Day 2 - Simulation of Diffraction Pattern - Duscher

This lecture by Professor Duscher focuses on simulating dynamic electron diffraction patterns, specifically Convergent Beam Electron Diffraction (CBED) and High-Angle Annular Dark Field (HAADF) images, using the pyTEM package. The core concept is the multislice method, which accounts for multiple scattering events by slicing the crystal into thin layers. The lecture begins by contrasting kinematic (single scattering) and dynamic (multiple scattering) regimes, explaining that dynamic effects are significant in electron microscopy. It introduces the Bloch wave approach for solving the Schrödinger equation in periodic potentials, leading to the concept of two-beam conditions and pendulum solutions. The multislice method is then detailed, involving alternating real-space and reciprocal-space calculations. The role of thermal diffuse scattering is emphasized, with the frozen phonon approximation used to simulate the effect of atomic vibrations. The lecture demonstrates simulations of diffraction patterns for strontium titanate, showing the emergence of Kikuchi lines and HOLZ lines with thermal ensembles. Practical considerations such as cell size, number of ensembles, and computational resources are discussed. The lecture concludes with a Q&A session addressing topics like the optimal number of thermal ensembles and the sensitivity of patterns to crystal defects.