Pushing atomic-scale detection limits with 4D STEM and ptychography*

A new generation of fast, high dynamic range pixelated electron detectors is enabling new research across physics and materials science. Here, we leverage the new capabilities of pixelated detectors in two projects: for atomic scale magnetic imaging and for sub-angstrom imaging in an uncorrected scanning transmission electron microscope (STEM). First, we demonstrate 4D-STEM methods to image the local magnetic moments in antiferromagnetic Fe2As with 6 angstrom spatial resolution. Our techniques utilize magnetic diffraction peaks combined with center-of-mass analysis to directly visualize the magnetic lattice. We illustrate how to robustly isolate the weak magnetic signals, a key challenge for atomic-scale magnetic imaging methods in the electron microscope. Second, we demonstrate sub-angstrom resolution electron ptychography of 2D materials in an uncorrected scanning transmission electron microscope. Counterintuitively, we find that geometric aberrations produce structured beams that are more dose-efficient for ptychography than focused, aberration-free probes. Most importantly, our results demonstrate that expensive aberration correctors are no longer required for atom-by-atom imaging, a significant step towards broadening access to high-end electron microscopy.