Prof. Christoph T. Koch
22/03/2026
אודיטוריום ע"ש דויד וואנג, בניין מידן, קומה 3
13:30
More than 20 years ago it was shown that proper modelling of phonons being thermally excited in a crystal yields highly quantitative electron diffraction simulations of bulk crystals [1]. More recently, it was demonstrated that quantitative fitting of 3D atomistic models to 4D-STEM data sets requires the thermal motion of the atoms to be included. Doing so allowed the atom positions to be determined with a precision in the single pm regime [2]. We have recently shown that, even without assuming any atomistic model, the effect of thermal motion can be incorporated into iterative ptychographic reconstruction schemes, allowing the contribution of all atoms in the sample to be included, i.e. also the amorphous surface layers or contamination [3]. In a parallel development the instrumental improvement of simultaneous spatial and energy-resolution in the (S)TEM has reached a level that makes it possible to study spatial variations in the excitation of atomic vibrations, at least for vibrational energies > 25 meV [4]. Both types of information about the thermal motion of atoms are complimentary to one another, and their accessibility within the same instrument makes it possible to combine them. This talk will present recent experimental results of atomically resolved observation of variations in the phonon spectrum at different grain boundaries in silicon [5]. It will also be experimentally demonstrated that multi-object ptychography, which accounts for thermal diffuse scattering, reveals spatially-resolved statistics of the motion of atoms from which also the low frequency regime of the phonon dispersion can be recovered [3].
References:
[1] D.A. Muller et al., Ultramicroscopy Vol. 86, (2001), 371
[2] B. Diederichs et al. Nature Communications Vol. 15, (2024), 101
[3] A. Gladyshev et al. arXiv:2309.12017 (2023)
[4] O.L. Krivanek et al., Ultramicros-copy 203 (2019) 60-67
[5] B. Haas et al., Nano Letters Vol. 23, (2023), 5975