to scattering to other vortex orders, but the m=1 component still remains dominant for a large range of thicknesses. It is noteworthy that the specific behavior depends
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on both the atomic weight of the scatterer and the size of the EVB. These results are also combined with inelastic simulations to calculate the (defocus-dependent) EMCD effect when using EVBs. Vortex-EMCD has the unique benefit over classical EMCD that the sample does not need to act as a beam splitter. Therefore, vortex-EMCD is directly applicable to amorphous materials. Hence, it is clear that understanding the scattering behavior of EVBs is not only required for analyzing experimental data, but also paves the way for the development and efficient use of novel, emerging techniques for nanoanalysis.