Symmetry is inherent to many physical systems. If we can identify the symmetries in a physical system, we can distill complex information into its irreducible components and laws.
In electron diffraction, the strong interaction of the electron with the electrostatic potential of the specimen means that the symmetry of the specimen is encoded in the symmetry of the scattered electron intensity distribution, e.g. [1]. This is illustrated beautifully in the vivid symmetries of convergent beam electron diffraction (CBED) patterns. In scanning CBED (or “4D-STEM”), a focused electron probe is raster scanned across the specimen and the corresponding CBED pattern for each probe position is recorded, providing a wealth of information about the specimen.
Here we propose a new image contrast mechanism for atomic resolution STEM based on a measurement of the degree of symmetry in the scattered intensity distribution at each point of a 4D-STEM scan: Symmetry STEM [2]. Due to the acute sensitivity of CBED patterns to local symmetry, the method generates local maxima at atomic positions that are extremely sharp. Furthermore, the method is robust with respect to specimen thickness, electron energy and defocus and sensitive to both light and heavy atomic columns. We demonstrate proof-of-principle of the method with high contrast atomic resolution images of CeB6 taken with a double-corrected FEI Titan 80-300 fitted with an EMPAD detector [3].
[1] M. Tanaka, Point-group and space-group determination by CBED, International Tables for Crystallography, Vol. B: Reciprocal space (Chester: International Union of Crystallography, 2010) Chap. 2.5.3, pp. 307-356.
[2] M. Krajnak, J. Etheridge, arXiv preprint arXiv:1903.04780 (2019) and submitted (2019).
[3] MW Tate, P Purohit, D Chamberlain, KX Nguyen, R Hovden et. al. Microscopy and Microanalysis 22, 237-249 (2016)
Acknowledgements:
This research was supported by the Australian Research Council (ARC) grant DP150104483 and equipment funded by ARC grant LE0454166.